fix: GPS缺失下的数据缺失问题

backend/monitoring/telemetry.py

@@ -105,13 +105,30 @@ class GpsDataService:
         }
 
     def _build_payload_from_metadata(self, metadata: FrameTrailerMetadata) -> dict[str, Any]:
-        timestamp_seconds = metadata.timestamp_ns / 1_000_000_000
         updated_at = _utc_from_epoch(metadata.received_at) or ""
+        if not metadata.has_gps_fix:
+            return {
+                "has_fix": False,
+                "utc_time": "--:--:--",
+                "latitude": None,
+                "longitude": None,
+                "raw_latitude_hex": f"0x{metadata.raw_latitude_hex}",
+                "raw_longitude_hex": f"0x{metadata.raw_longitude_hex}",
+                "satellites": None,
+                "altitude_m": None,
+                "coordinate_system": "WGS84",
+                "source_sentence": "VIDEO_TRAILER",
+                "raw_coordinate_format": VIDEO_TRAILER_COORDINATE_FORMAT,
+                "source_mode": "video-frame-trailer-no-fix",
+                "updated_at": updated_at,
+            }
+
+        timestamp_seconds = metadata.timestamp_ns / 1_000_000_000
         return {
             "has_fix": True,
             "utc_time": datetime.fromtimestamp(timestamp_seconds, timezone.utc).strftime("%H:%M:%S"),
-            "latitude": round(metadata.latitude, 6),
-            "longitude": round(metadata.longitude, 6),
+            "latitude": round(metadata.latitude, 6) if metadata.latitude is not None else None,
+            "longitude": round(metadata.longitude, 6) if metadata.longitude is not None else None,
             "raw_latitude_hex": f"0x{metadata.raw_latitude_hex}",
             "raw_longitude_hex": f"0x{metadata.raw_longitude_hex}",
             "satellites": None,