CA1188404A

CA1188404A - Imaging system with multiple, simultaneous transmission

Info

Publication number: CA1188404A
Application number: CA000387341A
Authority: CA
Inventors: Pierre Tournois
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1980-10-10
Filing date: 1981-10-06
Publication date: 1985-06-04
Also published as: EP0050060A1; US4456982A; AU7610081A; DE3174568D1; FR2492109A1; DK449181A; EP0050060B1; NO813419L; FR2492109B1; AU547216B2

Abstract

ABSTRACT OF THE DISCLOSURE

Bidimensional imaging system by echoes compris-ing a transmitting array and a receiving array. The trans-mitting array comprises a row of transmitters parallel to a direction Oz and the receiving array comprises a row of receivers in a direction Ox. All the transmitters si-multaneously transmit signals with different codes. The signals received are demodulated and the real and imagina-ry components of these signals are applied to two multi-plexing circuits. After modulation the multiplexed signals are applied to a Fourier transformation circuit, which sequentially supplies reception channels Vk (.THETA.k) for the direction referenced by angle .THETA.k and which are stored in memories. Other memories contain signals Si (0i) trans-mitted in an angular direction referenced by 0i. The two pairs of memories are connected and the signals are applied after modulation to a convolver, which supplies signals of the bidimensional image. Application to ultrasonic submarine cameras, submarine target classifiers, medical acoustic imaging and non-destructive tests.

Description

~R~SMISSIONS

~AC~ROU~D 0~ ~HE INVE~ION
~._ ~he present inYentiOn r~lates to improvement~
to radar ox sonar echo detec-tion system~ making it po~sible to provide an image o.~ the surrounding space in real time. ~he apparatu~ according to the inven-tion is more particul~rly applicable to high de~inition ~onar equipment by the ~o~mation o~
cha~nels, such as acoustic cameras .and target classi~ication s~tems in ~ubmarine technology~
imaging taki~g place in ~rontal visionO ~he i~vention also applie~ to medical acou~tic imaglng and to non-destructire tests~
Radar or ~o~ar s~stem~ are generally con~t~.tutcd by at least o~e tra~mitter9 who~e transmitted ~ignal~ ailluminate~ the angular fleld to be ob~erved a~d a receiring ant~nna or array, whoæo signals receiv~d ar~ used ~or ~orml~g a ~stem of ~ngular channel~ covering thl~ fieldO
To each chan~el ~ormed correæpond~ a distributio~
o~ the ~nergy received in the spaceg who3e angular width o~ the major lobe at the hal~ power d0fi~es the angular re~oluti~n~
It is kno~n to bring about imaging by means o~
tran~mitting and receiving arrays formed ~y lines o~ tran~ducers in orthogonal direction~. I'he signals xeceived about a ~requency ~ by the transducers of the receiving arraV are delayed or phase-di~placed relative to one another to ~orm channel~ ~ directions ~orming a constan-t angle with the direction o~ said recei~ing arr~y 2 .,~

r~he signals transmitted about frequ~ncy ~
by the transducers o~ the transmitting array are delayed or phase~displaced relative to one another to form channels in directions forming a constant angle with the direction o~ the tra~smitting ante~naO
~he formation o~ the transmission chan~el~ tak~s place ~equentiall~ in time~
Thus, fi~e txansmission and reception cha~nels are successi~ely obtained with angular widths of the 3 d~ atte~uation diagrams - A/~ which ~
is the ~avelength c~rrespo~ding to the frequency f in the propagation medium and ~ the length, whieh is assumed to be equal 9 of the transmitting and receiving ar~ays ~hi~ prior art apparatus for ~orming fine angular channels has the disa~va~-tage of a low in~ormation rate~ because transmission must take place on all the transmitters the same ~umber of times that there are transmission channels.
It is also known to carr~ out acoustic imaging with a single transmitter which gives the desired ~ngular field~ ~he receiving array in this arrangement is formed by several li~es of transducer~ Processing of all the signals received by the tran3ducer makes it possible to form fine an~ular channels in two di~ectionsO If a large n~ber of channel~ is to be ~ormed~ which i~ generally the case when it i~ desired ^to obtain a fine angular resolution9 there i~ a large number of receivers and the ~olume oP the electric circuits i~ al~o c.o~iderable~
It i~ known to ~orm ohannel~ by u~ing acou~tic len~e~. Howev~r with thi~ proc~s~ the angulc~r ~ield remains limited ~o a few degrees and require~ a I'retina'' in the ~ocu~ing planeO

