GPS

GPS stands for Global Positioning System. GPS uses satellite technology to enable a terrestrial terminal to determine its position on the Earth in latitude and longitude.

Navigation Using GPS

GPS receivers ascertain their position and height by measuring the signals from three or more satellites simultaneously and determining their position using the timing of these signals.

GPS operates using trilateration. Trilateration is the process of determining the position of an unknown point by measuring the lengths of the sides of an imaginary triangle between the unknown point and two or more known points.

In the GPS system, the two known points are provided by two GPS satellites. These satellites constantly transmit an identifying signal.

The GPS receiver measures the distance to each GPS satellite by measuring the time each signal took to travel between the GPS satellite and the GPS receiver.

The formula for this is:

Distance = Velocity * Time

Velocity of the GPS signal is the speed of light, approximately 300,000 Km/s.

GPS transmissions occur on two frequencies: 1575.42 and 1227.60 Mhz. Both of these frequencies are within the L Band.

GPS History

GPS was originally developed for the U.S. military, but is now provided as a public service for people all over the world by the U.S. government.

Deployment of the GPS system began on 22 February 1978 with the launch of the first Block I Navstar GPS satellite. Initial Operating Capability was declared in December 1993 with 24 operational GPS satellites in orbit. Full Operational Capability was declared in June of 1995.

GPS was developed by the U.S. military to help soldiers locate their positions. Civilian access to the GPS system was guaranteed by President Reagan as a response to the Soviet Union's shooting down of Korean Airline Flight KAL-007. President Reagan hoped that GPS technology would help to prevent such a tragedy from happening again.

GPS Architecture

The GPS system is divided into three segments:

The Space Segment
The Control Segment
The User Segment

The Space Segment

GPS uses more than two dozen operational satellites, with an additional three satellites in orbit as redundant backup.

GPS uses NAVSTAR satellites manufactured by Rockwell International. Each NAVSTAR satellite is approximately 5 meters wide (with solar panels extended) and weighs approximately 900Kg.

GPS satellites orbit the earth at an altitude of approximately 20,200Km.

Each GPS satellite has an orbital period of 11 hours and 58 minutes. This means that each GPS satellite orbits the Earth twice each day. Highly accurate atomic clocks are installed on these satellites, operating at a fundamental frequency of 10.23MHz each. With the help of these clocks, signals are generated from the satellite, to be broadcast to the Earth.

These twenty-four satellites orbit in six orbital planes, or paths. This means that four GPS satellites operate in each orbital plane.

Each of these six orbital planes is spaced sixty degrees apart. All of these orbital planes are inclined fifty-five degrees from the Equator.

The Control Segment

The Control Segment is comprised of a master control station, 5 monitor stations and 4 ground antennas. All of these are strategically located along the Equator.

The Master Control Station (MCS) of the GPS system is operated at Schriever Air Force Base in Colorado Springs, Colorado. The United States Air Force maintains redundant Master Control Stations in Rockville, Maryland and Sunnyvale, California.

The Air Force also maintains monitoring stations in Colorado Springs, Hawaii, The Ascension Islands, Diego Garcia, and Kwajalein.

Key Functions of the Control Segment

The Control Segment keeps track of the orbiting position of the GPS satellites, calibrating and synchronizing their clocks.

It also predicts the path of each satellite for the following 24 hours, and uploads this information to each satellite.

Communications with the space segment are conducted through ground antennas in the Ascension Islands, Diego Garcia, and Kwajalein. The satellite signals are read here and the measurements sent to the Master Control Station in Colorado. The signals are processed there to determine any errors, and sent back to the four monitor stations with ground antennas, after which the information is uploaded back to the satellites.