Slammin' the Jammin'

By Gavin Schrock, PLS

GNSS jamming and spoofing are serious subjects, as is radio frequency interference both intentional and unintentional. GNSS publications, seminars, and even advertisements have been highlighting the subject of jamming and interference especially in the past few years, as one of the key vulnerabilities of GNSS (space weather and policy are other potential hazards). But there are additional considerations that must be weighed when considering such hazards, including how far we should go in trying to prepare for what could amount to endless hypotheticals. To quote J.R.R. Tolkien, this subject includes an element of the saying, “The wolf that one hears is worse than the Orc that one fears.”

I don’t mean to open the entire subject of jamming; that would take volumes and volumes to cover, and there is plenty of press on it already. The subject is a gigantic can of worms. Some of the worms exist as a harsh reality for users such as the military, and some range from isolated and not readily verifiable anecdotes to well-documented and frightening incidents.  Two examples of the latter are the disruption of navigation GPS at the Newark Airport in 2009 from a trucker’s little GPS jammer, and the accidental jamming attributed to Navy equipment in San Diego in 2007. 

Rather than going in to even more details on these incidents and adding to the deluge of commentary that followed, this article focuses on solutions developed for hazards facing military users of GNSS in real-world situations, with an eye to how such development could benefit civilian users if we were ever to face actual Jammageddon.

A Visit to NovAtel

NovAtel is a GNSS manufacturer that also develops anti-jamming solutions. Visiting its headquarters and main development facility in Calgary, Canada, is real geek treat.

You may be using NovAtel technology and products without realizing it. With their own line of high-precision GNSS receivers and antennas, NovAtel produces OEM boards for other manufacturers such as Leica, and they produce hardened, reference station receivers for the FAA, such as those used to support the WAAS system and various landing systems in development.

The very modern facility was designed with GNSS development in mind; HVAC ducts, wiring, and piping are under-floor, to reduce incidental multipath and interference in rooftop GNSS antenna. Nearly every office is pre-wired with multiple coaxial cables leading from antenna mounts on the roof. I was shown the anechoic chamber used for testing equipment in a radio-frequency clean environment, and they have a dome in the roof with a robotic arm for antenna testing and modeling.

The “Gadget”

NovAtel product manager Neil Gerein continued the tour through research labs and testing and assembly rooms, past the office where the big guy sits—Nov-Atel CEO Michael Ritter—central to the development teams where he likes to be. I sat with Neil and got some details (or as much as could be shared) about their GAJT (dubbed the “gadget”), the GPS Anti-Jamming Antenna Technology antenna system. You may have seen pictures of these types of antennas mounted on military vehicles. Much bigger, heavier, and bulkier than your typical GPS antenna, it has a world of wizardry inside. Gerein showed the 14 chipsets (seven for L1 and seven for L2) installed with the spiral-patterned antenna. The M-code ready antenna can detect as many as six distinct jamming sources at once and produce antenna nulls for each.

At first glance, the hefty unit, weighing about as much as a bowling ball, appears to be armored. Along with the inherent hardening, the thick baffled casing acts primarily as a heat sink; anti-jamming can be a real power hog, and and the unit needs to operate in harsh and hot NATO desert environments. Gerein explained that a simple one-watt jammer can disrupt and deny GPS positioning over a wide area. To provide real-world jamming hazard simulations, NovAtel produces a military field jammer so that soldiers can experience what to expect before live operations.

Gerein gave examples of other field techniques used to mitigate jamming situations: users can even gain protection by shielding antennae with their bodies. Even commercial GNSS receivers and antennas can experience some default protection from rogue signals through their standard signal and multipath rejection features. Because jamming and interference sources are typically ground-based, masking serves to protect as well.

When asked about the little low-cost trucker-jammers (used by some commercial drivers to dupe the GPS tracking installed on their trucks by their employers), Gerein explained that the real hazard comes from poor design and manufacture. “Those who sell these jammers say they emit only a few milliwatts, but poor soldering and cheap components might cause it to emit much higher; this was something discovered [by those] investigating the Newark airport incident. That these devices are in many ways highly illegal to operate, and most certainly represent cause for termination by employers, has been the primary reason why they have not been as widely used as many feared.”

