AP_GPS: implement missing "RTK number of satellites" and "RTK correction age" information in the GPS2_RAW MAVLink message

libraries/AP_GPS/AP_GPS.cpp

@@ -900,8 +900,8 @@ void AP_GPS::send_mavlink_gps2_raw(mavlink_channel_t chan)
         ground_speed(1)*100,  // cm/s
         ground_course(1)*100, // 1/100 degrees,
         num_sats(1),
-        0,
-        0);
+        rtk_num_sats(1),
+        rtk_age_ms(1));
 }
 
 void AP_GPS::send_mavlink_gps_rtk(mavlink_channel_t chan)

libraries/AP_GPS/AP_GPS.h

@@ -142,6 +142,10 @@ public:
         bool have_horizontal_accuracy:1;    ///< does GPS give horizontal position accuracy? Set to true only once available.
         bool have_vertical_accuracy:1;      ///< does GPS give vertical position accuracy? Set to true only once available.
         uint32_t last_gps_time_ms;          ///< the system time we got the last GPS timestamp, milliseconds
+
+        // all the following fields must only all be filled by RTK capable backend drivers
+        uint32_t rtk_age_ms;               ///< GPS age of last baseline correction in milliseconds  (0 when no corrections, 0xFFFFFFFF indicates overflow)
+        uint8_t  rtk_num_sats;             ///< Current number of satellites used for RTK calculation
     };
 
     /// Startup initialisation.
@@ -305,6 +309,22 @@ public:
         return have_vertical_velocity(primary_instance);
     }
 
+    // return number of satellites used for RTK calculation
+    uint8_t rtk_num_sats(uint8_t instance) const {
+        return state[instance].rtk_num_sats;
+    }
+    uint8_t rtk_num_sats(void) const {
+        return rtk_num_sats(primary_instance);
+    }
+
+    // return age of last baseline correction in milliseconds
+    uint32_t rtk_age_ms(uint8_t instance) const {
+        return state[instance].rtk_age_ms;
+    }
+    uint32_t rtk_age_ms(void) const {
+        return rtk_age_ms(primary_instance);
+    }
+
     // the expected lag (in seconds) in the position and velocity readings from the gps
     // return true if the GPS hardware configuration is known or the lag parameter has been set manually
     bool get_lag(uint8_t instance, float &lag_sec) const;