Obviously, this must be winter and we have nothing better to talk about... What Eric described applies to 2D position (assuming (which not that far of) that the Earth is a sphere. Most GPS receivers require 4 satellite fixes to calculate the position (the 4th one gives you the error (the accuracy)). You need more satellites to get a 3D fix.
Marek C&C24 Fennel in Ottawa Message: 5 Date: Mon, 28 Jan 2013 08:23:51 -0500 From: "Eric Haberfellner" <e...@firstclass.com> To: cnc-list@cnc-list.com Subject: Re: Stus-List Sextant Message-ID: <fc.000086e905b9b9b13b9aca00b8aaf7aa.5b9b...@firstclass.com> Content-Type: text/plain; charset="utf-8" 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
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