The concept of GPS originated from the space race during the Cold War, when scientists tracked the position of satellites through changes in the "Doppler effect" of radio signals. The US Navy conducted satellite navigation tests in the mid-1960s to provide positioning services for US submarines equipped with nuclear missiles. Through six polar satellites, submarines are able to capture Doppler signals from satellites and determine their position within minutes, which is the precursor of GPS.

　　In the early 1970s, the US Department of Defense wanted to have a powerful and stable satellite navigation system, and based on the previous ideas of naval scientists, decided to use satellites to build the navigation system. Subsequently, in 1978, the United States Department of Defense launched the first navigation satellite with timing and ranging functions. The entire GPS system 2 was deployed as a constellation in 1994 and put into full operation and service.

　　Today, there are 32 GPS satellites in operation, covering the Earth and affecting every aspect of our lives. GPS currently provides two levels of services, namely the Standard Positioning Service (SPS) using coarse acquisition (C/A) codes on the L1 frequency, and the Precision Positioning Service (PPS) using P (Y) codes on both L1 and L2 frequencies. The use of precision positioning services is strictly restricted by the United States Air Force, federal agencies, and some allied armed forces and governments. All users worldwide can continuously use standard positioning services without charging any fees to direct users.

　　With the advancement of technology, the US government is also modernizing and updating its GPS satellite system. Currently, we are attempting to add two civilian signal channels to the GPS signal, 1) adding the second civilian signal L2C of 1227.60MHz to the GPSL2 frequency to meet the needs of high-precision scientific applications; 2) Add a third civilian signal L5 with a frequency of 1176.45MHz to meet the critical life and production safety needs of civil aviation and other industries. In addition, it is also planned to add the fourth civilian signal L1C to the GPS signal to achieve interoperability with other GNSS systems, such as Galileo from the European Union and GLONASS from Russia. This fourth signal will be added to the GPSBlock III satellite and is currently under development. GPS will be the main wireless navigation system provided by the US federal government and will be expanded and improved to meet future requirements for navigation positioning accuracy, coverage, availability, continuity, and integrity.

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