an improved basis model for the betssi submarinecradpdf.drdc-rddc.gc.ca/pdfs/unc18/p520811.pdf ·...

Defence R&D Canada

DEFENCE DÉFENSE&

An Improved BASIS Model for the BeTSSi

Submarine

Christopher W. Nell

Layton E. Gilroy

Technical Report

DRDC Atlantic TR 2003-199

November 2003

Copy No.________

Defence Research andDevelopment Canada

Recherche et développementpour la défense Canada

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An Improved BASIS Model for the BeTSSiSubmarine

Christopher W. Nell

Layton E. Gilroy

Defence R & D Canada – Atlantic

Technical Report


November 2003

Abstract

DRDC Atlantic and the US Naval Research Laboratory have developed a tool forOperations Research which will predict the low frequency acoustic target strengthof a rigid submarine hull. The BASIS (Bistatic Acoustic Simple Integrated Struc-ture) software uses analytical methods to combine the results from simple geometricshapes to approximate the response of the submarine hull.

A BASIS target strength simulation model was developed for the Benchmark TargetStrength Simulation Submarine (BeTSSi-Sub) and compared with reference data at200 Hz, 1 kHz, 4 kHz, and 8 kHz, to support an international workshop on targetstrength simulation methods. In general, it was found to outperform the genericmodel previously employed. However, there were three major discrepancies: atbow aspect angles and at higher frequencies, BASIS predicted significantly highertarget strength than reference; between beam and stern aspects and especially at 1kHz, a rise in target strength was present in the BASIS predictions but not reference;and at aspect angles normal to the rear of the sail and tailfins, BASIS target strengthpeaked higher and more sharply at high frequencies than reference.

The source of all three of these discrepancies is likely inaccurate modelling of thesloped sail and tailfin; the first two discrepancies were resolved quite well by re-moving the front side and back side of the sail and tailfin respectively (BASISassumes these components are true vertical), with the undesirable side effect oflowering target strength at beam incidence. The third issue was partially resolvedby segmenting the back side of the sail into multiple sections.

In general, full-3D component modelling must be incorporated to rely on the resultsof BASIS simulations of realistic submarines. Until then, BASIS is only reliablefor models which are well understood and for which modifications can be justified,or models which do not require true 3D for accuracy.

Resum e

RDDC Atlantique et le Naval Research Laboratory desEtats-Unis ont realise unoutil de recherche operationnelle pour prevoir l’intensite de cible acoustique bassefrequence d’une coque de sous-marin rigide. Le logiciel BASIS (de l’anglais Bi-static Acoustic Simple Integrated Structure) fait appela des methodes analytiquespour combiner les resultats obtenusa partir de formes geometriques simples, afinde determiner approximativement la reponse de la coque du sous-marin. Un modelede simulation d’intensite de cible BASIS aeteetabli pour le sous-marin BeTSSi (de

DRDC Atlantic TR 2003-199 i

l’anglais Benchmark Target Strength Simulation) et compare avec les donnees dereferencea 200 Hz, 1 kHz, 4 kHz, et 8 kHz en vue d’un atelier international surles methodes de simulation d’intensite de cible. En general, il s’est revele superieurau modele generique anterieur. Toutefois, on a releve trois importantsecarts : pourles angles d’aspect de proue et aux frequenceselevees, l’indice prevu par BASISdepassait sensiblement les valeurs de reference ; entre l’aspect de poupe et l’as-pecta la largeur maximale, et particulierementa 1 kHz, l’intensite de cible prevuepar BASISetait accru, mais non pas les valeurs de reference ; enfin, aux anglesd’aspect perpendiculairesa l’arriere du kiosque et des empennages du gouvernail,les valeurs maximales prevues par BASIS depassaient les valeurs de reference, enparticulier aux frequenceselevees. Ces troisecarts ont probablement pour sourcela modelisation inexacte du kiosque incline et des empennages du gouvernail ; lesdeux premiers ontete resolus aisement en supprimant la partie avant et la partiearriere du kiosque et des empennages du gouvernail respectivement (BASIS situeceselements sur une verticale vraie), ce qui a l’effet secondaire indesirable d’abais-ser l’intensite de la cible aux angles d’aspecta la largeur maximale. Le troisieme aete resolu en partie en morcelant l’arriere du kiosque. En general, la modelisation3D integrale deselements devraetre incorporee pour assurer la fiabilite des resultatsdes simulations BASIS de sous-marins realistes. D’ici la, BASIS ne sera fiable quepour les modeles qui sont bien compris et pour lesquels des modifications peuventetre justifiees, ou des modeles qui peuventetre exacts sans faire appela des donnees3D reelles.

