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Towed Antenna System Allows Two-Way, Real-Time Communication with UUVs
Module Floats on the Surface, Reducing Mission Survey Times And Increasing Underwater Maneuverability

By Roger E. Race
Program Executive
Advanced Programs

Jacob C. Piskura
Senior Mechanical Engineer
Davis Sanford
Design Engineer
Rolls-Royce Naval Marine Inc.
New Bedford, Massachusetts

When any unmanned underwater vehicle (UUV) is beneath the surface of the water, even a few meters down, it cannot use a global positioning system (GPS) signal and or radio frequency (RF) communication. This is a universal problem for all UUVs operators. The ability to communicate rapidly and in real time while still submerged is beneficial for many applications. A buoyant tethered antenna that supports two-way RF communication as well as Wi-Fi, Iridium satellite, a camera and GPS reception would greatly enhance the identification and localization capabilities of all UUVs. The added advantage is the UUV with a towed antenna will be much more maneuverable underwater at depths between three to five meters than it would be on the surface. In addition, successful creation of a universal towed antenna will be of great interest to the ever-growing world of commercial autonomous vehicles in use around the world.

Towed Antenna System Development
An Office of Naval Research Small Business Innovation Research (SBIR) Phase II contract sponsored development of the towed antenna system (TAS) for use on any 12.75-inch-diameter UUV. The TAS is modular and can bolt directly on to an existing UUV. The system is designed to be universal so it can be adapted to any underwater vehicle or structure. There are many scenarios where the TAS could prove invaluable. Homeland security can make use of the TAS on a UUV to allow rapid real-time inspection of harbors and ports, as well as the undersides of ships. Navy special operations can use the TAS to perform a covert robotic survey of an unfriendly shoreline. Additionally, survey ships engaged in gas and oil exploration can use the TAS for real-time data gathering, greatly reducing survey times.

The TAS body rides over waves during testing. (Photo credit: Rolls-Royce Naval Marine Inc.)

It is advantageous for a UUV that is hidden and covertly working underwater to have fast, reliable two-way communications with operators on shore. The two-way link means that images from a UUV swimming in a remote harbor could be sent back to a base station anywhere in the world via satellite. Currently, all UUVs must surface and have their dorsal antenna exposed above the waves in order to communicate. This makes the UUV vulnerable, and it requires extra energy to return to depth and perform its mission.

TAS Body Features
The Phase II TAS tow body builds on the development work performed on the buoyant tow body during the SBIR Phase I.

The length of the tow body is 14 inches overall and provides positive buoyancy. The internal volume of the body is 2,900 cubic centimeters, which results in a total of 22.2 Newtons of positive lift. The 3D design methodology was changed slightly so the body conforms to the UUV's hull shape, thereby reducing its impact on the overall hydrodynamics.

The TAS mast is raised above the deck, putting the GPS antenna above the water in order to reduce the effects of splash over by waves. An RF antenna was also integrated into the mast. This antenna is used for both Wi-Fi and UHF radio communication.

To facilitate the construction process, as well as the integration of the electronics, the body is built in three parts. The TAS body is split horizontally above the centerline of the National Advisory Committee for Aeronautics section. The antenna mast's top is also split horizontally to allow insertion of the GPS unit.

A docking receptacle is designed to fit into a 12.75-inch-diameter hull section. Using 3D computer-aided design, the tow body geometry was subtracted from a cylinder to form a well-fitting cavity. The top half is built with sterolithography (SLA), which is integrated into the 12.75-inch-diameter hull section to form the TAS module. The TAS module is 16 inches in length and contains the winch, the tow body and a three-inch-diameter sheave for the tow cable to roll over during launch and retrieval of the tow body from the UUV.

The tow body and cradle are made of tough plastic, with the durability of acrylonitrile-butadiene-styrene plastic and three times the impact strength of other SLA materials.

The electronics package consists of the communications modules, central multiplexor and UUV interface. The selection of communications modules can be tailored for each application, but could include any or all of the following: GPS, Wi-Fi, UHF, Iridium satellite, color camera and infrared (IR) strobe.

The goal in designing the towed communications system is to reproduce the functionality the UUV has on the surface. Typical UUV masts include a GPS antenna for location awareness, Wi-Fi module for short-range communications, UHF for medium-range communications, Iridium for long-range communications and IR strobe for spotting the vehicle during recovery. The state-of-the-art in all of these fields was selected for use in the TAS.

Unique environmental factors in this application require some special features. Because the GPS antenna operates very close to other antennae, RF interference needs to be addressed. Also, if the system is used in an area such as a harbor, multipath effects from the surrounding environment will cause the GPS signal to be degraded. The GPS device used in the TAS features high-accuracy differential GPS corrections, a sophisticated multipath mitigation routine that maintains the sub-meter accuracy for up to 30 minutes during temporary differential correction outages. The Wi-Fi antenna features dual-surface acoustic wave filters for zero out-of-band RF interference.

UUV applications of Wi-Fi include sending mission data to a user on shore, checking system status and full terminal access to the host computer via a secure shell tunnel. Data throughput for 802.11b can reach 11 megabits per second, and 802.11g can reach 54 megabits per second. Standard commercial equipment has a range of roughly 100 meters outdoors, and amplifiers can bring the range up to 1,000 meters or more. The TAS uses an RF amplifier and a similar setup on shore to significantly boost the range of the Wi-Fi signal.

In the TAS tow body, a 'virtual periscope' function is available. A small camera takes 640-by-480-pixel color images and sends them over the serial interface.

