Critical government and industry sectors (such as law enforcement, transportation, communication, and finance) are growing increasingly dependent on Global Navigation Satellite Systems (GNSS) for positioning, navigation, and timing. At the same time, the availability of low-cost GNSS jamming devices are presenting a serious threat to GNSS and increasing the likelihood of outages to infrastructures relying on GNSS. The attacks range from malicious parties intentionally jamming GNSS signals within a targeted geographical region to uninformed users causing accidental interference. This paper is an overview of different approaches adopted to date to mitigate GNSS disruption caused by intentional and unintentional jamming. The first approach outlined in this paper is the use of inertial systems to aid GNSS. The second and third approaches are the filtering of jamming/interference in the spatial and time-frequency domains, respectively. The fourth approach is vector tracking of GNSS signals in the receiver.

Protecting GNSS Receivers From Jamming and Interference

Global navigation satellite systems (GNSS) and, in particular, the global positioning system (GPS) have become ubiquitous in safety critical infrastructure. Vulnerability of GNSS to radio frequency interference (RFI) from either intentional (jamming) or unintentional sources is an ever growing concern. Hence, GNSS itself has become critical infrastructure which must be protected and its vulnerability to interference alleviated. As the RFI source is unknown a priori , passive localization systems are required; this adds an order of difficulty when compared with transmitter location systems with known and cooperative sources. The need for rapidly localizing the RFI leads to sensor network techniques which consist of spatially distributed sensor nodes (SNs). The localization systems typically use the received signal strength (RSS), source angle of arrival (AOA)/ direction of arrival (DOA), time difference of arrival (TDOA) or a combination of AOA(DOA)/TDOA or frequency difference of arrival (FDOA) measurements to estimate the RFI position. This paper provides an overview of existing systems from the literature and a comparison of these different interference geo-localization techniques.

Interference Localization for Satellite Navigation Systems

Secure reliable position information is indispensable for many transportation systems and services, such as traffic monitoring, fleet management, electronic toll collection, route guidance, vehicle telematics, and emergency response. Unfortunately, civil Global Positioning System (GPS) signals are vulnerable to spoofing attacks. This paper introduces a signal authentication architecture based on a network of cooperative GPS receivers. A receiver in the network correlates its received military P(Y) signal with those received by other receivers (hereinafter referred to as cross-check receivers) to detect spoofing attacks. This paper describes three candidate structures to implement this architecture and evaluates spoofing detection performance through theoretical analyses and field experiments. We show that the spoofing detection performance improves exponentially with increasing number of cross-check receivers. Even if the cross-check receivers are low cost, unreliable, and in challenging environment, cooperative authentication can match, if not outperform, a single high-quality reliable reference receiver in terms of spoofing detection performance.

GPS Signal Authentication From Cooperative Peers

Synchronized voltage and current phasor measurements provided by phasor measurement units (PMUs) have the potential to augment power system monitoring, control, and protection functions. PMUs use the Global Positioning System (GPS) to synchronize measurements across a wide geographical area. Unfortunately, low-received-power, unencrypted civil GPS signals are vulnerable to jamming and spoofing attacks. Accidental receiver malfunction can also lead to incorrect position/time solutions. This paper presents a multi-layered multi-receiver architecture that hardens GPS-based timing against jamming, spoofing, and receiver errors. Our architecture integrates eight countermeasures in all layers of receiver signal and data processing; most of the countermeasures exploit the static and networked nature of time reference receivers. We define five threat models, and qualitatively analyze the effectiveness of each countermeasure against each threat model. The analysis demonstrates that the redundant, independent but complementary countermeasures provide high reliability and robustness.

