The GPS (Global Positioning System) system is a global US-based satellite navigation system for high-precision positioning, navigation and timing. The Global Positioning System (GPS), the actual name is NAVSTAR, since the beginning worked with 24 orbiting satellites (21 operational and 3 spare satellites) on six different orbits at 20,180 km altitude. The rotational speed is 3.9 km / s, or 11 hours and 58 minutes for a mission in orbit. Meanwhile, 29 GPS satellites are active. The orbits are positioned above the MEO satellites, and in such a way that at the same four satellites can be received at all points on the Earth’s surface.
The GPS L1 band (1575.42 MHz) has turned to be the most important band for navigation purposes. Indeed most of the applications in the world nowadays are based on the signals transmitted at this frequency. Three signals are transmitted at the moment by GPS in L1: C/A Code, P(Y) Code and M-Code. In the future, an additional new civil signal, known as L1C, will also be transmitted.