Global Positioning System Reference
In-Depth Information
TABLE 5.13 Observed GPS Positioning Errors with Typical Standard Positioning
Service (SPS) Receiver
(
15
)
Typical Range Error Magnitude
(meters 1
σ
)
Error Source
Selective availability 24.0
Ionospheric
∗
7.0
Tropospheric
∗∗
0.7
Satellite clock & ephemeris 3.6
Receiver noise 1.5
∗
After applying Ionospheric model. Actual values can range between approximately 1 - 30 m.
∗∗
After applying tropospheric model.
5.17 SELECTIVITY AVAILABILITY (SA) AND TYPICAL POSITION
ERRORS
(
8,12-15
)
The selectivity availability is aimed to degrade the performance of the GPS. It
was put in effect on March 25, 1990. In accordance with the current policy of
the U.S. Department of Defense, the signal available from the GPS is actually
a purposefully degraded version of the C/A code. The signal degradation is
achieved by dithering the satellite clock frequency and providing only a coarse
description of the satellite ephemeris. This policy, known as selective availability,
effectively raises the value of the user range error by a factor of four or more.
The selectivity availability affects only the performance of a GPS receiver, it
does not impact the design of the receiver. A Presidential Decision Directive
(PDD) released in March 1996 states that the selectivity availability will be
turned off within 10 years. The SA signal was terminated on May 1, 2000 and
this information was obtained from the internet.
Some typical position errors caused by different effects are listed in Table 5.13.
The distance error given in meters for one standard deviation
(
1
σ)
is listed in
this table.
5.18 SUMMARY
In this chapter the C/A code signal of the L1 frequency is discussed. The radio
frequency and the C/A code length are important information for performing
acquisition and tracking by a receiver. The navigation data are in five subframes.
In order to obtain the data, the beginning of the subframe must be found. There
are parity data that must be checked before the data can be used. The information
in the first three subframes is enough to find the user position. The information
in the fourth and fifth subframes is support data. Ionospheric and tropospheric
models are introduced to improve receiver accuracy. Selectivity availability is
introduced in the L1 C/A code signal to deliberately degrade the user position
accuracy.
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