CA1189609A

CA1189609A - Diversified transmission multichannel detection system

Info

Publication number: CA1189609A
Application number: CA000389229A
Authority: CA
Inventors: Pierre Tournois
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1980-11-04
Filing date: 1981-11-02
Publication date: 1985-06-25
Also published as: DE3177278D1; FR2493528A1; EP0053048B1; EP0053048A1; FR2493528B1; US4458342A

Abstract

ABSTRACT OF THE DISCLOSURE

The invention relates to detection systems by the transmission of sonar or radar signals. The invention relates to a detection system associating diversified transmission means with an interferometric base. This base supplies angular channel formation means and each signal formed in this way is processed by matched filtering in a circuit containing copy sig-nals characterizing the space colouring obtained by the diversified transmission means. The invention is more particularly applicable to side or front looking detection sonars.

Description

~\
6Q~I
' Diversified transmission multichannel detection system BACKGROUND OF THE INVE~TION
The present invention relates to detection systems by echoes of the radar or sonar type in which an angular sector is the source of electro-magnet or ultrasonic radiation transmissions. The determination of the content of the sector follows from the detection and detailed analysis of the signals reflected by the points of the space which were irradiated or insonified. This analysis supplies information on the angular position and distance of the targets. On the basis of this information, it is possible to form on the screen of a cathode ray tube an image representing the scanned sec-tor. The distance of a target from the transmission - reception system is a function of the time elapsing between the transmission of a pulse and the reception of the echo corresponding thereto. The angular position of the target may depend on the directivity of the transmission and/or reception means. The invention more particularly relates to echo detection systems in which reception involves the use of two receivers, whilst the waves are simultaneously received by a system of radiating elements.
Such an association of transmission and reception means has already been proposed in U.S.
Patent 37716g824 granted on February 13th lg75. This U.S.Patent describes a sonar apparatus in which two receivers form a reception channel, whose directivity involves a fan-l;ke lobe arrangement. A line of transmitters supplied by a pulse-modulated mono-chromatic carrier supplies a very selective insonification of the sector to be monitored, which only covers one of the reception lobes. In this procedure, monitoring is essentially of a punctiform nature and much less extensive than the angular sector in which the reception means can detect the echoes. This system can only supply a detailed image of the sea bed by an appropriate mechanical scanning because only a single channel is used on transmission and reception.
~RIEF SUMMARY OF THE IN~ENTION
The present invention aims at obviating these disadvantages by offering the possibility of providing monodimensional front or side looking sonars, as well as bidimensional front loo~ing sonars. These extensive field sonars make it possible to display objects on the sea bed. Without passing beyond the

2~ scope of the invention9 reference is also made to medical imaging equipment and non-destructive testing equipment. In order to achieve this result with only two transducers as the reception means9 a diversified transmission is adopted consisting of applying differen-tiated excitations to the transmission transducers.Such a transmission is said to be coloured, because it assigns to each direction of the morlitoring sector an illumination law inherent thereto and which charac-terizes each echo as a function of its source. The thus personalized echo can be perceived in a univocal 39~

manner by a system o~ reception channels. Moreover, knowing a priori the diversified composition o~
the echo producing radiation, it is possible to carry out in the reception channels processing by correlation providing a greater resolution in connection with the processing of the angular data.
The present invention more specifically relates to a diversified transmission multichannel detection system comprising transmission means using a system of adjacent radiating elements positioned at the apex of a predetermined angular sector and reception means using a pair of receivers centered on said apex for detecting echoes from said sector, wherein the transmission means comprise means for the simultaneous pulse excitation of the radiating elements assigning to each of them a signal shape or form permitting its identification as an element of the system, the reception means comprise a plurality of ~ception channels connected to the ; 20 receivers by channel forming means associating with each of the receiving channels a surace with a constant step difference belonging to the said angular sector, each of the receiving channels being connected to a filter matched to the particular -configuration of the echoes coming from the surface with a constant step difference.
BRIEF ~ESCRIPTION OF THE DRAWINGS
The invention is described in greater detail hereinafter relative to non-limitative embodiments and the attached drawings, wherein show:

-3-~i39~ 9 Fig 1 a system of radiating elements making itpossible to perform a diversified transmission.
Figs 2, 3, 4 and 5 various band structures of signals which can be used in a diversified transmlssion. Fig 6 the diagram of a directivity-free reception making it possible to obtain a selection in association with the transmitter system of Fig 1~
Fig 7 a variant of the diagram of Fig 6. O Fig 8 a detection system using an interferometric base with a non-diversified transmission.
Fig 9 a detection system according to the invention.
Fig 10 an overall diagram of the system of Fig 9.
Fig 11 a first variant of the system of Fig 10. 5 Fig 12 a second constructional variant of the system of Fig 10~
Fig 13 a third constructional variant of the system of Fig 10.
Figs 14 and 15 explanatory drawings.
Fig 16 a side looking sonar.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Fig 1 shows an electromagnet or ultrasonic wave transmission system 1 making it possible to effect a detection by echoes of the radar or sonar type within the scope of a diversified transmission.
The radio space or the marine propagation medlum are related to a system of axes u~ v, W7 whose origin O located at the centre of system 1 also represents the apex of an angular monitoring sector. The trans~
mission system 1 comprising, for example, a matrix

-4 arrangement of radiating elements El l...El ...E.
...Em n which, in the case of a sonar, are electro-mechanical transducers able to sound the monitoring sector in a uniform manner. Each radiating element of system l is excited by a transmission generator 2 supplying a brief pulse used as a re~erence to an output 3. A system of connections 4 supplies to each of the radiating elements of system 1 pulse signals 5, whose path is shown in Eig 1 to the left of generator 2. The duration of excitation s;gnals

5 is T and their shape S(t) is chosen so as to be able to identify each radiating element by the content which it transmits. For a number m x n of radiating elements, it is possible, for example, to assign a separate transmission frequency to each radiating element in the manner shown in Fig 2. If the frequency band of the transmission covers a range B limited by the frequencies min and fmax~ it is clear that this procedure involves using m x n signals of band b with b - B~m x n)- l/T. Thus, m x n carrier 1,1' fl~2--- fm n are us2d separated from one another by the ~alue b. Frequency~shifted band pass filters are used for identifying these signals.
According to a different procedure, it is possible to use a single carrier f and distinguish the signals by modulation using m x n orthogonal codes, eacn occupying band B. This procedure is illustrated in Fig 3 in which Cl l....C represent 0 identifiable orthogonal functions of duration T.

~9~

As is shown in Fig 4, it is possible touse as other coding modes a code C of duration T
which modulates several carrier frequencies displaced by a value b' exceeding l/T. It is also possible to provide W carrier ~requencies modulated by p x q codes C ...C of duration T with m ~ W in 1,1 p,q p x q the manner shown in Fig 5.
Whatever the coding mode used for exciting the radiating elements of systern 1, it can be seen that the sounded space is "coloured". Thus, Fig 1 shows a dividing into squares of the space by r lines parallel to v and by s lines parallel to u.
One of the points Pi k of the s~stem of r x s intersections collects a transmission composed by m x n discernable values. The polar coordinates i,k' ~i,k~P i,k of the point Pi k deterrnine with the coordinates of the radiating elements E n a particular type of irradiation which it is possible to calculate.
In order to fix the ideas, on assuming that the radiating elements of Fig 1 are reduced to a - single row of M elements El, E~...Ej,... ~ and by designating a poin-t in the coloured space as Pi, it is possible to represent the irradiation received in Pi by the forrnula:
M

S(Pi)- j~l Cj(t ij) in which Cj(t) is the coded excitation applied to the radiating element Ej and ~ij is the propagation time along a line joining Ej and Pi. This simplified