- 2 - -~ he appara~u~ according to the ln~ention obviates these disadvantage~ by transmission~ at codes dif~ering ~xom t~e tra~mittera ~ormi~g the transmit-ting array. ~hiæ procedur~ make~ it po~sibl~ to ~orm tra~mis~ion channelæ with reduced m~an~. In additlo~ imaging i~ ob-tained i.n real time.
More speci~icall~ the pre~ent invention relate~
to an imaging sy~tem maki~g it possible to obtai.n P x Q resolution poirks along coordinates o~
parameters ~ and 0 comprising a transmitti~g array constituted by M transmittersp ~E1~ ~?~ -~9 E~ ) simultaneously tra~smitting coded (~17 C2~ c~ ~ ~, the codes all being dif~erent and separable~ a r~ceiving array constituted by ~ receiver~, (H1, H2~ -o Hj J~O H~) receiving the signal~ dif~used by the tar~eta~ means for proce~ing the sig~als received and means for e~ploiting the processed signals, wherein the processing means comprise in combi~ation means for formi~g P an~ular channels called "reception channels"
along the coordinate ~ and means for forming Q
a~gular channels called ~Itransmi~sion channels on reception" along coordinate ~

~ he in~e~tion is described in greater detail hereinaf~er relative to non-limitative embodiments ~nd with reference to the attached drawings~ ~herein 30 show:
Fig. 1 a diagrammatîc representatio~l of the ~ormation of transmission and reception channels~
~ig. 2 a general diagram o~ the circuits ~or processing the reception signals according to the invention~

3 -. _ _ ... .

~ig. 3 a diagram o~ these proee~sing circuit~
in an embodîment according to the invention.
Figs~ 4 timing signals obtained by processing and 5 the reoeption signal~
~ig. 6 the demodula~ion diagram of the ~ignals received~
Fig9 7 the dia~ram of bringing the processed ` signals on~ the carrier frequencyO
~igs~ 8 two diagrams relating to the ~ourier and 9 transformatio~ by elastic wave dispersive filters.
~ig~ 10 the circuit diagx~m o~ a con~olution device with a non linear e~ect by ~ur~ace wa~es~
~ig. 11 a diagra~ relating tv the fo~m~tion of the tran~miæsio~ channel~0 ~igo 12 an embodime~t of a tra~smltting receiving a~tenna ~ith tran~d~cer~ in a ring ~xrange;
ment~
Fig~ 13 an appaxatu~ according to the inv~ntion with a receiving array in ring ~o~m a~ a tran~mitting array i~ linear f3rm~

~he apparatus according to the i~vent.ion comprise~ a transmitting array con~tituted b~ M
tran~mis~ion tran~ducers E1, E2 ~ i 9 ~ ~nd a receivi~g . comprising ~ receptioll ~ransduce~8 H1, H2, 00~ ig. 1). On this figure i.s sh~wn a line~r transm~ting array ln accordance with a dixection Oy and a linear receiving antex~a i~ accoxda~ce with the perpendicular direction Ox~
According to the in~tion the ~ran~mitti~g~
~5 r0ceiYing ~r~ay c~n be con3tituted b~ a sy~tem -- 4 ~

~ (3Lf1 of transducexs, whereof a c0rtain numb~r caM have transmis~io~ function~, whereas other~ ca~ have reception ~unctions~ It is possible to have transducers having both transmission and reception ~ctlons~ ~hese transdueers appropriately sample the space i~ ~uch a way that channels can be ~oxmed both o~ transmission and ~n reception.
~ he di~tribution o~ the tra~ducer~ can be o~
a random nature9 provided that their coordinates are ~no~nO However, the most frequently used arrangements are trirectangular~ spherical, cylindrical, circularg or planar.
~ he tra~smission~reception apparatus of ~ig. 1 makes it possible to form reception channels in d~rectio~ h1 and transmission channel~ in a me~n direction ~2. With directions Ox and Oz straight lines ~1 and ~2 respectively form angles ~ and ~.
It is possible to ~ee lines D1 and D2 of the limit curves o~ the 3 d~ attenuation diagrams of the reception channel~. In ~he same way D3 and D4 are the line~ of the limit cur~es o~ the tra~smission channels ~or the 3 d~ a-ttenuation diagrams~ If ~
is the length of two arraysg the widths of these channel3 are equal to A/~.
According to the invention transmi~sion is ~uch that transd~ce~S E1- ~2~ ~ 9~ ~I
spatial coordinates simultaneously -tran~mit M coded ~ignals C1~ C~o~Ci~o~jCM ~hich are separable a~d of the same band b and sa~e duration ~0 ~he total band occupied by all the tra~mis~ion i~
equal tv B. ~he mal~ pos~ible transmi~ion type3 ar~p for example-1~ M ~ignals at pure frequencies ~P~-~ 9~i9 ~ ~ ~ fM
o~ duration ~ ~requency-separated by b = ~ ~
M signals of the ~ame ~entre ~r~quency fO coded ~ 5 --in the same ba~d b i~ accordance with ~ orthogonal code~ o~ duration ~ with b~ 7MID
~. M sig~al~ coded in the s~e band b in accorda~ce with the same code of d~ation ~ with bT ~1 and of 5 di~Iere~t centre Irequeneies ~requellc; r ~eparated by the ~alue b9

