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Abstract-The presence of a secondary code in modern global navigation satellite system signals complicates the acquisition of these signals, because there is a potential sign transition between each period of the primary code. Some previous works proposed to use the parallel code search by performing the correlation over the primary code several times and then combining the results according to the secondary code chips. In this article, we will focus on this method and compare different hardware implementations, to determine if it is better to do the combinations before or after the correlations, and to compare serial and parallel architectures. In a second part, we will show a simple method that manipulates the local secondary code to rearrange the equations, which approximately halves the theoretical number of operations related to the secondary code correlation and the processing time for hardware implementations, without any impact on the sensitivity.

The acquisition of global navigation satellite system signals can be performed using a fast Fourier transform (FFT). The FFT-based acquisition performs a circular correlation, and is thus sensitive to potential transitions between consecutive periods of the code. Such transitions are not occurring often for the GPS L1 C/A signal because of the low data rate, but very likely for the new GNSS signals having a secondary code. The straightforward solution consists in using two periods of the incoming primary code and using zeropadding for the local code to perform the correlation. However, this solution increases the complexity, and is moreover not efficient since half of the points calculated are discarded. This has led us to research for a more efficient algorithm, which discards less points by calculating several sub-correlations.It is applied to the GPS L5, Galileo E5a, E5b and E1 signals. Considering the radix-2 FFT, the proposed algorithm is more efficient for the L5, E5a and E5b signals, and possibly for the E1 signal. The theoretical number of operations can be reduced by 21 %, the processing time measured on a * Corresponding author Email address: jerome.leclere@epfl.ch (Jérôme Leclère)Preprint submitted to Signal Processing July 2, 2013 software implementation is reduced by 39 %, and the memory resources are almost halved for an FPGA implementation.

Abstract-The acquisition of Global Navigation Satellite Systems signals using Code Division Multiple Access can be performed through classical correlation or using a Fourier transform. These methods are well known but what is missing is a comparison of their performance for a given hardware area or target. This paper presents this comparison for FieldProgrammable Gate Arrays, describing the different parameters involved in the acquisition, detailing some optimized implementations where hardware elements are duplicated, and estimating and discussing the performances. The influence of the Doppler effect on the code, is also discussed as it plays an important role, particularly for new signals using a high chipping rate.

Global Navigation Satellite Systems (GNSSs) were originally introduced to provide positioning and timing services for terrestrial Earth users. However, space users increasingly rely on GNSS for spacecraft navigation and other science applications at several different altitudes from the Earth surface, in Low Earth Orbit (LEO), Medium Earth Orbit (MEO), Geostationary Earth Orbit (GEO), and feasibility studies have proved that GNSS signals can even be tracked at Moon altitude. Despite this, space remains a challenging operational environment, particularly on the way from the Earth to the Moon, characterized by weaker signals with wider gain variability, larger dynamic ranges resulting in higher Doppler and Doppler rates and critically low satellite signal availability. Following our previous studies, this paper describes the proof of concept “WeakHEO” receiver; a GPS L1 C/A receiver we developed in our laboratory specifically for lunar missions. The paper also assesses the performance of the receiver in two representative portions of an Earth Moon Transfer Orbit (MTO). The receiver was connected to our GNSS Spirent simulator in order to collect real-time hardware-in-the-loop observations, and then processed by the navigation module. This demonstrates the feasibility, using current technology, of effectively exploiting GNSS signals for navigation in a MTO.