Origins of GPS

GPS as we know today was originally created for military applications. Originally named NAVSTAR and started around 1978 with the launch of the first satellite. Satellite navigation greatly increased the precision of any military unit's ability to locate themselves and their targets reducing the "fog of war." Using the navigation data from the satellites GPS can enhance a guided weapon's ability to hit targets autonomously without having to be guided by an operator.

One of the predecessor of the GPS were fixed radio stations on the earth in known locations. A master station pulsing out a signal and then slave stations spread far apart pulsing out a signal after a precise amount of time. Receivers could compare the delay between the reception of the master signal and the slave signals and determine a position relative to the slave stations.

Since needing fixed ground broadcasting installations would be a problem for the military. They developed and deployed the first navigation satellite known as Transit in the 1960's. To calculate a location a receiver would calculate the Doppler effect on the precise frequency the satellite broadcast to the frequency actually received. This would inform the receiver which side of the satellite the receiver was on and subsequent readings would determine a location relative to the known position of the satellite.

Modern satellites have a different way of determining your position. The signal they send includes the position of the satellite and time the signal was transmitted. For each satellite received the ground unit is placed on a sphere from the satellite and with additional satellites where the spheres intersect is the location of the receiver. For more information on how this works look here

Since 1978 GPS has been restricted to military receivers, requiring a decryption key to decipher the data in the signal broadcast from an orbiting satellite. In 1983 Soviet interceptor aircraft downed a civilian airliner Korean Air Lines flight 007 killing all 269 people on board. It had wandered in and out of restricted Soviet airspace along the coast of U.S.S.R. Flying from Alaska to Korea. The plane had wandered off course on auto pilot and the crew had not performed navigational checks. After the shooting in 1983 President Reagan declared that the GPS system would be available to civilians when completed.

On January 17, 1994 a complete constellation of 24 satellites was in orbit. Currently there are 31 satellites in orbit.