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Chapter 2 - Introducing the 961
Absolute and
repeatable accuracy
Selective availability
Page 2-14
GPS's major advantage over all other commercially-available
navigation systems, including loran, is its all-weather, world-
wide availability, and its absolute accuracy. (Those readers
interested in loran should see "Understanding Loran-C" start-
ing on page 5-7.) Absolute accuracy refers to the ability of a
navigation device to determine your actual lat/lon coordinates
on the surface of the earth, as compared to position data deter-
mined from a paper chart or other reference. Repeatable accu-
racy, on the other hand, refers to the ability of a particular
navigation device to return to a position whose lat/lon coordi-
nates were previously recorded by this same navigation
device.
What is selective availability?
With GPS available to virtually anyone in the world, the DOD
wanted to avoid the possibility that GPS' precision could be
used by hostile forces as a source of accurate targeting data
against the United States. To guard against such actions, they
introduced into the GPS signals intentional errors, or selective
availability (SA), which only U.S. forces could bypass. Selective
availability involves a complex formula that can be turned on
or off.
At this time, the future of selective availability is uncertain.
Selective availability effectively reduces GPS' absolute and
repeatable accuracy from 30 meters (approximately 100 feet)
95 percent of the time to 100 meters (approximately 330 feet)
95 percent of the time. This reduction means that you can
expect to be within 330 feet of your measured position 95 per-
cent of the time and outside that area approximately five per-
cent of the time.
In the beginning of SA, the majority of end users accepted it as
an inevitable part of GPS, a trade-off against its worldwide
availability. This reduction in accuracy, however, wasn't
acceptable to the U.S. Coast Guard; it didn't meet its require-
ment of 8-20 meter accuracy for navigating in harbors. For this
reason, the Coast Guard developed a system of differential cor-
rections that let it effectively bypass selective availability's
built-in error.
961 Operations and Reference Manual Rev. D
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