How important it is to sync the timing for gnss and lidar in localization?

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I don't think that is important as much as long as GNSS Hz is below from a threshold such as 10Hz. In my case when I increase the GNSS Hz, it created 30 seconds delays.

I found gnss_pose is published based on the GPS time and ndt_pose is published based on the current timing. Why are they different?

I guess the fundamental reason for delay is your question, GPS time delays from the current_time when you incrase the speed of GNSS.

Of course, GNSS and LiDAR should be time synchronized.

However, since Autoware Localization does not perform processing that requires strict time synchronization between GNSS and LiDAR, I think that even if the timing is not synchronized, it will not affect the position estimation much.