The Global Positioning System (GPS) is a satellite-based navigation system owned by the U.S. Space Force and currently operated by Mission Delta 31 (the unit responsible for navigation warfare). GPS is one of the global navigation satellite systems (GNSS) providing geolocation and time information to a GPS receiver anywhere on (or near) the Earth, as long as the signal quality permits. The system does not require the user to transmit any data. It provides critical positioning capabilities to military, civil, and commercial users around the world. Despite being owned and operated by the United States government, GPS services are freely accessible by anyone with a relevant GPS receiver.
GPS Historical Development
Historically, the GPS project was launched in the U.S. in 1973 to overcome the limitations of the then-existing navigation systems by combining various ideas from several predecessors dating back to 1960s. The U.S. Department of Defense developed the system, which originally used 24 satellites, only for use by the United States military, and became fully operational in 1993. Civilian use was allowed starting from the 1980s. Currently, GPS usage is quite broad and it is estimated that nearly 7 billion active devices are in use.
Constellation and Frequency Utilization
GPS system has a constellation of more than 24 (currently 31) satellites at the so-called MEO (Mid-Earth Orbit) level flying 20,350 km above the surface of the earth. Each satellite circles the planet twice a day in one of six orbits to provide continuous, worldwide coverage. Each GPS satellite broadcasts radio signals from its on-board atomic clocks, providing its location, status, and precise time (denoted by t1). These relevant signals travel through space at the speed of light (denoted by c), more than 299,792 km/second. Whenever a GPS device receives these radio signals, notes their exact time of arrival (denoted by t2), and uses these to calculate its distance from each satellite in view. Once a GPS device knows its distance from at least four satellites, it can use geometry to determine its location on Earth in three dimensions.
The original GPS design contains two ranging codes inside the satellite-transmitted signals:
- the Coarse/Acquisition (C/A) code, which is freely available to the public, and
- the Precision (P) code, which is usually encrypted and referred to as the P(Y) code, generally reserved for military applications.
The signals must be modulated onto a carrier wave to be sent from the satellite to the receiver. Originally, for the GPS design, two frequencies were utilized:
- one at 1575.42 MHz (10.23 MHz × 154) called L1, which is the primary frequency used by GPS for navigation and timing. The L1 frequency is used to transmit the C/A and P(Y) code signals.
- and a second at 1227.60 MHz (10.23 MHz × 120), called L2, which is used to transmit the P(Y) code signal, an encrypted military code only available to authorized users. The L2 frequency is also used to transmit the L2C signal, which is a civil signal that is available to all users and is more precise than the C/A code and L1 frequency.
Later, additional frequencies have also been utilized. Among those, the so-called L5 (1176.45 MHz = 10.23 MHz × 115) is quite important for navigation and timing purposes. This frequency is used to transmit a civil signal that is available to all users; which is intended for high precision navigation and timing applications; and it’s more resistant to interference and multipath.
Additionally, GPS also uses the L3, L4 and L6 frequencies, but these are not used for navigation and timing, they are used for other purposes such as the Nuclear Detonation Detection System (NDS), experimental refraction correction studies and other military applications.
Note that all the utilized frequency values are determined as multiples of 10.23 MHz due to the usage of Rubidium-based oscillators, which have been quite stable and cost-effective during the design phase of the GPS system. The aforementioned standardized frequency relationships ensure that the system’s different signals are coherently tied together, all originating from a highly stable oscillator (like an atomic clock) running at the core 10.23 MHz rate within the relevant satellites.
Conclusion
Due to its free, open, and dependable nature, GPS has become an essential element of the global information infrastructure and led to development of numerous applications (requiring accurate position and time information) as regards every aspect of modern life. Currently, GPS technology has been incorporated inside everything from cell phones to wristwatches, shipping containers to ATMs, etc. Most navigation applications rely on the position information provided by the GPS system. Additionally, major communications networks, banking systems, financial markets, and power grids depend heavily on GPS for precise time synchronization.
