Abstract:
System and method for navigating an unmanned undersea vehicle (UUV) using three-dimensional acoustic reflectivity data sets and a beam steered downward looking sonar capturing sub-bottom features and creating three-dimensional representations to compare with the reflectivity data sets to general navigation corrections. Acoustically senses and exploits sub-bottom features for navigation of UUVs that can provide more reliable navigation than using surface features alone, since much of the sea floor is flat (e.g. on continental shelves and abyssal plains).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority based on U.S. Provisional Patent Application No. 61/777,094, entitled SYSTEM AND METHOD FOR NAVIGATION FOR UNMANNED UNDERSEA VEHICLES, filed on Mar. 12, 2013, the entirety of which is hereby incorporated by reference into the present application. 
    
    
     BACKGROUND 
     Methods and systems disclosed herein relate generally to unmanned underwater vehicles (UUVs) and more specifically to navigation for UUVs. 
     UUVs are expected to perform missions previously assigned to manned vessels. Autonomous navigation for these vehicles is a technical challenge, certainly a critical performance factor. The race to find accurate and robust navigation methods is a key part of UUV development. Use of UUVs to replace manned platforms can potentially save billions of dollars, remove sailors from risky environments, and reduce greenhouse gas emissions. Currently, navigation is performed by communication between the UUV and a host ship. 
     Terrain following navigation, which has worked well for unmanned aerial vehicles, has been proposed for UUVs. However, much of the ocean is devoid of terrain features. Terrain following vehicles transiting long distances would require powerful sonars which sense a wide footprint of the ocean floor. Certain bottom-following controllers designed for UUVs take into account the bathymetric characteristics ahead of the UUV as measured by echo sounders. Navigation off two-dimensional surface elevation terrain data sets has been performed, but in demonstrations, the elevation varied by more than 800 m in a 3.6 km distance, corresponding to at least a 13° slope (the average slope on the continental shelf is about 0.1°). 
     A normal incidence (unsteered) beam has been used that collects a depth profile of acoustic reflectivity along a track over which the UUV progresses. Being two-dimensional, this method cannot be used for navigation. 
     What are needed are systems and methods for navigating UUVs and ultimately for replacing manned platforms with UUVs. What are further needed are systems and methods for acoustically sensing and exploiting sub-bottom features (e.g. sand/mud layers, linear features like pipelines, or point scatterers, such as shells) for navigation of UUVs. What is still further needed is a system that includes a three dimensional data set provided by a sub-bottom profiler (SBP) and achieved through beam steering. Bottom-penetrating sonar could sense features in the sub-bottom to use for navigation. Depending on the morphology of a specific area, it is expected that the sub-bottom would be richer in features than much of the seafloor. Given that little of the seafloor sub-bottom structure has been mapped, navigation from sub-bottom features may require a pre-mission survey. This task can also be performed by a UUV in a less time-constrained period than an actual operational cruise. Known features, such as, for example, but not limited to, buried pipelines or cables, could be exploited. In this situation, a prior survey may not be required. The point of navigating off sub-bottom features to diminish the reliance on the rare areas of the ocean where there are terrain features distinct and dramatic enough to reliably navigate off of. 
     SUMMARY 
     The system and method of the present embodiment for acoustically sensing and exploiting sub-bottom features for navigation of UUVs can provide more reliable navigation than using surface features alone, since much of the sea floor is flat (e.g. on continental shelves and abyssal plains). Even when there are sea surface features, they may appear flat to a sensor, for example, a sonar, depending on the scale of the feature and the frequency and footprint of the sensor. 
     The method of the present embodiment for navigating a UUV can include, but is not limited to including, receiving a plurality of three dimensional acoustic reflectivity reference data sets, each of the plurality of reference data sets being associated with a reference location, receiving sonar data from a UUV equipped with a beam steered downward looking sonar, the sonar producing a sonar beam that penetrates the seafloor, the sonar beam producing several echoes from at least one sub-bottom feature, the sonar beam producing the sonar data from the echo, accumulating the sonar data to produce a three-dimensional representation of at least one sub-bottom feature, locating a match between the three-dimensional representation and one of the plurality of reference representations, and combining location of the matched reference feature with navigation data sensed by the UUV to navigate the UUV. 
