ekf2: Add support for use of multiple GPS receivers (#9765)

msg/CMakeLists.txt

@@ -52,6 +52,7 @@ set(msg_files
 	distance_sensor.msg
 	ekf2_innovations.msg
 	ekf2_timestamps.msg
+	ekf_gps_position.msg
 	esc_report.msg
 	esc_status.msg
 	estimator_status.msg
@@ -127,9 +128,9 @@ set(msg_files
 	vehicle_magnetometer.msg
 	vehicle_rates_setpoint.msg
 	vehicle_roi.msg
-	vehicle_trajectory_waypoint.msg
 	vehicle_status.msg
 	vehicle_status_flags.msg
+	vehicle_trajectory_waypoint.msg
 	vtol_vehicle_status.msg
 	wind_estimate.msg
 	vehicle_constraints.msg

msg/ekf_gps_position.msg

@@ -0,0 +1,16 @@
+# EKF blended position in WGS84 coordinates.
+int32 lat			# Latitude in 1E-7 degrees
+int32 lon			# Longitude in 1E-7 degrees
+int32 alt			# Altitude in 1E-3 meters above MSL, (millimetres)
+int32 alt_ellipsoid 		# Altitude in 1E-3 meters bove Ellipsoid, (millimetres)
+float32 s_variance_m_s		# GPS speed accuracy estimate, (metres/sec)
+uint8 fix_type # 0-1: no fix, 2: 2D fix, 3: 3D fix, 4: RTCM code differential, 5: Real-Time Kinematic, float, 6: Real-Time Kinematic, fixed, 8: Extrapolated. Some applications will not use the value of this field unless it is at least two, so always correctly fill in the fix.
+float32 eph			# GPS horizontal position accuracy (metres)
+float32 epv			# GPS vertical position accuracy (metres)
+float32 vel_m_s			# GPS ground speed, (metres/sec)
+float32 vel_n_m_s		# GPS North velocity, (metres/sec)
+float32 vel_e_m_s		# GPS East velocity, (metres/sec)
+float32 vel_d_m_s		# GPS Down velocity, (metres/sec)
+bool vel_ned_valid		# True if NED velocity is valid
+uint8 satellites_used		# Number of satellites used
+uint8 selected			# GPS selection: 0: GPS1, 1: GPS2. 2: GPS1+GPS2 blend