The Impact of Interference on GNSS Receiver Observables – A Running Digital Sum Based Simple Jammer Detector

A GNSS-based navigation system relies on externally received information via a space-based Radio Frequency (RF) link. This poses susceptibility to RF Interference (RFI) and may initiate failure states ranging from degraded navigation accuracy to a complete signal loss condition. To guarantee the integrity of the received GNSS signal, the receiver should either be able to function in the presence of RFI without generating misleading information (i.e., offering a navigation solution within an accuracy limit), or the receiver must detect RFI so that some other means could be used as a countermeasure in order to ensure robust and accurate navigation. Therefore, it is of utmost importance to identify an interference occurrence and not to confuse it with other signal conditions, for example, indoor or deep urban canyon, both of which have somewhat similar impact on the navigation performance. Hence, in this paper, the objective is to investigate the effect of interference on different GNSS receiver observables in two different environments: i. an interference scenario with an inexpensive car jammer, and ii. an outdoor-indoor scenario without any intentional interference. The investigated observables include the Automatic Gain Control (AGC) measurements, the digitized IF (Intermediate Frequency) signal levels, the Delay Locked Loop and the Phase Locked Loop discriminator variances, and the Carrier-to-noise density ratio (C/N0) measurements. The behavioral pattern of these receiver observables is perceived in these two different scenarios in order to comprehend which of those observables would be able to separate an interference situation from an indoor scenario, since in both the cases, the resulting positioning accuracy and/or availability are affected somewhat similarly. A new Running Digital Sum (RDS) -based interference detection method is also proposed herein that can be used as an alternate to AGC-based interference detection. It is shown in this paper that it is not at all wise to consider certain receiver observables for interference detection (i.e., C/N0); rather it is beneficial to utilize certain specific observables, such as the RDS of raw digitized signal levels or the AGC-based observables that can uniquely identify a critical malicious interference occurrence

Signal processing techniques for GNSS anti-spoofing algorithms

The Global Navigation Satellite Systems (GNSS) usage is growing at a very high rate, and more applications are relying on GNSS for correct functioning. With the introduction of new GNSSs, like the European Galileo and the Chinese Beidou, in addition to the existing ones, the United States Global Positioning System (GPS) and the Russian GLONASS, the applications, accuracy of the position and usage of the signals are increasing by the day. Given that GNSS signals are received with very low power, they are prone to interference events that may reduce the usage or decrease the accuracy. From these interference, the spoofing attack is the one that has drawn major concerns in the GNSS community. A spoofing attack consist on the transmission of GNSS-like signals, with the goal of taking control of the receiver and make it compute an erroneous position and time solution. In the thesis, we focus on the design and validation of different signal processing techniques, that aim at detection and mitigation of the spoofing attack effects. These are standalone techniques, working at the receiver’s level and providing discrimination of spoofing events without the need of external hardware or communication links. Four different techniques are explored, each of them with its unique sets of advantages and disadvantages, and a unique approach to spoofing detection. For these techniques, a spoofing detection algorithm is designed and implemented, and its capabilities are validated by means of a set of datasets containing spoofing signals. The thesis focuses on two different aspects of the techniques, divided as per detection and mitigation capabilities. Both detection techniques are complementary, their joint use is explored and experimental results are shown that demonstrate the advantages. In addition, each mitigation technique is analyzed separately as they require specialized receiver architecture in order to achieve spoofing detection and mitigation. These techniques are able to decrease the effects of the spoofing attacks, to the point of removing the spoofing signal from the receiver and compute navigation solutions that are not controlled by the spoofer and lead in more accurate end results. The main contributions of this thesis are: the description of a multidimensional ratio metric test for distinction between spoofing and multipath effects; the introduction of a cross-check between automatic gain control measurements and the carrier to noise density ratio, for distinction between spoofing attacks and other interference events; the description of a novel signal processing method for detection and mitigation of spoofing effects, based on the use of linear regression algorithms; and the description of a spoofing detection algorithm based on a feedback tracking architecture

