Time Synchronization

MAVSDK can estimate the offset between its own clock and the clock of the autopilot using the TIMESYNC protocol. The application can use the offset to convert between autopilot timestamps and local time, and MAVSDK uses it for the timestamps of the messages that the Mocap plugin sends.

Usage

Time synchronization is disabled by default and is enabled per system:

system->enable_timesync();

MAVSDK then sends a TIMESYNC request every 5 seconds. Only answers that arrive within 10 ms are used, so on slow links the offset may never become valid.

The state can be polled or subscribed to:

const auto status = system->timesync_status();
if (status.synced) {
    std::cout << "Autopilot clock offset: " << status.offset_ns << " ns, RTT: " << status.rtt_ns << " ns\n";
}

auto handle = system->subscribe_timesync_status([](System::TimesyncStatus status) {
    // Called for every TIMESYNC answer and when the system disconnects.
});

// ...

system->unsubscribe_timesync_status(handle);

The offset is defined as autopilot time minus local time, where local time is std::chrono::system_clock:

// Autopilot time (e.g. a time_boot_ms or time_usec field) to local time
const auto local = std::chrono::system_clock::time_point{
    std::chrono::nanoseconds(autopilot_time_ns - status.offset_ns)};

MAVSDK does not change the timestamps of incoming messages: they stay in the autopilot's time.

Timestamped values

These values carry the autopilot's timestamp, in microseconds, and can be converted to local time with the offset:

Value	Field	From MAVLink
Telemetry::Quaternion, Telemetry::EulerAngle	timestamp_us	ATTITUDE_QUATERNION, ATTITUDE
Telemetry::Imu	timestamp_us	HIGHRES_IMU, SCALED_IMU, RAW_IMU
Telemetry::ScaledPressure	timestamp_us	SCALED_PRESSURE
Telemetry::Altitude	timestamp_us	ALTITUDE
Telemetry::RawGps	timestamp_us	GPS_RAW_INT
Telemetry::Odometry	time_usec	ODOMETRY
Telemetry::GroundTruth	timestamp_us	HIL_STATE_QUATERNION
Telemetry::HomePosition	timestamp_us	HOME_POSITION
Gimbal::Attitude	timestamp_us	GIMBAL_DEVICE_ATTITUDE_STATUS
Winch::Status	time_usec	WINCH_STATUS

Other values, such as position, velocity or battery, have no timestamp. The time of reception is the best estimate for those.

MAVLink allows some of these fields to contain either time since boot or UNIX epoch time (see the MAVLink message definitions). Only time since boot needs to be converted; values larger than about 10¹⁵ µs are already UNIX epoch time.

The timesync example shows the status and converts an autopilot timestamp to local time.

INFO

The TIMESYNC time of PX4 is its time since boot, so the offset is roughly minus the time at which the autopilot booted.

Setting the system clock from the vehicle

TIMESYNC gives the offset to the autopilot's clock, not the current date and time, so it can't be used to set the clock of a companion computer. For that, use the wall clock time that the autopilot gets from GPS, which it sends in SYSTEM_TIME, available as Telemetry::subscribe_unix_epoch_time(). It can for example be fed to chrony as a reference clock, by an application that writes it to chrony's SOCK refclock. Only use it once the autopilot's clock is valid, e.g. after it got a GPS fix.

Language bindings

The same functions are available in the native bindings:

• C: mavsdk_system_enable_timesync(), mavsdk_system_timesync_status(), mavsdk_system_subscribe_timesync_status(), mavsdk_system_unsubscribe_timesync_status()
• Python: System.enable_timesync(), timesync_status(), subscribe_timesync_status(), unsubscribe_timesync_status(), and timesync_status_updates() in mavsdk.asyncio
• Kotlin: System.enableTimesync(), getTimesyncStatus(), subscribeTimesyncStatus()

Time synchronization is not available through mavsdk_server.