SoftFlex: An Advanced Approach to Design of GNSS Receiver with Software Correlator

The paper deals with GNSS receivers using software cor-relators (SWC). Such receivers implement all signal cor-relator functions not in hardware but in software. The main factor limiting the use of such receivers is high processor throughput required for sophisticated digital signal processing of comparatively wide-band signals. The main objective of the paper is to present enhance-ments to the known SWC technology, which allow practi-cal development of GNSS receivers with SWC not com-promising the main requirements (accuracy, TTFF) with affordable digital signal processor (DSP). A simple and effective idea of processor throughput flexi-ble utilization is described. According to the concept named SoftFlex, not all signal samples from the front-end are involved in signal processing. Only part of them re-quired and used to obtain reliable and accurate measure-ments, thus reducing the DSP computation load. The paper describes architecture of a SoftFlex GNSS re-ceiver and its main distinctions from a conventional re-ceiver. Satellites selection for SoftFlex receivers is more complicated than that for conventional ones. Constellation selection algorithms should account for (in addition to ge-ometry) signals tracking status and their contribution to the DSP computational burden. The SoftFlex approach saves the given DSP’s throughput, which may be used either to increase the number of tracking channels, to increase the input signal bandwidth, or to implement some additional user-specific functional-ity. More tracking channels may be particularly important for space-based receivers, which have to use compara-tively slow radiation-hardened DSPs. The increased input signal bandwidth may be required, for example, in the following cases: · Narrow-correlation · Combined reception of GPS/GLONASS signals · Reception of signals with higher modulation rates (P-codes of GPS and GLONASS, future GNSS-2). DSP throughput sufficient for SoftFlex GNSS receiver options has been evaluated. Not compromising TTFF re-quirement and implementing the whole functionality typi-cal for all-in-view receivers, DSPs with following throughput are acceptable: GPS/SBAS – 20 MIPS; GPS/GLONASS – 50-70 MIPS; GPS with narrow corre-lation – 100 MIPS. It is shown that the SoftFlex approach is promising for the applications, which require signifi-353 cant flexibility due to varying signal reception conditions, and are connected with HW limitations.