Expert Advice: Jamming: A Clear and Present Danger

April 1, 2010  - By

SallyBasker_120By Sallie Basker

A packed audience attended the National Physical Laboratory in the United Kingdom for a February 23 meeting titled, “GPS Jamming and Interference: A Clear and Present Danger,” organized by the Digital Systems Knowledge Transfer Network.

In his keynote address, David Last described a dark, silent and dangerous world without GPS. He regaled attendees with tales from his experience as a GPS forensic expert, assisting the police who beat a path to his door bearing interesting boxes that turned out to be all sorts of jammers: of GNSS, of mobile phones, and of other radio systems. Last pointed to the near future when he believes that spoofers will undoubtedly make an appearance. The defences are limited: detection, prosecution, and the use of alternative sources of positioning, navigation, and timing information, perhaps eLoran.

His final insight was this: “Navigation is no longer about how to measure where you are accurately. That’s easy. Now it’s how to do so reliably, safely, robustly.”

Jim Doherty, from the U.S. Institute of Defense Analyses, discussed the use of existing resources for time and frequency backup. Drawing on his experience, Doherty delivered three overarching thoughts:

  • use all available means;
  • re-use existing systems where possible; and
  • produce integrated time and navigation.

He advised the audience to be conservative with their designs and not to go too close to the boundary conditions. He also noted that there is an important trade-off between independence and cost when considering complementary systems. Finally, he identified a potential need for eLoran to support synchronisation in aviation’s multi-lateration systems.

Moving on, Alan Grant of the UK General Lighthouse Authorities (GLA) described recent GPS jamming trials. He demonstrated that GPS jamming has wildly different effects, ranging from total denial to hazardously misleading information (HMI). HMI was particularly problematic: it caused the ship’s GPS receivers to report a realistic course and speed well away from the truth that was provided by the GLA’s eLoran system. He noted that the impact depends on the ship’s bridge design.

Professor and consultant Martyn Thomas spoke on an ongoing Royal Academy of Engineering study on GPS vulnerability, which brings together experts from across the UK and will report in early June.

This was followed by three presentations on coverage prediction by Robert Watson of Bath University, on interference detection using the U.S. National Geospatial Intelligence Agency’s GPS Jammer Location (JLOC) system by Alison Brown of NavSys Corporation, and on the GNSS Availability, Accuracy, Reliability anD Integrity Assessment for Timing and Navigation (GAARDIAN) interference detection system by Charles Curry of Chronos Technology.

The conference audience learned that any system can be jammed, that JLOC detects thousands of jammers on a daily basis — nearly all of them unintentional — and that the GAARDIAN system has integrated GPS, eLoran, and clocks for interference detection and mitigation.

Tom Willems from Septentrio and Peter McIlroy from Raytheon gave a good overview of what can be done with receivers and antennas. Willems focused on pulse blanking and adaptive notch filtering. He saw a clear trend towards hybridization, and confirmed that manufacturers recognise that GNSS is not a golden bullet — they can mitigate some interference but not all.

Peter McIlroy told listeners to “defeat interference and jamming before you detect it.” This included hybridization with inertial systems, putting some form of barrier between the antenna and the jammer, and the use of controlled pattern-reception antennas. He suggested that controlled pattern-reception antennas might become available for civil use.

Finally, Paul Groves from the University College London gave a very useful overview on positioning without GNSS. He addressed radio and non-radio systems and presented a fascinating chart that related the various radio systems in terms of range and lifecycle (Figure 1). The message was very timely given the need for complementary systems expressed by all speakers.

FIGURE 1. Range and lifecycles of current radio systems (courtesy Paul Groves).


I then chaired a lively panel discussion with David Last, Martyn Thomas, Charles Curry, Jim Doherty, and Tom Willems. I led off by focusing the discussion on resilient PNT, referring to the UK Center for the Protection of National Infrastructure’s definition for resilience: the equipment and architecture used are inherently reliable, secured against obvious external threats, and capable of withstanding some degree of damage.

The panel agreed on the need for hybrid solutions with multiple technologies. It expressed concerns that cheap GPS receivers are components in many systems, and it is too easy to overlook them. Martyn Thomas brought insight from the computing world and noted that we need to avoid single points of failure and to demonstrate independence.

Do our governments understand and should they do more? The panel thought that different governments are at different points on a journey, and that very few policymakers understand how a loss of GPS impacts critical national infrastructure. It was suggested that the European Union lags behind, due to the focus on Galileo.

This led to an interesting discussion about economics and funding. Martyn Thomas said that GPS vulnerabilities have grown, and that GPS competitors have disappeared for economic reasons, leaving us dependent on GPS. He pointed out that there are limited mechanisms for sharing funding and questioned whether there are many (any) organisations that are prepared to take the risk.

If you have limited funding, should it be used for detection or mitigation? The panel agreed that both were needed, but the prevailing view was that mitigation is more important, and that this needs to be supported by human factors activity.

In Summary. GNSS interference is a real and present danger. It is probably more widespread than generally assumed, and it is here to stay. We can harden our GNSS systems with improved receiver and antenna design, but this will mitigate only some interference, not all. The problem is cost. Cheap — and vulnerable — GNSS receivers will inevitably find their way, unseen, to the heart of our critical infrastructure. We need resilient positioning, navigation, and timing based on independent and complementary systems and sensors. Demonstrating independence is vital but not necessarily straightforward, and true independence costs money. The greatest challenge is helping policymakers understand the risks of relying on vulnerable systems and the need for resilience.

Finally, I return to Jim Doherty’s overarching thoughts: use all available means; re-use existing systems where possible; and produce integrated time and navigation.

eLoran, anyone?

SALLY BASKER is director of research and radionavigation for the General Lighthouse Authorities of the United Kingdom and Ireland.

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