The quasi-permanent use of GPS has been possible since february 1994, when the constellation of 21 satellites (+3 in reserve) was declared operational.
These satellites are in quasi-polar orbit, at an altitude of 20,200 Km.
They are distributed over six orbital planes, offst by 60 degrees (Figure 2).
The peiod of revolution of the satellites is 12 sidereal hours (i.e. 11 hours and 58 minutes). Consequently, we find the same satellite, in the same positions, twice a day (a seemingly futile detail, but often very useful in the planning of field missions!).
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