behind GPS is relatively simple. Each of the 24 satellites, orbiting 11,000
miles above the surface of the earth, emits signals to receivers below.
By measuring the travel time of a signal transmitted from each satellite,
a receiver can calculate its distance from that satellite. Satellite positions
are used by a receiver as precise reference points to determine the location
of a receiver. When receiving the signals from at least 4 satellites, a
receiver can determine latitude, longitude, altitude, and time. The basic
GPS service provides users with approximately 100 meter accuracy 95% of
the time anywhere on or near the surface of the earth.