Abstract:
Methods and apparatus determine if an intruder passes into a security zone that is associated with a waterfront asset. An embodiment of the invention provides a harbor fence system that is designed to be deployed in water around ships or other waterfront assets to serve as a line-of-demarcation in order to provide protection. The harbor fence system comprises a series of spars that protrude above the water surface and that communicate with a computer with a telemetry subsystem. Each spar contains electronic sensors, e.g. water immersion sensors and accelerometers, and circuitry to detect an intrusion and to communicate the location of the intrusion to a computer control station. Spars may communicate wirelessly and may also be solar powered. Additionally, the embodiment may also determine whether an underwater intruder is passing under a protective boundary, in which the harbor fence system interfaces to an underwater sonar sensor subsystem.

Description:
[0001]     This application is a continuation of common-owned, co-pending U.S. application Ser. No. 10/365,357 filed on Feb. 12, 2003 naming Larry R. McDonald as inventor, the entire disclosure of which is hereby incorporated by reference. 
     
    
       [0002]     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of N41756-02-C-4682 awarded by the U.S. Navy. 
     
    
     FIELD OF THE INVENTION  
       [0003]     The present invention relates to a surface barrier to protect an asset such as a ship that abuts a body of water.  
       BACKGROUND OF THE INVENTION  
       [0004]     There are numerous situations in which a waterfront asset, such as military and civilian ships, that are situated in a harbor environment must be protected. Potential threats to the waterfront asset may originate at the surface of the water or below the surface of the water that abuts the asset. Typically, protective systems are passive barriers, such as oil booms or heavy fixed barriers to stop boats, or simple lines of small floats on the water. Security boom systems are typically heavy, usually difficult to deploy and moor, and are not intended to be portable. Moreover, security booms usually cannot be seen at night or in fog or rain, and do not provide any indications of intrusion.  
         [0005]     Consequently, a method and apparatus that may provide continuous protection for an asset by automatically warning personnel about a possible intruder, that has a reduced cost, that has mobility so that the protective system may be transported with the ship as the ship changes locations, that can be configured for a desired perimeter typology, and that uses less power while providing a required degree of protection from surface and underwater predators would be beneficial to advancing the art of protective systems for waterfront assets.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     A harbor fence system may be deployed in water around ships or other waterfront assets to serve as a line-of-demarcation (visible day or night or in fog) to warn boats to stay out of the enclosed “security zone” or exclusion zone” and to provide warnings and the location of any attempted intrusion across the harbor fence system. The harbor fence system may be lightweight and portable, capable of being transported on different sizes of ships (such as a navy ship), and deployed in different harbors where a ship may dock throughout the world in order to establish a security perimeter. The harbor fence may also be used to protect commercial ships, e.g. tankers and cruise lines) or other waterfront assets (e.g buildings and bridges) abutting harbors, lakes, or rivers.  
         [0007]     In one embodiment of the invention, a harbor fence system comprises a series of spars that protrude above the water surface, that are spaced approximately uniformly, and that are connected to an electrical computer with a telemetry subsystem. Each spar contains electronic sensors, e.g. water immersion sensors and accelerometers, and circuitry to detect intrusions and to communicate the location of the intrusion to a computer control station on shore or on the watch deck of the associated ship. The embodiment also facilitates deploying and retrieving the harbor fence system.  
         [0008]     Additionally, the embodiment may also determine whether an underwater intruder is passing under a protective boundary, in which the harbor fence system interfaces to an underwater sonar sensor subsystem. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     A more complete understanding of the present invention and the advantages thereof may be acquired by referring to the following description in consideration of the accompanying drawings, in which like reference numbers indicate like features and wherein:  
         [0010]      FIG. 1  illustrates a ship that is protected by a harbor fence system according to an embodiment of the invention;  
         [0011]      FIG. 2  shows a portion of the harbor fence system as shown in  FIG. 1 ;  
         [0012]      FIG. 3  shows a concentric hoop configuration that may be used as a flotation means for a harbor boom line according to an alternative embodiment of the invention;  
         [0013]      FIG. 4  shows a helix boom line configuration of a harbor fence system according to an alternative embodiment of the invention;  
         [0014]      FIG. 5  shows a scenario in which a harbor fence system is being breached;  
         [0015]      FIG. 6  shows a water crossing sensor circuit that is utilized in a harbor fence system according to an embodiment of the invention;  
         [0016]      FIG. 7  shows an excessive impact sensor circuit that is utilized in a harbor fence system according to an embodiment of the invention;  
         [0017]      FIG. 8  shows a boom line telemetry subsystem according to an embodiment of the invention;  
         [0018]      FIG. 9  shows deployment or retrieval of a harbor fence system according to an embodiment of the invention;  
         [0019]      FIG. 10  shows a variation of deployment or retrieval of a harbor fence system according to an embodiment of the invention;  
         [0020]      FIG. 11  shows retrieval of a harbor fence system according to an embodiment of the invention;  
         [0021]      FIG. 12  illustrates a ship that is protected by a sonar system;  
