AP_GPS: added backend specific get_lag() function

the ublox7 and 8 seem to produce lower lag, around 120ms. Separately
we should also look at running these at 10Hz and see if that helps.

libraries/AP_GPS/AP_GPS.cpp

@@ -861,3 +861,15 @@ void AP_GPS::inject_data_all(const uint8_t *data, uint16_t len)
     }
     
 }
+
+/*
+  return expected lag from a GPS
+ */
+float AP_GPS::get_lag(uint8_t instance) const
+{
+    if (drivers[instance] == nullptr || state[instance].status == NO_GPS) {
+        // return default;
+        return 0.2f;
+    }
+    return drivers[instance]->get_lag();
+}

libraries/AP_GPS/AP_GPS.h

@@ -314,7 +314,8 @@ public:
     }
 
     // the expected lag (in seconds) in the position and velocity readings from the gps
-    float get_lag() const { return 0.2f; }
+    float get_lag(uint8_t instance) const;
+    float get_lag(void) const { return get_lag(primary_instance); }
 
     // return a 3D vector defining the offset of the GPS antenna in meters relative to the body frame origin
     const Vector3f &get_antenna_offset(uint8_t instance) const {

libraries/AP_GPS/AP_GPS_UBLOX.cpp

@@ -1284,3 +1284,21 @@ AP_GPS_UBLOX::broadcast_configuration_failure_reason(void) const {
         }
     }
 }
+
+/*
+  return velocity lag in seconds
+ */
+float AP_GPS_UBLOX::get_lag(void) const
+{
+    switch (_hardware_generation) {
+    case UBLOX_5:
+    case UBLOX_6:
+    default:
+        return 0.22f;
+    case UBLOX_7:
+    case UBLOX_M8:
+        // based on flight logs the 7 and 8 series seem to produce about 120ms lag
+        return 0.12f;
+        break;
+    };
+}

libraries/AP_GPS/AP_GPS_UBLOX.h

@@ -111,6 +111,10 @@ public:
     }
 
     void broadcast_configuration_failure_reason(void) const override;
+
+    // return velocity lag
+    float get_lag(void) const override;
+
 private:
     // u-blox UBX protocol essentials
     struct PACKED ubx_header {

libraries/AP_GPS/GPS_Backend.h

@@ -52,6 +52,9 @@ public:
 
     virtual void handle_msg(const mavlink_message_t *msg) { return ; }
 
+    // driver specific lag
+    virtual float get_lag(void) const { return 0.2f; }
+
 protected:
     AP_HAL::UARTDriver *port;           ///< UART we are attached to
     AP_GPS &gps;                        ///< access to frontend (for parameters)