Merge pull request #84 from xdep/testing-branch

feat: Add VHF DSC Channel 70 monitoring and decoding for vessels page
2026-04-30 09:39:58 -07:00 · 2026-01-25 13:06:32 +00:00
parent 3b238c3c8f ab2d7bfe50
commit 7a2b90055a
20 changed files with 3718 additions and 11 deletions
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,7 +2,7 @@

 All notable changes to iNTERCEPT will be documented in this file.

-## [2.10.0] - 2026-01-24
+## [2.10.0] - 2026-01-25

 ### Added
 - **AIS Vessel Tracking** - Real-time ship tracking via AIS-catcher
@@ -11,17 +11,30 @@ All notable changes to iNTERCEPT will be documented in this file.
  - Navigation data: speed, course, heading, rate of turn
  - Ship type classification and dimensions
  - Multi-SDR support (RTL-SDR, HackRF, LimeSDR, Airspy, SDRplay)
+- **VHF DSC Channel 70 Monitoring** - Digital Selective Calling for maritime distress
+  - Real-time decoding of DSC messages (Distress, Urgency, Safety, Routine)
+  - MMSI country identification via Maritime Identification Digits (MID) lookup
+  - Position extraction and map markers for distress alerts
+  - Prominent visual overlay for DISTRESS and URGENCY alerts
+  - Permanent database storage for critical alerts with acknowledgement workflow
 - **Spy Stations Database** - Number stations and diplomatic HF networks
  - Comprehensive database from priyom.org
  - Station profiles with frequencies, schedules, operators
  - Filter by type (number/diplomatic), country, and mode
  - Tune integration with Listening Post
  - Famous stations: UVB-76, Cuban HM01, Israeli E17z
+- **SDR Device Conflict Detection** - Prevents collisions between AIS and DSC
+- **DSC Alert Summary** - Dashboard counts for unacknowledged distress/urgency alerts
 - **AIS-catcher Installation** - Added to setup.sh for Debian and macOS

 ### Changed
 - **UI Labels** - Renamed "Scanner" to "Listening Post" and "RTLAMR" to "Meters"
 - **Pager Filter** - Changed from onchange to oninput for real-time filtering
+- **Vessels Dashboard** - Now includes VHF DSC message panel alongside AIS tracking
+- **Dependencies** - Added scipy and numpy for DSC signal processing
+
+### Fixed
+- **DSC Position Decoder** - Corrected octal literal in quadrant check

 ---

--- a/README.md
+++ b/README.md
@@ -29,7 +29,7 @@ Support the developer of this open-source project
 - **Pager Decoding** - POCSAG/FLEX via rtl_fm + multimon-ng
 - **433MHz Sensors** - Weather stations, TPMS, IoT devices via rtl_433
 - **Aircraft Tracking** - ADS-B via dump1090 with real-time map and radar
- **Vessel Tracking** - AIS ship tracking via AIS-catcher with maritime map
+- **Vessel Tracking** - AIS ship tracking with VHF DSC distress monitoring
 - **ACARS Messaging** - Aircraft datalink messages via acarsdec
 - **Listening Post** - Frequency scanner with audio monitoring
 - **Satellite Tracking** - Pass prediction using TLE data
--- a/app.py
+++ b/app.py
@@ -37,6 +37,7 @@ from utils.constants import (
    MAX_WIFI_NETWORK_AGE_SECONDS,
    MAX_BT_DEVICE_AGE_SECONDS,
    MAX_VESSEL_AGE_SECONDS,
+    MAX_DSC_MESSAGE_AGE_SECONDS,
    QUEUE_MAX_SIZE,
 )
 import logging
@@ -145,6 +146,12 @@ ais_process = None
 ais_queue = queue.Queue(maxsize=QUEUE_MAX_SIZE)
 ais_lock = threading.Lock()

+# DSC (Digital Selective Calling)
+dsc_process = None
+dsc_rtl_process = None
+dsc_queue = queue.Queue(maxsize=QUEUE_MAX_SIZE)
+dsc_lock = threading.Lock()
+
 # TSCM (Technical Surveillance Countermeasures)
 tscm_queue = queue.Queue(maxsize=QUEUE_MAX_SIZE)
 tscm_lock = threading.Lock()
@@ -175,6 +182,9 @@ adsb_aircraft = DataStore(max_age_seconds=MAX_AIRCRAFT_AGE_SECONDS, name='adsb_a
 # Vessel (AIS) state - using DataStore for automatic cleanup
 ais_vessels = DataStore(max_age_seconds=MAX_VESSEL_AGE_SECONDS, name='ais_vessels')

+# DSC (Digital Selective Calling) state - using DataStore for automatic cleanup
+dsc_messages = DataStore(max_age_seconds=MAX_DSC_MESSAGE_AGE_SECONDS, name='dsc_messages')
+
 # Satellite state
 satellite_passes = []  # Predicted satellite passes (not auto-cleaned, calculated)

@@ -185,6 +195,7 @@ cleanup_manager.register(bt_devices)
 cleanup_manager.register(bt_beacons)
 cleanup_manager.register(adsb_aircraft)
 cleanup_manager.register(ais_vessels)
+cleanup_manager.register(dsc_messages)


 # ============================================
@@ -516,6 +527,7 @@ def health_check() -> Response:
            'aprs': aprs_process is not None and (aprs_process.poll() is None if aprs_process else False),
            'wifi': wifi_process is not None and (wifi_process.poll() is None if wifi_process else False),
            'bluetooth': bt_process is not None and (bt_process.poll() is None if bt_process else False),
+            'dsc': dsc_process is not None and (dsc_process.poll() is None if dsc_process else False),
        },
        'data': {
            'aircraft_count': len(adsb_aircraft),
@@ -523,6 +535,7 @@ def health_check() -> Response:
            'wifi_networks_count': len(wifi_networks),
            'wifi_clients_count': len(wifi_clients),
            'bt_devices_count': len(bt_devices),
+            'dsc_messages_count': len(dsc_messages),
        }
    })

@@ -531,7 +544,7 @@ def health_check() -> Response:
 def kill_all() -> Response:
    """Kill all decoder and WiFi processes."""
    global current_process, sensor_process, wifi_process, adsb_process, ais_process, acars_process
-    global aprs_process, aprs_rtl_process
+    global aprs_process, aprs_rtl_process, dsc_process, dsc_rtl_process

    # Import adsb and ais modules to reset their state
    from routes import adsb as adsb_module
@@ -580,6 +593,11 @@ def kill_all() -> Response:
        aprs_process = None
        aprs_rtl_process = None

+    # Reset DSC state
+    with dsc_lock:
+        dsc_process = None
+        dsc_rtl_process = None
+
    return jsonify({'status': 'killed', 'processes': killed})


--- a/bin/dsc-decoder
+++ b/bin/dsc-decoder
@@ -0,0 +1,13 @@
+#!/bin/bash
+# DSC (Digital Selective Calling) decoder wrapper
+# Invokes the Python DSC decoder module
+
+# Get the directory where this script is located
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+PROJECT_ROOT="$(dirname "$SCRIPT_DIR")"
+
+# Set PYTHONPATH to include project root
+export PYTHONPATH="${PROJECT_ROOT}:${PYTHONPATH}"
+
+# Run the decoder module
+exec python3 -m utils.dsc.decoder "$@"
--- a/config.py
+++ b/config.py
@@ -15,10 +15,10 @@ CHANGELOG = [
        "version": "2.10.0",
        "date": "January 2026",
        "highlights": [
-            "AIS vessel tracking with real-time maritime map",
+            "AIS vessel tracking with VHF DSC distress monitoring",
            "Spy Stations database (number stations & diplomatic HF)",
-            "Multi-SDR support for AIS (RTL-SDR, HackRF, LimeSDR, etc.)",
-            "UI improvements: renamed modes for clarity",
+            "MMSI country identification and distress alert overlays",
+            "SDR device conflict detection for AIS/DSC",
        ]
    },
    {
--- a/docs/FEATURES.md
+++ b/docs/FEATURES.md
@@ -57,6 +57,31 @@ Complete feature list for all modules.
  <img src="/static/images/screenshots/screenshot_radar.png" alt="Screenshot">
 </p>

+## AIS Vessel Tracking
+
+- **Real-time vessel tracking** via AIS-catcher or rtl_ais
+- **Full-screen dashboard** - dedicated popout with maritime map
+- **Interactive Leaflet map** with OpenStreetMap tiles (dark-themed)
+- **Vessel trails** - optional track history visualization
+- **Vessel details popup** - name, MMSI, callsign, destination, ship type, speed, heading
+- **Country identification** - flag lookup via Maritime Identification Digits (MID)
+
+### VHF DSC Channel 70 Monitoring
+
+Digital Selective Calling (DSC) monitoring on the international maritime distress frequency.
+
+- **Real-time DSC decoding** - Distress, Urgency, Safety, and Routine messages
+- **MMSI country lookup** - 180+ Maritime Identification Digit codes
+- **Distress nature identification** - Fire, Flooding, Collision, Sinking, Piracy, MOB, etc.
+- **Position extraction** - Automatic lat/lon parsing from distress messages
+- **Map markers** - Distress positions plotted with pulsing alert markers
+- **Visual alert overlay** - Prominent popup for DISTRESS and URGENCY messages
+- **Audio alerts** - Notification sound for critical messages
+- **Alert persistence** - Critical alerts stored permanently in database
+- **Acknowledgement workflow** - Track response status with notes
+- **SDR conflict detection** - Prevents device collisions with AIS tracking
+- **Alert summary** - Dashboard counts for unacknowledged distress/urgency
+
 ## Satellite Tracking

 - **Full-screen dashboard** - dedicated popout with polar plot and ground track
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [project]
 name = "intercept"
-version = "2.9.5"
+version = "2.10.0"
 description = "Signal Intelligence Platform - Pager/433MHz/ADS-B/Satellite/WiFi/Bluetooth"
 readme = "README.md"
 requires-python = ">=3.9"
--- a/requirements.txt
+++ b/requirements.txt
@@ -10,6 +10,10 @@ bleak>=0.21.0
 # Satellite tracking (optional - only needed for satellite features)
 skyfield>=1.45

+# DSC decoding (optional - only needed for VHF DSC maritime distress)
+scipy>=1.10.0
+numpy>=1.24.0
+
 # GPS dongle support (optional - only needed for USB GPS receivers)
 pyserial>=3.5

