Differential GPS - Understanding GPS: Principles and Applications - page 439

Table 8.5 SBAS Message Types

Type Contents

0 Don't use for safety applications (for SBAS testing)

1 PRN mask assignments, set up to 51 of 210 bits

2-5 Fast corrections

6 Integrity information

7 Fast correction degradation factor

8 Reserved for future messages

9 GEO navigation message ( X , Y , Z , time)

10 Degradation parameters

11 Reserved for future messages

12 SBAS network time/UTC offset parameters

13-16 Reserved for future messages

17 GEO satellite almanacs

18 Ionospheric grid point masks

19-23 Reserved for future messages

24 Mixed fast corrections/long-term satellite error corrections

25 Long-term satellite error corrections

26 Ionospheric delay corrections

27 SBAS service message

28 Clock-ephemeris covariance matrix message

29-61 Reserved for future messages

62 Internal test message

63 Null message

User Algorithms
SBAS user equipment are modified GPS L1 C/A code receivers. The equipment must
be modified to be able to generate and track the SBAS PRN codes described earlier;
demodulate the higher rate (250 bps), convolutionally encoded data; and include
modified software to apply the corrections and integrity data.
Application of the clock and ephemeris corrections is straightforward. Message
types 2-5 provide range domain clock corrections that are simply added to the
receiver's raw pseudorange measurements for all visible satellites. The SBAS data
does not include range rate corrections. These are generated within the user equip-
ment itself by differencing successive clock corrections [52, 54]. Message type 25
provides broadcast satellite position corrections in ECEF x , y , z coordinates. Satel-
lite broadcast position error rate terms and a clock bias term can also be provided, if
necessary, in message type 25 using a 1-bit velocity code flag.
As mentioned earlier, SBAS users are expected to be L1-only. Ionospheric cor-
rections for visible satellites are determined with an interpolation algorithm using
SBAS broadcast vertical ionospheric delay values. Applying the law of sines to Fig-
ure 8.32, the user first calculates the angle

pp , the Earth's central angle between the

user position and pierce point: