Comparison of acquisition techniques for GNSS signal processing in geostationary orbit
Résumé
GPS signal processing in geostationary environment is more difficult than for a classical receiver on Earth in normal conditions. There are numerous differences between the GPS signals that an Earth user receives and the signals that a geostationary satellite receives. The specific and main characteristics of the GPS signal received by a geostationary satellite are the following: high C/No values only for ray tangential to the earth, very important Doppler values (+/-15kHz), and poor Dilution Of Precision factor (usually higher than 5). A GPS/Galileo receiver onboard a geostationary satellite has to deal with these specific constraints. Several acquisition strategies can be envisaged so as to produce a point position. To reduce the delay and Doppler uncertainty and to get the navigation data, we could for example choose a strategy where some of the data (such as ephemeris or almanacs) are downloaded from Earth, but this increases the complexity and the cost of the receiver and of its integration in the satellite. Our aim in this paper is to use acquisition techniques which work in a global autonomous acquisition strategy. So the receiver does not use aiding data". In this case, the chosen acquisition technique must be really effective at least down to the demodulation threshold, and that threshold needs to be lowered to its minimum. The aim of this paper is to present 3 different unaided acquisition schemes and to compare their performances to process the GPS/Galileo signals in the particular context of a geostationary orbit. The first method consists in a classical FFT acquisition. This technique will be used as a reference to evaluate the performances of the other techniques. The second scheme called Half Bit Method (developed by M.Psiaki) is a method which allows long coherent integration time without knowing the data bit transition time. It avoids losses due to a bit transition occurring within the coherent integration time. The last acquisition technique studied is known as "double block zero padding" method. The main interest of this method is its rapidity and also, its low computational cost. The paper presents the test acquisition results over one standard day for a given geostationary orbit position. The work presented shows the statistics of successful acquisition as well as misdetection over one day for the three acquisition techniques. The length of signal required to achieve a minimum successful detection rate is also investigated. Due to the weakness of the considered signals and the power difference between the different received signals inducing cross-correlation, it is often necessary to process more than 1 second sometimes around 2 or 3 seconds. In this paper, we focus on an autonomous acquisition strategy, so we consider the receiver does not use "aiding data" downloaded from an earth link. Then, so as to perform a precise positioning, we need to demodulate the navigation message to extract the previous data. This is also necessary in order to reduce the Doppler uncertainty and so, to reduce the processing time. Thus, the paper considers several data demodulation thresholds (from 24dBHz to the usual 27dBHz value) and assesses the impact on the data demodulation and the consequence of a possible bit error on the final calculated position. Finally, we can compare the 3 acquisition methods in regards with their efficiency towards the different values when they are processed onboard a GEO satellite.
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