The GNSS satellites continuously transmit navigation signals in two or more frequencies. These signals contain ranging codes and navigation data to allow the users to compute the travelling time from the satellite to the receiver and the satellite coordinates at any epoch.
The main signal components are described as follows:
- Carrier: Radiofrequency sinusoidal signal at a given frequency.
- Ranging code: Sequences of 0s and 1s (zeroes and ones), which allow the receiver to determine the travel time of radio signal from the satellite to receiver. They are called Pseudo-Random Noise (PRN) sequences or PRN codes.
- Navigation data: A binary-coded message providing information on the satellite ephemeris (Keplerian elements or satellite position and velocity), clock bias parameters, almanac (with a reduced accuracy ephemeris data set), satellite health status, and other complementary information.
The use of multiple signals on different carriers helps significantly improve a receiver’s position estimation in terms of accuracy and reliability. It is the most effective way of eliminating the ionospheric error from the calculation of the position. The ionospheric error varies with frequency, so it has a different impact on different GNSS signals. By comparing the delays of two GNSS signals, L1 and L2, for example, the receiver can correct the impact of ionospheric errors.