On 16 Apr 2005 00:26:54 GMT, (Jay Masino)
wrote:

Newps wrote:


Suppose you are navigating solely by GPS. What are you going to do in
the event the military chooses to disable the GPS system while you're
airborne

Can't be done. There is no on/off switch.

I'm involved with supporting spacecraft operations of NASA's constellation
of EOS spacecraft. Somewhere, there's a satellite control center that
operates the constellation of GPS spacecraft. It may be true that the
military can't "throw a switch", but there's no doubt that they can
command each satellite to shut itself down and go into "safe hold",
effectively shutting down the GPS system.

I wrote some articles on GPS for Trimble in the early '80s. I don't
have my notes from then, but there are a few things I sort of remember
that sort of come down evenly on both sides of the debate.

One is that the Navstar system was a joint military/civilian effort,
implying a promise to keep it operational in most circumstances.

The other is that the satellites' orbits have to be tracked and
corrected ephemerides regularly updated with an uplink from the Naval
Observeratory, which I assume is how they can deliberately degrade C/A
coverage over specific geographic areas.

(The NO would have been a single point of failure. By now, there is
probably some redundancy.)

Feel free to bring me up to date.

Don