A Simple Model of a Rockan Mine in Processed Sidescan Sonar Imagery

Transcription

1 Copy No. Defence Research and Development Canada Recherche et développement pour la défense Canada DEFENCE & DÉFENSE A Simple Model of a Rockan Mine in Processed Sidescan Sonar Imagery Anna Crawford Defence R&D Canada Atlantic Technical Memorandum DRDC Atlantic TM January 2006

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3 A Simple Model of a Rockan Mine in Processed Sidescan Sonar Imagery Anna Crawford Defence R & D Canada Atlantic Technical Memorandum DRDC Atlantic TM January 2006

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5 Abstract During the joint Canada France minehunting trial in Brest Harbour, France, in June 2004, a sidescan sonar survey at multiple look angles was conducted over a Rockan shaped target. In the absence of physical verification of the orientation of the target on the seabed, it has been attempted to determine this by constructing a very simple model of how it would appear in processed sonar imagery depending on the look angle and parameters such as sonar range and altitude. The model results have been compared with the real sonar data collected during the trial and an estimate of the orientation of the Rockan target is made. Though the model is entirely lacking in physics, the results are surprisingly good at minimal computational cost. A tool such as this could be useful in interpreting what is seen in sidescan sonar imagery of Rockan targets, for example which surfaces produce highlights. Résumé Durant l essai de chasse aux mines conjoint Canada France effectué dans le port de Brest (France), en juin 2004, un levé par sonar à balayage latéral sous plusieurs angles d observation a été réalisé sur une cible en forme de mine Rockan. L orientation de la cible sur le fond marin n ayant pas fait l objet de vérification physique, on a tenté de la déterminer en construisant un modèle très simple montrant comment elle se présenterait sur des images de sonar traitées, selon l angle d observation et des paramètres tels que la portée et l altitude du sonar. Les résultats obtenus avec le modèle ont été comparés avec les données sonar réelles recueillies durant l essai et une estimation de l orientation de la cible Rockan a été faite. Même si le modèle est entièrement dépourvu de données physiques, on obtient des résultats d une qualité étonnante pour un coût peu élevé de traitement informatique. Un tel outil pourrait être utile pour l interprétation des images de cibles Rockan fournies par le sonar à balayage latéral, par exemple pour déterminer quelles surfaces produisent des hautes lumières. DRDC Atlantic TM i

6 Executive summary Background During the joint Canada France minehunting trial in Brest Harbour, France, in June 2004, a sidescan sonar survey at multiple look angles was conducted over a Rockan shaped target. The survey route passed the target on the starboard side at a range of 35 m at 24 headings in 15 increments over 360. In the absence of diver or ROV verification of the orientation of the target on the seabed, it has been attempted to determine this by constructing a very simple model of how it would appear in processed sonar imagery depending on the look angle and parameters such as sonar range and altitude. Significance of Results Though the model is entirely lacking in physics, the results are surprisingly good. A tool such as this could be useful in interpreting what is seen in sidescan sonar imagery of Rockan targets, for example which surfaces produce highlights. The illustration of the effect of the sonar resolution on discrimination of the shape of the shadow and target is also instructive. Clearly the model could not be relied upon in any operational scenario, however it might provide useful insights in training. Anna Crawford A Simple Model of a Rockan Mine in Processed Sidescan Sonar Imagery. DRDC Atlantic TM ii DRDC Atlantic TM

7 Sommaire Contexte Durant l essai de chasse aux mines conjoint Canada France effectué dans le port de Brest (France), en juin 2004, un levé par sonar à balayage latéral sous plusieurs angles d observation a été réalisé sur une cible en forme de mine Rockan. La route de levé passait à tribord de la cible à une distance de 35 m suivant 24 caps par accroissements de 15 sur 360. L orientation de la cible sur le fond marin n ayant pas fait l objet d une vérification par un plongeur ou par un engin télécommandé, on a tenté de la déterminer en construisant un modèle très simple montrant comment elle se présenterait sur des images de sonar traitées, selon l angle d observation et des paramètres tels que la portée et l altitude du sonar. Portée des résultats Même si le modèle est entièrement dépourvu de données physiques, on obtient des résultats d une qualité étonnante. Un tel outil pourrait être utile pour l interprétation des images de cibles Rockan fournies par le sonar à balayage latéral, par exemple pour déterminer quelles surfaces produisent des hautes lumières. L illustration de l effet de la résolution sonar sur la discrimination de la forme de l ombre et de la cible est également instructive. De toute évidence, on ne pourrait pas compter sur le modèle dans quelque scénario opérationnel que ce soit, mais on pourrait en tirer des connaissances utiles pour la formation. Anna Crawford Modèle simple d une mine Rockan dans des images traitées de sonar à balayage latéral. RDDC Atlantique TM DRDC Atlantic TM iii

