The first artificial satellite to be put into orbit was the Russian Sputnik 1, which was launched in 1957 to demonstrate the viability of artificial satellites. The transmitted radio signal of Sputnik 1 was monitored by researchers at the Applied Physics Laboratory (APL) of the John Hopkins University, who noticed that the satellite signal was Doppler (or frequency) shifted as result of the relative satellite motion. Following this observation, Dr. Frank T. McClure (of APL) realized that if the satellite orbit was known, a user's location could be determined based on the Doppler shift measurements. Using this concept, the APL proposed (and then developed) an innovative satellite Doppler navigation system [1, 2]. In April 1960, the first prototype satellite was successfully launched to examine the operational feasibility of the Doppler system, which was then called the Navy Navigation Satellite System (also known as Transit). Shortly after the development of the Transit system, a similar system, known as Cicada, was developed by the USSR [2]. The Transit launch program was ended in 1988, while the Transit system was retired in 1996 [1].

The Global Positioning System (GPS) is a satellite-based navigation system that was developed by the U.S. Department of Defense (DoD) in the early 1970s as the next generation replacement to the Transit system. Initially, GPS was developed as a military system to fulfill U.S. military needs. However, it was later made available to civilians, and is now a dual-use system that can be accessed by both military and civilian...