In December, 2016 the newest class of unmanned vehicles, Unmanned Underwater Vehicles (UUV) made international headlines after China, in an unprecedented act, seized an unclassified “ocean glider” operated by an oceanographic survey ship, the USNS Bowditch, about 50 nautical miles northwest of Subic Bay in the Philippines. According to the Pentagon, the captured UUV, which was soon returned to the US, was measuring salinity and temperature in the area.

While most people have become very familiar with unmanned aerial vehicles and unmanned ground vehicles, general awareness of the new class of Unmanned Maritime Systems (UMS), including UUVs and Unmanned Surface Vehicles (USV) has just begun. The US Navy currently deploys a variety of UMS platforms for use in mine warfare, mine neutralization, reconnaissance, surveillance, hydrographic surveying, environmental analysis, special operations, and oceanographic research missions. Ranging in size from man-portable systems to as much as 40 feet long. These UMSs can be launched and recovered from submarines or surface vessels.

As demand for UMS missions increases, the types of electronic subsystem payloads deployed on these platforms continues to expand. The question is what are the unique challenges faced by system designers tasked with developing rugged COTS electronics for use on these new unmanned platforms? Not surprisingly, like all unmanned platforms, size, weight and power (SWaP) is a critical concern. As a rule, the smaller one can make the onboard electronics, the smaller the unmanned platform can be, whether we are discussing airborne, ground or seaborne applications. In the case of UUVs, though, there is one key differentiator that works to the benefit of the subsystem integrator. Because UUVs operate in the ocean there is much less concern about thermal management than there is when designing solutions for UGV or UAS systems.

Thermal Management

The Data Transport System (DTS1) is a rugged Network Attached Storage (NAS) file server for use in Unmanned Aerial Vehicles (UAV), Unmanned Underwater Vehicles (UUV), and Intelligence Surveillance Reconnaissance (ISR) aircraft. Easily integrated into network centric systems, the DTS1 is a turnkey, rugged network File Server that houses one Removable Memory Cartridge (RMC) that provides quick off load of data. (Image courtesy of Curtiss-Wright Defense Solutions)

The reason for this reduced concern about heat dissipation is the simple fact that the ocean environment provides the world's largest heat-sink. Cooling comes for free. In many cases, UUV payloads are deployed in discrete pressurized tubes, similar to the way a torpedo is configured, and may in fact have direct contact with ocean water permitted into the UUV's interior. Most UUVs are not entirely pressurized. The tubes may house different electronic system payloads, and because the functions are compartmentalized and depending on functionality, a given tube might be pressurized or not. Because the housing tube is protected from the environment, there is often no need for demanding salt and fog protection for the electronics. Designers do, though, typically specify the use of salt/fog corrosion resistant connectors to provide protection against any ingress of water. The cooling advantage of the ocean environment often enables the use of passive cooling approaches, such as conduction/natural convective-cooled electronic subsystems, which can eliminate the need for fans or other weight-adding or active heat mitigation techniques.

Reducing size and weight for UUV electronics isn't just driven by the small size of the platforms themselves. It's also a goal driven by the fact that UUVs tend to be battery powered, unlike UGV and UAS platforms which are typically powered by gas engines. The goal is to extend the range and persistence that any UUV platform mission can undertake. The electrically rechargeable power cells favored in UUV designs limit their propulsion capability; when a UUV's battery runs out of power the platform is literally “dead-in-the-water.”

The overall weight of the platform is a key determinant for how long and how far the platform can travel. What's more, the desire is to significantly increase the autonomy of underwater drones and to extend their operating time to as much as a month at a time. Battery power is also challenged by the need, at times, to “sprint” in order to intercept a target of interest at distance or on the move.

Another advantage that UUVs have over unmanned air and land platforms is that they are much less susceptible to adverse weather or visibility conditions. A UUV's persistent surveillance mission is less likely to be affected or curtailed by a dust storm or hurricane. Unlike USVs that have to deal with surface vessel traffic and other obstacles, UUVs are liberated from navigation rules.

The designers of UUVs share many common goals with the designers of UGV and UAS platforms, including a preference for COTS electronics, the need to minimize SWaP, and a desire to reuse or leverage solutions for use in air, ground or seaborne platforms. The good news is that the same rugged COTS processors, network switches and data storage solutions that have been fielded and proven on SWaP-sensitive air and ground unmanned platforms have been found to equally excel at satisfying application requirements on board UUVs. These subsystems, optimized for low-power operation, are well suited for the demands of these battery-powered, long duration missions.

Because UUVs come in a wide range of sizes, we have seen demand both for standard, open architecture, small form factor 3U VPX modules and for ultra-small line replaceable unit (LRU) subsystems. For example, one UUV application for Software Defined Radio and Electronic Warfare specified a two-slot 3U VPX solution. In another UUV application, the customer selected a very low power (5W) Parvus DuraNET 20-11 8-port Gigabit Ethernet (GbE) switch with support for IEEE-1588 precision timing protocol (PTP). This ultra-miniature “pocket sized” subsystem (~10 cubic inches in volume, 0.50 lb in weight) required no additional modifications for use underwater.