corrections to the GPS broadcast
ephemerides, encoded, and are provided over the Internet to authorized
WAAS stands for Wide Area
Augmentation System. It's a system
of satellites and ground stations that
provide GPS signal corrections, giving you even better position accuracy, an average of up to five times
better accuracy. A WAAS-capable
receiver can give a position accuracy of better than three meters, 95
percent of the time. WAAS corrects
for GPS signal errors caused by
ionospheric disturbances, timing,
and satellite orbit errors, and it provides vital integrity information
regarding the health of each GPS
WAAS consists of approximately 25 ground reference stations positioned across the US that monitor
GPS satellite data. Two master stations, located on either coast, collect
data from the reference stations and
create a GPS correction message.
This correction accounts for GPS
satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then
broadcast through one of two geostationary satellites, or satellites with
a fixed position over the equator.
The information is compatible with
the basic GPS signal structure,
which means any WAAS-enabled
GPS receiver can read the signal.
Currently, WAAS satellite coverage is only available in North
America. There are no
ground reference stations in
South America, so even
though GPS users there can
receive WAAS, the signal
has not been corrected and
thus would not improve the
accuracy of their unit. For
some users in the US, the
position of the satellites over
the equator makes it difficult
to receive the signals when
trees or mountains obstruct
the view of the horizon.
WAAS signal reception is
ideal for open land and marine applications. WAAS provides extended
coverage both inland and offshore
compared to the land-based DGPS
(differential GPS) system. Another
benefit of WAAS is that it does not
require additional receiving equipment, while DGPS does.
Other governments are developing similar satellite-based differential
systems. In Asia, it's the Japanese
Multi-Functional Satellite Augmentation System (MSAS), while
Europe has the Euro Geostationary
Navigation Overlay Service (EGNOS).
Eventually, GPS users around the
world will have access to precise position data using these and other compatible systems.
GPS has a variety of applications on land, at sea, and in the air.
Basically, GPS is usable everywhere
except where it's impossible to
receive the signal such as inside
most buildings, in caves and other
subterranean locations, and underwater. The most common airborne
applications are for navigation by
general aviation and commercial aircraft. At sea, GPS is also typically
used for navigation by recreational
boaters, commercial fishermen, and
professional mariners. Land-based
applications are more diverse. The
scientific community uses GPS for its
precision timing capability and position information.
Surveyors use GPS for an
increasing portion of their work. GPS
offers cost savings by drastically
NUTS & VOLTS
reducing set-up time at the survey site
and providing incredible accuracy.
Basic survey units, costing thousands
of dollars, can offer accuracies down
to one meter. More expensive systems are available that can provide
accuracies to within a centimeter.
Recreational uses of GPS are
almost as varied as the number of
recreational sports available. GPS is
popular among hikers, hunters,
snowmobilers, mountain bikers, and
cross-country skiers, just to name a
few. Anyone who needs to keep
track of where he or she is, to find his
or her way to a specified location, or
know what direction and how fast he
or she is going can utilize the benefits
of the global positioning system.
GPS is now commonplace in
automobiles, as well. Some basic
systems are in place and provide
emergency roadside assistance at
the push of a button (by transmitting
your current position to a dispatch
center). More sophisticated systems
show your position on a street map.
Currently these systems allow a driver to keep track of where he or she
is, and suggest the best route to follow to reach a designated location.
GPS has become important for
nearly all military operations and
weapons systems. In addition, it is
used on satellites to obtain highly
accurate orbit data and to control
spacecraft orientation. Although the
GPS satellite constellation was completed only recently, it has already
proved to be a most valuable aid to
US military forces. Picture the desert,
with its wide, featureless expanses of
sand. The terrain looks much the
same for miles. Without a reliable navigation system, US
forces could not have performed the maneuvers of
Operation Desert Storm.
With GPS, the soldiers were
able to go places and
maneuver in sandstorms or
at night when even the
troops who lived there couldn't. Initially, more than 1,000
receivers were purchased for
their use. The demand was
so great that, before the end
of the conflict, more than
Figure 1. Internet based global differential GPS.