ii DRDC Atlantic TR 2003-199

Executive summary

Background

An important issue for both low frequency active sonar performance predictionsand the prediction of the vulnerability of naval platforms to these sonars is theevaluation of acoustic target strength. Techniques for predicting the acoustic tar-get strength of a given submarine model are under continuous development aroundthe world. One such method, the Bistatic Acoustic Simple Integrated Structure(BASIS) Target Strength Model was jointly developed by researchers at the NavalResearch Laboratory in the USA and at DRDC Atlantic in Canada. Establishedmethods for making high-fidelity target strength predictions are very computation-ally intensive and involve detailed geometrical models which require large amountsof time to develop. As a result, operations researchers often use a single mono-static target strength value, or a limited number of bistatic values, in their work.BASIS is intended to provide operations researchers with a simple way to quicklygenerate target strength values as required for their research. The BASIS softwareuses analytical methods to combine the results from simple geometric shapes toapproximate the response of the submarine hull.

In 2001, a generic submarine model was developed at Forschungsanstalt der Bun-deswehr fur Wasserschall und Geophysik (FWG) in Kiel, Germany. The Bench-mark Target Strength Simulation Submarine (BeTSSi-Sub) is designed to satisfyrequirements for a benchmark model and was specifically intended for testing at1, 4, and 8 kHz. Researchers from around the world were invited to participatein a 2002 workshop on numerical target strength simulation using BeTSSi-Sub. Avariety of codes were used to predict the target strength of this submarine includ-ing BASIS; however, the lack of a simple BASIS model to accurately approximateBeTSSi-Sub necessitated the use of a similar, pre-existing “Generic” model.

Principal results

A BASIS target strength simulation model was developed for the BeTSSi subma-rine and compared with reference data at 200 Hz, 1 kHz, 4 kHz, and 8 kHz. In gen-eral, it was found to outperform the generic model previously employed. However,there were three major discrepancies: at bow aspect angles and at higher frequen-cies, BASIS predicted significantly higher target strength than reference; betweenbeam and stern aspects and especially at 1 kHz, a rise in target strength was presentin the BASIS predictions but not reference; and at aspect angles normal to the rearof the sail and tailfins, BASIS target strength peaked higher and more sharply athigh frequencies than reference.

DRDC Atlantic TR 2003-199 iii

The source of all three of these discrepancies is likely inaccurate modelling of thesloped sail and tailfin; the first two discrepancies were resolved quite well by re-moving the front side and back side of the sail and tailfin respectively (BASISassumes these components are true vertical), with the undesirable side effect oflowering target strength at beam incidence. The third issue was partially resolvedby segmenting the back side of the sail into multiple sections.

Significance of results

In general, while BASIS does provide an extremely fast method for evaluatingacoustic target strength for operations research studies, full-3D component mod-elling must be incorporated to rely on the results of BASIS simulations of realisticsubmarines. Until then, BASIS is only reliable for models which are well under-stood and for which modifications can be justified, or models which do not requiretrue 3D for accuracy.

Future work

In order to overcome this limitation, the nature of BASIS must be fundamentallychanged to allow physical optics integrals to be calculated for the simple primi-tives in three full dimensions. Until this is achieved, BASIS is limited in its abilityto provide high fidelity target strength predictions for any model with componentsthat differ significantly from the primitives used to model them. BASIS does re-main a useful tool for quickly investigating a model at a relatively wide range offrequencies as long as there exists reliable target strength data with which to vali-date it.

Christopher W. Nell, Layton E. Gilroy; 2003; An Improved BASISModel for the BeTSSi Submarine; DRDC Atlantic TR 2003-199;Defence R & D Canada – Atlantic.

iv DRDC Atlantic TR 2003-199

Sommaire

Historique

L’ evaluation de l’intensite de cible acoustique joue un role important comme moyende prevoir la performance des sonars actifs basse frequence ainsi que la vulnerabilitedes plateformes navalesa ces sonars. Les techniques de prevision de l’intensite decible acoustique d’un modele de sous-marin donne font l’objet de developpementsconstantsa l’echelle mondiale. Une de ces techniques, le modele d’intensite decible BASIS (de l’anglais Bistatic Acoustic Simple Integrated Structure) est le fruitde la collaboration de chercheurs du Naval Research Laboratory auxEtats-Unis, etde RDDC Atlantique, au Canada. Les methodesetablies qui permettent de prevoiravec une grande fidelite l’intensite de cible exigent des calculs d’une grande com-plexite ainsi que des modeles geometriques detailles qui sont tres longsa etablir.Par consequent, les chercheurs operationnels utilisent souvent dans leur travail uneseule valeur d’intensite de cible monostatique, ou un nombre limite de valeurs bi-statiques. BASIS visea offrir aux chercheurs operationnels un moyen simple degenerer rapidement les valeurs d’intensite de cible dont ils ont besoin pour leursrecherches. Le logiciel BASIS fait appela des methodes analytiques pour combinerles resultats obtenusa partir de formes geometriques simples afin de determinerapproximativement la reponse de la coque du sous-marin. En 2001, un modele desous-marin generique aete realise au Forschungsanstalt der Bundeswehr fur Was-serschall und Geophysik (FWG) de Kiel, en Allemagne. Le sous-marin BeTSSi(de l’anglais Benchmark Target Strength Simulation) est concu pour repondre auxexigences d’une modele de reference et specifiquement pour repondre aux besoinsd’essaisa 1, 4 et 8 kHz. Des chercheurs du monde entier ontete invitesa participeren 2002a un atelier sur la simulation numerique d’intensites de cible au moyen dusous-marin BeTSSi. Divers codes ontete utilises pour prevoir l’intensite de ciblede ce sous-marin, y compris BASIS ; toutefois, en l’absence d’un modele BASISsimple pour obtenir une approximation exacte du sous-marin BeTSSi, il a fallu uti-liser un modele “generique” semblable deja etabli.