6~g .- ~. ,~

formul~ takes no account of the attenuation as a unction of the distance.
In general te~ns, the radiating elements can be posltioned in a random manner, provided that they appropriately sample the space and that it is possible to use identifiable codes for predicting and recognising without ambiguity the combinations represented by this colouring o~ the space.
The transmlssion diversity described herein-before with reference to Figs 1 to 5 plays a vital part in the detection of the echoes. Thus, a target receiving the coloured transmission reflects a coloured echo which can be separated from the other echoes by a correlation based on the knowledge a priori of the composition of the echo. At each point, Pi k of the space a signal S(Yi k) is received, which is formed by the sum in amplitude and phase of the codes transmitted by the transmitter El 19 ' . . E
These codes are received at point Pi k with a combination of delays dependent on the geometrical positions of the transmitters and of point Pi k.
For other points such as Pi k + 1' Pi 1 k~
Pi 1 k + 1~ the combinations of the delays differ and therefore the signals S assigned to these other points also differ. l`hus, the signal S(Pi k~
is dependent on the codes C transmitted, the spatial coordinates o the point in space forrning the source of an echo and the spatial coordinates of the radiating elements of t ansrnission system 1. If the

-7-3~i~9 points Pi k are far enough away from the transmission system, the signals S(Pi k) are dependent only on the coordinates ~i k and ~i k.
Fig 6 shows how, at reception7 it is possible to process a coloured transmission~ whilst wave receiver 6 has no inherent directivity. The echo signals are received by receiver 6, which supplies a complex electrical signal at the input of a system of angular channels. Each angular channel is allocated to an echo source direction, which in Fig 1 is the straight line passing through 0 and through Pi k.
For example, receiver 6 can be located at point 0 and the receiver system o Fig 6 may comprise r x s angular channels. ~ach angular channel is constituted by a filter 7 matched to the signals S(Pi k) from which emerges a correlation peak V(Pi k) The matched filter 7 of the angular channel (~i k~ i k~ i k) is designed to react to the theoretical combination S(Pi k) of the transmissions produced in Pi k by the transmitter system. Thus, diversified transmission leads to the a priori definition of a coloured space giving the system of Fig 6 reception properties provide~ wi~h an angular selectivity. Fig 7 illustrates a constructional variant of a receiver system having the same angular selectivity. Filters 8 matched to code C are supplied by the single receiver 6, which receives the echoes from the c~loured space. ~le filters 8 matched to each of the transmitted codes used for exciting the radiating elements of the transmission system 1 separate each of the codes of