         [0022]      FIG. 13  shows a sonar subsystem that protects a ship from underwater intruders in accordance with an embodiment of the invention;  
         [0023]      FIG. 14  shows a vertical coverage of adjacent sonar sensor modules;  
         [0024]      FIG. 15  shows apparatus for a sonar sensor module;  
         [0025]      FIG. 16  shows a sonar signal that is received by a sonar sensor module;  
         [0026]      FIG. 17  shows a telemetry configuration for a sonar system;  
         [0027]      FIG. 18  shows an example of a path of an underwater intruder through a sonar system;  
         [0028]      FIG. 19  shows an a path of an underwater intruder that is perpendicular to a protective boundary of a sonar system;  
         [0029]      FIG. 20  shows associated tracking data of adjacent sonar sensor modules for the example shown in  FIG. 19 ;  
         [0030]      FIG. 21  shows a method of determining the depth of an underwater intruder for the example shown in  FIG. 20 ;  
         [0031]      FIG. 22  shows a flow diagram for a sensor system;  
         [0032]      FIG. 23  shows an example of tracking data of a possible underwater intruder;  
         [0033]      FIG. 24  shows a first example of simulated tracking data;  
         [0034]      FIG. 25  shows a second example of simulated tracking data;  
         [0035]      FIG. 26  shows a third example of simulated tracking data;  
         [0036]      FIG. 27  shows tracking data of a target from adjacent sonar sensor modules; and  
         [0037]      FIG. 28  shows estimated paths of the target corresponding to  FIG. 27 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0038]      FIG. 1  illustrates a ship  101  that is protected by a harbor fence system  103  according to an embodiment of the invention. Ship  101  is moored along a pier that abuts a harbor. Variations of the embodiment may protect other types of waterfront assets (e.g. commercial ships, bridges, and buildings) that abut other types of bodies of water (e.g. rivers or lakes). Harbor fence  103  comprises a plurality of spars (“fenceposts”), e.g. spars  105 ,  107 , and  109 . The plurality of spars is connected together by a cable at the waterline containing multiple wires and by a thinner top line containing at least one wire (as shown in  FIG. 2 ). A shape of harbor fence  103  is maintained by moors, e.g. moor  111 . Moor  111  comprises a floating platform  151  that is anchored by anchors  153  and  155 , that provides a base for flag  157 , and that is connected to harbor fence  103  through connector  159 . Spar  109  comprises an upper section  161 , a LED strobe light  167 , a retractable keel  163 , and a counterweight  165 . Spar  109  floats essentially at a water surface, in which upper section  161  has buoyancy while keel  163  and counterweight  165  provide stability to spar  109 . LED strobe light  167  and flag  157  provide visible indications to anyone in the harbor (including potential intruders) about a presence of harbor fence system  103 . LED strobe light may obtain electrical power and activation instructions from a control module (e.g.  801  and  803  as shown in  FIG. 8 ) through a cabling arrangement. Upper section  161  may contain sensors that detect whether harbor fence system  103  is being impacted, lifted, or submerged by an intruder at a proximity of spar  109 . Also, the embodiment can detect an occurrence when an intruder cuts any section of harbor fence system  103  by detecting a loss of communications with any of the spars over a telemetry subsystem (which is discussed in the context of  FIG. 8 ) or by a detecting a cut in the top line. The plurality of spars communicates with a control unit  171  (that is located on shore or on ship  101 ) through cable  115 . A user  173  enters commands into control unit  171  in order to configure harbor fence system  103  and to monitor an output device in order determine a status (e.g. a detection of an intruder) of harbor fence system  103 .  
         [0039]      FIG. 2  shows a portion of harbor fence system  103  as shown in  FIG. 1 . As was shown in  FIG. 1 , spar  105  comprises upper section  161 , retractable keel  163 , counterweight  165 , LED strobe light  167 , immersion sensors  203  and  205 , and an accelerometer  207 . (Variations of the embodiment may use other types of lighting such as floodlights or may use audio sounds such as a sirens.) Immersion sensor  203  is normally above the water surface (i.e. not normally exposed to water) and immersion sensor  205  is normally below the water surface (i.e. normally exposed to water). A lighting pattern for associated LED strobe lights may be controlled by an associated control module that, in turn, may be configured by control unit  171 . Accelerometer  207  is sensitive to an impact by an intruder by sensing an acceleration imposed upon spar  105 . Sensors  203 ,  205 , and  207  provide inputs to electronic circuitry (as described in the context of  FIGS. 6 and 7 ) that is contained in spar  109 . The plurality of spars is connected together with a top line  211  and a primary cable  209 . In addition to providing physical cohesion of the spars, primary cable  209  (which is included in the cable arrangement) also provides electrical power and communications, including communications between control unit  171  and control modules (e.g  801  and  803  as shown in  FIG. 8 ) and between control modules and associated spars. The top line contains at least one wire that when cut will result in an intrusion alarm signal. In order to provide additional visibility of harbor fence system  103 , bright strips of visible material (e.g. plastic) may be attached to line  211  and large markings or letters may be painted on upper section  161 .  
         [0040]      FIG. 3  shows a concentric hoop configuration  300  that may be used as a flotation means for a harbor boom line according to an alternative embodiment of the invention. In some embodiments of the invention, other forms of flotation (other than spars as shown in  FIGS. 1 and 2 ) may maintain harbor fence system  103  at the water surface. Concentric hoop configuration  300  comprises a plurality of flotation elements comprising hoop  301  and  303  that are approximately attached at a perpendicular angle with respect to each other. In this embodiment, the sensors and lights may be mounted within the hoops.  
