The twenty-four (24) GPS satellites orbit at six
(6) levels (having a shape of an ellipse) which are
symmetrical. In each level there are three (3) satellites
which are in orbit at a period of 12 hours, at
a height of about 20,000 km above the earth surface
and at a distance of 120 between them. The
angle with the equator is equal to 55. The orbits
are in such a way so that, any two satellites which
are in orbit in adjacent levels have a distance of 40
between them. Using this set out, there are at least
four satellites available, at a height of 15–75, for
the horizontal and vertical positioning of a station
at any time.