a simple model of a rockan mine in processed sidescan...

Defence R&D Canada – Atlantic

DEFENCE DÉFENSE&

A Simple Model of a Rockan Mine in

Processed Sidescan Sonar Imagery

Anna Crawford

Technical Memorandum

DRDC Atlantic TM 2005-042

January 2006

Copy No.________

Defence Research andDevelopment Canada

Recherche et développementpour la défense Canada

This page intentionally left blank.

A Simple Model of a Rockan Mine inProcessed Sidescan Sonar Imagery

Anna Crawford

Defence R & D Canada – AtlanticTechnical Memorandum

DRDC Atlantic TM 2005–042

January 2006

Abstract

During the joint Canada–France minehunting trial in Brest Harbour, France, inJune 2004, a sidescan sonar survey at multiple look angles was conducted overa Rockan–shaped target. In the absence of physical verification of the orientationof the target on the seabed, it has been attempted to determine this by constructinga very simple model of how it would appear in processed sonar imagery dependingon the look angle and parameters such as sonar range and altitude. The model re-sults have been compared with the real sonar data collected during the trial and anestimate of the orientation of the Rockan target is made. Though the model is en-tirely lacking in physics, the results are surprisingly good at minimal computationalcost. A tool such as this could be useful in interpreting whatis seen in sidescansonar imagery of Rockan targets, for example which surfacesproduce highlights.

Resum e

Durant l’essai de chasse aux mines conjoint Canada–France effectue dans le port deBrest (France), en juin 2004, un leve par sonar a balayage lateral sous plusieurs an-gles d’observation a ete realise sur une cible en forme de mine Rockan. L’orientationde la cible sur le fond marin n’ayant pas fait l’objet de verification physique, on atente de la determiner en construisant un modele tres simple montrant comment ellese presenterait sur des images de sonar traitees, selon l’angle d’observation et desparametres tels que la portee et l’altitude du sonar. Les resultats obtenus avec lemodele ont ete compares avec les donnees sonar reelles recueillies durant l’essai etune estimation de l’orientation de la cible Rockan a ete faite. Meme si le modele estentierement depourvu de donnees physiques, on obtient des resultats d’une qualiteetonnante pour un cout peu eleve de traitement informatique. Un tel outil pourraitetre utile pour l’interpretation des images de cibles Rockan fournies par le sonara balayage lateral, par exemple pour determiner quellessurfaces produisent deshautes lumieres.

DRDC Atlantic TM 2005–042 i

Executive summary

Background

During the joint Canada–France minehunting trial in Brest Harbour, France, inJune 2004, a sidescan sonar survey at multiple look angles was conducted overa Rockan–shaped target. The survey route passed the target on the starboard sideat a range of 35 m at 24 headings in 15◦ increments over 360◦. In the absence ofdiver or ROV verification of the orientation of the target on the seabed, it has beenattempted to determine this by constructing a very simple model of how it wouldappear in processed sonar imagery depending on the look angle and parameterssuch as sonar range and altitude.

Significance of Results

Though the model is entirely lacking in physics, the resultsare surprisingly good.A tool such as this could be useful in interpreting what is seen in sidescan sonarimagery of Rockan targets, for example which surfaces produce highlights. Theillustration of the effect of the sonar resolution on discrimination of the shape of theshadow and target is also instructive.

Clearly the model could not be relied upon in any operationalscenario, however itmight provide useful insights in training.

Anna Crawford. 2005. A Simple Model of a Rockan Mine in Processed SidescanSonar Imagery. DRDC Atlantic TM 2005-042.

ii DRDC Atlantic TM 2005–042

Sommaire

Contexte

Durant l’essai de chasse aux mines conjoint Canada–France effectue dans le portde Brest (France), en juin 2004, un leve par sonar a balayage lateral sous plusieursangles d’observation a ete realise sur une cible en forme de mine Rockan. La routede leve passait a tribord de la cible a une distance de 35 m suivant 24 caps paraccroissements de 15◦ sur 360◦. L’orientation de la cible sur le fond marin n’ayantpas fait l’objet d’une verification par un plongeur ou par unengin telecommande, ona tente de la determiner en construisant un modele tres simple montrant commentelle se presenterait sur des images de sonar traitees, selon l’angle d’observation etdes parametres tels que la portee et l’altitude du sonar.