         [0041]      FIG. 4  shows a helix boom line configuration  401  of harbor fence system  103  according to another alternative embodiment of the invention. Helix boom line configuration  401  comprises a plurality of helix sections (one section being between connectors  411  and  413 . Adjacent sections are attached together at mating connectors (e.g. connectors  415  and  411 ). A shape of helix boom line configuration  401  is maintained by moorings, e.g. mooring  417 . Sensors (e.g. sensors  403 ,  405 , and  407 ) and lights are distributed along helix boom line configuration  401 . The helix material may be a transparent plastic to allow the lights to show through, or the lights may be mounted to protrude through holes in the helix wall. Communications and electrical power is provided by primary cable  409 , and a “snubber” cable may be included down the center of the helix to limit its extension. Hooks on elastic bands may be provided to keep the helix in the closed position for retrieval and storage.  
         [0042]      FIG. 5  shows a scenario in which harbor fence system  103  is being breached by intruders in surface craft.  FIG. 5  illustrates that an intruder in a surface craft must lift, submerge, or cut harbor fence system  103  the harbor fence line in order to breach it. Harbor fence system  103  provides an alarm and intrusion location for any of these actions, as well as in the event of an excessive impact on one of the spar “fenceposts”. In  FIG. 5 , an intruder  505  is lifting a section  501  of harbor fence system  103 , while an intruder  503  submerges section  501  in order to pass through the perimeter of system  103 . An intruder (not shown) may also pass under the surface of the water in an attempt to pass through the perimeter. (An embodiment of the invention addresses this latter possibility as discussed in the context of  FIG. 13 .) An embodiment of the invention may detect occurrences of such scenarios as will be discussed.  
         [0043]      FIG. 6  shows a water crossing sensor circuit  600  that is utilized in harbor fence system  103  according to an embodiment of the invention. In the embodiment, water crossing sensor circuit  600  is incorporated at each spar (e.g. spar  105  as shown in  FIG. 2 ). A wet detector  603  (corresponding to immersion sensor  205  in  FIG. 2 ) is normally submerged in water and detects an occurrence when wet detector  603  is not exposed to water (as with intruder  505  lifting section  501  in  FIG. 5 ). As an example, intruder  505  lifts the boom line, as shown in  FIG. 5 , and thus immersion sensor  205  is lifted from the water. Also, a dry detector  601  (corresponding to immersion sensor  203  in  FIG. 2 ) is normally above the water surface and detects an occurrence when dry detector  601  is exposed to water (as with intruder  503  submerging section  501  in  FIG. 5 ). As an example, intruder  503  runs over the boom line, as shown in  FIG. 5 , and thus immersion sensor  203  is submerged into the water. Outputs from dry detector  601  and wet detector  603  are combined by a logic gate  605 . (The output of logic gate  605  is a logically “1” only if both inputs are logically “1” or if both inputs are logically “0”.) An intrusion alarm is generated if the logic gate output is a logic “1”, indicating that either both sensors are underwater (submerged) or both sensors are out of the water (lifted). In order to reduce the possibility of false detections (such as when a large wave temporally submerges the corresponding spar), a pulse detector determines if a positive sensor output should be construed as an occurrence of an intruder penetrating the perimeter of harbor fence system  103  by “debouncing” the output of OR gate  605 . If pulse detector  607  determines the occurrence of an intruder, an alarm detector  609  is activated until water crossing sensor  600  is queried by a control module (not shown) selecting water crossing sensor circuit  600  by reading an alarm output  616  by selecting a driver  613  by activating a telemetry module (TM) select  611 .  
         [0044]      FIG. 7  shows an excessive impact sensor circuit  700  that is utilized in harbor fence system  103  according to an embodiment of the invention. An accelerometer  701  (corresponding to sensor  207  in  FIG. 2 ) detects an occurrence of excessive impact on its “fencepost”, such as when intruder  505  comes into contact with section  501  as shown in  FIG. 5 . (In variations of the embodiment, a hydrophone and amplifier may be used as an alternative sensor rather than an accelerometer.) An output of accelerometer  701  is integrated by a peak detector  703 . (Peak detector  703  determines if the output from accelerometer  701  exceeds a threshold to determine if an intruder is detected. Harbor fence system  103  configures the threshold level or sensitivity of the detector, by a command being sent by control unit  171 , in order to discriminate from erroneous detections such as when a spar is moved about by a wave or winds.) If an intruder is detected by the threshold of peak detector  703 , an output from peak detector  703  sets a one-shot alarm  705  that is activated until excessive impact sensor circuit  700  is queried by a control module (not shown). The control module selects excessive impact sensor  700  by activating telemetry module (TM) select line  707  and reads the alarm output  711 , after which the one-shot is cleared to be ready for the next impact measurement.  
         [0045]      FIG. 8  shows a boom line telemetry subsystem  800  according to an embodiment of the invention. In the embodiment, a spar is associated with a fence post node (e.g. fence post nodes  805 - 813 ). Each fence post is associated with water crossing circuit  600  and excessive impact detector  700 . Fence post nodes are multiplexed onto a local bus, in which a control module (e.g. control modules  801  and  803 ) can query each of the associated fence post nodes (e.g.  805 - 813 ). Moreover, each of the control modules may be queried by control unit  171  over a cabling configuration that comprises loop-around components  815  and  821 . The cabling arrangement distributes electrical power to the control modules and to fence post nodes, and provides communication between control unit  171  and the control modules, from both ends or from either end of harbor fence system  103 . In the embodiment, telemetry subsystem  800  uses two loop-around components in order to provide redundancy in a case in which one of the loop-around components becomes inoperative (e.g. when an intruder cuts one of the loop-around components). Electrical isolation of the cut wires will allow power and communications to all operative nodes on either side of the cut, even when the primary cable is cut, allowing multiple intrusions to be sensed.  