4 ~ M signals coded .in the ~ame band b in accordance with Q orthogonal code~ o~ duration ~
with b~ Q a~d P dif~er~nt centre frequencie~ ~uch 10 that PQ ~ M ~requency ~eparated by the value bo Reception consists o~ recei~ing on the di~fere~t receiYers Ha the echQe3 comi~g from the tranqmi~sions, each receiviex receiving during a recurrence all the echoes ~rom the waves tra~smitbed by all the tra~smitters E1 to ~ , the signals produced by thes~ echoes b~i~g processed in -two stageæ:
1. Formation o~ reception rhannels ~k in several direction~ o~ angles such that ~ = 0k.
20 ~'ormation o~ transmission channels on receiving each signal o~ channel Vk i~ se~eral directions defined by angle~ such as ~i~
~ he principle o~ forming transmlssion channels on reception accordi~g to the invention in gensral terms conæists o~ using omnidirectional tra~smitters E~ to ~ distributed in space in accordance wi-th a particular geometr;y and txansmitting 1~ separable signals. In each directio~ determined by angles ~ and 0 o~ the space is propagated a sign~ S(~
formed b~ the sum in amplitude a~d phase of the slgnals transmitt~d in ~hi~ direction~ ~his signal S(~ depends on the direction and the geometrical position o~ the transmitters~
~or each signal o~ reception char~el Vk(ak) the formation o~ a transmission cha~nel of angle di~ection ~ ~ 0i consists of correlating ~ignal Vk(~ ) b~ 6i~nals Si 1~0i ' ~ he general 8ynop~ic~ 0~ the ~ystem is ~hownin ~igo 2~ S~stem 1 receiving ~ signals from N
receivers of the .,array forms P reception channels of ~ngl~ direction~ 2~ ~ ~k~ ' P
supplying ~ignals of channels Yk(Qk).
~ ach 5ignal of a chan~el such as Vk(~k) is t~ansmitted into a processing system 2 9 which pe~fo~ms the filtering matched to Q signals 13 Si k(0i~ forming Q channels~ In the drawing this operation is ~ym~olized by ~ . At the output of the P sy~tems 2 P Y. Q signals of channels Wi k(~ k~ are obtained and are then exploited in the treatment unity 3.
~5 ~he processing system 1 foxming the reception channels Vk can be obtained by two known methods related to the type of receiving a~ray ~nd the frequency band of the ~ignals Hi receivedO
~he firs-t method i9 applicable to all types of receiving arrays. ~he processing means u~ed on the signals received by all the receivers are depende~t on whether the value of the depth of recei~ing transducer arrav p is greater or sma~ler than C/B, in which C is the velocity of the wa-Tes in the propa~ation mediump ~ beîng the band width of the transmitted signals, ~he depth of the receiving transdicer array is defined by the maximum path difference of the waves between two receivers:
a) if p ~ C/~,devices cornpensating delays be~ween the sig~als from the recei~ers for ~he direction of the pr~or.~e.d ch~nel ~re used;
b~ if p < 5J~,ph~se-disp~acement devices are usea for comp~ns~ir~ ~he p~ase displacements between the sl~nals from the recei~ers for the direction G~ the preformed GhannelO

~3 3~

The ~econd m~thod i8 applicabl~ whe~ the receivi~g array iS ~ormed by squidista~t r~ceiver~ ~d when the antenna depth p i~ le~ than C~B. The proce~sing mean~
u~ed o~ the signals re~eived by all the receiYers i~volve a multipleæi~g of the signal~
received in a ~mall interval of time compared with 1/~ supplying a sequence o~ the signal~ in serie~
in time 9 ~ollow~d by a digital or analog device which e~ects the ~ourier transform o~ thi~
~equence. Such a channel formatio~ device using a digital method is described ~n an article b~
J~ R. ~ IAMS in I~Journal o~ Acou~tical Society of America"~ volO 449 59 pp~ 1454-1455, 1968 ~he processing ~y~tem Z fo~ming the transmis~ion channel~ by matched ~ilter mg ~an be realized either by proces~i~g eaoh ~ignal of transmi~ion channel -~(9k) in parallel in Q
~ilters matched to Q signal~ Si ~(0i~ ek),i ~arying between 1 and Q~ or by proces~ing ~ach signal of reception channel ~k(~k~ ~equentially `by convoluting it with Q time rev~rsed copies o~
sig~a~g Sip~(0i~ 0k)~
~he preferred embodiment of the invention involves a transmitting arra~ ~ormed by a linear c~rray o~ transmitters and a receiving a.rray ~ormed by linear e~uidist~nt receivers.
~he two arrays are po~itioned orthogonally to one another~ a~ indicated in ~igo 1. ~he txansmis~ion band B a~d the ante~na depth p are cho~en so as to be able to form reception channels ~k(~k) by the method u~ing a ~ourier transformO
~ccording to the preferred embodiment o~ the inYention this ~ourier tr~ns~orm is produced in analog ma~er by the use of elastic wav@