Principaux r esultats

Un modele de simulation d’intensite de cible BASIS aete etabli pour le sous-marinBeTSSi et compare avec les donnees de referencea 200 Hz, 1 kHz, 4 kHz, et 8 kHz.En general, il s’est revele superieur au modele generique anterieur. Toutefois, on areleve trois importantsecarts : pour les angles d’aspect de proue et aux frequenceselevees, l’intensite de cible prevue pare BASIS depassait sensiblement les valeursde reference ; entre l’aspect de poupe et l’aspecta la largeur maximale, et parti-culierementa 1 kHz, l’intensite de cible prevue par BASISetait accru, mais non pasles valeurs de reference ; enfin, aux angles d’aspect perpendiculairesa l’arriere du

DRDC Atlantic TR 2003-199 v

kiosque et des empennages du gouvernail, les valeurs maximales prevues par BA-SIS depassaient les valeurs de reference, en particulier aux frequenceselevees. Cestrois ecarts ont probablement pour source la modelisation inexacte du kiosque in-cline et des empennages du gouvernail ; les deux premiers ontete resolus aisementen supprimant la partie avant et la partie arriere du kiosque et des empennages dugouvernail respectivement (BASIS situe ceselements sur une verticale vraie), ce quia l’effet secondaire indesirable d’abaisser l’intensite de la cible aux angles d’aspecta la largeur maximale. Le troisieme aete resolu en partie en morcelant l’arriere dukiosque.

Importance des r esultats

En general, bien que BASIS offre une methode extremement rapide pour l’evaluationde l’intensite de cible aux fins d’etudes de recherche operationnelle, la modelisation3D integrale deselements devraetre incorporee pour assurer la fiabilite des resultatsdes simulations BASIS de sous-marins realistes. D’ici la, BASIS ne sera fiable quepour les modeles qui sont bien compris et pour lesquels des modifications peuventetre justifiees, ou des modeles qui peuventetre exacts sans faire appel des donnees3D reelles.

Travaux futurs

Pour surmonter cette limitation, il faudra revoir en profondeur la nature de BA-SIS afin de permettre le calcul d’integrales d’optique physique pour les primitivessimples en 3D integrale. D’ici la, BASIS sera limite dans sa capacite de fournirdes previsions d’intensites de cible de grande fiabilite pour tout modele dont leselements different sensiblement des primitives utilisees pour la modelisation. BA-SIS demeure un outil utile pour l’etude rapide d’un modele a des frequences rela-tivement diversifiees dans la mesure ou il existe des donnees d’intensite de ciblepour le valider.

Christopher W. Nell, Layton E. Gilroy; 2003; Un modele BASISameliore pour le sous-marin BeTSSi; DRDC Atlantic TR 2003-199;R & D pour la defense Canada – Atlantique.

vi DRDC Atlantic TR 2003-199

Table of contents

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

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

Executive summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Sommaire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Table of contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

List of figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii

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

2 BASIS Description . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3 Model Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

4 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 5

5 Discussion and Recommendations . . . . . . . . . . . . . . . . . . . 7

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

DRDC Atlantic TR 2003-199 vii

List of figures

1 3D model of BeTSSi-Sub, as implemented in AVAST . . . . . . . . . 2

2 3D model of the Generic submarine, as implemented in AVAST . . . 2

3 Schematic diagrams of the Simple and Segmented BASISBeTSSi-Sub models . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4 Schematic diagram of the Generic BASIS submarine model . . . . . . 11

5 200 Hz monostatic target strength of original models . . . . . . . . . 11

6 1 kHz monostatic target strength of original models . . . . . . . . . . 12

7 200 Hz monostatic target strength of improved BASIS BeTSSi-Submodels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

8 90 ◦ Target strength of improved BASIS BeTSSi-Sub models;90 ◦

region at 200 Hz and92− 110 ◦ region at 1 kHz. The lack ofsmoothness here is due to the1 ◦ angular resolution used. . . . . . . . 13