         [0046]     The embodiment of harbor fence system  103  that is shown in  FIG. 8  may be interfaced with an underwater sonar subsystem  1300  that can detect underwater intruders that may dive beneath the perimeter of harbor fence system  103 . The cable arrangement interfaces to nodes  817  and  819  that may correspond to sonar sensor modules (e.g sensor modules  1307 ,  1309 ,  1321 ,  1323 , and  1325  as shown in  FIG. 13 ) as will be discussed in the context of  FIGS. 12-28 . Referring to  FIG. 8 , control unit  171  may query any of the diver sensor nodes (e.g.  817  and  819 ) in order to obtain a status relating to a detection of an underwater intruder.  
         [0047]      FIG. 9  shows deployment of harbor fence system  103  according to an embodiment of the invention. A plurality of spars (e.g. spar  903 ) and associated cabling of a deployed section of harbor fence system  103  is stored into container  901 . The keel of spar  903  is retracted into the upper section of spar  903  when spar  903  is stored in container  901  in order to facilitate the storing of the deployed section. As the deployed section is removed from container  901 , a keel (the shaft with the counterweight at the bottom) drops or is pulled from an upper section of an extracted spar (e.g spar  905 ). When the end of the deployed section is reached, harbor fence system  103  may be expanded by another section by connecting the deployed section to the other section by connecting associated connectors. Retrieval of each section or module of harbor fence system  103  is accomplished in the reverse manner.  
         [0048]      FIG. 10  shows a variation of deployment of harbor fence system  103  according to an embodiment of the invention. With the variation of the embodiment, a specially designed reel  1001  is used rather than container  901  when deploying a section of harbor fence system  103 . Retrieval may be accomplished by winding the fenceposts and the cables back onto reel  1001 .  
         [0049]      FIG. 11  shows retrieval of harbor fence system  103  according to an embodiment of the invention. Harbor fence system  103  is retrieved in sections (e.g.  1103 ,  1105 ,  1107 , and  1109 ). Multiple sections may be connected together and towed by a boat  1101  to minimize the number of trips to the “mother” ship during retrieval or deployment operations. This process may be repeated for retrieving other sections of harbor fence  103 . Sections of harbor fence system  103  are lifted by crane  1111  into ship  101  so that harbor fence system  101  may be transported with ship  101  to another location and redeployed.  
         [0050]      FIG. 12  illustrates a ship  1201  that floats at a water surface  1203  and that is protected by a sonar system. In  FIG. 12 , ship  1201  is located in a harbor with a water depth  1205 . The sonar system protects ship  1201  from intruders that pass under water (between water surface  1203  and a water bottom  1209 ) through a protection distance  1207 . Moreover, water depth  1205  may vary in the protected region of ship  1201 .  
         [0051]      FIG. 13  shows a sonar subsystem  1300  that protects ship  1201  from underwater intruders in accordance with an embodiment of the invention. Sonar subsystem  1300  protects ship  1201  with respect to a protective boundary  1301  (e.g. a perimeter around an asset such as ship  1201  or a line of protection across a harbor that is in close proximity to the asset). (In the embodiment, protective boundary  1301  has approximately a same shape as the perimeter of harbor fence system  103 .) Although the exemplary embodiment of the invention depicts ship  1201  being protected by sonar subsystem  1300 , sonar subsystem  1300  may protect other types of assets that border water, either partially or completely. Exemplary assets may include power plants, bridges, oil drilling rigs, river dams, military ships, and commercial ships. Protective boundary  1301 , as shown in the embodiment corresponding to  FIG. 13 , spans across an entrance to a mooring area for ship  1201  and may span protection distance  1207  in order to provide the same area of another sonar system. Although  FIG. 13  depicts an arc, the embodiment may protect a protective boundary corresponding to a different shape (that may enclose an area around ship  1201 ) by routing protective boundary  1301  to correspond to the different shape.  
         [0052]     Sonar subsystem  1300  comprises a plurality of sonar sensor modules (e.g. modules  1307 ,  1309 ,  1321 , and  1323 ), connections  1311 ,  1313 ,  1315 , and  1317 , and a central processor  1319 . In the embodiment, central processor  1319  may be integrated into the functionality of control unit  171  as shown in  FIG. 1 . (Although not shown, other sonar sensor modules along protective boundary  1301  have corresponding connections to central processor  1319 .) In the embodiment, connections  1311 ,  1313 ,  1315 , and  1317  may be bundled together into a cable and routed along protective boundary  1301  or may be arranged in a bus configuration to central processor  1319 . Sonar sensor modules  1307 ,  1309 ,  1321 ,  1323 , and  1325  are distributed along protective boundary  1301  in an approximately uniform manner. (In the embodiment, sonar sensor modules  1307 ,  1309 ,  1321 , and  1323  may correspond to diver sensor nodes, e.g. diver sensor nodes  817  and  819  as shown in  FIG. 8 .) Each sonar sensor module may correspond to a sonar radiation pattern (such as a radiation pattern  1303  corresponding to sensor module  1307  and a radiation pattern  1305  corresponding to sensor module  1309 ). The sonar power levels of each sonar sensor module (e.g. modules  1307 ,  1309 ,  1321 ,  1323 , and  1325 ) may be adjusted so that excessively strong sonar signals are not generated by each sonar sensor module beyond an associated coverage region.  
