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synced 2026-07-29 02:58:11 -07:00
Fix MMSI/position decode convention per spec; flag uncertain positions instead of clamping/discarding; deduplicate methods
This commit is contained in:
Jim Arndt
+817
-805
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+90
-181
@@ -527,49 +527,6 @@ class DSCDecoder:
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if eos is None:
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return None # message not complete yet
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def decode_mmsi(syms):
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if len(syms) != 5 or any(s < 0 or s > 99 for s in syms):
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return None
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digits = "".join(f"{s:02d}" for s in syms)
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return digits[:9]
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def decode_position(syms):
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"""
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Decode 5 coordinate symbols into a position dict, per ITU-R
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M.493 Annex 1 Sec 8.1.2/8.2.3 (Table A1-2), confirmed
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consistent across M.493-8 through -15. 10 digits total:
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digit 1 = quadrant (0=NE, 1=NW, 2=SE, 3=SW), digits 2-5 =
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latitude (2 degrees + 2 minutes), digits 6-10 = longitude
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(3 degrees + 2 minutes). All-9s (9999999999) is the spec-
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defined "position not available" sentinel.
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Returns a dict with "lat"/"lon" as signed decimal degrees
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(matching the pre-existing message["position"] shape this
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decoder's downstream consumers - parser.py/routes/dsc.py -
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expect) plus human-readable degree/minute text fields.
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Returns None if the position is unavailable or malformed.
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"""
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if len(syms) != 5 or any(s < 0 or s > 99 for s in syms):
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return None
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digits = "".join(f"{s:02d}" for s in syms)
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if digits == "9999999999":
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return None
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quadrant = int(digits[0])
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lat_deg = int(digits[1:3])
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lat_min = int(digits[3:5])
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lon_deg = int(digits[5:8])
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lon_min = int(digits[8:10])
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lat_sign = 1 if quadrant in (0, 1) else -1 # NE, NW -> North
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lon_sign = 1 if quadrant in (0, 2) else -1 # NE, SE -> East
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lat = lat_sign * (lat_deg + lat_min / 60.0)
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lon = lon_sign * (lon_deg + lon_min / 60.0)
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return {
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"lat": round(lat, 6),
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"lon": round(lon, 6),
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"lat_text": f"{lat_deg:02d}°{lat_min:02d}'{'N' if lat_sign > 0 else 'S'}",
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"lon_text": f"{lon_deg:03d}°{lon_min:02d}'{'E' if lon_sign > 0 else 'W'}",
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}
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message = {
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"type": "dsc",
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"format": format_code,
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@@ -581,11 +538,11 @@ class DSCDecoder:
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if is_distress_ack:
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message["format_text"] = "DISTRESS_ACK_OR_SELF_CANCEL"
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if "address" in fields:
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message["dest_mmsi"] = decode_mmsi(fields["address"])
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message["dest_mmsi"] = self._decode_mmsi(fields["address"])
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if "self_id" in fields:
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message["source_mmsi"] = decode_mmsi(fields["self_id"])
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message["source_mmsi"] = self._decode_mmsi(fields["self_id"])
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if "ship_in_distress_id" in fields:
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ship_mmsi = decode_mmsi(fields["ship_in_distress_id"])
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ship_mmsi = self._decode_mmsi(fields["ship_in_distress_id"])
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message["ship_in_distress_mmsi"] = ship_mmsi
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if "source_mmsi" not in message:
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# for a self-cancel, transmitting station IS the ship in
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@@ -603,7 +560,7 @@ class DSCDecoder:
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message["nature_text"] = NATURE_TEXT.get(nature_val, f"UNKNOWN-{nature_val}")
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if "coordinates" in fields:
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message["coordinates_raw"] = fields["coordinates"]
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message["position"] = decode_position(fields["coordinates"])
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message["position"] = self._decode_position(fields["coordinates"])
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if "time" in fields:
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message["time_raw"] = fields["time"]
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if "telecommand" in fields:
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@@ -718,143 +675,95 @@ class DSCDecoder:
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return (f"format={format_code}, all fields complete, but no valid EOS "
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f"found in remaining {len(dedup) - idx} symbols")
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def _diagnose_decode_failure(self) -> str:
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def _decode_mmsi(self, symbols: list[int]) -> str | None:
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"""
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Explain exactly why the current message_bits buffer failed to
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decode, for logging at timeout. Mirrors _try_decode_message's
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logic but reports where it stopped instead of returning None
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silently - normal accumulation still returns None quietly (this
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is only called once, at the moment of timeout, not on every
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partial-accumulation call).
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Decode 5 symbols into a 9-digit MMSI string.
