DTIC ADA459089: Acoustic Communication With Small UUVs Using a Hull-Mounted Conformal Array

7 Pages·0.67 MB·English

by  Defense Technical Information Center

Acoustic Communication with Small UUVs
Using a Hull-Mounted Conformal Array
Lee Freitag,MatthewGrund,JoskoCatipovic ,DanielNagle ,BrianPazol andJamesGlynn
y y z z
WoodsHoleOceanographicInstitution,WoodsHole,MA02543
NavalUnderseaWarfareCenter,Newport,RI02841
y
MaterialsSystemsIncorporated,Littleton,MA01460
z
Abstract—Anewconformalarraydesignedtoenhanceacousticcom- rectly to the pressure vessel. Recent advances in transducer
munication on small unmanned underwater vehicles (UUVs) is pre- technology have made it feasible to consider this approach
sented.Thearrayisintendedtoimprovethereliabilityandrateofacous-
on vehicles such as REMUS that have an outer diameter of
ticcommunicationtoUUVsunderconditionsofmultipath,multiuserin-
terference,broadbandornarrowbandjammingandlowsignal-to-noise only20cm. Theconformalarrayisfabricatedfromapiezo-
ratio. Thearrayelements areconstructed fromapiezocomposite ma- composite material that can be bonded to the hull and then
terialthatcanbeinjectionmoldedtonearlyanyshape,includinginto encapsulatedwithadurablepottingcompound.Thevehicle’s
curves that match the hull radius of the vehicle. The array is encap-
hullisboththestructuralbackboneofthearrayaswellasan
sulatedintoalow-profileassembly0.02mthickandbondeddirectlyto
thehullofaREMUSvehicle.Thefrequencyresponseandbeampatterns air-filled volume that increases the front-to-backratio of the
weremeasuredtodeterminethecharacteristicsofthecompletedarrays. arrayatnoadditionalcostinmaterialweight.
Thearraywasdeployedforinitialtrialsinbothhorizontalandvertical
Theperformanceof a prototypeconformalarrayis exam-
orientations in shallow water. The performance of thearray for both
singleandmultiuserphase-coherentcommunicationispresented. inedduringa test whereit is heldfixedanda mobilesource
changesrangeandbearingtothearray. Whilethearraywas
designedtobe usedhorizontally,it is instructivetocompare
I. INTRODUCTION
the performanceofboth horizontalandverticalorientations.
Reliable command and control of small underwater vehi- Thisrevealssomeofthetrade-offsassociatedwitharrayde-
clesisanessentialpartofsingleandmulti-vehicleoperations signforsmallUUVsandprovidessomeinsightintotheper-
in civilian and military applications. The downlink to the formanceofsmallarraysforacousticcommunicationsingen-
vehicle is just as important as the uplink to a surface ship, eral.
subsurfacenodeorbuoy,butcanbethemostdifficulttoim-
The paper is organizedas follows. In Section II the con-
plement reliably. Vehicle self-noise and limited space for
formalarray,includingthematerialstechnology,arraydesign
transducersarethemostcommonproblemsencounteredwhen
and tank testing are presented. Section III has results from
integratingacousticcommunicationsystems intounderwater in-water testing in both single and multiuser modes, as well
vehicles[1]. Carefuldesignofthepropulsionsystemcanre- asdifferentorientations.SectionIVincludesconclusionsand
ducetheimpact its noise mayhaveonacousticcommunica-
areasoffuturework.
tions,butitisalwaysachallengetoincorporateamultichan-
nelacousticarrayintothedesignofatypicalsmallvehicle. II. PIEZOCOMPOSITE CERAMIC ARRAYS
However,thereisconsiderablebenefittouseofanarrayas
The arrayelements incorporatea 1-3piezocompositema-
a communicationsreceiver. Noise and interferencefrom bi-
terialwhichisbuiltintoanacousticstackthatisattacheddi-
ological or man-made sources limit the SNR available from
rectlytothehullofaREMUSvehicle. Thedesigntakesthe
asingletransducer,andinmulti-usersituationstheremaybe
physicalpropertiesoftheair-backed,thin-wallaluminumhull
manysignalspresentsimultaneouslywhichcreatedirectional
into account, and uses it to enhance sensitivity and increase
interference.Whileamulti-channelarrayisknowntoincrease
thefront-to-backratio.The1-3piezocompositematerialcon-
thereliabilityofanacousticcommunicationlink[2],thelim-
sists of an array of small piezoelectricceramic rods that are
itedspaceaboardsmallvehiclesforarraysinthe10-30kHz
moldedusinganinjectionprocessthatissimilartothatused
rangehas preventedtheir usefrombecomingcommonplace.