Portee des resultats

Meme si le modele est entierement depourvu de donnees physiques, on obtient desresultats d’une qualite etonnante. Un tel outil pourrait etre utile pour l’interpretationdes images de cibles Rockan fournies par le sonar a balayagelateral, par exemplepour determiner quelles surfaces produisent des hautes lumieres. L’illustration del’effet de la resolution sonar sur la discrimination de la forme de l’ombre et de lacible est egalement instructive.

De toute evidence, on ne pourrait pas compter sur le modeledans quelque scenariooperationnel que ce soit, mais on pourrait en tirer des connaissances utiles pour laformation.

Anna Crawford. 2005. Modele simple d’une mine Rockan dans des images traiteesde sonar a balayage lateral. RDDC Atlantique TM 2005-042.

DRDC Atlantic TM 2005–042 iii

Table of contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i

Resume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i

Executive summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

Sommaire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Table of contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

List of figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Multi–aspect Sidescan Sonar Survey Route . . . . . . . . . . . . . .. 1

3 Rockan Shape Definition . . . . . . . . . . . . . . . . . . . . . . . . . 2

4 Modelling Sonar Images of the Target . . . . . . . . . . . . . . . . . . 4

4.1 Illuminating the Rockan Shape . . . . . . . . . . . . . . . . . . 5

4.2 Casting a Shadow . . . . . . . . . . . . . . . . . . . . . . . . . 5

4.3 Simulating Processed Sonar Data . . . . . . . . . . . . . . . . . 5

5 Matlab Implementation and Graphical User Interface . . . . .. . . . . 6

6 Comparisons With Real Data . . . . . . . . . . . . . . . . . . . . . . 7

7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Annex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

A Real and Modelled Sonar Data Animations . . . . . . . . . . . . . . . 12

Distribution List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

iv DRDC Atlantic TM 2005–042

List of figures

1 An example multi–aspect survey pattern in plan view. . . . . .. . . . 2

2 Photo of the GESMA Rockan target and perspective views of thesimple Rockan shape. . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3 Screen shot of the model calculation GUI. . . . . . . . . . . . . . . .. 6

4 Comparison of model results (left) and real data (right) for sonarheading of 268◦ and target orientation of 180◦, nose pointingSouthward. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

5 For sonar heading of 355◦ and target orientation of 180◦. . . . . . . . 8

6 For sonar heading of 87◦ and target orientation of 180◦. . . . . . . . . 8

7 For sonar heading of 211◦ and target orientation of 180◦. . . . . . . . 8

8 Illuminated model and shadow results for sonar headings of268◦

(left) and 87◦ (right) with target orientation of 180◦. . . . . . . . . . . 9

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vi DRDC Atlantic TM 2005–042

1 Introduction

During a joint Canada–France Remote Minehunting System (RMS) trial with re-searchers from le Groupe d’Etudes Sous–Marines de l’Atlantique (GESMA), sev-eral sidescan sonar surveys were performed over mine–shaped targets that weredeployed for that purpose. In addition to more traditional survey routes with longparallel legs, multi–aspect surveys were conducted over two targets, a cylinder andRockan. These are surveys designed to pass the target at a fixed range at headingsdistributed over 360◦. The orientation of the cylinder on the seabed is quite clearin the resulting survey data, however this is much more difficult to determine forthe 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 Rockantarget to the cable it was deployed along was such that it was most likely orientedwith the tail pointing Northward, head Southward.

Partly in order to determine the Rockan orientation, but more generally to assessthe feasibility of such an approach, a very simple model was constructed of howthe Rockan target might look in processed sidescan sonar imagery, with adjustableparameters such as look angle, sonar range and altitude. By comparing the modelresults 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 MatlabR©, and uses some of the functions of the Image Pro-cessing Toolbox [1]. The model–data comparison was facilitated by programminga graphical user interface (GUI) that allows interactive adjustment of model param-eters 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 beas simple as possible,with quick computation time and drastically simplified geometry. The method forconstructing the resulting modelled sonar images does not include any acoustics. Itis presented here only because it has worked surprising well.

