Real-time Pre-correlation Anti-jamming System for Civilian GNSS Receivers

Abstract

Global Navigation Satellite Systems (GNSS) have become a key technology that enables numerous location and navigation applications, thanks to the 24/7 worldwide availability of their signals, and the positioning accuracy that can be reached with them. However, GNSS have also some weaknesses, and the most relevant is that their signals reach the Earth’s surface with very low power. This makes them quite vulnerable to the effect of Radio-Frequency Interference (RFI), and particularly to intentional jamming, which degrades, or even disrupts, the performance of the receivers. This problem is particularly threatening for those critical GNSS-enabled applications that trust on the integrity and continuity of GNSS signals, whose reliability may be committed. Such liability/security-critical civilian applications include autonomous aerial/terrestrial navigation, automatic rail signaling, geo-localized toll/insurance payments, or network synchronization among others. Moreover, RFI is also troublesome for derived GNSS applications such as GNSS-Reflectometry (GNSS-R) where GNSS signals are used as signals of opportunity in a multi-static radar configuration for Earth observation purposes. A number of mitigation solutions have been proposed to increase the continuity of GNSS signals in the presence of jamming signals. Regarding the structure of a GNSS system, the mitigation process can be performed at different stages: antenna, front-end, pre-correlation, post-correlation, or measurement. Common antenna solutions can provide spatiotemporal selectivity, which can reach interference rejection ration higher than 40 dB, but this is only achieved in static scenarios. On the contrary, pre-correlation techniques work at signal level, between antenna and GNSS correlators, providing interference selectivity in time, frequency, statistical, or other sub-space signal domains, which can mitigate jamming signals regardless the scenario dynamics. Moreover, pre-correlation techniques usually have a high computational burden in order to achieve high performance with interference rejection ratios higher than 30 dB. However, last generation Field Programmable Gate Arrays (FPGA) can overcome this drawback, and they even enable real-time pre-correlation anti-jamming solutions. The Front-End GNSS Interference eXcisor (FENIX) is a GNSS anti-jamming technology based on the patented combination of statistical interference detection, and a multiresolution time-frequency blanking algorithm for jamming mitigation in DSSS-based multi-constellation GNSS receivers [1]. The main goal of FENIX is to increase the C/N0 in the presence of interference signals, thus improving the continuity of GNSS services. Moreover, the mitigation algorithm has been designed to be DSSS-based multi-constellation, frequency independent (i.e. it works at L1 and L2, L5…), and it is capable of mitigating almost all kinds of jamming signals. A general description of the major building blocks of FENIX was first presented in [2]. This work aims at showing the implementation and first results of a real-time L1/L2 GPS/Galileo lite version of FENIX (FENIX-lite) tested using commercial jammers and GNSS receivers. The FENIX-lite demonstrator has been implemented using a two Software Defined Radio (SDR) model USRP B200mini, the first covers the GPS L1 C/A and Galileo E1 OS services, whereas the second does the same with the GPS L2C signal. The SDR front-ends are tuned to the L1 and L2 bands, and the FENIX interference detection and mitigation algorithm is running in real-time in both FPGA. The FENIX algorithm detects the interference signal using a statistical domain analysis based on normality tests, while the samples containing most part of interference power signal are excised in the time-frequency space computed using the Multiresolution Fourier Transform (MFT). The use of the MFT allows to mitigate almost of kinds of jamming signals since it maximized the projection of the interference signal in the transformed domain as demonstrated in [3]. In order to evaluate the improvement in the continuity of the GNSS signals, the degradation of the SNR with and without the FENIX-lite is compared using jammers at different bands and commercial GNSS receivers. [1] J. Querol, and A. Camps, “System and method for detecting and eliminating radio frequency interferences in real time,” U.S. Patent Application 15 222 036, issued date July 28, 2016. [2] J. Querol, E. M. Julian, R. Onrubia, A. Alonso-Arroyo, D. Pascual and A. Camps, “Preliminary results of FENIX: Front-End GNSS Interference eXcisor,” 2016 IEEE International Geoscience and Remote Sensing Symposium (IGARSS), Beijing, 2016, pp. 5627-5630. [3] J. Querol; R. Onrubia; A. Alonso-Arroyo; D. Pascual; H. Park; A. Camps, “Performance Assessment of Time-Frequency RFI Mitigation Techniques in Microwave Radiometry,” in IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, in press.