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The three major segments of GPS are space, control and user.
The Space Segment:
An original constellation of 24 satellites in six orbital planes are used
to send coded satellite signals that can be processed in a GPS
receiver, enabling the receiver to compute position, velocity and time.
The satellites are spaced 60 degrees apart and are positioned at an
altitude of 20,200 km (12,552 miles) with a 55-degree inclination. In
addition to the 24 satellites in the constellation, three additional
satellites are in orbit and will eventually replace older space vehicles.

Control:
The Control segment consists of five monitor stations, three ground
antennas, and a Master Control Station. The monitor stations passively
track all satellites in view, accumulating ranging data. This data is
processed at the MCS and incorporated into satellite orbital models. The
updated orbital information is then transmitted to each satellite via the
ground antennas and is sent with each satellite's navigation message.

User Segment:
The GPS user segment consists of the GPS receivers and us, the
users. The 24 satellites in their respective orbits provide the users with
five to eight satellites visible anywhere on earth, able to receive data.
Satellite signal arrival times from at least four satellites are processed
to estimate four quantities.

One factor that complicates the situation is that the signals are
traveling at the speed of light. So the difference between the arrival
times of the signals is minute. Since the arrival times of the satellite
signals are such a critical factor in calculating a position, each satellite
is equipped with four atomic clocks, two cesium and two rubidium.
Satellite clocks are monitored by ground control stations and
occasionally reset to maintain time to within one-millisecond of GPS
time. This information is then transmitted to each satellite via the
ground Antennas and is sent with each satellite's navigation
message along with its ephemeris data.

The Signal:
The satellites transmit information via two radio waves that can be
picked up by GPS receivers. Each radio wave is modulated so that it
can carry specific information. The modulated signal resembles
random electrical noise, but since the signal is not random but coded
it is referred to as a pseudorandom code. The radio waves, which carry
the pseudorandom codes, are distinguished by the designations of L1
and L2, and each carries different information in its modulated code.
L1 carries the C/A Code used by the civilian sector free of charge, and
is also modulated to carry the Navigation Message and other satellite
system parameter information. L2 carries the P-Code used by the
military.

The pseudorandom code for each satellite is distinct, which makes it
easy for GPS receivers to distinguish between one satellite and
another. In this way GPS receivers can tell exactly which satellites
make up a given configuration. This is important since the signals are
very weak. So a GPS receiver identifies one signal and, using built in
almanacs, actually searches for signals from the other satellites it
thinks should be in the configuration. Once it has identified all of the
satellites in the configuration it then begins tracking their signals.

The GPS receiver then mimics or mirrors the pseudorandom code for
each of the satellites and compares the differences between its own
code and the one received. It is able to do this because it knows the
fluctuations in the pseudorandom code. It then matches up one
known point in the signal received and its own and begins to make
calculations.


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