9 1 kHz monostatic target strength of improved BASIS BeTSSi-Submodels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13



12 The effect of the sail and tailfin on target strength of SegmentedBeTSSi-Sub BASIS model at 200 Hz . . . . . . . . . . . . . . . . . . 15

13 The effect of the sail and tailfin on target strength of SegmentedBeTSSi-Sub BASIS model at 1 kHz . . . . . . . . . . . . . . . . . . 15



viii DRDC Atlantic TR 2003-199

1 Introduction

The underlying principle of submarine warfare has always been stealth. As such,techniques for predicting the acoustic target strength of a given submarine modelare under continuous development around the world. One such method, the BistaticAcoustic Simple Integrated Structure (BASIS) Target Strength Model [1] was jointlydeveloped by researchers at the Naval Research Laboratory in the USA and atDRDC Atlantic in Canada. Established methods for making high-fidelity targetstrength predictions are very computationally intensive and involve detailed geo-metrical models which require large amounts of time to develop. As a result, oper-ations researchers often use a single monostatic target strength value, or a limitednumber of bistatic values, in their work. BASIS is intended to provide operationsresearchers with a simple way to quickly1 generate target strength values as requiredfor their research.

In order to assess the accuracy of BASIS with respect to other target strength-modelling techniques, a benchmark submarine design is needed. Since target strengthdata on real submarines is kept confidential, this benchmark model must be suffi-ciently generic to avoid classification issues while being realistic enough to providean accurate measure of real-world performance.

In 2001, a generic submarine model was developed at Forschungsanstalt der Bun-deswehr fur Wasserschall und Geophysik (FWG) in Kiel, Germany [2]. The Bench-mark Target Strength Simulation Submarine (BeTSSi-Sub, Figure 1) is designed tosatisfy the aforementioned requirements for a benchmark model and was specifi-cally intended for testing at 1, 4, and 8 kHz; 200 Hz verification was later added.Researchers from around the world were invited to participate in a 2002 workshopon numerical target strength simulation using BeTSSi-Sub [3]. The majority of theparticipant codes utilized the Kirchhoff approximation to generate target strengthdata. Results from AVAST as well as other ”exact” simulations were included forcomparison at lower target frequencies. Target strength predictions from BASISwere also submitted, but the lack of a simple BASIS model to accurately approxi-mate BeTSSi-Sub necessitated the use of a similar, pre- existing ”Generic” model(Figure 2).

At certain sonar frequencies, portions of the submarine’s hull can become transpar-ent and internal features can have significant effects on target strength. While somecodes are able to model this, others (including BASIS) can not. Thus, one suite oftests involved the modelling of a rigid boat hull only. With respect to these runs,the various Kirchhoff codes agreed well throughout the four frequencies. Further,

1A typical BASIS runtime for monostatic target strength using a model like the ones describedin this report is on the order of 1 second.

DRDC Atlantic TR 2003-199 1

Figure 1: 3D model of BeTSSi-Sub, as implemented in AVAST

Figure 2: 3D model of the Generic submarine, as implemented in AVAST

they agreed reasonably well with the ”exact” results from boundary-element modelcodes (like AVAST), where such results are valid (200 Hz and 1 kHz). At theselower two frequencies, BASIS also produced target strength results similar to theothers. However, at higher frequencies, BASIS produced consistently high targetstrength values in the bow region. It was immediately speculated that this disparitywas due to the inaccurate BASIS submarine model used.

This report presents updated results using simple models designed to approximateBeTSSi-Sub. Results are referenced against Model 3, a typical Kirchhoff-approximationcode presented at the BeTSSi-Sub workshop by Australia’s Defence Science &Technology Organization (DSTO), and sources of target strength discrepancies areinvestigated.

2 DRDC Atlantic TR 2003-199

2 BASIS Description

A submarine model for BASIS is comprised of a small set of ” primitive” shapeswhich roughly approximate its hull. The contribution to monostatic target strengthof each of these primitives is considered independently. It is the square of thespecular reflection, and is given by:

TS =

∣∣∣∣k

2π

∫

S

ej2k∆rdA

∣∣∣∣2

,

wherek is the magnitude of the wavenumber vector pointing from the receiver tothe scatterer,∆r is the line-of-sight displacement of a point on the surface,dAis the line-of-sight differential area, andj =

√−1 (see [1] for derivation). Thisequation is valid only in the monostatic case and assumes perfect acoustic reflec-tion. For bistatic models, exact forward scatter contributions are also calculated andthe Bistatic Theorem is applied; as this report deals solely with monostatic targetstrength, these procedures will not be further described here.

The integral in the equation forTS is known as the physical optics integral (POI),and is evaluated over the illuminated portion of the surface of the primitive in ques-tion. It is not trivial to evaluate, and in some cases only approximates may befound. To improve ease-of-use, the POIs for several common primitives2 have beenpre-determined in BASIS. It is hence much easier and faster to create models usingexisting primitives than to use custom-made primitives.

