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It looks like the scan matcher isn't measuring velocity directly, but it is computing the delta in the laser position, which you could combine with the time between laser scans to produce a rough estimate of velocity, subject to jitter in your laser frequency and any errors in the scan matching.

Since laser_scan_matcher doesn't publish stamped poses, you'll have to modify laser_scan_matcher directly to compute this output.

LaserScan has a timestamp in the message header that you can use to compute the time between scans.