What is stealth warfare?

Modern conflict is very much a game of cat and mouse where combatants try to stay hidden. Stealth warfare - as it is called - is not just a tactical advantage, but a field of applied science. It draws on physics, signal processing, materials engineering and mathematics - all designed to outpace the systems built to detect and destroy. To understand how stealth works, I met Ramsey Faragher - CEO of the Royal Institute of Navigation - at Queens’ College, Cambridge. I began by asking him how submarines attempt to avoid detection…

Ramsey - The primary thing they try to do is stay incredibly quiet. They move very, very slowly, typically at just walking pace when they're trying to be quiet underwater. From the very beginning of the vessel’s design, silence has been carefully considered. So there are padded floors to reduce vibration from people moving around inside the submarine, and no big clunky, whirring, banging equipment on board. Another trick they employ is to hide among other sources of noise. If it's raining heavily, submarines can hide beneath the sound of the raindrops on the water. And even at certain times of the year, in certain parts of the world, they can hide behind the sound of plankton mating.

Chris - My goodness, what does that sound like?

Ramsey - Lots of loud chittering in the ocean.

Chris - How do they know where they are, though? Because when they're underwater, they don't have windows. You can't look out and search for landmarks. So how does a submarine actually navigate?

Ramsey - That is an excellent and very, very expensive question. They have the world's most expensive accelerometers and gyroscopes on board that monitor every change in speed and orientation, accumulating those changes in acceleration and rotation. That's called inertial navigation. Over time, the uncertainty about where they are increases, so they still need some kind of positional fix. If they're confident they can raise a periscope, they'll do so with a GPS antenna and get a quick GPS fix. Then they might not need to come up again for an entire day, relying on the gyroscopes and accelerometers. Submarines also have favourite particular places where they'll take depth readings. They'll go to known features, ping once, and measure the depth against an undersea mountain or similar structure to get periodic position fixes.

Chris - When we want to see where things are, we often use radar. In World War II it was a game-changer, wasn't it? Being able to see through mist and fog and detect things before they even knew you were there. How does radar do what it does?

Ramsey - Radar is a radio signal that is transmitted very, very loudly from a big transmitter with a very specific pattern. The radar then listens for that pattern to reflect off objects and come back. As you said, radar was invented just before World War II, but it's still the predominant way we detect aircraft at long range today. The vast majority of stealth technologies are about hiding from those pings.

Chris - When you want to hide something from radar, though, how effective is that? Is it really possible to hide from radar, and how does that work?

Ramsey - One method is to coat an aircraft or ship with radar-absorbent materials, usually advanced versions of iron filings in rubber or paint. Various tricks are used to absorb the radio signal. The second method is to design the shape of the vessel or aircraft so that radar pulses bounce away from the source rather than straight back. A classic example is the corner reflector: any L-shape or 90-degree angle, such as a traditional aircraft tail with its vertical fin and horizontal stabilisers, acts as a perfect corner reflector. A radar pulse will bounce straight back to the source, making the plane light up like a fighter-jet Christmas tree. Stealth aircraft avoid these right angles in their main structure.

Chris - Retroreflectors, aren't they? Isn't that how cat's eyes work in the road to reflect light from any angle? You see the cat's eyes illuminate the road ahead. It's the same principle.

Ramsey - Exactly. Bicycle reflectors work the same way, reflecting visible light. Large metal structures a few feet long are excellent for reflecting radar. There is also a third, rare and expensive trick called active suppression. The aircraft quickly captures the radar pulse, makes a copy, and sends it back out of phase to cancel the incoming signal.

Chris - Wouldn't that make a gap, though? If you've got an astute radar operator, they'd see an area darker than it should be, corresponding to a patch of sky. In fact, you'd be looking for the absence of something that tells you, “Whoa, something is trying to hide there.”

Ramsey - In terms of the direct radar return, it would look as though nothing is there, which is what you'd expect. But you're right that with bi-static or multi-static radars—where the pulse is emitted from a different location than the receiver—you can detect things that have absorbed energy. For example, if you usually see radar pulses bouncing off a mountain range and then notice a gap in that return, it could mean a stealth aircraft absorbed the energy that normally reflects back from that spot. All of these sorts of tricks are used both above and below the water.

Chris - Warfare has really shifted with the current conflict, for instance in Ukraine, towards much smaller aircraft. It's not dominated by huge jets and bombers above the skies of Ukraine and Russia. This is being fought by drones. This is really the first time we've seen this on a major scale, isn't it?

Ramsey - Yes, and stealth is quite different in this case and sometimes not needed at all. If you can launch thousands of incredibly cheap drones, they don't need to be stealthy. They can simply overwhelm the defences of the target. And because they're very small and often made of plastic or composite materials, they're often quite invisible to radar too. So you get a double advantage: they’re hard to detect, and there are so many of them.

Chris - The interesting development I've seen more recently is that they're now being flown via fibre optic cables because of jamming. Drones appear, people jam the radio signals controlling them, and now they're making fibre-optic remote-controlled drones.

Ramsey - Yes, exactly. It's the constant evolution of the weapon, the countermeasure, and then the counter-countermeasure. The original weapon was drones flown via radio using GPS. The countermeasure was jamming those signals. The counter-countermeasure was to attach 15 kilometres of fibre-optic cable to each drone and fly them via fibre. This shows just how complicated and rapidly evolving that theatre of war is.