The GPS satellite system for navigation was designed in the 1970s by the U.S. Department of Defense. It started out as a global navigation system for military land-based and air vehicles, with some limited use for the civilian population. But today, the tables have turned - there are more GPS receivers used by civilians than by the military!

Back in the 70's, the military employed a variety of navigation systems such as LORAN (acronym for Long Range Radio Navigation), VOR (acronym for VHF Omni-directional Radio) and OMEGA, and a need was felt for a more accurate navigational aid which would be functional all day, all year and everywhere on Earth, irrespective of day or night or weather conditions. The military's needs were to navigate its airplanes, tanks and personnel reliably. At that time, the U.S. Navy and the Air Force were in the pursuit of developing systems that would meet such requirements with programs such as Transit satellite system, Timation satellite system and Project 621B. The Department of Defense eventually formed a Joint Program Office (JPO), which consolidated the developments to date in to what we know today as the Global Positioning System (GPS).

The first GPS satellite was launched in 1974, and the GPS satellite constellation was declared fully operational in 1994. The satellites were/are designed and built by Rockwell International, Boeing and Lockheed-Martin.

Though GPS was designed by the DOD for the military, they did include some capabilities that could be useful for the civilian community. The GPS satellites transmit encrypted signals, which only the military has access to. The satellites also transmit non-encrypted signals, which can be detected in both military and civilian GPS receivers. The civilian signal is referred to as the C/A code signal (Coarse Acquisition).

A key parameter that describes GPS capability is positioning accuracy, which simply put, is the accuracy of the GPS receiver user's position computed in the GPS receiver. GPS system developers initially did not expect the civilian signal to provide position accuracy comparable with those obtained by the military signals. However, the designers soon realized that the civilian GPS receivers could employ several newly developed algorithms that could improve the position accuracy obtained in civilian GPS receivers dramatically. In order to limit the positional accuracy of civilian GPS receivers, they implemented degradations of the civilian GPS signal before being transmitted by the GPS satellites. The degradation capability is referred to as Selective Availability (SA). SA limited position accuracy in GPS receivers to ~50 meters. However, by late 1990s, President Clinton having realized the benefit of GPS to the civilian community, ordered SA degradation to be removed. With the removal of SA, the doors opened up for civilian users to use GPS for automotive tracking for driving assistance, fleet management, .......

For a more comprehensive knowledge of GPS today, refer to websites such as www.gadgetknowledge.com/gps1.html