“A military jammer may be only one watt; that can produce the desired disruption over an area of many miles,” explains Michael Ritter. “The case in Korea where the North probably used [about] a 100-watt jammer disrupted many [commercial and military flights in the south].” He continued, “A [broadband] tower at 1500 watts, especially if right next door to GPS [in the L-Band], could cause any number of problems.” (I decided not to pursue comments on recent telecommunications and GNSS controversies; that’s another huge can of worms.) “There can be interference that is deliberately produced, interference that is accidentally produced, and interference from signals deliberately transmitted that the [transmitter] does not have any idea will cause problems,” says Ritter.

How Many Deliberate and Inadvertent Hazards Are There?

The U.S. federal government’s Patriot Watch system involves monitoring, detection, analysis, and reporting. Through a web application, critical infrastructure sectors such as military, civil aviation, navigation, and timing can look at reports, frequency, and reported effects of jamming. These range from a relatively small number of frightening incidents to many inadvertent or deliberate trucker-jammer reports. Much of the non-formal reporting is anecdotal.

In a decade of operating a statewide real-time network in Washington, with tens of thousands of hours logged by field users, we have suspected only a few potential hazard spots near military installations, but even these have not undergone rigorous verification. Sometimes it only takes the “heightened awareness” that such hazards could exist to have a field user speculating that it might be jamming as the root of their RTK fixing and communications woes. Others know by experience that they have worked, without solution degradation, right in the middle of antenna farms and around high-powered transmission lines.

GNSS satellites are often described as “100-watt light bulbs in space,” and, given the right inadvertent circumstances or deliberate malfeasance, there are many possible scenarios of interference, but so far there has not been widespread reporting as such. I asked about such hazards as harmonics from otherwise harmless transmission like TV and radio towers; the response was, “This does happen, but it is rare” that it reaches a point where it disrupts GPS.
True Jammageddon may be just around the corner, but then again, it may never reach a level of wholesale disruption of users’ field operations. I asked the folks at NovAtel about the impact of so much publicity of the potential hazards in recent years and how possibly the sounding of alarms could be somewhat self-serving for GNSS developers, and the subtle answer was, “We leave that [sort of thing] to others.”

The Fire Hose

One tool that NovAtel developed to study jamming and interference is a very sensitive monitoring antenna coupled with analysis software developed by John B. Schleppe, NovAtel Engineering Fellow. The “Fire Hose” is a stock antenna, in a stock “bucket style” casing (for the pole-top style receivers they produce that surveyors use) with an Ethernet cable exiting the place where the antenna cable would be. This was designed so that these could be placed in and around the development facilities (or anywhere in the world, for that matter), and via Ethernet/internet the signals received can make it to the desks and lab benches of the developers.

The software analyzes combinations of in-band and out-of-band signals for potential interference. With the Nov-Atel facility so close to Alberta Highway 2 (the busy “gasoline alley” between Calgary and Edmonton), I asked Schleppe if he has noticed many trucker-jammer traces, he said, “Not very much, but once in a while we can see a wild spike in the graphs that we are not sure about.”
The Fire Hose has caught the attention of aviation GNSS clients, and it is a great alternative to the expensive spectrum analyzers commercially available. I have a vision of placing these around our real-time network to monitor over the internet what is really happening out there, if anything really is. Developers of consumer devices and even phone apps have looked at spectrum, the latter with the idea of “crowd sourcing” interference monitoring, but it is likely that such tools would not be sophisticated enough to yield any verifiable and actionable information.

A Good Strategy

Design and develop for current real-world hazards (like those facing the military), and keep such development and research at the ready should we face more widespread issues. The market will drive production of defensive solutions for GNSS, but until we are faced with enough interference to make day-to-day field operations unworkable, the best approach appears to be ready to act, and we can trust that manufacturers like NovAtel are keeping up the research and development. Don’t panic, but be ready to.

Gavin Schrock, PLS is a surveyor, technology writer, and operator of an RTN. He’s also associate editor of this magazine. 

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