Yes. I was talking about 2D position (which is what sailors are most interested in). In a 3D space 4 satellites (or more) are needed. For 2D using the earth itself as the fourth sphere is a simplifying assumption (it is round enough for purposes of practical navigation).
For aviators and astrogators, please include at least one more satellite :-) Eric Haberfellner cnc-list@cnc-list.com writes: >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" <[ mailto:e...@firstclass.com ]e...@firstclass.com> >To: [ mailto:cnc-list@cnc-list.com ]cnc-list@cnc-list.com >Subject: Re: Stus-List Sextant >Message-ID: ><[ mailto:fc.000086e905b9b9b13b9aca00b8aaf7aa.5b9b...@firstclass.com >]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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