For medium-range communications, a 900-megahertz spread-spectrum radio is utilized. This device will allow an operator to communicate with the UUV at ranges up to 60 miles at up to 1.2 megabits per second. The spread-spectrum radio supports a 128-bit advanced encryption standard, making it suitable for military use.

The TAS body is pictured underwater. (Photo credit: Rolls-Royce Naval Marine Inc.)

Tow Body Cable Design
In order for the UUV's host computer to talk to the devices in the tow body, both power and two-way communications must be sent over the coaxial tow cable. To accomplish this, an Ethernet/coaxial bridge originally intended for closed-circuit television Internet protocol (IP) camera networks is used.

This system consists of an upstream and downstream unit connected together with a coaxial cable. DC power is plugged into the downstream unit and into a twisted pair connection. The upstream unit is powered over the coaxial with 24 volts DC, which can also be used to power the other electronics in the tow body. Another twisted pair cable is plugged into the upstream unit to give the tow body Ethernet access to the host computer.

A cluster of serial device servers is attached to a managed Ethernet switch inside the tow body. Each device in the tow body, including GPS, Wi-Fi, UHF, Iridium and the camera, is connected to its own serial device server. This allows each serial device to be accessed by the UUV's host computer over the Ethernet link. Each device is accessible through its own unique IP address or a virtual COM port, set up by the device server's driver.

The tow cable that is used must support multiple two-way communications, transmitting the GPS data as well as the two-way data communication, while having minimal drag. The Ethernet-to-coaxial bridge requires the use of coaxial. Coaxial cable manufacturers typically use polytetrafluoroethylene or fluorinated ethylene propylene for the jacket material since it has excellent dielectric properties. Unfortunately, this makes it impossible to bond anything to it. In order to properly attach both the underwater connectors and the overmolded strain relief, a nonfluoropolymer jacket is required. The tiny tow cable utilized is just more than two millimeters in diameter and has a tightly bonded PVC outer jacket.

Underwater connectors are used to link the tow cable to the tow body and launch and recovery system (LARS) winch. The usual coaxial connectors available are too large and heavy for use in this system. The TAS uses standard two-pin connectors. The connector set is the smallest underwater connector available and is made of grade 5 titanium. The connectors are over-molded to the cable to provide strain relief.

The LARS is small and able to reliably launch and recover the tow body throughout the entire UUV mission. An underwater winch that will fit inside the 12.75-inch-diameter UUV module has been developed. The underwater winch is used to deploy and recover the tow body. The underwater winch is designed to hold 100 feet of the two-millimeter coaxial tow cable.

The underwater winch uses a small DC gear motor driven by a pulse-width modulation (PWM) driver located inside the UUV's electronics case. It features a subminiature 12-channel slip ring for sending the power and data over the tow cable. There is one underwater connector installed into the winch drum, one for the motor and one for the slip ring. The UUV host computer sends a serial command to the PWM driver, which will drive the motor forward, paying out the winch. When the TAS has completed its surface deployment, the computer will signal the winch to pull the tow body back in. A proximity switch in the cradle tells the computer when the tow body is in the cradle, which causes the winch to stop turning, thereby locking the tow body securely in the cradle.

Testing of the TAS System
A TAS system has been built and successfully tested in the waters of Buzzards Bay, Massachusetts. The test included launch and retrieval of the TAS buoyant tow body several times. The development team built a UUV out of 12.75 inch PVC and fitted the TAS module to it. For this set of tests, a 50-foot test tow cable was used.

The team then towed the UUV with the TAS module and launched the TAS while photographing the whole operation underwater. A waterproof digital camera was attached to a pole and put where the UUV could be seen and filmed the action.

Tow tests were at one to six knots. At higher speeds, the TAS was still able to come to the surface and ride over the waves. The team learned the TAS body floats up through the water column, but when it gets to the surface, it rides like a planing motorboat over the waves.

Another observation from the underwater video during testing was that during retrieval the TAS body would fly right up to the UUV, staying at the surface until the very last meter. Then the TAS body would seem to hover directly over the nesting cradle, landing always straight down into the recess. The buoyancy keeps it aloft as long as possible, and the TAS tow body is streamlined so there is very little drag, which is why it seems to hover right into its cradle.

The TAS reliably deploys 50 feet of cable to the tow body, which quickly receives GPS data and sends digital photos over a radio link for 1,000 meters.

Future Developments
Recently, Brooke Ocean Technology (Nova Scotia, Canada) was purchased by Rolls-Royce (London, England) Marine Group. The newly formed Rolls-Royce Advanced Programs Group is planning to continue development of the TAS system and anticipates utilizing the TAS with other underwater vehicles.

Roger Race is a program executive with the advanced programs group of Rolls-Royce Naval Marine. He has more than 20 years' experience designing and building towed underwater vehicles. He has degrees in both marine biology and mechanical engineering.

Jacob C. Piskura has held the title of senior mechanical engineer for Rolls-Royce Naval Marine Inc. (formerly BOT USA) for the past five years. He served as the technical lead during the development of the towed antenna system. He has a background in electromechanical system design and received a master's degree in mechanical engineering from the University of Massachusetts.

Davis Sanford graduated from Dalhousie University in Halifax, Canada, in 2005 with a bachelor's in biological engineering. Since then he has been a design engineer developing custom mechanical systems for the U.S. and international navies, seismic industry, universities and commercial companies. He is also a certified autonomous underwater vehicle operator and programmer.

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