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GPS navigation: The GPS Satellite System FAQ

GPS navigation systems appear to be the devices of the moment and everyone who is anyone owns a satellite navigation system in some shape or form. But what precisely is GPS technology and how does it function?


What is GPS?

GPS stands for "Global Positioning System", an advanced navigation system initially developed for use by the US military. GPS systems lock on to transmissions emitted by satellites far above the earth's surface. GPS receivers then use the data supplied by the satellites to determine their own positions either on, or in the case of in-flight systems, above the earth's surface.

How did GPS navigational technology come about?

GPS navigation was originally a military undertaking developed to improve the accuracy of target specific equipment. The first GPS satellite was launched by the American government in 1978 and today there are over two dozen GPS satellites orbiting the earth. In the 1980s the American government made GPS technology available for commercial use. Originally, GPS navigation was used predominantly for marine navigation, but air and land-based systems soon followed. Today GPS systems have taken on a number of different forms including marine units, in-car units, portable GPS units and handheld systems.

What constitutes the GPS satellite system?

The GPS satellite system is made up of 24 satellites that orbit the earth 24 hours a day, seven days a week. Each satellite travels at approximately 7000 miles per hour and orbits the earth twice each day. Specific data is transmitted by the GPS satellites to earth via low frequency radio signals. The first GPS satellite series, known as NAVSTAR, was developed and launched by the American Department of Defense. Although there are currently other GPS satellite constellations circling the earth, the NAVSTAR constellation remains the only series which transmits signals for commercial use.

What powers a GPS satellite?

GPS satellites function primarily on solar energy. In the event of a solar eclipse they are equipped with backup batteries.

How many satellites signals do GPS receivers need to function?

A GPS system is capable of operating on either a two-dimensional or a three-dimensional level. A two-dimensional system is capable of determining position based on longitude and latitude only, while a three-dimensional system will be able to establish longitudinal, latitudinal and altitudinal location. In order to determine location on a two-dimensional scale, receivers will need to lock onto at least three GPS satellites. Three-dimensional positioning requires data from a minimum of four satellites. In-car and marine GPS units usually function on a three-dimensional scale and are able to lock onto between four and twelve satellites. The majority of handheld units will only be capable of locking on to three satellite signals.

What causes satellite signals to weaken or disappear?

GPS units are not foolproof, and inaccurate readings do sometimes occur. Incorrect readings are usually the result of weak satellite transmissions that can be caused by a number of factors including: selective availability (the military purposely degrades satellite systems when in the event of threats to National security); atmospheric delays (signals slow down as they pass through the earth's atmosphere due to climatic factors); poor geometry (satellites are positioned too closely together and thus triangulation calculations are faulty) and

signal multipath (signals are blocked by large objects and reflected in a different direction, this results in signal delays that cause inaccurate readings.