--- a/routes/init.py
+++ b/routes/init.py
@@ -11,6 +11,7 @@ def register_blueprints(app):
    from .bluetooth_v2 import bluetooth_v2_bp
    from .adsb import adsb_bp
    from .ais import ais_bp
+    from .dsc import dsc_bp
    from .acars import acars_bp
    from .aprs import aprs_bp
    from .satellite import satellite_bp
@@ -30,6 +31,7 @@ def register_blueprints(app):
    app.register_blueprint(bluetooth_v2_bp)  # New unified Bluetooth API
    app.register_blueprint(adsb_bp)
    app.register_blueprint(ais_bp)
+    app.register_blueprint(dsc_bp)  # VHF DSC maritime distress
    app.register_blueprint(acars_bp)
    app.register_blueprint(aprs_bp)
    app.register_blueprint(satellite_bp)
--- a/routes/dsc.py
+++ b/routes/dsc.py
@@ -0,0 +1,575 @@
+"""VHF DSC (Digital Selective Calling) routes.
+
+DSC operates on VHF Channel 70 (156.525 MHz) for maritime
+distress and safety communications per ITU-R M.493.
+"""
+
+from __future__ import annotations
+
+import json
+import logging
+import os
+import pty
+import queue
+import select
+import shutil
+import subprocess
+import threading
+import time
+from datetime import datetime
+from typing import Any, Generator
+
+from flask import Blueprint, jsonify, request, Response
+
+import app as app_module
+from utils.constants import (
+    DSC_VHF_FREQUENCY_MHZ,
+    DSC_SAMPLE_RATE,
+    DSC_TERMINATE_TIMEOUT,
+)
+from utils.database import (
+    store_dsc_alert,
+    get_dsc_alerts,
+    get_dsc_alert,
+    acknowledge_dsc_alert,
+    get_dsc_alert_summary,
+)
+from utils.dsc.parser import parse_dsc_message
+from utils.sse import format_sse
+from utils.validation import validate_device_index, validate_gain
+from utils.sdr import SDRFactory, SDRType
+from utils.dependencies import get_tool_path
+
+logger = logging.getLogger('intercept.dsc')
+
+dsc_bp = Blueprint('dsc', __name__, url_prefix='/dsc')
+
+# Module state (track if running independent of process state)
+dsc_running = False
+
+
+def _get_dsc_decoder_path() -> str | None:
+    """Get path to DSC decoder."""
+    # Check for our custom decoder
+    project_bin = os.path.join(os.path.dirname(os.path.dirname(__file__)), 'bin', 'dsc-decoder')
+    if os.path.isfile(project_bin) and os.access(project_bin, os.X_OK):
+        return project_bin
+
+    # Check system PATH
+    system_decoder = shutil.which('dsc-decoder')
+    if system_decoder:
+        return system_decoder
+
+    return None
+
+
+def _check_dsc_tools() -> dict:
+    """Check availability of DSC decoding tools."""
+    rtl_fm_path = get_tool_path('rtl_fm')
+    decoder_path = _get_dsc_decoder_path()
+
+    # Check for scipy/numpy (needed for decoder)
+    scipy_available = False
+    try:
+        import scipy
+        import numpy
+        scipy_available = True
+    except ImportError:
+        pass
+
+    return {
+        'rtl_fm': {
+            'available': rtl_fm_path is not None,
+            'path': rtl_fm_path
+        },
+        'dsc_decoder': {
+            'available': decoder_path is not None,
+            'path': decoder_path
+        },
+        'scipy': {
+            'available': scipy_available,
+            'note': 'Required for DSC signal processing'
+        },
+        'ready': rtl_fm_path is not None and decoder_path is not None and scipy_available
+    }
+
+
+def stream_dsc_decoder(master_fd: int, decoder_process: subprocess.Popen) -> None:
+    """
+    Stream DSC decoder output to queue using PTY for unbuffered output.
+
+    Args:
+        master_fd: PTY master file descriptor
+        decoder_process: Decoder subprocess
+    """
+    global dsc_running
+
+    try:
+        app_module.dsc_queue.put({'type': 'status', 'status': 'started'})
+
+        buffer = ""
+        while dsc_running:
+            try:
+                ready, _, _ = select.select([master_fd], [], [], 1.0)
+            except Exception:
+                break
+
+            if ready:
+                try:
+                    data = os.read(master_fd, 1024)
+                    if not data:
+                        break
+                    buffer += data.decode('utf-8', errors='replace')
+
+                    while '\n' in buffer:
+                        line, buffer = buffer.split('\n', 1)
+                        line = line.strip()
+                        if not line:
+                            continue
+
+                        # Parse DSC message
+                        parsed = parse_dsc_message(line)
+                        if parsed:
+                            # Generate unique message ID
+                            msg_id = f"{parsed['source_mmsi']}_{int(time.time() * 1000)}"
+                            parsed['id'] = msg_id
+
+                            # Store in transient DataStore
+                            app_module.dsc_messages.set(msg_id, parsed)
+
+                            # Queue for SSE
+                            try:
+                                app_module.dsc_queue.put_nowait(parsed)
+                            except queue.Full:
+                                logger.warning("DSC queue full, dropping message")
+
+                            # Store critical alerts permanently
+                            if parsed.get('is_critical'):
+                                _store_critical_alert(parsed)
+                        else:
+                            # Raw output for debugging
+                            app_module.dsc_queue.put({
+                                'type': 'raw',
+                                'text': line
+                            })
+                except OSError:
+                    break
+
+            # Check if process is still running
+            if decoder_process.poll() is not None:
+                break
+
+    except Exception as e:
+        logger.error(f"DSC decoder error: {e}")
+        app_module.dsc_queue.put({
+            'type': 'error',
+            'error': str(e)
+        })
+    finally:
+        try:
+            os.close(master_fd)
+        except OSError:
+            pass
+        decoder_process.wait()
+        dsc_running = False
+        app_module.dsc_queue.put({'type': 'status', 'status': 'stopped'})
+
+        with app_module.dsc_lock:
+            app_module.dsc_process = None
+            app_module.dsc_rtl_process = None
+
+
+def _store_critical_alert(msg: dict) -> None:
+    """Store critical DSC alert (DISTRESS/URGENCY) to database."""
+    try:
+        store_dsc_alert(
+            source_mmsi=msg.get('source_mmsi', ''),
+            format_code=str(msg.get('format_code', '')),
+            category=msg.get('category', 'UNKNOWN'),
+            source_name=msg.get('source_name'),
+            dest_mmsi=msg.get('dest_mmsi'),
+            nature_of_distress=msg.get('nature_of_distress'),
+            latitude=msg.get('latitude'),
+            longitude=msg.get('longitude'),
+            raw_message=msg.get('raw_message')
+        )
+        logger.info(f"Stored {msg.get('category')} alert from {msg.get('source_mmsi')}")
+    except Exception as e:
+        logger.error(f"Failed to store DSC alert: {e}")
+
+
+def monitor_rtl_stderr(process: subprocess.Popen) -> None:
+    """Monitor rtl_fm stderr for errors."""
+    global dsc_running
+
+    try:
+        for line in process.stderr:
+            if not dsc_running:
+                break
+            err_text = line.decode('utf-8', errors='replace').strip()
+            if err_text:
+                logger.debug(f"[RTL_FM] {err_text}")
+
+                # Check for device busy error
+                if 'usb_claim_interface' in err_text.lower():
+                    app_module.dsc_queue.put({
+                        'type': 'error',
+                        'error': 'SDR device busy',
+                        'error_type': 'DEVICE_BUSY',
+                        'suggestion': 'Use a different SDR device or stop other SDR processes'
+                    })
+
+                # Check for other common errors
+                if 'no supported devices' in err_text.lower():
+                    app_module.dsc_queue.put({
+                        'type': 'error',
+                        'error': 'No SDR device found',
+                        'error_type': 'NO_DEVICE'
+                    })
+    except Exception:
+        pass
+
+
+@dsc_bp.route('/status')
+def get_status() -> Response:
+    """Get DSC decoder status."""
+    global dsc_running
+
+    with app_module.dsc_lock:
+        running = (
+            dsc_running and
+            app_module.dsc_process is not None and
+            app_module.dsc_process.poll() is None
+        )
+
+    # Get message counts
+    message_count = len(app_module.dsc_messages)
+    alert_summary = get_dsc_alert_summary()
+
+    return jsonify({
+        'running': running,
+        'frequency': DSC_VHF_FREQUENCY_MHZ,
+        'message_count': message_count,
+        'alerts': alert_summary
+    })
+
+
+@dsc_bp.route('/tools')
+def check_tools() -> Response:
+    """Check DSC decoder tool availability."""
+    tools = _check_dsc_tools()
+    return jsonify(tools)
+
+
+@dsc_bp.route('/start', methods=['POST'])
+def start_decoding() -> Response:
+    """Start DSC decoder."""
+    global dsc_running
+
+    with app_module.dsc_lock:
+        if app_module.dsc_process and app_module.dsc_process.poll() is None:
+            return jsonify({
+                'status': 'error',
+                'message': 'DSC decoder already running'
+            }), 409
+
+        # Check tools
+        tools = _check_dsc_tools()
+        if not tools['ready']:
+            missing = []
+            if not tools['rtl_fm']['available']:
+                missing.append('rtl_fm')
+            if not tools['dsc_decoder']['available']:
+                missing.append('dsc-decoder')
+            if not tools['scipy']['available']:
+                missing.append('scipy/numpy')
+
+            return jsonify({
+                'status': 'error',
+                'message': f'Missing required tools: {", ".join(missing)}'
+            }), 400
+
+        data = request.json or {}
+
+        # Validate device
+        try:
+            device = validate_device_index(data.get('device', '0'))
+        except ValueError as e:
+            return jsonify({
+                'status': 'error',
+                'message': str(e)
+            }), 400
+
+        # Validate gain
+        try:
+            gain = validate_gain(data.get('gain', '40'))
+        except ValueError as e:
+            return jsonify({
+                'status': 'error',
+                'message': str(e)
+            }), 400
+
+        # Check if device is in use by AIS
+        try:
+            from routes import ais as ais_module
+            if hasattr(ais_module, 'ais_running') and ais_module.ais_running:
+                # AIS is running - check if same device
+                if hasattr(ais_module, 'ais_device') and str(ais_module.ais_device) == str(device):
+                    return jsonify({
+                        'status': 'error',
+                        'error_type': 'DEVICE_BUSY',
+                        'message': f'SDR device {device} is in use by AIS tracking',
+                        'suggestion': 'Use a different SDR device or stop AIS tracking first',
+                        'in_use_by': 'ais'
+                    }), 409
+        except ImportError:
+            pass
+
+        # Clear queue
+        while not app_module.dsc_queue.empty():
+            try:
+                app_module.dsc_queue.get_nowait()
+            except queue.Empty:
+                break
+
+        # Build rtl_fm command
+        rtl_fm_path = tools['rtl_fm']['path']
+        decoder_path = tools['dsc_decoder']['path']
+
+        # rtl_fm command for DSC decoding
+        # DSC uses narrow FM at 156.525 MHz with 48kHz sample rate
+        rtl_cmd = [
+            rtl_fm_path,
+            '-f', f'{DSC_VHF_FREQUENCY_MHZ}M',
+            '-s', str(DSC_SAMPLE_RATE),
+            '-d', str(device),
+            '-g', str(gain),
+            '-M', 'fm',           # FM demodulation
+            '-l', '0',            # No squelch for DSC
+            '-E', 'dc'            # DC blocking filter
+        ]
+
+        # Decoder command
+        decoder_cmd = [decoder_path]
+
+        full_cmd = ' '.join(rtl_cmd) + ' | ' + ' '.join(decoder_cmd)
+        logger.info(f"Starting DSC decoder: {full_cmd}")
+
+        try:
+            # Start rtl_fm subprocess
+            rtl_process = subprocess.Popen(
+                rtl_cmd,
+                stdout=subprocess.PIPE,
+                stderr=subprocess.PIPE
+            )
+
+            # Start stderr monitor thread
+            stderr_thread = threading.Thread(
+                target=monitor_rtl_stderr,
+                args=(rtl_process,),
+                daemon=True
+            )
+            stderr_thread.start()
+
+            # Create PTY for decoder output
+            master_fd, slave_fd = pty.openpty()
+
+            # Start decoder subprocess
+            decoder_process = subprocess.Popen(
+                decoder_cmd,
+                stdin=rtl_process.stdout,
+                stdout=slave_fd,
+                stderr=slave_fd,
+                close_fds=True
+            )
+
+            os.close(slave_fd)
+            rtl_process.stdout.close()
+
+            # Store process references
+            app_module.dsc_process = decoder_process
+            app_module.dsc_rtl_process = rtl_process
+            dsc_running = True
+
+            # Start output streaming thread
+            output_thread = threading.Thread(
+                target=stream_dsc_decoder,
+                args=(master_fd, decoder_process),
+                daemon=True
+            )
+            output_thread.start()
+
+            return jsonify({
+                'status': 'started',
+                'frequency': DSC_VHF_FREQUENCY_MHZ,
+                'device': device,
+                'gain': gain,
+                'command': full_cmd
+            })
+
+        except FileNotFoundError as e:
+            return jsonify({
+                'status': 'error',
+                'message': f'Tool not found: {e.filename}'
+            }), 400
+        except Exception as e:
+            logger.error(f"Failed to start DSC decoder: {e}")
+            return jsonify({
+                'status': 'error',
+                'message': str(e)
+            }), 500
+
+
+@dsc_bp.route('/stop', methods=['POST'])
+def stop_decoding() -> Response:
+    """Stop DSC decoder."""
+    global dsc_running
+
+    with app_module.dsc_lock:
+        if not app_module.dsc_process:
+            return jsonify({'status': 'not_running'})
+
+        dsc_running = False
+
+        # Terminate rtl_fm process first
+        if app_module.dsc_rtl_process:
+            try:
+                app_module.dsc_rtl_process.terminate()
+                app_module.dsc_rtl_process.wait(timeout=DSC_TERMINATE_TIMEOUT)
+            except subprocess.TimeoutExpired:
+                try:
+                    app_module.dsc_rtl_process.kill()
+                except OSError:
+                    pass
+            except OSError:
+                pass
+
+        # Terminate decoder process
+        if app_module.dsc_process:
+            try:
+                app_module.dsc_process.terminate()
+                app_module.dsc_process.wait(timeout=DSC_TERMINATE_TIMEOUT)
+            except subprocess.TimeoutExpired:
+                try:
+                    app_module.dsc_process.kill()
+                except OSError:
+                    pass
+            except OSError:
+                pass
+
+        app_module.dsc_process = None
+        app_module.dsc_rtl_process = None
+
+        return jsonify({'status': 'stopped'})
+
+
+@dsc_bp.route('/stream')
+def stream() -> Response:
+    """SSE stream for real-time DSC messages."""
+    def generate() -> Generator[str, None, None]:
+        last_keepalive = time.time()
+        keepalive_interval = 30.0
+
+        while True:
+            try:
+                msg = app_module.dsc_queue.get(timeout=1)
+                last_keepalive = time.time()
+                yield format_sse(msg)
+            except queue.Empty:
+                now = time.time()
+                if now - last_keepalive >= keepalive_interval:
+                    yield format_sse({'type': 'keepalive'})
+                    last_keepalive = now
+
+    response = Response(generate(), mimetype='text/event-stream')
+    response.headers['Cache-Control'] = 'no-cache'
+    response.headers['X-Accel-Buffering'] = 'no'
+    response.headers['Connection'] = 'keep-alive'
+    return response
+
+
+@dsc_bp.route('/messages')
+def get_messages() -> Response:
+    """Get current DSC messages from transient store."""
+    messages = list(app_module.dsc_messages.values())
+
+    # Sort by timestamp (newest first)
+    messages.sort(key=lambda m: m.get('timestamp', ''), reverse=True)
+
+    return jsonify({
+        'count': len(messages),
+        'messages': messages
+    })
+
+
+@dsc_bp.route('/alerts')
+def get_alerts_endpoint() -> Response:
+    """Get stored DSC alerts (paginated)."""
+    # Parse query params
+    category = request.args.get('category')
+    acknowledged = request.args.get('acknowledged')
+    limit = min(int(request.args.get('limit', 50)), 200)
+    offset = int(request.args.get('offset', 0))
+
+    # Convert acknowledged param
+    ack_filter = None
+    if acknowledged is not None:
+        ack_filter = acknowledged.lower() in ('true', '1', 'yes')
+
+    alerts = get_dsc_alerts(
+        category=category,
+        acknowledged=ack_filter,
+        limit=limit,
+        offset=offset
+    )
+
+    summary = get_dsc_alert_summary()
+
+    return jsonify({
+        'alerts': alerts,
+        'count': len(alerts),
+        'summary': summary,
+        'pagination': {
+            'limit': limit,
+            'offset': offset
+        }
+    })
+
+
+@dsc_bp.route('/alerts/<int:alert_id>')
+def get_alert(alert_id: int) -> Response:
+    """Get a specific DSC alert by ID."""
+    alert = get_dsc_alert(alert_id)
+    if not alert:
+        return jsonify({
+            'status': 'error',
+            'message': 'Alert not found'
+        }), 404
+
+    return jsonify(alert)
+
+
+@dsc_bp.route('/alerts/<int:alert_id>/acknowledge', methods=['POST'])
+def acknowledge_alert(alert_id: int) -> Response:
+    """Acknowledge a DSC alert."""
+    data = request.json or {}
+    notes = data.get('notes')
+
+    success = acknowledge_dsc_alert(alert_id, notes)
+    if not success:
+        return jsonify({
+            'status': 'error',
+            'message': 'Alert not found'
+        }), 404
+
+    return jsonify({
+        'status': 'acknowledged',
+        'alert_id': alert_id
+    })
+
+
+@dsc_bp.route('/alerts/summary')
+def get_alerts_summary() -> Response:
+    """Get summary of unacknowledged DSC alerts."""
+    summary = get_dsc_alert_summary()
+    return jsonify(summary)
--- a/static/css/ais_dashboard.css
+++ b/static/css/ais_dashboard.css
@@ -899,3 +899,308 @@ body {
    line-height: 44px !important;
    font-size: 18px !important;
 }
+
+/* ============================================
+   DSC (Digital Selective Calling) Styles
+   ============================================ */
+
+/* DSC Control Group - Orange accent (warning/distress theme) */
+.control-group.dsc-group {
+    background: rgba(245, 158, 11, 0.05);
+    border-color: rgba(245, 158, 11, 0.2);
+}
+
+.control-group.dsc-group .control-group-label {
+    color: var(--accent-orange);
+}
+
+.control-group.dsc-group select,
+.control-group.dsc-group input[type="number"] {
+    border-color: rgba(245, 158, 11, 0.3);
+    color: var(--accent-orange);
+}
+
+.dsc-btn {
+    background: var(--accent-orange) !important;
+    font-size: 9px !important;
+    padding: 6px 12px !important;
+}
+
+.dsc-btn:hover {
+    background: #d97706 !important;
+    box-shadow: 0 0 20px rgba(245, 158, 11, 0.3) !important;
+}
+
+.dsc-btn.active {
+    background: var(--accent-red) !important;
+}
+
+/* DSC Panel */
+.panel.dsc-messages {
+    flex: 0 0 auto;
+    max-height: 250px;
+    display: flex;
+    flex-direction: column;
+    border-top: 1px solid rgba(245, 158, 11, 0.2);
+}
+
+.panel.dsc-messages::before {
+    background: linear-gradient(90deg, transparent, var(--accent-orange), transparent);
+}
+
+.panel.dsc-messages .panel-header {
+    background: rgba(245, 158, 11, 0.05);
+    border-bottom-color: rgba(245, 158, 11, 0.1);
+    color: var(--accent-orange);
+}
+
+/* DSC Alert Summary */
+.dsc-alert-summary {
+    display: flex;
+    gap: 8px;
+    padding: 6px 12px;
+    background: rgba(0, 0, 0, 0.2);
+    border-bottom: 1px solid rgba(245, 158, 11, 0.1);
+    font-family: 'JetBrains Mono', monospace;
+    font-size: 9px;
+}
+
+.dsc-alert-count {
+    padding: 2px 6px;
+    border-radius: 3px;
+    font-weight: 600;
+}
+
+.dsc-alert-count.distress {
+    background: rgba(239, 68, 68, 0.2);
+    color: var(--accent-red);
+}
+
+.dsc-alert-count.urgency {
+    background: rgba(245, 158, 11, 0.2);
+    color: var(--accent-orange);
+}
+
+/* DSC List Content */
+.dsc-list-content {
+    flex: 1;
+    overflow-y: auto;
+    padding: 8px;
+}
+
+.no-messages {
+    text-align: center;
+    padding: 20px 15px;
+    color: var(--text-secondary);
+    font-size: 11px;
+}
+
+/* DSC Message Items */
+.dsc-message-item {
+    position: relative;
+    background: rgba(0, 0, 0, 0.3);
+    border: 1px solid rgba(245, 158, 11, 0.15);
+    border-radius: 4px;
+    padding: 8px 10px;
+    margin-bottom: 6px;
+    cursor: pointer;
+    transition: all 0.2s ease;
+}
+
+.dsc-message-item:hover {
+    border-color: var(--accent-orange);
+    background: rgba(245, 158, 11, 0.05);
+}
+
+.dsc-message-item.distress {
+    border-color: var(--accent-red);
+    background: rgba(239, 68, 68, 0.1);
+    animation: distress-pulse 1.5s ease-in-out infinite;
+}
+
+.dsc-message-item.urgency {
+    border-color: var(--accent-orange);
+    background: rgba(245, 158, 11, 0.1);
+}
+
+@keyframes distress-pulse {
+    0%, 100% { box-shadow: 0 0 5px rgba(239, 68, 68, 0.3); }
+    50% { box-shadow: 0 0 15px rgba(239, 68, 68, 0.6); }
+}
+
+.dsc-message-header {
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+    margin-bottom: 4px;
+}
+
+.dsc-message-category {
+    font-family: 'JetBrains Mono', monospace;
+    font-size: 9px;
+    font-weight: 700;
+    text-transform: uppercase;
+    letter-spacing: 0.5px;
+    padding: 2px 6px;
+    border-radius: 3px;
+    background: rgba(245, 158, 11, 0.2);
+    color: var(--accent-orange);
+}
+
+.dsc-message-item.distress .dsc-message-category {
+    background: rgba(239, 68, 68, 0.2);
+    color: var(--accent-red);
+}
+
+.dsc-message-time {
+    font-family: 'JetBrains Mono', monospace;
+    font-size: 9px;
+    color: var(--text-dim);
+}
+
+.dsc-message-mmsi {
+    font-family: 'JetBrains Mono', monospace;
+    font-size: 11px;
+    color: var(--accent-orange);
+}
+
+.dsc-message-country {
+    font-size: 9px;
+    color: var(--text-secondary);
+}
+
+.dsc-message-nature {
+    font-size: 10px;
+    color: var(--accent-red);
+    font-weight: 500;
+    margin-top: 2px;
+}
+
+.dsc-message-pos {
+    font-family: 'JetBrains Mono', monospace;
+    font-size: 9px;
+    color: var(--text-secondary);
+}
+
+/* DSC Distress Alert Overlay */
+.dsc-distress-alert {
+    position: fixed;
+    top: 50%;
+    left: 50%;
+    transform: translate(-50%, -50%);
+    z-index: 10000;
+    background: rgba(15, 18, 24, 0.98);
+    border: 2px solid var(--accent-red);
+    border-radius: 8px;
+    padding: 24px 32px;
+    min-width: 300px;
+    text-align: center;
+    box-shadow: 0 0 40px rgba(239, 68, 68, 0.5);
+    animation: alert-flash 0.5s ease-in-out 3;
+}
+
+@keyframes alert-flash {
+    0%, 100% { border-color: var(--accent-red); box-shadow: 0 0 40px rgba(239, 68, 68, 0.5); }
+    50% { border-color: #ff6b6b; box-shadow: 0 0 60px rgba(239, 68, 68, 0.8); }
+}
+
+.dsc-distress-alert .dsc-alert-header {
+    font-family: 'Orbitron', 'JetBrains Mono', monospace;
+    font-size: 24px;
+    font-weight: 700;
+    color: var(--accent-red);
+    margin-bottom: 16px;
+    letter-spacing: 3px;
+}
+
+.dsc-distress-alert .dsc-alert-mmsi {
+    font-family: 'JetBrains Mono', monospace;
+    font-size: 16px;
+    color: var(--accent-cyan);
+    margin-bottom: 8px;
+}
+
+.dsc-distress-alert .dsc-alert-country {
+    font-size: 14px;
+    color: var(--text-secondary);
+    margin-bottom: 8px;
+}
+
+.dsc-distress-alert .dsc-alert-nature {
+    font-size: 18px;
+    font-weight: 600;
+    color: var(--accent-orange);
+    margin-bottom: 12px;
+}
+
+.dsc-distress-alert .dsc-alert-position {
+    font-family: 'JetBrains Mono', monospace;
+    font-size: 14px;
+    color: var(--accent-cyan);
+    margin-bottom: 16px;
+}
+
+.dsc-distress-alert button {
+    background: var(--accent-red);
+    border: none;
+    color: white;
+    padding: 10px 24px;
+    font-family: 'JetBrains Mono', monospace;
+    font-size: 12px;
+    font-weight: 600;
+    text-transform: uppercase;
+    letter-spacing: 1px;
+    border-radius: 4px;
+    cursor: pointer;
+    transition: all 0.2s ease;
+}
+
+.dsc-distress-alert button:hover {
+    background: #dc2626;
+    box-shadow: 0 0 20px rgba(239, 68, 68, 0.5);
+}
+
+/* DSC Map Markers */
+.dsc-marker {
+    background: transparent;
+    border: none;
+}
+
+.dsc-marker-inner {
+    width: 28px;
+    height: 28px;
+    border-radius: 50%;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    font-size: 14px;
+    color: white;
+    border: 2px solid white;
+    box-shadow: 0 2px 8px rgba(0, 0, 0, 0.5);
+}
+
+.dsc-marker-inner.distress {
+    animation: distress-marker-pulse 1s ease-in-out infinite;
+}
+
+@keyframes distress-marker-pulse {
+    0%, 100% { transform: scale(1); }
+    50% { transform: scale(1.2); }
+}
+
+/* Mobile adjustments for DSC */
+@media (max-width: 767px) {
+    .panel.dsc-messages {
+        max-height: 200px;
+    }
+
+    .dsc-distress-alert {
+        width: 90%;
+        min-width: auto;
+        padding: 16px 20px;
+    }
+
+    .dsc-distress-alert .dsc-alert-header {
+        font-size: 18px;
+    }
+}
--- a/templates/ais_dashboard.html
+++ b/templates/ais_dashboard.html
@@ -96,6 +96,23 @@
                    </div>
                </div>
            </div>
+
+            <div class="panel dsc-messages">
+                <div class="panel-header">
+                    <span>VHF DSC MESSAGES</span>
+                    <div class="panel-indicator" id="dscIndicator"></div>
+                </div>
+                <div class="dsc-alert-summary" id="dscAlertSummary">
+                    <span class="dsc-alert-count distress" id="dscDistressCount" title="Distress alerts">0 DISTRESS</span>
+                    <span class="dsc-alert-count urgency" id="dscUrgencyCount" title="Urgency alerts">0 URGENCY</span>
+                </div>
+                <div class="dsc-list-content" id="dscMessageList">
+                    <div class="no-messages">
+                        <div>No DSC messages</div>
+                        <div style="font-size: 10px; margin-top: 5px;">Start VHF DSC to monitor</div>
+                    </div>
+                </div>
+            </div>
        </div>