While BASIS employs 3D primitives, it does not truly allow 3D modelling. Instead,all primitives are placed on a 2-dimensional plane, with freedom only in thex−andy− directions. All primitives are necessarily symmetric about this plane. Asthe source and receiver are also both positioned on the plane, there is no possibilityfor horizontal plates; they would be inherently parallel to the incoming sound andhence make no contribution. These limitations impose significant restrictions onthe possible shape of BASIS models; especially important is that no componentmay exhibit vertical asymmetry.

Initial testing of BASIS was accomplished through a comparison with the Acous-tic Vibration and Strength Analysis (AVAST) program [4], a simulation packagedeveloped for DRDC-Atlantic. AVAST utilizes a boundary-element model to nu-merically determine the target strength of a complex panelled model; in principle,given a sufficiently detailed model, it should provide ”exact” results. However,

2POIs are calculated for the surfaces of prolate ellipsoids and hemiellipsoids, horizontal conesand trapezoids of revolution, vertical square and circular plates, vertical circular and elliptical cylin-ders and semi-cylinders, horizontal circular cylinders and semi-cylinders, and spheres and hemi-spheres


AVAST is a computationally intensive tool, and runtime increases exponentiallyas its model becomes more complex (which is necessary for high-frequency pre-dictions). A generic diesel-electric submarine model nominally valid up to 800Hzwas developed for AVAST, and approximated for BASIS. While the initial compar-isons showed reasonable agreement, especially in the 500 – 750 Hz range, AVASTmodel limitations meant BASIS performance at 1 kHz and beyond could not bedetermined.

3 Model Design

The BeTSSi-Sub BASIS models (Figure 3) were created directly from the GenericBASIS model (Figure 4) used in the FWG workshop3. In the first (”Simple”) ver-sion, the hemispherical nose section was replaced with a prolate hemi-ellipsoid,and a elliptical semi-cylinder ”front” was added to the tailfin, in addition to resiz-ing and shifting components to better match BeTSSi-Sub’s specifications. In thesecond (”Segmented”) version, the flat plates on the aft-end of the sail were dividedinto 3 segments per side, in order to more closely approximate the curvature of theactual sail. Similarly, the tail cone was divided into 2 sections.

One consideration when modelling BeTSSi-Sub with primitives is which radiusto use for the hull cylinder. BeTSSi-Sub’s hull is essentially a radially-symmetriccylinder of radius 3.5 m, with the top±45◦ off-vertical section extended another 0.5m upwards, forming the deck and transitioning to the sail. The main hull is hence7 m in beam and 7.5m in height, and exhibits symmetry only about the verticalcentreline plane. It was decided to preserve the height of the hull and implementa 3.75 m radius; this should optimize target strength at beam angles but impliesinaccuracy towards bow and stern aspects.

The tail fin in both improved versions is modelled as a single vertical elliptical half-cylinder trailed by angled plates, akin to the sail in the Simple model. In the Simplemodel, the tailfin is 7 m high overall. However, to account for the lack of occlusionin BASIS, the height of the tail in the Segmented version has been reduced to 5.2m; the removed fin area corresponds to the area of the 7 m fin which should beobscured by the tail cone.

In addition to physical optics contributions, bistatic target strength also receivescontributions from forward scattering. All components described above are in-cluded in forward scatter determination. Additionally, the bow planes and horizon-tal tailfins (which could not be modelled for POI calculation) are accounted for in

3Note that vertical displacement in these schematics is purely illustrative and has no meaning inthe actual implementation, as BASIS has no third dimension.


forward scatter. However, this has no effect on the solely monostatic cases reportedhere.

4 Results and Discussion

For reference purposes, Figures 5 and 6 compare the monostatic target strength pre-dictions of the Generic BASIS model with those of DSTO and AVAST for 200 Hzand 1 kHz sonar frequencies. The submarine’s bow is aligned at 0◦, so that beamincidence is at 90◦ and the stern is at 180◦. At 200 Hz, AVAST may be consideredcorrect, but the 1 kHz measurements are beyond the (nominal) 800 Hz maximumfrequency of the panelled model it uses.

At 200 Hz (Figure 7), there is not much distinction between the Simple and Seg-mented models, both of which typically follow the reference curve. Slight dis-crepancies are present towards stern angles (130-150◦), though. Both models alsoexhibit higher peak target strength values at direct beam incidence than the DSTOresult; this problem did not exist with the Generic model, and can be attributed tothe increased hull cylinder radius. Figure 8 shows that when the Segmented sub’scylinder is reduced to a radius of 3.5 m, the peak level once again agrees well. Cu-riously, though, the Generic model still fits best in the peak region, even after thisadjustment.

