Re: Questions

Earl Needham (KD5XB@AMSAT.ORG)
Wed, 27 Oct 1999 13:25:31 -0600

At 12:10 PM 10/27/1999 -0400, Edward S Light wrote:
>1) GPS-derived positions
>   I recall that, at least a few years ago, coordinates obtained from
>   the GPS system were intentionally degraded (I believe the term was
>   "SA"). If this is still true, what is the approximate magnitude of
>   the effect? Centimeters? Meters?  Worse? One frequently sees GPS-
>   derived positions quoted to 0.000 001 degrees, which is approximately
>   0.11 meters (or about 4 inches), and I wonder about the meaningful-
>   ness of all those significant digits. (My own coordinates, quoted
>   in my signature, are given to the nearest 11 meters and were based
>   on measuring a paper USGS 1:24000 map to the nearest 0.005 inch and
>   assuming absolutely no errors in the production of that map.)

	Yes, the term is "SA", which stands for Selective Availability, and the
error *I* usually see is around 60 meters or so.  Sometimes the satellites
get in really good positions and it gets down to around 30 meters, very
rarely it gets down to 10 meters.  That's why the "(or so)" is in my sig


Earl Needham, KD5XB    mailto:KD5XB@AMSAT.ORG
Clovis, New Mexico	DM84jk N34d 25.446 W103d 12.700 (or so)

Pet peeve: breath is a noun,  breathe is a verb (When you take a breath,
you breathe...)

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