Global Positioning System (GPS)^1,2

GPS devices, such as those sold under the brands Garmin, TomTom, and Magellan,  are becoming increasingly popular.  But how do they work? GPS, which has revolutionized navigation over last part of the 20^th century,  is based on a network of orbiting satellites above the earth that transmit signals to receivers via computers to generate accurate data for navigation.

The satellite component of the GPS system consists of 24 Navstar satellites that orbit the earth at 17,700 km above the earth in such a way that they circle the earth exactly 2x per day and that for a specific point on earth, 5 to 8 of them are above the horizon at any given time.  On board each Navstar satellite are an atomic clock, a computer, and a radio transmitter to broadcast signals back to earth at 2 frequencies: 1575 megahertz (mhz) and 1228 mhz. 

The other main element of the GPS system is the receiver unit, which also has a radio receiver, an atomic clock, and a computer. The receiver unit can range from a hand-held computer to a more complex one such as those found in the navigational systems of a ship or an airplane. 

Even before the Space Age, the principle that drives the GPS had already been in use. During World War II, the radio navigation system called LORAN (long-range navigation) was used and became the ancestor of later-generation instrument-based navigation systems. When the first satellites were launched in the late 1950’s, it became apparent that if the orbit of a satellite was accurately determined, a ground-based receiver would be able to calculate its own position based on a measurement of the Doppler shift in the satellite’s signal.

Other terms to remember related to GPS include SPS (standard positioning system), which is the civilian version, and the PPS (precise positioning system) which is restricted to military use.  GPS is a perfect example of military technology that has been made available to and benefited civilians.  Finally, prudence and common sense should be exercised when using GPS since, as with other modern marvels, these gadgets are not completely error-free or fail-safe. GPS devices are a great help and can be relied on – but please do not OVERrely on them!

1. Sparrow G. In: The Cutting Edge: An Encyclopedia of Advanced Technologies. 2000:121-124. Oxford University Press. New York NY
2. www.cuttingedge.oup.com