At 1 kHz (Figure 9), the new models both fit the DSTO results much better than thegeneric model, this time including the peak beam-incidence target strength4. Dif-ferences in results between the models are again minimal. However, target strengthspectra in regions corresponding to the back of the sail and fantail (92-105◦) andtail cone (105-110◦) are markedly flattened with the Segmented model, and agreebetter with the reference data (Figure 8). The is still room for some improvement,specifically in further smoothing the tail cone. Both models display significantdiscrepancies in the 130-150◦ interval; this behavior can be seen at 1 kHz in theGeneric version, also. It is ambiguous whether or not AVAST agrees with theseresults or DSTO’s.

It is evident that at both 4 kHz (Figure 10) and 8 kHz (Figure 11), the discrep-ancy between BASIS and DSTO at bow angles (0-90◦) remains – there is littleimprovement seen going to the new models from the Generic. Once again, theSegmented model provides the best results in the back- of-sail/tail cone regions,but target strength here is still significantly higher than in the DSTO model, anddistinct target strength peaks corresponding to each plate segment are present. The

4Upon investigation, little difference in peak target strength at 1 kHz was observed between hullradii of 3.5 and 3.75 m. At higher frequencies, 3.75 m provided better results, and so, was usedexclusively.


peaking suggests that the segmentation is not fine enough for these high sonar fre-quencies, but the significantly higher overall levels suggest another factor is alsopresent. Also, the target strength spectra over the entire stern region now has muchfiner structure; while it appears that the discrepancy at 130-150◦ has disappearedat these frequencies. it is possible that this fine structure has simply obscured theeffect.

It was suggested that the discrepancy between the as-modelled vertical sail andthe actual sail which exhibits an upward slant dependent on aspect angle (typicallyaround 3.25◦ from the vertical), and a similar effect relating to the tailfin, could beresponsible for some of the discrepancies observed. Specifically with respect to thediscrepancy at bow aspect angles, it was noted by D. Drumheller [5] that:

Assuming that the front of the sail contributes at allaspect angles between bow aspect (0 degrees) and beamaspect (+/- 90 degrees), and that the beam pattern ofthe sail approximately follows a sinc function withrespect to a perpendicular to its surface, then the "halfbeamwidths" of the physical optics integrals (3dB-downpoints) are:

Freq. in kHz Degrees------------ -------

0.2 27.1 5.434 1.368 0.68

Hence, one would expect the sloped front of the sail and tailfins to reflect the ma-jority of the incident energy up and away from the source at 4 and 8 kHz. Further,a similar argument could be applied to the rears of the sail and tailfins, with respectto the (albeit smaller) discrepancies observed at stern aspect angles.

To test this hypothesis, target strength of the Segmented model was re-calculatedtwice at each of the target frequencies; once without the elliptical semi-cylinders ofthe sail and tailfin, and once without the rear plate assemblies.

There is minimal difference between the results with and without sail/tail assem-blies at 200 Hz (Figure 12). Indeed, all results at this low frequency have beenreasonable. However, it does appear that at bow aspects, the model without the el-liptical semi-cylinders fares slightly better, and at stern aspects, the model withoutplate assemblies finds best agreement with reference.


At 1 kHz (Figure 13), the effect of removing these components becomes prominent.Removing the frontal sections drastically lowers target strength at forward aspects;as the Segmented model’s agreement had been quite good at 1 kHz, this modifica-tion causes target strength to be significantly lower than the reference. Removingthe rear plate assemblies, meanwhile, almost completely removes the discrepanciesfrom the 130-150◦ interval. These results support the hypothesis that inaccuratemodelling of the sail and fantail is the major limiting factor on result quality at highfrequencies.

At both 4 (Figure 14) and 8 kHz (Figure 15), the removal of the elliptical semi-cylinders results in a much better fit with DSTO’s results at bow aspects (althoughresults are still off from approximately 0-10◦). Removal of the plate assembliestended to lower target strength predictions to levels below those of the reference,and did not reduce their fine structure. The target strength peak at beam incidenceis also lowered and substantially narrowed in each case. Once again, then, it is seenthat the major source of discrepancy is inaccurate sail/fantail modelling.

5 Discussion and Recommendations

A model of BeTSSi-Sub was developed for monostatic target strength simulationin BASIS. In general, this new model improves the agreement between BASISpredictions and external predictions. However, there are a host of discrepancieswhich remain and can be confidently attributed to inaccurate modelling, especiallyof the sail and fantail regions.

At 200 Hz, these inaccuracies are not major, and would likely not be noticeablewithout additional data. However, at 1 kHz, it is observed that from 130-150◦

aspect angle, BASIS predicts a target strength significantly higher than is predictedby other methods. This behavior is hinted at, though is not unambiguously present,in the 200 Hz data as well.

At 4 and 8 kHz, the 130-150◦ discrepancy is no longer observed, possibly becausethe fine structure in the target strength prediction overpowers the effect. How-ever, an even larger effect takes over from 0-90◦; target strength predictions areconsistently much higher throughout this interval, and as frequency increases, thediscrepancy increases.