intheplastics industry. Theprocessincludesmixingthece-
However,previouswork on21-inchvehicleshas shownthat
ramic powder with a binder, heating the mixture and inject-
phase-coherentcommunicationusingmultichannelarrayscan
ing it into a cold mold, a low temperature firing to remove
beaccomplishedonUUVs[3].
thebinder,andthendensification(sintering)andpolarization.
Small vehicles provide considerable incentive to consider
Therodsareembeddedinapolymermatrixandmachinedto
novel approaches to array integration. Not only is space
the desired final thickness. Both hardand soft matrix mate-
aboardsmallUUVsatapremium,butminimizingbothweight
rials can be used depending upon the application. The soft
and drag is very importantif the primarymission of a vehi-
matrix provides the best receive sensitivity, typically 10 dB
cle is not to becompromised. The approachto arraydesign
over the equivalent ceramic, but operates only to moderate
describedinthispaperistobondtheindividualelementsdi-
depths. Hard matrixmaterials are usable to pressures above
1000psiandhave5dBbettersensitivitythansolidceramic.
This work was supported by the Naval Undersea Warfare Center, Division
Newport.Emailcontact:[email protected]. Hardmatrixmaterialshavetheaddedadvantagethattheycan
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1. REPORT DATE 3. DATES COVERED
2001 2. REPORT TYPE 00-00-2001 to 00-00-2001
4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Acoustic Communication with Small UUVs Using a Hull-Mounted
5b. GRANT NUMBER
Conformal Array
5c. PROGRAM ELEMENT NUMBER
6. AUTHOR(S) 5d. PROJECT NUMBER
5e. TASK NUMBER
5f. WORK UNIT NUMBER
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION
Naval Undersea Warfare Center,Newport,RI,02841 REPORT NUMBER
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14. ABSTRACT
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a. REPORT b. ABSTRACT c. THIS PAGE 6
unclassified unclassified unclassified
Standard Form 298 (Rev. 8-98)
Prescribed by ANSI Std Z39-18
be conformed to shapes such as cylinders or spheres. Ad- Parameter Specification
ditional advantagesof the injectionmoldingprocess include Elementwidth 31mm
better element to element uniformityand lower cost in high Elementheight 62mm
volume. Elementthickness 13mm
Encapsulationthickness 22mm
A. ArrayDesign Weight(air) 2.5Kg(perpair)
Weight(water) 1.2Kg(perpair)
ThearraysdesignedfortheREMUSvehiclewereintended
Overalllength 500mm
tosupportmultiplefrequencybandsasrequiredfordifferent
Overallheight 150mm
applications. These applications include compatibility with
acousticcommunicationsystemsthatusethe7-11kHzband TABLEI
(UQC), the 10-20 kHz band, and for certain systems, 20-30 CONFORMALARRAYPHYSICALSPECIFICATIONS.
kHz. While near-termapplicationswill just use onebandat
atime,inthefuture,avehiclemayneedtomonitormultiple
bands in an acoustic network. Future systems may also use
verybroadbandwidth(10kHzorgreater)formultiusercom-
municationortominimizetheprobabilityofdetection. Thus
thebandwidthgoalforthereceiverwas10-40kHz.
Thesizeoftheelementswasselectedtoprovide ver-
(cid:6)33Æ
ticalbeamwidthat20kHz. However,thisselectionisclearly
a compromisebecausethe operatingfrequencyspans sucha
wide range. At 20-30kHz this offers goodperformancefor
shalloworveryshallowwaterwherethedepthistypically3-
20mandtherangeis500-2000m. Almostalloftheenergy
is received at shallow angles and the directivity of the array
improvesitsresponseandthustheSNR.Inthe10kHzband
theverticalbeamwidthisdoubled,butbecausethisfrequency
is likely to be used for longer-rangecommunicationin deep
water,thebroaderangleisactuallyanadvantage.
