Geoscience Reference
In-Depth Information
LAAS is installed at individual airports and is effective over just a short range, with accuracy
of 1 m or less in all dimensions. The ground equipment includes four reference receivers, a LAAS
ground facility, and a VHF data broadcast transmitter. This ground equipment is complemented by
LAAS avionics installed on the aircraft. The GPS Reference Receivers and LAAS Ground Facil-
ity (or LGF) work together to measure errors in GPS-provided position. LAAS correction message
is then sent to a VHF data broadcast (VDB) transmitter. The VDB broadcasts the LAAS signal
throughout the LAAS coverage area to avionics in LAAS-equipped aircraft. The LAAS equipment
in the aircraft uses the corrections provided on position, velocity, and time to guide the aircraft
safely to the runway (http://gps.faa.gov/about/office_org/headquarters_offices/ats/service_units/
techops/navservices/nsss/Lass).
WAAS consists of a network of twenty-five ground reference stations and a number of geo-sta-
tionary (Inmarsat) satellites broadcasting a signal in the GPS band (Figure 9.12), also providing
additional ranging distances that are included in the user positioning solution. The WAAS signals
contain information including differential corrections and GPS satellite health status. WAAS has
been running 24 hours a day, 7 days a week since early 2000, providing high-integrity navigation
signals for nonaviation users such as boaters, precision agriculture, crop dusters, surveyors, vehicle
dispatchers and location services, cell phone 911 emergency services, hikers, and other personal
recreation uses within the conterminous United States. WAAS was officially commissioned by the
FAA for public aviation use on July 10, 2003. The published specifications of WAAS call for 7.6 m
in the vertical direction, which corresponds to better than 5 m horizontal accuracy 95 percent of the
time (for a single-frequency user receiver), but accuracies around 2 m horizontal RMS have already
been demonstrated (http://gps.faa.gov/programs/waas/waas-text.htm; http://www.navcen.uscg.gov/;
Lachapelle et al., 2002). Enhanced, dual-frequency WAAS significantly increases the accuracy to
approximately 30 to 70 cm in real time.
The GPS and WAAS signals are sent over the same frequency band, with the 50 bps (bits per
second) data rate from the normal GPS satellites for information like ephemeris and almanacs, and
500 bps of the raw signal data rate from the WAAS satellite. The increased data rate for WAAS
reduces the reliability of the WAAS data transmissions (increased bandwidth reduces SNR). The
GPS signals are also slightly stronger than the WAAS signals from the geo-stationary satellites
(http://waas.stanford.edu/tour.html).
NAVCEN (www.navcen.uscg.gov/dgps/default.htm) operates the U.S. Coast Guard Maritime
Differential GPS (DGPS) Service, consisting of two control centers and over sixty remote broadcast
sites, and the developing Nationwide DGPS Service (NDGPS). The Service broadcasts correction
signals on marine radio beacon frequencies to improve the accuracy of and integrity to GPS-derived
positions. The U.S. Coast Guard DGPS Service guarantees 10 m accuracy, while typical positional
error of 1 to 3 m is achieved. October 2007 coverage is presented in Figure 9.13.
9.7.2.2
dGpS Message format
DGPS receivers support the major international standards for GPS and DGPS (RINEX, RTCM,
and NMEA). RINEX (i.e., the Receiver Independent Exchange Format) is an ASCII format, estab-
lished for an easy exchange of the GPS data collected by different GPS receivers. The format has
been optimized for minimum space requirements independent from the number of different obser-
vation types of a specific receiver. Three primary types of RINEX files exist: observation data
file, navigation message file, and meteorological data file
(http://gps.wva.net/html.common/rinex.
html#rinex:_the_receiver_independent_exchange_format_version_2.10).
The most widely used international standards for DGPS message format were developed by the
Radio Technical Commission for Maritime Services (RTCM), a committee that governs standards for
passing data between different equipment used in the Marine Electronics industry. The RTCM Spe-
cial Committee No. 104 established “Recommended Standards for Differential Navstar GPS Service,”
dated January 3, 1994, referred to as RTCM SC104, which is a standard format for sending differential