8 Table of contents Abstract i Résumé i Executive summary Sommaire ii iii Table of contents iv List of figures v 1 Introduction Multi aspect Sidescan Sonar Survey Route Rockan Shape Definition Modelling Sonar Images of the Target Illuminating the Rockan Shape Casting a Shadow Simulating Processed Sonar Data Matlab Implementation and Graphical User Interface Comparisons With Real Data Conclusions References Annex A Real and Modelled Sonar Data Animations Distribution List iv DRDC Atlantic TM

9 List of figures 1 An example multi aspect survey pattern in plan view Photo of the GESMA Rockan target and perspective views of the simple Rockan shape Screen shot of the model calculation GUI Comparison of model results (left) and real data (right) for sonar heading of 268 and target orientation of 180, nose pointing Southward For sonar heading of 355 and target orientation of For sonar heading of 87 and target orientation of For sonar heading of 211 and target orientation of Illuminated model and shadow results for sonar headings of 268 (left) and 87 (right) with target orientation of DRDC Atlantic TM v

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11 1 Introduction During a joint Canada France Remote Minehunting System (RMS) trial with researchers from le Groupe d Études Sous Marines de l Atlantique (GESMA), several sidescan sonar surveys were performed over mine shaped targets that were deployed for that purpose. In addition to more traditional survey routes with long parallel legs, multi aspect surveys were conducted over two targets, a cylinder and Rockan. These are surveys designed to pass the target at a fixed range at headings distributed over 360. The orientation of the cylinder on the seabed is quite clear in the resulting survey data, however this is much more difficult to determine for the more stealthy Rockan target. Physical verification of the Rockan target heading, such as by diver or ROV, is unavailable. The design of the attachment of the Rockan target to the cable it was deployed along was such that it was most likely oriented with the tail pointing Northward, head Southward. Partly in order to determine the Rockan orientation, but more generally to assess the feasibility of such an approach, a very simple model was constructed of how the Rockan target might look in processed sidescan sonar imagery, with adjustable parameters such as look angle, sonar range and altitude. By comparing the model results with the real sonar data, an estimate of target orientation has been made. Some features of the model were tuned by matching with the data. The model is coded in Matlab R, and uses some of the functions of the Image Processing Toolbox [1]. The model data comparison was facilitated by programming a graphical user interface (GUI) that allows interactive adjustment of model parameters with display of the results for side by side comparison with the data. It should be kept in mind that this model was constructed to be as simple as possible, with quick computation time and drastically simplified geometry. The method for constructing the resulting modelled sonar images does not include any acoustics. It is presented here only because it has worked surprising well. 2 Multi aspect Sidescan Sonar Survey Route Collecting sidescan sonar data of realistic targets at a wide range of look angles supports basic research into MCM research areas such as computer aided detection/classification (CAD/CAC). The maneuverability and precise line following ability of the Remote Minehunting System semi submersible platform allow for great flexibility in the design of nontraditional survey routes. The multi aspect survey pattern was designed with short straight sections passing the target on one side DRDC Atlantic TM