Thedesigngoalforthehorizontalbeampatternwas toget
as close to as possible. Achieving this is difficult be-
(cid:6)90Æ
causethetubeislongerthanthearray.Thisproducesshading
that narrows the beampattern as discussed in the calibration
sectionbelow.
The array elements used in the transducer arrays were
shapedtomatchtheradiusofcurvatureoftheREMUShullto
makethemlow-profileandhydrodynamic.Flatelementswith
thesamebeampatternwouldbemuchthickerandincreasethe
dragofthevehicle. Theelementsare bondeddirectlyto the
aluminum tube and the signal from each element is brought
inside on a twisted, shielded pair using a feedthroughin the
hull.Thusnoadditionalwetconnectorsarerequired.
The conformalarray priorto final encapsulationis shown
in the top panel of Fig. 1. The pre-amplifier electronics are
abovethearray,andanoptionalside-scantransducerisatthe
bottom. ThelowerpanelofFig.1showsthecompletedunit.
The encapsulation is approximately 2 cm thick and the for-
wardandrearedgesareshapedtominimizeddrag.Thephys-
icalspecificationsforthearrayaresummarizedinTableI. Fig.1. ConformalarraysfabricatedfortheNUWCREMUSvehiclebyMate-
rialsSystemsIncorporated. Top:8elementcommunicationsarray,side-
scantransducer andpre-amplifierspriortoencapsulation. Bottom: the
B. CalibrationResults completedassembly.
The arrays were calibrated in a test facility operated by veryrepeatableandthroughoutmostofthebandthechannel-
Code8211attheNavalUnderseaWarfareCenterinNewport, to-channelmatchingis 3 dBorbetter. Channels1 and8 are
RI.Theobjectivesofthetestsweretoconfirmthedesigngoals different from the other group by 1-3 dB in the 15-30 kHz
andcharacterizeboththebeampatternandreceiveresponse. regionbutareotherwiseverysimilar. Thisdifferencemaybe
Thereceiveresponsefrom10kHzto40kHzforoneofthe due in partto edgeeffects at the two ends of the array. The
arrays is shown in Fig. 2. The response for channels 2-7 is roll-offat 40kHzincludesthelow-passresponseofthepre-
2
Frequency Response for Channels 1−4 inphysicalconstruction.
−155
III. RESULTS
−160 The arrays were tested by placing them on a fixed under-
Pa) water platformat the NUWC GouldIsland test range. They
µ
e wereinterfacedtoamulti-channeldataacquisitionsystemthat
dB r−165 performedrealtimedetection,demodulationandarchivingfor
p. ( off-lineanalysis.Thetestswereperformedwiththearrayori-
s Ch 1
e
R ented both horizontally and vertically.
−170
The primary acoustic communications signals of interest
arebroadband(greaterthan5kHz)PSK.Whiletheencoding
−175 andconstellationdensitymayvaryinthefinalapplication,itis
10 15 20 25 30 35 40
instructivetoexaminetheperformanceofa5000symbolper
second,rate1/7block-encodedQPSKsignalasdescribedina
relatedpaper[5]. Thissignalmaybeinterpretedasa10kb/s
Frequency Response for Channels 5−8
−155 uncodeddata stream, oras a coded1400bps signal suitable
forlowSNRormultiuseroperation.Signalscorrespondingto
sixdifferentusersweretransmitted. Asdescribedin[5],the
−160
differencebetweenthesignalsisthetrainingsequenceandthe
a)
µ P data.Thesameblockcodeisusedforallusers.
e
B r−165 Thesignalsweretransmittedfromasmallboatwithahigh-
p. (d Ch 8 frequencytransducerwithtoroidalbeampatternandnearlyflat
s frequencyresponse. Thecenterfrequencywas23kHz. One
e
R
−170 wavelengthatthisfrequencyis0.065m,sothatthearrayele-
mentspacingisalmostexactly . Additionaldetailsabout
theexperimentalsetupareprovi(cid:21)d=e2din[5].