It was speculated that the source of these discrepancies was that the actual sail isangled upward and should reflect high- frequency sound away from the receiver,whereas in this model it is perpendicular to incident sound and hence does not.This hypothesis was supported by disabling portions of the sail and tailfins andrepeating the target strength predictions. Indeed, when the front of the sail and


tail are disabled, the 0-90◦ discrepancy disappears, and when the rear portions aredisabled, the 130-150◦ disagreement is removed. However, such drastic omissionsreduce the accuracy of predictions at beam aspect angles (in the vicinity of 90◦),artificially lowering target strength and narrowing the peak.

One possible explanation for the observation that 130-150◦ discrepancy becomesprominent at 1 kHz but 0-90◦ discrepancy is only visible at 4 kHz is that the sail ismore slanted at stern aspects than it is at bow aspects. Figure 12 (not reproducedhere) from the BeTSSi-Sub specifications [2] visually suggests that this is indeedthe case.

However, while the front of the sail has components normal to all the aspect anglesaffected in 0-90◦, the rear of the sail does not have components normal to the 130-150◦ discrepancy region. No explanation for how the sail creates this target strengthpeak has been presented, although there is a demonstrable link between the two.

Also noted was an increase in the height of the peaks corresponding to the sail-and tail-plate and tail cone angles as frequency increased. As these componentsshould actually be smoothly curved, multiple sharp peaks should not occur; thereshould be a single, lower, wider peak for each. The Segmented model presentedhere fares reasonably well in addressing this at 1 kHz, but further segmentation ofthe plate region would be useful and is necessary for higher frequency predictions.Taking this to its logical limit results in the development of a detailed panelledmodel, as used by both AVAST and the reference, with the associated increase incomputational effort. As this issue is ultimately dominated by the aforementionedsail angle reflection effect, work on it is irrelevant until the latter issue is resolved.

As it exists, BASIS cannot accurately simulate BeTSSi-Sub (or any other realisticsubmarine) at or beyond 1 kHz for every aspect angle with one model. Knowingthe nature of the submarine allows these limitations to be somewhat relaxed; forexample, good results are produced for BeTSSi-Sub at 1 kHz when the rear of thesail/tail is disabled, and at 4 and 8 kHz when the front of the sail is disabled. How-ever, without prior detailed knowledge of the sub’s geometry and target strength,modifications such as these can not be justified.

In order to overcome this limitation, the nature of BASIS must be fundamentallychanged to allow physical optics integrals to be calculated for the simple primi-tives in three full dimensions. Until this is achieved, BASIS is limited in its abilityto provide high fidelity target strength predictions for any model with componentsthat differ significantly from the primitives used to model them. BASIS does re-main a useful tool for quickly investigating a model at a relatively wide range offrequencies as long as there exists reliable target strength data with which to vali-date it.


References

1. D.M. Drumheller, L.E. Gilroy, M.G. Hazen (2002). The Bistatic AcousticSimple Integrated Structure (BASIS) Target Strength Model. (TechnicalReport NRL/FR-MM/7140–02-10,019). US Naval Research Laboratory.

2. Ch. Fiedler, H.G. Schneider (2002). BeTSSi-Sub – Benchmark Target StrengthSimulation Submarine. Technical Report. Forschungsanstalt der Bundeswehrfur Wasserschall und Geophysik, Kiel.

3. Schneider, H.G. et al. (2003). Acoustic scattering by a submarine: results froma benchmark target strength simulation workshop. Proceedings of TenthInternational Congress on Sound and Vibration, Stockholm, Sweden.Forschungsanstalt der Bundeswehr fur Wasserschall und Geophysik, Kiel.

4. L.E. Gilroy, D.P. Brennan (2001). Predicting Acoustic Target Strength withAVAST. (DRDC Atlantic TM 2001-071). Defence R&D Canada – Atlantic.

5. Drumheller, D.M. (2003). Private communication.


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Figure 4: Schematic diagram of the Generic BASIS submarine model

Monostatic target strength at 200Hz, 0º elevation, for original BeTSSi-Sub models.

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Monostatic target strength at 1 kHz, 0º elevation, for original BeTSSi-Sub models.

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Figure 6: 1 kHz monostatic target strength of original models

Monostatic target strength at 200Hz, 0º elevation, for various BASIS BeTSSi-Sub models.

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Figure 7: 200 Hz monostatic target strength of improved BASIS BeTSSi-Submodels


Monostatic target strength at 200Hz, 0º elevation, for various BASIS BeTSSi-Sub models. (90º detail)

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Figure 8: 90 ◦ Target strength of improved BASIS BeTSSi-Sub models; 90 ◦ regionat 200 Hz and 92− 110 ◦ region at 1 kHz. The lack of smoothness here is due tothe 1 ◦ angular resolution used.

