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A Global Positioning (GPS) system is a radio-based, real-time navigation and positioning system with a global coverage that provides the exact position on the earth surface in almost any meteorological condition. It is formed by three components: satellite constellation, control centers, and a receiver.
There are various Global Navigation Satellite Systems (GNSS): NAVSTAR, GLONASS, and EG-NOS, precursor of the European Galileo. However, the system that coined the term and first developed the concept was NAVSTAR (NAVigation Satellite Time and Ranging). It was developed by the Department of Defense of the United States in 1978 and became completely operative in 1994. It was initially conceived for the military, though it was eventually opened for civilian use. Its constellation of 24 satellites, which hold a highly precise atomic clock, orbit at 12,000 miles (20,000 kilometers) above the Earth in six orbital planes with four satellites, each at an inclination of 55 degrees, making two complete orbits in less than 24 hours.
The geostationary satellites of the system emit a continuous signal with information on the exact time it has been sent out, which allows a receiver to calculate the distance to each satellite in real-time by calculating the time invested to arrive. When at least three satellites of the constellation are simultaneously visible, the receiver calculates (by triangulation) its local position in geodesic coordinates (x, y, z) in the WGS84 Geodetic datum; with the distance to a fourth satellite height or vertical position that can be further calculated.
The control segment is a network of five stations around the world that permanently monitor, estimate the satellites orbit and position (or ephemeredes) the status of their clocks and the conditions in the ionosphere that affect signal transmission. This operation is coordinated by the Master Control Station at Colorado Springs, Colorado.
The GPS satellite transmits the signal at two frequencies: L1 (1575.42 Megahertz), and L2 (1227.60 Megahertz) or carrier frequencies. The signal is altered as it travels through the atmosphere, interfering in system accuracy. An L1/L2 receiver can compare both signals and correct the alteration. In nondifferential mode, both L1 and L1/L2 receivers will obtain the same horizontal accuracy; while in differential mode, L1/L2 receivers will perform better since they can determine and correct the atmospheric-, ionospheric-, and tropospheric-propagation error. L1 and L2 are modulated by the Precise/Protected code (or “P code”) and the Course Acquisition code (or “C/A code”). L1 frequency carries the C/A code used for the standard positioning service (SPS), a service available without restrictions, and the P code used for the precise positioning service (PPS), a restricted service, while L2 frequency only carries P code.
The receiver holds a clock that calculates the difference between the time received from the satellite and the actual time, which allows it to calculate the distance to it, the positional sphere. The finest measurement is obtained when the satellites are 120 degrees to each other and the fourth in the vertical.
Signal quality was degraded by introducing digital noise to prevent military use by foreign countries with two security systems, Selective Availability (SA) and Antispoofing. SA is a process that limits accuracy to the SPS by changing information on satellite orbit data and/or clock frequency. SA was deactivated on May 1, 2000. Antispoofing (AS) is the encryption process of the P code, used for precise positioning, replaced by the Y code.
The Differential GPS (DGPS) performs a differential correction of the received signal to improve GPS accuracy. A base station with a known position tracks the satellites and receives the signal at the same time as the mobile receiver, calculates the error for each satellite, and allows the correction of the unknown locations collected. The correction can be accomplished in real-time, sending the information to the receiver or in post-processing, to obtain centimeter accuracy. Two modes are available, as single station or Local Area DGPS (LADGPS), and a network of stations or Wide Area DGPS (WADGPS).
- Ahmed El-Rabbany, Introduction to GPS: The Global Positioning System (Artech House, 2002);
- Bernhard Hofmann-Wellenhof, Herbert Lichtenegger, and James Collins, Global Positioning System: Theory and Practice (Springer, 2001);
- Elliott D. Kaplan and Christopher Hegarty, , Understanding GPS: Principles and Applications (Artech House, 2006);
- Bradford W. Parkinson and James J. Spilker, , Global Positioning System: Theory and Applications (AIAA, 1996).