Abstract
Low earth orbit (LEO) constellations for enhancing global navigation satellite systems (GNSS) have been investigated and studied due to their higher floor power and rapid geometric changes. Compared with GNSS based on medium orbit (MEO) and geostationary (GEO) satellites, the higher Doppler frequency is one of the problems faced by the LEO enhanced GNSS. For the traditional BPSK signal, the complexity has been greatly improved in acquisition, especially in the cold start state, which means more requests on the signal design. The periodic chirp signal modulated by the pseudo-noise code (chirp-PC) is proposed as the ranging signal of LEO enhanced GNSS. The coupling characteristic of Doppler and delay in chirp-PC can make the search frequency grid less than the traditional BPSK signal with the same acquisition performance, which can greatly reduce the complexity. However, the delay estimation obtained by the acquisition is biased. A tracking loop based on the chirp phase locked loop (CPLL) was proposed for removing the coupling of Doppler and time delay. The simulations show that the tracking loop can track chirp-PC signals stably and unbiasedly, and the tracking accuracy is similar to that of the corresponding BPSK signal. Therefore, the chirp-PC signal is a new choice for LEO enhanced GNSS with low acquisition complexity and equivalent tracking accuracy to BPSK.
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