Why GPS is in your future
Elimination of Selective Availability (SA)
To understand the impact of the elimination of Selective Availability upon the users of
the NAVSTAR GPS system, a brief introduction into how the system works is needed.
Please note that the following does not take into account the use of differential GPS
[DGPS] techniques; differential GPS is a process by which errors in accuracy are
minimized by the use of reference data to correct for anomalies in the data collected in
the field.
The NAVSTAR GPS system consists of three segments: space, control, and user. The
Space Segment consists of 24 operational satellites in six circular orbits 10,900 nautical
miles above the earth. The satellites continuously broadcast position and time data to
users all over the world.
The Control Segment consists of a master control station in Colorado Springs, with five
monitoring stations and three ground antennas located around the world. The monitor
stations track all GPS satellites in view and collect ranging information from the satellite
broadcasts.
The User Segment consists of the receivers and related equipment that allow users to
receive the GPS satellite broadcasts and determine their position, velocity and/or time.
There are two types of user groups: military or other governmental users and civilian
users. Different signals and different levels of accuracy are available to each group as
determined by the receiving equipment utilized.
Each GPS satellite transmits a highly accurate time signal. Distances from each of the
satellites are calculated by determining the differences between the time a signal is
transmitted from a satellite and the time it is received. This difference is a measure of the
apparent range to the satellite. By utilizing signals from three or more satellites and
triangulation, a GPS receiver can determine its location on the surface of the earth.
It is within the relationship between the space and user segments that a number of factors
affecting the accuracy of the locational solution arise. Some of the most important are:
Selective availability [SA]: Selective availability was implemented to intentionally
degrade the quality of the GPS signals available to civilian users. A primary intent behind
the implementation of SA was the belief that the signals could be easily used by enemy
states against the US. The potential accuracy of the system of approximately 20-30
meters is reduced to 100 meters by the presence of SA. (Dana, 1999)
Space vehicle clock errors: Each NAVSTAR GPS satellite contains a number of atomic
clocks that control broadcast signal generation. The clocks are highly stable but may
deviate from GPS time. This deviation is known as satellite clock error. (Kaplan, 1996)
Tropospheric delays: The troposphere is the lower part of the atmosphere (ground level to
8 to 13 km) that experiences the changes in temperature, pressure, and humidity associated with weather changes. These changes may affect the propagation of the
signals. (Dana, 1999)
Ionospheric delays: The ionosphere is the layer of the atmosphere from 50 to 500 km that
consists of ionized air. Changes in the ionosphere may also affect the propagation of
signals. (Dana, 1999)
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