−175
10 15 20 25 30 35 40 Thereceiveristhemulti-channeldecisionfeedbackequal-
Frequency (kHz) izer(DFE)modifiedtodirectlyincludeDopplercompensation
Fig.2. Measuredfrequencyresponseforoneofthefinishedarrays.Channels anderror-correctiondecoding.Reliablesymbolsarefedback
1and8exhibitsmalldifferencesfromtheotherelements. toupdatetheequalizerfiltersaftersoft-decisiondecodingso
thatthecodinggainmaybeeffectivelyused. Thustheequal-
amplifier. The response of the piezocomposite itself is flat izeroperatesreliablyatsymbolsignal-to-noiseratiosthatare
wellabove40kHz. severaldBlowerthanwouldotherwisebepossible.
Thehorizontalandverticalbeampatternsofthearrayswere
A. Single-UserCommunication
also measured in the calibration facility. Three frequencies
wereused,10,20and30kHz.Theverticalresponseisshown The signals corresponding to the six different users were
inFig.3. At10kHztheverticalresponseis ,andat20 firstprocessedindividuallytocheckthedifferenceinperfor-
(cid:6)75Æ
kHzitdecreasestoapproximately .At30kHzthe-3dB mancebetweenthetwoarrayorientations.Fourelements(ev-
(cid:6)25Æ
pointsaresimilartothoseat20kHz,buttheoverallwidthof ery other one from the array) are used in this analysis. The
themainlobeisnarrowerandtheresponseis20dBdownat resultsareinFig.5, wherethehistogramofMSEforall the
. Thewidebeamwidthat10kHzallowsthearraytobe datapacketstakenduringeachtestareshown. Thespreadof
(cid:6)50Æ
used in the UQC band in deep water where the source may MSEisbroadbecausethesourceismovingatdifferentspeeds
be at a steep verticalanglewith respect to the vehiclearray. andchangingrange,butingeneralthesamepathistracedby
Whilethefront-to-backratioisonly8dBat10kHz,at20and thesourcevessel. TheaverageMSEfortheverticalarrayge-
30kHzitisgreaterthan25dB. ometryis about3 dB lower than that of the horizontalarray
The horizontal beampatterns at 10, 20 and 30 kHz are tests. This translates to a significant differencein the possi-
shown in Fig. 4. The horizontal response varies with fre- ble rateorreliabilitylevelof theacousticlink usingthetwo
quency as expected, but is nominally . While the low orientations. However,the single-userperformancewith the
(cid:6)45Æ
response to aft of the vehicle helps to reject noise from the arrayhorizontalismorethanadequateformanyapplications,
propellerorcontrolsurfaces,thesamereducedresponselook- evenifitisnotasgoodasthevertical.
ingforwardleaves a gapin theoveralldirectionalcapability Thedifferencebetweenthetwomaybeexplainedatleastin
of the acoustic communications receiver. While this beam- partbythespatialcoherenceordiversityfunctionsinshallow
patternissignificantlydifferentthanthedesigngoal,itisnot water. The adaptive equalizer takes advantage of the differ-
completelyunexpectedfromalinearrayfabricatedonalong encesinthereceivedsignalacrossthearray,andinthisprop-
air-backed tube. Methods for broadening the horizontal re- agation environment the output SNR of the adaptive equal-
sponseareunderconsiderationbutmayrequireacompromise izer is limited primarily by inter-symbol interference (ISI),
3
90 90
120 60 120 60
−25 −25
150 30 150 30
180 0 180 0
210 330 210 330
240 300 240 300
270 270
90 90
120 60 120 60
−25 −25
150 30 150 30
180 0 180 0
210 330 210 330
240 300 240 300
270 270
90 90
120 60 120 60
−25 −25
150 30 150 30
180 0 180 0
210 330 210 330
240 300 240 300
270 270
Fig.3. Verticalbeampatternsforchannels1-4. Top: 10kHz. Middle: 20 Fig.4. Horizontalbeampatternsforchannels1-4.Top:10kHz.Middle: 20
kHz. Bottom: 30kHz. Theoverallscaleis50dB.Zerodegreesishori- kHz.Bottom:30kHz.
zontaland330represents-30degrees.
