Introduction - Understanding GPS: Principles and Applications - page 10

SBAS is still under development. The BNTS provides limited coverage and only sup-
ports users in and around China. The BNTS should be operational through the end
of the decade. In the long term, the Chinese plan is to deploy a regional or worldwide
navigation constellation of 14-30 satellites under the BeiDou-2 program. The Chi-
nese did not plan to finalize the design for BeiDou-2 until sometime in 2005 [14, 15].
Section 11.2 provides further details about BeiDou.

1.8 Augmentations

Augmentations are available to enhance stand-alone GPS performance. These can
be space-based, such as a geostationary satellite overlay service that provides satel-
lite signals to enhance accuracy, availability, and integrity, or they can be ground-
based, as in a network that assists embedded GPS receivers in cellular telephones to
compute a rapid position fix. Other forms of augmentations make use of inertial
sensors for added robustness in the presence of interference. Inertial sensors are also
used in combination with wheel sensors and magnetic compass inputs to provide
vehicle navigation when the satellite signals are blocked in urban canyons (i.e., city
streets surrounded by tall buildings). GPS receiver and sensor measurements are
usually integrated by the use of a Kalman filter. (Chapter 9 provides in-depth treat-
ment of inertial sensor integration and assisted-GPS network methods.)
Some applications, such as precision farming, aircraft precision approach, and
harbor navigation, require far more accuracy than that provided by stand-alone GPS.
They may also require integrity warning notifications and other data. These applica-
tions utilize a technique that dramatically improves stand-alone system performance,
referred to as differential GPS (DGPS). DGPS is a method of improving the position-
ing or timing performance of GPS by using one or more reference stations at known
locations, each equipped with at least one GPS receiver to provide accuracy enhance-
ment, integrity, or other data to user receivers via a data link. There are several types
of DGPS techniques, and, depending on the application, the user can obtain accura-
cies ranging from meters to millimeters. Some DGPS systems provide service over a
local area (10-100 km) from a single reference station, while others service an entire
continent. The European Geostationary Navigation Overlay Service (EGNOS) and
U.S. Wide Area Augmentation System (WAAS) are examples of wide area DGPS ser-
vices. EGNOS coverage is shown in Figure 1.7. Chapter 8 describes the underlying
concepts of DGPS and details a number of operational and planned DGPS systems.

1.9 Markets and Applications

The first publication of this topic referred to GPS as an enabling technology. It has
truly become that but it is also a ubiquitous technology. Technology trends in com-
ponent miniaturization and large-scale manufacturing have led to a proliferation of
low-cost GPS receiver components. GPS receivers are embedded in many of the
items we use in our daily lives. These items include cellular telephones, personal digi-
tal assistants (PDAs), and automobiles. Applications range from the provision of a
reference time source for synchronizing computer networks to guidance of robotic