USAF and Honeywell Test Quantum Sensors for GPS-Free Aircraft Navigation

An aircraft flew without relying on GPS navigation signals, instead using quantum sensing technology to measure its movement and maintain an estimate of its position. It may sound like something from a particularly ambitious science-fiction script, but this was very much a real flight test conducted by the US Air Force Research Laboratory (AFRL) and Honeywell Aerospace.

Perhaps the most interesting part of the story is what wasn't happening during those four hours. There was no continual GPS position update coming from satellites overhead telling the aircraft where it was. Instead, the experimental system was measuring the aircraft's own movements and using those measurements to work out where it had gone. In other words, rather than asking the satellites for directions, the aircraft was attempting to keep track of itself.

That could become rather important in a world where GPS is no longer the infallible technological miracle we once treated it as. Modern aviation has become heavily dependent on satellite navigation, but GPS signals are relatively weak and can be disrupted. Jamming can prevent a receiver from getting useful information, while spoofing can go a step further by providing false information and potentially convincing the aircraft that it is somewhere it isn't.

For an airliner operating its normal route, that is already something worth taking seriously. For a military aircraft operating in contested airspace, it is considerably less amusing. An aircraft that depends on an external signal for its position is rather like a person who has memorised every road in the city but still needs Google Maps to tell them which street they are standing on.

The obvious solution is to give the aircraft a way of navigating independently, and that is where quantum sensing enters the picture. The AFRL system uses quantum sensors to measure motion and changes in an aircraft's movement with extremely high sensitivity. Those measurements can then be combined to estimate the aircraft's position without continuously checking its location against GPS satellites.

The basic idea isn't actually new. Aircraft have been using inertial navigation systems for decades, with accelerometers and gyroscopes measuring movement and rotation so that onboard computers can calculate where the aircraft should be. The problem is that no measurement system is perfect. Tiny errors inevitably creep in, and over time those errors accumulate, causing the calculated position to drift away from the aircraft's actual position.

Quantum sensors are being investigated as a way of tackling that problem. Quantum sensing makes use of the behaviour of atoms and other quantum systems to make exceptionally precise measurements. In navigation applications, researchers are working towards sensors that can measure acceleration and rotation accurately enough to allow an aircraft to maintain a reliable internal estimate of where it is travelling.

That distinction is important because this isn't simply a case of replacing the GPS antenna with something that looks more futuristic. The objective is to create a navigation system that can continue providing useful information when the outside world is no longer providing a convenient reference. If the aircraft can accurately measure every acceleration, turn and movement, it can theoretically continue calculating its position without having to phone home to a constellation of satellites.

The four-hour flight over the ocean was therefore a significant practical test. The aircraft operated without GPS navigation inputs while the quantum navigation technology was evaluated under real flight conditions. Flying over the open ocean also gave the researchers an environment in which the aircraft could operate for an extended period without the system being constantly checked against obvious external references such as terrain.

Four hours, of course, isn't exactly an impressive endurance figure when compared with the length of a modern transoceanic airliner flight. Nobody is suggesting that tomorrow's Boeing 787 will leave Johannesburg for London with the GPS switched off and a quantum sensor politely telling the crew which way to go. The significance is that the technology has moved beyond simply being something that works under carefully controlled laboratory conditions and has been demonstrated during an actual flight.

There is still a considerable amount of work to be done. One of the biggest challenges with GPS-free navigation is that even very small measurement errors can build up over time. Quantum sensors may reduce that drift, but the technology will have to demonstrate that it can maintain sufficient accuracy over much longer periods and under a much wider range of operating conditions before it can become a practical replacement or supplement for established navigation systems.

And that last word — supplement — may ultimately be more important than replacement. The future of navigation is unlikely to involve throwing away GPS and putting all our faith in quantum physics. A much more sensible approach is to use several independent systems that can cross-check one another and keep an aircraft navigating when one source of information becomes unavailable.

The US military is already looking at other techniques, including navigation using terrain, celestial observations and other external references. Combining those methods with conventional inertial navigation, GPS and potentially quantum sensors could provide aircraft with several independent ways of establishing where they are.

That sort of redundancy is particularly attractive for military aviation, where an adversary actively trying to interfere with navigation systems is a fairly reasonable assumption rather than a particularly gloomy thought exercise. An aircraft that can continue navigating accurately despite GPS being jammed or spoofed becomes considerably more difficult to disrupt.

There could also be civilian applications. Ships, autonomous vehicles and other systems operating in environments where satellite navigation is unreliable could potentially benefit from GPS-independent navigation. The technology could eventually provide an additional layer of resilience wherever knowing exactly where you are is rather important — which, admittedly, covers a surprisingly large portion of modern transportation.

The latest flight test therefore represents another step towards a future in which an aircraft doesn't necessarily need to rely on a satellite to tell it where it is. The US Air Force will now need to establish how well the technology performs during longer flights, under different conditions and during more demanding missions.

For now, GPS remains firmly entrenched as the backbone of modern navigation. But the Americans are clearly thinking about the day when it isn't available, isn't reliable or, worse, is deliberately lying to the aircraft. After all, there are few things more embarrassing for a highly sophisticated military aircraft than discovering that the world's most advanced navigation system has suddenly become the electronic equivalent of a passenger asking, “Are we there yet?”