     The system of the present teachings can include, but is not limited to including, an input processor receiving, for example, but not limited to, a narrow beam, electronically steered and bottom-penetrating sonar sensing sub-bottom features, a CPU processing UUV data and reference data, and producing navigation information for the UUV. The system of the present teachings can allow a UUV to follow known/manmade sub-surface features for example, but not limited to, pipelines. This system can also include attitude sensors, a waveform generator, a power supply, and a UUV controller. The narrow angle, downward looking sonar can be mounted on the bottom of a UUV. As the UUV moves forward, the sonar beam sweeps at athwartships angles and penetrates the seafloor, where sub-bottom features are revealed in the returned echo. Each ping of the sonar ensonifies a different area of the seafloor—a “footprint”. As the UUV moves forward, a three dimensional image of the sea floor is compiled, the three dimensions being length (in direction of motion), width (athwartship sweep angle) and depth. Attitude sensors provide estimates of vehicle roll and pitch. These and vehicle navigation data (position, speed, heading) are required for navigation. The system can provide sub-bottom feature recognition, course correction to match expected features, and electronic steering for the narrow-beam, bottom-penetrating sonar. The system can provide highly accurate navigation, robust terrain-following, covert sensing, and reduced power consumption compared to the prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
         FIG. 1  is a schematic block diagram of the system of the present embodiment; 
         FIG. 2  is a schematic block diagram of the UUV data processor of the present embodiment; 
         FIG. 3  is a flowchart of the method of the present embodiment; 
         FIG. 4  are side and front views of the UUV of the present embodiment; 
         FIG. 5  is a plan view of the UUV sensing the seafloor in search of navigation aids; 
         FIG. 6  is a comparison plan view of an operational use of the system of the present embodiment; and 
         FIG. 7  is a graphical view of a simulation of based on the system of the present embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     These solutions and other advantages are achieved by the various embodiments of the teachings described herein below. 
     Referring now to  FIG. 1 , system  100  for acoustically sensing and exploiting sub-bottom features for navigation of unmanned undersea vehicles (UUVs) can include, but is not limited to including, beam steered downward looking sonar  102 , attitude sensors  109 , and control/processing unit (CPU)  101  for navigating UUV  11  through UUV controller  111 . UUV controller  111 , pre-amplifier  119 , receiver  115 , transducer  117 , amplifiers  113 , waveform generator  105 , and analog to digital converter  107  are conventionally available and perform conventional functions. Sonar  102  ( FIG. 4 ) of the present embodiment operates as follows. Waveform generator  105  creates signals  13  of the proper temporal and spectral form for bottom penetration and echolocation, as well as provides temporal delays used in beam steering. Amplifiers  113  amplify signals  13 . Transducer  117  converts amplified signals  27  into sonar sound beam  15  that penetrates seafloor  23  ( FIG. 4 ). Sonar beam  15  results in several echoes  29  from at least one sub-bottom feature  43  ( FIG. 6 ). Receiver  115  converts echo  29  into electrical signals  31 . Pre-amplifier  119  provides amplified electrical signals  33  for digitization by analog to digital converter  107 , producing sonar data  145 . UUV data processor  103  can compute UUV navigation data  176  based on UUV reference data  141  and UUV reference data locations  143  (provided by, for example, data storage  106 ), attitude data  149  provided by attitude sensors  109 , and digitized data  145  and other data  147  provided by analog to digital converter  107 . UUV data processor  103  can provide UUV navigation data  176  to UUV controller  111 . Power supply  121  powers devices in system  100 . 