dispersive fil~ersO ~he transmission channel~
are fo~ned by sequential correlation of recsptio~ channel sig~als with æignals Si k obtai~ed b~ their con~olution with time-reversed ~ig~als Si k. Accordirg to thepreferred embodiment o~ the invention thi~
con~olution iæ carried out in analog m~nner by the use of an elastic wave convolvers. ~his has the advantage o~ permitting high operating ~peed~
and leading to reduced o~erall dimen~ion~
~ he pre~e~red embodiment o~ the l~ention i9 æho~m i~ ~g~ 3 and Figs. 4 and 5 ~how the Main tim~ng signal~ relati~g theretoO
~ he n~t ~how~ tranæmitting array comprise~
a linear a~ray of ~ tran mitteræ receivi~g ths ~oded signal~ C1 to CM from the ge~erator. ~he M tra~æmi~sio~s are produced s~multanevu~ly, their duration being T and thelr ba~d b~ ~he po~sible tran~mi~is~ ~ypes are as de~cribed herei.nbe~ore~
~he centxe ~requency o~ these tra~smi~sions is io and the total ba~d occupied i3 ~O
~ he æignals are received by a linear receiving array fo~med 0~ ~
equidistant receivers ~1 to HN~ e~g~ hydrophoneæ
i~ ~he ca~e o~ a æo~ æO ~hese signal~ are demodulated by N circuits 60.1, 60.2~D~60.N
shown in ~ig~ 6, ~ach ~i~nal received by a receiver such as H~ is mult.iplied in M~ and M2 by two ~ignals co~ t) and ~in(2~f phase quadratul~e, æupplied by a local o~cillator~
In this way sig~als Hj ~ and H~ 2 are ob-tained af-ter low pa~ filteri~g in 161 and 167 with a cut-o~ ~requency cloæe to ~/2~ said two ~ig~al~
being called the real component and the imaginary component. ~wo synchronous mul~iplexers 61,1 and ; ~ ~

61.2 multiplex the real and imaginary compo~en~s of the ~ demodulat~d ~ignal~ H;j 1 a~d Mj 2 These multiplexing cireuits ar~ controlled by a timing sig~al H of p~riod ~H" l,Yhose value is chosen so

5 that N ~ignals by a receiver such as Hj are multiple~ed in a time interval N~ ~ which is small compared with l/Bo F~ hermore the signal o:~ each hydrophone iæ sampled at frequency :B and at ~he output o~ each multiple~cing cixcuit a 10 signal is obtai~ed which is form~d by group~ of signals o~ duxation N~ ~eparated by a time 1/~
~hese two signals 62 ,.1 and 62 0 2 obtained at the output o~ the two multiple~ers 61 ,1 and 61. 2 are modulated in a circuit 62 shown in ~ig. 7D ~hey 15 are multiplicd in multiplying circuits M~ a~d M3 b~r the two signals cos (2 ~zt~ ~d sin~2 al~zt~
~upplied by a local oscillator, then the two ~ignal~ oktained are added in an adding circul-t ` 7001 and the ~ignal obtained i~ filtered in a low pa~s ~iltering circui~ 70.2 o~ width ~ and o~
centre ~requency ~z~ ~and ~z i~ a ~unction of the multiplexi~g ~requency ~f ~ignal~ such a~
H~ 1 and Ej 2 and the celltre freq~ency f~ i~ made appro~imately three times the value of band B~o ~he output ~ig~al 6203 from modulator 62 i~
diagra~matically shown in ~ig~ 4aO ~ver a time ~ it is ~ormed by E = ~1 group~ of ~ig~als 400l5 40~2~o~o 40.~0 Each group corresponds to the ~ignal~ recei~ed by recei~er~ H19 H~g~oH~ in ~erie~ with period TH9 These ~i.gnal~ have a band B~, a cen~re ~requenc~ fz and a duration ~
equal ~o ~9 which i~ small compared with 1/~o ~ hi~ ou~put ~ignal 620~ ~xom modulator 62 i~
transmitted into an analog ~ourier transformer 63 u~ing ela~tic wa~e dispersive ~ilter3 an~ fo~