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Per ITU-R M.493-16: identities are transmitted as five
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characters C5C4C3C2C1, comprising ten digits X1...X10, "whereas
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digit X10 is always the digit 0" (unless M.1080 extended
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addressing is in use, not handled here). X10 is the LAST digit
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of the sequence, not the first - so the real 9-digit MMSI is
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digits 1-9, dropping the trailing padding digit. Confirmed
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against real over-the-air captures: this produces MMSIs with
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recognizable, valid Maritime Identification Digits (e.g. "338"
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= USA) consistently across independent real recordings.
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Args:
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symbols: 5 raw symbol values (0-99 each)
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Returns:
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9-digit MMSI string, or None if malformed input.
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"""
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FORMAT_SPECIFIERS = {102, 112, 114, 116, 120, 123}
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FIELD_LAYOUTS = {
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120: [("address", 5), ("category", 1), ("self_id", 5),
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("telecommand", 2), ("frequency_1", 3), ("frequency_2", 3)],
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123: [("address", 5), ("category", 1), ("self_id", 5),
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("telecommand", 2), ("frequency_1", 3)],
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116: [("category", 1), ("self_id", 5), ("telecommand", 2),
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("frequency_1", 3)],
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114: [("address", 5), ("category", 1), ("self_id", 5),
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("telecommand", 2), ("frequency_1", 3), ("frequency_2", 3)],
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102: [("address", 5), ("category", 1), ("self_id", 5),
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("telecommand", 2), ("frequency_1", 3), ("frequency_2", 3)],
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112: [("self_id", 5), ("nature", 1), ("coordinates", 5),
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("time", 2), ("telecommand_msg4", 1)],
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if len(symbols) != 5 or any(s < 0 or s > 99 for s in symbols):
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return None
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digits = "".join(f"{s:02d}" for s in symbols)
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return digits[:9]
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def _decode_position(self, symbols: list[int]) -> dict | None:
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"""
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Decode 5 coordinate symbols into a position dict, per ITU-R
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M.493 Annex 1 Sec 8.1.2/8.2.3 (Table A1-2), confirmed
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consistent across M.493-8 through -15. 10 digits total:
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digit 1 = quadrant (0=NE, 1=NW, 2=SE, 3=SW), digits 2-5 =
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latitude (2 degrees + 2 minutes), digits 6-10 = longitude
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(3 degrees + 2 minutes). All-9s (9999999999) is the spec-
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defined "position not available" sentinel.
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Physically-impossible values (degrees/minutes outside valid
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range - e.g. from a residual bit error the diversity-combining
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fallback in _process_bit didn't catch) are NOT silently
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discarded or clamped: the computed position is still returned
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(a partial position can still be useful - e.g. for search and
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rescue, where even an approximate position beats none at all),
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but flagged via "position_uncertain"/"position_issue" so
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downstream consumers can decide how much to trust it rather
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than being silently given a wrong-but-plausible-looking value.
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Returns a dict with "lat"/"lon" as signed decimal degrees
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(matching the pre-existing message["position"] shape this
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decoder's downstream consumers - parser.py/routes/dsc.py -
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expect) plus human-readable degree/minute text fields and the
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uncertainty flag described above. Returns None only if the
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position is unavailable (all-9s sentinel) or the input itself
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is malformed (wrong length/out-of-range symbols).
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Args:
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symbols: 5 raw symbol values (0-99 each)
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"""
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if len(symbols) != 5 or any(s < 0 or s > 99 for s in symbols):
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return None
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digits = "".join(f"{s:02d}" for s in symbols)
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if digits == "9999999999":
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return None
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quadrant = int(digits[0])
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lat_deg = int(digits[1:3])
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lat_min = int(digits[3:5])
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lon_deg = int(digits[5:8])
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lon_min = int(digits[8:10])
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issues = []
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if lat_deg > 90:
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issues.append(f"latitude degrees ({lat_deg}) exceeds valid range (0-90)")
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if lon_deg > 180:
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issues.append(f"longitude degrees ({lon_deg}) exceeds valid range (0-180)")
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if lat_min > 59:
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issues.append(f"latitude minutes ({lat_min}) exceeds valid range (0-59)")
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if lon_min > 59:
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issues.append(f"longitude minutes ({lon_min}) exceeds valid range (0-59)")
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lat_sign = 1 if quadrant in (0, 1) else -1 # NE, NW -> North
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lon_sign = 1 if quadrant in (0, 2) else -1 # NE, SE -> East
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lat = lat_sign * (lat_deg + lat_min / 60.0)
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lon = lon_sign * (lon_deg + lon_min / 60.0)
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return {
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"lat": round(lat, 6),
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"lon": round(lon, 6),
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"lat_text": f"{lat_deg:02d}°{lat_min:02d}'{'N' if lat_sign > 0 else 'S'}",
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"lon_text": f"{lon_deg:03d}°{lon_min:02d}'{'E' if lon_sign > 0 else 'W'}",
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"position_uncertain": bool(issues),
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"position_issue": "; ".join(issues) if issues else None,
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}
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VALID_EOS = {117, 122, 127}
