"Larry W4CSC" wrote in message
...
GPS receivers only provide 3 pieces of information. Latitude,
Longitude and Altitude....the 3 dimensions in space. All other
information (speed, heading, COG, VMG, etc.) are all derived by the
computer monitoring the changes in these 3 dimensions over
time.....many seconds. That's why it takes so long for it to change
heading or speed or VMG or other derived outputs when you make that
turn or come about.

I have heard otherwise: A GPS derives it's speed from the doppler shift on
the received carriers. A very slow averaging filter smoothes the output,
which causes the delay in indicated speed. The filter is there to prevent
erroneous speed indication due to atmospheric disurbances and multi-path
signals.

Meindert