Abstract - This paper will describe techniques and equipment that aid public safety dive teams in performing their Maritime Homeland Security missions particularly when they are operating in the typical poor visibility conditions that are found in our harbors and rivers. The equipment described will focus on diver held sonars that are available to the public safety divers bearing the majority of the initial search and location responsibilities.
Maritime transportation systems and the marine infrastructure are potential recipients of terrorist actions. Emphasis on maritime security has increased to help counter this potential. This has resulted in increased workloads on US public safety maritime patrols and dive teams. The municipal, county, and state police organizations that perform marine recovery tasks in response to criminal activities and accidents are carrying this load.
Organizations that performed recoveries of evidence in criminal cases or recover bodies under bridges or in recreational swimming and boating areas are now being called upon to investigate and identify items of interest found in commercial ports, waterways, and approaches. The scope of responsibilities for public safety organizations has the potential of increasing many times as the operational tasks increase. The option of hiring additional staff is seldom an available solution. Organizations are turning to technology to address this increase in workload.
There is no one device or even one technology that can satisfy all the operational needs of our maritime security guardians. Equipment and technology must be selected to best meet the needs and requirements of the mission, the operating environment and the characteristics of the perceived threat, while at the same time minimizing the amount of time that divers must be underwater. The underwater environment in and around our ports and harbors is a dangerous one and even with all the training and protective gear, the less time actually in the water the better.
Side scan sonar and inspection class remotely operated vehicles (ROVs) are technologies that under proper conditions can help. Side scan sonar can, in many cases, provide wide areas of bottom information. Because of the need to maintain a constant tow fish height above the bottom, harbors and channels with large bottom features can make the use of side scan difficult. Once an object is located, ROVs can, under proper conditions, provide information on unknown objects without having to put a diver in the water. More often than not, however, a diver is needed to bring specialized training and experience to bear to make a judgment as to an object’s threat level. While most people know of side scan sonar and ROVs, fewer are aware of the important role diver held sonars play in helping public safety divers accomplish this mission.
Diver held sonars have been in use by the US Navy since the early 1980’s to locate sea mines. The US Navy developed the AN/PQS-2A specifically for this purpose. To reduce the possibility of triggering the magnetic influence fuzing that often is in use on large sea mines, this device was designed and manufactured to low magnetic permeability (low µ) requirements. This sonar is still in production and in active use by the US Navy. It has seen recent use in Iraq in clearing the waterways and sea approaches of Iraqi laid mines. They may very well be in use in that area today.
II. MISSION & TEAMS
Former Secretary Tom Ridge has described the Coast Guard as the lead agency in the Maritime Security domain . Within the Coast Guard, the Maritime Safety and Security Team (MSST) has the primary mission to provide Maritime Law Enforcement. Their goal is to detect and intercept criminal or catastrophic events well before they threaten our shores. Each team consists of approximately 100 personnel, both active duty and reservists, and at present, are deployed in thirteen locations on the Atlantic, Gulf and Pacific coasts and in Hawai`i. These teams’ posses specialized skills, capabilities, and expertise to perform a broad range of port security and harbor defense missions. They have Physical Security Teams, Canine Handling Teams and highly qualified Dive Teams as well as the boats, dedicated equipment, and available air support to respond to any threat. The recognized importance of keeping a threat as far from our shores as possible results in many members of these teams being deployed away from their home port as much as half time .
Being deployed on temporary duty leaves the homeports in need of alternative coverage, particularly in the area of underwater operations. There are two Department of Justice law enforcement agencies with diving responsibilities that provide additional support to the MSST mission. They are the Federal Bureau of Investigation Underwater Search and Evidence Recover Teams (FBI US-ERT) and the Alcohol, Tobacco, Firearms and Explosives divers (ATF). The FBI US-ERT teams are located in New York, NY, Washington, DC, Miami, FL, and in Los Angeles, CA. For the most part, the US-ERT members are primarily Special Agents with all the normal caseloads and work assignments as any FBI Special Agent. The diving responsibilities are collateral duty. The ATF divers are far fewer in number compared to the size of the FBI teams but they have the additional operational advantage of all being qualified bomb technicians. The ATF divers are also located in various parts of the country. The MSST, FBI US-ERT, and ATF divers are well trained and equipped and are capable of responding to high level, real and perceived threats in our more than three hundred active ports. The day to day port and harbor issues that are one on hand routine but on the other may be a precursor to a major incident are however beyond their modest levels of manpower.
