Global Positioning System Reference
In-Depth Information
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Sitka, Alaska;
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Winnipeg, Manitoba.
By 2004, the network had evolved into 20 sites:
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Billings, Montana;
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Cold Bay, Alaska;
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Juneau, Alaska;
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Albuquerque, New Mexico;
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Anchorage, Alaska;
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Boston, Massachusetts;
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Washington, D.C.;
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Honolulu, Hawaii;
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Houston, Texas;
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Mauna Loa, Hawaii;
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Kansas City, Kansas;
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Los Angeles, California;
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Salt Lake City, Utah;
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Miami, Florida;
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Minneapolis, Minnesota;
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Oakland, California;
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Cleveland, Ohio;
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Seattle, Washington;
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San Juan, Puerto Rico;
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Atlanta, Georgia.
Until May 2000, when SA was discontinued, observed 95% HPE and VPE were
on the order of 50m and 75m, respectively. Since SA has been discontinued, the
average values of the 95% HPE and VPE levels have been 7.1m and 11.4m, respec-
tively. The 99.99% HPE and VPE values, since SA discontinuance, have generally
been below 50m with a few notable exceptions. In July 2001, for instance, a failure
of one GPS satellite (PRN 22) caused instantaneous GPS position errors to exceed
well over 100 km over a significant portion of the western hemisphere. This event is
noted for its magnitude, but also for the rarity of occurrence for such an event. In
general, the GPS constellation has provided stable and consistent service since GPS
was declared operational.
Saying that GPS is generally stable and consistent should not be taken to mean
that all users view the performance they achieve using GPS in the same manner. Dif-
ficulties arise when we try to ascertain exactly what is meant when we refer to GPS
operational performance and what conclusions we can draw from its measurement.
These difficulties are based on the large number of possible GPS receiver configura-
tions and integrations, operating within a wide range of environmental conditions.
Figure 7.28 represents a detailed report of performance from the NSTB's
Hawaii reference station for July 19, 2004. The dual-frequency SPS (i.e., commer-
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