GNSS Markets and Applications - Understanding GPS: Principles and Applications - page 651

Stationary users can derive their own differential corrections and transmit them
to specific locations using almost any means of communications, from the Internet
to cellular to VLF-HF-microwave radios to satellite links. It makes better sense,
however, to share such a system with other users, and there are market opportuni-
ties in providing such signals. In addition to the freely provided satellite transmitted
corrections of the FAA's WAAS, the U.S. Coast Guard provides correction signals,
broadcast over an existing network of nondirectional beacon transmitters around
the coast of the United States and in the Great Lakes. This NDGPS system is being
expanded to cover all of the U.S. landmass so it can be used by vessels in all inland
waterways and by railroads for positive train control. The latter application alone is
expected to accrue $3 billion/year in economic benefits for the railroad industry and
its customers. NDGPS has many U.S. government participants; notably, the U.S.
Army Corps of Engineers in conjunction with the Coast Guard provides similar cov-
erage for the Mississippi and Ohio River valleys, and other countries' authorities
are implementing similar systems. These broadcasts are provided free of charge, but
require the purchase of specialized receivers and demodulators to decode the
correction signals, sent at 283-325 kHz.
12.6.1 Precision Approach Aircraft Landing Systems
Most instrument approaches carried out by commercial air carriers are precision
approaches.
Unlike NPA, these procedures give glideslope guidance to the aircraft on
approach. The lack of signal integrity precludes the use of unaided GPS for demand-
ing aviation applications. These applications require the use of either code differen-
tial or kinematic carrier-phase tracking techniques. Precision landing systems
require not only better integrity (warnings of system failure or inaccuracy within 6
seconds or less) but also better accuracy than is provided by the basic GPS service.
The FAA's WAAS provides this warning, and sufficient accuracy to perform close to
category I precision landing requirements. This allows about 90% of the airline
approaches currently performed to use a GPS approach augmented in this way. Cat-
egory II and III approaches, involving lower weather minima, also require improved
accuracy and integrity warnings, which will be provided by airport-based differen-
tial stations broadcasting GPS corrections directly to the aircraft on approach (i.e.,
LAAS). It is expected that when GPS III and GALILEO are deployed, there will be
both an improvement in overall accuracy and additional integrity.
12.6.2 Other Differential Systems
Surveyors, cartographic and hydrographic agencies, as well as a host of other users
require accuracy better than that available from GPS's SPS. These users can either
set up their own BSs and datalink facilities or purchase correction signals from a
supplier or cooperative of similar users. Many services are presently available from
which one can purchase or otherwise obtain differential corrections. Some of these
services operate in real time, broadcasting their signals to users, and some provide
time-tagged data with which one can correct field data after gathering it. This is
known as postprocessing and is common in surveying applications. State survey and