Amanda Smith

Meeting the Threat of Maritime Improvised Explosive Devices The PROSAS Surveyor: An Operational Case Study for Harbor Protection

By Andy Wilby Chief Engineer of Sensor Systems Applied Signal Technology Inc. Torrance, California

M.I rit i me improvised explosivedevices (MIEDs) present one of tlie most potent asymmelric threats to port security posed in the post- September Í1 era. Every month, around seven million Ions of freight pass through the Port of New York/New Jersey. Fifty to 100 times per year, tankers—each containing around one million barrels of crude oil—arrive in San Pedro, California. Several times per day, cruise ships carrying thousands of passengers leave through the narrow straits of Fort Lauderdale, Florida. These are simple examples, and they repre- sent the tip ofthe iceberg when consid- ering how much infrastructure, business and way of life in the U.S. depends on travel through busy seaports.

An MIED, though it costs only a few thousand dollars, threatens to disrupt lives, cause ecological and environ- mental disaster on a scale not seen since the grounding of the Exxon Valdez and also cause economic fallout run- ning into hundreds of millions, if not billions, of dollars when placed in any busy port. Even a credible threat of such a placement will causea similar level of turmoil. An idea of the financial and ecological impact that an MIED threat/attack might cause can be esti- mated by looking back at historical data, like when the 2002 dockworker strike shut down all 29 West Coast ports in the United States for 11 days, result- ing in direct and indirect costs of $1,9 billion per day. In 1989, the Exxon

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Valdez ran aground in Alaska's Prince William Sound, spilling more than 270,000 barrels of crude oil and caus- ing devastation to the Alaskan coastal flora and fauna. The clear up cost Exxon (Irving, Texas) more than $2.5 billion.

The protection of U.S. ports cannot be put on hofcl until the development of some magic bullet sensor system, déployable in times of heightened risk and able to infallibly seek out and destroy threatening objects in our sea- ways. It must instead rely on the rigor- ous and continual management of har- bors and harbor approach sea-lanes and provide a detailed knowledge of the underwater environment and an ability to assess its vulnerability to any prevailing threat.

To a war fighter, the prosecution of a target can be SLJmmarized in five dis- tinct phases: detection, classification, localization, threat evaluation and tacti-

cal advice. The needs of a harbor underwater security program may be expressed in completely analogous terms.

First is surveying and target marking, or the ability to detect, resolve and clas- sify to a reasonable level of confidence ptjtential threatening objects on the seabed.

Second is location, or the ability to locate identified objects on the seabed with a precision high enough to provide comparative reports from successive surveys and to allow extraction/disposal of threatening objects.

Last is change management, or the ability to assess, understand and cata- log the seabed; understand areas where the seabed is stable and those where it changes rapidly; database objects with- in the scene; compare new data against historical records; and highlight change.

www.se.i-lechnology.ciim

Inertial navigator withi. the vehicle refines USBL

solution to provide an accurate track

These three requirements are set .ig.iinsi the background of a difficult working environment and a time and hudgol-liniiled resoLirce pool.

Harbors and their approaching sea- ways represent one oí the most chal- lenging operational environments for any hydrographie survey system. The requirement to operate in shallow water limits the performance of sonar systems, with surface reverberation and multipath interference limiting the operational range of the sensor. Strong tidal flows and currents in the vicinity of inlets and river estuaries, where many harbors are located, provide a difficult

(Above) A control system uses nav- igation data from the USBL to aid the vehicle inertial navigator. The resulting position feeds an autopi- lot which steers the vehicle.

(Left) Seabed clutter is character- ised using high-resolution sonar and catalogued for future refer- ence. Here, an anchor and chain lay next to a lobster pot.

working environment. The ^ presence of large amounts of

shipping traffic and pleasure craft limit maneuverability. The mixing of fresh and salt water can provide other challenges.

Applied Signal Technology has been working over the past several years to put together a system able to provide the capabilities necessary for the rou- tine underwater management of a port environment. While the PROSAS Surveyor system is only one of a num- ber of systems capable of addressing such a task, the ideas behind ihe devel- opment of the system and the concepts for its operation in this environment provide a good framework to the deci- sion makers within a port authority.

who are charged with provitling lools and processes for the protection of their environment.

Tethered/Autonomous Operation One of the key decisions that needs

to be made in developing a survey capabilily for a port is what Iransporta- tion vehicle to use. Autonomous under- water vehicles (AUVs) are becoming more accepted in a mine-hunting envi- ronment but are possibly not ideal for harbor security.

The advantages of AUVs are that the lack of a tether makes them highly maneuverable and less susceptible to motion artifacts associated with vehicle tow behavior. Unfortunately, AUVs are sl(iw moving and havi« very limited nav- igation and obstacle avoidance capabil- ities, and the legal issues associated with the collision of an autonomous vehicle and a commercially operated pleasure craft have not been resolved. In addition, the current generation of survey-capable AUVs are operated in deep water and, usually, under the close supervision of a support boat. This is not a reasonable ttincept of opera- tion in a harbor or approach environ- ment.

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For these reasons, the PROSAS Surveyor system was developed using a towed vehicle. "iTie advantages of this approach are that data gathered are immediately available for viewing and the vehicle is capable of operation in high current flows, unlike AUVs. The presence of a tow platform, together with suitable limited maneuverability markings, provides a good platform for work in busy shipping channels.

