Differential GPS - Understanding GPS: Principles and Applications - page 428

msb lsb

msb msb lsb lsb

Pseudorange correction Parity

UDRE Satellite ID

*

1 234 8 9 24 25 30

Word 3

msb msb lsb

msb msb lsb lsb

Range-rate correction Parity

Issue of data UDRE Satellite ID

*

1 8 9 16 20 24 25 30

Word 4

msb lsb msb msb lsb lsb

Range-rate correction Parity

Pseudorange correction

1 16 17 24 25 30

Word 5

msb lsb msb msb lsb lsb

Pseudorange correction
(1st byte)

Parity

Issue of data UDRE Satellite ID

*

1 8 9 10 11 16 17 24 25 30

Word 6

msb lsb

msb lsb

Pseudorange correction
(2nd byte)

Range-rate
correction

Issue of data Parity

1 8 9 24 25 30

16 17

Word 7

* Scale factor

Figure 8.22 RTCM SC-104 version 2 message 1 format: words 3 to 7.

similar, but they convey carrier-phase and pseudorange measurements, respectively,
that have been corrected using the GPS broadcast ephemerides.

8.5.2 Version 3.0
The development of the SC-104 version 3.0 standard [41, 42] was driven primarily
by a need to treat RTK carrier-phase operations. The format is radically different
from version 2.3, in part to provide a more efficient parity scheme designed to pro-
tect against bursts errors as well as random bit errors and in part to overcome limita-
tions of the version 2.3 format, including an increased efficiency that will allow
more timely broadcasts for RTK operations.
Version 3.0 messages are broadcast in variable length frames shown in Figure
8.23. Each frame begins with an 8-bit preamble, followed by 6 reserved bits and a
10-bit message length field. The data message, ranging from 0 to 1,023 bytes, is then
broadcast followed by 24 bits of parity for error detection referred to as a cyclic
redundancy check (CRC) code. This message format is much more efficient than
version 2.3, which devotes more than 20% of the data link throughput to overhead
(e.g., parity). Furthermore, the version 3.0 parity scheme is much stronger than that
used for version 2.3.