China poses several awkward challenges for the US Army.
First, the US Army is a land-based military service and China is an ocean away. Second, the wide-open spaces of the Pacific Ocean would seem to make it more difficult to hide in-bound helicopters and tiltrotors from radar.
Nonetheless, the US Army is making the case that its Future Vertical Lift aircraft, combined with clever tactics, can bring something unique to the Indo-Pacific region. The service thinks its in-development Future Long Range Assault Aircraft (FLRAA), a replacement for the Sikorsky UH-60 Black Hawk utility helicopter, and the Future Attack Reconnaissance Aircraft (FARA), to replace the retired Bell OH-58 Kiowa Warrior scout helicopter, will create new dilemmas for the Chinese military.
New flight automation technologies will enable these rotorcraft to fly faster and closer to the surface of the ocean than ever before, it says. New missiles and loitering munitions will increase the aircrafts’ lethal reach. Tactics, techniques and procedures to match will create a new dimension to joint warfare within the US Indo-Pacific Command, says Brigadier General Walter Rugen, director of the Future Vertical Lift Cross Functional Team within US Army Futures Command.
The challenge is that the Pacific Ocean offers few obvious places to hide. Over land, rotorcraft can hide from enemy radar in river valleys, behind hills or between buildings. The featureless expanse of the ocean seemingly makes spotting, targeting and firing at rotorcraft much easier for units armed with surface- or air-to-air missiles guided by radar or infrared sensors.
The US Army acknowledges there are fewer obvious places to hide over the ocean, but insists there are ways to avoid detection and enemy fire. For starters, rotorcraft can take off vertically and thus can be stationed and concealed on almost any island.
FARA and FLRAA also promise greater range, and thus the ability to be stationed beyond the reach of some Chinese missiles and artillery. Rugen notes that the service’s “pacing threat” are Chinese medium-range ballistic missiles. The US Army wants FLRAA to have a combat radius of 300nm (555km), for instance.
“What the advanced rotorcraft configurations really do for us is they allow us to operate from relative sanctuary, outside of those long range fires that a peer threat may have,” says Rugen. “Then, because we’re operating disaggregated, [we can] quickly aggregate, and then penetrate multiple layers of A2/AD [Anti-Access/Area Denial].”
RADAR HORIZON AND OCEAN CLUTTER
The US Army believes it can also hide rotorcraft in the radar shadow behind the curvature of the Earth. By flying below 300ft above sea level for hundreds of nautical miles, FARA and FLRAA ought to remain concealed until reaching an adversary’s weapons engagement zone or before making the final dash to the target. The service wants FARA to have a cruise speed of at least 180kt (333km/h) and a dash speed of 200kt or greater. And, it wants FLRAA to have a 280kt cruise speed.
The service also believes there are opportunities to hide in radar “clutter” over the Pacific Ocean. Rugen declines to say what he means by clutter, other than to note it does not mean pretending to be civilian shipping, for example.
“We understand the electromagnetic spectrum to a high degree. We understand where the gaps are,” says Rugen. “We understand what ranges we need to operate at to be able to stand outside any enemy weapon engagement zones and bring lethal and non-lethal effects on their systems that can see and maybe hear us in the electromagnetic spectrum.”
The US Army won’t go into detail, but radar engineers and analysts say the wide-open spaces of the ocean are not as hostile to rotorcraft as it might seem. In fact, there are ways to hide while flying over the ocean’s surface.
For starters, though the ocean is flat compared to the mountains and valleys of dry land, it can still reflect as radar clutter, especially at close ranges. That’s because as an adversary beams radar out over the water it not only sees echoes returned from aircraft, but also will have to deal with the signal bouncing off the surface of the ocean, among other objects in its path.
“On the ocean, waves create a dense carpet of clutter interference,” says Christian Wolff, a former officer in the German air force, who now works as a radar engineering consultant and runs the website radartutorial.eu.
Special radar filters can account for sea clutter, says Wolff.
“But each additional filter decreases the power from the weak echo signal of a small target,” he says. “It may therefore be detected later. A few seconds are often important here.”