Monostatic target strength at 1 kHz, 0º elevation, for various BASIS BeTSSi-Sub models.

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Figure 9: 1 kHz monostatic target strength of improved BASIS BeTSSi-Sub models


Monostatic target strength at 4 kHz, 0º elevation, for various BASIS BeTSSi-Sub models.

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Figure 10: 4 kHz monostatic target strength of improved BASIS BeTSSi-Submodels

Monostatic target strength at 8 kHz, 0º elevation, for various BeTSSi submarine models.

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Figure 11: 8 kHz monostatic target strength of improved BASIS BeTSSi-Submodels


Effect of sails/fantails on monostatic target strength at 200Hz, 0º elevation, for segmented BASIS BeTSSi-Sub model.

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Figure 12: The effect of the sail and tailfin on target strength of SegmentedBeTSSi-Sub BASIS model at 200 Hz

Effect of sails/fantails on monostatic target strength at 1 kHz, 0º elevation, for segmented BASIS BeTSSi-Sub model.

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Figure 13: The effect of the sail and tailfin on target strength of SegmentedBeTSSi-Sub BASIS model at 1 kHz



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DOCUMENT CONTROL DATA(Security classification of title, body of abstract and indexing annotation must be entered when 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 acontractor’s report, or tasking agency, are entered in section 8.)

Defence R & D Canada – AtlanticPO Box 1012, Dartmouth, NS, Canada B2Y 3Z7

2. SECURITY CLASSIFICATION(overall security classification of the documentincluding special warning terms if applicable).

UNCLASSIFIED

3. TITLE (the complete document title as indicated on the title page. Its classification should be indicated by the appropriateabbreviation (S,C,R or U) in parentheses after the title).

An Improved BASIS Model for the BeTSSi Submarine

4. AUTHORS(Last name, first name, middle initial. If military, show rank, e.g. Doe, Maj. John E.)

Nell, Christopher W. ; Gilroy, Layton E.

5. DATE OF PUBLICATION (month and year of publication of document)

November 2003

6a. NO. OF PAGES (totalcontaining information. IncludeAnnexes, Appendices, etc).

20

6b. NO. OF REFS (total cited indocument)

5

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Technical Report

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Defence R & D Canada – AtlanticPO Box 1012, Dartmouth, NS, Canada B2Y 3Z7

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11cj12

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10b. OTHER DOCUMENT NOs. (Any other numbers which may beassigned 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 DocumentAvailability (11). However, where further distribution beyond the audience specified in (11) is possible, a wider announcement audience may beselected).

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 theabstract of classified documents be unclassified. Each paragraph of the abstract shall begin with an indication of the security classification of theinformation in the paragraph (unless the document itself is unclassified) represented as (S), (C), (R), or (U). It is not necessary to include here abstracts inboth official languages unless the text is bilingual).

DRDC Atlantic and the US Naval Research Laboratory have developed a tool for Operations Research whichwill predict the low frequency acoustic target strength of a rigid submarine hull. The BASIS (Bistatic AcousticSimple Integrated Structure) software uses analytical methods to combine the results from simple geometricshapes to approximate the response of the submarine hull.

A BASIS target strength simulation model was developed for the Benchmark Target Strength SimulationSubmarine (BeTSSi-Sub) and compared with reference data at 200 Hz, 1 kHz, 4 kHz, and 8 kHz, to supportan international workshop on target strength simulation methods. In general, it was found to outperform thegeneric model previously employed. However, there were three major discrepancies: at bow aspect anglesand at higher frequencies, BASIS predicted significantly higher target strength than reference; between beamand stern aspects and especially at 1 kHz, a rise in target strength was present in the BASIS predictions butnot reference; and at aspect angles normal to the rear of the sail and tailfins, BASIS target strength peakedhigher and more sharply at high frequencies than reference.

The source of all three of these discrepancies is likely inaccurate modelling of the sloped sail and tailfin;the first two discrepancies were resolved quite well by removing the front side and back side of the sail andtailfin respectively (BASIS assumes these components are true vertical), with the undesirable side effect oflowering target strength at beam incidence. The third issue was partially resolved by segmenting the backside of the sail into multiple sections.

In general, full-3D component modelling must be incorporated to rely on the results of BASIS simulations ofrealistic submarines. Until then, BASIS is only reliable for models which are well understood and for whichmodifications can be justified, or models which do not require true 3D for accuracy.

14. KEYWORDS, DESCRIPTORS or IDENTIFIERS (technically meaningful terms or short phrases that characterize a document and could be helpful incataloguing the document. They should be selected so that no security classification is required. Identifiers, such as equipment model designation, tradename, 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, theclassification of each should be indicated as with the title).

BeTSSiBASISTarget strengthSubmarineOperations researchPhysical opticsHelmholtz-Kirchhoff theory

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