If you've seen GPS equipment, you've seen only a part of what is GPS.
Because GPS (Global Positioning System) is a satellite-based navigational
network that makes it possible to find your position anywhere on earth. Commercially
available GPS receivers are capable of accuracy to within a few meters or yards.
And with GPS devices at a relatively low cost, accurate navigation or positioning
is available to most anyone.
The GPS Satellite
The heart of the GPS is a group of 24 satellites in unique orbits
approximately 11,000 miles above the earth. The satellites are maintained by
the U.S. Department of Defense (DoD) which conceived of the system
in the 1970s. GPS was created to provide highly accurate positioning
for submarines before they fired their missiles. The first satellite was
launched in 1978 and replacements are launched periodically as needed.
The satellites are monitored by five ground stations to assure they maintain
their precise positioning.
The Satellite's GPS Information
Each GPS satellite has a ultra-high-frequency radio transmitter
that sends out a unique stream of information. A GPS receiver uses the signals
from three or more satellites to determine its location by means of triangulation.
The satellite signal has three types of information; a pseudorandom code,
ephemeris data and almanac data. The pseudorandom code identifies
which satellite is sending the information. Ephemeris data consists of
the current date and time as well as whether the satellite is functioning
properly. And the almanac data relays the expected position of all the
satellites during the day.
Satellites emit two signals, one for military use and a less accurate
signal for civilian use. When the military first allowed civilians access
to GPS in the 1980's it didn't anticipate how accurate commercial GPS
receivers would be. So, out of security concerns, the DoD created "Selective
Availability (S/A)" which randomly decreased the civilian signal's accuracy.
However, other ways to maintain security were developed and Selective Availability
was stopped on May 2, 2000. Current uncorrected GPS accuracy using the civilian
signal is less than 15 meters or 49 feet.
The GPS position calculations are based on comparing the reading of
a clock in a GPS satellite with one in a GPS receiver. Ideally, the
comparison is made almost instantaneously, since radio waves travel
at the speed of light. But since the signal passes through charged particles
and water vapor in the atmosphere, it slows by a variable amount.
And anything which slows the speed of the signal affects the position
calculations. The signal also takes longer to travel if it bounces off a building
or other obstacle (multipath error) before reaching the GPS receiver.
Further errors can come from a satellite slightly out of position, an inaccurate
receiver clock or other radio signals causing interference. Compensation is built
into the system to help correct for these possible errors, but some inaccuracy
will always be present.
Differential GPS is a simple method to improve GPS accuracy to within
a few meters or yards. The U.S. Coast Guard created it as a response to the
accuracy loss caused by Selective Availability. Differential GPS relies on the
use of two GPS receivers. One is permanently located at a precisely
determined spot. This static receiver can compare its known position with
the position calculated from the GPS satellite signals. Signal timing error
corrections can then be transmitted to the mobile GPS receiver. Differential
GPS can eliminate most accuracy problems except multipath errors or signal
The two usual sources for corrective Differential GPS signals are the
free transmissions provided by the Coast Guard and the more widely
available private subscription services. A separate receiver is required
to pick up the correction signals and pass the information to the GPS receiver. And,
of course, the GPS receiver must be capable of processing the correction signal.
Although the accuracy possible with Differential GPS is attractive, for
most uses the gain is unnecessary.
Wide Area Augmentation System (WAAS)
The Wide Area Augmentation System provides the accuracy of Differential
GPS without the need for extra equipment or cost. WAAS was created by the
Federal Aviation Administration to provide precision guidance for aircraft
at smaller airports. It's currently only available in the United States and border
areas, although other countries are working on similar systems.
WAAS consists of two geo-stationary satellites, which operate on the
same frequency as GPS satellites, and twenty-five ground stations to provide
signal correction. Newer GPS receivers are WAAS enabled and can automatically
process the signal correction information. WAAS can reduce position errors to
less than three meters or 10 feet.