2 Multi–aspect Sidescan Sonar SurveyRoute

Collecting sidescan sonar data of realistic targets at a wide range of look anglessupports basic research into MCM research areas such as computer aided detec-tion/classification (CAD/CAC). The maneuverability and precise line followingability of the Remote Minehunting System semi–submersibleplatform allow forgreat flexibility in the design of nontraditional survey routes. The multi–aspect sur-vey pattern was designed with short straight sections passing the target on one side

DRDC Atlantic TM 2005–042 1

35.90’ 35.80’ 4oW 35.70’

35.60’ 35.50’ 35.40’ 17.90’

48oN 18.00’

18.10’

18.20’

35.90’ 35.80’ 4oW 35.70’

35.60’ 35.50’ 35.40’ 17.90’

48oN 18.00’

18.10’

18.20’

Figure 1: An example multi–aspect survey pattern in plan view. The first 4passes are shown in black for emphasis.

at a fixed range, connected by turns of minimal radius, as shown in Figure 1. Thisparticular route has 24 passes at headings ranging over the full 360◦ at 15◦ incre-ments with the target at a range of 35 m on the starboard side each time. The outerdiameter of the pattern is about 600 m. The time required for the RMS to completeone of these surveys was about 75 minutes, as compared to morethan a day for asimilar set of passes performed by GESMA researchers surveying from a conven-tional vessel. Two of these multi–aspect surveys were performed during the trial inFrance, and it was also used during a more recent RMS trial in Esquimalt (May of2004).

3 Rockan Shape Definition

Measurements of the shape of the Rockan target were obtainedfrom two sources,Jane’s (online) [2] and physical measurements of a fibreglass replica molded fromthe 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 showsa photograph of the GESMA Rockan that was surveyed, and several perspective

2 DRDC Atlantic TM 2005–042

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 thetwo 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 thetop surfaces are included as these determine the shape of theacoustic shadow andcontribute most to the highlight. The model does not includeany tilt of the Rockan,as if it has been placed on a level surface. It is centred on a 5 mby 5 m planarsurface, oriented with its nose pointing Northward.

Aside from being quite simplified, the shape of the target hasbeen modified. Someof the small fin faces have been angled so as to be visible in plan view. This isa concession to the visualization method which is not a true representation of theview from the sonar. This will be discussed further later.

Figure 2: Photo of the GESMA Rockan target and perspective views of thesimplified geometric Rockan shape. The axis units are meters.


Table 1: Rockan dimensions used in the simplified model.

Dimension (cm)Nose to tail (horizontal) 99.6Tail width 82.5Nose width 47.0Head width at top 21.0Head height 40.0Head set back from nose 18.0Tail fin width at tail 0.30Tail fin length along top edge 40.0Tail fin height 19.5Corner fin length 18.0Corner fin height 15.0Corner fin top edge set in from side10.0

4 Modelling Sonar Images of theTarget

The main features identifying a target in sidescan sonar images are its highlight andshadow. In the case of a target designed to be stealthy such asthe Rockan, a high-light is only visible at a limited range of sonar look angles.The shadow is almostalways present in most regular sonar–target geometries, however in real situations,irregularities in the seabed or tilt of the target can resultin unrecognizable shadowshapes. Even with the high resolution of the Klein 5500 sidescan sonar, it is oftendifficult to resolve shadow shape clearly even under ideal conditions.

The approach taken here has been to create an illuminated image of the surfaceof the target and the acoustic shadow that it casts in plan view, then to pixellatethis image with a resolution consistent with the processed sidescan sonar data. Thesonar parameters relevant to constructing the illuminatedtarget and shadow are therange to the sonar, sonar altitude, and the beam angle, whichis the Cartesian angleto the sonar from the target at the origin of the model domain.In order to makecomparisons 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 andGaussian noise can be added to the image. The following sections describe this inmore detail.


4.1 Illuminating the Rockan ShapeThe illumination of each of the Rockan faces is calculated from the vector dot prod-uct of the unit normal to that surface and the unit vector pointing to the sonar. Thisvalue is then thresholded to determine the grey level colourfor that surface: lessthan 0.5 (including negative values for surfaces facing more than 90◦ away fromthe sonar) is set to very dark grey, 0.5 to 0.9 to mid–range grey and 0.9 to 1 towhite, which gives a highlight.

4.2 Casting a ShadowThe vertices defining the shape of the shadow are determined by tracing singlerays from the location of the sonar past the upper corners of the Rockan to theplane seabed. Each of the shadows of the main body of the Rockan and the threefins are created separately and drawn as polygons on the seabed. The vertices ofthe shadow of the main body include the four outer corners of the main body inplan view and two vertices traced from the upper corners of the head. The shadowis made convex by excluding the vertices falling in the interior of the outermostpolygon. The shadows of the fins are calculated in a similar manner, includingvertices traced from both corners of the upper edge (for the main tail fin, one corneris located at its junction with the back of the Rockan). The shadow polygons arecoloured black. Fin shadows that fall within the outline of the main body are stillcalculated and drawn, but are obscured in plan view by the target itself.

