The number of GPS outages reported by aircrews rose dramatically to 3,000 in Europe in 2018 and now aircraft operators and airlines are starting to pay attention to the problem.
That is in contrast to a decade ago, when Gerhard Berz, the senior expert for navigation systems at Eurocontrol, said it seemed that GPS always worked, so pilots and international air transport associations needn't spend much time thinking about potential problems. “Now aircraft operators and airlines are starting to pay attention to issues related to GPS outages,” he says. These problems are often briefed now as part of the industry's safety oversight efforts.
“So there is quite a lot going on with GNSS outages and we are quite busy,” Berz says. The term global navigation satellite systems (GNSS) encompasses several systems: US Air Force GPS, European Galileo, Russian Glonass and Chinese BeiDou.
Jamming consequences
The FAA says that GPS jamming and spoofing events in the US are rare occurrences, but that the agency has participated in numerous GPS jamming and spoofing tests with the Department of Defense over the past 20 years.
Jamming is when another signal overpowers GPS, which is easy to do since GPS has a very weak signal – less than a millionth of a billionth of a watt, according to Brad Parkinson who is known as the father of GPS. Parkinson served in the US Air Force and as a colonel led the development of GPS. He also founded the Stanford Center for Position, Navigation and Time in the USA and still consults for the FAA.
As recently as June, NASA's Aviation Safety Reporting System reported that an aircraft wandered off course and narrowly avoided crashing into a mountain after the pilot involved and others nearby reported GPS jamming and outages. There was also an abundance of smoke in the area along the approach course to Friedman Memorial Airport in Sun Valley, Idaho. The aircraft was cleared for a GPS type of approach then strayed off course at an altitude of 10,700ft near a 10,900ft mountain. The report states if the radar controller had not noticed and intervened the flight crew and passengers would have died.
Spoofing involves transmitting a signal that allows a GPS receiver to calculate a false position. A more sophisticated version, known as smart jamming, is when a false signal is transmitted that can't be used by a receiver to calculate a position at all. Receivers in this case may continue to function and not show they have detected any fault. Russian use of jamming and smart jamming was recounted in a report, “Above Us Only STARS”, published earlier this year by the Washington DC based NGO the C4ADS group, which analyzes international security issues.
The C4ADS report cites 10,000 Russian GPS spoofing incidents detected over 2 years affecting 1,300 ships in the Mediterranean, Black Sea, Gulf of Finland and in the waters off Vladivostok, Russia. Some of these incidents near Russia and Syria occurred in international waters and may be a by-product of the Russian government protecting Russian President Vladimir Putin and other VIPs from drones.
The report also cites incidents discovered by Professor Todd Humphreys of the University of Texas in Austin. Humphreys has devised a way to use sensors on the International Space Station to detect and record GPS signal disruption in real time.
Another report published last year by the UK Government Office of Science, titled “Time and Position: A Study of Critical Dependencies”, found a growing dependence on GNSS, including in aviation. The report cites GNSS interference measured by two sensors over four years. One was mounted on the roof of a building in London's financial district and another near a suburban airport where light aircraft use GNSS to navigate. Both recorded around four cases of interference every day, believed to coming from road vehicles with jammers. The report found that the vulnerabilities of GNSS, both natural and man-made, are poorly understood and that a backup system is needed.
That report followed one published by the UK Government in 2017, “The Economic Impact on the UK of a Disruption to GNSS” which estimated that a five-day loss of GNSS would create a loss of £5.1 billion (US$6.4 billion). The report also shows that aviation is only receiving about 1% of the £6.7 billion (US$8.3 billion) worth of benefits from GNSS, with roads receiving 49% and emergency and justice services 30%.
Countering the threat
Civil aviation can be affected by jamming that isn't even intended to disrupt its operations. Potential sources include GPS jammers fitted to road vehicles and even solar activity. In addition, nation states and other groups that may want to disrupt civil aviation can directly target aviation activities and disrupt air navigation.
