Acquisition of modern GNSS signals using a modified parallel code-phase search architecture

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 zero-padding 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 software implementation is reduced by 39%, and the memory resources are almost halved for an FPGA implementation

Comparison Framework of FPGA-based GNSS Signals Acquisition Architectures

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 Field Programmable 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

Modified parallel code-phase search for acquisition in presence of sign transition

One of the method to have a fast acquisition of GNSS signals is the parallel code-phase search, which uses the fast Fourier transform (FFT) to perform the correlation. A problem with this method is the potential sign transition that can happen between two code periods due to data or secondary code and lead to a loss of sensitivity or to the non-detection of the signal. A known straightforward solution consists in using two code periods instead of one for the correlation. However, in addition to increasing the complexity, this solution is not efficient since half of the points calculated are discarded. This led us to look for a more efficient algorithm. The algorithm proposed in this article transforms the initial correlation into two smaller correlations. When the radix-2 FFT is used, the proposed algorithm is more efficient for half of the possible sampling frequencies. It is shown for example that the theoretical number of operations can be reduced by about 21 %, and that the memory resources for an FPGA implementation can be almost halved

Improving the Performance of the FFT-based Parallel Code-phase Search Acquisition of GNSS Signals by Decomposition of the Circular Correlation

This paper proposes alternative architectures to perform a circular correlation using the Fast Fourier Transform (FFT) by decomposing the initial circular correlation into several smaller circular correlations. The approach used is similar to the Fast Finite Impulse Response (FIR) Algorithms (FFAs). These architectures improve the performance in terms of reduced processing time or resource usage, and consequently lower the energy consumption. The results can be applied to any system that performs circular convolution or correlation. In this paper, the application is the acquisition of Global Navigation Satellite System (GNSS) signals with the FFT-based Parallel Code-phase Search (PCS), and more precisely on the GPS L1 C/A signal, when the target considered is a Field Programmable Gate Array (FPGA). In this context, it is for example shown that it is possible with one of the proposed architectures to reduce the logic usage by 11 %, the memory usage by 41 %, and the Digital Signal Processing (DSP) block usage by 32 %, while keeping the same processing time. With another architecture, it is shown that the processing time can be halved by increasing the logic usage by only 35 %, while reducing the memory usage and keeping the same DSP usage. Note that the proposed approach is not based on an approximation of the traditional method, but a modified implementation providing the same result. Thus, there is no loss of sensitivity

Implementing super-efficient FFTs in Altera FPGAs

In this article, an alternative method is proposed to compute a fast Fourier transform (FFT) on Altera FPGAs. This method is using the Altera FFT intellectual property (IP) core, but it is more efficient than the direct use of the Altera FFT IP core, in the sense that the processing time or the resources can be reduced. For the FPGA user, the implementation of the proposed method is more complex than using directly the Altera FFT IP core because additional elements are required, such as a numerically controlled oscillator (NCO) or a memory, a complex multiplier, adders and scaling, but it may be worth it since the decrease in processing time or resources is significant, especially regarding the memory with large FFTs. The proposed method can also be applied to the computation of the convolution or correlation using FFTs

FFT Splitting for Improved FPGA-Based Acquisition of GNSS Signals

With modern global navigation satellite system (GNSS) signals, the FFT-based parallel code search acquisition must handle the frequent sign transitions due to the data or the secondary code. There is a straightforward solution to this problem, which consists in doubling the length of the FFTs, leading to a significant increase of the complexity. The authors already proposed a method to reduce the complexity without impairing the probability of detection. In particular, this led to a 50% memory reduction for an FPGA implementation. In this paper, the authors propose another approach, namely, the splitting of a large FFT into three or five smaller FFTs, providing better performances and higher flexibility. For an FPGA implementation, compared to the previously proposed approach, at the expense of a slight increase of the logic and multiplier resources, the splitting into three and five allows, respectively, a reduction of 40% and 64% of the memory, and of 25% and 37.5% of the processing time. Moreover, with the splitting into three FFTs, the algorithm is applicable for sampling frequencies up to 24.576 MHz for L5 band signals, against 21.846 MHz with the previously proposed algorithm. The algorithm is applied here to the GPS L5 and Galileo E5a, E5b, and E1 signals

An Assisted-GNSS Solution for Demanding Road Applications using the EGNOS Data Access System (EDAS)

This paper describes a prototype solution which has been developed to provide both the positioning availability and integrity required for this application. The SIGNATURE (SImple GNSS Assisted & TrUsted REceiver) solution includes an assistance service which provides ephemeris data and corrections from the EGNOS Data Access Service (EDAS), optimized for the user location. Assistance messages are sent to OBUs which can either host an experimental receiver or a Commercial-Off-The-Shelf (COTS) receiver. Measurement data from the receiver is then processed with application-specific navigation algorithms on the OBU which aim to improve the integrity of the position solution relative to standard solutions. The paper describes the SIGNATURE solution and how it is being tested in the course of the project. It then presents initial results from field trials which are assessing its performance in a range of representative conditions. The tests assess the contribution that assistance can make to positioning performance, and illustrate options for enhancing standard assistance solutions. Enhancements to assistance encompass modifications to the message content and alternative means of communications, showing the benefits and feasibility of a broadcast service. The impact of including EGNOS corrections through a broadcast assistance service in urban areas is also under investigation

An Assisted-GNSS Solution for Demanding Road Applications using the EGNOS Data Access System (EDAS)

Roads right across Europe are becoming more congested and governments and regional authorities are looking for ways to better manage the existing road network. One tool that is being increasingly promoted to tackle the congestion challenge is Road User Charging (RUC). Recording journey information using a GNSS receiver embedded in an OBU is a convenient and flexible solution to support the automated fee collection process. If GNSS positioning is to be used as the basis for billing drivers though it must meet stringent reliability and availability requirements, and at the same time, be based on low-cost equipment. This paper describes a prototype solution which has been developed to provide both the positioning availability and integrity required for this application. The SIGNATURE (SImple GNSS Assisted & TrUsted REceiver) solution includes an assistance service which provides ephemeris data and corrections from the EGNOS Data Access Service (EDAS), optimized for the user location. Assistance messages are sent to OBUs which can either host an experimental receiver or a Commercial-Off-The-Shelf (COTS) receiver. Measurement data from the receiver is then processed with application-specific navigation algorithms on the OBU which aim to improve the integrity of the position solution relative to standard solutions

Aspects actuels des représentations de connaissances par objets et de la classification

napoli1997aNational audienceCet article présente certains thèmes de recherches étudiés par les membres du groupe "Objets et classification" du PRC-IA. Ces thèmes concernent essentiellement la théorie des systèmes de représentation de connaissances par objets (RCPO), la révision d'une base de connaissances dans les systèmes de RCPO, la classification de classes et d'instances, et la mise en oeuvre d'applications, illustrée ici par le système RESYN. Les travaux présentés montrent une certaine continuité avec les préoccupations des membres du groupe depuis qu'il existe. L'article se termine par la présentation d'éléments de définition d'un système de RCPO, et de perspectives de recherches découlant des thèmes explicités dans l'article