Read custom MAVLink topic


Is it possible to look up if something is published to a certain MAVLink topic? I am publishing a message via mavros to a custom MAVLink topic and try to read it with PX4. I am not sure if mavros does not publish the topic or PX4 does not read it.

Best Nils

Did you write the code to handle it in PX4? I would start on that side and add some basic debugging.

Hi @dagar,

I just followed this guide:

I created two messages in uORB, one to send data to my custom MAVLink topic and one to receive data from it. When I try to compilate it it works just fine. But I receive nothing from the MAVLink topic. Do I have to announce it somewhere that I use a custom MAVLink topic too?


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Are you talking about within PX4, ROS, or QGC? Where are you filling the mavlink message and sending?

I am also having some trouble with a custom MAVLink message.
I’m trying to send a second message using TELEM2 on a PixRacer (so px4fmu-v4). As practice I just decided to use the ca_trajectory help there.
I can compile and upload just fine using
make px4fmu_default upload

What I’ve been trying to figure out for days is that I added the following to my rcS file:
if param compare SYS_COMPANION 1499
mavlink start -d ${MAVLINK_COMPANION_DEVICE} -b 57600 -s CA_TRAJECTORY

What do all of these options mean? I’m guessing -d is device, -b is baudrate, what is -s? I can’t seem to find where the code that parses that command is.

I realized after writing this that the SYS_COMPANION code was under an
if ver hwcmp PX4FMU_V1
statement. Probably a part of why my Arduino is seeing nothing when connected to the TELEM2 port. Nonetheless I’m curious what all of the options are for starting mavlink. Is there a problem with having two “mavlink start” in the rcS code?


I tried it within PX4. I added two different topics (gazebo_pos.msg and gazebo_pos_test.msg). In an example app I publish data to the gazebo_pos_test topic

send_pos.x = 1.0;
send_pos.y = 2.0;
send_pos.z = 3.0;
orb_publish(ORB_ID(gazebo_pos_test), gazebo_pos_pub, &send_pos);

In MAVlink messages I added therefore the following class in mavlink_messages.cpp

class MavlinkStreamGazeboPos : public MavlinkStream
  const char *get_name() const
	return MavlinkStreamGazeboPos::get_name_static();

  static const char *get_name_static()
	return "GAZEBO_POS";

  static uint16_t get_id_static()

  uint16_t get_id()
	return get_id_static();

  static MavlinkStream *new_instance(Mavlink *mavlink)
	return new MavlinkStreamGazeboPos(mavlink);

  unsigned get_size()

MavlinkOrbSubscription *_gazebo_sub;
uint64_t _gazebo_time;

// do not allow top copying this class
MavlinkStreamGazeboPos(MavlinkStreamGazeboPos &);
MavlinkStreamGazeboPos &operator = (const MavlinkStreamGazeboPos &);

explicit MavlinkStreamGazeboPos(Mavlink *mavlink) : MavlinkStream(mavlink),

void send(const hrt_abstime t)
	struct gazebo_pos_test_s gazebo;

	if (_gazebo_sub->update(&_gazebo_time, &gazebo)) {
		mavlink_gazebo_pos_t msg = {};

		msg.time_usec = gazebo.timestamp;
		msg.qw = gazebo.qw;
		msg.qx = gazebo.qx;
		msg.qy = gazebo.qy;
		msg.qz = gazebo.qz;
		msg.x = gazebo.x;
		msg.y = gazebo.y;
		msg.z = gazebo.z;

		mavlink_msg_gazebo_pos_send_struct(_mavlink->get_channel(), &msg);

In addition I added in mavlink_receiver.cpp the follwoing function in order to receive the data from the MAVLink topic and put it into the gazebo_pos uORB topic

MavlinkReceiver::handle_message_gazebo_pos(mavlink_message_t *msg)
mavlink_gazebo_pos_t gazebo;
mavlink_msg_gazebo_pos_decode(msg, &gazebo);

struct gazebo_pos_s gazebo_pos = {};

//att_pos_mocap.timestamp = sync_stamp(mocap.time_usec);
//att_pos_mocap.timestamp_received = hrt_absolute_time();

gazebo_pos.qw = gazebo.qw;
gazebo_pos.qx = gazebo.qx;
gazebo_pos.qy = gazebo.qy;
gazebo_pos.qz = gazebo.qz;

gazebo_pos.x = gazebo.x;
gazebo_pos.y = gazebo.y;
gazebo_pos.z = gazebo.z;

if (_gazebo_pos_pub == nullptr) {
	_gazebo_pos_pub = orb_advertise(ORB_ID(gazebo_pos), &gazebo_pos);

} else {
	orb_publish(ORB_ID(gazebo_pos), _gazebo_pos_pub, &gazebo_pos);

In my example app I subscribe to this uORB topic (gazebo_pos). But when I print the receiving data I dont get the right values for for x,y,z (1.0,2.0,3.0). Instead I get for y = 0 and fpr x and z really high numbers such as 5468798798.
Do you know what I am doing wrong?


Hi all & Nils,

I’m trying to do the same. I am able to read all messages coming from the px4 -> mavlink -> mavros -> ros. But i am not able to send my custom message to px4. Almost the same problem like you, but with the only difference, that I don’t need a separate uORB msg. I tried to send some set_actuator_control messages to px4 ( from ros->mavros->mavlink->px4) and they worked fine for me, after offboard configuration was done. The code for my message is almost the same.

Are you now able to read mavlink messages?


HI danusanth,

Have you sovlved your problem? We are able to read messages comming from ros -> mavros ->mavlink ->px4, but not for the inverse way. Could you please list all steps that you have done for reading messages coming from the px4 -> mavlink -> mavros -> ros.

Thanks in advanced!

Lam Hung