         [0053]     Each radiation pattern may be non-directional with respect to underwater coverage (oriented in the downward position) and may have an approximate coverage range from 50 to 100 feet, thus requiring a reduced transmitted power. However, the distance of protective boundary  1301  may be substantially greater than the coverage distance of a sensor module in order to provide a total coverage range that may be as great or greater than what is provided in prior art. In the embodiment, adjacent radiation patterns (e.g  1303  and  1305 ) overlap at least 50% in coverage area. Adjacent sensor modules (e.g.  1307  and  1309 ) are separated by approximately the minimum expected water depth  1205 . However, in other embodiments of the invention, the separation between sensor modules may vary as a function of the corresponding water depth.  
         [0054]     In the embodiment, the sensors (e.g. sensors  1307 ,  1309 ,  1321 ,  1323 , and  1325 ) of sonar system  1300  are activated (in which a sensor generates a sonar signal that may be referred as a “ping”) such that a degree of interference among the sensors is limited to a level that does not cause a false detection of a target. (For example, adjacent sensors may be activated at different times if the adjacent sensors are operating at the same frequency.) The amount of adjacent interference may be controlled by adjusting a sequence of activating each sensor and by configuring different operating frequencies with different sensors.  
         [0055]      FIG. 14  shows a vertical coverage of adjacent sonar sensor modules  1307  and  1309 .  FIG. 14  shows coverage regions  1401  and  1403  of adjacent sonar sensor modules  1307  and  1309 , in which the distance between adjacent sensors is distance (S)  1405 . Sensor modules  1307  and  1309  are situated in the proximity of water surface  1203 . Sensor modules  1307  and  1309  have unidirectional coverage beams spanning coverage regions  1401  and  1403 , respectively. In the embodiment, adjacent sonar sensor modules  1307  and  1309  are separated by a distance that is approximately equal to or less than water depth  1205 , and coverage regions  1401  and  1403  overlap by at least 50%. However, the embodiment may be configured for different harbor topologies in which the distance between adjacent sonar sensor modules  1307  and  1309  and the degree of overlap of coverage regions  1401  and  1403  may be adjusted. Moreover, water depth  1205  may vary along protective boundary  1301 . In the embodiment, the distance between adjacent sonar sensor modules is approximately equal to the minimum water depth around protective boundary  1301  (as shown in  FIG. 13 ). However, in other embodiments of the invention, the distance between adjacent sonar sensors (e.g. sonar sensor modules  1307  and  1309 ) may be adjusted according to the water depth in the proximity of the adjacent sonar sensors.  
         [0056]      FIG. 15  shows an apparatus  1500  for a sonar sensor module, e.g. sonar sensor module  1307 . Apparatus  1500 , as may be instructed by central processor  1319  (that may be integrated with the functionality of control unit  171 ), generates a transmitted sonar signal  1502  with a pulse generator  1501 , a power amplifier  1503 , a transmit-receive (T/R) switch  1505 , and a transducer  1506 . Typically, transmitted sonar signal  1502  has a time duration between 100 and 600 microseconds, with a carrier frequency between 100 KHz to 200 KHz, but other embodiments of the invention may utilize other pulse parameters.  
         [0057]     After sonar signal  1502  has been transmitted, T/R switch  1505  changes its state so that apparatus  1500  receives a sonar signal, resulting from reflections of transmitted sonar signal  1502 . The received sonar signal is received by transducer  1506  (which functions in both the transmit mode and the receive mode) and is amplified by a preamplifier  1507 . A sonar signal  1553  shows the received sonar signal at the output of preamplifier  1507 . Sonar signal  1553  is characterized by three signal regions: a surface reverberation (SR) region corresponding to sonar reflections from water surface  1203  (as shown in  FIG. 12 ), a diver (D) region corresponding to sonar reflections from a target that may be an underwater intruder, and a bottom reverberation region (BR) corresponding to sonar reflections from water bottom  1209 .  
         [0058]     A time varied gain (TVG) amplifier  1511  reduces the amplitude of the SR region of sonar signal  1553  by starting at a lower gain immediately after TR switch  1505  reverts into the receive mode (i.e. after the transmission of transmit sonar signal  1502 ), and by increasing its gain with time so that sonar signal  1553  from surface reverberation is equalized to approximately constant amplitude until the bottom reflections begin. The resulting sonar signal is shown as a sonar signal  1555 . (The sonar signal during the BR-region is typically not equalized because the received sonar signal is subsequently gated off before the occurrence of the BR-region by a gate  1517 .) Providing at least partial amplitude equalization enhances the ability to detect a target during the D-region of sonar signal  1553  by applying a threshold criteria. (Reducing the amplitude variation of sonar signal  1502  also enhances the resolution of analog to digital conversion as performed by an analog to digital converter  1519 .)  
         [0059]     A rectifier  1513  removes the sonar carrier component of sonar signal  1555  in order to obtain the corresponding envelope that is further processed by a low pass filter  1515 . Gate and threshold module  1517  determines if sonar signal is above a threshold (which is indicative of a target) during a search window that spans betweens the initiation of sonar reception and the return of sonar reflections from water bottom  1209 .  
