Pure Pilot Signals: How Short Can We Choose GNSS Spreading Codes?

Abstract

With the modernization of global navigation satellite systems (GNSS), a trend towards wideband navigation signals with high symbol rates and long spreading codes can be observed. However, low-complexity signals resembling the legacy coarse/acquisition (C/A) signals are still preferred for the purpose of rapid/low-energy signal acquisition. "Pure pilot" signals with short primary spreading codes and virtually no data or secondary code allow a particularly simple acquisition with few code-search bins and flexible predetection integration time. However, to allow reliable acquisition of any particular satellite, it must be ensured that self-interference received from other satellites is not excessive and can be decorrelated from the desired signal by simple coherent integration. To minimize the number of required code/Doppler acquisition bins, we formulate a dual-objective optimization problem to minimize both the spreading code length and the coherent integration time, under a number of constraints representing the acquisition reliability, satellite constellation or power levels and chip waveform. Results suggest that the spreading code length can be chosen as short as 600 to 800 chips, when combined with binary phase-shift keying (BPSK) at 0.5 to 1.0 MHz chipping rate. This provides a very promising perspective for the future design of fast-acquisition signals.