Using GPS
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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.
