BendixKing SKYMAP IIIC User Manual

Page 134

HOW DOES GPS WORK?

There is the possibility however of this being intentionally degraded (by feeding random

errors into the satellite clocks) to 100 meters for strategic reasons. This is called Selective
Availability or S/A and can be implemented at will by the DoD.
Day to day accuracy is monitored by Falcon Air Force Base, Colorado, who receive
information from a group of ground stations positioned at Ascension Island, Diego Garcia,

Kwagale and Hawaii.
These stations receive the satellite signals and check them for timing accuracy. If any

errors are detected, error information is transmitted back to the satellite, which then
corrects itself.
Reliability should be good when the full constellation of 24 satellites are in orbit.
Each satellite has three separate clocks on board (one in use and two standby) and in the

event of the total failure of a satellite one of the spares already in orbit can be manoeuvred
into position within a few hours.
The only time the DoD can predict there may be a substantial break in coverage for civilian

users, is at a time of National Emergency, when the whole system may be reverted to P

code (military) operation only, for strategic reasons.
Having said this, the system remained fully available to civilian users during the Gulf

conflict.

Rev 9 Jul 2006

D-3

Skymap/Tracker IIIC Pilot's Guide