         [0060]     From sonar signal  1557 , apparatus  1500  determines the corresponding range and amplitude of the received sonar signal as well as the width of a detected target echo during the D-region of sonar signal  1557  from a range register  1525 , an amplitude register  1521 , and a width register  1527 , respectively that are gated by gated counters  1523 . The corresponding data are collected by a microcontroller  1529 . Microcontroller  1529  may provide this data to central processor  1319  through an interface  1531  and a serial telemetry bus  1533 . The embodiment supports the RS-485 standard, which is a differential data transmission standard that is specified by Electronic Industries Association (EIA) and Telecommunications Industry Association (TIA). Sonar data may be collected in a variety of ways, including after each transmission of sonar signal  1502  or after a plurality of transmission of sonar signal  1502 . Data may be collected autonomously, in which a sonar sensor module (e.g. module  1307 ) automatically sends the data, or may be collected in a polled manner, in which central processor  1319  queries each sonar sensor module to return sonar data.  
         [0061]     The embodiment may utilize different higher layer protocols with respect to the physical layer as provided by the RS-485 standard. For example, the embodiment may support an Internet Protocol (IP) in conjunction with Transmission Control Protocol (TCP) or a customized protocol. Also, other embodiments may utilize a different physical layer such as Ethernet.  
         [0062]     After processing the received sonar signal in response to transmitting a sonar signal at a time instance, apparatus  1500  may transmit a subsequent transmitted sonar signal  1502  at a subsequent time instance and process a received sonar signal in order to determine a range, amplitude, and width of a target corresponding to the subsequent time instance. This process is typically repeated during the detection mode of sonar subsystem  1300 .  
         [0063]      FIG. 16  shows sonar signal  1557  that is received by a sonar sensor module. Apparatus  1500  determines whether amplitude  1603  of sonar signal  1557  during D-region  1605  exceeds a threshold  1611  during search window  1609 . Sonar signal  1557  is gated off at time  1613 , corresponding to the beginning of BR-region  1607 . In the embodiment, central processor  1319  that is integrated with control unit  171  may set threshold  1611  by sending a command.  
         [0064]      FIG. 17  shows a telemetry configuration for a sonar subsystem  1300 . Central processor  1319  collects target data (e.g. range, amplitude and target width) from each of the sonar sensor modules (e.g. modules  1307 ,  1309 ,  1321 ,  1323 ,  1325 , and  1701 ) through telemetry bus  1533  (as shown in  FIG. 15 ) or through a “backup” telemetry bus  1703 . Telemetry busses  1533  and  1701  support two-way communication between central processor  1319  and the sonar sensor modules so that central processor  1319  may send commands to the sonar sensor modules and so that the sonar sensor modules may send information about received sonar signals to central processor  1319 .  
         [0065]     In the embodiment, telemetry bus  1533  and telemetry bus  1703  each may comprise a twisted pair of wires in order to reduce common mode noise that may be injected by noise sources along telemetry busses  1533  and  1703 . Also, telemetry busses  1533  and  1703  may each provide electrical power for each of the sonar sensor modules or may provide electrical power through a separate pair of wires. Sonar subsystem  1300  supports two telemetry busses (bus  1533  and bus  1703 ) in order to support transmission redundancy. For example, if an intruder cuts telemetry bus  1533  or  1703 , fuses or switches will isolate each side of the cut so that both telemetry busses  1533  and  1703  remain partially operational. Telemetry bus  1533  may still operate the modules before the cut, while telemetry bus  1703  operates modules after the cut. In the embodiment, if both telemetry busses  1533  and  1703  are fully operational, approximately half of the sonar sensor modules may communicate with central processor  1319  through telemetry bus  1533  while the other approximate half of the sonar sensor modules may communicate to central processor  1319  through telemetry bus  1703  in order to distribute the message traffic load.  
         [0066]      FIG. 18  shows an example of a path  1801  of an underwater intruder traversing through sonar subsystem  1300 . (In the discussion regarding  FIGS. 18-21 , a target is assumed to be an underwater intruder, and is referred as such. However, sonar subsystem  1300  may determine if the target should be considered to be an underwater intruder as may be performed in step  2205  in  FIG. 22 .) In  FIG. 18 , the underwater intruder traverses through coverage areas  1303 ,  1305 , and  1306  of sonar sensor modules  1307 ,  1309 , and  1321 , respectively. An underwater intruder may traverse different paths, such as a path  1803 . With path  1803 , only two adjacent sonar sensor modules (i.e. modules  1305  and  1306 ) detect the intruder. Even though the example shown in  FIG. 18  illustrates linear path  1803 , an underwater intruder may traverse a non-linear path such as path  1805  or a zigzag path (not shown).  
         [0067]      FIG. 19  shows a path  1901  of an underwater intruder that is essentially perpendicular to protective boundary  1301  of a sonar subsystem  1300 . Path  1901  traverses through coverage regions  1305  and  1306 , corresponding to sonar sensor modules  1309  and  1321 , respectively. Sonar sensor module  1309  is approximately situated at a location A  1903  and sonar sensor module  1321  is approximately situated at a location B  1905 . As the underwater intruder traverses path  1901 , the horizontal distance to sonar sensor module  1309  is horizontal distance (S A )  1907  and the horizontal distance to sonar sensor module  1321  is horizontal distance (S B )  1909 . The distance between sonar sensor modules  1309  and  1321  is distance (S)  1405 . In geometric configuration shown in  FIG. 19 , S  1405  is approximately equal to S A    1907  plus S B    1909 . In the embodiment, a sonar sensor module may detect the underwater intruder only if the intruder is within the coverage region of the sonar sensor module (e.g. within region  1305  for sonar sensor module  1309 ). Thus, sonar senor module  1309  detects the intruder between points  1911  and  1917 , and sonar sensor module  1321  detects the intruder betweens points  1913  and  1915 . Moreover, the speed of the intruder may be approximated by dividing the distance between points  1911  and  1917  by the time interval for the intruder to traverse between points  1911  and  1917 . One can also perform the same calculation for points  1913  and  1915 . (The approximation is more accurate the more constant the intruder&#39;s velocity is.)  