GNSS Vulnerabilities and Existing Solutions:A Review of the Literature

This literature review paper focuses on existing vulnerabilities associated with global navigation satellite systems (GNSSs). With respect to the civilian/non encrypted GNSSs, they are employed for proving positioning, navigation and timing (PNT) solutions across a wide range of industries. Some of these include electric power grids, stock exchange systems, cellular communications, agriculture, unmanned aerial systems and intelligent transportation systems. In this survey paper, physical degradations, existing threats and solutions adopted in academia and industry are presented. In regards to GNSS threats, jamming and spoofing attacks as well as detection techniques adopted in the literature are surveyed and summarized. Also discussed are multipath propagation in GNSS and non line-of-sight (NLoS) detection techniques. The review also identifies and discusses open research areas and techniques which can be investigated for the purpose of enhancing the robustness of GNSS

Mass-Market Receiver for Static Positioning: Tests and Statistical Analyses

Nowadays, there are several low cost GPS receivers able to provide both pseudorange and carrier phase measurements in the L1band, that allow to have good realtime performances in outdoor condition. The present paper describes a set of dedicated tests in order to evaluate the positioning accuracy in static conditions. The quality of the pseudorange and the carrier phase measurements let hope for interesting results. The use of such kind of receiver could be extended to a large number of professional applications, like engineering fields: survey, georeferencing, monitoring, cadastral mapping and cadastral road. In this work, the receivers performance is verified considering a single frequency solution trying to fix the phase ambiguity, when possible. Different solutions are defined: code, float and fix solutions. In order to solve the phase ambiguities different methods are considered. Each test performed is statistically analyzed, highlighting the effects of different factors on precision and accurac

GNSS Signal Authentication via Power and Distortion Monitoring

We propose a simple low-cost technique that enables civil Global Positioning System (GPS) receivers and other civil global navigation satellite system (GNSS) receivers to reliably detect carry-off spoofing and jamming. The technique, which we call the Power-Distortion detector, classifies received signals as interference-free, multipath-afflicted, spoofed, or jammed according to observations of received power and correlatio n function distortion. It does not depend on external hardware or a network connection and can be readily implemented on many receivers via a firmware update. Crucially, the detector can with high probability distinguish low-power spoofing from ordinary multipath. In testing against over 25 high-quality empirical data sets yielding over 900,000 separate detection tests, the detector correctly alarms on all malicious spoofing or jamming attack s while maintaining a <0.5% single-channel false alarm rate.Aerospace Engineering and Engineering Mechanic

Radio Frequency Interference Impact Assessment on Global Navigation Satellite Systems

The Institute for the Protection and Security of the Citizen of the EC Joint Research Centre (IPSC-JRC) has been mandated to perform a study on the Radio Frequency (RF) threat against telecommunications and ICT control systems. This study is divided into two parts. The rst part concerns the assessment of high energy radio frequency (HERF) threats, where the focus is on the generation of electromagnetic pulses (EMP), the development of corresponding devices and the possible impact on ICT and power distribution systems. The second part of the study concerns radio frequency interference (RFI) with regard to global navigation satellite systems (GNSS). This document contributes to the second part and contains a detailed literature study disclosing the weaknesses of GNSS systems. Whereas the HERF analysis only concerns intentional interference issues, this study on GNSS also takes into account unintentional interference, enlarging the spectrum of plausible interference scenarios.JRC.DG.G.6-Security technology assessmen

GNSS Integrity Monitoring assisted by Signal Processing techniques in Harsh Environments