4.3 Simulating Processed Sonar DataThe resulting plan view image is captured as a raster image and resampled at alower resolution specified by the user. The user can adjust the background greylevel and can add a specified level of Gaussian noise to the pixels in the resampledimage. In order to make comparisons with real data easier, the user can rotate theresulting image.

The graininess of the pixellation should be determined by the resolution of the pro-cessed sonar imagery that is being compared, which in turn isdetermined by thealongtrack sonar resolution setting. The Klein sonar operates in two modes: a highresolution “classification” mode with nominal alongtrack resolution of 10 cm and a“detection” mode with 20 cm alongtrack resolution. (The acrosstrack resolution ofthe raw beam data is always about 3 cm, determined by the digitization rate of thesonar receiver and the local sound speed.) The resolution ofKlein sonar imageryprocessed using DRDC’s in–house SIPS3 software is 11 cm for high resolutiondata, or 22 cm for low resolution data. Setting the georeferencing grid spacing


Figure 3: Screen shot of the model calculation GUI showing results fortypical sonar and target orientation settings. Parts of the display arelabelled by blue and yellow text.

slightly larger than the alongtrack resolution reduces theoccurrence of small gapsin the imagery.

It should be emphasized that this isnot a true as–seen–by–the–sonar image. It iswhat seems to be a reasonable approximation produced in a reasonable amount ofcomputing time. Creating an accurate representation of simulated sonar data wouldrequire starting from the equivalent of beam data, including the beam pattern andmodelled backscattering from the seabed and target, then georeferencing it.

5 Matlab Implementation andGraphical User Interface

Figure 3 is a screen shot of the GUI written as a front end for user input of parame-ters 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 illuminatedRockan model and shadow. The user sets the range to the sonar and the sonar


Figure 4: Comparison of model results (left) and real data (right) for sonarheading of 268◦ and target orientation of 180◦, nose pointing Southward.

altitude using text input boxes. The beam angle is adjusted using a slider or textinput. The display on the right shows the result of pixellating the image on the leftand applying the various display options set by the user. Thecurrent beam angleand sonar heading are displayed as white and red coordinate axes in the lower rightcorners of each display, white for sonar heading and red for the beam angle. Thesonar resolution and background grey level are input through text boxes, and therotation angle of the image is set using a slider or text box. Changes in the text boxsettings are affected by adjusting the sliders. The Exit button is in the upper right.

The computation time for rendering the model and pixellatedimages is 8.5 s on asingle processor 860 MHz PC or 1.5 s on a dual processor 3 GHz PC(both run-ning Windows 2000 and Matlab v. 6.1). Note that Matlab does not utilize parallelprocessing.

6 Comparisons With Real Data

Following are a few examples of comparisons between real processed sidescan im-agery and the model results with settings that best illustrate the comparison. Thesonar altitude is set to 11 m, the average altitude during themulti–aspect survey(varying between 10 and 12 m) and range to sonar is 35 m in all cases. The back-ground gray level, noise variance and sonar resolution are set to 0.3, 0.01 and 11cm. The small gaps in the processed sonar imagery are betweenconsecutive pings,due to the fast towing speed during the multi–aspect survey.Processed sidescansonar imagery is georeferenced so that North is Up. The results are referred to bythe sonar heading, instead of beam angle, and target orientation, which is equivalentto the rotation angle of the model domain in the simulated sonar data.


Figure 5: For sonar heading of 355◦ and target orientation of 180◦.




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 thenose surface of thetarget (Figure 4), the side–on and end–on highlights (Figures 5 and 6), and the lackof highlight as seen from most angles (Figure 7). Figure 8 shows the illuminatedmodel results that correspond to the head–on and end–on lookangles that corre-spond to the views in Figures 4 and 6 (these have been rotated 180◦ for comparisonwith the other Figures). These images indicate which surfaces the highlights in themodelled sonar data originate from.