which two embodiment~ are po~ible~ ~ourier tra~ormers by dispersive ~ilters are more particularly described in the article b~
C~ IARDA~ in '7Prooeedi~s 1978 IEEE Ultra~o~ic S;ymposium~7 pp~ 518--521~ 19780 ~he fir~t embodiment ~hown in ~ig~ 8 i~ a so called ~;C-M arrangement ~multiplication-convolu-tioll-multiplication~. In this arrangemen~
the input signal 62~3 i~ pre multiplied in a multiplication cir~uit 82 by a ~ignal called a chirp,which is linearly modulated in ~requency and whose band is Bz and ~hose duration i~ ~O
This signal is obtained by ~upplyi~g a very short pulse to the i~put of a dispersive filter 81 ~he signal at the ou~put of multiplier 82 is convoluted in a dispersive ~ilter 83, who~e band i~ 2~z a~ ~hose duration is 2~z e ~he signal obtai~ed is then post~multiplied i~ a ci~cuit 84 by a ehirp signal which is identical to the a~orementione~ sig~al genexated by dispersive filter 850 The Fourier tra~sfo~m signal of the input signal 62.3 i~ obtained at output 64 o~ multipliex 840 ~he second em~odiment shown in ~ig. 9 i~ a so-call~d C-M-C a~rangement (convolutlon-multipl~cation-convolu~ion). In this arrangement the input signal 62~3 i~ firstly con~olu~ed i~ a ~ispersive filter 91 of band ~ and dul~atio~ ~z 9 then multiplied in 92 by a chirp ~ignal supplied by a dispersive filter 93 of band 2~z and d-~ration 2~zr The signal ob-tained at the ou-tput of multiplier 92 is then ~ransmitted into a dispersive ~ilter 93 identical to ~ilter 91.
~he ~ourier transform signal of signal 620~ is obtained at the ou~put o~ dispersive ~ilter 94 at 64i The output signal 64 OI a ~ouriar transform 63 15 dia~;rammatica].ly ~hown ln ~ig. 4bo I,ike the i~put signal i~ is Iormed over time ~ by E- :BT
group~ oX ~ig~als 41 o 19 41.2, . 9 . 9 41~ ~o Each 5 gxoup co2l~ist~ o~ ~ signals o~ channel~ V1 to V~
in s~ries in time o~ period TH alld period N~ o ~hey are ~taggered by the time take~ for carrying out the FotLri~r tra~ o rmation :~ompared with input signal~s 62 1û ~he ~ourier tran~orm signal 64 i~ the demodulated in circuit 164 (~ 3) ha~ing a~
identical ~tructure to circuit 60 in which the phaee quadratlLre signals are cos ~ 2 T~ and 5in ( 2 ~ ~2t ~ . The two compo~e~s obtai~ed are 15 converted i~Lto digita~ orm ~ ~wo ~oll~rerters 6501 and 65.2 controlled by timi:~g eircu~t lI alld 8 tored ~eparately in digital memorieæ 66 " 1 and 66.2, ~aid memorie3 bei~g ~ar e:~ample o:i~ the RAM
tgpeO :Each group of cha~el 3ignals ~uch as 410i 20 i~ e~tered in the memory in i;he :Eorm o~ a lineO
The memor3r is thu~ :Eilled ~eguentially line by `` line at rate 1/B and contaiIl3 1~ lines. ~he signal~ stored in this way are ~equentiall~ read column by column at 3peed 1/~ h0 memory is~ fox 25 e~ample ;, su~di~rided into two block~ which are use alte~nately i~vol~i~g filling one block line b~
line and simultaneo~sly reading, a~ well as the o~her bloc-k column b~ ¢olumng ~ollowed by the ~witchi~g o~ blocks.
The txan~mitting array i~ formed by a lineax array o~ transm.itters; signals S~ 0k3 defined hereinbefore bei~g depe~dent o~ly on the directions defined b~ angl8 ~i with the tran3mitting ante~na a~d c~n the~ be expressed according to S~ 0 ~ he ~ystem comprises digital memories 70,1 a~d 70.2, e~g. of the RAM type and which contaI~
demodulated~ time-reversed copies o~ the Q signals Si~0i) in the form of real and imaginary compone~ts~ These Q ~ignals are stored for time T at frequency ~ a~d thus compri~e K sample~
Two signals V~(~k~ and Si(~i) are read in synchronism xespecti~ely Lnto memories 66,1~ 66.2 and 70.1~ 7002 u~der the control of timi~g circuit H. Their dura~io~ is identical and equal to R~.
~he ~ame signal of channel V~(~k~ ~ read Q time~
~ynchron~usly with the Q signal~ S~ to SQ0 ~igS3 5a a~d 5b respectivel~ show the signals o~ channels Vk and the ~ignal~ Si a~ a function of timei ~ach ~ignal is composed of Q groups of ~ signal~ i~
series in timeO ~he reading ~peed 1~ of ~ignals Vk or Si must there~ore be e~u~l to or greater than QR~H e ~he output signal3 66~6, 66~7 and 70.6, 70~'l ~rom memories 66 and 70, as shown i~ ~igs. 5a and 5b are co~verted into a~alog form by co~verter~
67~1, 67,2, ~odula~ed by the two cixcuits 68,1~ 68,2 and applied to the two input~ of a c~nvolver 69 using the propaga-tion of elastic waves.
~hese signal~ read into the m~ories are modulated ab~ut the ~requenc~ fæ and in band ~æ~
circuits 68.1 a~d 68~2 bei~g identical to circuit 62 of .Fig. 7. ~onvolver - ~9 operate~ arou~d the ccntre ~requency ~z and around band ~z 9 being ~hown in Fi~ 10~
~ he knQwn acoustic ~ur~ace wave deviGe 100 (Figo 10~ ls pr~vided with ~wo electr~d~ 102 and 103 o~ le~g~h ~i ~he ~ignals to be con~ol~ted F(t)ei ~t and G(t~ej~ in which ~ i~ the angular frequency o~ the carrier are applied t~ interdigital .. . .

