Hello,
I have a Jetson Xavier NX as a companion computer for my drone which uses a pixhawk flight controller. I have the latest firmware beta version on the pixhawk as I’m using ROS2 to communicate with the Jetson thanks to the XRCE-DDS bridge. I have a USB camera connected to my Jetson and a ROS topic that publishes detections from this camera’s images at certain timestamps. I need to synchronize these timestamps with the timestamps coming from the pixhawk (for example a topic like /fmu/out/vehicle_local_position).
Thus, I’m looking for a way to synchronize the timestamps. In dds_topics.yaml I noticed the topic /fmu/out/timesync_status but I don’t understand how it works and what I need to do with it even though I looked at https://mavlink.io/en/services/timesync.html
When echoing this timesync_status topic I get such values :
timestamp: 1685627282799767
source_protocol: 2
remote_timestamp: 1685627282796707
observed_offset: -1685624282890066
estimated_offset: -1685624282890795
round_trip_time: 4654
I’m not sure how to interpret those values given that when I publish for example to the topic /fmu/in/vehicle_command, my code sets the timestamps with the following command : uint64_t timestamp = node->get_clock()->now().nanoseconds() / 1000;
and the corresponding published message looks like this :
timestamp: 1685627425075143
param1: 1.0
param2: 4.0
param3: 2.0
param4: 0.0
param5: 0.0
param6: 0.0
param7: 0.0
command: 176
target_system: 1
target_component: 1
source_system: 1
source_component: 1
confirmation: 0
from_external: true
and when I listen to a topic like /fmu/out/vehicle_local_position coming from the pixhawk, the timestamps have a very similar value :
timestamp: 1685626860535678
timestamp_sample: 1685626860535269
xy_valid: false
z_valid: true
v_xy_valid: false
v_z_valid: true
x: 0.0004316429840400815
y: -0.00031042000045999885
z: -2.9434313774108887
delta_xy:
- 3.0614144552743028e-09
- 1.0718685494737201e-08
xy_reset_counter: 1
delta_z: 0.0
z_reset_counter: 1
vx: 0.0009532887488603592
vy: -0.0001496115728514269
vz: -0.0008669407689012587
z_deriv: -0.0017052473267540336
delta_vxy:
- 6.991126610955689e-07
- 2.4477471924910787e-06
vxy_reset_counter: 1
delta_vz: -0.00020352080173324794
vz_reset_counter: 1
ax: -0.00023102709383238107
ay: 0.03141145408153534
az: -0.03113688714802265
heading: -2.064100742340088
delta_heading: -2.2383649349212646
heading_reset_counter: 1
heading_good_for_control: false
xy_global: false
z_global: false
ref_timestamp: 0
ref_lat: .nan
ref_lon: .nan
ref_alt: .nan
dist_bottom: 0.10035395622253418
dist_bottom_valid: false
dist_bottom_sensor_bitfield: 0
eph: 0.01077322755008936
epv: 0.2710666060447693
evh: 0.03419855237007141
evv: 0.07012168318033218
dead_reckoning: true
vxy_max: .inf
vz_max: .inf
hagl_min: .inf
hagl_max: .inf
Thus I am confused by the crazy offset given by TimeSyncStatus (estimated_offset: -1685624282890795).
Can anyone explain to me how this works please ?
Thanks a lot 
