In non aviation GPSs, at least the older ones, there really is a limitation
on speed. These units receive the signals from satellites one at a time. You
have to receive 4 satellites in order to calculate a 3 dimensional position.
If you are traveling extremely fast, than the signals from each satellite
are received at different physical positions. This makes it hard for the GPS
to resolve your position. Now here is the clincher : The speed at which the
GPS unit can switch between satellites determines the top speed at which the
GPS unit will work.

Aviation GPS units have simultaneous receive channels so that they can
receive up to 7 ( usually ) simultaneous signals. This eliminates the speed
restriction ( unless you are traveling close to the speed of light). I think
the simultaneous receive channels is starting to be used in higher end
non-aviation GPSs, perhaps like the one in the Land Rover.

In response to another posters comments about the reception inside the
plane: Most auto GPSs do have external antennae ports. It is just as hard to
get a signal through the top of a Land rover as it is a C130. My handheld
hiking GPS also has an antenna port.

"Peter Duniho" wrote in message
...
"Jim Macklin" wrote in message
news:Wksbg.22187$ZW3.9577@dukeread04...
As for the speed limit on the GPS, I presume it is the rate
at which the computer can identify new positions and the CPU
can calculate, it is just a cheap $125 hand held.

That doesn't make any sense. You can measure your position every 30
seconds and still get an accurate speed measurement.

Whatever the reason for an upper bound on the displayed speed, it has
nothing to do with the rate at which the computer in the GPS can do
anything, whether identifying new positions, calculating, etc.

Pete