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GPS is a satellite-based technology developed for and funded by the US Department of Defense over more than four decades. Its primary purpose is military – positioning infantry brigades and armoured divisions in a theatre of war, guiding aircraft and munitions such as missiles and artillery shells to their targets, allowing military units operating behind enemy lines to navigate their way through hostile territory and provide accurate position-based intelligence back to headquarters.

Following the success of the United State’s GPS satellite constellation, other nations have started deploying their own satellite systems. Russia is close to completing the deployment in space of the GLONASS constellation, the European Union is working on a satellite system called Galileo and China has a constellation called BeiDou (a.k.a. Compass). Collectively these systems are known as Global Navigation Satellite Systems (GNSS), but in common parlance the technology is widely known as GPS.

GPS was purposely designed as to be global system. That is, a constellation of 24 orbiting GPS satellites broadcasts its positioning signal over the entire planet. When detected by any GPS receiver capable of acquiring signals from a minimum of 4 satellites, it provides latitude, longitude and altitude information, which can then be overlayed on a map or other geographic information system (GIS).

The USA’s GPS system is broadcast on two frequencies – a civilian signal known as L1 (1575.42 MHz), and a highly encrypted military signal, L2 (1227.6 MHz). The L1 signal generally used by civilian receivers initially had large position errors intentionally induced by the US military, whereas the L2 military signal remained accurate for the US and its allies. This intentional inaccuracy, known as Selective Availability, was scrapped by President Clinton in 2000 when it became clear that L1 users were finding new ways to remove the errors through terrestrial augmentation systems. The decision to scrap Selective Availability accelerated the rapid development of affordable GPS receivers for civilian use, e.g., in-car navigation systems and hand-held receivers.

Today, civilian applications of GPS, based on sales of GPS receivers, are part of a multi-billion dollar global industry – from professional-grade receivers for applications such as high precision surveying to inexpensive hand-held devices for social navigation activities – e.g., playing games such as geocaching, or finding the way to a children’s sports field on a Saturday morning.

In fact, GPS is now used in a very wide variety of applications – applications that couldn’t possibly have been imagined by the US Department of Defense’s engineers when they first worked on the development of the science and technology of GPS in the 1970s.

Other reference material

Basic explanation of GPS technology
GPS’ military heritage