        <div class="controls-bar">
@@ -131,6 +148,17 @@
                    <button class="start-btn" id="startBtn" onclick="toggleTracking()">START</button>
                </div>
            </div>
+
+            <div class="control-group dsc-group">
+                <span class="control-group-label">VHF DSC</span>
+                <div class="control-group-items">
+                    <select id="dscDeviceSelect" title="DSC SDR device (secondary)">
+                        <option value="0">SDR 0</option>
+                    </select>
+                    <input type="number" id="dscGain" value="40" min="0" max="50" style="width: 50px;" title="Gain">
+                    <button class="start-btn dsc-btn" id="dscStartBtn" onclick="toggleDscTracking()">START DSC</button>
+                </div>
+            </div>
        </div>
    </main>

@@ -142,6 +170,13 @@
        let selectedMmsi = null;
        let eventSource = null;
        let isTracking = false;
+
+        // DSC State
+        let dscEventSource = null;
+        let isDscTracking = false;
+        let dscMessages = {};
+        let dscMarkers = {};
+        let dscAlertCounts = { distress: 0, urgency: 0 };
        let showTrails = false;
        let vesselTrails = {};
        let trailLines = {};
@@ -290,18 +325,37 @@
            fetch('/devices')
                .then(r => r.json())
                .then(devices => {
-                    const select = document.getElementById('aisDeviceSelect');
-                    select.innerHTML = '';
+                    // Populate AIS device selector
+                    const aisSelect = document.getElementById('aisDeviceSelect');
+                    aisSelect.innerHTML = '';
                    if (devices.length === 0) {
-                        select.innerHTML = '<option value="0">No devices</option>';
+                        aisSelect.innerHTML = '<option value="0">No devices</option>';
                    } else {
                        devices.forEach((d, i) => {
                            const opt = document.createElement('option');
                            opt.value = d.index;
                            opt.textContent = `SDR ${d.index}: ${d.name}`;
-                            select.appendChild(opt);
+                            aisSelect.appendChild(opt);
                        });
                    }
+
+                    // Populate DSC device selector
+                    const dscSelect = document.getElementById('dscDeviceSelect');
+                    dscSelect.innerHTML = '';
+                    if (devices.length === 0) {
+                        dscSelect.innerHTML = '<option value="0">No devices</option>';
+                    } else {
+                        devices.forEach((d, i) => {
+                            const opt = document.createElement('option');
+                            opt.value = d.index;
+                            opt.textContent = `SDR ${d.index}: ${d.name}`;
+                            dscSelect.appendChild(opt);
+                        });
+                        // Default to second device if available
+                        if (devices.length > 1) {
+                            dscSelect.value = devices[1].index;
+                        }
+                    }
                })
                .catch(() => {});
        }
@@ -758,6 +812,238 @@
            }
        }

