AP_NavEKF2: Utilize the GPS drivers estimate for lag

libraries/AP_NavEKF2/AP_NavEKF2.cpp

@@ -188,15 +188,7 @@ const AP_Param::GroupInfo NavEKF2::var_info[] = {
     // @Units: m
     AP_GROUPINFO("GLITCH_RAD", 7, NavEKF2, _gpsGlitchRadiusMax, GLITCH_RADIUS_DEFAULT),
 
-    // @Param: GPS_DELAY
-    // @DisplayName: GPS measurement delay (msec)
-    // @Description: This is the number of msec that the GPS measurements lag behind the inertial measurements.
-    // @Range: 0 250
-    // @Increment: 10
-    // @User: Advanced
-    // @Units: ms
-    // @RebootRequired: True
-    AP_GROUPINFO("GPS_DELAY", 8, NavEKF2, _gpsDelay_ms, 220),
+    // 8 was used for GPS_DELAY
 
     // Height measurement parameters
 

libraries/AP_NavEKF2/AP_NavEKF2.h

@@ -362,7 +362,6 @@ private:
     AP_Float _accNoise;             // accelerometer process noise : m/s^2
     AP_Float _gyroBiasProcessNoise; // gyro bias state process noise : rad/s
     AP_Float _accelBiasProcessNoise;// accel bias state process noise : m/s^2
-    AP_Int16 _gpsDelay_ms;          // effective average delay of GPS measurements relative to inertial measurement (msec)
     AP_Int16 _hgtDelay_ms;          // effective average delay of Height measurements relative to inertial measurements (msec)
     AP_Int8  _fusionModeGPS;        // 0 = use 3D velocity, 1 = use 2D velocity, 2 = use no velocity
     AP_Int16  _gpsVelInnovGate;     // Percentage number of standard deviations applied to GPS velocity innovation consistency check

libraries/AP_NavEKF2/AP_NavEKF2_Measurements.cpp

@@ -423,9 +423,12 @@ void NavEKF2_core::readGpsData()
             // get current fix time
             lastTimeGpsReceived_ms = gps.last_message_time_ms();
 
+
             // estimate when the GPS fix was valid, allowing for GPS processing and other delays
             // ideally we should be using a timing signal from the GPS receiver to set this time
-            gpsDataNew.time_ms = lastTimeGpsReceived_ms - frontend->_gpsDelay_ms;
+            float gps_delay = 0.0;
+            gps.get_lag(gps_delay); // ignore the return value
+            gpsDataNew.time_ms = lastTimeGpsReceived_ms - (uint32_t)(1e3f * gps_delay);
 
             // Correct for the average intersampling delay due to the filter updaterate
             gpsDataNew.time_ms -= localFilterTimeStep_ms/2;