How Many GNSS Satellites are Too Many?

Global Navigation Satellite Systems (GNSS) are growing from the current US GPS and Russian GLONASS to additional European Galileo and Chinese Compass systems. Along with the growth of the systems, the number of satellites will also increase. The whole family of GNSS is projected to consist of about 120 satellites by 2030. Moreover, the new satellites are capable of transmitting multiple signals in multiple frequency bands. Altogether there will be more than 300 GNSS signals broadcast in the future. The growing number of GNSS satellites and signals enable greater redundancy for positioning. On the other hand, the signals interfere with each other due to overlapping frequency bands. Here we answer the question: how many satellites are too many? We assess the self-interference within GNSS, and hence establish their multiple access capacity, by examining the code interactions between satellites. This analysis considers cross-correlation properties of the codes at all possible Doppler frequency offsets between satellites. We first approach the question theoretically by calculating auto- and cross-correlation properties of random sequences with binary phase shift keying (BPSK) modulation and binary offset carrier (BOC) modulation. With the theoretical result of pure random sequences as a guideline, we then use real broadcast pseudorandom noise (PRN) codes of the current Galileo GIOVE and Compass-M1 satellites to further analyze various correlation properties over a range of Doppler frequency offset. We ultimately establish the multiple access capacity of GNSS.