Reliable GPS-based timing for power systems: A multi-layered multi-receiver architecture

Adaptive antenna array processing is widely known to provide significant anti-interference capabilities within a Global Navigation Satellite Systems (GNSS) receiver. A main challenge in the quest for such receiver architecture has always been the computational/processing requirements. Even more demanding would be to try and incorporate the flexibility of the Software-Defined Radio (SDR) design philosophy in such an implementation. This paper documents a feasible approach to a real-time SDR implementation of a beam-steered GNSS receiver and validates its performance. This research implements a real-time software receiver on a widely-available x86-based multi-core microprocessor to process four-element antenna array data streams sampled with 16-bit resolution. The software receiver is capable of 12 channels all-in-view Controlled Reception Pattern Antenna (CRPA) array processing capable of rejecting multiple interferers. Single Instruction Multiple Data (SIMD) instructions assembly coding and multithreaded programming, the key to such an implementation to reduce computational complexity, are fully documented within the paper. In conventional antenna array systems, receivers use the geometry of antennas and cable lengths known in advance. The documented CRPA implementation is architected to operate without extensive set-up and pre-calibration and leverages Space-Time Adaptive Processing (STAP) to provide adaptation in both the frequency and space domains. The validation component of the paper demonstrates that the developed software receiver operates in real time with live Global Positioning System (GPS) and Wide Area Augmentation System (WAAS) L1 C/A code signal. Further, interference rejection capabilities of the implementation are also demonstrated using multiple synthetic interferers which are added to the live data stream.

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Design and implementation of real-time software radio for anti-interference GPS/WAAS sensors.