The routine responses that may blossom into a full blown maritime directed attack are met by the state, county, and municipal police, fire and rescue and recovery dive teams that are local to our more than 300 ports and harbors.
The United States Marine Transportation System includes more than 300 ports, more than 1,000 harbor channels, 3,700 passenger and cargo terminals and over 25,000 miles of inland, intracoastal and coastal waterways .
The Los Angeles/Long Beach port complex in San Pedro Bay, CA, is shown below in a satellite image.
The Port of Los Angeles and the Port of Long Beach are the first and second busiest container ports in the United States. Taken together, they are ranked as the third largest container port in the world by volume, exceeded only by Hong Kong and Singapore (2001 data) .
In addition to containerized cargo, these ports handle dry bulk, liquid bulk, break bulk, and Ro-Ro cargos. The Port of Los Angeles has a very large cruise passenger complex and Long Beach has its own cruise terminal next to the permanently moored Queen Mary. Both ports are recreational and tourist destinations in their own right.
These port complexes are mentioned to try to give a sense of scale to the problems confronting the small numbers of personnel that are directly involved in the underwater environments in and around our ports. As busy as these two ports are, in terms of total tonnage, they only rank 8 and 13 in the US. The leading US Ports in 2003 in terms of millions of short tons of cargo are :
Of particular concern among our ports are the four domestic liquid natural gas (LNG) import terminals located in Everett, Massachusetts (near Boston, MA), Cove Point, Maryland, (near Washington, DC), Elba Island, Georgia (near Savannah, GA), and Lake Charles, Louisiana (between Houston, TX, and New Orleans, LA). There is also a terminal in Puerto Rico, and additional domestic terminals are being proposed, one in Port Arthur, Texas, a terminal in Fall River, Massachusetts, and a third in Providence, RI. LNG is highly volatile and the concern is that an LNG tanker damaged in a terrorist attack will likely loose some or most of its liquefied cargo. There has been much discussion about the consequence of such a hull structure failure, whether an explosion or fire will result, and how widespread the damages will be, particularly in an urban harbor like Boston. One thing is certain, if a serious incident involving an LNG tanker were to occur, it would have a profound and immediate impact on our maritime transportation system..
Ideally, the goal of the use of technology in the support of Maritime Security Missions, particularly underwater sensor technology, is to remove the diver from the water altogether. As that is not likely to happen in the near term, we can hope that technology makes strides in minimizing the amount of time a diver spends in the water without making mission sacrifices. For the purposes of this paper, I am going to arbitrarily divide the world of underwater sensor technology into four categories that are related to their method of deployment: Surface Ship Hull Mounted, Towed Devices, Swimming Vehicles: (Tethered and Autonomous), and Diver Carried. It is generally the deployment device carrying the sensor that establishes the amount of area coverage per unit time. To be complete, I could include satellite and fixed and rotary wing borne sensors, but these airborne assets, except for some very specialized applications, have limited practical use in sensing objects smaller than a few meters on the surface and even less so for submerged objects.
A. Surface Ship Hull Mounted Sensors
The usefulness of surface ships is as a sonar platform. Single beam and multibeam hull mounted sonars can be effectively operated at survey speeds up to sixteen knots and in some instances higher. This allows for large areas of coverage particularly with multibeam sonar since the there is a larger swath than in the case of a single beam. A single beam sonar, such as a Fathometer or a bathymetric charting device is of value in only well charted areas where gross changes to a baseline depth survey can be detected. Because of the generally narrow beamwidth of the transmitted acoustic energy there is a risk that a single beam bottom sounder will miss a target. A multibeam sonar avoids this problem by creating a broad swath of acoustic energy that is directed down and out to either side as much as a total of 150 degrees. Generally, hull mounted multibeam systems use two sets of transducers mounted orthogonal to each other in a Mills Cross arrangement. The transmitting projectors are an array mounted in the athwartships orientation and the receiving hydrophones are mounted in the fore/aft axis, usually on the center line (ship’s keel).
