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im doing a individual info search on GPS. i can understand everything (well its just a basic overview of GPS) but the one thing i cant get my head around is why the need for a fourth satellite? i know it corrects the timing as the receiver does not have an atomic clock. ive read 4 or 5 different explanations about it but i just dont get it..

here is an excerpt from wikipedia (i know its unreliable..), if someone can, can you elaborate on it and how they derive the equation? i can tell its REALLY simple, but its just not going through to me.

"It is likely that the surfaces of the three spheres intersect, since the circle of intersection of the first two spheres is normally quite large, and thus the third sphere surface is likely to intersect this large circle. It is very unlikely that the surface of the sphere corresponding to the fourth satellite will intersect either of the two points of intersection of the first three, since any clock error could cause it to miss intersecting a point. However, the distance from the valid estimate of GPS receiver position to the surface of the sphere corresponding to the fourth satellite can be used to compute a clock correction. Let denote the distance from the valid estimate of GPS receiver position to the fourth satellite and let denote the pseudorange of the fourth satellite. Let . is the distance from the computed GPS receiver position to the surface of the sphere corresponding to the fourth satellite. Thus the quotient, , provides an estimate of

(correct time) − (time indicated by the receiver's on-board clock),

and the GPS receiver clock can be advanced if is positive or delayed if is negative. However, it should be kept in mind that a less simple function of may be needed to estimate the time error in an iterative algorithm as discussed in the Navigation section."

http://en.wikipedia.org/wiki/Global_Positioning_System#Correcting_a_GPS_receiver.27s_clock

edit: also theres an image on wiki that is explaining it. hope it helps you help me with it :)

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# Homework Help: GPS - the fourth satellite?

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