mavlink_timesync.cpp

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/**
 * @file mavlink_timesync.cpp
 * Mavlink timesync implementation.
 *
 * @author Mohammed Kabir <mhkabir98@gmail.com>
 */

#include "mavlink_timesync.h"
#include "mavlink_main.h"

#include <stdlib.h>

MavlinkTimesync::MavlinkTimesync(Mavlink *mavlink) :
    _mavlink(mavlink)
{
}

void
MavlinkTimesync::handle_message(const mavlink_message_t *msg)
{
    switch (msg->msgid) {
    case MAVLINK_MSG_ID_TIMESYNC: {

            mavlink_timesync_t tsync = {};
            mavlink_msg_timesync_decode(msg, &tsync);

            const uint64_t now = hrt_absolute_time();

            if (tsync.tc1 == 0) {           // Message originating from remote system, timestamp and return it

                mavlink_timesync_t rsync;

                rsync.tc1 = now * 1000ULL;
                rsync.ts1 = tsync.ts1;

                mavlink_msg_timesync_send_struct(_mavlink->get_channel(), &rsync);

                return;

            } else if (tsync.tc1 > 0) {     // Message originating from this system, compute time offset from it

                // Calculate time offset between this system and the remote system, assuming RTT for
                // the timesync packet is roughly equal both ways.
                int64_t offset_us = (int64_t)((tsync.ts1 / 1000ULL) + now - (tsync.tc1 / 1000ULL) * 2) / 2 ;

                // Calculate the round trip time (RTT) it took the timesync packet to bounce back to us from remote system
                uint64_t rtt_us = now - (tsync.ts1 / 1000ULL);

                // Calculate the difference of this sample from the current estimate
                uint64_t deviation = llabs((int64_t)_time_offset - offset_us);

                if (rtt_us < MAX_RTT_SAMPLE) {  // Only use samples with low RTT

                    if (sync_converged() && (deviation > MAX_DEVIATION_SAMPLE)) {

                        // Increment the counter if we have a good estimate and are getting samples far from the estimate
                        _high_deviation_count++;

                        // We reset the filter if we received 5 consecutive samples which violate our present estimate.
                        // This is most likely due to a time jump on the offboard system.
                        if (_high_deviation_count > MAX_CONSECUTIVE_HIGH_DEVIATION) {
                            PX4_ERR("[timesync] Time jump detected. Resetting time synchroniser.");
                            // Reset the filter
                            reset_filter();
                        }

                    } else {

                        // Filter gain scheduling
                        if (!sync_converged()) {
                            // Interpolate with a sigmoid function
                            double progress = (double)_sequence / (double)CONVERGENCE_WINDOW;
                            double p = 1.0 - exp(0.5 * (1.0 - 1.0 / (1.0 - progress)));
                            _filter_alpha = p * ALPHA_GAIN_FINAL + (1.0 - p) * ALPHA_GAIN_INITIAL;
                            _filter_beta = p * BETA_GAIN_FINAL + (1.0 - p) * BETA_GAIN_INITIAL;

                        } else {
                            _filter_alpha = ALPHA_GAIN_FINAL;
                            _filter_beta = BETA_GAIN_FINAL;
                        }

                        // Perform filter update
                        add_sample(offset_us);

                        // Increment sequence counter after filter update
                        _sequence++;

                        // Reset high deviation count after filter update
                        _high_deviation_count = 0;

                        // Reset high RTT count after filter update
                        _high_rtt_count = 0;
                    }

                } else {
                    // Increment counter if round trip time is too high for accurate timesync
                    _high_rtt_count++;

                    if (_high_rtt_count > MAX_CONSECUTIVE_HIGH_RTT) {
                        PX4_WARN("[timesync] RTT too high for timesync: %llu ms", rtt_us / 1000ULL);
                        // Reset counter to rate-limit warnings
                        _high_rtt_count = 0;
                    }

                }

                // Publish status message
                timesync_status_s tsync_status{};

                tsync_status.timestamp = hrt_absolute_time();
                tsync_status.remote_timestamp = tsync.tc1 / 1000ULL;
                tsync_status.observed_offset = offset_us;
                tsync_status.estimated_offset = (int64_t)_time_offset;
                tsync_status.round_trip_time = rtt_us;

                _timesync_status_pub.publish(tsync_status);
            }

            break;
        }

    case MAVLINK_MSG_ID_SYSTEM_TIME: {

            mavlink_system_time_t time;
            mavlink_msg_system_time_decode(msg, &time);

            timespec tv = {};
            px4_clock_gettime(CLOCK_REALTIME, &tv);

            // date -d @1234567890: Sat Feb 14 02:31:30 MSK 2009
            bool onb_unix_valid = (unsigned long long)tv.tv_sec > PX4_EPOCH_SECS;
            bool ofb_unix_valid = time.time_unix_usec > PX4_EPOCH_SECS * 1000ULL;

            if (!onb_unix_valid && ofb_unix_valid) {
                tv.tv_sec = time.time_unix_usec / 1000000ULL;
                tv.tv_nsec = (time.time_unix_usec % 1000000ULL) * 1000ULL;

                if (px4_clock_settime(CLOCK_REALTIME, &tv)) {
                    PX4_ERR("[timesync] Failed setting realtime clock");
                }
            }

            break;
        }

    default:
        break;
    }
}

uint64_t
MavlinkTimesync::sync_stamp(uint64_t usec)
{
    // Only return synchronised stamp if we have converged to a good value
    if (sync_converged()) {
        return usec + (int64_t)_time_offset;

    } else {
        return hrt_absolute_time();
    }
}

bool
MavlinkTimesync::sync_converged()
{
    return _sequence >= CONVERGENCE_WINDOW;
}

void
MavlinkTimesync::add_sample(int64_t offset_us)
{
    /* Online exponential smoothing filter. The derivative of the estimate is also
     * estimated in order to produce an estimate without steady state lag:
     * https://en.wikipedia.org/wiki/Exponential_smoothing#Double_exponential_smoothing
     */

    double time_offset_prev = _time_offset;

    if (_sequence == 0) {           // First offset sample
        _time_offset = offset_us;

    } else {
        // Update the clock offset estimate
        _time_offset = _filter_alpha * offset_us + (1.0 - _filter_alpha) * (_time_offset + _time_skew);

        // Update the clock skew estimate
        _time_skew = _filter_beta * (_time_offset - time_offset_prev) + (1.0 - _filter_beta) * _time_skew;
    }

}

void
MavlinkTimesync::reset_filter()
{
    // Do a full reset of all statistics and parameters
    _sequence = 0;
    _time_offset = 0.0;
    _time_skew = 0.0;
    _filter_alpha = ALPHA_GAIN_INITIAL;
    _filter_beta = BETA_GAIN_INITIAL;
    _high_deviation_count = 0;
    _high_rtt_count = 0;

}