Analysis of a software-based A-GPS acquisition performance using statistical processes

Damien Kubrak is a Ph.D. student in the field of personal positioning in the signal processing lab of the Ecole Nationale de l'Aviation Civile (ENAC) in Toulouse, France. He graduated in 2002 as an electronics engineer from the ENAC, and received the same year his Master research degree in signal processing. Christophe Macabiau graduated as an electronics engineer in 1992 from the ENAC (Ecole Nationale de l'Aviation Civile) in Toulouse, France. Since 1994, he has been working on the application of satellite navigation techniques to civil aviation. He received his Ph.D. in 1997 and has been in charge of the signal processing lab of the ENAC since 2000. Michel Monnerat graduated from the ENSICA (Ecole Nationale Superieure d'Ingenieur de Constructions Aeronautiques) engineering school. After being involved within Alcatel Space in many radar programs, in charge of the onboard processing of the ARGOS / SARSAT payload, he has been involved in the Galileo program since 1998, for the signal design and performance aspects. ABSTRACT The efforts of the semiconductor industry to produce small, low consumption and powerful chipsets bear fruit, and many portable devices are now equipped with them. Indeed, the miniaturized chipsets with increasing processing power make them suitable for supporting software-based applications. On the other hand, demand for indoor navigation and location based services applications, whose interoperability with GNSS technology is being more and more obvious, is expected to grow rapidly in the next few years. In this context, because of their maximum flexibility and minimum hardware modifications, software-based positioning techniques embedded in portable devices become very interesting. The remaining key point is the time needed to acquire these rare and weak indoor signals and give a position solution. The aim of this study is to present results on the time needed to acquire one satellite for such embedded GPS applications, and more specifically, for Assisted-GPS ones. In this paper, an RF front-end chipset combined with a processor embedded in a portable device is considered. Only the acquisition process using FFT correlations will be studied here. The classical structure for L1 code FFT acquisition is first described and acquisition performance, such as the mean acquisition time, is discussed. A statistical process is used to simulate acquisition procedures, according to probabilities of detection, acquisition structure and the implemented acquisition strategy. The estimated times are then deduced from these tests. A special study is carried out to measure the impact of the Doppler affecting the GPS signal. An analysis of the effect of the computational power is also conducted, as well as the influence of the signal-to-noise ratio on the acquisition procedure, mostly dedicated to indoor and deep indoor environments. Results point out the fact that to achieve the requirements of the application supporting the software-based acquisition process, a trade-off has to be found between performance relative to the acquisition of weak signals, and the rapidity of the processing, especially in deep indoor environments. The evaluation of the Doppler component due to user dynamics and the acquisition strategy are also key points that would speed up the process.