tra~sducer~ ~ 13 ~ placed at the two ends o~
device 100. ~ a result of no~-linear e~fect~
between ~he two electrodes is collected the convoluted ~ignal ~(t3 such that R~t3 ~ Ce2a ~t J~(t3G~2t~)d7 in which C i~ a constant~
If v is the ela~tic wavs velocit~ the i~teraetio~ length ~ equal to ~HX v. ~he output 9ignal of the convolver i~ repre~e~ted ~0 in Figo 5c~ I~ consists of channel ~k ~ Q
correlation signals separated b~ a time equal to K~H and of ~entxe frequency 2~z relating to channel ~ignals ~ormed in accordance with Q dixections ~i ~or direction ~k or W1,k to WQ~k.
lea~e the cQnvolver with a time di~placeme~t equal to the dura~i~n ~T~I ~ the input signalæ~
~ he ou~put sig~al .~from the ~onvolver is then demodulated in circuit 71, ide~tical to circuit 60 shown in Figo 6 in ~hich the ~requency ~0 is replaced by the frequency 2~z æupplied by a local o~cilla~ox~ It is then converted into digital ~orm by co~verter~ 71 D 1 and 71.2 for ~torage in a memory or ~or e~ploitation. ~hese sig~als suppl~ an image in two dimen~io~s ~7 p for a duration ~.
If the transmitting arr~y is o~ a random nature in this case memories 70.1~ 7002 must 3tore the copies o~ Q x N signals Si k(0i9 ~k) and their ~ize is grea-~er. ~henever a signal o~ channel V~ read in memories 66~1 ~nd 66.2r Q oorresponding signals S~ ~(01~ ~k~ to SQ k(~Q~ ~) are read i memorie~ 70~1 a~d 70~2.