         [0068]      FIG. 20  shows associated tracking data  2005  and  2007  obtained from adjacent sonar sensor modules  1309  and  1321 , respectively, for the example shown in  FIG. 19 . Each data point on tracking data  2005  corresponds to a range measurement of a target from sonar sensor module  1309  (as shown in  FIG. 21 ) and each data point on tracking data  2007  corresponds to a range measurement of the intruder from sonar sensor module  1321  (as shown in  FIG. 21 ) as a function of time. Because the sonar coverage of a sonar sensor module is essentially omnidirectional, an individual measurement from a sonar sensor module is not indicative of the direction of an intruder&#39;s path. However, central processor  1319  may correlate data from a plurality of sonar sensor modules (e.g. modules  1309  and  1321 ) in order to deduce the direction of the intruder&#39;s path. In  FIG. 20 , a closest point of approach of the intruder (CPA)  2009  to sonar sensor module  1309  has a range R A    2013  and closest point of approach of the intruder  2011  to sonar sensor module  1321  has a range R B    2015  at approximately the same time T x    2010  for paths approximately perpendicular to the line between modules. The underwater intruder traverses between points  1911  and  1917  in a time (ΔT A )  2019  and between points  1913  and  1915  (as shown in  FIG. 19 ) in a time (ΔT B )  2021 .  
         [0069]      FIG. 21  shows a method of determining a water depth  2101  of an underwater intruder for the example shown in  FIGS. 19 and 20 . In this example, the intruder is moving in a perpendicular direction to protective boundary  1301 , which corresponds to a shortest path to ship  1201 . In fact, from this observation, the path of the intruder may be determined. (The intruder moving in the perpendicular direction to protective boundary  1301  corresponds to CPA  2009  occurring at essentially the same time as CPA  2011 .) Sonar sensor module  1309  is separated from sonar sensor  1321  by distance S  1405 . Because the intruder is approaching protective boundary in the perpendicular direction, distance S  1405  is essentially equal to horizontal distance S A    1907  plus horizontal distance S B    1909 .  
         [0070]     Applying the Pythagorean theorem to a triangle corresponding to distance S A    1907 , range R A    2013 , and target depth D  2101  and to a triangle corresponding to distance S B    1909 , range R B    2015 , and water depth D  2101 , one may determine target depth D by the following equations (other algorithms may be possible as well):
 
 S   A   =S ( K /( K +1))  (EQ. 1)
 
 S   B   =S (1/( K +1))  (EQ. 2)
 
 D ={square root}[( R   B ) 2 −( S   B ) 2 ] or  D ={square root}[( R   A ) 2 −( S   A ) 2 ]  (EQ. 3),
 
 where K=R A /R B . 
 
         [0071]      FIG. 22  shows a flow diagram  2200  for sonar sensor subsystem  1300 . In step  2201 , (after a transmit pulse has been sent on command), sonar signals are received by a sonar sensor module (e.g. module  1307 ) from sonar reflections from the target. In step  2203 , subsystem  1300  applies criteria to the signals to determine if a significant reflecting body is present between surface and bottom. If not, subsystem  1300  waits for another command to “ping” again, in which step  2201  is repeated. If a significant echo is received, in step  2205  sonar sensor subsystem  1300  measures parameters of the received sonar echo from the potential target. In the exemplary embodiment, sonar sensor subsystem  1300  collects tracking data (as exemplified in  FIG. 13 , in which measured ranges to potential targets are collected in relation to time), as well as size and amplitude data related to the echo. This data is then sent from the sonar sensor module (or modules) receiving potential target echoes to central processor  1319  through telemetry busses  1533  and  1703 .  
         [0072]     In step  2207 , central processor  1319  collects and stores the recent sonar data measurements from the modules receiving echoes and uses the data to calculate at least one estimator about the target and/or the target&#39;s path (e.g. path  1801  or path  1803 ). In the embodiment, an estimator pertains to an initial guess of a parameter that is associated with the target or it&#39;s path(e.g. path consistency, closest point of approach, depth, speed, size, etc). In step  2209 , central processor  1319  utilizes one or more estimators in order to facilitate the determining of an estimated target path. In the embodiment, as will be discussed in the context of  FIGS. 23-26 , central processor  1319  searches a collection of simulated tracking data and attempts to match a set of simulated tracking data to the actual sonar data. This approach is similar to a technique known as matched-field tracking. In a variation of the embodiment, as will be discussed in the context of  FIGS. 27 and 28 , central processor  1319  adjusts the estimated path in order to minimize an error measure between corresponding tracking data (i.e. corresponding to the estimated path) and actual tracking data. This approach is referred as error-function minimization, and may be used to improve the speed and efficiency of the target path estimation and prediction of future target locations over time.  