-8-6~1~

the system of codes C~ C1~2' Cl,n CmnThe correlation peaks supplied by the matched Eilters 8 are then processed by an angula~ channel formation circuit 9 having m x n inputs connected S to r x s outputs. Dotted lines represent the group within circuit 9 for compensating delays ~ij of the codes used for orming the channel V(Pl 1) This channel is 3erved by sign~ls from the m~x n filters 8 added to the time lags which, added to the delays with which the elementary transmission arrive at point Pl 1~ give a constant total delay or time lag.
The receiver system of Fig 7 is equivalent to that of Fig 6 but better illustrates the "imaging1' function of circuit 9 which~ although located in the reception section of an ecno system, provides a separating power and an energy cont-rast based on the geometry of the transm-Lssion means. To this end, forming circuit 9 is a so-called "transmission channel formation circuit on reception".
Before defining a system according to the invention which is based on the use of a diversified transmission and on a detector sys~em using two receivers Rl and R2, it is of interest to demonstrate the ambiguity of a system having two receivers using a non-diversified transmission produced by a single non~directional transmitter E.
The diagram of such a system is illustrated by Fig 8. The single transmitter E is excited by a generator 2 which, for example, emits pulses of dura~ion T and carrier frequency fO. The receiver ~l~1396~9 system comprises two receivers Rl and R2 positioned on either side of transmitter E. ~ delay line 10 producing a delay ~ and connected to a su~mating circuit 11 forms an angular channel able to select echoes from a hyperbolic surface 15, which is a geometrical locus of step difference ~ for receptions effected by the two receivers Rl and R2~
Signal D leaving summatin~ circuit 11 is associated with any echo from layer 5, but ln the absence of a colouring of the space by diversified transmission signal D can be the source of serious confusion in locating the echoes. In order to illustrate this amhiguity it is merely necessary to consider two echoes coming from separate targets lo and 17.
Receiver Rl receives the echo from target 16 with a de~y ~ 1 after the transmission vf a pulse by transmitter E~ This delay ~ 1 defines a geometrical locus in the form of an ellipsoid of revolution 12 with sources-Rl and E. In the same way, receiver R2 receives the echo ~rom target 17 with a delay ~ 2 after the transmission of the same pulse by transmitter E. This delay ~2 defines another geometrical locus in the form of an ellipsoid of revolution 13 having the sources R2 and E. If ~2 ~ , the signal D due to the reflections on the two targets will be formed in the same way as if it resulted from a single echo coming from one point of the hyperbolic surface 15. This explanation shows thak the device of Fig 8 is unsuitable for unambiguously locating echoes.
-i0-~l~896~3 In order to obviate this disadvantage the invention proposes effecting univocal location by us;ng the diversified transmission method.
In Fig 9, it is possible to see that the transmission is obtained ^rom the system of radiating elements E ...E forming, for example, a line. In 1 m addition,- the angular channel formed is provîded with a matched filter 19. Generator 2 supplies M
separable e~citations producing at point Pi a composite insonification characterizing this point.
It is possible to calculate this insonification tak:ing account o the codes used and the dotted line paths towards point Pi. When this insonification has been calculated, it makes it possible to perfo~m a matched filtering in filter 19 in such a way that the signal of channel V obtained cannot correspond to other points of the coloured space.
Thus, the system is no longer subject to the detect-ion ambiguity referred to in connection with Fig 8.
The two receivers Rl, R2 make it possible to subdivide the space into several hyperbolic surfaces 15, 20, 21 corresponding to constant step differences. It is therefore possible to forll several angular channels by associating with each layer a delay line 10, whose delay ~ corresponds to the step difference PiR2 ~ PiRl. When point Pi is far enough away from the transmitter line, each hyperbolic surface, whose path lS can coincide with the ?ath 18 of the asymptotic cone, whose axis is the line joining receivers Rl and R2. This cone is characterized by the angle ~i which lt forms with the plane 1~ constituting the plane of symmetry of ~he two receivers Rl, R2O
If transnission takes place by a se.ies of radiating elements positioned along the line of the two - 5 receivers, it can be accepted that a rerllote point Pi receives an insonification which is only dependent on the angle ~1 and the prior choice of the trans~
mitted codes. However, at short distance, the insonification can be adapted to each point of the considered hype.bolic surface.
Fig lO gives an overall view of a diversified transmission multichannel protec-tion sys em. It comprises a system 1 of radiating elements cooperating with two receivers Rl and R2. In a constructional variant, there can be two other receivers R3 and R4. This other line of receivers fo-rming an angle with the line RlR2 determines a division of the space into squares by the intersections of the two groups of hyperbolic surfaces. It should be noted that each hyperbolic surface such as 15 (Fig 9) is a surface of revolution whose axis is the line passing through the two receivers. The space is coloured by a generator 2 supplying the radiating elements of system l with separating excitation signals, e.g. signals carrying orthogonal codes.
A plurality of reception channels is formed by a circuit 22 comprising several groups realising the summatio~ delay function of elements 10 and 11 of Fig 9. Circuit 22 produces the same num~er of signals D as there are subdivisions in the angular -12~