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num_symbols = len(self.message_bits) // 10
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symbols = [self._bits_to_symbol(self.message_bits[i * 10:i * 10 + 10]) for i in range(num_symbols)]
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invalid_count = sum(1 for s in symbols if s == -1)
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anchor = None
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for i in range(len(symbols) - 2):
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if symbols[i] in FORMAT_SPECIFIERS and symbols[i] == symbols[i + 2]:
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anchor = i
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break
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if anchor is None:
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return (f"no anchor found ({num_symbols} symbols, {invalid_count} invalid, "
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f"first 15 symbols={symbols[:15]})")
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dedup = symbols[anchor::2]
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dedup_b = symbols[anchor + 1::2]
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def value_at(k):
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primary = dedup[k] if k < len(dedup) else -1
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if primary != -1:
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return primary
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fb = k + 2
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return dedup_b[fb] if 0 <= fb < len(dedup_b) else -1
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format_code = value_at(0)
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layout = FIELD_LAYOUTS.get(format_code)
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if layout is None:
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return f"anchor found at {anchor}, but format specifier {format_code} not recognized"
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idx = 2
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for name, count in layout:
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if idx + count > len(dedup):
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have = len(dedup) - idx
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return (f"format={format_code}, stalled waiting for field "
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f"'{name}' (need {count} symbols, have {max(have, 0)})")
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idx += count
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for k in range(idx, len(dedup)):
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if value_at(k) in VALID_EOS:
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return "all fields complete, EOS found - should have decoded (unexpected)"
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return (f"format={format_code}, all fields complete, but no valid EOS "
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f"found in remaining {len(dedup) - idx} symbols")
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def _diagnose_decode_failure(self) -> str:
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"""
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Explain exactly why the current message_bits buffer failed to
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decode, for logging at timeout. Mirrors _try_decode_message's
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logic but reports where it stopped instead of returning None
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silently - normal accumulation still returns None quietly (this
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is only called once, at the moment of timeout, not on every
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partial-accumulation call).
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"""
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FORMAT_SPECIFIERS = {102, 112, 114, 116, 120, 123}
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FIELD_LAYOUTS = {
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120: [("address", 5), ("category", 1), ("self_id", 5),
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("telecommand", 2), ("frequency_1", 3), ("frequency_2", 3)],
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123: [("address", 5), ("category", 1), ("self_id", 5),
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("telecommand", 2), ("frequency_1", 3)],
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116: [("category", 1), ("self_id", 5), ("telecommand", 2),
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("frequency_1", 3)],
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114: [("address", 5), ("category", 1), ("self_id", 5),
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("telecommand", 2), ("frequency_1", 3), ("frequency_2", 3)],
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102: [("address", 5), ("category", 1), ("self_id", 5),
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("telecommand", 2), ("frequency_1", 3), ("frequency_2", 3)],
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112: [("self_id", 5), ("nature", 1), ("coordinates", 5),
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("time", 2), ("telecommand_msg4", 1)],
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}
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VALID_EOS = {117, 122, 127}
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num_symbols = len(self.message_bits) // 10
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symbols = [self._bits_to_symbol(self.message_bits[i * 10:i * 10 + 10]) for i in range(num_symbols)]
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invalid_count = sum(1 for s in symbols if s == -1)
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anchor = None
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for i in range(len(symbols) - 2):
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if symbols[i] in FORMAT_SPECIFIERS and symbols[i] == symbols[i + 2]:
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anchor = i
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break
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if anchor is None:
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return (f"no anchor found ({num_symbols} symbols, {invalid_count} invalid, "
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f"first 15 symbols={symbols[:15]})")
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dedup = symbols[anchor::2]
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dedup_b = symbols[anchor + 1::2]
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def value_at(k):
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primary = dedup[k] if k < len(dedup) else -1
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if primary != -1:
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return primary
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fb = k + 2
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return dedup_b[fb] if 0 <= fb < len(dedup_b) else -1
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format_code = value_at(0)
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layout = FIELD_LAYOUTS.get(format_code)
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if layout is None:
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return f"anchor found at {anchor}, but format specifier {format_code} not recognized"
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idx = 2
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for name, count in layout:
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if idx + count > len(dedup):
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have = len(dedup) - idx
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return (f"format={format_code}, stalled waiting for field "
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f"'{name}' (need {count} symbols, have {max(have, 0)})")
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idx += count
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for k in range(idx, len(dedup)):
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if value_at(k) in VALID_EOS:
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return "all fields complete, EOS found - should have decoded (unexpected)"
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return (f"format={format_code}, all fields complete, but no valid EOS "
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f"found in remaining {len(dedup) - idx} symbols")
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def _bits_to_symbol(self, bits: list[int]) -> int:
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"""
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