fix(gps): smooth Doppler measurements before velocity WLS

soc/software/gps_system/src/gps/gps_interrupt_handlers.c

@@ -91,6 +91,7 @@ static gps_external_irq_handler_t gps_external_irq_handler = NULL;
 	(((uint64_t)GPS_PPS_CLOCK_HZ * GPS_NAV_ABS_SAMPLE_SYNC_WINDOW_MS) / 1000u)
 #define GPS_NAV_HOLDOFF_LOCAL_TICKS \
 	(((uint64_t)BSP_CLINT_TIMEBASE_HZ * GPS_NAV_EPOCH_HOLDOFF_MS) / 1000u)
+#define GPS_TRACK_DOPPLER_SMOOTH_ALPHA 0.05
 
 _Static_assert(GPS_NAV_ABS_SAMPLE_SYNC_WINDOW_MS <
                (GPS_NAV_SOLVE_PERIOD_MS / 2u),
@@ -147,6 +148,20 @@ static void gps_update_tracking_measurement_isr(
 		measurement.cn0_dbhz =
 			0.2 * measurement.cn0_dbhz +
 			0.8 * previous->cn0_dbhz;
+		/*
+		 * The carrier loop's 20 ms snapshots contain several hertz of
+		 * zero-mean jitter.  At L1 that is metres/second of range-rate noise,
+		 * which poor satellite geometry amplifies heavily in velocity WLS.
+		 * A 0.05 pole at the 50 Hz navigation-bit cadence has an approximately
+		 * 0.4 s time constant while retaining real receiver dynamics.
+		 */
+		measurement.doppler_hz =
+			GPS_TRACK_DOPPLER_SMOOTH_ALPHA * measurement.doppler_hz +
+			(1.0 - GPS_TRACK_DOPPLER_SMOOTH_ALPHA) *
+			previous->doppler_hz;
+		measurement.pseudorange_rate_mps =
+			gps_tracking_doppler_to_range_rate_mps(
+				measurement.doppler_hz);
 	}
 	measurement.local_timestamp_ticks = local_timestamp_ticks;
 	gps_latest_tracking_measurement[tracking_index] = measurement;