Key point: Drones offer a lot of interesting possibilities for how warfare might change. However, they aren't yet able to displace traditional systems.

After a post-Cold War hiatus, navies across the planet are pursuing new anti-submarine capabilities as a submarine arms race accelerates in the Pacific Ocean. Developing technologies like quantum magnetometers and satellite-based optical sensors are leading to forecasts that submarines may be on the verge of losing their stealthy edge by the mid-twenty-first century.

But swarms of cheap drones both above and below the water (unmanned underwater vehicles, or UUVs) may pose the biggest and most proximate threat to submarines.

This first appeared earlier in 2019 and is being reposted due to reader interest.

Swarming drones are distinct from larger, higher capability (and more expensive) long-range autonomous unmanned vehicles like the Large-Diameter HSU-001 submarine, recently displayed by China, or the Extra-Large Displacement Orca being built for the U.S. Navy.

Swarming systems, by contrast, are small, lightweight, cheap, numerous—and networked together to cooperate like bees from the same hive.

Submarines, larger UUVs, aircraft or warships could flood the ocean with hundreds of such cheap and expendable drone sensor platforms to form a long-endurance surveillance capability that could move based on new intelligence and changing operational requirements.

They could also be deployed by seemingly non-military commercial shipping, such as watercraft used by China’s vast maritime militias. This could make it virtually impossible to tell which ships are deploying and controlling the drones.

Submarine analyst Peter Coates observes in a blog post that underwater swarming drones could particularly improve China’s surveillance capabilities.

“Think of a flotilla of 20 low cost Chinese ‘trawlers’ al peacefully ‘catching fish’, but actually acting as a mother-ship each for 200 low-cost mass-produced mini-UUVs…This flotilla could be assisted by the fixed seabed and tethered anti-submarine sonar sensors China is already stringing across East Asian seas.”

Coates points out that cheap and numerous underwater mini-drones could be strung across hundreds of miles in “nets” to detect and then even follow slow-moving submarines.

“The possibilities are endless, cheap and can occur in peacetime—no need for expensive naval assets. They can ‘cue’ naval assets to our [Australian] submarines, if and when conflict breaks out.”

Worse, as submarine-hunting drones mature they could theoretically acquire offensive kamikaze capabilities similar to existing Switchblade ground-attack mini-drones.

Limitations of Drone Swarms

Drone swarms, if operationalized, are too numerous and cheap to be efficiently destroyed using most current military systems.

But small drones also have big limitations: they can’t carry much fuel or weaponry. That limits their range and speed, meaning they will likely require some sort of long-endurance mothership (or “motherplane”) to deploy from.

Furthermore, in an offensive role, the payload limitation of small drones means they couldn’t carry a large explosive with good odds of a crippling a sub. A Mark 48 heavyweight torpedo has a 650-pound warhead. By comparison, a swarming drone may be limited to 20 pounds or less, making them more likely to sabotage than deliver outright lethal attacks, similar to the limpet-like buzz droids depicted in Star Wars.

Maintaining underwater communication links is also difficult through the dense medium of water. Though autonomous drones can bypass this problem, it doesn’t obviate the need for a surveillance system to transmit back useful data. For offensive autonomous drones, it also raises the ethical issue of whether to trust the sub’s AI to correctly assess if it should launch a potentially lethal attack.

Finally, small UUVs eek out endurance by traveling at very slow speeds. But those speeds would make it very difficult to pursue or intercept a submarine cruising at 6-12 knots or sprinting at up to 30.

Distributed Underwater Wireless Sensor Networks (DUWSNs)

Todays swarming UUV designs are focused primarily on locating mines and submerged objects.

One example is the 3.7-pound SwarmDiver drone developed by Aquabotix, which can be tossed over the side of a boat and remotely controlled by a human operator as shown in this video. The current system already has an application for mine-countermeasure missions, recovery of unexploded ordnance on the seafloor, port security and port defense. 

A system that can detect underwater mines could potentially be evolved to track submarines as well. But submarine hunting drones would need to sweep much greater areas, thus invoking aforementioned communication-link difficulties.