+        // ============================================
+        // DSC (Digital Selective Calling) Functions
+        // ============================================
+
+        function toggleDscTracking() {
+            if (isDscTracking) {
+                stopDscTracking();
+            } else {
+                startDscTracking();
+            }
+        }
+
+        function startDscTracking() {
+            const device = document.getElementById('dscDeviceSelect').value;
+            const gain = document.getElementById('dscGain').value;
+
+            fetch('/dsc/start', {
+                method: 'POST',
+                headers: { 'Content-Type': 'application/json' },
+                body: JSON.stringify({ device, gain })
+            })
+            .then(r => r.json())
+            .then(data => {
+                if (data.status === 'started') {
+                    isDscTracking = true;
+                    document.getElementById('dscStartBtn').textContent = 'STOP DSC';
+                    document.getElementById('dscStartBtn').classList.add('active');
+                    document.getElementById('dscIndicator').classList.add('active');
+                    startDscSSE();
+                } else if (data.error_type === 'DEVICE_BUSY') {
+                    alert('SDR device is busy.\n\n' + data.suggestion);
+                } else {
+                    alert(data.message || 'Failed to start DSC');
+                }
+            })
+            .catch(err => alert('Error: ' + err.message));
+        }
+
+        function stopDscTracking() {
+            fetch('/dsc/stop', { method: 'POST' })
+            .then(r => r.json())
+            .then(() => {
+                isDscTracking = false;
+                document.getElementById('dscStartBtn').textContent = 'START DSC';
+                document.getElementById('dscStartBtn').classList.remove('active');
+                document.getElementById('dscIndicator').classList.remove('active');
+                if (dscEventSource) {
+                    dscEventSource.close();
+                    dscEventSource = null;
+                }
+            });
+        }
+
+        function startDscSSE() {
+            if (dscEventSource) dscEventSource.close();
+
+            dscEventSource = new EventSource('/dsc/stream');
+            dscEventSource.onmessage = function(e) {
+                try {
+                    const data = JSON.parse(e.data);
+                    if (data.type === 'dsc_message') {
+                        handleDscMessage(data);
+                    } else if (data.type === 'error') {
+                        console.error('DSC error:', data.error);
+                        if (data.error_type === 'DEVICE_BUSY') {
+                            alert('DSC: Device became busy. ' + (data.suggestion || ''));
+                            stopDscTracking();
+                        }
+                    }
+                } catch (err) {}
+            };
+
+            dscEventSource.onerror = function() {
+                setTimeout(() => {
+                    if (isDscTracking) startDscSSE();
+                }, 2000);
+            };
+        }
+
+        function handleDscMessage(data) {
+            const msgId = data.id || data.source_mmsi + '_' + Date.now();
+            dscMessages[msgId] = data;
+
+            // Update alert counts
+            if (data.category === 'DISTRESS') {
+                dscAlertCounts.distress++;
+            } else if (data.category === 'URGENCY') {
+                dscAlertCounts.urgency++;
+            }
+
+            // Show prominent alert for distress/urgency
+            if (data.is_critical) {
+                showDistressAlert(data);
+            }
+
+            // Add position marker if coordinates present
+            if (data.latitude && data.longitude) {
+                addDscPositionMarker(data);
+            }
+
+            updateDscMessageList();
+            updateDscAlertSummary();
+        }
+
+        function showDistressAlert(data) {
+            // Create alert notification
+            const alertDiv = document.createElement('div');
+            alertDiv.className = 'dsc-distress-alert';
+            alertDiv.innerHTML = `
+                <div class="dsc-alert-header">${data.category}</div>
+                <div class="dsc-alert-mmsi">MMSI: ${data.source_mmsi}</div>
+                ${data.source_country ? `<div class="dsc-alert-country">${data.source_country}</div>` : ''}
+                ${data.nature_of_distress ? `<div class="dsc-alert-nature">${data.nature_of_distress}</div>` : ''}
+                ${data.latitude ? `<div class="dsc-alert-position">${data.latitude.toFixed(4)}, ${data.longitude.toFixed(4)}</div>` : ''}
+                <button onclick="this.parentElement.remove()">ACKNOWLEDGE</button>
+            `;
+            document.body.appendChild(alertDiv);
+
+            // Auto-remove after 30 seconds
+            setTimeout(() => {
+                if (alertDiv.parentElement) alertDiv.remove();
+            }, 30000);
+
+            // Play alert sound if available
+            try {
+                const audio = new Audio('data:audio/wav;base64,UklGRnoGAABXQVZFZm10IBAAAAABAAEAQB8AAEAfAAABAAgAZGF0YQoGAACBhYqFbF1yc3R3eXx+foCAfn59fHt5d3VzcWxnYlxVT0hCOzUuJx8YEAkDAP/+/v7+/v7+/v8AAAECAwUHCQsOEBMWGRwfIiUoKy4xNDc6PT9CRUdKTE5QUlRVV1hZWlpbW1taWVhXVlRTUU9NSkdEQT47ODUyLywpJiMgHRoXFBEOCwgFAwEA/v38+/r5+Pf29fTz8vHw7+7t7Ovq6ejn5uXk4+Lh4N/e3dzb2tnY19bV1NPS0dDPzs3MzMvLy8vMzM3Nzs/Q0dLT1NXW19jZ2tvc3d7f4OHi4+Tl5ufp6uvs7e7v8PHy8/T19vf4+fr7/P3+');
+                audio.volume = 0.5;
+                audio.play().catch(() => {});
+            } catch (e) {}
+        }
+
+        function addDscPositionMarker(data) {
+            const mmsi = data.source_mmsi;
+
+            // Remove existing marker
+            if (dscMarkers[mmsi]) {
+                vesselMap.removeLayer(dscMarkers[mmsi]);
+            }
+
+            // Create marker with distress icon
+            const isDistress = data.category === 'DISTRESS';
+            const color = isDistress ? '#ef4444' : (data.category === 'URGENCY' ? '#f59e0b' : '#4a9eff');
+
+            const icon = L.divIcon({
+                className: 'dsc-marker',
+                html: `<div class="dsc-marker-inner ${isDistress ? 'distress' : ''}" style="background: ${color};">
+                    <span>&#9888;</span>
+                </div>`,
+                iconSize: [28, 28],
+                iconAnchor: [14, 14]
+            });
+
+            dscMarkers[mmsi] = L.marker([data.latitude, data.longitude], { icon })
+                .addTo(vesselMap)
+                .bindPopup(`
+                    <strong>${data.category}</strong><br>
+                    MMSI: ${mmsi}<br>
+                    ${data.source_country ? `Country: ${data.source_country}<br>` : ''}
+                    ${data.nature_of_distress ? `Nature: ${data.nature_of_distress}<br>` : ''}
+                    Position: ${data.latitude.toFixed(4)}, ${data.longitude.toFixed(4)}
+                `);
+
+            // Pan to distress position
+            if (isDistress) {
+                vesselMap.setView([data.latitude, data.longitude], 12);
+            }
+        }
+
+        function updateDscMessageList() {
+            const container = document.getElementById('dscMessageList');
+            const msgArray = Object.values(dscMessages)
+                .sort((a, b) => (b.timestamp || '').localeCompare(a.timestamp || ''));
+
+            if (msgArray.length === 0) {
+                container.innerHTML = `
+                    <div class="no-messages">
+                        <div>No DSC messages</div>
+                        <div style="font-size: 10px; margin-top: 5px;">Start VHF DSC to monitor</div>
+                    </div>
+                `;
+                return;
+            }
+
+            container.innerHTML = msgArray.slice(0, 50).map(msg => {
+                const isDistress = msg.category === 'DISTRESS';
+                const isUrgency = msg.category === 'URGENCY';
+                const categoryClass = isDistress ? 'distress' : (isUrgency ? 'urgency' : '');
+
+                return `
+                    <div class="dsc-message-item ${categoryClass}" data-id="${msg.id}">
+                        <div class="dsc-message-header">
+                            <span class="dsc-message-category">${msg.category}</span>
+                            <span class="dsc-message-time">${formatDscTime(msg.timestamp)}</span>
+                        </div>
+                        <div class="dsc-message-mmsi">MMSI: ${msg.source_mmsi}</div>
+                        ${msg.source_country ? `<div class="dsc-message-country">${msg.source_country}</div>` : ''}
+                        ${msg.nature_of_distress ? `<div class="dsc-message-nature">${msg.nature_of_distress}</div>` : ''}
+                        ${msg.latitude ? `<div class="dsc-message-pos">${msg.latitude.toFixed(4)}, ${msg.longitude.toFixed(4)}</div>` : ''}
+                    </div>
+                `;
+            }).join('');
+        }
+
+        function formatDscTime(timestamp) {
+            if (!timestamp) return '--:--';
+            try {
+                const d = new Date(timestamp);
+                return d.toLocaleTimeString('en-GB', { hour: '2-digit', minute: '2-digit', second: '2-digit' });
+            } catch (e) {
+                return timestamp.slice(11, 19) || '--:--';
+            }
+        }
+
+        function updateDscAlertSummary() {
+            document.getElementById('dscDistressCount').textContent = `${dscAlertCounts.distress} DISTRESS`;
+            document.getElementById('dscUrgencyCount').textContent = `${dscAlertCounts.urgency} URGENCY`;
+        }
+
+        // Cross-reference DSC MMSI with AIS vessels
+        function crossReferenceDscWithAis(mmsi) {
+            const vessel = vessels[mmsi];
+            if (vessel) {
+                return {
+                    name: vessel.name,
+                    callsign: vessel.callsign,
+                    ship_type: vessel.ship_type,
+                    destination: vessel.destination
+                };
+            }
+            return null;
+        }
+
        // Initialize
        document.addEventListener('DOMContentLoaded', initMap);
    </script>
--- a/tests/test_dsc.py
+++ b/tests/test_dsc.py
@@ -0,0 +1,467 @@
+"""Tests for DSC (Digital Selective Calling) utilities."""
+
+import json
+import pytest
+from datetime import datetime
+
+
+class TestDSCParser:
+    """Tests for DSC parser utilities."""
+
+    def test_get_country_from_mmsi_ship_station(self):
+        """Test country lookup for standard ship MMSI."""
+        from utils.dsc.parser import get_country_from_mmsi
+
+        # UK ships start with 232-235
+        assert get_country_from_mmsi('232123456') == 'United Kingdom'
+        assert get_country_from_mmsi('235987654') == 'United Kingdom'
+
+        # US ships start with 303, 338, 366-369
+        assert get_country_from_mmsi('366123456') == 'USA'
+        assert get_country_from_mmsi('369000001') == 'USA'
+
+        # Panama (common flag of convenience)
+        assert get_country_from_mmsi('351234567') == 'Panama'
+        assert get_country_from_mmsi('370000001') == 'Panama'
+
+        # Norway
+        assert get_country_from_mmsi('257123456') == 'Norway'
+
+        # Germany
+        assert get_country_from_mmsi('211000001') == 'Germany'
+
+    def test_get_country_from_mmsi_coast_station(self):
+        """Test country lookup for coast station MMSI (starts with 00)."""
+        from utils.dsc.parser import get_country_from_mmsi
+
+        # Coast stations: 00 + MID
+        assert get_country_from_mmsi('002320001') == 'United Kingdom'
+        assert get_country_from_mmsi('003660001') == 'USA'
+
+    def test_get_country_from_mmsi_group_station(self):
+        """Test country lookup for group station MMSI (starts with 0)."""
+        from utils.dsc.parser import get_country_from_mmsi
+
+        # Group call: 0 + MID
+        assert get_country_from_mmsi('023200001') == 'United Kingdom'
+        assert get_country_from_mmsi('036600001') == 'USA'
+
+    def test_get_country_from_mmsi_unknown(self):
+        """Test country lookup returns None for unknown MID."""
+        from utils.dsc.parser import get_country_from_mmsi
+
+        assert get_country_from_mmsi('999999999') is None
+        assert get_country_from_mmsi('100000000') is None
+
+    def test_get_country_from_mmsi_invalid(self):
+        """Test country lookup handles invalid input."""
+        from utils.dsc.parser import get_country_from_mmsi
+
+        assert get_country_from_mmsi('') is None
+        assert get_country_from_mmsi(None) is None
+        assert get_country_from_mmsi('12') is None
+
+    def test_get_distress_nature_text(self):
+        """Test distress nature code to text conversion."""
+        from utils.dsc.parser import get_distress_nature_text
+
+        assert get_distress_nature_text(100) == 'UNDESIGNATED'
+        assert get_distress_nature_text(101) == 'FIRE'
+        assert get_distress_nature_text(102) == 'FLOODING'
+        assert get_distress_nature_text(103) == 'COLLISION'
+        assert get_distress_nature_text(106) == 'SINKING'
+        assert get_distress_nature_text(109) == 'PIRACY'
+        assert get_distress_nature_text(110) == 'MOB'  # Man overboard
+
+    def test_get_distress_nature_text_unknown(self):
+        """Test distress nature returns formatted unknown for invalid codes."""
+        from utils.dsc.parser import get_distress_nature_text
+
+        assert 'UNKNOWN' in get_distress_nature_text(999)
+        assert '999' in get_distress_nature_text(999)
+
+    def test_get_distress_nature_text_string_input(self):
+        """Test distress nature accepts string input."""
+        from utils.dsc.parser import get_distress_nature_text
+
+        assert get_distress_nature_text('101') == 'FIRE'
+        assert get_distress_nature_text('invalid') == 'invalid'
+
+    def test_get_format_text(self):
+        """Test format code to text conversion."""
+        from utils.dsc.parser import get_format_text
+
+        assert get_format_text(100) == 'DISTRESS'
+        assert get_format_text(102) == 'ALL_SHIPS'
+        assert get_format_text(106) == 'DISTRESS_ACK'
+        assert get_format_text(108) == 'DISTRESS_RELAY'
+        assert get_format_text(112) == 'INDIVIDUAL'
+        assert get_format_text(116) == 'ROUTINE'
+        assert get_format_text(118) == 'SAFETY'
+        assert get_format_text(120) == 'URGENCY'
+
+    def test_get_format_text_unknown(self):
+        """Test format code returns unknown for invalid codes."""
+        from utils.dsc.parser import get_format_text
+
+        result = get_format_text(999)
+        assert 'UNKNOWN' in result
+
+    def test_get_telecommand_text(self):
+        """Test telecommand code to text conversion."""
+        from utils.dsc.parser import get_telecommand_text
+
+        assert get_telecommand_text(100) == 'F3E_G3E_ALL'
+        assert get_telecommand_text(105) == 'DATA'
+        assert get_telecommand_text(107) == 'DISTRESS_ACK'
+        assert get_telecommand_text(111) == 'TEST'
+
+    def test_get_category_priority(self):
+        """Test category priority values."""
+        from utils.dsc.parser import get_category_priority
+
+        # Distress has highest priority (0)
+        assert get_category_priority('DISTRESS') == 0
+        assert get_category_priority('distress') == 0
+
+        # Urgency is lower
+        assert get_category_priority('URGENCY') == 3
+
+        # Safety is lower still
+        assert get_category_priority('SAFETY') == 4
+
+        # Routine is lowest
+        assert get_category_priority('ROUTINE') == 5
+
+        # Unknown gets default high number
+        assert get_category_priority('UNKNOWN') == 10
+
+    def test_validate_mmsi_valid(self):
+        """Test MMSI validation with valid numbers."""
+        from utils.dsc.parser import validate_mmsi
+
+        assert validate_mmsi('232123456') is True
+        assert validate_mmsi('366000001') is True
+        assert validate_mmsi('002320001') is True  # Coast station
+        assert validate_mmsi('023200001') is True  # Group station
+
+    def test_validate_mmsi_invalid(self):
+        """Test MMSI validation rejects invalid numbers."""
+        from utils.dsc.parser import validate_mmsi
+
+        assert validate_mmsi('') is False
+        assert validate_mmsi(None) is False
+        assert validate_mmsi('12345678') is False  # Too short
+        assert validate_mmsi('1234567890') is False  # Too long
+        assert validate_mmsi('abcdefghi') is False  # Not digits
+        assert validate_mmsi('000000000') is False  # All zeros
+
+    def test_classify_mmsi(self):
+        """Test MMSI classification."""
+        from utils.dsc.parser import classify_mmsi
+
+        # Ship stations (start with 2-7)
+        assert classify_mmsi('232123456') == 'ship'
+        assert classify_mmsi('366000001') == 'ship'
+        assert classify_mmsi('503000001') == 'ship'
+
+        # Coast stations (start with 00)
+        assert classify_mmsi('002320001') == 'coast'
+
+        # Group stations (start with 0, not 00)
+        assert classify_mmsi('023200001') == 'group'
+
+        # SAR aircraft (start with 111)
+        assert classify_mmsi('111232001') == 'sar'
+
+        # Aids to Navigation (start with 99)
+        assert classify_mmsi('992321001') == 'aton'
+
+        # Unknown
+        assert classify_mmsi('invalid') == 'unknown'
+        assert classify_mmsi('812345678') == 'unknown'
+
+    def test_parse_dsc_message_distress(self):
+        """Test parsing a distress message."""
+        from utils.dsc.parser import parse_dsc_message
+
+        raw = json.dumps({
+            'type': 'dsc',
+            'format': 100,
+            'source_mmsi': '232123456',
+            'dest_mmsi': '000000000',
+            'category': 'DISTRESS',
+            'nature': 101,
+            'position': {'lat': 51.5, 'lon': -0.1},
+            'telecommand1': 100,
+            'timestamp': '2025-01-15T12:00:00Z'
+        })
+
+        msg = parse_dsc_message(raw)
+
+        assert msg is not None
+        assert msg['type'] == 'dsc_message'
+        assert msg['source_mmsi'] == '232123456'
+        assert msg['category'] == 'DISTRESS'
+        assert msg['source_country'] == 'United Kingdom'
+        assert msg['nature_of_distress'] == 'FIRE'
+        assert msg['latitude'] == 51.5
+        assert msg['longitude'] == -0.1
+        assert msg['is_critical'] is True
+        assert msg['priority'] == 0
+
+    def test_parse_dsc_message_routine(self):
+        """Test parsing a routine message."""
+        from utils.dsc.parser import parse_dsc_message
+
+        raw = json.dumps({
+            'type': 'dsc',
+            'format': 116,
+            'source_mmsi': '366000001',
+            'category': 'ROUTINE',
+            'timestamp': '2025-01-15T12:00:00Z'
+        })
+
+        msg = parse_dsc_message(raw)
+
+        assert msg is not None
+        assert msg['category'] == 'ROUTINE'
+        assert msg['source_country'] == 'USA'
+        assert msg['is_critical'] is False
+        assert msg['priority'] == 5
+
+    def test_parse_dsc_message_invalid_json(self):
+        """Test parsing rejects invalid JSON."""
+        from utils.dsc.parser import parse_dsc_message
+
+        assert parse_dsc_message('not json') is None
+        assert parse_dsc_message('{invalid}') is None
+
+    def test_parse_dsc_message_missing_type(self):
+        """Test parsing rejects messages without correct type."""
+        from utils.dsc.parser import parse_dsc_message
+
+        raw = json.dumps({'source_mmsi': '232123456'})
+        assert parse_dsc_message(raw) is None
+
+        raw = json.dumps({'type': 'other', 'source_mmsi': '232123456'})
+        assert parse_dsc_message(raw) is None
+
+    def test_parse_dsc_message_missing_mmsi(self):
+        """Test parsing rejects messages without source MMSI."""
+        from utils.dsc.parser import parse_dsc_message
+
+        raw = json.dumps({'type': 'dsc'})
+        assert parse_dsc_message(raw) is None
+
+    def test_parse_dsc_message_empty(self):
+        """Test parsing handles empty input."""
+        from utils.dsc.parser import parse_dsc_message
+
+        assert parse_dsc_message('') is None
+        assert parse_dsc_message(None) is None
+        assert parse_dsc_message('   ') is None
+
+    def test_format_dsc_for_display(self):
+        """Test message formatting for display."""
+        from utils.dsc.parser import format_dsc_for_display
+
+        msg = {
+            'category': 'DISTRESS',
+            'source_mmsi': '232123456',
+            'source_country': 'United Kingdom',
+            'dest_mmsi': '002320001',
+            'nature_of_distress': 'FIRE',
+            'latitude': 51.5074,
+            'longitude': -0.1278,
+            'telecommand1_text': 'F3E_G3E_ALL',
+            'channel': 16,
+            'timestamp': '2025-01-15T12:00:00Z'
+        }
+
+        output = format_dsc_for_display(msg)
+
+        assert 'DISTRESS' in output
+        assert '232123456' in output
+        assert 'United Kingdom' in output
+        assert 'FIRE' in output
+        assert '51.5074' in output
+        assert 'Channel: 16' in output
+
+
+class TestDSCDecoder:
+    """Tests for DSC decoder utilities."""
+
+    @pytest.fixture
+    def decoder(self):
+        """Create a DSC decoder instance."""
+        # Skip if scipy not available
+        pytest.importorskip('scipy')
