No Signal: Combat Aviation in the Time of GPS Denial
How to fly a combat mission when your GPS signal is jammed or spoofed

A true and robust GPS signal is essential to our lives today. Without it we’d be unable to summon a ride on Über or track our pizza delivery on our smartphones.

This GPS signal is even more critical in conflict. A widespread GPS-outage, especially in the context of military conflicts, is no longer simply a matter of time. It’s already happening. And air forces around the world have to get a fix on this problem, especially since they are responsible for primary surveillance and combat response in conflicts, today.

Why a GPS Signal is Important in Conflicts

Each side in a conflict today will rely heavily on the integrity of its signal from a multi-constellation GNSS (Global Navigation Satellite System) like the Global Positioning System (GPS) or satellite system counterparts of other origins like the Russian GLONASS, Chinese BeiDou or the European Union’s Galileo. And rivals are doing their best to deny a signal to their adversaries to prevent them from carrying out their operations, freely.

This barely-restrained scuffle is taking place because the availability, integrity or denial of a GPS signal has a disproportionate impact on the outcome of a conflict, given how critical it is for military operations like navigation of manned and unmanned aircraft, and targeting and deployment of weapons. A basic objective for rivals in any conflict is to deny a GPS signal to the other, by jamming or spoofing; with anyone who controls it, able to bend or manipulate the actions of their adversary.

And while there could be other reasons why GNSS coverage is absent over an area, like solar activity, jamming or spoofing of GNSS signals, the prospect of conducting operations in GNSS-denied conditions is no longer as distant eventuality.

What does GPS do?

Industry has been pondering the issue of how military aviation can carry out their missions in GNSS-denied, GNSS-free or GNSS-independent conditions, irrespective of the reason behind the absence of GNSS.

Specifically, what they aim to substitute from the absent GNSS are its Positioning, Navigation and Timing (PNT) capabilities. These three components are what allow us to order an Über and what fighter pilots use to fly to the right spot and carry out their missions, whether it is surveillance or deployment of weapons.

A Rand Corporation study last year began by defining PNT capability as ‘the ability to locate specific points on the earth (positioning), accurately develop and execute routing to move people and goods from place to place (navigation), and synchronize events and actions across wide locations (timing)’.

Sweden’s defence and aviation company, Saab AB, has been working on the problem and they say they’ve devised methods and technologies that would allow military aircraft to operate in GNSS-denied environments.

Speaking to StratPost, Tobias Jansson, Product Manager at the Gripen E fighter aircraft program and Jussi Halmetoja, former Swedish Air Force pilot and operations advisor at Saab, explained the importance of PNT in military aviation.

They say that what’s fundamental for a tactical formation of military aircraft is having the correct data and, importantly, having the same data among them.

“In the modern context where you operate with several aircraft in the air — in a tactical air unit, all these platforms need to have the same information about time, about navigation and positioning. Because if those things differ, that will then adversely affect your collective situational awareness in collaboration. So if you acquire something — a target in the air or sites on the ground; if you have different navigational accuracy, or time or positioning in comparison to your friends in the air, then you have differences -— you will indicate an error in the acquisition of that target,” Halmetoja said.

These errors, unless checked, grow larger over time, area covered and complexity of mission. Timing, according to him, is something that can be corrected via tactical datalink collaboration. And Saab has a solution ready to deal with the challenge of positioning and navigation, with their GNSS-independent solution which can determine positioning and navigation to a high degree of precision. The solutions for all three PNT elements are ready to be part of the Gripen E fighter.

“So that’s what we are trying to kind of solve here so we can be as efficient as possible. If you don’t have a GPS signal you will have less accuracy with time in your mission due to the drift of the gyros — this is physics. So we have to find different ways to do it but the problem of declining accuracy in positioning and navigation remains. Operationally, what’s important is to keep the accuracy of your situational awareness track data as high as possible,” explained Halmetoja.

Combat Missions Without GPS

Saab has developed a system that combines the capabilities of Odometry, Terrain Navigation and Image-Based Navigation that includes 3-D Mapping, in addition to the Inertial Navigation System (INS), to not only complement, but in some conditions, supplant the gaps left by denial of GNSS.

“We need to go back to basics,” said Jansson. In normal circumstances, he explained, the Inertial Navigation System (INS) and ‘traditional radio navigation’, vaguer in accuracy, begin to drift in the absence of a GNSS-aided system. “The GPS and INS work very well together, but now we need to find something else to keep the drift down,” he said.

And while modern INS systems, ‘for instance in the Gripen E’, are very effective, “Still, if you fly for a long period of time, you will have drift for sure — it’s the laws of physics,” clarified Jansson, explaining, “So we have introduced various other navigation technologies. Terrain navigation, with which we have a lot of experience at Saab, and Image-Based Navigation, which is newer. We fuse these together and use it with the INS to fix a good position —- a robust position, as we call it.”