4
Vertical Array Equalizer MSE Summary Mean Square Error vs. Channel Use: Horizontal
50 −4
Linear Selection
Sparse Selection
40 −6
s
et
ack30 B) −8
P d
of E (
ber 20 MS−10
m
u
N
−12
10
−14
0 1 2 3 4 5 6 7 8
−20 −15 −10 −5 0
Average Mean Square Error (dB)
Mean Square Error vs. Channels: Vertical
Horizontal Array Equalizer MSE Summary −4
50
Linear Selection
Sparse Selection
−6
40
s
of Packet30 SE (dB) −8
ber 20 M−10
m
u
N
−12
10
−14
1 2 3 4 5 6 7 8
0
−20 −15 −10 −5 0 Number of Channels
Average Mean Square Error (dB)
Fig. 6. Equalizer mean square error for the horizontal (top) and vertical
Fig.5. Equalizermeansquareerror(MSE)histogramforaseriesofsingle- (bottom)arrayorientationinthesingle-usercase.Linearselectionis[1],
usertestsinshallowwater. TheaverageMSEfortheverticalarray(top) [1,2],[1,2,3],etc.Sparseselectionis[1],[1,8],[1,4,8],etc.
islowerthanthatofthehorizontal(bottom)arrayorientation.
B. MultiuserCommunication
not noise. The multipath structure varies more in the verti-
calthanthehorizontal,andthustheadaptiveequalizerworks Comparative analysis of different arrays or approaches to
betterwhenthearrayisorientedvertically. multiusercommunicationiscomplicatedbecauseofthenum-
berofvariables. Forexample,despitethefactthatthetrans-
In Fig. 6 a comparison of typical results using different mitter vessel is following the same track while sending the
numbers of elements in both orientations is presented. The signalsforallusers,thechanneldoesvary. Selectivelycom-
objectiveofprocessingthedatainthiswayistodeterminethe biningthedifferentusers’signalscanresultineitheroverly-
optimalnumberofchannelsfromthearray.Thearraymaybe optimistic performance estimates, or pessimistic worst-case
drawnfrominnumericalorder,ormaybeusedwithmaximal performance(i.e. decodingfailure). Rather than attempting
channelseparation.Twochannellinearselectioncorresponds to reach globally-applicableconclusions, we instead present
tochannels1and2,whilesparseselectionischannels1and8. someindividualcasesthathelptoillustratetherangeofper-
InthetoppanelofFig.6itisapparentthat2channelsselected formancethat might be expected. In this case two users are
withmaximumseparationprovidesthelowestMSEwhenthe examined.Thusthetotaldatarateis2800bpsusingthe5000
arrayishorizontal. Whenthearrayisorientedverticallyuse Hzbandwidth.Resultswithfourusersarepresentedin[5].
ofeveryotherelementgivesthebestMSE. Two-user performance for two example cases of horizon-
talandverticalarrayplacementareshowninFig.7.Twodata
Itshouldbenotedthatthesedatapacketswerenotcollected packetsfromdifferentusersthathavesimilarsingle-userMSE
at the same time, so that the MSE cannot be compared di- wereselectedandthencombinedwithoutweighting. Inboth
rectly, ratherit is thetrendthat is important. Thehorizontal ofthetestcasesthepowerdifferencesbetweenthetwousers
coherenceishighandthusthegainfromusingtwochannels was approximately 6 dB. The number of channels used for
ishighestwhentheyarelocatedasfarapartaspossible.More equalizationwasvariedfromonetoeight,withchannelselec-
arrayelementscontributetothebestsolutionwhenthearrayis tiondonesparsely,i.e.maximalseparationacrossthearray.
vertical. Theshapeofthesecurveswillchangeasconditions InthetoppanelofFig.7theperformanceoftheequalizer
change. forahorizontalarrayorientationisshown. Thetwo-usersig-
5
Horizontal Array with Two Users allowedshapingoftheelementstomatchthevehicle’sradius
0 of curvature. Evaluation of adaptive equalizer performance
Weaker User resultsdemonstratesthatthearrayprovidesreliablecommu-
Stronger User
−2 nicationwhenasubsetofthearrayisusedineitherverticalor
B) horizontalconfigurations.
d
E ( −4 For the single-user case with the array oriented horizon-
S
M tally,asfewastwoelements(eitherend)providebestresults.
ge Whenthearrayisvertical,everyotherelementtypicallypro-
a −6
er videsbestperformance.Theseresultsareforsignalswithcar-
v
A
rierfrequencyselectedsuchthattheelementsareatone-half
−8
wavelength. Thedataconfirmheuristicargumentsthatcom-
municationsarraysonsmallvehiclesshouldattempttouseall
−101 2 3 4 5 6 7 8 possibleverticalaperture,andthenallhorizontalaperture.
