Performance of Stand-Alone GPS - Understanding GPS: Principles and Applications - page 361

7.7 Measured Performance

The purpose of this section is to discuss assessments of GPS accuracy, to include but
not be limited to direct measurements. This is a complex topic due to the global
nature of GPS, the wide variety of receivers and how they are employed by their
users, and the complex environment in which the receivers must receive GPS sig-
nals. The discussion in this section leads the reader through an overview of the fac-
tors that affect GPS performance measurements. Techniques to combine
measurement and estimation to generate regional and even global assessments of
GPS performance are discussed, and performance trends resulting from such tech-
niques are provided. The section ends with a description of the range of typical per-
formance users can expect from a cross-section of receivers in use today, given
current GPS constellation performance.
Long-term accuracy measurements provide an indication of the stability and
consistency of GPS, as evidenced in the following plot. The data provided in Figure
7.27 was collected from the U.S. FAA National Satellite Test Bed (NSTB) [52]. Hor-
izontal position errors (HPE) and vertical position errors (VPE) were computed at
the 95% and 99.99% levels at each NSTB site, and averaged to generate the figure.
The network in 1999 consisted of eight sites:

• Bangor, Maine;
•

Dayton, Ohio;

Elko, Nevada;

•

Gander, Newfoundland;

•

Honolulu, Hawaii;

•

Seattle, Washington;

•

250

HPE (95%)
HPE (99.99%)
VPE (95%)
VPE (99.99%)
Time transfer (95%)

200

150

100

50

0

Dates

Figure 7.27 GPS SPS performance as observed within the United States from 1999-2004.

( After: [52].)