Quote:
Originally posted by Cory
The only thing about GPS that I see as a problem is that the government only allows commercial users to fix their position with 3(I think?) satellites, which gets you fairly close to where you actually are, as opposed to military GPS, which I believe uses 5 satellites, and can get you to within a few meters of your location. What I am getting at is what if some sort of environmental feature were in the way, and the GPS is not accurate enough to stay away from it. This would bring up the need for redundant systems of tracking location/terrain avoidance as others have said.
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The biggest functional difference between civilian and military GPS is that the civilian signal may be "degraded" (called selective availability) at any time the government deems it necessary. When selective availability is in force, the satellites transmit intentionally misleading information. The effect is that the position calculated by the receiver is only within about 100 meters of its actual position.
Selective availability was disabled on May 2, 2000. Since then, the acuracy is supposed to be within 20m.
There is no limitation on the number of satellites that a GPS receiver is allowed to use. (Other than the number of satellites that are "in view" at any one time.) I think the civilian and military signals are carried by separate constellations of satellites, so it may be that the military constellation has a larger number of satellites, and so a larger number of satellites may be visible at any given time. This could be where you got the idea that civilian GPS could only use 3 satellites at a time.