     Referring now primarily to  FIG. 2 , UUV data processor  103  for navigating a UUV can include, but is not limited to including, data set receiver  131  receiving a plurality of three dimensional acoustic reflectivity reference data sets  141  from UUV data storage  106 , each of the plurality of reference data sets  141  being associated with a reference location  143 . UUV data processor  103  can also include UUV data receiver  133  receiving sonar data  145  and other sensed data  147  from sonar  102 . UUV data processor  103  can also include 3D representation processor  135  accumulating sonar data  145  to produce three-dimensional representation  171  of the at least one sub-bottom feature  43  ( FIG. 6 ). UUV data processor  103  can still further include match processor  137  locating match  173  between three-dimensional representation  171  and one of the plurality of reference representations  141  at reference location  143 . Match  173  is associated with match location  175 . UUV data processor  103  can even still further include combination processor  139  combining match location  175  with other sensed data  147  to create UUV navigation data  176  and send it to UUV controller  111  to navigate UUV  11  ( FIG. 4 ). 
     Referring now primarily to  FIG. 3 , method  150  for navigating UUV  11  ( FIG. 4 ) can include, but is not limited to including, receiving  151  a plurality of three dimensional acoustic reflectivity reference data sets  141  ( FIG. 2 ), each of the plurality of reference data sets  141  ( FIG. 2 ) being associated with a reference location  143  ( FIG. 2 ), receiving  153  sonar data  145  ( FIG. 2 ) and other sensed data  147  ( FIG. 2 ) from beam steered downward looking sonar  11  ( FIG. 4 ), the sonar producing a sonar beam  15  ( FIG. 4 ) that penetrates the seafloor  23  ( FIG. 4 ), the sonar beam  15  ( FIG. 4 ) resulting in several echoes  29  ( FIG. 2 ) from at least one sub-bottom feature  43  ( FIG. 6 ), the echoes  29  ( FIG. 2 ) resulting in the sonar data  145  ( FIG. 2 ), the UUV  11  ( FIG. 4 ) collecting the other sensed data  147  ( FIG. 2 ). Method  150  can also include accumulating  159  the sonar data  145  ( FIG. 2 ) to produce a three-dimensional representation  171  ( FIG. 2 ) of the at least one sub-bottom feature  43  ( FIG. 6 ), locating  161  a match  173  ( FIG. 2 ) between the three-dimensional representation  171  ( FIG. 2 ) and one of the plurality of reference representations  141  ( FIG. 2 ), the match  173  ( FIG. 2 ) having a match location  175  ( FIG. 2 ), and combining  163  the match location  175  ( FIG. 2 ) with navigation data to navigate the UUV  11  ( FIG. 4 ). 
     Referring now primarily to  FIG. 4 , narrow angle, downward looking sonar  102  is mounted on the bottom of UUV  11 , shown in side view  19  and front view  21 . As UUV  11  moves forward, sonar beam  15  sweeps at athwartships angles  17  and penetrates seafloor  23 , where sub-bottom features  43  ( FIG. 6 ) can be revealed in the returned echo. Three sweep angles  17  are shown, but more sweep angles  17  could provide proportionally greater navigation assistance. 
     Referring now to  FIG. 5 , each ping of sonar  102  ( FIG. 4 ) can ensonify a different area of seafloor  23 —“footprint”  25  of acoustic beams  15  ( FIG. 4 ) on seafloor  23 . As UUV  11  moves forward, a three dimensional image of seafloor  23  is compiled, the three dimensions being length (in direction of motion), width (athwart ship sweep angle) and depth. Attitude sensors  109  ( FIG. 1 ) can provide attitude data  149  ( FIG. 2 ) which can include, for example, estimates of vehicle roll and pitch. UUV navigation is based on attitude data  149  ( FIG. 2 ) and other sensed data  147  ( FIG. 2 ) (position, speed, heading). 
