Implementation Location

MAVLink transmission and control in ArduPilot are primarily implemented in libraries/GCS_MAVLink and vehicle-specific directories (e.g., Ardusub/). Library modules contain core functionality, while vehicle directories implement specialized subclasses inheriting from base classes.

Transmission Control

Interface Parameter Definition

The GCS_MAVLink class manages MAVLink transmission, while the GCS class emulates ground station functionality internally. In GCS.h:

• _chan is a pointer array to GCS_MAVLink objects
• Maximum channels: 5 (hardware) or 7 (SITL)

#if CONFIG_HAL_BOARD == HAL_BOARD_SITL
#define MAVLINK_CHANNEL_COUNT 7
#else
#define MAVLINK_CHANNEL_COUNT 5
#endif

MAVLink Initialization Sequence

1. AP_Vehicle::setup() invokes gcs().init()
2. System ID configuraton:

void GCS::initialize() {
    mavlink_system.sysid = fetch_current_system_id();
}

3. Default system ID (7,1) overridden by vehicle parameter SYSID_THISMAV
4. Serial port initialization via serial_manager.init()
5. Primary channel setup:

void GCS::configure_primary_port() {
    AP_HAL::UARTDriver* serial_device = 
        AP::serial_manager().find_serial(
            AP_SerialManager::MAVLinkProtocol, 0);
    if(serial_device) {
        initialize_mavlink_backend(port_params[0], *serial_device);
    }
}

6. Secondary channels initialized via setup_auxiliary_ports()
7. Delay callback registration for packet processing:

hal.scheduler->register_delay_handler(packet_processor, 5);

Message Handling

Reception Process

1. Periodic execution (400Hz) via scheduler tasks:

SCHED_TASK(update_receive, 400, 180),
SCHED_TASK(update_transmit, 400, 550)

2. Channel processing:

void GCS::process_incoming() {
    for(uint8_t i=0; i<channel_count(); i++) {
        channel(i)->parse_incoming();
    }
}

3. Packet decoding and routing:

void MAVLinkHandler::decode_packet(const mavlink_status_t& status, 
                                  const mavlink_message_t& msg) {
    if(valid_packet(status, msg)) {
        route_message(msg);
        process_message(msg);
    }
}

4. Vehicle-specfiic message handling:

void CopterGCS::process_message(const mavlink_message_t& msg) {
    switch(msg.msgid) {
        case MAVLINK_MSG_ID_MANUAL_CONTROL:
            decode_manual_input(msg);
            break;
        // Additional cases
    }
}

Transmission Mechanism

1. Message scheduling:

void GCS::update_transmit() {
    for(uint8_t i=0; i<channel_count(); i++) {
        channel(i)->queue_messages();
    }
}

2. Payload preparation:

bool MAVLinkHandler::prepare_message(ap_message_id msg_type) {
    switch(msg_type) {
        case ATTITUDE_DATA:
            return encode_attitude();
        // Additional cases
    }
}

3. Attitude message encoding:

void MAVLinkHandler::encode_attitude() {
    const AHRS& orientation = AP::ahrs();
    mavlink_msg_attitude_send(
        channel_index,
        AP_HAL::millis(),
        orientation.roll,
        orientation.pitch,
        orientation.yaw,
        gyro.x, gyro.y, gyro.z
    );
}

Custom Message Support

1. XML definition per MAVLink specification
2. Message registry updates:

#include <custom_messages/mavlink_msg_custom.h>

3. Transmission implementation:

void transmit_custom_data(float param1, int16_t param2) {
    mavlink_msg_custom_send(
        channel_index,
        param1, param2
    );
}