Some features of the model were tuned based on comparison with the data. Thethresholding level for highlight illumination of the faceswas set by comparing withthe real images containing highlights and the images at the closest beam anglesto either side (the head–on look angle primarily). It is interesting that the nosehighlight in the model can be eliminated by raising or lowering the sonar altitudeby 1 m or changing the beam angle by 10◦ in either direction. The fins contributeto both the shadow and highlight. The corner fins and rear faceof the tail fin wereangled inward at the top in order to have sufficient cross section in plan view to bevisible if highlighted. This changes the component of the surface normal directedtoward the sonar, which changes the level of illumination, so these effects werebalanced by comparing images such as Figure 6.

Based on this work, the estimated orientation of the Rockan is with the head di-rected Southward. It is difficult to assign a rigorous estimate of uncertainty, how-ever the 15◦ angular resolution of the real data provides a rough guide (say +/-15◦).Based on the small asymmetry between the processed sidescanimages at headings15◦ to either side of head–on and tail–on, it is possible that theRockan had a smallpositive heading offset from Southward, however it would require a much moresophisticated model to resolve this, or to differentiate this from possible tilt of thetarget on the seabed.


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 bymatching with the multi–aspect data at the head–on look angle, the values werefound to be suitable for the other angles as well. A few modifications to the shape ofthe 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 resultssuggest that the mostlikely orientation of the Rockan target is with the head directed Southward, withan uncertainty of about +/-15◦. This agrees with the most likely orientation basedon the mechanics of the deployment of the target. There is no physical verificationavailable.

A more important aspect of this work is that it more generallydemonstrates thatthe approach taken here provides a useful tool for better understanding what is seenin the processed sidescan images. By adjusting the sonar–target geometry, the usercan create highlights from the different surfaces of the target or change the shape ofthe shadow. It is also instructive to observe the effect of varying the resolution ofthe modelled image.

There are undoubtedly many improvements that can be made to the model, but thatwould be contrary to the idea of keeping it simple and is not supported by the lackof physics in the model.


References

1. The Mathworks Inc. (2000).Using Matlab, Version 6. Also,http:\\www.mathworks.com.

2. http:\\online.janes.com (subscription required), Jane’s Underwater WarfareSystems, Underwater Weapons – Mines, Sweden, BGM 100 (Rockan), Feb 112004.


Annex AReal and Modelled Sonar DataAnimations

The distribution of this document on CD–ROM is accompanied by three animationsin 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 softwareson a PC running Win-dows 2000.

The animation named “RealRockan.avi” is a compilation of the processed multi–aspect sidescan sonar imagery from 24 passes by the Rockan target, ordered byincreasing sonar heading angle. The sonar heading and look angle are indicated bya set of arrows in the upper right of each frame (sonar headingin white, look anglein red).

The animation named “FakeRockan.avi” is a similar compilation of model resultsat the same look angles as the real data. The target orientation is set at 180◦ (noseSouthward), and the other model sonar and display parameters are the same as forthe examples shown in Figures 4 to 7 in the document.

The animation named “ModelRockan.avi” is the corresponding illuminated modelresults at the same angles, though not rotated to the estimated target orientation.


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Document No. DRDC Atlantic TM2005–042

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

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Anna Crawford

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(U) During the joint Canada–France minehunting trial in Brest Harbour, France, in June2004, a sidescan sonar survey at multiple look angles was conducted over aRockan–shaped target. In the absence of physical verification of the orientation of thetarget on the seabed, it has been attempted to determine this by constructing a very simplemodel of how it would appear in processed sonar imagery depending on the look angleand parameters such as sonar range and altitude. The model results have been comparedwith the real sonar data collected during the trial and an estimate of the orientation of theRockan target is made. Though the model is entirely lacking in physics, the results aresurprisingly good at minimal computational cost. A tool such as this could be useful ininterpreting what is seen in sidescan sonar imagery of Rockan targets, for example whichsurfaces produce highlights.

14. KEYWORDS, DESCRIPTORS or IDENTIFIERS (technically meaningful terms or short phrases that characterize adocument and could be helpful in cataloguing the document. They should be selected so that no security classification isrequired. Identifiers, such as equipment model designation, trade name, military project code name, geographic location mayalso be included. If possible keywords should be selected from a published thesaurus. e.g. Thesaurus of Engineering andScientific Terms (TEST) and that thesaurus-identified. If it not possible to select indexing terms which are Unclassified, theclassification of each should be indicated as with the title).

Remote Minehunting SystemCA/FR SA#21Sidescan sonar imagery

a simple model of a rockan mine in processed sidescan...

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