The Global Navigation Satellite Systems (GNSS) applications are growing and more pervasive in the modern society. The presence of multi-constellation GNSS receivers able to use signals coming from different systems like the american Global Positioning System (GPS), the european Galileo, the Chinese Beidou and the russian GLONASS, permits to have more accuracy in position solution. All the receivers provide always more reliable solution but it is important to monitor the possible presence of problems in the position computation. These problems could be caused by the presence of impairments given by unintentional sources like multipath generated by the environment or intentional sources like spoofing attacks. In this thesis we focus on design algorithms at signal processing level used to assist Integrity operations in terms of Fault Detection and Exclusion (FDE). These are standalone algorithms all implemented in a software receiver without using external information. The first step was the creation of a detector for correlation distortion due to the multipath with his limitations. Once the detection is performed a quality index for the signal is computed and a decision about the exclusion of a specific Satellite Vehicle (SV) is taken. The exclusion could be not feasible so an alternative approach could be the inflation of the variance of the error models used in the position computation. The quality signal can be even used for spoofinng applications and a novel mitigation technique is developed and presented. In addition, the mitigation of the multipath can be reached at pseudoranges level by using new method to compute the position solution. The main contributions of this thesis are: the development of a multipath, or more in general, impairments detector at signal processing level; the creation of an index to measure the quality of a signal based on the detector’s output; the description of a novel signal processing method for detection and mitigation of spoofing effects, based on the use of linear regression algorithms; An alternative method to compute the Position Velocity and Time (PVT) solution by using different well known algorithms in order to mitigate the effects of the multipath on the position domain

Multipath Propagation, Mitigation and Monitoring in the Light of Galileo and the Modernized GPS

