Differential GPS - Understanding GPS: Principles and Applications - page 438

GPS codes that have zero Doppler difference. In the octal notation for the first 10
chips of the SBAS code as shown in the table, the first digit on the left represents
a “0” or “1” for the first chip. The last three digits are the octal representation
of the remaining 9 chips. For example, the initial G2 setting for PRN 120 is
1001000110. Note that the first 10 SBAS chips are simply the octal inverse of the
initial G2 setting.
Some future SBAS satellites will also be capable of transmitting a signal on the
GPS L5 frequency. Such signals will likely use PRN codes from the same family as
the GPS L5 signals (see Section 4.5.2), but they may not include a dataless compo-
nent. A baseline data rate of 250 bps is anticipated, convolutionally encoded into a
500-symbol/s stream.

SBAS Message Format and Contents
The 250-bps data from each SBAS GEO is packed into 1-second blocks of 250 bits,
as shown in Figure 8.31. Each block includes an 8-bit preamble (one of three parts of
a 24-bit unique word, 01010011 10011010 11000110, that is distributed over three
blocks), a 6-bit message-type field (allowing for up to 64 message types), a 212-bit
payload with unique meaning specifically defined for each message type, and 24 bits
of CRC parity for error detection, as shown in Figure 8.31. The start of every other
24-bit preamble is synchronous with a 6-second GPS subframe epoch. The pream-
bles and timing information provided in the messages facilitate data acquisition.
They also aid the user receiver to perform time synchronization during initial acqui-
sition before GPS satellites are acquired, thus aiding the receiver in subsequent GPS
satellite acquisitions.
Table 8.5 lists the message types that have been defined thus far for SBAS.
These message types support the basic wide area GPS concepts discussed in Section
8.3.3. Message types 2-5 provide broadcast clock corrections. Message type 25
provides broadcast orbit corrections. Message type 26 provides the L1-only user
with vertical ionospheric delay values over a grid of locations with predefined lati-
tude and longitude values. Each user receiver calculates the latitude and longitude
of the intersection points between each GPS signal and the ionosphere, which is
modeled as a thin shell at 350-km altitude above the surface of the Earth. The ver-
tical ionospheric delays at these intersection points, referred to as ionospheric
pierce points (IPPs), are determined for each visible satellite by interpolating the
delays from the three or four nearest grid points, as discussed later in this section.
The reader is referred to [52] for a complete description of the messages and their
applications.

Direction of data flow from satellite;
most significant bit (MSB) transmitted first

250 bits - 1 second

24-bits
parity

212-bit data field

6-bit message type identifier (0 - 63)

8-bit preamble of 24 bits total in 3 contiguous blocks

Figure 8.31 SBAS data block format.