The signal broadcast by the navigation satellites is usually represented as Signal-In-Space (SIS). these signals must: Allow the user to estimate the pseudo distance user-satellite, carry some useful data, be robust to the transmission through the atmosphere and identify in a unique way the satellites. GPS and Galileo use CDMA (Code Division Multiple Access) to identify the satellites without ambiguity. each satellite transmits a different code (ranging codes), using the same carrier frequencies, without time division. Codes are mutually orthogonal in order to permit to the receiver to separate the signal of the satellite of interest from the others and the data signals are modulated by the ranging codes.