Using quantum mechanics to safeguard GPS navigation

But first, when people say things are “up in the air”, that’s often a bad sign; but not this week because the UK has successfully completed flight trials of an advanced quantum-based navigation system that can’t be confused or jammed by hostile states. It’s so accurate it means that even if GPS signals are lost for long periods of time, it can still keep track of our location. To find out more, I caught up with Ramsey Faragher, a fellow at Queen’s College in Cambridge, and fellow at the Royal Institute of Navigation. First we talked about how present day GPS works, and how it can be vulnerable…

Ramsey - The main tool that we all use, whether we're a person on the way to the shops or whether we're a submarine captain or a fighter pilot, we rely very heavily on the global navigation satellite systems of which GPS is the most famous. These are a lot of satellites that orbit the earth. They transmit radio waves down to us here. We have little receivers in our smartphones, in our cars, and in our submarines and fighter jets that use those radio signals to work out where you are and when you are. They're a metre level accurate positioning system and a nanosecond accurate timing system.

Chris - And how does my GPS receiver, whether it's in a phone, it's in my car, how does it know where I am?

Ramsey - Fundamentally, they are atomic clocks in space that transmit the time and because the speed of light is constant, when you are different distances from each satellite, you get those copies of time at slightly different times. So you collect up these numbers from the satellites and you do a little bit of maths that calculates, in order for me to have picked up this particular sequence of the time from these particular satellites, I must be at this location on the Earth's surface. We've become very reliant on these signals and because these signals travel a very long distance to get to us, 20,000 kilometres to get to us, if you have a nearby jammer, which is just spraying noise out on the same radio frequency, it completely blankets the signal and it removes the ability to use GPS anymore.

Chris - Are they vulnerable as well? If someone were to create false signals, could it be fooled into thinking you are where you're not?

Ramsey - Yes, that's called spoofing. It used to be just the realm of Hollywood movies. There was a James Bond film all about spoofing GPS. Nowadays, unfortunately, it's possible to download software on the internet and buy a cheap little software defined radio and actually spoof your own GPS. People do this to cheat at Pokemon Go and to do research into GPS and to test GPS receivers in smartphone companies and car companies and things before they ship them in their products. So it's an unfortunate feature that in order to test these systems well before we give them out to the public, we design them so that they can be spoofed easily so that they can be tested well. It's a difficult catch 22 but, yes, it's easier than we would like for people to broadcast fake GPS signals into receivers and trick them into thinking that they are somewhere where they're not.

Chris - And the new system that they're now testing, what's the vulnerability that that's designed to protect against and how does it work?

Ramsey - So, because GPS can be jammed, it can be spoofed and it can simply be blocked. If you drive through a tunnel, for example, the signals can't penetrate through the tunnel covering. We use what are called inertial sensors to bridge those gaps. This consists of a little box containing accelerometers and gyroscopes. When the GPS signal is unavailable, the accelerometers measure your changes in acceleration. The gyroscopes measure your changes in heading, and you can keep adding up the measurements from them to work out how you've moved since your last GPS fix. Your smartphone has little accelerometers and gyroscopes in it as well that are predominantly used to play games and to make the screen turn when you turn the phone. But even the tiny cheap accelerometers and gyroscopes inside your phone can do a really good job of bridging GPS fixes for say, 10 seconds. So this new quantum navigator is a much, much more accurate and sensitive accelerometer and gyroscope set than we've ever had before. That means you can survive very big GPS outages, maybe hours.

Chris - How does it work?

Ramsey - It uses cold atom interferometry. A chap called Bose, who's less famous, and Einstein, who we all know, they wrote a seminal paper a long time ago on how, if you cool down atoms enough, they enter a new state that we call a Bose Einstein condensate. What happens is, all of the atoms of the gas collapse down to a single energy state, and they all now start to act as waves instead of particles. And so because they're now at the quantum limits where they can all be treated like waves instead of particles, you can do wave stuff with them, like interfere with them and defract them and make use of their wavelength. Atoms don't normally have the concept of a wavelength until you bring them down to this point. The wavelength of these cold atom systems is about ten to a hundred thousand times finer than the wavelengths of lasers, and so the laser based gyroscopes we've been using for years are about a hundred thousand times less accurate than these ones. So it's very exciting. That's why we're using the new technology. It's a whole new level of sensitivity and performance.

Chris - How big is it though, Ramsey? And is this easy to do, easy to build, easy to apply? Because the report I read was of a whole bank of material and equipment on aircraft.

Ramsey - Yeah. So I had the pleasure, about 15 years ago, to go and visit early work in Imperial College in London on these cold atom interferometers and I was pretty much standing inside the device at the time. I was inside a lab where the entire room was dedicated simply to trapping, cooling and using these atoms. There were lasers everywhere and power supplies and lots of tubing and piping, and it's still container sized from what I gather from some of the experiments that have been going on, but the aim is to get tabletop and eventually shoebox sized versions of these sensors.

Chris - Can they cope with going round corners really quickly, though, because aircraft, it's not the gentlest experience, and if you're a military aircraft, these guys are going to be pulling multiple Gs to take evasive actions and so on. How do their clouds of cold atoms cope with that?

Ramsey - Yes, that's a very good question. So the different designs, the different types of accelerometer and gyroscopes that we've used over the years all have different levels of sensitivity, accuracy, and dynamic range, which is how big a force they can survive and still operate. At the moment, the cold atom technologies don't have high dynamic ranges. It's literally a cloud of atoms, tiny little clouds of atoms that are being thrown up and down inside a tube. So, if you're midway through chucking these tiny little atoms up and down the tube, and then you give the tube a big kick, they all smash against the side of the tube and you can't measure them anymore. This is the current problem that they'll be working on overcoming. There are various ideas on combining the cold atom technologies with traditional technologies so that when the cold atoms are available you've got all of that accuracy and in high dynamics you free wheel on a normal inertial navigation system. These are the main engineering challenges, Chris. Us physicists don't worry about such things.