Differential GPS - Understanding GPS: Principles and Applications - page 426

msb lsb

msb lsb

Frame ID Parity

Preamble Station ID

1 8 9 14 15 24 25 30

Word 1

msb lsb

msb lsb

Sequence
number

Frame
length

Station
health

Parity

Modified Z-count

1 13 14 16 17 21 22 24 25 30

Word 2

Figure 8.21 RTCM SC-104 version 2.3 message header.

are used to provide a scale factor for a field that appears in various message types
referred to as user differential range error (UDRE) that will be described later.
For code-based DGPS systems, message types 1 and 9 are among the most
important messages. The content of message type 1 is shown in Figure 8.22 (note
that the two-word header that is appended at the beginning of every message type is
not explicitly shown). For every visible satellite, the type 1 message includes the fol-
lowing parameters:

• Scale factor: 1 bit to indicate the resolution of the pseudorange and range-
rate corrections to follow. If unset (set), resolutions of 0.02m (0.32m) and
0.002 m/s (0.032 m/s) apply for the pseudorange and range-rate corrections,
respectively.
• UDRE: 2 bits that indicate ranges of expected 1-sigma errors of the
pseudorange corrections. As mentioned earlier, 6-bit patterns in the station
health field of the header are used to provide a scale factor for UDRE. UDRE
values ranging from 8m are possible with the scale factor applied.

• Satellite ID: 5 bits to indicate the satellite number for which DGPS corrections
are being provided.
• Pseudorange correction: 16-bit correction

≤ 0.1m to >

∆ρ i t ( 0 for the indicated satellite,
applicable at the time t 0 provided by the Z -count in the header.
• Range-rate correction: 8-bit rate correction

∆& ρ i () t 0 (see discussion in Section

8.3.1.2).
• Issue of data (IOD): The IOD indicates the specific set of GPS navigation data
that was used in generating the corrections. As noted in Chapter 4, approxi-
mately every 2 hours, the broadcast clock and ephemeris data from each GPS
satellite is changed. The GPS navigation message tags each set of clock and
ephemeris data with IOD values referred to as IODC and IODE. IODC is a
10-bit parameter, and IODE is an 8-bit parameter (the 8 LSBs of IODC). IOD
in the SC-104 messages are equal to IODE in the GPS broadcast message.