Among the numerous potential sources of GNSS signal degradation, multipath takes on a prominent position. Unlike other errors like ionospheric or tropospheric path delays which can be modeled or significantly reduced by differential techniques, multipath influences cannot be mitigated by such approaches. Although a lot of multipath mitigation techniques have been proposed and developed in the past among them many receiver internal approaches using special signal processing algorithms multipath (especially multipath with small geometric path delays) still remains a major error source. This is why multipath has been a major design driver for the definition of the Galileo signal structure carried out in the past years and the subsequent signal optimization activities. This thesis tries to provide a broad and comprehensive insight into various aspects of multipath propagation, mitigation and monitoring (without claiming to be exhaustive). It contains an overview of the most important aspects of multipath propagation, including the discussion of different types of multipath signals (e.g. specular vs. diffuse multipath, satellite vs. receiver multipath or hardware-induced multipath), typical characteristics such as periodic signal variations whose frequency depends on the satellite-antenna-reflector geometry and the impact on the signal tracking process within a GNSS receiver. A large part of this thesis is dedicated to aspects of multipath mitigation, first providing a summary of the most common multipath mitigation techniques with a special focus on receiver-internal approaches such as the narrow correlation technique, double-delta correlator implementations, the Early-Late Slope (ELS) technique or Early/Early tracking implementations. However, other mitigation approaches such as using arrays of closely spaced antennas or multipath-limiting antennas are discussed as well. Some of these techniques are used for subsequent multipath performance analyses considering signals of the (modernized) GPS and Galileo. These analyses base on a new methodology to estimate typical and meaningful multipath errors making use of multipath error envelopes that are scaled in a suitable way to account for different multipath environments. It will be shown that typical (mean) multipath errors can be derived from these scaled envelopes by computation of the envelopes running average and that these mean multipath errors are of the same order as multipath errors obtained from complex statistical channel models. Another part of this thesis covers various aspects of multipath detection and monitoring. First, current techniques for multipath detection and monitoring are described and discussed with respect to their benefits and drawbacks or their real-time capability. Among the considered approaches are techniques like code minus carrier monitoring, SNR monitoring, the use of differenced observations or spectral and wavelet analysis. Following this introductory overview, a completely new approach for real-time multipath monitoring by processing multi-correlator observations will be introduced. Previously being used primarily for the detection of Evil Waveforms (signal failures that originate from a malfunction of the satellites signal generation and transmission hardware), the same basic observations (linear combinations of correlator outputs) can be used for the development of a multi-correlator-based real-time multipath monitoring system. The objective is to provide the user with instant information whether or not a signal is affected by multipath. The proposed monitoring scheme has been implemented in the form of a Matlab-based software called RTMM (Real-Time Multipath Monitor) which has been used to verify the monitoring approach and to determine its sensitivity.Die Qualität eines Satellitensignals wird durch eine Vielzahl potenzieller Fehlerquellen negativ beeinflusst. Neben atmosphärischen Einflüssen tragen Mehrwegeeinflüsse einen wesentlichen Anteil zum Gesamtfehlerbudget der Satellitennavigation bei. Während eine ganze Reihe von Fehlereinflüssen durch geeignete Modellierung oder differenzielle Verfahren deutlich reduziert werden können, ist dies durch die räumliche Dekorrelation der Mehrwegeeffekte nicht möglich. Obwohl in der Vergangenheit eine Vielzahl von Verfahren zur Mehrwegereduzierung vorgeschlagen und entwickelt wurden, stellen Mehrwegesignale noch immer eine wesentliche, stets zu berücksichtigende Fehlerquelle dar. Aus diesem Grund spielten die zu erwartenden Mehrwegefehler auch eine sehr wichtige Rolle im Zuge der Definition sowie der Optimierung der Galileo-Signalstruktur und können somit als wesentliches Design-Kriterium angesehen werden. Die vorliegende Arbeit gibt einen umfassenden Einblick in verschiedene Aspekte der Mehrwegeausbreitung, -reduzierung sowie der Detektion und der Überwachung auftretender Mehrwegeeffekte. Die Arbeit beschreibt zunächst die wichtigsten Aspekte der Mehrwegeausbreitung, wobei beispielsweise unterschiedliche Arten von Reflexionen oder unterschiedliche Entstehungsarten ebenso diskutiert werden wie typische Auswirkungen von Mehrwegesignalen wie die Entstehung periodischer Signalvariationen. Solche Signalvariationen sind in starkem Maße abhängig von der durch die Satellitenposition, dem Antennenstandpunkt und der Lage des Reflexionspunktes definierten Geometrie. Die Frequenz dieser Signalvariationen wird für unterschiedliche geometrische Verhältnisse berechnet. Zudem werden der Einfluss bzw. die Auswirkungen einer Mehrwegeausbreitung auf den Signalverarbeitungsprozess in einem GNSS Empfänger aufgezeigt. Einen weiteren Schwerpunkt dieser Arbeit bilden die derzeit gebräuchlichen Methoden zur Reduzierung von Mehrwegeeinflüssen. Dabei werden zunächst die wichtigsten empfängerinternen Ansätze vorgestellt. Aber auch Methoden wie die Verwendung von Antennenarrays oder spezieller Antennen bleiben nicht unberücksichtigt. Einige dieser Methoden bilden im Folgenden die Grundlage für die Bestimmung von typischen Mehrwegefehlern. Dazu wird eine neuartige Methodik vorgestellt, um aus Hüllkurven des Mehrwegefehlers aussagekräftige mittlere Mehrwegefehler zu bestimmen. Hierzu werden die Hüllkurven mit Hilfe einiger aus statistischen Kanalmodellen abgeleiteter Parameter in geeigneter Weise skaliert, um unterschiedlichen Mehrwegeumgebungen Rechnung zu tragen. Es wird gezeigt, dass die mit Hilfe dieser relativ einfachen und effizienten Methode ermittelten Mehrwegefehler in derselben Größenordnung liegen wie die aus komplexen statistischen Kanalmodellen ermittelten Fehler. Einen weiteren Themenkomplex stellen Methoden zur Detektion und zum Monitoring von Mehrwegeeinflüssen dar. Dabei werden zunächst derzeit verwendete Ansätze vorgestellt und hinsichtlich ihrer Vor- und Nachteile sowie hinsichtlich ihrer Echtzeitfähigkeit diskutiert. In Anschluss daran wird ein neuartiger Ansatz zur Detektion und zum Monitoring von Mehrwegesignalen in Echtzeit vorgestellt, der auf der Auswertung von Multikorrelatorbeobachtungen basiert. Ziel dieser Entwicklung ist es, einen potenziellen Nutzer sofort darüber informieren zu können, wenn ein Signal mit Mehrwegefehlern behaftet ist. Der vorgeschlagene Ansatz wurde in Form einer Matlab-basierten implementiert, welche im Folgenden zur Verifizierung und zur Bestimmung der Empfindlichkeit des Verfahrens verwendet wird