~ n acoustic lmagin~; system according to th~
invention with orthogonal linear arra~Js :'or txansmission and reception haæ the ~ollowing ~ha;racteri3tic~:
5 - n~mber of trallsmitters M = 50 number OI receivers ~ = 50 - ~pacing between tra~smi tters a~d recei~er~
d - 0~75 mm - mea~ ~requency of transm~ ions ~0 = 3 MHæ
10 ~- pulse ~ime ~ a 10 ms - total band :B = 5 kHæ, i.eO 1/:B - 200 ,~s ~gulæ ~leld in ~3 a~d in ~ = ~ 20 degreeæ
- depth of arrays in Itime - 20 ~s - clock period TH ~ 50 ns 15 ~ ba~d Bæ ~ ~ MH~
- ~requency ~z o 100 I~Hz.
~ or this embodime~t the number o~ çha;nnels Q
is take~ aæ equal to ~. Moreover product :B~ ~s equal to Q ~ ~uch a way that ~ ~ M - Q - ~ ¢ ~0 The condi~io~ <1/B is satisfiedO ~he dura~ion o~ the ~ignals a~ the i~put and outp~t o~
A the Fourier txan~former 9 a~ w~l a~ at the con~oluto~ i~pu~ is equal to 205~D
A con~tx~ctional ~ariant is ~how~ in ~ig. 110 I~ applies ~he parallel proce~si~g re~erred to .~ereinbe~ore. ~he output ~ig~al ~or eaeh channel ~uch a~ Vk(0k) ~rom mod~lator 6~2 ~o 68~ igo3) is simultaneously transmitted to the input of Q
filters 110~1k, 1~002k, 99 o110~Qk matchedto signals Sl k ~ SQ k ~he~e filters are, ~or example9 reali~ed b~ using the propagation of t~e elastic wa~es, the duration of ~he pulse response of ~ach ~ilter correspo~ds to tIme ~ ~nd each filter lead~
to a proce~sing equivale~t to one convolution; whil~t the outpu~ signals ~rom each matched filter have a __ maximum representing the signal from a direction ~i 9 ~k-In the case where the tra~mit~ing array is formed b~ a linear array of ~ran~mltters the ~y~tem of Q filters 1100~k to 110~Qk remains identical for the N signal~ of ch~nnel~ Vk(~k) and can be multiplexed, ~ variant of the inventiQn is ~hown in ~ig.
12~ which represents an array formed by a ring of tran~ducers of diameter Do Each transducer is both a transmitter and a recei~r. In the tr~nsmission function the tran~ducers simultaneously supply N coded tr~nsmis~ions. Reception channals are formed in a plane perpendicular to the plane of the antenna by means in accordance with the first method described hereinbefore9 In this plane the direct.ional pattern obtained by the reception channel has an angular width of appro~imately ~o if ~0 is the mean wavelength of the transmissions, ~ransmission channels are then formed in the plane orthogonal to the reception channel formation plane by one of the two mean~ referred to hereinbefore. In thi~
plane the directional pattern obtained by the transmission channel also ha~ an an~ular width of approximately ~ .
~ ig. 13 shows-~n array formed on the one hc~nd b~ a ring of trc~n3ducers 30 of di ameter D and on the other by~a colu~n of transducers 120 of height h2.
Colu~n 13.1 is formed by omnidirectional tr~nsmitters in c~ng-e direetlon ~ said trarlsmitters simulta~eously ~upplying cQded transmi~sions~ Ring 120 is formed by receivers of height h1 in ~uch a wa~ that the directional patte~n in ~ccordance with o~

angle ~ ha~ an angle width o~ appro~imately ~o~
R~ception ch~n~els in accordance with the1 ang~e ~ are ~o~med over the complete horizon7 iOe.
O to ~60 by mean~ corxe3ponding to the first method de~cribed hereinbe~ore~ ~he directional pa~tern obtained by the receptio~ cha~nel has an ~gular width of approximately ;~oO ~ran~mission cha~nels in accordance with the angle ~ are the~
formed b~ one of the two means described herei~efore, the directlonal pattern obtained by the tra~smis~ion channel has a~ a~gular width of approximately ~ and its value is well below To increas~ the trarlsmitted ~ower it i~
possibie to replace each transmitter sueh a~ 13 0 of the column by a ring identical to that fo~med bg the tran~ducers in such a way that a cylindrieal arr~y is obtained.
It is also possible to re~exse the func~ion~
of the ring and column~ the ring then being formed by transmi~ters, each supplyi~g a ~ifferent code a~d ~he co7umn being formed by receiversO In ~hi.s case the reception channels can be fo~med by a ~ourier transform in the case of a narrow band~
Xn this way a panoramic imaging sy~tem is obtained.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An imaging system comprising:

a transmitting array including plural omnidirectional transducers each simultaneously transmitting respective differently coded signals;

a receiving array including plural tranducers each adapted to receive signals transmitted by said transmitting array and reflected off a target, each said receiving array transducer producing a respective output signal; and means for processing the output signals produced by said receiving array tranducers, comprising, means for forming P reception channels and generating respective reception signals V1(.theta.), V2(.theta.2),...Vk(.theta.k),...Vp(.theta.p) corresponding respectively to predetermined increments .theta.1, .theta.2,....theta.k,....theta.p along a predetermined angular coordinate .theta., means for forming Q transmission channels along a second angular coordinate .PHI. and for generating respective signals Si,k(.PHI.i, .theta.k) corresponding respectively to predetermined increments .PHI.1, .PHI.2,....PHI.i....PHI.q along a predetermined angular coordinate .PHI. for each of said reception channels, wherein the signals Si,k(.PHI.i, .theta.k) are the sum in amplitude and phase of the coded signals transmitted by said transmitting array in the direction referenced by angular coordinates .PHI.i and .theta.k, means for performing matched filtering of each of said signals Vk(.theta.k) with a respective set of said signals Si,k(.PHI.i, .theta.k) and for producing respective output signals, and means for forming P x Q image points based on the output signals produced by said matched filtering performing means.

2. An imaging system according to claim 1, wherein the matched filtering performing means comprises:
Q filters in parallel for each signal of channel Yk(.theta.k) .