12 o N o W Figure 1: An example multi aspect survey pattern in plan view. The first 4 passes are shown in black for emphasis. at a fixed range, connected by turns of minimal radius, as shown in Figure 1. This particular route has 24 passes at headings ranging over the full 360 at 15 increments with the target at a range of 35 m on the starboard side each time. The outer diameter of the pattern is about 600 m. The time required for the RMS to complete one of these surveys was about 75 minutes, as compared to more than a day for a similar set of passes performed by GESMA researchers surveying from a conventional vessel. Two of these multi aspect surveys were performed during the trial in France, and it was also used during a more recent RMS trial in Esquimalt (May of 2004). 3 Rockan Shape Definition Measurements of the shape of the Rockan target were obtained from two sources, Jane s (online) [2] and physical measurements of a fibreglass replica molded from the GESMA Rockan target used in the CA/FR trials. The shape has been simplified, having straight edges up to the point of the head, for example. Figure 2 shows a photograph of the GESMA Rockan that was surveyed, and several perspective 2 DRDC Atlantic TM

13 views of the Rockan shape used in the model. The shape measurements are listed in Table 1. In the table, nose and tail refer to the two opposite ends of the Rockan. The tail has three fins, the tail fin in the center, and two smaller corner fins that have top edges set in from the sides. The head has the highest elevation, and is set back toward the tail from the nose. Only the top surfaces are included as these determine the shape of the acoustic shadow and contribute most to the highlight. The model does not include any tilt of the Rockan, as if it has been placed on a level surface. It is centred on a 5 m by 5 m planar surface, oriented with its nose pointing Northward. Aside from being quite simplified, the shape of the target has been modified. Some of the small fin faces have been angled so as to be visible in plan view. This is a concession to the visualization method which is not a true representation of the view from the sonar. This will be discussed further later. Figure 2: Photo of the GESMA Rockan target and perspective views of the simplified geometric Rockan shape. The axis units are meters. DRDC Atlantic TM

14 Table 1: Rockan dimensions used in the simplified model. Dimension (cm) Nose to tail (horizontal) 99.6 Tail width 82.5 Nose width 47.0 Head width at top 21.0 Head height 40.0 Head set back from nose 18.0 Tail fin width at tail 0.30 Tail fin length along top edge 40.0 Tail fin height 19.5 Corner fin length 18.0 Corner fin height 15.0 Corner fin top edge set in from side Modelling Sonar Images of the Target The main features identifying a target in sidescan sonar images are its highlight and shadow. In the case of a target designed to be stealthy such as the Rockan, a highlight is only visible at a limited range of sonar look angles. The shadow is almost always present in most regular sonar target geometries, however in real situations, irregularities in the seabed or tilt of the target can result in unrecognizable shadow shapes. Even with the high resolution of the Klein 5500 sidescan sonar, it is often difficult to resolve shadow shape clearly even under ideal conditions. The approach taken here has been to create an illuminated image of the surface of the target and the acoustic shadow that it casts in plan view, then to pixellate this image with a resolution consistent with the processed sidescan sonar data. The sonar parameters relevant to constructing the illuminated target and shadow are the range to the sonar, sonar altitude, and the beam angle, which is the Cartesian angle to the sonar from the target at the origin of the model domain. In order to make comparisons with the real multi aspect sonar data, the resulting image is pixellated, and can then be rotated. The background is adjustable with a base grey level and Gaussian noise can be added to the image. The following sections describe this in more detail. 4 DRDC Atlantic TM

