GPS design is based partly on ground-based radio navigation systems developed in the early 1940s that were used in World War II. These systems were named LORAN and Decca Navigator and were focused on knowing where the enemy was so they could either attack or retreat depending on the size of the forces.

Additional inspiration for modern day GPS systems came when Sputnik was launched by the Soviet Union in 1957. A team of scientists monitored Sputniks radio transmissions and discovered that because of the Doppler Effect, the frequency of the signal being transmitted was high as the satellite approached and lower as it moved away.

The Doppler Effect is the change in frequency and wave length of a wave as it is perceived by an observer moving relative to the source of the waves. This team of scientists that was observing Sputniks radio transmissions soon realized that since they knew their exact location on the globe, they could pin point where the satellite was along its orbit by measuring the Doppler distortion. This was groundbreaking and very exciting for the military at the time.

The United States Navy used the first satellite navigation system called Transit. It was first successfully test in 1960 and was quite mind-boggling for everyone in the military. When the Navy tested Transit, they did so hoping for some quite specific results. Using a constellation of five satellites, they found that the system could provide a navigational fix approximately once per hour.

In 1967, the Navy developed the Timation satellite which proved the ability to place accurate clocks in space. This is a technology that the GPS system relies on. In the 1970s, the groundbased Omega Navigation System, based on signal phase comparison, became the first worldwide radio navigation system.

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