Galileo E5a/E5b and GPS L5 Acquisition Time Statistical Characterization and Application to Civil Aviation

摘要

The initial task of a receiver is to acquire at least four satellites and then track them to compute a first fix. The acquisition process is a two-dimensional search both in time and frequency. Indeed neither the received code delay nor the carrier Doppler frequency are perfectly known and so must be searched. In the literature, only the mean acquisition time and its standard deviation can be found for acquisition strategies such as the classical single dwell or double dwell serial time search processes. However this characterization is not sufficient for applications such as civil aviation for which time to first fix requirements are paramount. Given RTCA MOPS 229, the acquisition process performance may be stated in terms of probability of success so statistical characterization of the acquisition time such as the probability density function (Pdf) or, equivalently, the cumulative density function (Cdf) are required. The effect of key parameters such as the signal quality, the dwell time or the number of available correlators on the acquisition performance has been assessed. rnThe future GPS L5 signal and GALILEO E5a and E5b signals are of particular interest for the civil aviation community. Indeed they will be broadcast in an ARNS band and are expected to increase accuracy, availability, integrity and continuity of service. The PRN spreading codes selected for these signals can be acquired directly using a traditional search scheme although a strategy combining both the data and pilot codes is shown to be more efficient as described in [1]. However E5a/L5 and E5b bands interfering environment is severe and mainly composed of pulsed signals transmitted by DME/TACAN and JTIDS/MIDS systems. They constitute a real threat causing large signal-to-noise ratio degradations especially at high altitude and so potentially jeopardize the compliance to the acquisition timing requirements. It is particularly true when the GNSS receiver experiences a power outage or brownout during en-route at FL 400 and must afterwards satisfy initial acquisition performance. rnThe aim of this paper is to present Monte Carlo simulation results enabling to estimate the acquisition time Pdf and Cdf for the classical single dwell serial time search process. The spreading codes for the considered signals are transmitted at high frequency (10.23 Mcps) causing possible high code Doppler frequencies as compared to GPS L1 C/A code for instance. The acquisition performance degradation, particularly for large dwell times, is quantified accordingly. These results are used to verify in which conditions the civil aviation requirements for initial acquisition can be satisfied.