The office of Naval Research and the Naval Research Laboratory experimentation in underwater acoustic communications research: U.S. Navy and Air Force reservists integrated seamlessly in at-sea experiments in the past year on board NRL research vessels

CHIPS, Jan-March, 2006 by Michael T. McCord

Underwater digital acoustic communications (Acomms) with added networking capability is one of the leading research programs at the Naval Research Laboratory (NRL)--one that supports advances in the Sea Basing concept of the Navy's Sea Power 21 initiative. Radio frequency and laser communications have limited ocean range, but Acomms provide strategic communications at ranges up to 20 nautical miles.

The goal is to improve the technologies, used by the Office of Naval Research (ONR) and the Space and Naval Warfare Systems Command (SPAWAR),to enhance communications with submarines and between autonomous undersea vehicles. Improvements are delivered to the fleet in a series of milestones. ONR raises the bar each year for increased Acomms data rates, and we are in a continuous process of pushing technology advancements to meet these goals.

To this end, the NRL civilian staff teamed with members of the Navy and Air Force Reserves to take Acomms research to the ocean. Advances directly support future Navy capabilities for communications and networking between submarines, autonomous undersea vehicles, surface ships and test ranges. Applications for Acomms technology include rapidly deployable systems, minehunting and mine countermeasure systems, tactical communications, advanced weapon systems and undersea networking.

Technical Description

NRL's research concentrates on understanding the underwater medium and developing techniques that improve the communications efficiency under less-than-optimal channel conditions.

Low signal-to-noise ratio, multipath, reverberation and motion-induced Doppler frequency shift are examples of adverse channel conditions that limit data rates and lead to higher bit-error rates. The Acomms team tests new modulation techniques and evaluates the ability of new algorithms to improve communication rates.

The team also characterizes the ocean's acoustic channels so the theoretical maximum communication rate can be determined under varying conditions. For one-to-one acoustic data telemetry, focus is on achieving the highest data rate using phase-coherent acoustic communication techniques over a given bandwidth and a given set of acoustic channel conditions. For networking, focus is on the robustness of acoustic handshaking and maximizing channel capacity for multiple users.

Highly specialized equipment is used to conduct at-sea experiments. In a typical deployment, two subsurface systems are placed in the water column and moored about 30 meters below the surface. A third system, modified for towing behind the vessel, simulates an autonomous undersea vehicle for researching communications between submerged vessels.

The two moored systems are loosely tethered to surface buoys that provide radio frequency communications with the research vessel over a wireless local area network (LAN). To simulate an autonomous undersea vehicle, NRL developed a hydrodynamic frame for the Acoustic Communications Data Storage system. Controlled from the ship's lab, the two moored systems and the towed system conduct digital networking using underwater acoustic communications.

Space-efficient PC104 computers provide the brains and interface with acoustic projectors for transmitting and hydrophones for receiving. A typical experiment is conducted in three days and ends when the batteries are exhausted or the on-board 300 GB data storage drives are full.

Communications between the moored Acoustic Communications Data Storage buoy systems and a NRL-chartered research vessel require temporary installation of a 2.4 GHz antenna. With special Federal Communications Commission (FCC) authorization, the wireless LAN operates with 6 watts of power, providing communications out to 8 nautical miles.

The NRL Civilian Team

Dr. Tsih C. Yang leads one of the high-visibility research programs at NRL. He is a world-class expert in acoustic communications research. The team also includes signal-processing experts, Paul Gendron, Wen-Bin Yang and Jeff Schindall, with engineering services provided by Michael McCord.

Research results have appeared in more than 20 publications. Two patents have been granted with two more in process. The team has participated in eight at-sea experiments in various parts of the world under different sound propagation conditions.

Naval Reservists integrated seamlessly in several at-sea experiments in the past year on board NRL research vessels. Michael McCord, an NRL engineer (and Naval Reservist) coordinates the use of reservists in these experiments. Reservists are usually service members attached to units supporting the ONR or Naval Sea Systems Command. During the experiments, there were many long days, little time off and intense pressure to meet the schedule.

The weather was poor at times, creating challenges in port and at sea, but reservists stepped up to each task often suggesting improved ways of using the equipment, and always setting the highest example for their respective services. It was evident early on that they were making significant contributions to the program.