Hi Bob.

14-Sep-03 10:15:02, Bob Lawrence wrote to John Tserkezis


 >> When it first became available, kit was 100m, and the military
 >> version that called in another satellite to get 1m, but I think
 >> they all do that now...

 JT>> Not quite. You're thinking of Selective Availability. It was a
 JT>> system that effectively scrambled the lower order accuracy of the
 JT>> signal to increase the error to 100m. They used a psudeo random
 JT>> encryption system that cycled every two days or so, so, if you
 JT>> averaged a stationary reading over that time period you would get
 JT>> the 15m accuracy they originally quote. Military equipment
 JT>> decrypts that, so they get the 15m accuracy.

 BL> Ahh. God bless Amurrica and our masters the CIA.

 JT>> Selective Availability was turned off about May 2000? (can't
 JT>> remember exactly when) by the president at that time making
 JT>> available the full accuracy of the system to civilians.

 BL> Either that, or they found a sneakier way to fuck it up.

tight beam satellite transmitters. - they can selectively jam GPS from orbit.

 BL> Do you know if there are other GPS systems up and running? The
 BL> Japs have launched heaps of satellites lately...

Glonass (Russian), Magellan (European), I've not heard of a japanese one.

 -=> Bye <=-

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