Source signal modulation To other frequencies (carrying signals), the functions include: 1. Reduce the noise in transmission;
2、 Frequency division multiplexing That is, multiple signals are transmitted on the same channel between the same frequencies without Aliasing ； 3. It can spread over a longer distance and is conducive to reception.
There are two carriers used in GPS satellites, because they are located in the microwave L-band , so they are called L1 carrier and L2 carrier respectively. Satellite navigation and positioning systems usually use L-band radio signal as carrier, and the frequency is too low (f<1GHz) the ionosphere Serious delay; If the frequency is too high, the signal is seriously resonant due to water vapor absorption and oxygen absorption, while the L-band signal is moderate. 