Clutter often can be tricky to deal with at close range. Wolff notes that visual clutter is a problem human eyes deal with as well. “If you stand close to a house, the field of view is restricted,” he says. “If this house is far away, it may be just a dot on the horizon.”
Radar also has issues distinguishing between ocean clutter and an aircraft flying below a minimum detectable velocity, says Nicholas O’Donoughue, a senior engineer at think tank RAND Corp, who is a radar signal processing expert. “If their velocity is on the order of the ocean speed, then it’s going to be hard to tell them apart from the waves beneath them,” he says.
Radar is very good at measuring the speed of objects moving towards or away from it by gauging the change in frequency in reflected waves, known as the Doppler effect. However, it can struggle to accurately gauge objects moving perpendicular to it. And so, while a rotorcraft flying perpendicular to a radar emitter may in fact be flying quickly, to the radar receiver its apparent velocity might be only a few knots. At that speed it might be indistinguishable from the ocean current, which is also just a few knots.
And then there are atmospheric ducts. These pockets of weather, such as temperature inversions, refract radar.
“Depending on the weather, there may be this zone where radar waves below it are trapped and radio waves above it start to bend up away from the Earth a little bit, that can actually create this kind of blind spot that you can use to hide in,” O’Donoughue says. “You might be able to penetrate a little bit further than the typical radar horizon calculation because some of the signals are being pushed up.”
It would likely require flying at an altitude range of 130ft to 230ft to hide in an atmospheric duct, he says.
The angle of an airborne radar relative to the target aircraft matters too, O’Donoughue says.
“Particularly for airborne look-down radars clutter is a really big problem,” he says. “And as you go to steeper angles, where you’re looking more and more straight down, you get a much stronger return from the clutter.”
SHOWING UP UNINVITED
When it comes to dealing with Chinese look-down/shoot-down radars, the US Army believes part of its advantage is the element of surprise.
“The strength of vertical lift is to show up where they least expect you to,” says Rugen. “That form of deception, that ability to hide in that lower tier gives us a chance to deal with those look-down radars. And then, if we have to deal, we’ll see who has to kind of blink first, based on the ranges.”
Besides FARA and FLRAA are not necessarily the threats that airborne look-down/shoot-down radars are searching for, he says.
“Are the algorithms tuned and all that? It becomes something that they have to contend with if they want to see us,” says Rugen.
Rugen sees a couple of mission roles for FARA. “I think our critical path – and I use those words very specifically – is the deliberate attack [and] the screen,” he says. “And then, obviously, any exploitation of that deliberate attack via disintegration or penetrating multiple layers of A2/AD.”
A deliberate attack is an operation executed based on detailed plans. Missions that aim to exploit gaps in an adversary’s radar coverage and weapons’ range based on intelligence would typically fall into that category. A screen is a security operation mostly aimed at providing early warning to a protected force, for example, by guarding its flanks.
Rugen says air assault, medical evacuation and resupply also fall within the mission profile of FLRAA.
FARA and FLRAA would typically operate at night. “In that period of darkness, we go there, do our actions on the objective and come back under the cover of darkness,” says Rugen.
There isn’t going to be a single cloaking agent for US Army rotorcraft. The service has outlined a plan to use radar hiding techniques, in coordination with an overwhelming joint force, decoys and electronic warfare, to protect aircraft. “You’re seeing the army and Army Aviation come into a space with electronic warfare that it hasn’t been in since the Cold War,” says Rugen.
Novel air-launched effects, a sort of multi-purpose drone launched from FARA, will act as reconnaissance platforms, decoys, radar jammers and even loitering munitions many miles in advance of FARA. For example, in August and September 2020, during its Project Convergence exercises, the service practiced using the Area-I Altius-600 air-launched effect, which has a range of 238nm.
Ultimately, the US Army wants FARA and FLRAA to be closely integrated into a joint force that includes the US Navy, US Marine Corps and US Air Force. All of the services plan to practice futuristic concepts later this year as part of the Project Convergence 2021 exercises, says Rugen.