Originally Posted by Ian W
I think you will find that messages are timestamped probably synched to GPS clock.
I think AT1 in his post
https://www.pprune.org/showthread.php?p=7124 explains that the satellite sends out a precise timestamp as part of its ping and the receiver returns it along with the receiver's unique iD, this allowing the lag to be precisely calculated irrespective of how good the clock on the receiver is.