15 4.1 Illuminating the Rockan Shape The illumination of each of the Rockan faces is calculated from the vector dot product of the unit normal to that surface and the unit vector pointing to the sonar. This value is then thresholded to determine the grey level colour for that surface: less than 0.5 (including negative values for surfaces facing more than 90 away from the sonar) is set to very dark grey, 0.5 to 0.9 to mid range grey and 0.9 to 1 to white, which gives a highlight. 4.2 Casting a Shadow The vertices defining the shape of the shadow are determined by tracing single rays from the location of the sonar past the upper corners of the Rockan to the plane seabed. Each of the shadows of the main body of the Rockan and the three fins are created separately and drawn as polygons on the seabed. The vertices of the shadow of the main body include the four outer corners of the main body in plan view and two vertices traced from the upper corners of the head. The shadow is made convex by excluding the vertices falling in the interior of the outermost polygon. The shadows of the fins are calculated in a similar manner, including vertices traced from both corners of the upper edge (for the main tail fin, one corner is located at its junction with the back of the Rockan). The shadow polygons are coloured black. Fin shadows that fall within the outline of the main body are still calculated and drawn, but are obscured in plan view by the target itself. 4.3 Simulating Processed Sonar Data The resulting plan view image is captured as a raster image and resampled at a lower resolution specified by the user. The user can adjust the background grey level and can add a specified level of Gaussian noise to the pixels in the resampled image. In order to make comparisons with real data easier, the user can rotate the resulting image. The graininess of the pixellation should be determined by the resolution of the processed sonar imagery that is being compared, which in turn is determined by the alongtrack sonar resolution setting. The Klein sonar operates in two modes: a high resolution classification mode with nominal alongtrack resolution of 10 cm and a detection mode with 20 cm alongtrack resolution. (The acrosstrack resolution of the raw beam data is always about 3 cm, determined by the digitization rate of the sonar receiver and the local sound speed.) The resolution of Klein sonar imagery processed using DRDC s in house SIPS3 software is 11 cm for high resolution data, or 22 cm for low resolution data. Setting the georeferencing grid spacing DRDC Atlantic TM

16 Figure 3: Screen shot of the model calculation GUI showing results for typical sonar and target orientation settings. Parts of the display are labelled by blue and yellow text. slightly larger than the alongtrack resolution reduces the occurrence of small gaps in the imagery. It should be emphasized that this is not a true as seen by the sonar image. It is what seems to be a reasonable approximation produced in a reasonable amount of computing time. Creating an accurate representation of simulated sonar data would require starting from the equivalent of beam data, including the beam pattern and modelled backscattering from the seabed and target, then georeferencing it. 5 Matlab Implementation and Graphical User Interface Figure 3 is a screen shot of the GUI written as a front end for user input of parameters for the model calculation and for display of the results. The Matlab window has two displays and a number of controls for adjusting sonar target geometry and display settings. The display on the left shows the illuminated Rockan model and shadow. The user sets the range to the sonar and the sonar 6 DRDC Atlantic TM

17 Figure 4: Comparison of model results (left) and real data (right) for sonar heading of 268 and target orientation of 180, nose pointing Southward. altitude using text input boxes. The beam angle is adjusted using a slider or text input. The display on the right shows the result of pixellating the image on the left and applying the various display options set by the user. The current beam angle and sonar heading are displayed as white and red coordinate axes in the lower right corners of each display, white for sonar heading and red for the beam angle. The sonar resolution and background grey level are input through text boxes, and the rotation angle of the image is set using a slider or text box. Changes in the text box settings are affected by adjusting the sliders. The Exit button is in the upper right. The computation time for rendering the model and pixellated images is 8.5 s on a single processor 860 MHz PC or 1.5 s on a dual processor 3 GHz PC (both running Windows 2000 and Matlab v. 6.1). Note that Matlab does not utilize parallel processing. 6 Comparisons With Real Data Following are a few examples of comparisons between real processed sidescan imagery and the model results with settings that best illustrate the comparison. The sonar altitude is set to 11 m, the average altitude during the multi aspect survey (varying between 10 and 12 m) and range to sonar is 35 m in all cases. The background gray level, noise variance and sonar resolution are set to 0.3, 0.01 and 11 cm. The small gaps in the processed sonar imagery are between consecutive pings, due to the fast towing speed during the multi aspect survey. Processed sidescan sonar imagery is georeferenced so that North is Up. The results are referred to by the sonar heading, instead of beam angle, and target orientation, which is equivalent to the rotation angle of the model domain in the simulated sonar data. DRDC Atlantic TM

18 Figure 5: For sonar heading of 355 and target orientation of 180. Figure 6: For sonar heading of 87 and target orientation of 180. Figure 7: For sonar heading of 211 and target orientation of DRDC Atlantic TM