sector to be monitored~ Each signal D corresponds to a particular value of the delay ~, i.e. to a particular hyperbolic surface having the property of representing a geomei-rical locus with a constant step dif-ference.
The signals D of reception ~eams for~ed by circuit 22 are applied ~o a transmission beam fonnation system 23 on reception constituted by elements like the channel element 19 of Fig 9. Each matched filtering element transforms each signal D
into a correlation peak V when the theoretical shape imposed by the simultaneous, diversified transmission at the echo formation point coincides with the fonn effectively received by -the two receivers. Thus, system ~3 comprises a generator of all the copies resulting from the dive-,-sified transmission in the monitoring sector. These copies are used in the matched filtering relative to each reception channel.
Consideration will be given to an echo from a point Pi of the monitoring sector. This point Pi belongs to a hyperbolic su--face corresponding to a delay ~i. The signal of chamlel Di is obtained by the synchronous addition oE two signals received by the two receivers Rl and R2 and its amplitude is 2 if the amplitllde of a single si~nal is taken as equal to 1. For ano~her signal of-channel Dk, there is another delay ~ which differs from ~i in such a way that the two signals are added together 0 in quadratic manner ~o obtain am amplitude of ~.
~13-6~9 The diEference between the optimum signal Di andthe other signals of the channel is close to 3dB. In system 23, the filter lnatched to signal S(Pi) and the only signal if Pi is far enough away, is the filter which processes the signal of channel Di.
Thls processing is equivalent to the synchronous addition of M signals of the M transmitters from polnt Pi and forming the signal S(Pi) in such a way that an amplitude increase equal to M is obtained.
This same processing summates in a quadratic man~er the M signals coming from anoLher point Pk forming a signal S(Pk)-in such a way that the increase obtained is equal to ~ is amplitude. Thus the final amplitude contrast on signal Vi compared with another signal Vk is equal to ~M, M being the number of transmitters.
Fîg 10 shows a device 24 for processing signals V supplied by circuit 23. This device comprises a screen 25 on which are dispLayed the echoes received in the monitoring sector. A connection 26 is a pulse indicating the start of transmission to circuit 23 and to processing device 24. This pulse initiates the supply of copies to the dlfferent matched filters of system 23. It also serves as a reference for transcribing the outward and return time into a distance measurement. The image fonned on the screen 25 can be sectorial in order to rep-resent the angles and distances of the targets.
However, for a given distance, it can also be frontal, when it represents the polar coordinates ~ of Fig 1.
-14~

n~
~` ~

As a non-limitative embodlment, it is possible to see in Fig ll a detection system according to the invention which is more particularly applicable to side looking sonars. The transmission system 1 comprises a line of radiating elements E
at the ends of which are located two receivers R
and R~.
The transmission circuits make it possible to excite each radiating element by means of a separable code modulating~ for example, a single carrier comprising a brief pulse generator 2, which supplies a system of coding circuits 27, whose pulse responses are the sought codes. Each coding circuit 27 excites a radiating element via a power amplifier 28. The chosen codes are,of ~he P.S oK~ type, i~e.
Phase Shifting Keyed. In the case of coding by pure frequencies, circuits 2, 27 and 28 are replaced by a frequency generator of the synthesizer type.
According to an advantageoils feature of the invention9 the formation of the reception channels is ensured by an elastic surface wave device.
This device comprises a substrate 31 made from piezoelectric material carrying two sets of electrodes 32 and 33 in the form of interdigital transducers. These electrode sets are located at the two ends of substrate 3l and form electro-mechanical transdu~ers transmitting counter-progressive acoustic waves. The signal supplied by receivers R
and R2 are amplified by amplifiers which are not shown in Fig 11~ The amplified signals are applied ~L~8~

