Niels Blaauw wrote: > As far as I understand, DGPS is a way of correcting the Selective > Availability, the deliberate inaccuracy that was put in the data from the > satellites. Since Selective Availability is OFF since May last year, DGPS is > not needed anymore. Actually, Selective Availability is only part of the "error budget". The removal of SA has eliminated MOST of the benefit from DGPS, but not all of it. There are still other sources of error, including atmospheric refraction. Here is the error budget: GPS Error Budget Source Uncorrected Error Level Ionosphere 0-30 meters Troposphere 0-30 meters Measurement Noise 0-10 meters Ephemeris Data Error 1-5 meters Clock Drift 0-1.5 meters Multipath 0-1 meter (in rare cases much more) Selective Availability 0-70 meters Note that these are not always "additive" (one component can partially cancel another component), but this gives you an idea of what is possible. Nonetheless, I certainly agree that DGPS is MUCH less useful in the absence of SA. Still, it can be of some help even with SA off. (Note that DGPS does not correct for multipath error, which is highly localized.) >My Etrex does not show any drift while standing still. That's because it cannot zoom in very far. :-) The lowest zoom setting is "200 feet". If you had a receiver that could zoom in to ten times this scale ("20 feet"), you would see drift. Again, I agree with your basic point, which is that DGPS has become much less useful since SA was turned off. Dan Hagen *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************Received on Tue May 08 2001 - 07:08:16 PDT
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