GPS - Global
Positioning System
GPS - (Global Positioning System) is a constellation of satellites that
orbit the earth twice a day, transmitting precise time and position (latitude, longitude
and altitude) information. With a GPS receiver, users can determine their location
anywhere on the Earth. Position and navigation information is vital to a broad range of
professional and personal activities, including hiking, hunting, camping, boating,
surveying, aviation, national defense, vehicle tracking, navigation and more.
The complete system consists of 24 satellites orbiting about 12,000 miles above the
Earth, and five ground stations to monitor and manage the satellite constellation. These
satellites provide 24-hour-a-day coverage for both two-and three- dimensional positioning
anywhere on Earth.
Development of the $10 billion GPS satellite navigation system was begun in the 1970s
by the US Department of Defense, which continues to manage the system, to provide
continuous, worldwide positioning and navigation data to US military forces around the
globe. However, GPS has an even broader civilian, commercial application. To meet these
needs, GPS offers two levels of service, one for civilian access and the second encrypted
for exclusive military use.
The GPS signals are available to an unlimited number of users simultaneously, and there
is no charge for using the GPS Satellites either.
The basis of GPS technology is precise time and position information. Using atomic
clocks and location data, each satellite continuously broadcast the time and its position.
A GPS receiver receives these signals, listening to three or more satellites at once, to
determine the users position on earth.
By measuring the time interval between the transmission and the reception of a
satellite signal, the GPS receiver calculates the user and each satellite. Using the
distance measurements of at least three satellites in an algorithm computation, the GPS
receiver arrives at an accurate position fix. Information must be received from three
satellites in order to obtain two-dimensional fixes(latitude and longitude), and four
satellites are required for three-dimensional positioning (latitude, longitude and
altitude).
Under normal conditions, the GPS signal will provide a civilian user an accuracy of
better than 15 meters (50 feet). However, using a technique called differential GPS
(DGPS), the user can increase the overall accuracy of the GPS receiver to approximately
1-3 meters. With DGPS, one GPS receiver unit is placed in a known location and the
position information from that receiver is used to calculate correction in the position
data transmitted to other GPS receivers in the area. The resulting real-time accuracy is
in the 10 foot range. Sub-meter accuracy can be obtained by using DGPS and post-processing
calculations in static positioning.
