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pirateofcapeann 03-30-2003 04:16 PM

GPS Selective Availability
Just heard a nasty rumor. I heard that the commercial GPS selective availability was turned back on or was going to be turned back on. I was using my GPS 111 tonight and it was telling me that its EPE was 12 15 feet and the DOP was 1.3ish. That looks pretty good to me. Has anyone else heard anything?


gpsguy 04-16-2003 08:11 AM

GPS Selective Availability
Hello ''Pirate''-

According to Uncle Sam, there is no intention of ever utilizing SA again. You can take that statement for what you might make of it...

The reason SA was instituted to begin with was because back in the infancy of the GPS program it was thought essential to prevent the "bad guys", who did not have the hardware necessary to decipher some encrypted codes,to be able to pinpoint accuracies with the precision the "good guys" could. Now, it didn''t take long at all before the university types and others figured out how to over ride these encrypted codes; as a result Selective Availability was obselete not long after it was instituted. (The GPS carrier phase is utilized instead of the raw code message. Quite simple; but in order for you to position yourself with precision inside this system you must apply a differential correction to your own data. This is where a base station such as CORS or WAAS comes in).

Anyway, the Dilution of Precision (DOP) will not change whether SA is on or off - it is an indicator of satellite geometric positioning. Much as a celestial navigator desires a good ''spread'' of observable bodies throughout the sky it is more desirable for the GPS space vehicles to be positioned advantageously as well.

I hope this helps -


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