to frequency changing circuits 29 with a localoscillator signal supplied by generator 30. This frequency chan~e places the signals received in the operating band of transducers 32 and 33. Transducer 32 transmits to ~he right a surface wave having ~he characteristics of the signal received by receiver Rl. Transducer 33 transmitstO the let a surface wave having the signal characteristics received by receiver R2. A system of intermediate transducers 34 detects and summates with the different time lags, the surface waves from transducers 32 and 33.
Thus, transducers 34 directly constitute the output members of the angular reception channelsO By placing a transducer 34 at the cen~re of the distance separating transducers 32 and 33, it is possible to deEine an average.transmission delay corresponding to a zero step difference~ This position corresponds to echoes coming from the plane of symmetry 14 (Fig 9). To obtain a reception channel corresponding to a hyperbolic surface such as 15, i~ is necessary to displace transducer 34 in order to introduce the desired step difference.
The delay line with elastic surface waves and multiple connections provides the double advan~age of simplicity and processing speedO It is highly suitable for systems requiring del.ays or time lags of approximately 10 ~S. This is applicable with side looking sonars and in the f;eld of medical imaging~
Thus, for a maximum delay of 10 ~S the 36~

distance separating transducers 32 and 33 is approcimately 7cm, when using a substrate 31 made from lithium niobate. Ihe receptlon channels are connected to the frequency changing circuits 35, wh;ch receive a local oscillator signal produced by circuit 36. This second frequency change serves to place the spectrum of the channel signals as low as possible in order to make the matched filtering operation simpler. The outputs of the fr~quency changing circuits 35 are connected to the first inputs of correlation circuits 37. These correlation circuits receive by second inputs copies of the signals S(Pi) supplied by a digital memory 40. A
control circuit 39 supplies memory 40 with the addresses of the copies. For each transmitted pulse, generator 2 supplies address generator 9 with a pulse for starting the cycle of correlations. At the start of the cycle, the copy is of a short-range field insonification and then progressively changes to assume the fixed form charac~erizing the long-range field sounding. This operating mode ;s illustrated in Fig 14 showing the addresses of P 1' K2, K3, K4 and K5 ranging from near field to far field. It can be seen that as from the transmission time to, che address change evolves increasingly slowly to reach an asymptotic value K5 in Fig 14. The func~ion Gf the correlation circuits 37 is to recognise among all the pulse shapes received from the different points of the 0 monitoring sector that characterizing a point Pi ~ 9 6 ~ ~located on a hyperbolic surface with a given step difference.
In the envisaged appllcation such as side looking sonars, the pulse times T of the transmitted signals are between a few milliseconds and a few dozen milliseconds.
According to the preferred embodirnrnt of the invention, each correlation circuit 37 is realised in known manner by a correlator with charge-coupled devices (CCD), whose operation is the same as that of a shift register. Each correlator is matched to a signal S(Pi) and at the outpu' a signal having a correlation peak is obtained for signal S(Pi) onl-y.
The signal supplied to the CCD correlators is sampled at frequency fH0 supplied by a clock 38.
The number of stages K of the correlator is such that K - T.fHo. The sampling frequency must be at least twice the frequency of the input signal. The centre frequency Fc of the signal applied to the correlator is made as low as possible, i.e. close to B/2 in order to obtain-a m;nimllm numbPr of stages K. According to the lnvention9 khe CCD correlators can be programmed and the copy applied evolves as a function of time in the manner shown in Fig 14.
It is pointed out that a programmable CCD convolvers has the structure of a transverse filter and comprises a memory into which is loaded-the copy, khe actual CCD register9 analog multipliers and a summator. To obtain the correlation from a convolVer , it is necessary to invert the time variable for one of the two wave shapes.