To maintain accuracy and maximize spatial resolution, hull mounted multibeam sonars require motion compensation systems to account for ship motions. A comparison of hull mounted multibeam sonars, demonstrated at the 1997 US/Canada Hydrographic Commission Coastal Multibeam Surveying Course showed an inability for the sonars being operated to resolve mine-like targets at bottom depths of approximately 25 meters. At shallower depths, around 14 meters, plausible positive bathymetric anomalies were detected in the target areas but were indistinguishable from false targets likely generated from random noise in the same area. 
Digitized receiving systems, advances in computing power and real time signal processing, and new detection algorithms have resulted in improved performance since the 1997 test. About the best resolution a high frequency, motion compensated, hull mounted, shallow water multi beam sonar can ideally obtain is likely the size of a typical bottom mounted sea mine, ~1.0 meters in length. Certainly a twenty-foot container could be reliably detected with a hull mounted multibeam system under typical harbor conditions. The minimum reliably detected, low-false alarm rate target size for multibeam sonars under real world conditions probably lies between these two sizes. The way to improve upon these limitations is to decouple the sensor from ship motions by submerging the sensor beneath the waves, and by moving the sensor closer to the target so that higher frequencies can be used. This sensor configuration is a towed system and the most common towed system is side scan sonar.
B. Towed Devices
By mounting the sensors on a towed device, they are moved closer to the area of interest, the bottom, and they are removed from the motion effects experienced by the surface ship. Putting the sensor on a tow body that is towed beneath the ship generally results in a lower survey speed when compared to the hull mounted systems. Tow cable stress and tow body stability are but two of the issues that contribute to this reduction.
Towed bodies can come in a number of forms. An imaging vehicle named the Argo on its first deep-sea cruise made the initial 1985 discovery of the RMS Titanic while being towed by the Woods Hole RV Knorr. The Argo was fifteen feet long by three and a half feet square and weighted over four thousand pounds. Smaller imaging sleds are still in use but the relatively shallow water of ports and harbors have limited visual ranges and make them ineffective. A more typical towed sensor system is the side scan sonar. These tow bodies’ range in length from three to more than six feet.
Side scan sonar was developed in the 1960’s in a large part based upon development work initiated by H. Edgerton of the Massachusetts Institute of Technology. Side scan sonars produce a fan shape acoustic beam to either side of the tow body. Unlike hull mounted single beam or multibeam sonars that use time of arrival of the reflected acoustic pulse to determine water depth, the side scan system measures the intensity of the reflected signal and generates and displays a record of this. The time of arrival information of the reflected energy is used to determine the slant range of the signal being received and thus the position off the tow track of the object that created the reflection. It is important to maintain a stable tow attitude so that the geometry can be accurately determined. High-end side scan systems contain heading, pitch and roll sensors to more accurately present the received images and typically can be towed at speeds up to 5 knots with excellent results. Tow speeds as high as 15 & 16 knots are claimed, particularly with systems that generate more than a single beam, but these systems are expensive to own and operate  . Multi-beam, high speed, side scan systems are sophisticated geophysical instruments that require considerable topside equipment and an experienced, well-trained dedicated crew. These logistic and personnel requirements place these equipments beyond the reach of all but the largest and best-funded public safety organizations. A simple, bare bones side scan system that is typically in use in local level public safety marine patrols, like the Imagenex Model 881 SPORTSCAN dual frequency unit, has a maximum recommended tow speed of 2 to 3 knots .
Because side scan uses intensity information in generating images, the lack of intensity, that is, no reflected return, is plotted or displayed as a blank area. The blank area is readily interpreted as a shadow. This can convey a wealth of information about the object being ensonified, and is a major factor in its popularity among public safety dive teams conducting searches preliminary to diving.
Another towed device is a magnetometer. The magnetometer detects ferromagnetic metals (iron, nickel and some steels) by monitoring variations in the earth’s magnetic field caused by their field distorting influence.
The following are some typical detection ranges:
- large trash truck 150 feet
- an automobile 75 feet
- a 45 cal automatic 6 to 8 feet
Towed magnetometers are likely to have little use in most underwater maritime security missions in part due to the modest detection ranges, but mostly due to the low likelihood of substantial amounts of ferromagnetic materials in the device or container being searched for.
