Differential GPS - Understanding GPS: Principles and Applications - page 437

synchronized to GPS system time to emulate a GPS satellite and provide a ranging
capability. A data rate of 250 bps is used. The data is convolutionally encoded using
a rate half constraint length 7 encoder to generate an overall symbol rate of 500
symbols/s. The SBAS data symbols are synchronized with the 1-kHz GPS C/A code
epochs.
The C/A codes used by SBAS belong to the same family of 1,023-bit Gold codes
as the 37 PRN codes reserved by the GPS system described in Chapter 4. The SBAS
C/A codes were specifically selected to not adversely interfere with GPS signals (e.g.,
see [53]). The 39 current SBAS C/A codes and the associated geostationary satellites
are shown in Table 8.4. The listing of PRN code assignments is maintained by the
GPS Joint Program Office at http://gps.losangeles.af.mil. The SBAS C/A codes are
identified by the PRN number, the G2 delay in chips, and the initial G2 state. The
definition of either the G2 delay or initial G2 setting is required for implementation
of the generation of the SBAS C/A codes. Like the GPS C/A codes, the PRN number
is arbitrary, but they start with 120 instead of 1. The actual codes are defined by
either the G2 delay or the initial G2 register setting. The codes are ranked by the
average number of cross-correlation peaks when correlating these codes with the 36

Table 8.4 SBAS Ranging C/A Codes

Initial G2
Setting
(Octal)

First 10
SBAS Chips
(Octal)

G2 Delay
(Chips)

Geostationary Satellite PRN
Allocations PRN Orbital Slot

120 145 1106 0671 INMARSAT 3F2, AOR-E 15.5°W

121 175 1241 0536 INMARSAT 4F2 53°W

122 52 0267 1510 INMARSAT 3F4, AOR-W 54°W

123 21 0232 1545 Unallocated

124 237 1617 0160 ARTEMIS 21.5°E

125 235 1076 0701 Unallocated

126 886 1764 0013 INMARSAT 3F5, IND-W 25°E

127 657 0717 1060 GAGAN-1 TBD 1

TBD 1 128 634 1532 0245 GAGAN-2

129 762 1250 0527 MTSAT-1R (or MTSAT-2) 2 TBD

130 355 0341 1436 INMARSAT 4F1 63°E

131 1012 0551 1226 INMARSAT 3F1, IOR 64°E

132 176 0520 1257 Unallocated

133 603 1731 0046 INMARSAT 4F3 N/A

134 130 0706 1071 INMARSAT 3F3, POR 178°E

135 359 1216 0561 Unallocated

136 595 0740 1037 INMARSAT Reserved 8°E

137 68 1007 0770 MTSAT-2 (or MTSAT-1R) 2 TBD

138 386 0450 1327 Unallocated

1. The proposed GAGAN geostationary orbital longitudes are 55ºE, 74ºE, and 93.5ºE.
2. When MTSAT-2 is unavailable, MTSAT-1R will broadcast two PRN signals—each of which is received from an inde-
pendent uplink station—in order to maintain continuity in case of uplink signal attenuation or equipment failure at either
uplink station. Similarly, MTSAT-2 will broadcast two PRN signals when MTSAT-1R is unavailable. When MTSAT-1R
and MTSAT-2 are available, MTSAT-1R will broadcast PRN 129 signal only and MTSAT-2 will broadcast PRN 137 sig-
nal only.