+        pytest.importorskip('numpy')
+        from utils.dsc.decoder import DSCDecoder
+        return DSCDecoder()
+
+    def test_decode_mmsi_valid(self, decoder):
+        """Test MMSI decoding from symbols."""
+        # Each symbol is 2 BCD digits
+        # To encode MMSI 232123456, we need:
+        # 02-32-12-34-56 -> symbols [2, 32, 12, 34, 56]
+        symbols = [2, 32, 12, 34, 56]
+        result = decoder._decode_mmsi(symbols)
+        assert result == '232123456'
+
+    def test_decode_mmsi_with_leading_zeros(self, decoder):
+        """Test MMSI decoding handles leading zeros."""
+        # Coast station: 002320001
+        # 00-23-20-00-01 -> [0, 23, 20, 0, 1]
+        symbols = [0, 23, 20, 0, 1]
+        result = decoder._decode_mmsi(symbols)
+        assert result == '002320001'
+
+    def test_decode_mmsi_short_symbols(self, decoder):
+        """Test MMSI decoding handles short symbol list."""
+        result = decoder._decode_mmsi([1, 2, 3])
+        assert result == '000000000'
+
+    def test_decode_mmsi_invalid_symbols(self, decoder):
+        """Test MMSI decoding handles invalid symbol values."""
+        # Symbols > 99 should be treated as 0
+        symbols = [100, 32, 12, 34, 56]
+        result = decoder._decode_mmsi(symbols)
+        # First symbol becomes 00
+        assert result == '003212345'[-9:]
+
+    def test_decode_position_northeast(self, decoder):
+        """Test position decoding for NE quadrant."""
+        # Quadrant 10 = NE (lat+, lon+)
+        # Position: 51°30'N, 0°10'E
+        symbols = [10, 51, 30, 0, 10, 0, 0, 0, 0, 0]
+        result = decoder._decode_position(symbols)
+
+        assert result is not None
+        assert result['lat'] == pytest.approx(51.5, rel=0.01)
+        assert result['lon'] == pytest.approx(0.1667, rel=0.01)
+
+    def test_decode_position_northwest(self, decoder):
+        """Test position decoding for NW quadrant."""
+        # Quadrant 11 = NW (lat+, lon-)
+        # Position: 40°42'N, 74°00'W (NYC area)
+        symbols = [11, 40, 42, 0, 74, 0, 0, 0, 0, 0]
+        result = decoder._decode_position(symbols)
+
+        assert result is not None
+        assert result['lat'] > 0  # North
+        assert result['lon'] < 0  # West
+
+    def test_decode_position_southeast(self, decoder):
+        """Test position decoding for SE quadrant."""
+        # Quadrant 0 = SE (lat-, lon+)
+        symbols = [0, 33, 51, 1, 51, 12, 0, 0, 0, 0]
+        result = decoder._decode_position(symbols)
+
+        assert result is not None
+        assert result['lat'] < 0  # South
+        assert result['lon'] > 0  # East
+
+    def test_decode_position_short_symbols(self, decoder):
+        """Test position decoding handles short symbol list."""
+        result = decoder._decode_position([10, 51, 30])
+        assert result is None
+
+    def test_decode_position_invalid_values(self, decoder):
+        """Test position decoding handles invalid values gracefully."""
+        # Latitude > 90 should be treated as 0
+        symbols = [10, 95, 30, 0, 10, 0, 0, 0, 0, 0]
+        result = decoder._decode_position(symbols)
+        assert result is not None
+        assert result['lat'] == pytest.approx(0.5, rel=0.01)  # 0 deg + 30 min
+
+    def test_bits_to_symbol(self, decoder):
+        """Test bit to symbol conversion."""
+        # Symbol value is first 7 bits (LSB first)
+        # Value 100 = 0b1100100 -> bits [0,0,1,0,0,1,1, x,x,x]
+        bits = [0, 0, 1, 0, 0, 1, 1, 0, 0, 0]
+        result = decoder._bits_to_symbol(bits)
+        assert result == 100
+
+    def test_bits_to_symbol_wrong_length(self, decoder):
+        """Test bit to symbol returns -1 for wrong length."""
+        result = decoder._bits_to_symbol([0, 1, 0, 1, 0])
+        assert result == -1
+
+    def test_detect_dot_pattern(self, decoder):
+        """Test dot pattern detection."""
+        # Dot pattern is alternating 1010101...
+        decoder.bit_buffer = [1, 0] * 25  # 50 alternating bits
+        assert decoder._detect_dot_pattern() is True
+
+    def test_detect_dot_pattern_insufficient(self, decoder):
+        """Test dot pattern not detected with insufficient alternations."""
+        decoder.bit_buffer = [1, 0] * 5  # Only 10 bits
+        assert decoder._detect_dot_pattern() is False
+
+    def test_detect_dot_pattern_not_alternating(self, decoder):
+        """Test dot pattern not detected without alternation."""
+        decoder.bit_buffer = [1, 1, 1, 1, 0, 0, 0, 0] * 5
+        assert decoder._detect_dot_pattern() is False
+
+
+class TestDSCConstants:
+    """Tests for DSC constants."""
+
+    def test_format_codes_completeness(self):
+        """Test that all standard format codes are defined."""
+        from utils.dsc.constants import FORMAT_CODES
+
+        # ITU-R M.493 format codes
+        assert 100 in FORMAT_CODES  # DISTRESS
+        assert 102 in FORMAT_CODES  # ALL_SHIPS
+        assert 106 in FORMAT_CODES  # DISTRESS_ACK
+        assert 112 in FORMAT_CODES  # INDIVIDUAL
+        assert 116 in FORMAT_CODES  # ROUTINE
+        assert 118 in FORMAT_CODES  # SAFETY
+        assert 120 in FORMAT_CODES  # URGENCY
+
+    def test_distress_nature_codes_completeness(self):
+        """Test that all distress nature codes are defined."""
+        from utils.dsc.constants import DISTRESS_NATURE_CODES
+
+        # ITU-R M.493 distress nature codes
+        assert 100 in DISTRESS_NATURE_CODES  # UNDESIGNATED
+        assert 101 in DISTRESS_NATURE_CODES  # FIRE
+        assert 102 in DISTRESS_NATURE_CODES  # FLOODING
+        assert 103 in DISTRESS_NATURE_CODES  # COLLISION
+        assert 106 in DISTRESS_NATURE_CODES  # SINKING
+        assert 109 in DISTRESS_NATURE_CODES  # PIRACY
+        assert 110 in DISTRESS_NATURE_CODES  # MOB
+
+    def test_mid_country_map_completeness(self):
+        """Test that common MID codes are defined."""
+        from utils.dsc.constants import MID_COUNTRY_MAP
+
+        # Verify some key maritime nations
+        assert '232' in MID_COUNTRY_MAP  # UK
+        assert '366' in MID_COUNTRY_MAP  # USA
+        assert '351' in MID_COUNTRY_MAP  # Panama
+        assert '257' in MID_COUNTRY_MAP  # Norway
+        assert '211' in MID_COUNTRY_MAP  # Germany
+        assert '503' in MID_COUNTRY_MAP  # Australia
+        assert '431' in MID_COUNTRY_MAP  # Japan
+
+    def test_vhf_channel_70_frequency(self):
+        """Test DSC Channel 70 frequency constant."""
+        from utils.dsc.constants import VHF_CHANNELS
+
+        assert VHF_CHANNELS[70] == 156.525
+
+    def test_dsc_modulation_parameters(self):
+        """Test DSC modulation constants."""
+        from utils.dsc.constants import (
+            DSC_BAUD_RATE,
+            DSC_MARK_FREQ,
+            DSC_SPACE_FREQ,
+        )
+
+        assert DSC_BAUD_RATE == 100
+        assert DSC_MARK_FREQ == 1800
+        assert DSC_SPACE_FREQ == 1200
--- a/tests/test_dsc_database.py
+++ b/tests/test_dsc_database.py
@@ -0,0 +1,422 @@
+"""Tests for DSC database operations."""
+
+import tempfile
+import pytest
+from pathlib import Path
+from unittest.mock import patch
+
+
+@pytest.fixture(autouse=True)
+def temp_db():
+    """Use a temporary database for each test."""
+    with tempfile.TemporaryDirectory() as tmpdir:
+        test_db_path = Path(tmpdir) / 'test_intercept.db'
+        test_db_dir = Path(tmpdir)
+
+        with patch('utils.database.DB_PATH', test_db_path), \
+             patch('utils.database.DB_DIR', test_db_dir):
+            from utils.database import init_db, close_db
+
+            init_db()
+            yield test_db_path
+            close_db()
+
+
+class TestDSCAlertsCRUD:
+    """Tests for DSC alerts CRUD operations."""
+
+    def test_store_and_get_dsc_alert(self, temp_db):
+        """Test storing and retrieving a DSC alert."""
+        from utils.database import store_dsc_alert, get_dsc_alert
+
+        alert_id = store_dsc_alert(
+            source_mmsi='232123456',
+            format_code='100',
+            category='DISTRESS',
+            source_name='MV Test Ship',
+            nature_of_distress='FIRE',
+            latitude=51.5,
+            longitude=-0.1
+        )
+
+        assert alert_id is not None
+        assert alert_id > 0
+
+        alert = get_dsc_alert(alert_id)
+
+        assert alert is not None
+        assert alert['source_mmsi'] == '232123456'
+        assert alert['format_code'] == '100'
+        assert alert['category'] == 'DISTRESS'
+        assert alert['source_name'] == 'MV Test Ship'
+        assert alert['nature_of_distress'] == 'FIRE'
+        assert alert['latitude'] == 51.5
+        assert alert['longitude'] == -0.1
+        assert alert['acknowledged'] is False
+
+    def test_store_minimal_alert(self, temp_db):
+        """Test storing alert with only required fields."""
+        from utils.database import store_dsc_alert, get_dsc_alert
+
+        alert_id = store_dsc_alert(
+            source_mmsi='366000001',
+            format_code='116',
+            category='ROUTINE'
+        )
+
+        alert = get_dsc_alert(alert_id)
+
+        assert alert is not None
+        assert alert['source_mmsi'] == '366000001'
+        assert alert['category'] == 'ROUTINE'
+        assert alert['latitude'] is None
+        assert alert['longitude'] is None
+
+    def test_get_nonexistent_alert(self, temp_db):
+        """Test getting an alert that doesn't exist."""
+        from utils.database import get_dsc_alert
+
+        alert = get_dsc_alert(99999)
+        assert alert is None
+
+    def test_get_dsc_alerts_all(self, temp_db):
+        """Test getting all alerts."""
+        from utils.database import store_dsc_alert, get_dsc_alerts
+
+        store_dsc_alert('232123456', '100', 'DISTRESS')
+        store_dsc_alert('366000001', '120', 'URGENCY')
+        store_dsc_alert('351234567', '116', 'ROUTINE')
+
+        alerts = get_dsc_alerts()
+
+        assert len(alerts) == 3
+
+    def test_get_dsc_alerts_by_category(self, temp_db):
+        """Test filtering alerts by category."""
+        from utils.database import store_dsc_alert, get_dsc_alerts
+
+        store_dsc_alert('232123456', '100', 'DISTRESS')
+        store_dsc_alert('232123457', '100', 'DISTRESS')
+        store_dsc_alert('366000001', '120', 'URGENCY')
+        store_dsc_alert('351234567', '116', 'ROUTINE')
+
+        distress_alerts = get_dsc_alerts(category='DISTRESS')
+        urgency_alerts = get_dsc_alerts(category='URGENCY')
+
+        assert len(distress_alerts) == 2
+        assert len(urgency_alerts) == 1
+
+    def test_get_dsc_alerts_by_acknowledged(self, temp_db):
+        """Test filtering alerts by acknowledgement status."""
+        from utils.database import (
+            store_dsc_alert,
+            get_dsc_alerts,
+            acknowledge_dsc_alert
+        )
+
+        id1 = store_dsc_alert('232123456', '100', 'DISTRESS')
+        id2 = store_dsc_alert('366000001', '100', 'DISTRESS')
+        store_dsc_alert('351234567', '100', 'DISTRESS')
+
+        acknowledge_dsc_alert(id1)
+        acknowledge_dsc_alert(id2)
+
+        unacked = get_dsc_alerts(acknowledged=False)
+        acked = get_dsc_alerts(acknowledged=True)
+
+        assert len(unacked) == 1
+        assert len(acked) == 2
+
+    def test_get_dsc_alerts_by_mmsi(self, temp_db):
+        """Test filtering alerts by source MMSI."""
+        from utils.database import store_dsc_alert, get_dsc_alerts
+
+        store_dsc_alert('232123456', '100', 'DISTRESS')
+        store_dsc_alert('232123456', '120', 'URGENCY')
+        store_dsc_alert('366000001', '100', 'DISTRESS')
+
+        alerts = get_dsc_alerts(source_mmsi='232123456')
+
+        assert len(alerts) == 2
+        for alert in alerts:
+            assert alert['source_mmsi'] == '232123456'
+
+    def test_get_dsc_alerts_pagination(self, temp_db):
+        """Test alert pagination."""
+        from utils.database import store_dsc_alert, get_dsc_alerts
+
+        # Create 10 alerts
+        for i in range(10):
+            store_dsc_alert(f'23212345{i}', '100', 'DISTRESS')
+
+        # Get first page
+        page1 = get_dsc_alerts(limit=5, offset=0)
+        assert len(page1) == 5
+
+        # Get second page
+        page2 = get_dsc_alerts(limit=5, offset=5)
+        assert len(page2) == 5
+
+        # Ensure no overlap
+        page1_ids = {a['id'] for a in page1}
+        page2_ids = {a['id'] for a in page2}
+        assert page1_ids.isdisjoint(page2_ids)
+
+    def test_get_dsc_alerts_order(self, temp_db):
+        """Test alerts are returned in reverse chronological order."""
+        from utils.database import store_dsc_alert, get_dsc_alerts
+
+        id1 = store_dsc_alert('232123456', '100', 'DISTRESS')
+        id2 = store_dsc_alert('366000001', '100', 'DISTRESS')
+        id3 = store_dsc_alert('351234567', '100', 'DISTRESS')
+
+        alerts = get_dsc_alerts()
+
+        # ORDER BY received_at DESC, so most recent first
+        # When timestamps are identical, higher IDs are more recent
+        # The actual order depends on the DB implementation
+        # We just verify all 3 are present and it's a list
+        assert len(alerts) == 3
+        alert_ids = {a['id'] for a in alerts}
+        assert alert_ids == {id1, id2, id3}
+
+    def test_acknowledge_dsc_alert(self, temp_db):
+        """Test acknowledging a DSC alert."""
+        from utils.database import (
+            store_dsc_alert,
+            get_dsc_alert,
+            acknowledge_dsc_alert
+        )
+
+        alert_id = store_dsc_alert('232123456', '100', 'DISTRESS')
+
+        # Initially not acknowledged
+        alert = get_dsc_alert(alert_id)
+        assert alert['acknowledged'] is False
+
+        # Acknowledge it
+        result = acknowledge_dsc_alert(alert_id)
+        assert result is True
+
+        # Now acknowledged
+        alert = get_dsc_alert(alert_id)
+        assert alert['acknowledged'] is True
+
+    def test_acknowledge_dsc_alert_with_notes(self, temp_db):
+        """Test acknowledging with notes."""
+        from utils.database import (
+            store_dsc_alert,
+            get_dsc_alert,
+            acknowledge_dsc_alert
+        )
+
+        alert_id = store_dsc_alert('232123456', '100', 'DISTRESS')
+
+        acknowledge_dsc_alert(alert_id, notes='Vessel located, rescue underway')
+
+        alert = get_dsc_alert(alert_id)
+        assert alert['acknowledged'] is True
+        assert alert['notes'] == 'Vessel located, rescue underway'
+
+    def test_acknowledge_nonexistent_alert(self, temp_db):
+        """Test acknowledging an alert that doesn't exist."""
+        from utils.database import acknowledge_dsc_alert
+
+        result = acknowledge_dsc_alert(99999)
+        assert result is False
+
+    def test_get_dsc_alert_summary(self, temp_db):
+        """Test getting alert summary counts."""
+        from utils.database import (
+            store_dsc_alert,
+            get_dsc_alert_summary,
+            acknowledge_dsc_alert
+        )
+
+        # Create various alerts
+        store_dsc_alert('232123456', '100', 'DISTRESS')
+        store_dsc_alert('232123457', '100', 'DISTRESS')
+        store_dsc_alert('366000001', '120', 'URGENCY')
+        store_dsc_alert('351234567', '118', 'SAFETY')
+        acked_id = store_dsc_alert('257000001', '100', 'DISTRESS')
+
+        # Acknowledge one distress
+        acknowledge_dsc_alert(acked_id)
+
+        summary = get_dsc_alert_summary()
+
+        assert summary['distress'] == 2  # 3 - 1 acknowledged
+        assert summary['urgency'] == 1
+        assert summary['safety'] == 1
+        assert summary['total'] == 4
+
+    def test_get_dsc_alert_summary_empty(self, temp_db):
+        """Test alert summary with no alerts."""
+        from utils.database import get_dsc_alert_summary
+
+        summary = get_dsc_alert_summary()
+
+        assert summary['distress'] == 0
+        assert summary['urgency'] == 0
+        assert summary['safety'] == 0
+        assert summary['routine'] == 0
+        assert summary['total'] == 0
+
+    def test_cleanup_old_dsc_alerts(self, temp_db):
+        """Test cleanup function behavior."""
+        from utils.database import (
+            store_dsc_alert,
+            get_dsc_alerts,
+            acknowledge_dsc_alert,
+            cleanup_old_dsc_alerts
+        )
+
+        # Create and acknowledge some alerts
+        id1 = store_dsc_alert('232123456', '100', 'DISTRESS')
+        id2 = store_dsc_alert('366000001', '100', 'DISTRESS')
+        id3 = store_dsc_alert('351234567', '100', 'DISTRESS')  # Unacknowledged
+
+        acknowledge_dsc_alert(id1)
+        acknowledge_dsc_alert(id2)
+
+        # Cleanup with large max_age shouldn't delete recent records
+        deleted = cleanup_old_dsc_alerts(max_age_days=30)
+        assert deleted == 0  # Nothing old enough to delete
+
+        # All 3 should still be present
+        alerts = get_dsc_alerts()
+        assert len(alerts) == 3
+
+        # Verify unacknowledged one is still unacknowledged
+        unacked = get_dsc_alerts(acknowledged=False)
+        assert len(unacked) == 1
+        assert unacked[0]['id'] == id3
+
+    def test_cleanup_preserves_unacknowledged(self, temp_db):
+        """Test cleanup preserves unacknowledged alerts regardless of age."""
+        from utils.database import (
+            store_dsc_alert,
+            get_dsc_alerts,
+            cleanup_old_dsc_alerts
+        )
+
+        # Create unacknowledged alerts
+        store_dsc_alert('232123456', '100', 'DISTRESS')
+        store_dsc_alert('366000001', '100', 'DISTRESS')
+
+        # Cleanup with 0 days
+        deleted = cleanup_old_dsc_alerts(max_age_days=0)
+
+        # All should remain (none were acknowledged)
+        alerts = get_dsc_alerts()
+        assert len(alerts) == 2
+        assert deleted == 0
+
+    def test_store_alert_with_raw_message(self, temp_db):
+        """Test storing alert with raw message data."""
+        from utils.database import store_dsc_alert, get_dsc_alert
+
+        raw = '100023212345603660000110010010000000000127'
+
+        alert_id = store_dsc_alert(
+            source_mmsi='232123456',
+            format_code='100',
+            category='DISTRESS',
+            raw_message=raw
+        )
+
+        alert = get_dsc_alert(alert_id)
+        assert alert['raw_message'] == raw
+
+    def test_store_alert_with_destination(self, temp_db):
+        """Test storing alert with destination MMSI."""
+        from utils.database import store_dsc_alert, get_dsc_alert
+
+        alert_id = store_dsc_alert(
+            source_mmsi='232123456',
+            format_code='112',
+            category='INDIVIDUAL',
+            dest_mmsi='366000001'
+        )
+
+        alert = get_dsc_alert(alert_id)
+        assert alert['dest_mmsi'] == '366000001'
+
+
+class TestDSCDatabaseIntegration:
+    """Integration tests for DSC database operations."""
+
+    def test_full_alert_lifecycle(self, temp_db):
+        """Test complete lifecycle of a DSC alert."""
+        from utils.database import (
+            store_dsc_alert,
+            get_dsc_alert,
+            get_dsc_alerts,
+            acknowledge_dsc_alert,
+            get_dsc_alert_summary
+        )
+
+        # 1. Store a distress alert
+        alert_id = store_dsc_alert(
+            source_mmsi='232123456',
+            format_code='100',
+            category='DISTRESS',
+            source_name='MV Mayday',
+            nature_of_distress='SINKING',
+            latitude=50.0,
+            longitude=-5.0
+        )
+
+        # 2. Verify it appears in summary
+        summary = get_dsc_alert_summary()
+        assert summary['distress'] == 1
+        assert summary['total'] == 1
+
+        # 3. Verify it appears in unacknowledged list
+        unacked = get_dsc_alerts(acknowledged=False)
+        assert len(unacked) == 1
+        assert unacked[0]['source_mmsi'] == '232123456'
+
+        # 4. Acknowledge with notes
+        acknowledge_dsc_alert(alert_id, 'Rescue helicopter dispatched')
+
+        # 5. Verify it's now acknowledged
+        alert = get_dsc_alert(alert_id)
+        assert alert['acknowledged'] is True
+        assert alert['notes'] == 'Rescue helicopter dispatched'
+
+        # 6. Verify summary updated
+        summary = get_dsc_alert_summary()
+        assert summary['distress'] == 0
+        assert summary['total'] == 0
+
+        # 7. Verify it appears in acknowledged list
+        acked = get_dsc_alerts(acknowledged=True)
+        assert len(acked) == 1
+
+    def test_multiple_vessel_alerts(self, temp_db):
+        """Test handling alerts from multiple vessels."""
+        from utils.database import store_dsc_alert, get_dsc_alerts
+
+        # Simulate multiple vessels in distress
+        vessels = [
+            ('232123456', 'United Kingdom', 'FIRE'),
+            ('366000001', 'USA', 'FLOODING'),
+            ('351234567', 'Panama', 'COLLISION'),
+        ]
+
+        for mmsi, country, nature in vessels:
+            store_dsc_alert(
+                source_mmsi=mmsi,
+                format_code='100',
+                category='DISTRESS',
+                nature_of_distress=nature
+            )
+
+        # Verify all alerts stored
+        alerts = get_dsc_alerts(category='DISTRESS')
+        assert len(alerts) == 3
+
+        # Verify each has correct nature
+        natures = {a['nature_of_distress'] for a in alerts}
+        assert natures == {'FIRE', 'FLOODING', 'COLLISION'}
--- a/utils/constants.py
+++ b/utils/constants.py
@@ -237,3 +237,20 @@ HANDSHAKE_CAPTURE_PATH_PREFIX = '/tmp/intercept_handshake_'