Metal detectors are different from magnetometers and although they can be towed at very slow speeds (1 to 3 knots), they are not considered suitable for mobile searches. Their short detection ranges (nine feet for a car sized object in salt water) limits their effectiveness. Metal detectors generate a local magnetic field that creates current flow in all nearby conductors (eddy currents). The induced eddy current in the conductor in turn creates a magnetic field normal to the inducing field. Sensitive detectors that null out the inducing field can sense the very, very much smaller induced field. These devices can be effective in limited searches when a diver or a shallow water wader can pass the metal detector’s search coil within a couple of inches of the bottom.
C. Swimming Vehicles
Tethered and Autonomous Tethered swimming vehicles are more common known as Remotely Operated Vehicles, or ROVs. Autonomous swimming vehicles are known by a number of names, but most frequently are referred to as Autonomous Underwater Vehicles, or AUVs.
ROVs are an additional step in the progression that started with hull-mounted sensors. They are similar to towed sensors in that they are removed from the effects of the surface motions of hull mounted ones and they are brought down closer to the bottom. The ROVs go one step further in that most can hover motionless in the water and, if necessary, set down on the bottom. This is an important capability for close inspection of an object of interest, or to place a marker on an object or in some cases to actually retrieve an object using an ROV manipulator arm.
ROVs capable of deep ocean depths are as large small automobiles. These work class and heavy work class vehicles require large topside winch and deployment mechanisms, sub sea-garages and tether management systems and are intended to be used in deep ocean environments. These vehicles are far too expensive and complex for use in our ports and harbors to support homeland security missions. Smaller inspection class ROVs have found a niche in support of public safety underwater work. These small devices have tether lengths that range from 100 meters to as long as 750 meters. They range in weight from 4 or 5 kilograms to greater than 250 kilograms. One person can deploy the smallest over the side of a rigid inflatable boat. The medium size ROVs can be deployed from simple davits. Although cable reels and slip rings are not always needed they simplify operation even for the smallest of units and make for a safer deployment and retrieval. All but the smallest of units will need a generator for prime power and for the shipboard instrumentation. At a minimum, ROVs will have a television camera and thrusters for horizontal propulsion and vertical control. Larger units can have multiple cameras with sophisticated underwater lighting and pan and tilt controls. Most units, even small ones can have one or more types of sonars. These can range from simple forward-looking, collision avoidance sonar to multibeam, high resolution, scanning sonar. Side scan sonar is also a commonly included sensor. ROVs frequently have manipulating arms of varying degrees of complexity that are remotely activated through the video system. However, one of the shortcomings of ROVs is that because they are generally ballasted very close to neutral buoyancy, unless they have releasable ballast, they have a very limited capability of adding payload while underway.
An un-tethered swimming vehicle is more commonly called an Autonomous Underwater Vehicle (AUV). It is considered the next step beyond ROVs for the military, science and commercial world for survey and data gathering. For sometime now, universities have been developing unique, purpose-built vehicles for oceanographic applications. Recent university spin-offs such as Bluefin and Hydroid are building and selling AUVs for military and commercial applications. The primary limitation is the amount of power that an AUV can carry. For deep-water applications, gliders, which make small changes in buoyancy to effect large depth changes that can be translated into horizontal motion by use of lifting wings, minimize the amount of power used in propulsion, saving most of the power for the sensor payload. Given the shallow nature of ports and harbors, gliders are not going to be very effective in most harbors. A network of bottom mounted docking stations that provide power to the AUV to recharge its energy storage system and accepts data downloads from the AUV sensors is a likely solution. Wide spread use of AUVs in Homeland Security applications is likely some years off, but it has the potential to be a solution to the large scale repetitive monitoring needs of port and harbor protection. The AUV advantage is that after deployment and before retrieval, it needs no on site personnel to be involve in control or monitoring.
