Differential GPS - Understanding GPS: Principles and Applications - page 425

spheric refraction effects can be mitigated through modeling (e.g., the use of Snell's
law in conjunction with a slab model for the troposphere [38]). More comprehen-
sive treatments of GPS attitude determination concepts may be found in [38, 39].

8.5 Message Formats

Many messaging protocols have been developed throughout the industry for the
dissemination of code- and carrier-based DGPS data between reference stations and
users. This section will present, as an important example, DGPS messages devel-
oped by the RTCM Study Committee 104 (SC-104). Although originally developed
for maritime applications, RTCM SC-104 messages are now supported by the vast
majority of commercial GPS receivers, including low-cost recreational devices.
Until recently, there was only one set of SC-104 messages to support both code-
and carrier-based LADGPS services. This message set has evolved over time with
version 2.3 published in August 2001 [40] being the most recent version. In Febru-
ary 2004, RTCM published guidelines for a new set of messages that use a more
efficient protocol [41]. This later protocol, referred to as version 3.0, is currently
focused on carrier-phase DGPS, although expansion for other applications is envi-
sioned. Both protocols (version 2.3 and 3.0) describe digital message formats that
can be broadcast from a reference station to a user using any arbitrary data link.

8.5.1 Version 2.3
Figure 8.20 shows the basic frame format of version 2.3, which consists of a vari-
able number of 30-bit words. The last 6 bits in every word are parity, and the 30-bit
word format is derived from the GPS navigation message. The first two words of
each frame are referred to as the header . The content of the header is shown in Fig-
ure 8.21. The first word of the header contains an 8-bit preamble, consisting of the
fixed sequence 01100110, followed by the 6-bit frame ID , which identifies 1 of 64
possible message types (see Table 8.2). Next, a 10-bit station ID identifies the refer-
ence station. The first 13 bits of the second word in the header, the modified
Z-count , comprise the time reference for the message. The following three bits form
the sequence number , which increments on each frame and is used to verify frame
synchronization. The frame length is needed to identify the beginning of the next
frame, since the length of the frame is variable, depending on the message type and
the number of visible satellites. The 3-bit station health indicates whether or not the
reference station is functioning properly and whether the reference station transmis-
sions are unmonitored. Six of the possible eight patterns of the 3-bit station health

msb lsb

msb lsb

Word n+2

Word 1 Word 2 Word 3

1 30 31 60 61 90 91 30 (n+2)

Header

Figure 8.20 RTCM SC-104 version 2 message frame.