 # PMKID capture path prefix
 PMKID_CAPTURE_PATH_PREFIX = '/tmp/intercept_pmkid_'
+
+
+# =============================================================================
+# DSC (Digital Selective Calling)
+# =============================================================================
+
+# VHF DSC frequency (Channel 70)
+DSC_VHF_FREQUENCY_MHZ = 156.525
+
+# DSC audio sample rate for rtl_fm
+DSC_SAMPLE_RATE = 48000
+
+# Maximum age for DSC messages in transient store
+MAX_DSC_MESSAGE_AGE_SECONDS = 3600  # 1 hour
+
+# DSC process termination timeout
+DSC_TERMINATE_TIMEOUT = 3
--- a/utils/database.py
+++ b/utils/database.py
@@ -352,6 +352,39 @@ def init_db() -> None:
            ON tscm_cases(status, created_at)
        ''')

+        # =====================================================================
+        # DSC (Digital Selective Calling) Tables
+        # =====================================================================
+
+        # DSC Alerts - Permanent storage for DISTRESS/URGENCY messages
+        conn.execute('''
+            CREATE TABLE IF NOT EXISTS dsc_alerts (
+                id INTEGER PRIMARY KEY AUTOINCREMENT,
+                received_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+                source_mmsi TEXT NOT NULL,
+                source_name TEXT,
+                dest_mmsi TEXT,
+                format_code TEXT NOT NULL,
+                category TEXT NOT NULL,
+                nature_of_distress TEXT,
+                latitude REAL,
+                longitude REAL,
+                raw_message TEXT,
+                acknowledged BOOLEAN DEFAULT 0,
+                notes TEXT
+            )
+        ''')
+
+        conn.execute('''
+            CREATE INDEX IF NOT EXISTS idx_dsc_alerts_category
+            ON dsc_alerts(category, received_at)
+        ''')
+
+        conn.execute('''
+            CREATE INDEX IF NOT EXISTS idx_dsc_alerts_mmsi
+            ON dsc_alerts(source_mmsi, received_at)
+        ''')
+
        logger.info("Database initialized successfully")


