Unmanned Vessels as Sensors in U.S. Naval Operations​

By Jonathan Panter

During the wars in Afghanistan and Iraq, images of aerial drones flashed across television screens all over the world. Today, unmanned systems are appearing in the news once again, but in a new context: warfare on and below the surface of the ocean. The Armed Forces of Ukraine have repeatedly attacked Russian warships with explosive-laden “naval drones,” even crowd-funding these efforts. Leading media outlets have covered the U.S. Navy’s newest unmanned vessel initiatives, especially its testing of the “Saildrone” under Task Force 59 in the Persian Gulf. And most recently, under the “Replicator” initiative, the U.S. military has pledged to field thousands of unmanned platforms within two years.

Assessing the promise of unmanned maritime systems, as with any emerging military technology, is difficult. Unmanned ships and submarines have not been operationally employed at scale, and lessons from aerial surveillance and strike do not readily transfer to the physical operating environments and tactical demands of surface and submarine warfare. In addition, military operations are a complex system-of-systems: interlinked platforms, people, and practices. This system-of-systems promises many “use cases” for unmanned systems. But it also raises many pitfalls, since changes in one area can have cascading effects on another.

This article narrows the aperture. It focuses on one specific capability: surface and subsurface vessels employed as sensors. Such vessels are still a nascent set of platforms. The U.S. Navy’s fielding of unmanned aircraft – such as the MQ-8 Fire Scout (a small reconnaissance helicopter) and MQ-25 Stingray (a refueling drone) – is much further along. There are two main reasons for this. First, because their required mission endurance is measured in hours rather than in days or weeks, aerial drones need not be as physically robust to marine environmental conditions and normal wear-and-tear as unmanned vessels. Second, other service branches have already widely tested and deployed aerial drones. Accordingly, efforts to predict the future of unmanned surface and subsurface vessels in fleet operations should proceed with caution. The roll-out period for these platforms may take longer than expected, and may face unforeseen setbacks, including effects on manned operations (especially if the Navy prematurely delegates responsibilities from manned to unmanned systems).

The Role of Sensing

The Navy plans to use unmanned maritime systems to support three broad mission areas: maritime security, power projection, and sea control. Maritime security operations comprise efforts to monitor and prevent piracy, sanctions evasion, terrorism, and other illegal activities. Power projection refers to attacking land targets with shipborne missiles or carrier-based aircraft, or transporting troops and supplies by sea. “Sea control” means fighting enemy ships, submarines, and aircraft so that friendly states can use the sea for power projection or commerce, or so that the enemy cannot do so.

The Navy is developing a variety of unmanned vessels with “kinetic applications” (the ability to use destructive force) to support these three missions. For instance, several classes of large and medium unmanned surface and undersea vessels that carry missiles or mines are under development. But kinetic effects, whether delivered by manned or unmanned platforms, are impossible without sensing.

In naval operations, the three critical forms of sensing are visual, acoustic, and electromagnetic (identifying energy emitted by radars or communications equipment). The foremost purpose of these is to find the enemy and ascertain his disposition and intentions. An attack may follow but not always. It may be desirable to prioritize some targets, or even not to destroy a vessel but to monitor, board, or capture it.

The nature of operations at sea makes accurate sensing vital but uniquely difficult. The oceans are vast, and some areas teem with civilian traffic. As a result, ships that limit their electromagnetic emissions can be hard to find; submarines can hide even more easily. Ships’ magazine capacities are limited, and during wartime, resupply is perilous. Finally, defense against anti-ship missiles is very difficult. Accordingly, in maritime warfare, there is a high premium on striking first, and on hitting the right targets, in the right order, on the first try.

Enter Unmanned Vessels

The U.S. Navy’s investment in unmanned vessels has its origins in a very specific set of strategic circumstances. First, after the Cold War, the U.S. Navy’s surface fleet shrank, even as the demands on it remained constant or grew in the following decades. The Navy needs a larger fleet, but cannot afford one composed largely of exquisite, multi-mission platforms. Second, beginning in the early 2000s, China accelerated its development of “anti-access, area-denial” (“A2/AD”) capabilities, such as attack submarines and anti-ship ballistic missiles, which threaten U.S. forces at great ranges from the Chinese mainland. The Navy needs a way to re-assert sea control so that it can still project power ashore.

Unmanned vessels, according to their advocates, help solve both these problems. They are ostensibly cheaper than manned ships, so the Navy can build a larger fleet under constrained budgets. And, since unmanned vessels do not risk human life, they can be deployed within those areas where A2/AD capabilities pose a threat. By “flooding the battlespace” with swarms of attritable unmanned vessels, the Navy can more readily threaten an adversary’s forces. Doing so also makes it more difficult for an enemy to pick the right targets to strike.

The Navy’s future, large, geographically-dispersed hybrid (manned-unmanned) fleet will need to gather data, share it, and use it to synchronize fires. Small unmanned surface vessels can help spot enemy vessels and distinguish them from civilian shipping or listen for enemy electromagnetic emissions. Unmanned undersea vessels can use acoustic sensors to search for underwater anomalies, or collect data on ocean depth, salinity, and temperature to assist with anti-submarine warfare efforts. Having a greater number of sensors may also make the fleet more resilient, especially if an adversary disables satellites or denies airspace to airborne sensors.

Distributed unmanned sensors can also contribute to maritime security operations. Drug runners, terrorists, and other criminals routinely hide among civilian maritime traffic, and unmanned sensors persistently deployed en masse can help find them. And such capabilities do not solely apply to monitoring criminal activity. China continues to enforce its excessive maritime claims using military, paramilitary, and government-sponsored civilian actors. U.S. and allied efforts to counter this agenda depend heavily on identifying vessels that obscure their activities and intentions.

Challenges

While the utility of unmanned vessels for naval operations as a whole remains under debate, sensing is among the least controversial use cases. Unmanned sensors avoid the ethical and legal issues associated with kinetic applications. And because sensor payloads are lighter than weapons and electromagnetic warfare equipment, the platforms that carry them are smaller. This means they can, in some cases, rely on solar panels rather than diesel engines for propulsion and electrical power generation, extending their endurance.

At the same time, there are notable limits to the value of unmanned vessels as sensors. In particular, electro-optical sensing by small unmanned surface vessels is less revolutionary than sometimes claimed. The smaller a vessel’s “height of eye,” the shorter distance it can see. But scaling up size makes hull, mechanical, and electrical engineering more challenging (and incurs cyber and electromagnetic targeting vulnerabilities). Furthermore, if vessels are intended to be attritable, then their software and hardware cannot be too sophisticated, lest an adversary seize and exploit them. Finally, the widespread use of unmanned sensors will generate huge quantities of data. This is the goal, of course, but also raises risks related to data storage, human oversight over the artificial intelligence used to make predictions and secured communications capacity.

The U.S. Navy rightly recognizes that more information, and greater dispersion of platforms, will be essential to future naval warfare and maritime security operations. Unmanned vessels deployed en masse as sensors are part of the solution, but represent only one narrow capability that should not be confused with other projections about how unmanned systems will affect future naval operations.

Jonathan Panter is a Ph.D. candidate in the Department of Political Science at Columbia University. His dissertation examines political influences on the U.S. Navy’s commitment to forward presence. Prior to attending Columbia, Mr. Panter served as a Surface Warfare Officer in the U.S. Navy. He holds an M.Phil. and M.A. in political science from Columbia, and a B.A. in government from Cornell University. He can be found on LinkedIn.

The views expressed in this piece are the sole opinions of the author and do not necessarily reflect those of the Center for Maritime Strategy or other institutions listed.