The system o Fig 11 is completed by a processing circuit 24 making it possible to obtain an image in the V mode (~, time)O
Without passing beyond the scope of the invention, it is possible to use other technologies both for side looking sonars and other applications.
Thusg Fig 11 shows a system in which a group of correlating circuits 37 is used. It ls also possible to sequentially multiplex in time the signals of the reception channels and carry out matched filtering with a single correlating circuit, which involves replacin~ parallel processing by series processing.
Another constructional variant of the invention is illustrated in Fig 12 and relates to applications such as imaging by echography in the medical field. Thus, it uses elastic surface wave convolving devices of the channel signals implying that the duration T of the pulse supplied to each radiating element is appro~imately 10 microseconds.
Fig 12 once again has elements Rl, R2, 29, 30 31, 32, 33, 34 and 2 of Fig 11. The elastic surface wave devices generally operate with a carrier wave of a few dozen megahertz. The signal supplied by transducers 34 are sequentially multiplexed in time by a parallel - series converter 41. The channel signals D therefore arrive in series in time at the inpu_ of the convolver and there is a return to the as first channel signal at each transmission. Fig 12 does not show the transmission circuits. I'he multiplexing timing is chosen so as to take account o the subsequent correlation processing period.
Parallel processing with the same number of convolutors as there are chan-nel signals D is also possible.
Correlation processing involves the use o~
an elastic wave convolver comprising a piezoelectric material substrate 42 having two terminal transdusers 43 and 44. These transducers transmit co~-nter-progressive elastic waves which interact in a multiplicative manner, The colleci,ing electrode 45 collects a signal representative of the convolution integral of the electrical signals applied to transducers 43, 44. To obtain the desired correlation function of output terminal 46, there is a time reversal of the copy serving as the comparison element'for the signal transmitted by converter 41 and applied to transducer 43. The copies are stored by real and imaginary values in two memory zones 48, 49. The two componen~s of each copy are extracted from memory 48, 49 by a control circuit 47 initiated by a pulse from excitation generator 2. Digital -analog converters 50, 51 under the control of aclock 52 supply sine ancl cosine multipliers contained ' in a modulating circuit 53, which receives a local oscillator signal from oscillator 30. The signal supplied by the modulating circuit 53 ls applied to transducer 4. The correlation peak available at olltput ~896~ 9 46 has a frequency which is twice as high as the frequency of the input signals. The time necessary for obtaining convolution function is approximately the same as the duration of the input signals.
The embodiments of the systern according to the invention described hereinbefore use analog circuits for performing the channel formatLon and matched filtering.
Fig 13 shows a diagram of the system usîng digital calculation means. The same references designate the same elements as in the preceding drawings. The signals supplied by recei~ers Rl and R2 are sampled and digitized at frequency fH0 of a clock 55 in two analog - digi~al converters 54.
A microprocessor circ~it 56 recei~es the digitized signals from converters 54, as well as an ini~iation pulse produced by excitation generator 2. The operation of microprocessor circuit 56 is timed by clock 55.
In per se kno~n marmer, microprocessor circuit 56 R.A.M. and POR.O.M.-type memories, operators and input -'output means. This circuit is programmed for digitally performing the formation of recepi,ion channels D and matched filtering. The copies are stored in the memories of the microprocessor circuit and are recalled as the processing cycle proceeds.
The processing system 24 receives the output si~nals of microprocessor circuit 56, ~hich can be directly processed in a memory storing samples represencing an -image. System 24 may also comprise digital - analog con~erters perrnitting the control of a cathode ray tube.