@@ -1455,3 +1488,192 @@ def get_sweep_capabilities(sweep_id: int) -> dict | None:
            'limitations': json.loads(row['limitations']) if row['limitations'] else [],
            'recorded_at': row['recorded_at']
        }
+
+
+# =============================================================================
+# DSC (Digital Selective Calling) Functions
+# =============================================================================
+
+def store_dsc_alert(
+    source_mmsi: str,
+    format_code: str,
+    category: str,
+    source_name: str | None = None,
+    dest_mmsi: str | None = None,
+    nature_of_distress: str | None = None,
+    latitude: float | None = None,
+    longitude: float | None = None,
+    raw_message: str | None = None
+) -> int:
+    """
+    Store a DSC alert (typically DISTRESS or URGENCY) to permanent storage.
+
+    Returns:
+        The ID of the created alert
+    """
+    with get_db() as conn:
+        cursor = conn.execute('''
+            INSERT INTO dsc_alerts
+            (source_mmsi, source_name, dest_mmsi, format_code, category,
+             nature_of_distress, latitude, longitude, raw_message)
+            VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)
+        ''', (
+            source_mmsi, source_name, dest_mmsi, format_code, category,
+            nature_of_distress, latitude, longitude, raw_message
+        ))
+        return cursor.lastrowid
+
+
+def get_dsc_alerts(
+    category: str | None = None,
+    acknowledged: bool | None = None,
+    source_mmsi: str | None = None,
+    limit: int = 100,
+    offset: int = 0
+) -> list[dict]:
+    """
+    Get DSC alerts with optional filters.
+
+    Args:
+        category: Filter by category (DISTRESS, URGENCY, SAFETY, ROUTINE)
+        acknowledged: Filter by acknowledgement status
+        source_mmsi: Filter by source MMSI
+        limit: Maximum number of results
+        offset: Offset for pagination
+
+    Returns:
+        List of DSC alert records
+    """
+    conditions = []
+    params = []
+
+    if category is not None:
+        conditions.append('category = ?')
+        params.append(category)
+    if acknowledged is not None:
+        conditions.append('acknowledged = ?')
+        params.append(1 if acknowledged else 0)
+    if source_mmsi is not None:
+        conditions.append('source_mmsi = ?')
+        params.append(source_mmsi)
+
+    where_clause = f'WHERE {" AND ".join(conditions)}' if conditions else ''
+    params.extend([limit, offset])
+
+    with get_db() as conn:
+        cursor = conn.execute(f'''
+            SELECT * FROM dsc_alerts
+            {where_clause}
+            ORDER BY received_at DESC
+            LIMIT ? OFFSET ?
+        ''', params)
+
+        results = []
+        for row in cursor:
+            results.append({
+                'id': row['id'],
+                'received_at': row['received_at'],
+                'source_mmsi': row['source_mmsi'],
+                'source_name': row['source_name'],
+                'dest_mmsi': row['dest_mmsi'],
+                'format_code': row['format_code'],
+                'category': row['category'],
+                'nature_of_distress': row['nature_of_distress'],
+                'latitude': row['latitude'],
+                'longitude': row['longitude'],
+                'raw_message': row['raw_message'],
+                'acknowledged': bool(row['acknowledged']),
+                'notes': row['notes']
+            })
+        return results
+
+
+def get_dsc_alert(alert_id: int) -> dict | None:
+    """Get a specific DSC alert by ID."""
+    with get_db() as conn:
+        cursor = conn.execute(
+            'SELECT * FROM dsc_alerts WHERE id = ?',
+            (alert_id,)
+        )
+        row = cursor.fetchone()
+        if not row:
+            return None
+        return {
+            'id': row['id'],
+            'received_at': row['received_at'],
+            'source_mmsi': row['source_mmsi'],
+            'source_name': row['source_name'],
+            'dest_mmsi': row['dest_mmsi'],
+            'format_code': row['format_code'],
+            'category': row['category'],
+            'nature_of_distress': row['nature_of_distress'],
+            'latitude': row['latitude'],
+            'longitude': row['longitude'],
+            'raw_message': row['raw_message'],
+            'acknowledged': bool(row['acknowledged']),
+            'notes': row['notes']
+        }
+
+
+def acknowledge_dsc_alert(alert_id: int, notes: str | None = None) -> bool:
+    """
+    Acknowledge a DSC alert.
+
+    Args:
+        alert_id: The alert ID to acknowledge
+        notes: Optional notes about the acknowledgement
+
+    Returns:
+        True if alert was found and updated, False otherwise
+    """
+    with get_db() as conn:
+        if notes:
+            cursor = conn.execute(
+                'UPDATE dsc_alerts SET acknowledged = 1, notes = ? WHERE id = ?',
+                (notes, alert_id)
+            )
+        else:
+            cursor = conn.execute(
+                'UPDATE dsc_alerts SET acknowledged = 1 WHERE id = ?',
+                (alert_id,)
+            )
+        return cursor.rowcount > 0
+
+
+def get_dsc_alert_summary() -> dict:
+    """Get summary counts of DSC alerts by category."""
+    with get_db() as conn:
+        cursor = conn.execute('''
+            SELECT category, COUNT(*) as count
+            FROM dsc_alerts
+            WHERE acknowledged = 0
+            GROUP BY category
+        ''')
+
+        summary = {'distress': 0, 'urgency': 0, 'safety': 0, 'routine': 0, 'total': 0}
+        for row in cursor:
+            cat = row['category'].lower()
+            if cat in summary:
+                summary[cat] = row['count']
+            summary['total'] += row['count']
+
+        return summary
+
+
+def cleanup_old_dsc_alerts(max_age_days: int = 30) -> int:
+    """
+    Remove old acknowledged DSC alerts (keeps unacknowledged ones).
+
+    Args:
+        max_age_days: Maximum age in days for acknowledged alerts
+
+    Returns:
+        Number of deleted alerts
+    """
+    with get_db() as conn:
+        cursor = conn.execute('''
+            DELETE FROM dsc_alerts
+            WHERE acknowledged = 1
+              AND received_at < datetime('now', ?)
+        ''', (f'-{max_age_days} days',))
+        return cursor.rowcount
--- a/utils/dsc/init.py
+++ b/utils/dsc/init.py
@@ -0,0 +1,34 @@
+"""
+DSC (Digital Selective Calling) utilities.
+
+VHF DSC is a maritime distress and safety calling system operating on 156.525 MHz
+(VHF Channel 70). It provides automated calling for distress, urgency, safety,
+and routine communications per ITU-R M.493.
+"""
+
+from .constants import (
+    FORMAT_CODES,
+    DISTRESS_NATURE_CODES,
+    TELECOMMAND_CODES,
+    CATEGORY_PRIORITY,
+    MID_COUNTRY_MAP,
+)
+
+from .parser import (
+    parse_dsc_message,
+    get_country_from_mmsi,
+    get_distress_nature_text,
+    get_format_text,
+)
+
+__all__ = [
+    'FORMAT_CODES',
+    'DISTRESS_NATURE_CODES',
+    'TELECOMMAND_CODES',
+    'CATEGORY_PRIORITY',
+    'MID_COUNTRY_MAP',
+    'parse_dsc_message',
+    'get_country_from_mmsi',
+    'get_distress_nature_text',
+    'get_format_text',
+]
--- a/utils/dsc/constants.py
+++ b/utils/dsc/constants.py
@@ -0,0 +1,468 @@
+"""
+DSC (Digital Selective Calling) constants per ITU-R M.493.
+
+This module contains all DSC-specific constants including format codes,
+distress nature codes, telecommand definitions, and MID (Maritime
+Identification Digits) country mappings.
+"""
+
+from __future__ import annotations
+
+# =============================================================================
+# DSC Format Codes (Category)
+# Per ITU-R M.493-15 Table 1
+# =============================================================================
+
+FORMAT_CODES = {
+    100: 'DISTRESS',       # All ships distress alert
+    102: 'ALL_SHIPS',      # All ships call
+    104: 'GROUP',          # Group call
+    106: 'DISTRESS_ACK',   # Distress acknowledgement
+    108: 'DISTRESS_RELAY', # Distress relay
+    110: 'GEOGRAPHIC',     # Geographic area call
+    112: 'INDIVIDUAL',     # Individual call
+    114: 'INDIVIDUAL_ACK', # Individual acknowledgement
+    116: 'ROUTINE',        # Routine call
+    118: 'SAFETY',         # Safety call
+    120: 'URGENCY',        # Urgency call
+}
+
+# Category priority (lower = higher priority)
+CATEGORY_PRIORITY = {
+    'DISTRESS': 0,
+    'DISTRESS_ACK': 1,
+    'DISTRESS_RELAY': 2,
+    'URGENCY': 3,
+    'SAFETY': 4,
+    'ROUTINE': 5,
+    'ALL_SHIPS': 5,
+    'GROUP': 5,
+    'GEOGRAPHIC': 5,
+    'INDIVIDUAL': 5,
+    'INDIVIDUAL_ACK': 5,
+}
+
+
+# =============================================================================
+# Nature of Distress Codes
+# Per ITU-R M.493-15 Table 3
+# =============================================================================
+
+DISTRESS_NATURE_CODES = {
+    100: 'UNDESIGNATED',    # Undesignated distress
+    101: 'FIRE',            # Fire, explosion
+    102: 'FLOODING',        # Flooding
+    103: 'COLLISION',       # Collision
+    104: 'GROUNDING',       # Grounding
+    105: 'LISTING',         # Listing, in danger of capsizing
+    106: 'SINKING',         # Sinking
+    107: 'DISABLED',        # Disabled and adrift
+    108: 'ABANDONING',      # Abandoning ship
+    109: 'PIRACY',          # Piracy/armed robbery attack
+    110: 'MOB',             # Man overboard
+    112: 'EPIRB',           # EPIRB emission
+}
+
+
+# =============================================================================
+# Telecommand Codes (First and Second)
+# Per ITU-R M.493-15 Tables 4-5
+# =============================================================================
+
+TELECOMMAND_CODES = {
+    # First telecommand (type of subsequent communication)
+    100: 'F3E_G3E_ALL',        # F3E/G3E all modes (VHF telephony)
+    101: 'F3E_G3E_DUPLEX',     # F3E/G3E duplex
+    102: 'POLLING',            # Polling
+    103: 'UNABLE_TO_COMPLY',   # Unable to comply
+    104: 'END_OF_CALL',        # End of call
+    105: 'DATA',               # Data
+    106: 'J3E_TELEPHONY',      # J3E telephony (SSB)
+    107: 'DISTRESS_ACK',       # Distress acknowledgement
+    108: 'DISTRESS_RELAY',     # Distress relay
+    109: 'F1B_J2B_FEC',        # F1B/J2B FEC NBDP telegraphy
+    110: 'F1B_J2B_ARQ',        # F1B/J2B ARQ NBDP telegraphy
+    111: 'TEST',               # Test
+    112: 'SHIP_POSITION',      # Ship position request
+    113: 'NO_INFO',            # No information
+    118: 'FREQ_ANNOUNCEMENT',  # Frequency announcement
+    126: 'NO_REASON',          # No reason given
+
+    # Second telecommand (additional info)
+    200: 'F3E_G3E_SIMPLEX',    # Simplex VHF telephony requested
+    201: 'POLL_RESPONSE',      # Poll response
+}
+
+
+# =============================================================================
+# DSC Symbol Definitions
+# Per ITU-R M.493-15
+# =============================================================================
+
+# Special symbols
+DSC_SYMBOLS = {
+    120: 'DX',      # Dot pattern (synchronization)
+    121: 'RX',      # Phasing sequence RX
+    122: 'SX',      # Phasing sequence SX
+    123: 'S0',      # Phasing sequence S0
+    124: 'S1',      # Phasing sequence S1
+    125: 'S2',      # Phasing sequence S2
+    126: 'S3',      # Phasing sequence S3
+    127: 'EOS',     # End of sequence
+}
+
+
+# =============================================================================
+# MID (Maritime Identification Digits) Country Mapping
+# First 3 digits of MMSI identify the country
+# Per ITU MID table (partial list of common codes)
+# =============================================================================
+
+MID_COUNTRY_MAP = {
+    # Americas
+    '201': 'Albania',
+    '202': 'Andorra',
+    '203': 'Austria',
+    '204': 'Azores',
+    '205': 'Belgium',
+    '206': 'Belarus',
+    '207': 'Bulgaria',
+    '208': 'Vatican City',
+    '209': 'Cyprus',
+    '210': 'Cyprus',
+    '211': 'Germany',
+    '212': 'Cyprus',
+    '213': 'Georgia',
+    '214': 'Moldova',
+    '215': 'Malta',
+    '216': 'Armenia',
+    '218': 'Germany',
+    '219': 'Denmark',
+    '220': 'Denmark',
+    '224': 'Spain',
+    '225': 'Spain',
+    '226': 'France',
+    '227': 'France',
+    '228': 'France',
+    '229': 'Malta',
+    '230': 'Finland',
+    '231': 'Faroe Islands',
+    '232': 'United Kingdom',
+    '233': 'United Kingdom',
+    '234': 'United Kingdom',
+    '235': 'United Kingdom',
+    '236': 'Gibraltar',
+    '237': 'Greece',
+    '238': 'Croatia',
+    '239': 'Greece',
+    '240': 'Greece',
+    '241': 'Greece',
+    '242': 'Morocco',
+    '243': 'Hungary',
+    '244': 'Netherlands',
+    '245': 'Netherlands',
+    '246': 'Netherlands',
+    '247': 'Italy',
+    '248': 'Malta',
+    '249': 'Malta',
+    '250': 'Ireland',
+    '251': 'Iceland',
+    '252': 'Liechtenstein',
+    '253': 'Luxembourg',
+    '254': 'Monaco',
+    '255': 'Madeira',
+    '256': 'Malta',
+    '257': 'Norway',
+    '258': 'Norway',
+    '259': 'Norway',
+    '261': 'Poland',
+    '262': 'Montenegro',
+    '263': 'Portugal',
+    '264': 'Romania',
+    '265': 'Sweden',
+    '266': 'Sweden',
+    '267': 'Slovakia',
+    '268': 'San Marino',
+    '269': 'Switzerland',
+    '270': 'Czech Republic',
+    '271': 'Turkey',
+    '272': 'Ukraine',
+    '273': 'Russia',
+    '274': 'North Macedonia',
+    '275': 'Latvia',
+    '276': 'Estonia',
+    '277': 'Lithuania',
+    '278': 'Slovenia',
+    '279': 'Serbia',
+
+    # North America
+    '301': 'Anguilla',
+    '303': 'USA',
+    '304': 'Antigua and Barbuda',
+    '305': 'Antigua and Barbuda',
+    '306': 'Curacao',
+    '307': 'Aruba',
+    '308': 'Bahamas',
+    '309': 'Bahamas',
+    '310': 'Bermuda',
+    '311': 'Bahamas',
+    '312': 'Belize',
+    '314': 'Barbados',
+    '316': 'Canada',
+    '319': 'Cayman Islands',
+    '321': 'Costa Rica',
+    '323': 'Cuba',
+    '325': 'Dominica',
+    '327': 'Dominican Republic',
+    '329': 'Guadeloupe',
+    '330': 'Grenada',
+    '331': 'Greenland',
+    '332': 'Guatemala',
+    '334': 'Honduras',
+    '336': 'Haiti',
+    '338': 'USA',
+    '339': 'Jamaica',
+    '341': 'Saint Kitts and Nevis',
+    '343': 'Saint Lucia',
+    '345': 'Mexico',
+    '347': 'Martinique',
+    '348': 'Montserrat',
+    '350': 'Nicaragua',
+    '351': 'Panama',
+    '352': 'Panama',
+    '353': 'Panama',
+    '354': 'Panama',
+    '355': 'Panama',
+    '356': 'Panama',
+    '357': 'Panama',
+    '358': 'Puerto Rico',
+    '359': 'El Salvador',
+    '361': 'Saint Pierre and Miquelon',
+    '362': 'Trinidad and Tobago',
+    '364': 'Turks and Caicos',
+    '366': 'USA',
+    '367': 'USA',
+    '368': 'USA',
+    '369': 'USA',
+    '370': 'Panama',
+    '371': 'Panama',
+    '372': 'Panama',
+    '373': 'Panama',
+    '374': 'Panama',
+    '375': 'Saint Vincent and the Grenadines',
+    '376': 'Saint Vincent and the Grenadines',
+    '377': 'Saint Vincent and the Grenadines',
+    '378': 'British Virgin Islands',
+    '379': 'US Virgin Islands',
+
+    # Asia
+    '401': 'Afghanistan',
+    '403': 'Saudi Arabia',
+    '405': 'Bangladesh',
+    '408': 'Bahrain',
+    '410': 'Bhutan',
+    '412': 'China',
+    '413': 'China',
+    '414': 'China',
+    '416': 'Taiwan',
+    '417': 'Sri Lanka',
+    '419': 'India',
+    '422': 'Iran',
+    '423': 'Azerbaijan',
+    '425': 'Iraq',
+    '428': 'Israel',
+    '431': 'Japan',
+    '432': 'Japan',
+    '434': 'Turkmenistan',
+    '436': 'Kazakhstan',
+    '437': 'Uzbekistan',
+    '438': 'Jordan',
+    '440': 'South Korea',
+    '441': 'South Korea',
+    '443': 'Palestine',
+    '445': 'North Korea',
+    '447': 'Kuwait',
+    '450': 'Lebanon',
+    '451': 'Kyrgyzstan',
+    '453': 'Macao',
+    '455': 'Maldives',
+    '457': 'Mongolia',
+    '459': 'Nepal',
+    '461': 'Oman',
+    '463': 'Pakistan',
+    '466': 'Qatar',
+    '468': 'Syria',
+    '470': 'UAE',
+    '471': 'UAE',
+    '472': 'Tajikistan',
+    '473': 'Yemen',
+    '475': 'Yemen',
+    '477': 'Hong Kong',
+    '478': 'Bosnia and Herzegovina',
+
+    # Oceania
+    '501': 'Adelie Land',
+    '503': 'Australia',
+    '506': 'Myanmar',
+    '508': 'Brunei',
+    '510': 'Micronesia',
+    '511': 'Palau',
+    '512': 'New Zealand',
+    '514': 'Cambodia',
+    '515': 'Cambodia',
+    '516': 'Christmas Island',
+    '518': 'Cook Islands',
+    '520': 'Fiji',
+    '523': 'Cocos Islands',
+    '525': 'Indonesia',
+    '529': 'Kiribati',
+    '531': 'Laos',
+    '533': 'Malaysia',
+    '536': 'Northern Mariana Islands',
+    '538': 'Marshall Islands',
+    '540': 'New Caledonia',
+    '542': 'Niue',
+    '544': 'Nauru',
+    '546': 'French Polynesia',
+    '548': 'Philippines',
+    '550': 'Timor-Leste',
+    '553': 'Papua New Guinea',
+    '555': 'Pitcairn Island',
+    '557': 'Solomon Islands',
+    '559': 'American Samoa',
+    '561': 'Samoa',
+    '563': 'Singapore',
+    '564': 'Singapore',
+    '565': 'Singapore',
+    '566': 'Singapore',
+    '567': 'Thailand',
+    '570': 'Tonga',
+    '572': 'Tuvalu',
+    '574': 'Vietnam',
+    '576': 'Vanuatu',
+    '577': 'Vanuatu',
+    '578': 'Wallis and Futuna',
+
+    # Africa
+    '601': 'South Africa',
+    '603': 'Angola',
+    '605': 'Algeria',
+    '607': 'St. Paul and Amsterdam Islands',
+    '608': 'Ascension Island',
+    '609': 'Burundi',
+    '610': 'Benin',
+    '611': 'Botswana',
+    '612': 'Central African Republic',
+    '613': 'Cameroon',
+    '615': 'Congo',
+    '616': 'Comoros',
+    '617': 'Cabo Verde',
+    '618': 'Crozet Archipelago',
+    '619': 'Ivory Coast',
+    '620': 'Comoros',
+    '621': 'Djibouti',
+    '622': 'Egypt',
+    '624': 'Ethiopia',
+    '625': 'Eritrea',
+    '626': 'Gabon',
+    '627': 'Ghana',
+    '629': 'Gambia',
+    '630': 'Guinea-Bissau',
+    '631': 'Equatorial Guinea',
+    '632': 'Guinea',
+    '633': 'Burkina Faso',
+    '634': 'Kenya',
+    '635': 'Kerguelen Islands',
+    '636': 'Liberia',
+    '637': 'Liberia',
+    '638': 'South Sudan',
+    '642': 'Libya',
+    '644': 'Lesotho',
+    '645': 'Mauritius',
+    '647': 'Madagascar',
+    '649': 'Mali',
+    '650': 'Mozambique',
+    '654': 'Mauritania',
+    '655': 'Malawi',
+    '656': 'Niger',
+    '657': 'Nigeria',
+    '659': 'Namibia',
+    '660': 'Reunion',
+    '661': 'Rwanda',
+    '662': 'Sudan',
+    '663': 'Senegal',
+    '664': 'Seychelles',
+    '665': 'Saint Helena',
+    '666': 'Somalia',
+    '667': 'Sierra Leone',
+    '668': 'Sao Tome and Principe',
+    '669': 'Swaziland',
+    '670': 'Chad',
+    '671': 'Togo',
+    '672': 'Tunisia',
+    '674': 'Tanzania',
+    '675': 'Uganda',
+    '676': 'Democratic Republic of Congo',
+    '677': 'Tanzania',
+    '678': 'Zambia',
+    '679': 'Zimbabwe',
+
+    # South America
+    '701': 'Argentina',
+    '710': 'Brazil',
+    '720': 'Bolivia',
+    '725': 'Chile',
+    '730': 'Colombia',
+    '735': 'Ecuador',
+    '740': 'Falkland Islands',
+    '745': 'Guiana',
+    '750': 'Guyana',
+    '755': 'Paraguay',
+    '760': 'Peru',
+    '765': 'Suriname',
+    '770': 'Uruguay',
+    '775': 'Venezuela',
+}
+
+
+# =============================================================================
+# VHF Channel Frequencies (MHz) for DSC follow-up
+# =============================================================================
+
+VHF_CHANNELS = {
+    6: 156.300,   # Intership safety
+    8: 156.400,   # Commercial working
+    9: 156.450,   # Calling
+    10: 156.500,  # Commercial working
+    12: 156.600,  # Port operations
+    13: 156.650,  # Bridge-to-bridge navigation safety
+    14: 156.700,  # Port operations
+    16: 156.800,  # Distress, safety and calling (VHF voice)
+    67: 156.375,  # UK small craft safety
+    68: 156.425,  # Marina/yacht club
+    70: 156.525,  # DSC distress, safety and calling
+    71: 156.575,  # Port operations
+    72: 156.625,  # Intership
+    73: 156.675,  # Port operations
+    74: 156.725,  # Port operations
+    77: 156.875,  # Intership
+}
+
+
+# =============================================================================
+# DSC Modulation Parameters
+# =============================================================================
+
+DSC_BAUD_RATE = 100  # 100 baud per ITU-R M.493
+
+# FSK tone frequencies (Hz)
+DSC_MARK_FREQ = 1800  # B (mark) - binary 1
+DSC_SPACE_FREQ = 1200  # Y (space) - binary 0
+
+# Audio sample rate for decoding
+DSC_AUDIO_SAMPLE_RATE = 48000
+
+# Frame structure
+DSC_DOT_PATTERN_LENGTH = 200  # 200 bits of alternating pattern
+DSC_PHASING_LENGTH = 7        # 7 symbols phasing sequence
+DSC_MESSAGE_MAX_SYMBOLS = 180 # Maximum message length in symbols
--- a/utils/dsc/decoder.py
+++ b/utils/dsc/decoder.py
@@ -0,0 +1,514 @@
+#!/usr/bin/env python3
+"""
+DSC (Digital Selective Calling) decoder.
+
+Decodes VHF DSC signals per ITU-R M.493. Reads 48kHz 16-bit signed
+audio from stdin (from rtl_fm) and outputs JSON messages to stdout.
+
+DSC uses 100 baud FSK with:
+- Mark (1): 1800 Hz
+- Space (0): 1200 Hz
+
+Frame structure:
+1. Dot pattern: 200 bits alternating 1/0 for synchronization
+2. Phasing sequence: 7 symbols (RX or DX pattern)
+3. Format specifier: Identifies message type
+4. Address/Self-ID fields
+5. Category/Nature fields (if distress)
+6. Position data (if present)
+7. Telecommand fields
+8. EOS (End of Sequence)
+
+Each symbol is 10 bits (7 data + 3 error detection).
+"""
+
+from __future__ import annotations
+
+import argparse
+import json
+import logging
+import struct
+import sys
+from datetime import datetime
+from typing import Generator
+
+import numpy as np
+from scipy import signal as scipy_signal
+
+from .constants import (
+    DSC_BAUD_RATE,
+    DSC_MARK_FREQ,
+    DSC_SPACE_FREQ,
+    DSC_AUDIO_SAMPLE_RATE,
+    FORMAT_CODES,
+    DISTRESS_NATURE_CODES,
+)
+
+# Configure logging
+logging.basicConfig(
+    level=logging.WARNING,
+    format='%(asctime)s [%(levelname)s] %(message)s',
+    stream=sys.stderr
+)
+logger = logging.getLogger('dsc.decoder')
+
+
+class DSCDecoder:
+    """
+    DSC FSK decoder.
+
+    Demodulates 100 baud FSK audio and decodes DSC protocol.
+    """
+
+    def __init__(self, sample_rate: int = DSC_AUDIO_SAMPLE_RATE):
+        self.sample_rate = sample_rate
+        self.baud_rate = DSC_BAUD_RATE
+        self.samples_per_bit = sample_rate // self.baud_rate
+
+        # FSK frequencies
+        self.mark_freq = DSC_MARK_FREQ  # 1800 Hz = binary 1
+        self.space_freq = DSC_SPACE_FREQ  # 1200 Hz = binary 0
+
+        # Bandpass filter for DSC band (1100-1900 Hz)
+        nyq = sample_rate / 2
+        low = 1100 / nyq
+        high = 1900 / nyq
+        self.bp_b, self.bp_a = scipy_signal.butter(4, [low, high], btype='band')
+
+        # Build FSK correlators
+        self._build_correlators()
+
+        # State
