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Each GPS satellite
transmits data that indicates its location and the current
time. All GPS satellites synchronize operations so that
these repeating signals are transmitted at the same instant.
The signals, moving at the speed of light, arrive at a GPS
receiver at slightly different times because some satellites
are farther away than others. The distance to the GPS
satellites can be determined by estimating the amount of
time it takes for their signals to reach the receiver. When
the receiver estimates the distance to at least four GPS
satellites, it can calculate its position in three
dimensions.
A GPS receiver "knows" the location of
the satellites, because that information is included in
satellite transmissions. By estimating how far away a
satellite is, the receiver also "knows" it is located
somewhere on the surface of an imaginary sphere centered at
the satellite. It then determines the sizes of several
spheres, one for each satellite. The receiver is located
where these spheres intersect.
Anyone who drives a
car, travels, or spends time outdoors will soon own a GPS
unit. At the GPS Source, our experts introduce people to GPS
every day,
There are 5 basic steps to utilizing GPS
technology. First the basis is "triangulation" from the satellites (Trimble
1999). The second step is to triangulate the GPS by measuring the travel time of
the radio signals (Trimble 1999). The third step is to measure the travel time,
since the GPS system needs accurate timing, the atomic clock is used (Trimble
1999). The fourth step is that with distance you need to know the exact position
of the satellites in space, the key here is high orbits and careful monitoring
(Trimble 1999). The last step is to correct any delays in the signal as it
travels through the atmosphere (Trimble 1999).
The first part of the system is the Space Segment.
The GPS technology is based on a constellation of 24
satellites in space, the space segment of the GPS system
(Aerospace Corp.. 1999, Dana 1999, Mentor 1999, GMS 1999).
These 24 satellites are in 6 circular orbits about 20,200 Km
above the earth at 55 degrees with 12 hour period (GMS
1999). The satellites are high enough to bypass the problems
encountered by land-based systems (Mentor 1999) They send
wireless radio signals from space (Dana 1999, Mentor 1999).
Their configuration provides the user with between 5 and 8
space vehicles anywhere on the earth (Dana 1999). They are
spaced to give at any time a minimum of 6 satellites in view
anywhere in the world (GMS 1999). They also continuously
broadcast position and time throughout the world (GMS 1999).
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