Geoscience Reference
In-Depth Information
to mariners or shipping, regarding information on the status of the local DGPS service which is not
provided in other message types. Additionally, the type 16 message may provide limited information
on service outages in adjacent coverage areas or planned outages for scheduled maintenance at any
broadcast site. In order to keep data link loading to a minimum, type 16 messages contain only
system information that is crucial to the safety of navigation. For more details on the RTCM format
and message types, the reader is directed to www.ccg-gcc.gc.ca/dgps/main_e.htm.
The recently updated standards, Version 3.0 RTCM [RTCM, 2004], consist primarily of mes-
sages designed to support real-time kinematic (RTK) operations for both GPS and GLONASS (see
Section 9.11), including broadcasting of code and carrier phase observables, antenna parameters,
and ancillary system parameters by the reference station to the user's location. Unlike the earlier
version, this standard does not include tentative messages; it is designed to accommodate modifica-
tions to GPS and GLONASS (e.g., new L2C and L5 signals) and to the new systems that are under
development (e.g., Galileo, see Section 9.11). In addition, augmentation systems that use geostation-
ary satellites that provide ranging signals and operate in the same frequency bands are now in the
implementation stages (RTCM, 2004). The primary reason for this update included the following
shortcomings of the earlier versions: (1) parity scheme that uses words with 24 bits of data fol-
lowed by 6 bits of parity was wasteful of bandwidth; (2) parity was not independent from word to
word; (3) with so many bits devoted to parity, the actual integrity of the message was not as high
as it should be; and (4) 30-bit words are awkward to handle. Version 3.0 is intended to correct these
weaknesses.
Message types contained in the current Version 3.0 standard have been structured in several
groups: (1) observations (GPS L1, GPS L1/L2, GLONASS L1, GLONASS L1/L2)—message type
1001-1004, 1009-1012; (2) station coordinates (antenna reference point coordinates and antenna
height)—message type 1005-1006; (3) antenna description—message type 1007-1008; and (4) aux-
iliary operation information—message type 1013 (RTCM, 2004).
Aside from the differentially corrected position coordinates in NMEA (National Marine Elec-
tronics Association) format, DGPS receivers might also offer the possibility of storing all of the
raw data and correction signals for postprocessing. The raw data at the receiver can generally be
stored in either receiver-specific or standard RINEX format. NMEA is an industry association that
sets data transmission standards (www.gpsinformation.org/dale/nmea.htm; www.nmea.org) and
has developed specifications that define the interface between various pieces of marine electronic
equipment, including a set of standard messages defining the possible outputs of a GPS receiver.
The idea of NMEA is to send a line of data called a sentence that is totally self-contained and
independent from other sentences. There are standard sentences for each device category, and there
is also the ability to define proprietary sentences for use by the individual company. All of the
standard sentences have a two-letter prefix that defines the device that uses that sentence type (e.g.,
for GPS receivers the prefix is GP), which is followed by a three-letter sequence that defines the
sentence contents (www.gpsinformation.org/dale/nmea.htm).
There are several GP sentences, each one containing some unique data associated with them.
They are all in ASCII format and are in the form of comma delimited strings. The character string
lengths vary from 30 to 100 characters and are output at the selected intervals. The most common
string (or sentence) is called the GGA string that provides essential fix data containing the Time of
the Fix, Latitude, Longitude, Height, Number of Satellites used in the fix, DOP, Differential Status,
and the Age of the Correction. Other strings may contain Speed, Track, Date, and so forth. NMEA
is available in virtually all GPS receivers and is the most commonly used data output format. It is
also the format used in most software packages that interface to a GPS receiver.
Some other sentences that have applicability to GPS receivers are as follows:
•
AAM—Waypoint Arrival Alarm
•
ALM—Almanac data
