Acquisition systems for GNSS signals with the same code and bit rates

Abstract

In this paper we address the problem of the acquisition of GNSS signals for the next-generation systems (as Galileo, and GPS modernization), which will use signals with code rate equal to the bit rate. In this case there is a potential sign transition in each segment of the signal processed in the acquisition schemes. The search space could result greatly impaired, especially with the methods based on FFT. These methods are very efficient, but a bit transition leads to a modification of the shape of the Cross Ambiguity Function evaluated in the search space. Here we propose a method which mitigates this impairment, which fits the requirements of the new GNSS signals. We will show that the bit transition does not destroy the information on the presence of the satellite, but introduces an error in the estimation of the Doppler frequency and code delay. In fact the effect of the sign transition on the correlation peak is a splitting of the peak into two asymmetric lobes along the correct delay bin, so leading to a potential null value at the correct cell. However the integral of these lobes exhibits an invariance property, which can be exploited to recover the bit transition position, from which the acquisition process becomes feasible. In the paper we will provide the mathematical background of a method based on this invariance property and we will show some simulation results which prove the validity of the method.