Micaela Troglia Gamba

The disruptive effect of radio frequency interference (RFI) on global navigation satellite system (GNSS) signals is well known, and in the last four decades, many have been investigated as countermeasures. Recently, low-Earth orbit (LEO) satellites have been looked at as a good opportunity for GNSS RFI monitoring, and the last five years have seen the proliferation of many commercial and academic initiatives. In this context, this paper proposes a new spaceborne system to detect, classify, and localize terrestrial GNSS RFI signals, particularly jamming and spoofing, for civil use. This paper presents the implementation of the RFI detection software module to be hosted on a nanosatellite. The whole development work is described, including the selection of both the target platform and the algorithms, the implementation, the detection performance evaluation, and the computational load analysis. Two are the implemented RFI detectors: the chi-square goodness-of-fit (GoF) algorithm for non-GNSS-like interference, e.g., chirp jamming, and the snapshot acquisition for GNSS-like interference, e.g., spoofing. Preliminary testing results in the presence of jamming and spoofing signals reveal promising detection capability in terms of sensitivity and highlight room to optimize the computational load, particularly for the snapshot-acquisition-based RFI detector.

Many GNSS applications have been experiencing some constantly growing needs in terms of security and reliability. To address some of them, both GPS and Galileo are proposing evolutions of their legacy civil signals, embedding features of authentication. This paper focuses on the Galileo Open Signal Navigation Message Authentication (OSNMA) and describes its implementation within a real-time software receiver for ARM-based embedded platforms. The innovative contributions of the paper include the software profiling analysis for the OSNMA add on, along with the comparison among performances obtained with different platforms. In addition, specific evaluations on the computational load of the whole receiver complete the analysis. The receiver used for the implementation belongs to the NGene receivers family—real-time fully-software GPS and Galileo receivers, tailored for different platforms and sharing the same core processing. In detail, the paper deals with the introduction of the OSNMA support inside the eNGene, the version of the receiver executable by ARM-based embedded platforms.

With the fast growing diffusion of the real-time high accuracy applications based on the Global Navigation Satellite System (GNSS), the robustness of the GNSS receiver performance has become a compelling requirement. Disruptive effects can be induced to the signal processing stages of GNSS receivers due to the disturbances from Radio-Frequency Interference (RFI), even leading to a complete outage of the positioning and timing service. A typical RFI threat to the GNSS signals is represented by portable jammers which transmit swept-frequency (chirp) signals in order to span the overall GNSS bandwidth. The implementation in the receivers of Adaptive Notch Filters (ANFs) for chirp cancellation has been extensively investigated and proved to be an efficient countermeasure. However, the performance of ANF is strongly dependent on its configuration setup. Inappropriate parameter settings of the ANF for interference removal may induce severe distortion to the correlation process. In addition, an effective mitigation will still introduce a vestigial signal distortion contributed by the residual unmitigated chirp and the ANF operation itself, being not negligible for high accuracy solutions. This paper addresses the detailed analysis for assessing the effects of interference mitigation by notch filtering. A bias compensation strategy is proposed, wherein for each Pseudo Random Noise (PRN) the biases due to the parameter settings of the notch filter are estimated and compensated. The impact of using the ANF operation on chirp signals at the acquisition and tracking stages of GNSS receivers is analyzed. On the basis of the three proposed metrics, the effects can be quantitatively estimated to depict a complete picture of the most influential parameters of the chirp and the ANF configurations, as well as the optimal achievable performance at the acquisition and tracking stages.

Adaptive Notch Filters have been widely investigated in literature for their low complexity and good performance, and used in many signal processing applications. More recently they have been proven to be an effective solution against jamming interferences, considered more than ever a real threat to Global Navigation Satellite System (GNSS) signals.

Adaptive notch filters in general, and frequency lock loop-equivalent adaptive notch filters in particular, have been shown to be an effective solution to counteract jamming signals. Much effort has been spent at analyzing their adaptive capability, while detection approaches in charge of activating/deactivating the filter have not been properly addressed. This paper presents two new detection strategies, namely discriminator-based and output-input power ratio-based, suitable to be applied to FLL-equivalent ANFs. The performance evaluation for several interference-to-noise ratio conditions demostrates their suitability to be combined with such kind of filters and their effectiveness in mitigating jamming signals.

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.

In the past years, many techniques have been researched and developed to detect and identify the interference sources of Global Navigation Satellite System (GNSS) signals. In this paper, we utilize a simple and portable application to map interference sources in real-time. The results are promising and show the potential of the crowdsourcing for monitoring and mapping GNSS interference distribution.