19 Figure 8: Illuminated model and shadow results for sonar headings of 268 (left) and 87 (right) with target orientation of 180. The examples shown were chosen to show the highlight from the nose surface of the target (Figure 4), the side on and end on highlights (Figures 5 and 6), and the lack of highlight as seen from most angles (Figure 7). Figure 8 shows the illuminated model results that correspond to the head on and end on look angles that correspond to the views in Figures 4 and 6 (these have been rotated 180 for comparison with the other Figures). These images indicate which surfaces the highlights in the modelled sonar data originate from. Some features of the model were tuned based on comparison with the data. The thresholding level for highlight illumination of the faces was set by comparing with the real images containing highlights and the images at the closest beam angles to either side (the head on look angle primarily). It is interesting that the nose highlight in the model can be eliminated by raising or lowering the sonar altitude by 1 m or changing the beam angle by 10 in either direction. The fins contribute to both the shadow and highlight. The corner fins and rear face of the tail fin were angled inward at the top in order to have sufficient cross section in plan view to be visible if highlighted. This changes the component of the surface normal directed toward the sonar, which changes the level of illumination, so these effects were balanced by comparing images such as Figure 6. Based on this work, the estimated orientation of the Rockan is with the head directed Southward. It is difficult to assign a rigorous estimate of uncertainty, however the 15 angular resolution of the real data provides a rough guide (say +/-15 ). Based on the small asymmetry between the processed sidescan images at headings 15 to either side of head on and tail on, it is possible that the Rockan had a small positive heading offset from Southward, however it would require a much more sophisticated model to resolve this, or to differentiate this from possible tilt of the target on the seabed. DRDC Atlantic TM

20 7 Conclusions The visual comparison between the model results and real data is surprisingly good. Once model parameters such as the illumination level thresholds were tuned by matching with the multi aspect data at the head on look angle, the values were found to be suitable for the other angles as well. A few modifications to the shape of the Rockan are required due to the method of creating the simulated sonar imagery, however these are small. Based on the comparison with the real data, the model results suggest that the most likely orientation of the Rockan target is with the head directed Southward, with an uncertainty of about +/-15. This agrees with the most likely orientation based on the mechanics of the deployment of the target. There is no physical verification available. A more important aspect of this work is that it more generally demonstrates that the approach taken here provides a useful tool for better understanding what is seen in the processed sidescan images. By adjusting the sonar target geometry, the user can create highlights from the different surfaces of the target or change the shape of the shadow. It is also instructive to observe the effect of varying the resolution of the modelled image. There are undoubtedly many improvements that can be made to the model, but that would be contrary to the idea of keeping it simple and is not supported by the lack of physics in the model. 10 DRDC Atlantic TM

21 References 1. The Mathworks Inc. (2000). Using Matlab, Version 6. Also, (subscription required), Jane s Underwater Warfare Systems, Underwater Weapons Mines, Sweden, BGM 100 (Rockan), Feb DRDC Atlantic TM

22 Annex A Real and Modelled Sonar Data Animations The distribution of this document on CD ROM is accompanied by three animations in AVI format. These can be viewed with software such as Windows Media Player (Microsoft), Quicktime (Apple Computer Inc.) or RealPlayer (Real Networks Inc.). The animation files have been tested with all three softwares on a PC running Windows The animation named RealRockan.avi is a compilation of the processed multi aspect sidescan sonar imagery from 24 passes by the Rockan target, ordered by increasing sonar heading angle. The sonar heading and look angle are indicated by a set of arrows in the upper right of each frame (sonar heading in white, look angle in red). The animation named FakeRockan.avi is a similar compilation of model results at the same look angles as the real data. The target orientation is set at 180 (nose Southward), and the other model sonar and display parameters are the same as for the examples shown in Figures 4 to 7 in the document. The animation named ModelRockan.avi is the corresponding illuminated model results at the same angles, though not rotated to the estimated target orientation. 12 DRDC Atlantic TM

23 Distribution List Document No. DRDC Atlantic TM Internal Distribution 5 copies DRDC Atlantic LIBRARY 4 copies Anna Crawford DRDC Atlantic External Distribution 1 copy NDHQ/DRDKIM DRDC Atlantic TM