D Diver carried Diver carried sensors are those devices that can be used directly by the diver to locate an object of interest. With one exception, the ones described here are all sonar-based devices. Once an object of interest is located by one of the broad area search methods, side scan sonar for example, its position is marked by GPS or by tossing a weighted marker float overboard. If an ROV is available and visibility is good enough, it is used to visually inspect the object of interest. Even though the object is marked by GPS or by an external method, the ROV will need to establish a search method using its sensors to be able to get close enough to use its TV cameras to observe the object. The typical visibility on the bottom of a commercial port usually ranges from poor to very bad . If the bottom silt has been disturbed, as is often the case, visibility can be less than 15 cm. At times, visibility is so bad as to be described as black water. Under conditions of reduced visibility, a diver must make an on-site inspection.
In the early 1980’s, the US Navy designed and the Harris division of General Instrument manufactured the AN/PQS-2A. This assembly is low magnetic signature, underwater sonar that uses a continuous tone frequency modulated (CTFM) signal to locate sea mines and mine-like objects. This sonar became the mainstay of Navy Explosive Ordnance Disposal (EOD) divers. The Harris division underwent a number of name changes in the 1980s and 1990s and is now Harris Acoustic Products, a member of the Channel Technology family of companies. The “Two Alpha”, as it has become known amongst divers, became so popular that Harris adapted the design for commercial use and produces the DLS-1 and DLS-2A hand held sonars. The DLS-2A has the same low µ signature as the “Two Alpha” and is screened to the same military specifications. Foreign EOD divers and domestic bomb technician divers use it. The DLS-1 does not have the low mu characteristics and therefore is less expensive.
Although the hand held sonars were well known and accepted among Navy trained divers, the public safety diving community was generally unaware of them until TWA Flight 800 crashed off the south coast of Long Island in July 1993. In response to this tragedy, the National Transportation Safety Board (NTSB) requested assistance from the Navy in finding the cockpit voice recorder (CVR), the flight data recorder (FDR) and mapping the debris fields on the ocean bottom. The scope of work for the Navy salvage experts and divers quickly expanded beyond this to recover victims and eventually to locate, note the position of, and then recover all the pieces, large and small of the downed aircraft. As a side note, all of these recovered items had to treated as evidence of a potential crime. The FBI ensured that proper chain of custody rules were observed throughout the process. As a further aside, although this requirement further complicated an already complex task, it will most certainly be a requirement in any potential underwater post-blast recovery and investigation and should be considered while developing current training scenarios. Navy scuba diving operations were supplemented by public safety divers from the New York State Police, New York City Police, Suffolk County Police, New York City Fire Department, and the FBI. Diving operations proceeded around the clock from 18 July to 2 November halting only for severe weather.
The Navy had a large number of “Two Alphas” on hand and not only made them available to any of the public safety divers, they strongly encouraged them to try them out. These hand-held sonars were very good at identifying acoustically reflective objects on the rolling sandy bottom. One New York State Trooper that dove throughout the entire search related one of his experiences with the hand held sonar in finding a ball point pen sticking out of the sand in near zero bottom visibility. Although he never found out if it was associated with the crash or an incidental item dropped by someone on the surface, he told me he was very impressed that the sonar was able to lead right to such a small thing.
From that start and other common experiences between the US Navy and Public Safety Diving organizations, the use of the hand held sonars has slowly grown so that the DLS-1 is owned by public safety dive teams in New York, Connecticut, South Carolina, Florida and California. Because of recent increased interest in use of the hand held sonar coupled with a desire to have a more affordable unit, Harris Acoustic Products has developed a new version designated the DLS-3. This new hand held sonar, which is shown in use on the following page, has been developed specifically for use by public safety dive teams. Prototype testing has been accomplished by state, county and municipal police teams during training exercises on both the West and East coasts in salt water, fresh water and even under a foot of ice.
The DLS-3 has two ranges, 20 m and 40 m. Unlike the “Two Alpha” and the DLS-1 and DLS-2A, this new unit only operates in the active mode. The unit was designed to be smaller, have simpler controls and to operate from standard type “C” alkaline batteries. During two recent FBI/LA County Sheriff’s Underwater Post Blast Crime Scene Investigators courses, the DLS-3 was used successfully by a number of police divers and diving bomb technicians to locate and recover both large and small pieces of evidence. Very few of the divers had prior experience with any hand held sonars. It is expected that this unit will find wide spread use among public safety dive teams.
