fix: Preserve GPS satellites across DOP-only SKY messages

gpsd sends multiple SKY messages per cycle — some contain only DOP
values with an empty satellites array. Previously this would overwrite
the satellite list, causing the sky view to flicker empty. Now DOP-only
SKY messages preserve the existing satellite list. Also adds a 5-second
polling fallback for satellite data since SSE can miss sky updates due
to queue contention.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

static/js/modes/gps.js

@@ -8,6 +8,7 @@ const GPS = (function() {
     let connected = false;
     let lastPosition = null;
     let lastSky = null;
+    let skyPollTimer = null;
 
     // Constellation color map
     const CONST_COLORS = {
@@ -41,6 +42,7 @@ const GPS = (function() {
                         updateSkyUI(data.sky);
                     }
                     subscribeToStream();
+                    startSkyPolling();
                     // Ensure the global GPS stream is running
                     if (typeof startGpsStream === 'function' && !gpsEventSource) {
                         startGpsStream();
@@ -58,6 +60,7 @@ const GPS = (function() {
 
     function disconnect() {
         unsubscribeFromStream();
+        stopSkyPolling();
         fetch('/gps/stop', { method: 'POST' })
             .then(() => {
                 connected = false;
@@ -77,6 +80,34 @@ const GPS = (function() {
         }
     }
 
+    function startSkyPolling() {
+        stopSkyPolling();
+        // Poll satellite data every 5 seconds as a reliable fallback
+        // SSE stream may miss sky updates due to queue contention with position messages
+        pollSatellites();
+        skyPollTimer = setInterval(pollSatellites, 5000);
+    }
+
+    function stopSkyPolling() {
+        if (skyPollTimer) {
+            clearInterval(skyPollTimer);
+            skyPollTimer = null;
+        }
+    }
+
+    function pollSatellites() {
+        if (!connected) return;
+        fetch('/gps/satellites')
+            .then(r => r.json())
+            .then(data => {
+                if (data.status === 'ok' && data.sky) {
+                    lastSky = data.sky;
+                    updateSkyUI(data.sky);
+                }
+            })
+            .catch(() => {});
+    }
+
     function subscribeToStream() {
         // Subscribe to the global GPS stream instead of opening a separate SSE connection
         if (typeof addGpsStreamSubscriber === 'function') {
@@ -395,6 +426,7 @@ const GPS = (function() {
 
     function destroy() {
         unsubscribeFromStream();
+        stopSkyPolling();
     }
 
     return {

utils/gps.py

@@ -318,6 +318,8 @@ class GPSDClient:
 
                     except json.JSONDecodeError:
                         logger.debug(f"Invalid JSON from gpsd: {line[:50]}")
+                    except Exception as parse_err:
+                        logger.error(f"Error handling gpsd {msg_class} message: {parse_err}")
 
             except socket.timeout:
                 continue
@@ -371,19 +373,33 @@ class GPSDClient:
         self._update_position(position)
 
     def _handle_sky(self, msg: dict) -> None:
-        """Handle SKY (satellite sky view) message from gpsd."""
-        sats = []
-        for sat in msg.get('satellites', []):
-            prn = sat.get('PRN', 0)
-            gnssid = sat.get('gnssid')
-            sats.append(GPSSatellite(
-                prn=prn,
-                elevation=sat.get('el'),
-                azimuth=sat.get('az'),
-                snr=sat.get('ss'),
-                used=sat.get('used', False),
-                constellation=_classify_constellation(prn, gnssid),
-            ))
+        """Handle SKY (satellite sky view) message from gpsd.
+
+        gpsd sends multiple SKY messages per cycle: some contain only DOP
+        values while others include the full satellites array.  When a
+        DOP-only SKY arrives, preserve the most recent satellite list
+        instead of overwriting it with an empty one.
+        """
+        raw_sats = msg.get('satellites', [])
+        has_satellites = len(raw_sats) > 0
+
+        if has_satellites:
+            sats = []
+            for sat in raw_sats:
+                prn = sat.get('PRN', 0)
+                gnssid = sat.get('gnssid')
+                sats.append(GPSSatellite(
+                    prn=prn,
+                    elevation=sat.get('el'),
+                    azimuth=sat.get('az'),
+                    snr=sat.get('ss'),
+                    used=sat.get('used', False),
+                    constellation=_classify_constellation(prn, gnssid),
+                ))
+        else:
+            # DOP-only SKY message — keep existing satellites
+            with self._lock:
+                sats = list(self._sky.satellites) if self._sky else []
 
         sky_data = GPSSkyData(
             satellites=sats,