Performance Analysis of FLL Schemes to Track Swept Jammers in an Adaptive Notch Filter

Abstract

In the field of Global Navigation Satellite System (GNSS), jamming signals are more than ever a real threat, demanding for an increasing need of countermeasures. Among several proposed detection and mitigation methods developed for specific types of interference, the Adaptive Notch Filter (ANF) is still an attractive solution to mitigate very narrowband interference signals, for its ease of implementation, low computational cost and reduced activation time. ANFs have been widely exploited in literature, with many variations: albeit well known to be particularly effective for Continuous Wave (CW) interferes, an early proof of their suitability also against certain types of GNSS jammers has been shown. In this paper, two recently proposed Frequency Lock Loop (FLL) models to implement the adaptive capability of the ANF are considered: the standard FLL and the FLL with exponential filtering. While both have already been analyzed in the presence of CW interfering signals, with constant carrier frequency, here an evaluation in case of fast sweeping central frequency is conducted, targeting the features of some of the most common jammers detected in the GPS LI/Galileo E1 band. A comparison in terms of tracking capability and noise performance is presented. While the standard FLL shows to be able to successfully track and mitigates jamming signals, the exponential filtering FLL proves it is not up to, suffering particularly from frequency discontinuities, which commonly characterize such kind of interferers.