GPS satellites broadcast radio frequency signals at two carrier frequencies; L1 (1575.42 MHz) and L2 (1227.60 MHz). The beams that can be used by the general public are encoded using C/A (rough/acquisition) codes, while beams used solely by the US military are encoded using P (precise) codes.

　　The C/A code includes the identification codes and navigation messages of each satellite. The L1 signal from the GPS satellite is phase modulated using C/A code. C/A code is a digital signal in which 0 and 1 are encoded by 1023 consecutive digital pulse patterns. The navigation message consists of 25 frames, each with 5 subframes of 300 bits. Therefore, each frame has 1500 bits (0 and 1). The data length of each bit is 20 milliseconds (i.e., the total pulse width of a C/A signal of one bit is 20 milliseconds).

　　The orbital data of each satellite is called an ephemeris. This data is used to generate the precise position of the satellite, which is required for the GPS receiver to calculate position information. The orbital data of all satellites is called the almanac. It includes rough orbit and status information of all satellites in the network. This data is used to locate available satellites for the GPS receiver so that it can find their current time and position on Earth.

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