A solution proposed in the US to help counter the threat to GPS is to develop an eLoran (enhanced Long range navigation) system. As a terrestrial based system operating in a low frequency band, eLoran would be almost impossible to jam or spoof, according to Dana Gower, who leads the Resilient Navigation and Timing Foundation in Washington DC. The group lobbies the government to protect, toughen and augment GPS. Defense IT supplier Harris Corporation is a member.
Loran was originally developed as a navigation system during World War 2. By the end of the conflict it covered 30% of the earth's surface and eventually covered 70%. The last Loran C system, a more accurate version than the original, was decommissioned in the US in 2010.
The US Transportation Department's Volpe Center did a study on the vulnerability of GPS in 2001 which stated that an improved Loran C system might provide a backup to GPS for positioning and timing, but that more study was needed. Later the government decommissioned Loran C, but the idea of Loran as a backup has been batted around in Washington government circles ever since. The question of whether a system will actually be built remains to be seen, although there has been some positive action towards the development of a GPS-backup politically in the USA.
GPS is a critical timing source for computer networks and the US financial system. Parkinson says some advocates for eLoran are trying to sell the idea of building a system based on its value as a timing backup alone. However, eLoran can only provide timing backup at the microsecond (one millionth of a second) level, which is sufficient for computer and financial network synchronization but not for the nanosecond (one billionth of a second) timing that scientific users need and already receive from GPS.
Alternative backup
But does civil aviation need eLoran? There is already an array of ground based navaids available that could guide an airline aircraft to a safe landing in the case of a widespread GPS outage. The FAA sees DME/DME (Distance Measuring Equipment) area navigation (RNAV) and ILS (Instrumented Landing Systems) as suitable backups for airline aircraft air navigation and VOR (Very High Frequency (VHF) Omni-Directional Range) for other aircraft. The FAA's NextGen program is adding 130 new DMEs to improve coverage and resiliency in the event of a GPS outage. This program is expected to be completed by 2033.
But spectrum pressure may also pose a threat to aviation's use of DME in the future, Berz notes. International Mobile Telecommunications providers maintain aviation is wasting valuable spectrum by using DME in the L band because it is a high-frequency sweet spot in terms of its range and ability to carry large amounts of data. “A lot of radio services would like to have this part of the spectrum because it is prime real estate,” he says. “At some time in the future pressure could increase to provide mobile broadband to rural communities and governments could decide this is a high priority so maybe the use of DME will not be sustainable over the next couple of decades.”
Pressure also comes from adjacent frequencies. Wireless company Ligado wants to place powerful transmitters on the ground in the US as part of a 5G mobile network that will supersede LTE networks. Ligado, formerly named LightSquared before bankruptcy and reorganization, has not been able to secure access to these frequencies from the US Government, but has not given up trying. Goward and Parkinson are concerned about possible interference with GPS signals and receivers if the government grants permission for adjacent frequency operations.
Parkinson points out that if an eLoran system was built and used by civil aviation then the FAA could retire some of its ground based navaids. And if the US Government, not the FAA, builds an eLoran system for overall GPS backup, most experts believe civil aviation would probably use it along with a dozen other industries that depend on GPS for time synchronization or positioning.
Drones ahead
Airliners are not the only air vehicles expected to be occupying civil airspace in the years ahead. Drones and air taxis will depend a lot on GPS and are unlikely to be equipped with VOR and DME, according to Parkinson. Advocates for drone delivery envision thousands of them flying simultaneously over large urban areas. Air taxis may also be hopping autonomously from one skyscraper helipad to another or directly to a passenger's back yard in the suburb. What if GPS goes out in this scenario?”
You have the problem of navigation and control of the air vehicle but the second problem, which is equivalent and totally unresolved is the air traffic management for these things,” says Parkinson. “The key in both cases is GPS and the pertinent question is, do you have a backup for it?”
