Instead of true random numbers, which are difficult to generate, in practice one works with pseudorandom numbers, pseudorandom numbers(PRN), which are generated, for example, via the noise ofrandom generators.
The pseudorandom number technique is used to generate pseudorandom codes whose ones and zeros appear to be randomly distributed but are reproducible.
Pseudorandom codes are used in encryption, the spread-spectrum technique( DSSS), and the GPS system. In the spread spectrum technique, individual data bits of the signal to be modulated are XORed with a PRN signal. The output signal is Differential PhaseShift Keying( DPSK) resampled and transmitted over the individual frequency channels.
In GPS systems, each GPS satellite has unique pseudo-random codes in the form of the C/A codes (Coarse/Acquisition) and Precision codes (P code), whose low autocorrelation is important for the delays and shifts between the GPS satellites.