GPS is a satelite navigation system developed by the US DoD, composed of between 24 and 32 satellites (also called SV, for Space Vehicle) orbiting at approx. 20200 km. The SVs continuously transmit (at 50 bits-per-second) a CDMA-modulated signal, encoded using a different pseudo-random number for each SV. The Navigation Message (1500 bits, taking 30 seconds to transmit) is composed of 3 parts: the time the message was sent, the ephemeris (the satellite’s orbit), and the almanac (rough orbit and status of the other SVs). At any given time, from a point on Earth not obstructed by buildings or mountains, approx. 10 sattelites are visible. 4 SVs are necessary for successful trilateration, given the fact that the receiver’s clock is inaccurate and has to be corrected too. Several methods for increasing the positioning accuracy exist, such as DGPS (using ground-based stations that transmit signal corrections) and A-GPS (using GSM cell towers for a quick rough initial position).