Geo-Location and Navigation The issues associated with geo-loca-

tion and navigation fall into two main areas. First, it is important to know where objects seen in a sonar record really are on the seabed. This knowl- edge is needed to allow the comparison of one data set with another and to allow the reacquisition of previously identified targets for further investiga- tion and/or removal.

A second, equally important consid- eration is the dependence of a target image and the shadow structure behind that image on the aspect at which it is vieu/ed. In a system where the most important question to be asked is "What has changed since the last sur-

vey?" retention of the same aspect to the seabed from one survey to the next dramatically simplifies the task of change detection and improves the reli- ability of the resulting list of objects identified.

With the variations in tide, current and wind direction and the general dif- ficulties in navigating a vessel, it is extremely difficult to guarantee that a tow fish travels the same path through the water on successive surveys. The PROSAS Surveyor system utilizes a MacArtney (Esbjerg, Denmark) Focus-2 dynamically controlled tow fish to sig- nificantly improve the track-to-track repeatability.

The tow fish provides control surfaces to dictate roll, side-to-side and vertical position within the water column. This allows the vehicle to fly independent of the track of the tow vehicle (within the limits of the tow cable) and allows repeatable survey tracks to be run regardless of the prevailing conditions.

The PROSAS Surveyor system has a highly sophisticated navigation system with an inertial navigator, aided by a Doppler velocity logger and an ultra- short baseline (USBL) tracking system.

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which allows precise navigation of the tow fish. Autopilot facilities allow tracks to be accurately repeated from one run to the next.

Using a system such as this, it is pos- sible to geo-locate objects on the seabed to within around two meters and to repeat track lines from survey to survey with a similar level of accuracy.

Imaging Performance The use of synthetic aperture sonar

(SAS), as opposed to conventional side scan technology, allows the imaging oí the seabed at a constant resolution across the whole swath. With a high- frequency side scan, resolutions of the order of three to five centimeters are achievable at close range. At longer ranges, performance quickly drops, owing to the absorption of sound in the water channel. Lower frequency side scan systems can provide longer range operation as well as short range, because they use multielement locus- ing techniques that can provide high- resolution images. At longer ranges, the array apertures needed to focus a beam to the three to five-centimeter level required for classification are prohibi- tive, and the resolution ol the system falls off linearly with range.

To effectively keep a port secure, the seabed must be imaged with an ade- quate resolution lo allow (hopefully) positive classification and (at a mini- mum) a reduction in the clutter that can be falsely classified as an object of interest.

The limitation in range performance of a conventional side scan means that they are typically operated with very high levels of overlap between adjacent tracks. With an SAS system, the resolu- tion remains constant at all ranges, so the track spacing can be set according to the available water depth. In harbor and estuarine environments, this ability saves significant time in carrying out thu survey. Typical area coverage rates in excess of two square kilometers pt-r hour can be achieved in 40 feet of water using the PROSAS.

Environmental Awareness Once the area surrounding a harbor

has been surveyed, an enormous num- her of contacts will be generated. These represent the clutter and debris which have accumulated on the seabed over the operational lifeof the portas well as naturally occurring objects such as

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rocks and outcrops. A high-resolut ion sontir system, such as is provided by the SAS, will allow the operator to work through these contacts and hopefully reduce the number to a manageable (]uantity to be investigated or removed.

Key to successful monitoring of the area is understanding the hopefully iienign clutter structure on the seabed at the current time and having the abil- ity to alert the operator to new items of potential threat.

Within lhe PRĈ SAS Surveyor system, a database of contacts is maintained. Every time the system travels over the same area, existing contacts within the database are automatically updated with new images of the objects. Every time a new object is identified, the database of previous surveys can be scanned to find what that particular area of seabed looked like on those past occasions.

Building up knowledge of which parts ot the environment remain fairly static and which parts are subject to rapiti change takes time. Tasks such as ihest' need tu bv repetitively undertaken to inform decisions in times of increased threat.

Conclusions The maritime environment is vital to

the United States' way of life. In an era of intense terrorist threat, the Insertion of MlEDs into this environment repre- sents a significant risk to the security, safety, economy and ecology of a port. Key to containing this risk is the man- agement of the port and approaching sea-lane environment. This requires knowledge that has to be built up over a period of time before a credible threat is posed. RAdm. John Christenson, vice commander of the U.S. Naval Mine and Anti-Submarine Warfare Com- mand, recently said, "If you want to get a port opened quickly, you have to do your homework in advance. You have to know what's on the bottom before you start."

To achieve this mission, a holistic approach must be taken to system spec- ification, including imaging perfor- mance, navigation accuracy, interac- tion with port traffic, track repeatability and change management.

Tine utilization of a system such as the PROSAS Surveyor and some ot the con- cepts of operation discussed above pro- vide the port authority with a cost-effec-

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Acknowledgments The author would like to acknowl-

edge the support of MacArtney Underwater Technology A/S, EdgeTech Inc. (Boca Raton, Elorida) and the Ocean Works Group (Eernandina Beach, Elorida) for their support in the development and testing of the PROSAS Surveyor system. •

Visit our Web site at www.sea-tech- nology.com, and click on the title of this article in the Table of Contents to be linked with the respective company's Web site.

Andy Wilhy /us been a c77ie/' f/igH)(ff with Applied Sigrial Tech- nology Inc. since 2004 and works out of its Torrance, California, office. Formerly an employee of Ultra Electronics in the United Kingdom, he has been working within (/je sonar industry for 22 years, developing sen- sors and sensor prœpssing systems for mine hunting, antisubmarine warfare and com- mercial applications.

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