3. An imaging system according to claim 1, wherein the matched filtering performing means comprises:
means for performing the convolution of each signal of channel Vk(.theta.k) with Q time reversed signals Si,k(.PHI.i,.theta.k)

4. An imaging system according to claim 3, wherein the convolution performing means comprises:
an analog convolver using the propagation of elastic waves in solids, means for modulating the signals Vk(.theta.k) and Si,k(.PHI.i, .theta.k) , about a center frequency and in a band permitting the use of such a convolver, and means for sequentially convoluting channel Vk(.theta.k) Q times with the Q time-reverse signal Si,k(.PHI.i,.theta.k) in a time less than 1/B, B being the frequency band of the signals received.

5. An imaging system according to claim 1, wherein the receiving array comprises:
a linear array of N equidistant receivers wherein the depth of the receiving array tranducers in the most inclined direction is less than C/B, C being the propagation velocity of the signals received and B
being the frequency band of the signals received.

6. An imaging system according to claim 5, comprising:
a demodulation circuit, having applied thereto the signals received by each of said receiving array transducers, for supplying respective real and imaginary components of the corresponding receiving array transducer signal, and first and second multiplexers respectively having applied thereto real and imaginary components supplied by said demodulation circuit, the period TH of the multiplexing operations being such that NTH < 1/B.

7, An imaging system according to claim 6, comprising:
a Fourier transform circuit having the two multiplexed signals applied thereto for supplying the Fourier transform of these multiplexed signals.

8. An imaging system according to claim 7, comprising:
analog means for performing the Fourier transform in an analog manner, including, a system of dispersive filters using the propagation of the elastic waves in solids supplying N
signals of channels Vk(.theta.k) sequentially in time for a duration equal to NTH and which is well below 1/B.

9. An imaging system according to claim 8, comprising:
means for demodulating the two multiplexed real and imaginary components about a center frequency and in a band permitting the use of said dispersive filters.

10. An imaging system according to claim 8, comprising:
means for demodulating, the signals of channels Vk(.theta.k), and means for converting the demodulated real and imaginary components obtained into digital form.

11. An imaging system according to claim 10, comprising:
a digital memory for storing the real and imaginary components of the N channel signals.

12. An imaging system according to claim 6, comprising:
means for sampling the signal from each receiver at frequency 1/B such that N channel signals are supplied in the time interval 1/B.

13. An imaging system according to claim 11, comprising:
means for writing data into the digital memory line by line, each line storing N channel signals corresponding to N directions .theta.1 to .theta.N, wherein the memory is filled at the end of time T and has BT lines.

14. An imaging system according to claim 13, comprising:
means for reading the digital memory column by column at speed 1/B and successively supplying the signals of channels Vk(.theta.k) for the duration T in the form of two real and imaginary components having BT
samples, wherein the BT samples are read at period TH.

15. An imaging system according to claim 14, comprising:
means for converting the two components of each signal of channel Vk(.theta.k) into analog form, and means for modulating the converted analog signals about the center frequency and in the band of the convolver.

16. An imaging system according to claim 15, comprising:
another digital memory for storing the N x Q
copies of the signals Si,k(.PHI.i, .theta.k) of two real and imaginary components, each signal having BT samples, including, means for reading in each signal in the memory at speed 1/B and the BT samples of each signal at TH, means for converting the two components into analog form, and means for modulating the two components converted into analog form about the center frequency and in the band of the convolver.

17. An imaging system according to claim 16, comprising:
the elastic wave convolver having applied thereto in synchronism the two modulated signals Vk(.theta.k) and Si,k(.PHI.i, .theta.k), their duration being equal to BTTH, wherein QBTTH is chosen lower than 1/B.

18. An imaging system according to claim 5, wherein the transmitting array comprises:
a linear array of transmitters positioned orthogonally to the receiving array.

19. An imaging system according to claim 2, comprising:
means for transmitting each signal Vk(.theta.k) to said set of Q filters matched to signals Si,k(.PHI.i, .theta.k) whose pulse response has a duration T.

20. An imaging system according to claim 19, comprising:

means for modulating each signal Vk(.theta.k) about the center frequency and in the band of the matched filters, and said matched filters comprising analog filters based on the propagation of elastic waves in solids.

21. An imaging system according to claim 1, comprising:
means for controlling said transducers to successively fulfil the functions of transmitters and receivers, and at least of the transducers arrays arranged in ring-like manner.

22. An imaging system according to claim 1, comprising:
said transmitting array transducers arranged in linear manner, and the receiving array transducers arranged in ring-like manner.

23. An imaging system according to claim 1, comprising:
said receiving array transducers arranged in ring-like manner, and the transmitting array transducers arranged in linear manner.--