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Politics : Formerly About Advanced Micro Devices -- Ignore unavailable to you. Want to Upgrade?

To: Petz who wrote (109404)	5/4/2000 8:23:00 PM
From: pgerassi	Read Replies (1) \| Respond to of 1578969

Dear John: Since more than four satellites are used for fixes, (in a flat field mine does track and calculate position from 12 satellites), these sort of delays can be calculated out. Remember, only four satellites are used to be able to establish a minimum fix (four equations in four unknowns (X, Y, Z, and t (time))). Using twelve minimizes the delay. The slow track of the position fix, is exactly what the military does to "diddle" the signal. A varying signal where the offset falls inside a range of +/- 100 Meters for Longitude (X) and Latitude (Y) and +/- 150 Meters for Altitude (Z) is in the displayed range for 90% of the time (the way the offset is specified by the DOD) for each of the three axes (X, Y, Z). This means that the real offset is in the box, (True X +/- 100M, True Y +/- 100M, and True Z +/- 150M), at least 90% cubed or 72.9% of the time. If the propagation delay is not calculated out, the bias would always be below the current position (due to the delay being that the speed of light is slower in air than in space and those satellites below the horizon can not be received). This is not what I am seeing in the plot. The signal has the same accuracy as before the Executive Order. (the plot looks like a kids freehand of a star (highly concave polygon)) When one has say 8 satellites being received, 8 equations in 5 or six unknown gets a highly accurate fix because you do get the satellites rough positions to you. SA means the ability of the military to completely turn off the system. The manuals I have state, that the SA means (Selective Availability). Now, have you taken any GPS receiver, went into a field (or large shopping center parking lot), turned it on and stayed at a single location for 4-8 hours? This is where the proof can be found. I would agree to your premise, if my receiver did not receive 12 channels at once. Some consumer units track only 5 satellites at once. According to my manual, my receiver has an accuracy of +/- 5 meters when SA is turned off. Since I am not getting that accuracy (even +/- 100 meters), "diddling" is not off, however the system is running (not blacked out (I have seen the later happen)). Pete