     Referring now to  FIG. 6 , navigation is accomplished by comparing the 3D mission images  144  of the sub-bottom with reference images  141 . At each point along track  57  ( FIG. 7 ) of UUV  11 , reference image  141 , either compiled by a pre-mission survey using similar sonar, or a map of sub-bottom features  43 , can be used for comparison with mission image  144 . When a map is available, sub-bottom features  43  like buried pipelines or communication cables may provide paths for navigation. As UUV  11  moves along its track  57  ( FIG. 7 ), it compares each new look from sonar  102  at bottom  23  ( FIG. 4 ) to reference image  141  of what bottom  23  ( FIG. 4 ) should look like. Object a  36  and object b  37  can cause distinct returns in the sub-bottom echoes. Each ping at each different steer angle produces a history of energy reflected versus time, and can be diagrammed as one column  39  for each sweep angle  17 . After each ping hits the bottom  41 , sound continues as it travels deeper into seafloor  23 , until it is reflected off sub-bottom feature  43 . In reference image  141 , the sub-bottom features  43  appear in beam −1. However, in mission image  144 , during which UUV  11  approached at an offset as compared to reference image  141 , sub-bottom feature  43  shows up in beam −2. By comparing, the offset is accounted for, and UUV  11  is directed to yaw so as to eliminate the offset. Note this example reduced the problem to two dimensions. In general, offsets occur in all three directions, and UUV  11  can be controlled along six degrees of freedom. 
     Referring now to  FIG. 7 , simulated image  51  of randomly distributed point scatterers  53  can be used as an exemplary reference image  141  ( FIG. 5 ). A beam-steered sub-bottom sonar  102  ( FIG. 6 ) can sense this type of image. If UUV  11  ( FIG. 5 ) begins at track starting point  55  and attempts to maintain position, the result is navigated track  57 . For this simulation, the vehicle is assumed to begin at pre-selected position, and has a 1° heading error and 1% speed error. Even using a crude tracking algorithm, the track can be maintained with less than 1 m RMS error. In this simulation, circles  59  mark the sonar footprint  25  ( FIG. 5 ), there are eleven steering angles  61 , one normal (steered directly below the vessel) and five on each side of track  57 , and feature locations  63  are shown. 
     Embodiments of the present teachings are directed to computer systems for accomplishing the methods discussed in the description herein, and to computer readable media containing programs for accomplishing these methods. The raw data and results can be stored for future retrieval and processing, printed, displayed, transferred to another computer, and/or transferred elsewhere. Communications links can be wired or wireless, for example, using cellular communication systems, military communications systems, and satellite communications systems. In an exemplary embodiment, the software for the system can be written in any programming language. The system can operate on a computer having a variable number of CPUs. Other alternative computer platforms can be used. The operating system can be, for example, but is not limited to, WINDOWS® or LINUX®. 
     The present embodiment is also directed to software for accomplishing the methods discussed herein, and computer readable media storing software for accomplishing these methods. The various modules described herein can be accomplished on the same CPU, or can be accomplished on different computers. In compliance with the statute, the present embodiment has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the present embodiment is not limited to the specific features shown and described. 
     Referring again primarily to  FIG. 3 , method  150  can be, in whole or in part, implemented electronically. Signals representing actions taken by elements of system  100  ( FIG. 1 ) and other disclosed embodiments can travel over at least one live communications network. Control and data information can be electronically executed and stored on at least one computer-readable medium, for example, but not limited to, data storage  106  ( FIG. 1 ). The system can be implemented to execute on at least one computer node, for example, but not limited to, CPU  101  ( FIG. 1 ) in at least one live communications network. Common forms of at least one computer-readable medium can include, for example, but are not be limited to, a floppy disk, a flexible disk, a hard disk, magnetic tape, or any other magnetic medium, a compact disk read only memory or any other optical medium, punched cards, paper tape, or any other physical medium with patterns of holes, a random access memory, a programmable read only memory, and erasable programmable read only memory (EPROM), a Flash EPROM, or any other memory chip or cartridge, or any other medium from which a computer can read. 
     Although the present teachings have been described with respect to various embodiments, it should be realized these teachings are also capable of a wide variety of further and other embodiments.