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XMap User Guide
342
The GPS constellation consists of 24 satellites, each carrying several atomic clocks to
ensure the most accurate time possible. The satellites broadcast low-power radio
waves containing the satellite's identity code and the exact time (to the nanosecond)
that the message was sent.
When a GPS receiver picks up a satellite signal, it identifies the satellite and
compares the signal time with its own clock. The time difference represents the time
it has taken for that radio wave to travel from the satellite to the receiver. Since
radio waves travel at the speed of light, the time difference can be used to calculate
the distance from the satellite to the GPS receiver.
The satellite's identity code provides the location of the satellite, and the distance to
the receiver creates a sphere of possible locations for the GPS receiver. Without
more information, the receiver only knows that it is located somewhere on that
sphere.
Two additional satellites are necessary to narrow down the receiver's possible
position. Each of these satellites sends a similar radio message containing time and
identity information. The GPS receiver checks the orbital location of each satellite
and uses the elapsed time to create two additional spheres of possible locations.
These three spheres intersect at two points; however, one of these points is
eliminated because it is far away from the earth's surface. Therefore, the second
point is assumed to be correct. The data from these three satellites provides the
receiver with a two-dimensional location.
Data acquired from a fourth satellite pinpoints the receiver's exact location. This
additional positioning information allows the GPS receiver to calculate its elevation,
which is particularly important for GPS users in mountainous locations.
GPS Position Accuracy
The accuracy of the data your GPS receiver provides is dependent upon many
factors, including the quality of your equipment. A low-quality clock within the
receiver decreases the accuracy of your location. The atmosphere, the ionosphere,
and the number of channels your receiver can handle all affect the accuracy of your
system. Consult your GPS hardware manuals for information on how your receiver
adjusts for errors.
Any buildings, natural structures, or heavy foliage that obstruct the GPS antenna's
view of the sky prevent satellite signals from reaching the receiver and decreases the
accuracy of your position.
Your accuracy will also depend on your level of clearance with the U.S. Department
of Defense. There are two available radio signals that receivers can use: the
Standard Positioning Service (SPS) for civilians and the Precise Positioning Service
(PPS) for military and authorized personnel.