US Navy supported research laboratories have recently developed advanced capability diver held sonars that generate and record visual images. Two approaches have been successful, in one, a multibeam sonar is manipulated by the diver to build up a line by line image. The second approach uses an acoustic lens technique to produce the return. Both techniques utilize high frequency projectors to obtain the desired resolution but must operate at reduced ranges compared to the CTFM sonars. Because divers will have difficulty observing the images in poor visibility conditions, either small helmet mounted “heads-up displays” or topside observers may be needed during operation in “black water”. The scan type sonar is just now reaching operational use in the Navy and the acoustic lens type is available commercially under the trade name “DIDSON”. Although these units have advanced capabilities beyond what is in current use, both units are significantly more expensive than the CFTM types of sonars and it remains to be seen if they make much of an impact in the area of Maritime Homeland Security.
A non-sonar sensor that can be carried by divers is the metal detector. Both the metal detector and the magnetometer operate with magnetic fields. Unlike electric fields, which are shorted out by a conductive medium, magnetic fields are attenuated but not shorted out by water. As mentioned earlier, a metal detector must be very close to the target object to create a reading. However, if the target object is metal and is suspected to be in a small area, then the metal detector can be an effective tool. Because of its range limitations, it is not used as a general search tool in ports and harbors.
There is another sonar tool that although divers do not carry it, it can be installed by divers and is useful in bottom searches. This is the bottom-mounted scanning sonar, one example being the Kongsberg Mesotech MS-1000. That unit consists of a single fan beam, 675 kHz sonar that is mechanically scanned through 360 degrees. Since it is bottom mounted on a tripod it is best suited to a moderately flat bottom with little or no current. It has a typical range of 100 meters, although there are resolution/scan rate/range tradeoffs. It is particularly well suited to work with divers who have topside communications so that the topside sonar operator can direct them to items of interest.
There is no one technology that will satisfy all the needs of public safety divers in support of Maritime Homeland Security missions. However, experience with large scale searches such as TWA 800 in New York, or recovery of a classified payload from an aborted rocket launch off of Cape Canaveral, FL, or the search for wreckage of the Space Shuttle Columbia in the Toledo Bend Reservoir, TX, has demonstrated the value of using side scan sonar to survey large areas and then to deploy divers with hand held sonars to follow-up on targets of interest. Even when GPS based navigation systems are used to mark items of interest, divers usually need additional tools to search for and find their targets. The hand held sonars like the AN/PQS-2A and the DLS types help the divers locate those targets.
 T. Ridge, “Statement of Secretary Tom Ridge Department of Homeland Security before the Senate Committee on Commerce, Science and Transportation”, April 9, 2003
 Maritime Safety and Security Team 91103, San Pedro, CA Home Page, Welcome Aboard web page, http://www.uscg.mil/pacarea/msst91103/welcome_aboard.htm
 J. A. Facett, Ph.D., J. Kruse, “Ports, Harbors and the Urban Coast, An Introduction to the Marine Transportation System”
 The U.S. Waterway System – Transportation Facts, 2003, US Army Corps of Engineers, Navigation Data Center, December 2004, http://www.iwr.usace.army.mil/ndc/factcard/fc04/factcard.pdf
 Delineation of Mine-Like Targets 1997 US/Canada Hydrographic Commission Coastal Multibeam Surveying Course, John E. Hughes Clarke http://www.omg.unb.ca/~jhc/uschc97/comp_mines.html
 EdgeTech 4300-MPX MultiPulse Side Scan Sonar System brochure, undated, EdgeTech Marine, 4 Little Brook Road, West Wareham, Massachusetts 02576,
 Klein System 5800 product brochure, undated, L3 Communications, Klein Associates, Inc., 11 Klein Drive, Salem, New Hampshire 03079
 Imagenex Model 881 SPORTSCAN Digital Sidescan Sonar, Frequently Asked Questions, Imagenex Technology Corp., 209 - 1875 Broadway Street, Port Coquitlam, BC, V3C 4Z1, Canada
 US Navy Diving Manual, Revision 4, Change A, SS521-AG-PRO-10/NSN 0910—LP-708-8000, -par 6-6.1 Underwater Visibility (1 March 2001)