Just to clarify. The only GPS satellites that are in geosynchronous orbit are the ones that provide WAAS correction data. The ones used to generate a position fix are not in geosynchronous orbit.
The constellation of about 24 GPS satellites orbits at about 12,600 miles and these are not in equatorial orbit. If fact in order to generate a fix, it is critical that the satellites not be arranged in a straight line as all geosynchronous satellites are along the equator. This would be a classic case of bad satellite geometry. The fact that the satellites are not in geosynchronous orbit and are therefore moving relative to the earth's surface is critical in GPS calculations. This relative movement allows the GPS receiver to calculate the satellite's true position by using the Doppler shift. The receiver can now calculate its distance from the satellite. Once you know the distance you know that the receiver has to be on a point on the surface of a sphere with a radius of that distance with the satellite at the center of the sphere. By limiting the points on the surface of the sphere to points that intersect the surface of the earth gives you a circle of position on the earth's surface that the receiver lies on. By then repeating the calculation for at least two more satellites and seeing where the circles of position intersect you get a position fix. Just like using a sextant and three lines of position. -- Eric Haberfellner [ http://EricHaberfellner.com ]http://EricHaberfellner.com 1979 C&C 34 Arioso National Yacht Club Toronto Ontario, Canada 43° 37.9' N, 079° 24.4' W [ http://Ariosolog.com ]http://AriosoLog.com 1972 Marshall Sanderling Caper West Dennis Yacht Club West Dennis, Mass., USA [ http://Caperslog.com ]http://CapersLog.com cnc-list@cnc-list.com writes: >Actually you get exact time from your GPS 8 times each day. > >One of the consequences of Relativity is that the faster you go, the slower >time progresses for your frame of reference. (Hence "relativity". > >The GPS satellites are 27000 miles up in geosyncronus orbit, and trasveling >about 18000MPH faster than the spot just below them on earth. The clocks on >the satellites lose several seconds each day relative to the spot b elow >them on the surface. > >There is an Air Force facility out west (Nebraska, I think) that adjusts the >clocks on the satellite 8 times a day to bring them back in line with UTC. >There was a segment on a NOVA episode about the GPS system and how it >worked, and they mentioned that one time, when the USAF could not make the >adjustment because of a software or equipment problem, that the average >error of a GPS system using the US satellites got up to 8 miles > >-----Original Message----- >From: CnC-List [mailto:cnc-list-boun...@cnc-list.com] On Behalf Of Wally >Bryant >Sent: Sunday, January 27, 2013 8:25 PM >To: cnc-list@cnc-list.com >Subject: Re: Stus-List Sextant > >You can always get accurate time from your GPS... (sorry, I couldn't help >myself.) > >Wal > > > > >_______________________________________________ >This List is provided by the C&C Photo Album >http://www.cncphotoalbum.com >CnC-List@cnc-list.com
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