Each satellite transmits two signals, namely L1 (1575.42 MHz) and L2 (1227.60 MHz). L1 signal is modulated with two pseudo-random signal that is the code P (Protected) code and C / A (coarse / Acquisition). L2 signal only bring the code P. Each satellite transmits a unique code so that the receiver (GPS devices) to identify the signal from each satellite. At the time of the "Anti-Spoofing" is activated, the code P will be encrypted and subsequently known as the code P (Y) or Y-code
GPS devices that are specific for only civilian to receive the code C / A on L1 signal (although in a sophisticated GPS devices can take advantage of the L2 signal to obtain a more precise measurement.
GPS devices receive signals transmitted by GPS satellites. In determining the position, we need at least 3 satellites for the determination of 2-dimensional position (latitude and longitude) and 4 satellites for the determination of 3-dimensional position (latitude, longitude, and altitude). More and more satellites are obtained, the accuracy of our position will be higher. To get these signals, the GPS device must be open space. If your GPS device we are in a room or a dense canopy and the area we are surrounded by tall buildings, the signal obtained will be further reduced so that it would be difficult to determine precisely the position or even not able to determine position.
