Finding Mobile GPS Jammers using ad-hoc networks

GPS/GNSS jamming systems (sometimes referred to as personal privacy devices) are illegal, and for good reason - they're potentially dangerous.

As reported by InsideGNSS: GPS/GNSS technology constitutes a fundamental element for new intelligent transport systems (ITSs) and their applications, such as advanced driver assistance, dangerous goods tracking, and distance- based toll systems. Due to the weak strength of navigation signals from distant satellites, however, these applications are threatened by malicious as well as unintentional interference.

In particular, so-called in-car jammers — also known as “personal privacy devices” — represent a serious threat for GNSS-based systems and applications.

A common Chinese made GPS Jammer - illegal in the USA
purchasable in the UK, but illegal to use.

These jammers are cheap devices able to obscure, partially or totally, the navigation signals received not only by the operator’s vehicle but also by other receivers in the vicinity.

The introduction of standards for vehicular ad-hoc networks (VANETs) sometimes referred to Car2Car or V2V systems, enables the exchange of data regarding detected interference events. This data exchange can be used to enhance the awareness of interference sources, allowing their localization and mitigation, thus increasing the reliability of future ITSs.

Jamming devices could be thwarted using ad-hoc
vehicle networks - which are being developed in the USA
as well as the EU

GNSS Interference Detection and Localization 
Interest in developing GNSS interference detection and localization capabilities has increased steadily in the last few years. The first efforts in this direction have come for military applications, like the jammer detection and location (JLOC) system developed and propsed by Dr. Alison Brown with NAVSYS. This system provides situational awareness on GPS interference events to U.S. military personnel and predicts effects worldwide to enhance battlefield situational awareness and mission planning.

In the case of identified GPS threats, the system disseminates alerts and reports to subscribed JLOC users through the SIPRNet (Secure Internet Protocol Router Network), a system of interconnected computer networks used by the U.S. departments of defense and state to transmit classified information. The core element is the JLOC master station, operated by the U.S. National Geospatial-Intelligence Agency (NGA), which collects information on interference events through dedicated sensors incorporating GPS receivers. These sensors generate reports when they detect signal degradation as measured by carrier-to-noise ratio (C/N0) readings.

For the actual localization of the interference source, angle-of-arrival (AOA) information is collected from receivers capable of digital beam-forming, while time-difference-of-arrival (TDOA) measurements are computed with snapshots of data from multiple locations.

Further studies have been done to integrate mobile devices, such as cell phones, into the network of sensors. Also airborne sensors that can be carried on small unmanned aerial vehicles (UAVs) are envisaged for possible use in future jammer location systems.

Small UAV (drones) could be used to identify GPS jamming
sources in the USA

Interference monitoring and localization systems have been developed to protect safety critical aviation services as required by the International Civil Aviation Organization (ICAO) for GPS-based approach procedures.

In this application, location of interference sources is commonly based on direction-finding sensors. For example, in the United States the localization of interference is performed by the combined use of airborne (AIMDS), transportable (TIMDS), portable (PIMDS), handheld (HIMDS), and fixed (FIMDS) interference monitoring and detection systems. In Europe, similar systems are operated, such as the so-called GIMOS (GNSS Interference Monitoring System) in Germany.

A GBAS reference station outside of an airport.

The problem that can be caused for aviation by GPS jammers became very visible in November 2009 at the Newark Liberty International Airport (New York City). Without an appropriate interference localization system in place, it took more than three months to find the reason for periodic outages of the airport’s ground-based augmentation system (GBAS) reference stations: a GPS jammer transmitting from a vehicle passing near the airport. The airport eventually tackled the problem by making infrastructural changes, such as relocating the GBAS reference antennas over a wider area and enhancing interference detection and localization capabilities.

For maritime applications, the threat represented by jammers became evident in 2008, when the General Lighthouse Authorities of the United Kingdom and Ireland (GLA) conducted a series of sea-trials with the aim of characterizing the full effects of GPS jamming on safe navigation at sea. The test unveiled serious effects on GLA differential GPS (DGPS) reference stations and GPS receivers on ships, as well as radar systems using GPS as their time reference.

These results gave rise to the GAARDIAN/SENTINEL projects. The GAARDIAN project developed and deployed probe sensors at various locations around the United Kingdom and Ireland to continuously report on the integrity, continuity, accuracy, and reliability of GPS signals. The monitoring network was then further expanded by including the Ordnance Survey’s OS Net, consisting of more than 100 continuously operating GNSS receivers. This system bases detection of anomalies on a mask for C/N0 measurements.

The SENTINEL project later added interference localization capability to this network through the use of handheld GPS interference detection devices. AOA, TDOA, and differential-received-signal-strength (DRSS)–based geolocation are currently under investigation.

A down-to-earth alternative

Another way to cope with jammers is to deploy backup systems that do not depend on satellite signals, but rely on terrestrial signals instead. In America radio-navigation and air-traffic-control systems based on terrestrial beacons, which predate GPS, were supposed to be phased out by 2018 in favor of satellite-based alternatives, under a modernization program called NextGen, overseen by the Federal Aviation Administration (FAA). Switching to satellite-based air-traffic control would, for example, allow more direct routes and save fuel, because aircraft would no longer have to follow a 'wiggly' route from one ground-based beacon to another.

In a paper presented at the NAV10 conference in London in December, Mitch Narins, chief systems engineer at the FAA, and colleagues described the Newark jamming incident as “a valuable lesson” because it highlighted the risks of becoming too dependent on satellite-based systems that were vulnerable to disruption. Mr Narins and his team are now investigating whether the old-style terrestrial systems can be modernized and extended to provide a backup that could take over in the event of GPS failures. They expect to make their recommendations in 2013 or 2014, in time for implementation to begin in 2016.

An eLoran reference station

Elsewhere, eLoran is another non-satellite-based alternative which has many cheerleaders. It is an enhanced version of Loran-C, which is itself an improved version of the original Loran (“long-range navigation”) system developed in the 1940s. Once widely used in America, Japan and parts of Europe, Loran fell out of favor with the emergence of satellite-based systems. But its proponents have continued to develop the technology, and eLoran is now accurate to within 10 meters, which is comparable to GPS. “It is terrestrial as opposed to spaced-based, uses very high-powered signals rather than low-powered ones and it's very low frequency instead of high,” says Sally Basker, president of the International Loran Association. “All of which means its failure mechanisms are different to GPS and other satellite-navigation systems.”

Enthusiasm for eLoran is strongest in Britain, where the government awarded a 15-year contract in 2007 to develop eLoran for use by shipping in western Europe. A ministerial decision to move from the development to the operational phase is expected shortly. In the United States and Canada, however, Loran-C transmitters were switched off last year. After a long debate about the merits of keeping the system going, Barack Obama declared it outdated. The House of Representatives has given the Department of Homeland Security until April to decide whether a single, national GPS backup system is required. Which technology would be used to build such a system remains to be seen.

In a way, GPS has become a victim of its own success. Because it is used for such a wide range of civilian purposes, when somebody wishes to disable one GPS-based system, their actions can also disrupt other, unrelated systems. The benefits of satellite positioning are undeniable, and they are only likely to increase in future. But it is now clear that fully realizing those benefits depends on putting systems in place to mitigate against deliberate and accidental interference, and to provide an independent backup that does not rely on the delicate trilling of distant satellites.