Today, most of us would probably rate cellphones, ATMs and the Internet as the three most useful modern gadgets we use regularly. We likely wouldn’t rank GPS up there, and maybe not even in the top 10. Yet without GPS, those three wouldn’t work too well, if at all, and neither would a host of other things that we depend on (reliable electrical power; banking systems; national and worldwide telecommunications, including air traffic control; and car navigation, to name a few). And with NextGen slowly approaching, aviation’s dependence on GPS will grow exponentially.

But their dependability is not assured for the future. Considering their importance, the GPS satellite signals are improbably weak: someone compared them to detecting a Christmas tree light bulb in New York, when viewed from Los Angeles. The reason is that in the 1970s USAF developers had classified technology that could hide them from adversary detection deep in the atmospheric “noise.” But parallel civil technology eventually caught up and public GPS access was granted in 1983. On Sept. 10, 2001, the Department of Transportation warned that the weakness of the signals made them vulnerable to deliberate jamming. Events the very next day demonstrated the reality of ill intent.

Today, the worldwide GPS market and the system’s applications have expanded beyond anyone’s imagination, with the number of receivers in use estimated to be “close to half a billion.” Accompanying the rise in GPS usage have been ever increasing reports of signal interference and jamming. One UK report stated that over a six-month period in 2011, twenty dedicated GPS signal monitors spread across the country had recorded between 50 and 450 deliberate interference events every day. Of these, almost all were attributed to small, low-powered devices selling for around $50 on the Internet since, as in the U.S., GPS jammers are illegal in the UK. Nevertheless, several thousand are reportedly in use in Britain and more than100,000 in the U.S., their main purpose being to defeat GPS tracking systems installed in trucks by their company owners. But their numbers are said to be steadily increasing and, unfortunately, their low power levels make them difficult to detect, especially in moving traffic. Last November, the Department of Homeland Security reportedly introduced a U.S. nationwide GPS monitoring project called Patriot Watch, similar to the system in the UK.

What does GPS jamming mean for aviation? Because the most common jammers today are low powered, their main threat is to lower-altitude aircraft on a GPS, GPS/Waas or GPS/RNP approach and on the airport surface, where GPS-driven airport maps are being used. AIN reported last year on the collateral jamming of the ground-based augmentation system (GBAS) at Newark, which suffered random and unpredictable shutdowns that were eventually found to be caused by jammers in trucks travelling along the nearby New Jersey Turnpike. The only cure for the problem was to move the four GBAS antennas farther infield to a point out of range of the jammers.

But this is unlikely to be a permanent cure. Inevitably, under a “bigger is better” mindset, some buyers will feel more protected with a more powerful, longer-range jammer, and these too are now available on the Internet. This of course raises the threat level since, depending on their antenna configuration, they increase the likelihood of higher-altitude interference, with a consequent impact on ADS-B.

Is There Safety in Numbers?

Will other, more recently assembled GNSS constellations such as Europe’s Galileo be less affected by GPS jammers? The answer, unfortunately, is no. All GNSS satellites transmit in the same quite narrow set of frequencies, with each satellite assigned a unique coded ID. On the plus side, the DOD’s next-generation GPS III will transmit more powerful signals than at present, but jamming follows the military practice of escalating countermeasures as the adversary increases his.

It was also once felt that when Europe and China joined the U.S. and Russia with full worldwide GNSS constellations, along with smaller regional constellations over India and Japan, the 130 or more satellites would always provide superior performance by allowing user receivers to select the optimum satellite geometry, with maximum redundancy. That is still true, but with all those satellites continuously transmitting, initial recent research suggests that the ambient atmospheric noise “floor” also rises, potentially further weakening those already weak satellite signals, to the jammers’ advantage.

Spoofing

Spoofing is an essentially military technique that can allow control of a UAV to be taken over electronically in flight and directed elsewhere on false GPS data. Earlier this year, Iran claimed that it used spoofing to capture an unmanned U.S. RQ-170 Sentinel, a claim the USAF has challenged. However, the fact that it appears to have made a successful wheels-up landing with relatively little damage has created concern in the Pentagon and elsewhere, since both Russia and China are known to have conducted extensive spoofing research. Spoofing is a complex process and, as far as is known, has not been achieved with an unmanned aircraft in the West. If Iran’s claim is true, it would have a profound effect on future unmanned aircraft warfare strategies.

The Future of Aviation GPS

Unquestionably, GPS jamming will increase and, as we move further into a satellite-based NextGen environment, its interference will become more noticeable. This raises two key questions (not only for NextGen but also for all other critical GPS applications). For aviation, will GPS reach the point of no longer being totally dependable? Second, if so, are the backup systems proposed for NextGen adequate for the long-term future?

AIN will explore some of the options ahead in a future issue.

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