Saab TerNav

The Saab TerNav algorithm solution is something that grew from developments in the eighties for ‘missile applications’, which went into service on the Viggen – the predecessor to Gripen – even though computer capability then was far from what it is today. “It was ready, so it was always a functionality you could have in the Gripen C/D, as well,” said Jansson.

The TerNav software has been constantly updated. “We have continued with a lot of research and technology studies related to UAVs, to fighters and so on, and we now have completely new, updated algorithms for this functionality, that are ready to go into fighter and missile applications. And this is what we’ve planned for the Gripen E, now,” he explained.

Odometry + Image-Based Navigation

The next component of GNSS-free operations is Image-Based Navigation coupled with Odometry. “Odometry does not require any database. You simply find pixels in your image that have rich terrain information and then the algorithm follows where that pixel goes in the next couple of nanoseconds. During the flight this is compared and cross-referenced with the INS system, and corrects any drift in it,” said Jansson.

“We’ve already flown this application and the algorithms do everything, calculating the drift in the INS and then correcting it, without the pilot needing to do anything. We are now ready to integrate this into the Gripen E and are offering this together with Terrain Navigation to future customers,” he said.

3D Mapping

And finally, Saab relies on 3D Mapping to complete its calculations.

“You have an electro-optical sensor, an image database and an image-map algorithm that compares what the camera sees with the 3D database. You have an electro-optical camera, which could be visual or infra-red. But you don’t have any active sensors, so this is a passive system. It’s obviously dependent on weather and you need to have some kind of rich terrain that is a source of rich information to compare with the database,” he explained.

The 3D database comes from the satellite company, Maxar that produces a satellite imagery-based 3D database that has an accuracy of 3 meters and resolution of 0.5 meters. Jansson pointed out that is only from satellite data, which means that ‘you can have a database of an area where you have never been before’ like North Korea, for example.

We’ve taken the database in collaboration with Maxar and integrated it in the Gripen,” he said. And while the camera didn’t generate any data to work on while the aircraft was in the clouds, Jansson said their team was really surprised at how quickly the algorithm found a correlation with the 3D surface model once it was out of the clouds.

“The system does the fixes automatically. The pilot doesn’t have to do anything. So we integrated this software and we could actually fly it, connected with the rest of the Gripen software, and validated its functionality. And the algorithm implements a very immediate re-alignment of navigational accuracy,” he said.

Mix and Match

These three different technologies for navigational integrity provide options to pilots to choose what to deploy depending on the constraints imposed by flying conditions and operational requirements, said Janssen.

“There could be adverse weather conditions, you might be flying at night or you may or may not want to use your radar because of your tactical scenario. So you need to decide which combination of these technologies to use, in collaboration with your tactical formation,” prescribed Jansson.

You have the three technologies — Terrain Navigation, Odometry and 3D Mapping, and you fuse these together in the aircraft navigational solution. For example, if you’re flying below clouds, you can bring your position error down by enabling the Image Navigation aid. And with both Odometry and 3D Mapping, you can turn off your radar altimeter for Terrain Navigation so that you’re silent, if required. If you’re flying above the clouds, switch the radar altimeter on for Terrain Navigation. All of these will keep correcting the INS,” explained Jansson.

He said, “We’re now offering the combination of these three technologies on the Gripen E, since we’re at a very advanced TRL (Technology Readiness Level). Meanwhile, our research continues and we’re looking at other, new algorithms as well. There are no magic solutions to replacing GNSS capability. Fortunately, we have a very good avionics system, which you can always update with newer elements of these navigation technologies in future. But these three technologies can cover a lot of the gaps that emerge in a GNSS-denied environment.”

Real-World Application

Former Swedish Air Force pilot, Jussi Halmetoja, explained why this is important from a fighter pilot’s point of view. “Imagine you have a platform flying over land, somewhere in an area where you don’t have GPS signals and you have four aircraft — four fighters working collaboratively, to try and actively or passively acquire targets, in the air and on the ground. All those four aircraft will have fairly good navigational accuracy. However over time as they work together, the navigational uncertainty will add up, incrementally and cumulatively, and will be transferred into anything you acquire. And you don’t really know what is the actual accuracy – or essentially, positional error – of that target you are locking your radar on or the target you’re choosing on the ground. Is it going to be good enough to actually employ a weapon against, effectively? So the biggest takeaway from his technology is the ability to maintain the integrity of your accuracy, which directly transfers into a minimised error of your track data quality and the integrity of your entire situational awareness. That’s the biggest takeaway,” he said, adding, “The other one of course, to be able to easily land on dispersed runways where you may not have instrument landing approach technology helping you. So those are the two kind of biggest operational takeaways — why this is so important.”

Halmetoja pointed out, “It’s an easy way for an adversary to deny you of your navigation, positioning and timing to make your missiles fail. And if you don’t have that it will directly affect your probability of effectiveness in terms of intercept of any weapon.”

