The Global Positioning System(GPS) is a worldwide satellite navigation system. More than 20 MEO satellites orbit the earth in various orbits and transmit their position data to an earth station. GPS systems transmit their signals on three different frequencies: on 1,575.42 MHz GPS signals are transmitted for civil use, on 1.57542 GHz precision signals are transmitted for military use, and on 1,176.45 MHz signals are transmitted for aviation use.
There are two GPS carrier frequencies: One for the c/A code for civil use, and the other for the P code for high-precision military applications. The GPS frequency for the C/A code is 1,575.42 MHz, and that for the precision code is 1,227.6 MHz. Both of these and also a third microwave are in the L-band and are designated L1-band and L2-band. They are derived from frequencies of atomic frequency standards. The L-band was chosen for the navigation satellites because the transmission can be done without parabolic antennas and the interference of the transmission by ionospheric disturbances is lower than in other frequency ranges. In addition, wave absorption is not affected by weather conditions.
The GPS carrier frequency of the L1 band at 1.57542 GHz has a wavelength of 19.05 cm and transmits the GPS frames that can be used by civilians. The GPS frame contains the navigation message with all important information about the satellite, with a time stamp, the ID number and the orbit data with correction data.
The third microwave frequency of 1.17645 GHz provides higher reception quality and is used in aviation and rescue applications. This frequency is referred to as the L5 band.