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

Large spatial variations in ionospheric delay of Global Navigation Satellite System signals observed during severe ionospheric storms pose potential threats to the integrity of the Ground-Based Augmentation System, which supports aircraft precision approaches and landing. Range-domain monitoring within the Ground-Based Augmentation System ground facility cannot completely eliminate all possible ionospheric threats, because ionospheric gradients are not observable to the ground monitor if they impact the satellite-to-ground lines of sight with the worst-possible geometry and velocity. This paper proposes an algorithm called position-domain geometry screening to remove potentially hazardous satellite geometries under worst-case ionospheric conditions. This is done by inflating one ormore integrity parameters broadcast by the ground facility. Hence, the integrity of the system can be guaranteedwithout anymodification of existing avionics. This paper develops an algorithm that allows the ground station to conservatively estimate the worst-case ionospheric errors for Ground-Based Augmentation System users. The results of this algorithm determine which potential aircraft satellite geometries are safe and which are unsafe, and inflation of the broadcast vig parameter is used to make all unsafe geometries unusable for the Ground-Based Augmentation System. Although the elimination of unsafe geometries reduces system availability, this paper shows that acceptable availability for category I precision approaches is attainable at Memphis International Airport and Newark Liberty International Airport while guaranteeing system integrity under anomalous ionospheric gradients.

Ionospheric threat mitigation by geometry screening in ground-based augmentation systems

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

This paper describes a Ground Based Augmentation System (GBAS) ground-based monitor capable of instantly detecting anomalous ionospheric gradients at the time of satellite acquisition. The monitor utilizes differential carrier phase measurements across multiple reference station baselines as the basis for detection. Performance analysis shows that the monitor is highly sensitive to the quality of the carrier phase measurements. Therefore, data collected from a GBAS prototype ground facility is used to quantify the measurement quality and validate the concept monitor.

Carrier Phase Ionospheric Gradient Ground Monitor for GBAS with Experimental Validation

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

The Federal Aviation Administration (FAA) has
developed a Local Area Augmentation System (LAAS)
Test Prototype (LTP) derived from a previously
sponsored FAA university grant. Test results are
presented from this FAA owned and operated Differential
Global Positioning System (DGPS) landing system.
These tests represent an independent evaluation of FAA
sponsored work.
A total of 156 approaches have been flown using the
system described in this report. Approaches were
completed in two FAA-owned Convair 580 aircraft, an
FAA-owned Boeing 727, and a National Research
Council Canada (NRC)-owned Falcon 20. These
procedures were conducted at four operational airports.
The LTP operational flight test results show that this
code-based DGPS landing system performs with a 95%
vertical accuracy which is consistent with previous flight
test results conducted at the WJHTC.
The current system accuracy levels as well as the current
measurement screening integrity algorithm will not be
sufficient to meet the requirements for all categories of
precision approach. Therefore, ongoing FAA/university
research projects have addressed the system deficiencies
and corrective modifications will soon be incorporated
into the LTP for evaluation in further operational tests.

Flight Test Results of the FAA Local Area Augmentation System Test Prototype

The Federal Aviation Administration (FAA) is currently developing a specification for the Local Area Augmentation System (LAAS) Ground Facility. The FAA William J. Hughes Technical Center has been tasked with validation of this specification. As part of the validation, the LAAS Test Prototype (LTP) was used in a series of tests to evaluate LAAS performance at operational airports selected from the LAAS Requirements Document (RD). This paper summarizes the LTP ground and flight tests conducted in Pennsylvania at the Philadelphia International Airport, in Alaska at the Fairbanks International and Cold Bay Airports, and in Minnesota at the Minneapolis–St. Paul International Airport. Over 240 approaches were conducted using the system described in this report. Approaches were completed in both an FAA-owned Boeing 727 and a Falcon 20 owned by the National Research Council of Canada. The results of the LTP tests show that the current LAAS architecture is capable of providing the level of service required by the LAAS RD.

John Warburton

Papers

Carrier Phase Ionospheric Gradient Ground Monitor for GBAS with Experimental Validation

The impact of uninformed RF interference on GBAS and potential mitigations

Carrier Phase Ionospheric Gradient Ground Monitor for GBAS with Experimental Validation

Flight Test Results of the FAA Local Area Augmentation System Test Prototype

Validation of the FAA LAAS Specification Using the LAAS Test Prototype (LTP)