Managing a New, GPS-Based Anti-Access Area Denial

Saab has designed a three-technology solution for successful operations in GNSS-denied conditions – jammed or spoofed signal – against adversaries that have the capability to create ‘Anti Access Area Denial (A2AD) bubbles’, according to Tobias Jansson, and is envisaged to be operated from ‘all modern systems that operate in such an environment, including fighters, Airborne Early Warning (AEW) platforms, long-range weapons or Manned-Unmanned Teaming concepts’.

The former fighter pilot, Jussi Halmetoja, also noted out that the system works especially well in mountainous regions. “The mountainous areas like the primary operational areas for the Indian Air Force (IAF) have a lot of rich terrain and it will work perfectly, there,” he said, in a nod to Saab’s offer of the Gripen to India.

Jansson said an important design constraint while building this system was to avoid adding to the pilot’s workload. “The man doesn’t need to be in the loop,” he said, adding, “The pilot doesn’t need to do anything. The algorithm will do it. When there is rich terrain, it will update the aircraft’s navigational solution, which means that if you’re flying over clouds, the algorithm will make corrections and get a fix directly with the Terrain Navigation System. This leaves the pilot free to focus on the mission, whether it’s BVR (Beyond Visual Range) or air-to-ground, since it’s a swing role fighter.”

Halmetoja summarised the capability, “You’ve seen the three different technologies. Of course, looking at the capabilities and limitations of some of those sensors, you quickly realise that not all of them will work in clouds, for instance. Some of them will have limitations in night flying, so that’s why we’ve introduced all three together, and we’re also exploring other sensors to complement this capability. The important thing is that the war-fighter will be aware of any possible limitations and plan accordingly, like always, depending on terrain, weather and emission control constraints.”

Jansson outlined the inevitability of operations in a GNSS-denied environment, “There could be many threats to satellite-based navigation systems. There could be natural causes behind interruptions like solar activity. They could be jammed, or worse, spoofed by another country or terrorists. It could be military-grade high-power jamming, it could be ground-based, or an airport jammer. It can be easy for terrorists and other organisations to build jamming capabilities. We have known that if you have an area with GPS interference, you need to plan for that. You can fly around that area. But sometimes you cannot avoid flying into an area and operate in it because of mission requirements. There will always be areas where you will not have any GPS. So this is the context of the modern Anti-Access Area Denial environment in which the Gripen is operating. This will also be the case in the neighbouring countries around India, for sure. And though you might have military-grade technology that protects you from jamming or spoofing, you can be certain that someone will try. You don’t always know where you will be jammed, or if you will be jammed. An adversary might not be able to jam all your aircraft in a very large area, all the time. But you have to be prepared for the prospect of going into an area where your GNSS will fail.”

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Here’s how the integrity of a GPS signal has become crucial in all kinds of conflict today, from passive-aggressive harassment of adversaries to grey-zone conflicts up to all-out war.

Without it, the UAVs (Unmanned Aerial Vehicles) deployed by Azerbaijan wouldn’t have been able to devastate and overwhelm Armenia’s armoured defences in 2020.

GPS spoofing has been deployed in the conflict in Syria with an adverse spillover in Cyprus and Israel, which recently complained about it to Russia.

Norway has also complained about this. And a British warship got its location spoofed in the Black Sea last year.

Jamming systems have been acknowledged by Russia to be part of its electronic warfare inventory and have been deployed in Ukraine since the early days of the conflict in 2014.

Russia has emerged as a pioneer with recurring incidents attributed to its GPS-denial capability, especially affecting maritime traffic.

Even Ukrainian civilians are planning for this eventuality, with printed maps part of their survival kits. And GPS interference is already being monitored by insurance companies as a risk indicator.

The simmering conflict in Ukraine, in particular, has shown the role of GPS spoofing and jamming become common to the point of ubiquitous. Such tactics, especially, are standard for the penultimate stage of the conflict, according to David Kilcullen and increase in such activity would be an indicator of an escalation in conflict.

Commentators in Russia were quoted baring threats to destroy the 32 satellites that GPS relies on with Anti-Satellite (ASAT) weapons late last year.

GPS signal interruptions have also been reported by vessels in the Straits of Hormuz and Persian Gulf, which, are said to be intended to lure shipping into specific territorial waters and national jurisdictions by misdirecting them with jammed or spoofed signals for tactical political objectives, planting seeds of crises or potential casus belli.

Messing with GPS signals doesn’t have to be expensive, either, nor are capabilities limited to state actors. Off-the-shelf GPS jamming devices out there in the world, for example, include sizes as small as to fit a car cigarette lighter socket.

ZDNet reported that ‘Thanks to the rise of cheap software-defined radios (SDRs) and publicly available and open-source GPS spoofing code, the cost of building GPS spoofers has gone down to as low as $300.’

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