GPS jamming hard to combat

The recently opened air link between Helsinki and Tartu has been put on an extended hiatus due to the GPS satellite navigation system's inability to provide pilots with full assurance for approaches to Tartu Airport. "Aktuaalne kaamera. Nädal" investigated the circumstances under which GPS should assist pilots and what can be done to counter GPS interference.
While the seats on the flight from Tallinn to Kuressaare are filling up, pilots in the cockpit perform their final checks, and just a few minutes later, the wheels are in the air. Tallinn quickly shrinks to a model-sized view. About ten minutes after takeoff, however, the GPS signal begins to fluctuate.
Loss of GPS signal is nothing new for pilots. Such incidents have been occurring daily recently.
"Whether it lasts two minutes, three minutes or five minutes depends on the day, the weather and the altitude, but mostly here, towards Kuressaare, it's considerably less. The main problem remains around Tartu. For us, it's a problem when we are high up, the signal restores as we descend," said NyxAir pilot Tanel Pajo.
GPS is a satellite navigation system owned by the government of the United States. It began development in the 1970s and is primarily based on a constellation of 24 satellites located 20,000 kilometers above the Earth.
"Each satellite has precise atomic clocks and constantly sends out signals stating 'I am satellite such and such, I am currently at this location, and the time is so much.' If a receiver on the ground can capture signals from at least four such satellites, it can calculate your position and, as an added bonus, you also get the exact time," explained Ivo Müürsepp, a senior lecturer at the Tallinn University of Technology.
There is no connection to the electrical grid in space, and the energy available is quite limited. Consequently, the signals that reach Earth are actually very weak.
"For example, GPS doesn't work very well indoors, and similarly under dense vegetation, in narrow streets and in all such places, the signal may disappear or be so poor that we can no longer pinpoint an accurate location," said Müürsepp.
For the same reasons, GPS is easy to jam. A jammer on the ground simply produces a stronger signal and effectively 'shouts over' the satellites in space. Simple jammers are sold online for a few hundred euros. Some run on batteries, while others can be plugged into a car's cigarette lighter.
"It's generally intended to suppress the signal within a range of a few dozen meters, for example, if you are a prisoner with an ankle monitor trying to sneak away. However, the larger jammers used for security and military reasons are certainly more complex and expensive, but there's nothing overly complicated about them," Müürsepp explained.
Russian jammers, which have caused GPS connectivity issues in Estonia and neighboring countries, are usually placed in higher locations, such as on cell towers. According to technical data, one jammer should cover an area of about 25 kilometers, but multiple units can be combined to create larger affected areas.
"There isn't a very good way to limit this interference. Turning off the source always helps, but since the source is located in a foreign country's territory, this is complicated. In this case, the sector is regulated by the International Telecommunication Union (ITU), of which both Estonia and Russia are members. Their charter includes a clause that one should not cause radio interference to neighbors, and they also have mechanisms to combat these disturbances," said Erko Kulu, head of the frequency management department at the Consumer Protection and Technical Regulatory Authority (TTJA).
"It's quite difficult to limit [GPS interference]. One good way is to simply use geography. If the jammer is beyond the curvature of the Earth, then the signal simply doesn't reach us. That's why we don't perceive any jamming on the ground here," Müürsepp explained.
Therefore, there shouldn't be concerns that someone might lose GPS signal due to a jammer in a border forest and get lost.
"We haven't received such reports from people – from those who hike in forests and so on – at the TTJA. We've had a couple of instances where drone operators reported disturbances, and once from private companies that offer geolocation services. /.../ Maybe more so for drones because they fly higher, but there's also likely a factor that drones are not allowed to fly very close to the border, so the impact isn't as significant," said Kulu.
GPS disruptions in airplanes are also due to the fact that they fly high and their line-of-sight range is broad. Although NyxAir pilots have noticed problems daily, GPS has always eventually returned, and they have managed to land.
"Approach is very important. Here, in Kuressaare, we don't have this issue, but otherwise, we'd be in exactly the same situation as Tartu if we were closer to the border," said Tanel Pajo.
Finnair has initially suspended flights to Tartu for a month due to GPS disruptions. There's a possibility that this suspension might extend. Various solutions have been discussed, including more qualified air traffic control, but currently, the decision has been made to proceed with installing radio beacons, which Pajo mentioned were once removed due to cost savings and the allure of new systems.
"That's actually the solution to this problem. Then we don't need to use GPS-based landing systems, and then it's back to basics, as they say – back to the old ways and everything fits," said Pajo.
According to the pilot, it's possible to land based on visual observation, but not always. Therefore, it's understandable why Finnair pilots had to return to Helsinki twice.
"The problem with Tartu seems to be that it's at night. At night, this airport is new for pilots. The runway lighting, I can't say how good it is there. Certainly, on the days it was canceled, there were clouds, and there was simply nothing the pilots could do," Pajo speculated.
In good weather, visual landing wouldn't be a problem at all.
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Editor: Merili Nael, Marcus Turovski
Source: "AK. Nädal"