There
are three parts to a GPS system: a constellation of between 24 and 32
solar-powered satellites orbiting the earth in orbits at an altitude
of approximately 20000 kilometers, a master control station and four
control and monitoring stations (on Hawaii, Ascension Islands, Diego
Garcia and Kawajale) and GPS receivers such as the one in a car.
Each
of the satellites is in an orbit that allows a receiver to detect at
least four of the operational satellites. The satellites send out
microwave signals to a receiver where the built-in computer uses
these signals to work out your precise distance from each of the four
satellites and then triangulates your exact position on the planet to
the nearest few meters based on these distances.
In
fact, signals from just three satellites are needed to carry out this
trilateration process; the calculation of your position on earth
based on your distance from three satellites. The signal from the
fourth satellite is redundant and is used to confirm the results of
the initial calculation. If the position calculated from distances to
satellites “A-B-C” do not match the calculation based on “A-B-D”
then other combinations are tested until a consistent result is
obtained.
The
process of measuring the distance from satellite to GPS receiver is
based on timed signals. For example, at 16h45m precisely, the
satellite may begin broadcasting its signal. The GPS receiver will
also begin running the same random sequence at 16h45m local time, but
does not broadcast the sequence. When the receiver picks up the
signal from the different satellites, there will be a time lag,
because the microwaves take a fraction of a second to travel from the
satellite to the receiver. The time lag is easily converted into
distance to each satellite. The slight difference between signals
from each satellite is then used to calculate the receiver's
position.