+        self.buffer = np.array([], dtype=np.int16)
+        self.bit_buffer = []
+        self.in_message = False
+        self.message_bits = []
+
+    def _build_correlators(self):
+        """Build matched filter correlators for mark and space frequencies."""
+        # Duration for one bit
+        t = np.arange(self.samples_per_bit) / self.sample_rate
+
+        # Mark correlator (1800 Hz)
+        self.mark_ref = np.sin(2 * np.pi * self.mark_freq * t)
+
+        # Space correlator (1200 Hz)
+        self.space_ref = np.sin(2 * np.pi * self.space_freq * t)
+
+    def process_audio(self, audio_data: bytes) -> Generator[dict, None, None]:
+        """
+        Process audio data and yield decoded DSC messages.
+
+        Args:
+            audio_data: Raw 16-bit signed PCM audio bytes
+
+        Yields:
+            Decoded DSC message dicts
+        """
+        # Convert bytes to numpy array
+        samples = np.frombuffer(audio_data, dtype=np.int16)
+        if len(samples) == 0:
+            return
+
+        # Append to buffer
+        self.buffer = np.concatenate([self.buffer, samples])
+
+        # Need at least one bit worth of samples
+        if len(self.buffer) < self.samples_per_bit:
+            return
+
+        # Apply bandpass filter
+        try:
+            filtered = scipy_signal.lfilter(self.bp_b, self.bp_a, self.buffer)
+        except Exception as e:
+            logger.warning(f"Filter error: {e}")
+            return
+
+        # Demodulate FSK using correlation
+        bits = self._demodulate_fsk(filtered)
+
+        # Keep unprocessed samples (last bit's worth)
+        keep_samples = self.samples_per_bit * 2
+        if len(self.buffer) > keep_samples:
+            self.buffer = self.buffer[-keep_samples:]
+
+        # Process decoded bits
+        for bit in bits:
+            message = self._process_bit(bit)
+            if message:
+                yield message
+
+    def _demodulate_fsk(self, samples: np.ndarray) -> list[int]:
+        """
+        Demodulate FSK audio to bits using correlation.
+
+        Args:
+            samples: Filtered audio samples
+
+        Returns:
+            List of decoded bits (0 or 1)
+        """
+        bits = []
+        num_bits = len(samples) // self.samples_per_bit
+
+        for i in range(num_bits):
+            start = i * self.samples_per_bit
+            end = start + self.samples_per_bit
+            segment = samples[start:end]
+
+            if len(segment) < self.samples_per_bit:
+                break
+
+            # Correlate with mark and space references
+            mark_corr = np.abs(np.correlate(segment, self.mark_ref, mode='valid'))
+            space_corr = np.abs(np.correlate(segment, self.space_ref, mode='valid'))
+
+            # Decision: mark (1) if mark correlation > space correlation
+            if np.max(mark_corr) > np.max(space_corr):
+                bits.append(1)
+            else:
+                bits.append(0)
+
+        return bits
+
+    def _process_bit(self, bit: int) -> dict | None:
+        """
+        Process a decoded bit and detect/decode DSC messages.
+
+        Args:
+            bit: Decoded bit (0 or 1)
+
+        Returns:
+            Decoded message dict if complete message found, None otherwise
+        """
+        self.bit_buffer.append(bit)
+
+        # Keep buffer manageable
+        if len(self.bit_buffer) > 2000:
+            self.bit_buffer = self.bit_buffer[-1500:]
+
+        # Look for dot pattern (sync) - alternating 1010101...
+        if not self.in_message:
+            if self._detect_dot_pattern():
+                self.in_message = True
+                self.message_bits = []
+                logger.debug("DSC sync detected")
+                return None
+
+        # Collect message bits
+        if self.in_message:
+            self.message_bits.append(bit)
+
+            # Check for end of message or timeout
+            if len(self.message_bits) >= 10:  # One symbol
+                # Try to decode accumulated symbols
+                message = self._try_decode_message()
+                if message:
+                    self.in_message = False
+                    self.message_bits = []
+                    return message
+
+            # Timeout - too many bits without valid message
+            if len(self.message_bits) > 1800:  # ~180 symbols max
+                logger.debug("DSC message timeout")
+                self.in_message = False
+                self.message_bits = []
+
+        return None
+
+    def _detect_dot_pattern(self) -> bool:
+        """
+        Detect DSC dot pattern for synchronization.
+
+        The dot pattern is at least 200 alternating bits (1010101...).
+        We look for at least 20 consecutive alternations.
+        """
+        if len(self.bit_buffer) < 40:
+            return False
+
+        # Check last 40 bits for alternating pattern
+        last_bits = self.bit_buffer[-40:]
+        alternations = 0
+
+        for i in range(1, len(last_bits)):
+            if last_bits[i] != last_bits[i - 1]:
+                alternations += 1
+            else:
+                alternations = 0
+
+            if alternations >= 20:
+                return True
+
+        return False
+
+    def _try_decode_message(self) -> dict | None:
+        """
+        Try to decode accumulated message bits as DSC message.
+
+        Returns:
+            Decoded message dict or None if not yet complete/valid
+        """
+        # Need at least a few symbols to start decoding
+        num_symbols = len(self.message_bits) // 10
+
+        if num_symbols < 5:
+            return None
+
+        # Extract symbols (10 bits each)
+        symbols = []
+        for i in range(num_symbols):
+            start = i * 10
+            end = start + 10
+            if end <= len(self.message_bits):
+                symbol_bits = self.message_bits[start:end]
+                symbol_value = self._bits_to_symbol(symbol_bits)
+                symbols.append(symbol_value)
+
+        # Look for EOS (End of Sequence) - symbol 127
+        eos_found = False
+        eos_index = -1
+        for i, sym in enumerate(symbols):
+            if sym == 127:  # EOS symbol
+                eos_found = True
+                eos_index = i
+                break
+
+        if not eos_found:
+            # Not complete yet
+            return None
+
+        # Decode the message from symbols
+        return self._decode_symbols(symbols[:eos_index + 1])
+
+    def _bits_to_symbol(self, bits: list[int]) -> int:
+        """
+        Convert 10 bits to symbol value.
+
+        DSC uses 10-bit symbols: 7 information bits + 3 error bits.
+        We extract the 7-bit value.
+        """
+        if len(bits) != 10:
+            return -1
+
+        # First 7 bits are data (LSB first in DSC)
+        value = 0
+        for i in range(7):
+            if bits[i]:
+                value |= (1 << i)
+
+        return value
+
+    def _decode_symbols(self, symbols: list[int]) -> dict | None:
+        """
+        Decode DSC symbol sequence to message.
+
+        Message structure (symbols):
+        0: Format specifier
+        1-5: Address/MMSI (encoded)
+        6-10: Self-ID/MMSI (encoded)
+        11+: Variable fields depending on format
+        Last: EOS (127)
+
+        Args:
+            symbols: List of decoded symbol values
+
+        Returns:
+            Decoded message dict or None if invalid
+        """
+        if len(symbols) < 12:
+            return None
+
+        try:
+            # Format specifier (first non-phasing symbol)
+            format_code = symbols[0]
+            format_text = FORMAT_CODES.get(format_code, f'UNKNOWN-{format_code}')
+
+            # Determine category from format
+            category = 'ROUTINE'
+            if format_code == 100:
+                category = 'DISTRESS'
+            elif format_code == 106:
+                category = 'DISTRESS_ACK'
+            elif format_code == 108:
+                category = 'DISTRESS_RELAY'
+            elif format_code == 118:
+                category = 'SAFETY'
+            elif format_code == 120:
+                category = 'URGENCY'
+            elif format_code == 102:
+                category = 'ALL_SHIPS'
+
+            # Decode MMSI from symbols 1-5 (destination/address)
+            dest_mmsi = self._decode_mmsi(symbols[1:6])
+
+            # Decode self-ID from symbols 6-10 (source)
+            source_mmsi = self._decode_mmsi(symbols[6:11])
+
+            message = {
+                'type': 'dsc',
+                'format': format_code,
+                'format_text': format_text,
+                'category': category,
+                'source_mmsi': source_mmsi,
+                'dest_mmsi': dest_mmsi,
+                'timestamp': datetime.utcnow().isoformat() + 'Z',
+            }
+
+            # Parse additional fields based on format
+            remaining = symbols[11:-1]  # Exclude EOS
+
+            if category in ('DISTRESS', 'DISTRESS_RELAY'):
+                # Distress messages have nature and position
+                if len(remaining) >= 1:
+                    message['nature'] = remaining[0]
+                    message['nature_text'] = DISTRESS_NATURE_CODES.get(
+                        remaining[0], f'UNKNOWN-{remaining[0]}'
+                    )
+
+                # Try to decode position
+                if len(remaining) >= 11:
+                    position = self._decode_position(remaining[1:11])
+                    if position:
+                        message['position'] = position
+
+            # Telecommand fields (usually last two before EOS)
+            if len(remaining) >= 2:
+                message['telecommand1'] = remaining[-2]
+                message['telecommand2'] = remaining[-1]
+
+            # Add raw data for debugging
+            message['raw'] = ''.join(f'{s:03d}' for s in symbols)
+
+            logger.info(f"Decoded DSC: {category} from {source_mmsi}")
+            return message
+
+        except Exception as e:
+            logger.warning(f"DSC decode error: {e}")
+            return None
+
+    def _decode_mmsi(self, symbols: list[int]) -> str:
+        """
+        Decode MMSI from 5 DSC symbols.
+
+        Each symbol represents 2 BCD digits (00-99).
+        5 symbols = 10 digits, but MMSI is 9 digits (first symbol has leading 0).
+        """
+        if len(symbols) < 5:
+            return '000000000'
+
+        digits = []
+        for sym in symbols:
+            if sym < 0 or sym > 99:
+                sym = 0
+            # Each symbol is 2 BCD digits
+            digits.append(f'{sym:02d}')
+
+        mmsi = ''.join(digits)
+        # MMSI is 9 digits, might need to trim leading zero
+        if len(mmsi) > 9:
+            mmsi = mmsi[-9:]
+
+        return mmsi.zfill(9)
+
+    def _decode_position(self, symbols: list[int]) -> dict | None:
+        """
+        Decode position from 10 DSC symbols.
+
+        Position encoding (ITU-R M.493):
+        - Quadrant (10=NE, 11=NW, 00=SE, 01=SW)
+        - Latitude degrees (2 digits)
+        - Latitude minutes (2 digits)
+        - Longitude degrees (3 digits)
+        - Longitude minutes (2 digits)
+        """
+        if len(symbols) < 10:
+            return None
+
+        try:
+            # Quadrant indicator
+            quadrant = symbols[0]
+            lat_sign = 1 if quadrant in (10, 11) else -1
+            lon_sign = 1 if quadrant in (10, 0) else -1
+
+            # Latitude degrees and minutes
+            lat_deg = symbols[1] if symbols[1] <= 90 else 0
+            lat_min = symbols[2] if symbols[2] < 60 else 0
+
+            # Longitude degrees (3 digits from 2 symbols)
+            lon_deg_high = symbols[3] if symbols[3] < 10 else 0
+            lon_deg_low = symbols[4] if symbols[4] < 100 else 0
+            lon_deg = lon_deg_high * 100 + lon_deg_low
+            if lon_deg > 180:
+                lon_deg = 0
+
+            lon_min = symbols[5] if symbols[5] < 60 else 0
+
+            lat = lat_sign * (lat_deg + lat_min / 60.0)
+            lon = lon_sign * (lon_deg + lon_min / 60.0)
+
+            return {'lat': round(lat, 6), 'lon': round(lon, 6)}
+
+        except Exception:
+            return None
+
+
+def read_audio_stdin() -> Generator[bytes, None, None]:
+    """
+    Read audio from stdin in chunks.
+
+    Yields:
+        Audio data chunks
+    """
+    chunk_size = 4800  # 0.1 seconds at 48kHz, 16-bit = 9600 bytes
+    while True:
+        try:
+            data = sys.stdin.buffer.read(chunk_size * 2)  # 2 bytes per sample
+            if not data:
+                break
+            yield data
+        except KeyboardInterrupt:
+            break
+        except Exception as e:
+            logger.error(f"Read error: {e}")
+            break
+
+
+def main():
+    """Main entry point for DSC decoder."""
+    parser = argparse.ArgumentParser(
+        description='DSC (Digital Selective Calling) decoder',
+        epilog='Reads 48kHz 16-bit signed PCM audio from stdin'
+    )
+    parser.add_argument(
+        '-r', '--sample-rate',
+        type=int,
+        default=DSC_AUDIO_SAMPLE_RATE,
+        help=f'Audio sample rate (default: {DSC_AUDIO_SAMPLE_RATE})'
+    )
+    parser.add_argument(
+        '-v', '--verbose',
+        action='store_true',
+        help='Enable verbose logging'
+    )
+    args = parser.parse_args()
+
+    if args.verbose:
+        logger.setLevel(logging.DEBUG)
+
+    decoder = DSCDecoder(sample_rate=args.sample_rate)
+
+    logger.info(f"DSC decoder started (sample rate: {args.sample_rate})")
+
+    for audio_chunk in read_audio_stdin():
+        for message in decoder.process_audio(audio_chunk):
+            # Output JSON to stdout
+            try:
+                print(json.dumps(message), flush=True)
+            except Exception as e:
+                logger.error(f"Output error: {e}")
+
+    logger.info("DSC decoder stopped")
+
+
+if __name__ == '__main__':
+    main()
--- a/utils/dsc/parser.py
+++ b/utils/dsc/parser.py
@@ -0,0 +1,322 @@
+"""
+DSC message parser.
+
+Parses DSC decoder JSON output and provides utility functions for
+MMSI country resolution, distress nature text, etc.
+"""
+
+from __future__ import annotations
+
+import json
+import logging
+import re
+from datetime import datetime
+from typing import Any
+
+from .constants import (
+    FORMAT_CODES,
+    DISTRESS_NATURE_CODES,
+    TELECOMMAND_CODES,
+    CATEGORY_PRIORITY,
+    MID_COUNTRY_MAP,
+)
+
+logger = logging.getLogger('intercept.dsc.parser')
+
+
+def get_country_from_mmsi(mmsi: str) -> str | None:
+    """
+    Derive country from MMSI using Maritime Identification Digits (MID).
+
+    The first 3 digits of a 9-digit MMSI identify the country.
+
+    Args:
+        mmsi: The MMSI number as string
+
+    Returns:
+        Country name if found, None otherwise
+    """
+    if not mmsi or len(mmsi) < 3:
+        return None
+
+    # Normal ship MMSI: starts with MID (3 digits)
+    mid = mmsi[:3]
+    if mid in MID_COUNTRY_MAP:
+        return MID_COUNTRY_MAP[mid]
+
+    # Coast station MMSI: starts with 00 + MID
+    if mmsi.startswith('00') and len(mmsi) >= 5:
+        mid = mmsi[2:5]
+        if mid in MID_COUNTRY_MAP:
+            return MID_COUNTRY_MAP[mid]
+
+    # Group ship station MMSI: starts with 0 + MID
+    if mmsi.startswith('0') and len(mmsi) >= 4:
+        mid = mmsi[1:4]
+        if mid in MID_COUNTRY_MAP:
+            return MID_COUNTRY_MAP[mid]
+
+    return None
+
+
+def get_distress_nature_text(code: int | str) -> str:
+    """Get human-readable text for distress nature code."""
+    if isinstance(code, str):
+        try:
+            code = int(code)
+        except ValueError:
+            return str(code)
+
+    return DISTRESS_NATURE_CODES.get(code, f'UNKNOWN ({code})')
+
+
+def get_format_text(code: int | str) -> str:
+    """Get human-readable text for format code."""
+    if isinstance(code, str):
+        try:
+            code = int(code)
+        except ValueError:
+            return str(code)
+
+    return FORMAT_CODES.get(code, f'UNKNOWN ({code})')
+
+
+def get_telecommand_text(code: int | str) -> str:
+    """Get human-readable text for telecommand code."""
+    if isinstance(code, str):
+        try:
+            code = int(code)
+        except ValueError:
+            return str(code)
+
+    return TELECOMMAND_CODES.get(code, f'UNKNOWN ({code})')
+
+
+def get_category_priority(category: str) -> int:
+    """Get priority level for a category (lower = higher priority)."""
+    return CATEGORY_PRIORITY.get(category.upper(), 10)
+
+
+def parse_dsc_message(raw_line: str) -> dict[str, Any] | None:
+    """
+    Parse DSC decoder JSON output line.
+
+    The decoder outputs JSON lines with fields like:
+    {
+        "type": "dsc",
+        "format": 100,
+        "source_mmsi": "123456789",
+        "dest_mmsi": "000000000",
+        "category": "DISTRESS",
+        "nature": 101,
+        "position": {"lat": 51.5, "lon": -0.1},
+        "telecommand1": 100,
+        "telecommand2": null,
+        "channel": 16,
+        "timestamp": "2025-01-15T12:00:00Z",
+        "raw": "..."
+    }
+
+    Args:
+        raw_line: Raw JSON line from decoder
+
+    Returns:
+        Parsed message dict or None if parsing fails
+    """
+    if not raw_line or not raw_line.strip():
+        return None
+
+    try:
+        data = json.loads(raw_line.strip())
+    except json.JSONDecodeError as e:
+        logger.debug(f"Failed to parse DSC JSON: {e}")
+        return None
+
+    # Validate required fields
+    if data.get('type') != 'dsc':
+        return None
+
+    if 'source_mmsi' not in data:
+        return None
+
+    # Build parsed message
+    msg = {
+        'type': 'dsc_message',
+        'source_mmsi': str(data.get('source_mmsi', '')),
+        'dest_mmsi': str(data.get('dest_mmsi', '')) if data.get('dest_mmsi') else None,
+        'format_code': data.get('format'),
+        'format_text': get_format_text(data.get('format', 0)),
+        'category': data.get('category', 'UNKNOWN').upper(),
+        'timestamp': data.get('timestamp') or datetime.utcnow().isoformat(),
+    }
+
+    # Add country from MMSI
+    country = get_country_from_mmsi(msg['source_mmsi'])
+    if country:
+        msg['source_country'] = country
+
+    # Add distress nature if present
+    if 'nature' in data and data['nature']:
+        msg['nature_code'] = data['nature']
+        msg['nature_of_distress'] = get_distress_nature_text(data['nature'])
+
+    # Add position if present
+    position = data.get('position')
+    if position and isinstance(position, dict):
+        lat = position.get('lat')
+        lon = position.get('lon')
+        if lat is not None and lon is not None:
+            try:
+                msg['latitude'] = float(lat)
+                msg['longitude'] = float(lon)
+            except (ValueError, TypeError):
+                pass
+
+    # Add telecommand info
+    if 'telecommand1' in data and data['telecommand1']:
+        msg['telecommand1'] = data['telecommand1']
+        msg['telecommand1_text'] = get_telecommand_text(data['telecommand1'])
+
+    if 'telecommand2' in data and data['telecommand2']:
+        msg['telecommand2'] = data['telecommand2']
+        msg['telecommand2_text'] = get_telecommand_text(data['telecommand2'])
+
+    # Add channel if present
+    if 'channel' in data and data['channel']:
+        msg['channel'] = data['channel']
+
+    # Add EOS (End of Sequence) info
+    if 'eos' in data:
+        msg['eos'] = data['eos']
+
+    # Add raw message for debugging
+    if 'raw' in data:
+        msg['raw_message'] = data['raw']
+
+    # Calculate priority
+    msg['priority'] = get_category_priority(msg['category'])
+
+    # Mark if this is a critical alert
+    msg['is_critical'] = msg['category'] in ('DISTRESS', 'DISTRESS_ACK', 'DISTRESS_RELAY', 'URGENCY')
+
+    return msg
+
+
+def format_dsc_for_display(msg: dict) -> str:
+    """
+    Format a DSC message for human-readable display.
+
+    Args:
+        msg: Parsed DSC message dict
+
+    Returns:
+        Formatted string for display
+    """
+    lines = []
+
+    # Header with category and MMSI
+    category = msg.get('category', 'UNKNOWN')
+    mmsi = msg.get('source_mmsi', 'UNKNOWN')
+    country = msg.get('source_country', '')
+
+    header = f"[{category}] MMSI: {mmsi}"
+    if country:
+        header += f" ({country})"
+    lines.append(header)
+
+    # Destination if present
+    if msg.get('dest_mmsi'):
+        lines.append(f"  To: {msg['dest_mmsi']}")
+
+    # Distress nature
+    if msg.get('nature_of_distress'):
+        lines.append(f"  Nature: {msg['nature_of_distress']}")
+
+    # Position
+    if msg.get('latitude') is not None and msg.get('longitude') is not None:
+        lat = msg['latitude']
+        lon = msg['longitude']
+        lat_dir = 'N' if lat >= 0 else 'S'
+        lon_dir = 'E' if lon >= 0 else 'W'
+        lines.append(f"  Position: {abs(lat):.4f}{lat_dir} {abs(lon):.4f}{lon_dir}")
+
+    # Telecommand
+    if msg.get('telecommand1_text'):
+        lines.append(f"  Request: {msg['telecommand1_text']}")
+
+    # Channel
+    if msg.get('channel'):
+        lines.append(f"  Channel: {msg['channel']}")
+
+    # Timestamp
+    if msg.get('timestamp'):
+        lines.append(f"  Time: {msg['timestamp']}")
+
+    return '\n'.join(lines)
+
+
+def validate_mmsi(mmsi: str) -> bool:
+    """
+    Validate MMSI format.
+
+    MMSI is a 9-digit number. Ship stations start with non-zero digit.
+    Coast stations start with 00. Group stations start with 0.
+
+    Args:
+        mmsi: MMSI string to validate
+
+    Returns:
+        True if valid MMSI format
+    """
+    if not mmsi:
+        return False
+
+    # Must be 9 digits
+    if not re.match(r'^\d{9}$', mmsi):
+        return False
+
+    # All zeros is invalid
+    if mmsi == '000000000':
+        return False
+
+    return True
+
+
+def classify_mmsi(mmsi: str) -> str:
+    """
+    Classify MMSI type.
+
+    Args:
+        mmsi: MMSI string
+
+    Returns:
+        Classification: 'ship', 'coast', 'group', 'sar', 'aton', or 'unknown'
+    """
+    if not validate_mmsi(mmsi):
+        return 'unknown'
+
+    first_digit = mmsi[0]
+    first_two = mmsi[:2]
+    first_three = mmsi[:3]
+
+    # Coast station: starts with 00
+    if first_two == '00':
+        return 'coast'
+
+    # Group call: starts with 0
+    if first_digit == '0':
+        return 'group'
+
+    # SAR aircraft: starts with 111
+    if first_three == '111':
+        return 'sar'
+
+    # Aids to Navigation: starts with 99
+    if first_two == '99':
+        return 'aton'
+
+    # Ship station: starts with MID (2-7)
+    if first_digit in '234567':
+        return 'ship'
+
+    return 'unknown'