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25 DOCUMENT CONTROL DATA (Security classification of title, body of abstract and indexing annotation must be entered when the overall document is classified) 1. ORIGINATOR (the name and address of the organization preparing the document. Organizations for whom the document was prepared, e.g. Centre sponsoring a contractor's report, or tasking agency, are entered in section 8.) 2. SECURITY CLASSIFICATION (overall security classification of the document including special warning terms if applicable). DRDC Atlantic UNCLASSIFIED 3. TITLE (the complete document title as indicated on the title page. Its classification should be indicated by the appropriate abbreviation (S,C,R or U) in parentheses after the title). A Simple Model of a Rockan Mine in Processed Sidescan Sonar Imagery 4. AUTHORS (Last name, first name, middle initial. If military, show rank, e.g. Doe, Maj. John E.) Anna Crawford 5. DATE OF PUBLICATION (month and year of publication of document) January a. NO. OF PAGES (total containing information Include Annexes, Appendices, etc). 13 (approx.) 6b. NO. OF REFS (total cited in document) 2 7. DESCRIPTIVE NOTES (the category of the document, e.g. technical report, technical note or memorandum. If appropriate, enter the type of report, e.g. interim, progress, summary, annual or final. Give the inclusive dates when a specific reporting period is covered). Technical Memorandum 8. SPONSORING ACTIVITY (the name of the department project office or laboratory sponsoring the research and development. Include address). Defence R&D Canada Atlantic PO Box 1012 Dartmouth, NS, Canada B2Y 3Z7 9a. PROJECT OR GRANT NO. (if appropriate, the applicable research and development project or grant number under which the document was written. Please specify whether project or grant). 11cl 9b. CONTRACT NO. (if appropriate, the applicable number under which the document was written). 10a ORIGINATOR'S DOCUMENT NUMBER (the official document number by which the document is identified by the originating activity. This number must be unique to this document.) DRDC Atlantic TM b OTHER DOCUMENT NOs. (Any other numbers which may be assigned this document either by the originator or by the sponsor.) 11. DOCUMENT AVAILABILITY (any limitations on further dissemination of the document, other than those imposed by security classification) ( x ) Unlimited distribution ( ) Defence departments and defence contractors; further distribution only as approved ( ) Defence departments and Canadian defence contractors; further distribution only as approved ( ) Government departments and agencies; further distribution only as approved ( ) Defence departments; further distribution only as approved ( ) Other (please specify): 12. DOCUMENT ANNOUNCEMENT (any limitation to the bibliographic announcement of this document. This will normally correspond to the Document Availability (11). However, where further distribution (beyond the audience specified in (11) is possible, a wider announcement audience may be selected). DRDC Atlantic mod. May 02

26 13. ABSTRACT (a brief and factual summary of the document. It may also appear elsewhere in the body of the document itself. It is highly desirable that the abstract of classified documents be unclassified. Each paragraph of the abstract shall begin with an indication of the security classification of the information in the paragraph (unless the document itself is unclassified) represented as (S), (C), (R), or (U). It is not necessary to include here abstracts in both official languages unless the text is bilingual). (U) During the joint Canada France minehunting trial in Brest Harbour, France, in June 2004, a sidescan sonar survey at multiple look angles was conducted over a Rockan shaped target. In the absence of physical verification of the orientation of the target on the seabed, it has been attempted to determine this by constructing a very simple model of how it would appear in processed sonar imagery depending on the look angle and parameters such as sonar range and altitude. The model results have been compared with the real sonar data collected during the trial and an estimate of the orientation of the Rockan target is made. Though the model is entirely lacking in physics, the results are surprisingly good at minimal computational cost. A tool such as this could be useful in interpreting what is seen in sidescan sonar imagery of Rockan targets, for example which surfaces produce highlights. 14. KEYWORDS, DESCRIPTORS or IDENTIFIERS (technically meaningful terms or short phrases that characterize a document and could be helpful in cataloguing the document. They should be selected so that no security classification is required. Identifiers, such as equipment model designation, trade name, military project code name, geographic location may also be included. If possible keywords should be selected from a published thesaurus. e.g. Thesaurus of Engineering and Scientific Terms (TEST) and that thesaurus-identified. If it not possible to select indexing terms which are Unclassified, the classification of each should be indicated as with the title). Remote Minehunting System CA/FR SA#21 Sidescan sonar imagery DRDC Atlantic mod. May 02

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