Channels Used (sparse selection) In the two-user case the performance difference between
thetwoorientationsappearstonarrow,withthehorizontalar-
Vertical Array with Two Users rayshowinggoodtwo-userdatarecovery. Theseparationin
0 bearingbetweenthe usersmaybebest exploitedbythehor-
Weaker User izontal array,indicatingthat for multi-userreceptionthis ar-
Stronger User
−2 raydesignmayofferthebestperformance.Byextension,the
B) horizontalarraywillalsooffergoodinterferencesuppression
d
E ( −4 fromnoisesourcesatanglesseparatedfromthedesireduser.
S
M Future transducer work includes development of an array
ge withabroaderhorizontalbeamandthedesignoftwodimen-
a −6
er sional arrays that work well for both single and multiuser
v
A
communication. Future algorithm development and evalua-
−8
tionincludesreduced-complexityprocessingthatmayprovide
more effective use of all elements of the array as described
−10
1 2 3 4 5 6 7 8 in[6].
Channels Used (sparse selection)
ACKNOWLEDGEMENTS
Fig. 7. Equalizer mean square error for the horizontal (top) and vertical
(bottom)arrayorientationfortwo-usercases.Channelselectionissparse The authors would like to thank Ben Allen, Tom Austin
inallcases.
andChrisvonAltoftheWHOIOceanographicSystemsLab
nalisprocessedforbothuserssothattheperformanceofthe fortheirhelpinintegratingtheconformalarraysontotheRE-
weakerusercanbeexaminedwithrespecttothearrayusage. MUSvehicle.
A4dBdifferenceisapparentbetweenthetwoforoneandtwo The array beampattern and response measurements were
channels,butwhenfourchannelsareusedtheperformanceis donebyWalter Boober,Code 8211,NUWC, DivisionNew-
within1dB. port.
Intheverticalcase(bottomofFig.7)theseparationinper-
formance between weaker and stronger users also starts out REFERENCES
atapproximately4dBandnarrowsasadditionalchannelsare [1] L.Freitag, M.JohnsonandJ.Preisig, “Acoustic communications for
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[2] M.Stojanovic,J.CatipovicandJ.Proakis,“Phasecoherentdigitalcom-
izontalcase,andonlywitheightchannelsdoesitnarrowto2 municationsforunderwateracousticchannels”,IEEEJ.OceanicEng.,
dB. Vol.16,pp.100-111,Jan.1994.
[3] L.Freitag,M.Grund,S.Singh,S.Smith,R.Christenson,L.Marquis,
Whilethesetestsarenotbyanymeansexhaustive,thepre-
and J. Catipovic, “A Bidirectional coherent acoustic communication
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viding as good performanceas the vertical array for single- [4] L.Freitag,M.Grund,S.SinghandM.Johnson,“Acousticcommunica-
tioninveryshallowwater:Resultsfromthe1999AUVFest,”inProc.
user reception, actually does as well (and perhaps better in
Oceans2000,Providence,Sept.2000.
somecases)astheverticalarrayfortwo-userreceptionfrom [5] M.StojanovicandL.Freitag,“Multiuserunderseaacousticcommuni-
sourcesseparatedinbearing. cationsinthepresenceofmultipathpropagation,”Proc.Oceans2001,
Honolulu,Nov.2001.
[6] M.Stojanovic, J.A.Catipovic and J.G.Proakis, “Reduced-complexity
IV. CONCLUSIONS AND FUTURE WORK multichannel processing of underwater acoustic communication sig-
nals,” JournaloftheAcousticalSocietyofAmerica,vol.98(2),Pt.1,
A conformal array suitable for a small (20 cm diameter)
pp.961-972,Aug.1995.
UUVmayusethevehicle’spressurecaseasastructuralmem-
berandexploittheair-filledtubetoincreasethe arrayfront-
to-back ratio without adding heavy or bulky backing mate-
rial. Piezocompositeceramicarraysoffersexcellentsensitiv-
itycomparedtotraditionalceramicsolutionsandinthiscase
6