How GPS Works:
GPS is a space-based radionavigation system designed
to provide worldwide, all-weather, passive, three-dimensional position,
velocity, navigation, and time data to a variety of civilian and military users
(figure 1). GPS does this by providing a constellation of at least 24
satellites, in an orbit 12,000 miles high and inclined at 55˚, which continuously broadcast
their position, a timing signal, and other information. By measuring the time it takes this signal
to travel from a given satellite to the user’s receiver, the receiver can
determine how far from the satellite it is.
For a single satellite, the receiver can determine the range, but not
the direction, from the satellite.
Thus, he receiver has identified a sphere, centered on the satellite, on
which the receiver is located. From a
second satellite, the receiver could can identify a second such sphere with the
receiver location being somewhere on the circular intersection of these
spheres. A third satellite provides a
third sphere, which reduces the possible receiver location to only two
points. A fourth satellite narrows this
to a single point. Figure 2 provides a
visual interpretation of this process.
Thus, by combining the measurements from four different satellites,
users can determine their three dimensional position.
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Figure 1. GPS Space Segment
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Figure 2. Determining Your Navigation Solution
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GPS
currently provides a Standard Positioning Service (SPS) available to civilian
users worldwide. US and allied military organizations have access to the
Precise Positioning Service (PPS) that offers increased jamming resistance and
other functions not typically needed by civilian users today. The quality of a radionavigation service is
typically specified by three parameters, accuracy, availability, and
integrity.
Accuracy is defined as the degree of
conformance between the estimated or measured position and velocity of a
platform at a given time and its true position or velocity. Radionavigation
system accuracy is usually presented as a statistical measure of system error
and is specified as:
Integrity is the ability of a system to provide timely
warnings to users when the system should not be used for navigation.
The availability of a navigation system is the percentage
of time that the services of the system are usable. Availability is an
indication of the ability of the system to provide usable service within the
specified coverage area. Signal availability is the percentage of time that
navigation signals transmitted from external sources are available for use.
Availability is a function of both the physical characteristics of the environment
and the technical capabilities of the transmitter facilities.[i]
While GPS offers many advantages over other
navigation and positioning services, many emerging applications require better
accuracy, availability, and integrity than is provided by GPS SPS (currently
6-40 meters, 95%, and two hours respectively).
