px4的mavlink指令

### PX4 MAVLink Commands List and Usage

In the context of integrating custom MAVLink messages with PX4, understanding how to utilize existing MAVLink commands is crucial. The MAVLink protocol supports a wide range of message types that can be used for communication between ground control stations (like QGroundControl) and autonomous systems such as drones running on PX4 autopilot.

#### Commonly Used MAVLink Messages in PX4

Several key MAVLink messages are frequently utilized when working with PX4:

- **HEARTBEAT**: This message provides information about the system's status and type. It helps maintain a connection between the vehicle and the ground station[^1].

- **SET_POSITION_TARGET_LOCAL_NED**: Sends position setpoints relative to the local frame origin. For setting positions or velocities within the NED coordinate system locally around the drone. An important correction was made from `MAVLINK_MSG_SET_POSITION_TARGET_LOCAL_NED_TAKEOFF` to `MAVLINK_MSG_SET_POSITION_TARGET_LOCAL_NED_POSITION`, which ensures proper functionality beyond just unlocking but also taking off correctly[^2].

- **COMMAND_LONG / COMMAND_INT**: These generic command messages allow sending various predefined actions like arming/disarming motors, changing modes, etc., by specifying different command IDs defined under `mav_cmd.h`.

- **MISSION_ITEM_INT**: Used for defining waypoints or mission items including takeoff points, landing sequences, and other specific tasks during flight missions.

Below demonstrates an example snippet showing how one might define a new stream handler class named `MAVLINK_TEST_HPP` inside the specified directory structure mentioned earlier:

// src/modules/mavlink/streams/MAVLINK_TEST.hpp
#ifndef MAVLINK_STREAMS_MAVLINK_TEST_HPP_
#define MAVLINK_STREAMS_MAVLINK_TEST_HPP_

#include <uORB/PublicationMulti.hpp>
#include "mavlink_stream.h"

class MavlinkStreamTest : public MavlinkStream {
public:
    static MavlinkStream *new_instance(Mavlink *mavlink);
    
private:
    uORB::PublicationMulti<mavlink_message_t> _test_pub;
};

#endif /* MAVLINK_STREAMS_MAVLINK_TEST_HPP_ */

This code defines a basic framework where additional functionalities related to handling custom MAVLink streams could be implemented based upon project requirements.

--related questions--

1. How does HEARTBEAT contribute to maintaining stable communications?
2. What modifications should be applied while using SET_POSITION_TARGET_LOCAL_NED for precise navigation?
3. Can you explain more about implementing MISSION_ITEM_INT into actual flight plans?
4. Are there any best practices recommended when creating custom MAVLink streams similar to MAVLINK_TEST_HPP?