Parkinson believes if the US government gives the go ahead to build an eLoran system to act as a backup for the GPS system, a GPS/eLoran computer chip could be developed for small drones and autonomous air taxis that would seamlessly feed eLoran data from the vehicle for air navigation in case GPS goes down. “Industry is not even going to try to develop that unless the government decides to bathe the critical areas with eLoran signals,” he says.
Is eLoran likely?
Parkinson believes building an eLoran system would cost about as much as one GPS satellite, around US$300 million and that it would have cost less if the US Government had not dismantled the Loran C system.
He also believes that eLoran may be about to happen finally after 15 years of discussion because of several political developments. Goward agrees with him.
In Washington, the National Space-Based Positioning, Navigation, and Timing (PNT) Advisory Board, which provides advise on GPS-related policy, has been recommending that the US government give the go ahead to build an eLoran system for several years. The PNT Board comprises of 25 industry, academic and international GPS experts, who advise the US government on GPS policy and programs. They last met in early June. Former PNT advisory board member Scott Pace became the executive secretary of the National Space Council when it was revived by the Trump administration in 2017. Diana Furchtgott-Roth, the DOT Deputy Assistant Secretary for Research and Technology, also supports a GPS backup. Several US Senators including Ted Cruz also back the initiative to build a GPS backup.
Suitable backup
The FAA says that eLoran would be a suitable backup for aviation but using it would require all aircraft to be equipped with new avionics which makes it unaffordable for aviation compared to DME/DME and VOR. However, if eLoran becomes an operational system in the future, the FAA says it would assess its viability as a backup for aviation. However, the FAA also highlights that the current reversion to DME/DME is seamless. Current avionics on aircraft automatically switch to DME/DME RNAV if GPS becomes unavailable and aircrews currently fly RNAV arrivals, departures and conventional airway routes.
Parkinson believes that eLoran is a cheap insurance policy. “Granted there are no commercial receivers right now. You need the signal first and then you will get a user. I'm convinced we are going to end up with some simple augmentation to a computer chip and it is going to cost very little. The interfaces are going to be the same if you do it right. It will not be as accurate. It might be 25 meters, maybe something like that.”
European hurdles
The USA is in the enviable position of being one country covering a large part of a continent. That makes it much easier to establish an eLoran backup system. Berz believes that establishing an eLoran system in Europe is a nonstarter because it would face major political and financial hurdles. Europe did have a Loran system that covered the British Isles and France. Stations in Russia might still be working but information from the country is unclear. A new common system with continental coverage in Europe would have to include installations in the UK, Scandinavia, France, all of the Mediterranean states and also Russia among others.
However, Berz believes that if the USA built an eLoran backup system and aircraft started to be equipped there, the situation in Europe could change. The maritime industry could also be a prime mover on the system. And these two are not the only industries benefiting from GPS. A study released in June by the National Institute of Standards and Technology, “Economic Benefits of the Global Positioning System”, identifies US$1.4 trillion in benefits from GPS over 33 years to private industries in the USA. These industries included agriculture, electricity, finance, location-based services, mining, maritime, oil and gas, surveying and telecommunications. There are three billion GPS receivers in use and the study estimates a widespread outage would cost the United States US$1 billion per day.
But eLoran would not be a panacea. Goward says that systems engineering says you need to have three to five sources for something you can't live without, so the PNT advisory board has been advocating a multilayered approach. A single backup to GPS is better than no backup, but the advisory board has advocated a balance of using GPS/GNSS, an inertial measurement unit for navigation, a clock for timing, and a difficult to disrupt terrestrial signal such as eLoran.
“The government instituting a single backup and encouraging the use of others is the first step to a more complete architecture,” says Goward. The foundation he leads has also approached the US Government about establishing a more complete system for detecting and warning mariners and other users about GPS disruptions around the world.
“Others see us as calling the GPS baby ugly, but we see it as calling the GPS baby precious and deserving of as much protection as we can give it,” Goward says. We haven't been doing enough to protect it in terms of the signals and with complementary capability that would dissuade people from messing with GPS and provide users some capability when GPS is not available.