         [0073]     In step  2211 , central processor  1319  processes the sonar data and path estimations in order to determine if the target echo should be perceived as an dangerous (human) underwater intruder as opposed to a marine mammal, fish, or other reflector. In the exemplary embodiment, central processor  1319  develops a threat level estimate (a measure of a probability or likelihood that the target is an human underwater intruder on a relatively consistent path toward the protected asset) by comparisons with potential threat characteristics and capabilities. In the embodiment, central processor  1319  may use a target motion threat score that is based upon depth, speed, and path (track) consistency; a course direction threat score that is based on an angle of crossing protective boundary  1301 ; the amplitude of the received sonar signal reflected from the target in relation to the range of the target as compared with an expected “target strength”; a target echo width, relating to target size; and other criteria that may be derived from the sonar data. In step  2213 , different levels of alarms may be initiated depending on the threat level estimate, and the predicted track of the target is calculated and can be provided to response forces.  
         [0074]      FIG. 23  shows an example of tracking data  2300  of a target. Tracking data  2300  comprises tracking data  2301 , tracking data  2303 , and tracking data  2305  that central processor  1319  collects from adjacent sonar sensor modules, e.g. modules  1307 ,  1309 , and  1321 , respectively.  
         [0075]      FIG. 24  shows a first example of simulated tracking data  2400 . In an example of the embodiment, simulated tracking data  2401 ,  2403 , and  2405  that are simulated “off-line” (i.e. previous to receiving tracking data  2300  by sonar sensor modules  1307 ,  1309 , and  1321 ) for a first path of the target. Simulated tracking data are simulated for different simulated paths, and the sets of simulated tracking data (e.g sets  2400 ,  2500 , and  2600 ) are stored in a memory that is associated with central processor  1319 .  
         [0076]      FIG. 25  shows a second example of a set of simulated tracking data  2500 , in which simulated tracking data  2501 ,  2503 , and  2505  are simulated sonar data from adjacent modules  1307 ,  1309 , and  1321  corresponding to a second simulated path.  
         [0077]      FIG. 26  shows a third example of a set of simulated tracking data  2600 , in which simulated tracking data  2601 ,  2603 , and  2605  are simulated sonar data from adjacent modules  1307 ,  1309 , and  1321  corresponding to a third simulated path. In the embodiment, typically more simulated tracking data, corresponding to different simulated paths, are stored for central processor  1319  to access and to compare with tracking data  2300 . Central processor  1319  may compare selected simulated tracking data to tracking data  2300  and choose a matched simulated tracking data that is “closest” to tracking data  2300 . In the embodiment, the matched simulated tracking data has the smallest error when compared with tracking data  2300 . Central processor  1319  consequently determines the simulated path that is associated with the matched simulated tracking data, which is consequently selected as the estimated path of the target.  
         [0078]     For an environment, many simulated tracking data may be stored for comparison by central processor  1319 . Moreover, with a variation of the embodiment, sonar subsystem  1300  may store simulated tracking data for non-linear paths so that sonar subsystem  1300  may discern a target that traverses a non-linear path such as path  1805  as shown in FIG.  18 . Central processor may utilize target parameter estimations (as determined in step  2207  in  FIG. 22 , e.g. the target&#39;s depth) to reduce the number of memory accesses and to reduce the execution time for determining the matched simulated tracking data.  
         [0079]      FIG. 27  shows tracking data  2701 ,  2703 , and  2705  of a target from adjacent sonar sensor modules  1307 ,  1309 , and  1321 , respectively. (In the example shown in  FIG. 27 , tracking data  2700  is the same as tracking data  2300  as shown in  FIG. 23 .) In  FIG. 27 , the target has a closest point of approach (CPA) to module  1307  corresponding to data point  2711 . The target has a closest point of approach to module  1321  corresponding to data point  2713 . A difference in time  2707  (t 1 ) and a difference in range  2709  (r 1 ) are determined from data points  2711  and  2713 . Central processor  1319  may also determine corresponding time differences and range differences for the other tracking data (i.e.  2707  and  2703 , and  2703  and  2705 ).  
         [0080]      FIG. 28  shows initial estimated path  2801  and final estimated path  2803  of the target corresponding to  FIG. 27 . Central processor  1319  uses the time history of range differences from preferably two or more sonar modules to obtain an initial estimated path  2801 . An estimated path corresponds to a set of tracking data that may be compared with tracking data  2700  in order to determine an error measure. The initial estimated path is adjusted in order to reduce the error measure using a multi-parameter search method. In this method, the estimated path is perturbed in each of several parameters related to the path in a sequence based on the greatest slope until a desired minimum error measure is achieved. This procedure results in a “best” estimate of the target&#39;s actual path from the sonar data in a relatively time-efficient manner. In summary, it can be said that a “matched-field” approach matches the simulated tracking data with actual tracking data, from which a best guess of a target&#39;s path is determined. An “error-function minimization” approach adjusts the estimated path to improve the accuracy and speed of calculation of the path estimate using an efficient search method.  
         [0081]     As can be appreciated by one skilled in the art, a computer system with an associated computer-readable medium containing instructions for controlling the computer system can be utilized to implement the exemplary embodiments that are disclosed herein. The computer system may include at least one computer such as a microprocessor, microcontroller, digital signal processor, and associated peripheral electronic circuitry.  
         [0082]     While the invention has been described with respect to specific examples including presently preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described systems and techniques that fall within the spirit and scope of the invention as set forth in the appended claims.