Performance of Stand-Alone GPS - Understanding GPS: Principles and Applications - page 357

The geostationary satellites downlink the integrity and differential corrections
for each monitored satellite using the GPS L1 frequency with a modulation similar
to that used by GPS. Therefore, the geostationary satellite also can serve as an addi-
tional GPS ranging signal. Based on this information, the user receiver forms hori-
zontal and vertical protection levels based on a weighted solution. The initial phase
of the U.S. WAAS system consists of two geostationary satellites, Atlantic Ocean
Region West (AORW) and Pacific Ocean Region (POR), provided by Inmarsat.
SBAS systems are discussed in much greater detail in Chapter 8.
The initial four INMARSAT satellites that broadcast the SBAS signal as an aug-
mentation to GPS can be included in a similar analysis to that performed in Section
7.2 to determine the distribution of HDOP and PDOP. The same analysis was per-
formed as before with a 5º mask angle and up to three satellites removed from the
constellation in order to investigate the overall improvement in the availability of
GPS. The locations of the ranging geostationary satellites are POR at 179.5ºE,
Indian Ocean region (IOR) at 64.5ºE, AORW at 55.5ºW, and Atlantic Ocean
Region East (AORE) at 15.5ºW.
The cumulative distributions of HDOP and PDOP augmented with the four
geostationary satellites are displayed in Figures 7.25 and 7.26, respectively. Again
applying a PDOP threshold of 6.0 to determine the availability of these satellite con-
stellations, it can be seen that both the 24- and 23-satellite constellations provide
coverage 100% of the time. With two satellites removed from the constellation, the
availability is 99.97%, and with 21 satellites the constellation provides an availabil-
ity of 99.65%. This is an improvement from an availability of 99.9% and 99.2%
that the 22 and 21 satellite constellations respectively provided without the aiding
of the geostationary satellites.
Next, the improvement in the availability of RAIM and FDE over the conter-
minous United States (CONUS) is examined when the constellation is augmented

99.999
99.99

99.9

99

90

70
50
30

24 satellites with 4 GEOS
23 satellites with 4 GEOS
22 satellites with 4 GEOS
21 satellites with 4 GEOS

10

1

0.1

4 5 6

0 1 2 3 7 8 9 10

HDOP

Figure 7.25 Cumulative distribution of HDOP with four geostationary satellites.