3 ms·

You need four because you must solve for 4 unknowns, the fourth being time. Note that getting the time is the whole point in this case. None of the satellites

• onemoreact 15y ago
  Suppose you have 3 points in 3d space. P1 = (1,0,0),P2 = (0,1,0), P3 = (0,0,1) You know point the distance from U to P1 is x, from U to P2 is x +1 and from U to P2 is x + 2) You know know U is on a 1d curve (c1) though 3d space. (Or as you say you don't know time aka distance.) Now suppose P1, P2, P3 velocity is more than 2x that of point U and you have you have new offsets from U to {P1,P2,P3} you are now going to have a new curve (c2) in 3d space, but you know U's vector must take it from c1 to c2. Now add another time cycle, and you know it's vector must take it from C1 to C2 to C3. As you keep adding C's you constrain both the valid vectors(direction and velocity) as well as positions. Now measurement accuracy as well as change in velocity limit the accuracy but GPS satellites are moving a lot faster than you are so you can get reasonably accurate information this way. Edit: Also you can basically ignore the sections of the curve that whose altitude is unreasonable so the solution space is fairly constrained to start with and for each subsequent curve. But you can also use an internal clock/oscillator, accelerometer, gyroscope to further constrain reasonable accelerations etc.