Scientists Michael Kobold and Keith Aliberti at the Australian Naval Institute describe those challenges in an article, while suggesting that acoustic signaling methods could overcome them.

“Underwater acoustic channels, however, feature large-latency and low-bandwidth. In addition, the sensor nodes of a DUWSN are mobile and will shift position relative to one another due to dispersion and shear thereby resulting in coordinated networking amongst large numbers (potentially hundreds to thousands) of densely-deployed sensors a challenge.”

“The ‘rules’ by which the sensor nodes communicate, otherwise known as communication protocols, are key to advancing near-real-time delivery of information. The U.S. Navy has conducted research and development on novel network architectures to meet the needs of short-term, time-critical and long-term, non-time-critical mine-countermeasure operational requirements.”

LOCUSTs

The Navy is also pursuing aerial swarming LOCUST drones (standing for Low-Cost UAV Swarming Technology) to skim low over the ocean hunting subs. These could be dropped from patrol planes. The Navy has also developed cannon-like surface-based launch systems that can rapidly catapult multiple LOCUST drones into the sky.

One example of how drones similar to LOCUST could aid anti-submarine operations concerns the P-8 maritime patrol plane, which lacks its own Magnetic Anomaly Detector (MAD) for use on low-altitude passes, and normally operates at higher altitudes anyway to take better advantage of its other sensors. 

However, small drones dropped by a P-8 could not only handle those low-altitude electromagnetic scans, but allow searching of much wider areas, as MADs have fairly small detection radii.

Navy SHARKs?

In 2013, the Navy also tested a drone developed by Bluefin Robotics called Submarine Hold-At-Risk, or SHARK—part of a larger DARPA program called Distributed Agile Submarine Hunting (DASH). 

In theory, a SHARK-type drone would be deployed after detecting a submarine using passive sensors. SHARK would then employ an active sonar to continuously track that submarine’s movements. 

Active sonar—which transmits soundwaves that bounce off objects underwater—offers superior tracking to passive sonar. But submarines rarely use active sonar because it likely reveals their own position as well, like turning on a flashlight in a dark room. 

Thus, an unmanned system could assume the “risky” job of tracking the enemy submarine with active sonar, while the mothership which deployed it could still reap the benefits by listening in using passive hydrophones.

Submarines versus Swarms

Future submarines will require capabilities to counter drone swarms. They can’t expend their limited supply of expensive torpedoes to pick off underwater drones, and they lack close-defense weapons like those on surface warships.

However, they could themselves deploy interceptor drones from torpedo tubes. One could envision a higher-end recoverable drone designed to rove further ahead of the submarine and disable distant, less-agile drones or static surveillance systems. These might be complemented by cheaper, single-use interceptor drones released in a burst that could also attempt to decoy torpedoes.

Electronic warfare has also already proven highly effective at disabling aerial drones in combat in Ukraine and Syria. Perhaps similar systems could be developed for submarine employment.

A recent article in The Drive highlights that since at least 2013 the Navy has been working on a countermeasure system called the Netted Emulation of Multi-Element Signature against Integrated Sensors, or NEMESIS. 

NEMESIS employs distributed systems including decoys, jammers, acoustic countermeasures, and multiple-input/output radios (MIMOs) to create dozens of fake sensor returns so that opposing operators can’t tell which are real and which are fake.

Thus, as sensors become more diverse and powerful, the future of stealth both above and below the surface may shift from “cloaking” against detection in favor of over-saturating sensors with plausible targets, like a wizard creating illusory duplicates to confound enemies.

While rapidly increasing capacity for cheap but affordable swarming sensor platforms poses an existential challenge to submarines and other stealth platforms, these new systems may in turn prove susceptible to countermeasures also employing networked swarming technology.

Sébastien Roblin holds a master’s degree in conflict resolution from Georgetown University and served as a university instructor for the Peace Corps in China. He has also worked in education, editing, and refugee resettlement in France and the United States. He currently writes on security and military history for War Is Boring. This first appeared earlier in 2019 and is being reposted due to reader interest.

Image: Reuters