A two-steps GNSS acquisition algorithm

The next generation of Global Navigation Satellite Systems (GNSS), such as Galileo [1] and GPS modernization [2], will use signals with equal code and bit periods, which will introduce a potential bit sign transition in each segment of the received signal processed in the acquisition block. The presence of bit sign transitions in the data record is a critical aspect in all the acquisition methods where the data are processed in blocks, like the fast acquisition method based on Fast Fourier Transformation (FFT) where the bit sign transition could occur in any position within the data block. A bit sign transition occurring within an integration time causes a splitting of the main peak of the Cross Ambiguity Function (CAF) into two smaller lobes along the Doppler shift axis, which causes a great performance degradation of the acquisition system [3]. In this paper an innovative two steps acquisition algorithm is proposed to mitigate the CAF peak splitting effect caused by the presence of bit sign transitions in the signal segments, which fits the requirements of the new generation of GNSS signals. The bit sign transition problem is described in detail in this paper and the CAF peak splitting effect dependent on the bit sign transition position in the signal segment is also clearly explained. It is able to prove that the presence of bit sign transition does not destroy the information on the presence of the satellite in view, but it introduces an erroneous Doppler frequency shift estimation. When the proposed two steps acquisition technique is adopted in the signal acquisition in presence of bit sign transitions, it could overcome the CAF peak splitting effect due to the presence of bit sign transitions and provide improved performance in comparison with the classical fast acquisition scheme. In order to validate the proposed technique, simulation campaigns have been performed on the simulated Galileo Open Service (OS) BOC 1,1 signals to evaluate the performances in terms of histograms of the estimated Doppler shift and code phase delay, Receiver Operative Characteristics (ROC) and Signal to Noise Ratio (SNR) curves. The simulation results show that the proposed two steps acquisition method provides superior performance over the classical fast acquisition approach, which prove the advantages of the proposed technique and consolidate its validity and effectiveness to solve the CAF peak splitting problem in presence of bit sign transitions.