RF interference (RFI) has been and will continue to be a significant worry for GNSS users. This paper introduces several different types of RFI, categorizes them based upon the intent (if any) of the RFI transmitter, and then examines a relatively new and growing source of RFI: Personal Privacy Devices (PPDs) that aim to prevent people and vehicles from being tracked by GNSS within a limited area. Unfortunately, signals from PPDs are not well-controlled and can interfere with GNSS receivers several hundred meters away. The impact of PPDs on the GBAS reference station site at Newark Airport, New Jersey and the WAAS reference station at Leesburg, Virginia are illustrated. While GBAS ground station monitoring prevents PPDs from posing a significant integrity threat, PPDs can force the sudden loss of service and thus harm continuity and availability. The hardware and software modifications made to the Newark GBAS installation to reduce this impact are described, and the future benefits of more-flexible ground-station siting and GNSS modernization are also identified. 1.0 Introduction and RF Interference Categorization Because Global Navigation Satellite System (GNSS) signals are very weak when received by user equipment, they are vulnerable to radio frequency interference (RFI). Signals that overlap with GNSS frequencies are likely to come from transmitters much closer than the GNSS satellites. Therefore, these signals can easily "overpower" the GNSS signals and make them unusable. To protect GNSS, existing ITU and FCC regulations prohibit the intentional broadcast of any non-GNSS signals on or near GPS L1/Galileo E1, while lesser protections apply to the GPS L2 and GPS L5/Galileo E5A frequencies. Despite these protections, RFI affecting GNSS is occasionally observed, and its apparent frequency has increased significantly with the number of civil GNSS users. In order to better understand the many possible sources of RFI and their potential effects on GNSS, this paper suggests a means of classifying RFI affecting GNSS into three categories. These categories are not all-inclusive, nor do their names fit all possibilities, but they help to separate RFI scenarios in a way that makes it easier to forecast impacts and design mitigations. The first category is malicious interference, meaning RFI that is intentionally transmitted to prevent the use of GNSS (or make its use hazardous) for as many users as possible. Coordinated hostile broadcast of RFI, while hopefully very rare, has the potential to make GNSS unusable over large regions and is difficult to defeat. Therefore, it makes sense to provide non-GNSS backup services to support transportation and other critical infrastructure needs [1]. The second category, and the focus of this paper, is uninformed interference, which results from the intentional transmission of signals at or near GNSS frequencies but without the desire to cause harm. At first, it may seem that signals deliberately broadcast on or near GNSS frequencies are likely aimed at harming GNSS users, but this is not true of the vast majority of cases, as will be illustrated in the following sections. Personal Privacy Devices, or PPDs, fall into this category and are of particular concern because they have become numerous in the last few years. The third category is accidental interference, which results from unintentional transmissions at or near GNSS frequencies. This usually is due to malfunctions of equipment that is designed to transmit at other frequencies or not to transmit at all. It is less common than uninformed interference both because malfunctions are rare and because they are more rapidly detected now that many GNSS receivers are likely to be in use nearby. On the other hand, accidental interference is more variable because it is not designed to prevent harm to users. Section 2.0 of this paper provides past and recent examples of accidental and uninformed RFI. Section 3.0 focuses on PPD interference and illustrates the Figure 1: RF Interferer Location on Stanford Campus Figure 2: Area at Stanford where GPS was Unusable characteristics of PPDs. Section 4.0 shows the impact of PPDs on the Wide Area Augmentation System (WAAS) Reference Station (WRS) in Leesburg, Virginia, while Section 5.0 describes their more-severe impact on the Ground Based Augmentation System (GBAS) ground facility at Newark Airport in New Jersey. Section 6.0 describes the GBAS hardware and software modifications being pursued to limit the impact of PPDs at Newark Airport so that acceptable GBAS Category I precision approach service can be provided. Section 7.0 concludes the paper and looks forward to the additional mitigation steps that will be made possible by GNSS modernization. 2.0 Examples of RF Interference to GPS Figures 1 and 2 show an instance of RF interference to GPS that occurred at Stanford University in 1999. At the time, construction was occurring in the Engineering section of the Stanford campus. A camera had been Figure 3: RF Interference at Moss Landing Harbor [2] installed on the Durand Building with a good view of the construction site, and an attached datalink transmitted digital pictures of the site to the construction headquarters trailer to allow progress to be monitored. This proceeded without incident until, for some reason, the datalink transitioned from its primary frequency of 1530 MHz to its secondary one of 1570 MHz, which is very close to the GPS L1 frequency of 1575.45 MHz. The GPS lab at Stanford discovered that GPS was suddenly "gone"  we could not acquire or track any signals. We also discovered that other GPS users in the area were affected, including the helicopters that transported severe cases to Stanford Hospital. This made it clear that the outage zone had a radius of at least 1 km. The cause was not immediately evident, but the use of directional antennas and signal analyzers allowed us to track down the offending device and (manually) remove its power source, after which GPS became usable again. Once the offending data transmitter was discovered, Todd Walter of Stanford communicated with the device's designers, who knew that their secondary transmission frequency was close to GPS L1 but thought that such transmissions were legal as long as they remained below a certain power level. In other words, they had no intention of interfering with GPS and had no idea that they were capable of doing so. In any case, this company's understanding was incorrect no intentional transmissions (regardless of power level) are allowed this close to L1. At this time, the civil use of GPS was relatively new, and it is not surprising that the regulations protecting it were not well understood. Uninformed interference due to misunderstandings of this sort should be less likely now that GNSS is well-established. Figure 3 shows another, well-known RFI incident from 2001 that was previously described in [2]. This was an example of accidental interference caused by amplifiers attached to UHF/VHF antennas for receiving over-the-air Figure 4: RFI Generated by GPS Repeater at German

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The impact of uninformed RF interference on GBAS and potential mitigations

Pandemos es un grupo híbrido, atravesado por la inquietud de enfrentar los desafíos del mundo contemporáneo en el contexto de la pandemia. Discutiendo los modos en los que la lógica del capital opera en los campos en los que desarrollan sus actividades, se proponen intervenir con investigaciones y creaciones que no sólo en sus contenidos sino, sobre todo, en sus propias modalidades construyan vínculos que resistan a sus efectos más negativos. Así, en este texto que dialoga con la tradición de los manifiestos como estrategia para hacer visible las indagaciones de los grupos críticos, asumen el desafío de expresar las distintas voces que configuran puntos de vista simultáneos sobre la pregunta que se formula: “¿Qué es Pandemos?”. Por ello, la respuesta no emerge tanto en su literalidad, como en las cesuras que mantienen unida su potencia sin anular las singularidades que la constituyen.

Carmen Tedeschi

Papers

The impact of uninformed RF interference on GBAS and potential mitigations

Manifiesto Pandemos: restos de una grupalidad antiviral.