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Clock/Time Synchronisation

ROS / ros eco / packages / mavros

The flight controller and companion computer clocks can be synchronised using MAVLink’s SYSTEM_TIME and TIMESYNC messages. This is important so that each side can blend data from the other side into its EKF.

SYSTEM_TIME (2)

The system time is the time of the master clock. The message send both way and the master clock set by BRD_RTC_TYPES in ardupilot

Field Name Type Description
time_unix_usec uint64_t Unix time in microseconds (UTC). 0 if unknown.
time_boot_ms uint32_t Time since system boot in milliseconds. Starts at 0 on every system reset.

Autopilots (e.g., PX4, ArduPilot) may set time_unix_usec based on GPS time once available .

The message publish by the fcu if no gps the time_unix_usec are zero (1/1/1970)

TM: Wrong FCU time.

if pixhawk has no time source like gps, and his unix data < Sat Feb 14 02:31:30 MSK 2009 mavros not publish SYSTEM_TIME to topic /mavros/time_reference and we got [WARN] [] [mavros.time]: TM: Wrong FCU time.

BRD_RTC_TYPES parameter

In Ardupilot we can control the time source using BRD_RTC_TYPES parameter

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  • GPS (0): Time init from GPS
  • MAVLINK_SYSTEM_TIME(1): for example mavros send system time implement from sys_time plugin the rate control by system_time_rate 0 disabled sending
  • HW(2): Using external hardware like DS3231 or RV3028 there is no driver implementation, i read about LUA driver implementation

TIMESYNC (111)

The MAVLink TIMESYNC message is used to synchronize time between two systems — typically between a companion computer and an autopilot (FCU) like Pixhawk.

Field Description
tc1 Remote system’s time in microseconds
ts1 Local system’s time in microseconds

Sync sequence

sequenceDiagram
    participant Companion
    participant FCU

    Note right of Companion: Companion initiates timesync
    Companion->>FCU: TIMESYNC (tc1 = 0, ts1 = T_local_send)

    Note right of FCU: FCU receives and replies with its own time
    FCU-->>Companion: TIMESYNC (tc1 = T_fcu_now, ts1 = T_local_send)

    Note right of Companion: Companion receives response
    Note right of Companion: Computes RTT = T_local_recv - ts1
    Note right of Companion: Computes offset = tc1 + RTT/2 - T_local_recv

    Note over Companion, FCU: Repeat periodically to refine offset
demo code 
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from pymavlink import mavutil
import time

# Connect to MAVLink
# master = mavutil.mavlink_connection('udp:localhost:14550')
master = mavutil.mavlink_connection('/dev/ttyACM0', baud=115200)

def current_time_us():
    """Return current time in microseconds"""
    return int(time.time() * 1_000_000)

print("Waiting for heartbeat...")
master.wait_heartbeat()
print("Heartbeat received")

# Send initial TIMESYNC message to start sync loop (optional)
master.mav.timesync_send(0, current_time_us())

while True:
    msg = master.recv_match(type='TIMESYNC', blocking=True, timeout=5)
    if msg is None:
        print("No TIMESYNC message received")
        continue

    print(f"Received TIMESYNC: tc1={msg.tc1}, ts1={msg.ts1}")

    if msg.tc1 == 0:
        # Autopilot is requesting time sync — reply with our time
        master.mav.timesync_send(0, current_time_us())
        print("Responded with timesync")
    else:
        # Autopilot responded to our request, calculate offset
        now = current_time_us()
        rtt = now - msg.ts1
        offset = msg.tc1 + rtt // 2 - now
        print(f"RTT: {rtt} µs, Time offset estimate: {offset} µs")

MAVROS

Parameters

name default description
time_ref_source fcu just a string for source filed in sensor_msgs::msg::TimeReference message (SYSTEM_TIME)
timesync_mode mavlink - none, passthrough, mavlink, onboard
system_time_rate 0.0 (disabled) send SYSTEM_TIME rate from CC to FCU
timesync_rate 0.0 (disabled) send timesync rate , sync mode control by timesync_mode

timesync_mode

  • mavlink: Send time sync request tc1=0, tc2=CC time
  • onboard: Run time sync on board (TODO: explain more)

Reference