Quoting DiamondFlyer (Reply 6): IIRC, you only need 4, and 5 to throw one out, if you have a Baro-aided GPS system. |
Correct, in a very basic description of how the system works, a GPS satellite transmits a time signal, and also a list of orbits the satellites (bit like a street directory, except they are always moving in a predictable orbit planes).
The receiver gets the different transmissions from each satellite in view, and knows which one sent what time signal. It also has in its memory a list of where they are in space.
Knowing where a satellite is in space, and the amount of time it takes to get the signal from the satellite, gives the receiver a sphere around the satellite where the aircraft must be, The distance from the satellite is basic maths, d=speed of light/time, bit like a constant
DME from a ground based
DME.
Do this calculation 4 times over, the position is the intersection of the 4 spheres. It is solving 4 simultaneous equations, for x,y,z, and time.
Baro-aiding achieves additional redundancy as the receiver puts a pseudo satellite extending from the center of the earth through the aircraft, and solves one of the equations, Z is known, so only 3 satellites are needed to solve for x,y, and time. The receiver can then compare the position generated by the 4 satellites, and the various combinations of 3 satellites and the baro-aiding. if an error exists between the calculated positions, they can discount the bad satellite from further position calculations.
This is very simplistic overview.
We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar