MAVLink: Queue transponder reports and send them at full data rate

This will ensure that no transponder reports are dropped and that all received reports are passed on to the GCS and other devices.

msg/transponder_report.msg

@@ -11,3 +11,5 @@ uint8 emitter_type 	# Type from ADSB_EMITTER_TYPE enum
 uint8 tslc 		# Time since last communication in seconds
 uint16 flags 		# Flags to indicate various statuses including valid data fields
 uint16 squawk 		# Squawk code
+
+uint32 ORB_QUEUE_LENGTH = 3

src/modules/mavlink/mavlink_messages.cpp

@@ -1178,6 +1178,11 @@ public:
 		return new MavlinkStreamADSBVehicle(mavlink);
 	}
 
+	virtual bool const_rate()
+	{
+		return true;
+	}
+
 	unsigned get_size()
 	{
 		return (_pos_time > 0) ? MAVLINK_MSG_ID_ADSB_VEHICLE_LEN + MAVLINK_NUM_NON_PAYLOAD_BYTES : 0;
@@ -1201,7 +1206,7 @@ protected:
 	{
 		struct transponder_report_s pos;
 
-		if (_pos_sub->update(&_pos_time, &pos)) {
+		while (_pos_sub->update(&_pos_time, &pos)) {
 			mavlink_adsb_vehicle_t msg = {};
 
 			msg.ICAO_address = pos.ICAO_address;

src/modules/mavlink/mavlink_receiver.cpp

@@ -2087,7 +2087,7 @@ void MavlinkReceiver::handle_message_adsb_vehicle(mavlink_message_t *msg)
 	//warnx("code: %d callsign: %s, vel: %8.4f", (int)t.ICAO_address, t.callsign, (double)t.hor_velocity);
 
 	if (_transponder_report_pub == nullptr) {
-		_transponder_report_pub = orb_advertise(ORB_ID(transponder_report), &t);
+		_transponder_report_pub = orb_advertise_queue(ORB_ID(transponder_report), &t, transponder_report_s::ORB_QUEUE_LENGTH);
 
 	} else {
 		orb_publish(ORB_ID(transponder_report), _transponder_report_pub, &t);