GPS Know-How
 Location of S2
 Position of S1
 R1
 R2
 Position of S3
 Receiver (R) sends a signal to Satellite 1 (S1)
 And S1 sends the signal back, but with a time lapse, say 0.04s.
 Therefore, distance between R and S1 would be 0.04s/2 x 3.0x10   ms   = 6.0x10  m.
 R also sends a (different) signal to Satellite 2 (S2).
 And S2 sends the signal back, but with a time lapse, say 0.06s
 And S3 sends the signal back with a time lapse, say 0.02s..
 Therefore, distance between R and S3 would be 0.02s/2 x 3.0x10   ms   = 3.0x10   m.
 Now, R has 3 sets of data to calculate its location.Distance of R from S1 = 6.0x10   m Distance of R from S2 = 9.0x10  m Distance of R from S3 = 3.0x10  m Note: 1. all views are from above the terrestrial position of R.            2. The possible locations of R from each of the satellites forms a sphere, not a                   circle.            3. The intersections of 2 spheres forms a circle.
 The intersection of the possible positions of R using the possible distances of R from S1, S2 and S3 lead to two possible locations of R : R1 and R2. Which one to choose? By simple calculation, the receiver R will eliminate one of these two values on the basis of their altitude ie how high they are from sea level. This is easily done as one of these readings puts you more than 100km from the surface of the Earth.          We thus see that although the GPS is a complex system that costs a whole lot of money, it relies on some basic principles of geometry (intersection of objects, triangulation). The big problem is getting the accuracy required.
 Therefore, distance between R and S2 would be 0.06s/2 x 3.0x10   ms   = 9.0x10  m.
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 R also sends a (different) signal to Satellite 3 (S3).
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 the speed of light ie 299 792 458 ms
 » 3 x 10
 ms
 (for ease of calculation)
 GPS, a godsend for hopeless navigators