-- Last Updated: Jan-29-13 8:16 PM EST --
The GPS processes the incoming satellite signals
and calculates the difference in timing and
equates that to distance over time which equals speed.
I don't believe the equipment and conditions can
be set up or calibrated in a way to truly record
differences that can't be explained away by the
inaccuracies of measurement.
Try mowing your lawn and see if the GPS records
every turn, every variance, every nuance of your
moves in an extremely small area like a residential lot.
Handheld GPS units don't like slow speeds with small movements