The "overlapping spheres" model is NOT how GPS works. It's the same as
the
stupid flashing light analogy for VOR. It perhaps gives someone some
concept,
but it's not accurate at all. The way GPS works is to use pairs of
satellites
to determine hyperbolids which are intersected with ones generated from
other
pairs.



Ron,

I looked up some information. I have not had time to completely digest it,
but I do see how a GPS system could be designed to use pairs of satellites
with hyperboloids. I think that my explanation is sufficient for a pilot
level operational understanding of how GPS works. Among other sources
propagating a similar explanation are Trimble and Garmin, so at least I am
in good company. My purpose is to provide a brief conceptual idea of how
GPS works. The primary purpose of my text is to provide practical guidance
as to how to actually use GPS.

Trimble's explanation is at http://www.trimble.com/gps/how.html. And
Garmin's "GPS for Beginners" is available at
http://www.garmin.com/support/userMa...ory=53&product
=999-99999-20

I did a Google search,
http://www.google.com/search?hl=en&i...id+GPS&spell=1 , and
did find some sources indicating that positioning could be done using pairs
of satellites and the intersection of hyperboloids. I am not sure if GPS
receivers actually use the intersecting hyperploids or intersecting sphere
model in their actual design. It appears that both are different techniques
to solve the same problem.

I am not an electrical engineer and am not sure which explanation is correct
or if they are merely different solutions to the same problem. I appreciate
you responding back to explain why you think my explanation was wrong.


John Bell
www.cockpitgps.com