~8~6(~

The iNvention is also applicable to the formation of a b;dlmensional frontal image of the sertor being monitored. :[n this type of application, the system comprises two interferometric bases, In Fig~1s, it is possible to see a first line oriented in accordance with v and comprising receivers Rl and R2 an~ a second line oriented according to u comprising receivers R3 and R4. The transmission system l may be constituted by a matrix arrangement of radiating elements. However, it may merely constitute one row and one column of radiating elements positioned between the receivers along axes u and v, The beam forming circuits 22l, .~22 supply signals Dv and D which are summated in a forming circuit 223, whlch supplies signals Du v which then undergo matched filtering.
Circuits 221,222 and 223 may be constituted by a single circuit.
Fig 16 shows a typical arrangement of a side looking sonar system~ A support vehicle 59 towed by a cable 60 moves in direction 58 in the vicinity of the sea bed 61. A generally linear transmission system l and two receivers Rl, R2 formin~ the inter-ferometric base are installed along the side of vehicle 59. Due to the angular channels formed, a beam of directivity F emanates from centre 0 and intercepts the sea bed 6l along lines of width J
perpendicular to direction 58.
The hatched area Z corresponds to an angular 0 channel formed and at the projection distance a certain resolution r2 is obtained for a cerLainangular width of the channel. The transmitted frequency band B determ.ines the distance resoluti.on rl. Bearing in mind the speed of the vehicle, the range and the resolution r2 it is necessary to form a number of angular channelsO An image is obtained of the sea bed as a result of the movement of vehicle 59~
The characteristics of a sonar of this type are, for example, as followsO
Range: 200 mekres R~solution: rl - 0.2 metresS r2 ~ metres Transmission antenna length: 3 5 met.res Support vehicle movement speed: 5m/s.
Receivers Rl and R2 are at a distance of 2.5m from one another and the values obtained for the main parameters are as fol.lows:
- centre.transmission frequency: fO - 2S0 kHz - frequency band: B - 7.5 kHz - distance between consecutive radiating elements:
d - 20mm - number of channels formed: 7 - angular field: 0.4 - length of radiating elements: 1 ~ 20mm - rumber of radiating.eleme-nts: M ~ 128 - pulse duration: T - 10 nls - - CCD clock :Erequency: EHo ~ lOkHz - number of CCD stages: K = 100 per filter.
The coding circuits on ~ransmisslon generate 128 or~hogonal codes with 128 bits. In order to obtain -23~

a good rejection oE transmlssion image lobes, the two receivers are advantageollsly directional.
The;r length is, for example~ approximately 50mm.
The proposed system greatly simplifies -the recep~ion circuits and makes it possible to obtain a given angular resolution with reduced dimensions of the transmission and r~ception means. Moreover, this sys~m functions in a wide frequency band and makes it possible to obtain in real time even short~
range images.

Claims

WHAT IS CLAIMED IS:
l. A diversified transmission multichannel detection system comprising transmission means using a system of adjacent radiating elements positioned at the apex of a predetermined angular sector and reception means using a pair of receivers centered on said apex for detecting echoes from said sector, wherein the transmission means comprise means for the simultaneous pulse excitation of the radiating elements assigning to each of them a signal shape or form permitting its identification as an element of the system, the reception means comprise a plurality of reception channels connected to the receivers by channel forming means associating with each of the receiving channels a surface with a constant step difference belonging to the said angular sector, each of the receiving channels being connected to a filter matched to the particular configuration of the echoes coming from the surface with a constant step difference.
2. A system according to claim 19 wherein the channel forming means comprise a delay line with elastic surface waves.
3. A system according to claims 1or 2, wherein each of the said channels is provided with a matched filter,
4. A system according to claims l or 2, wherein the channels are sequentially multiplexed for series processing by a single matched filter.
5. A system according to claim 1, wherein the matched filter is constituted by a CCD correlator.
6. A system according to claim 5, wherein the signal supplied by the reception channels are frequency transposed by frequency changing circuits.
7. A system according to claim 17 wherein generating means of copy signals of irradiations received by the points of the angular sector are associated with the matched filter, which is programmed.
8. A system according to claim 7, wherein the generating means comprise an address generator and a memory containing the copy signals.
9. A system according to claim 8, wherein the addresses produced by the address generator evolve as from the transmission time so as to permit the matching of the copy signals to the long-range and short-range echoes in real time.
10. A system according to claim 1, wherein the matched filter is an elastic wave convolver
11. A system according to claim 1, wherein sampling and digitization means connect the receivers to an arithmetic unit which is cabled by a programme on the formation of the receiving channels and on matched filtering by a correlation function calculation process applied to copy data contained in an R.O.M-type memory and to the intermediate results stored in an R.A.M. memory.
12. A system according to claim 11, wherein the arithmetic unit comprises at least one microprocessor circuit.
13. A system according to claim l, wherein it comprises two crossed interferometric bases, which are each provided with two receivers, the receiving channel forming means collect the signals supplied by the four receivers and each channel is then processed by matched filtering.
14. A system according to claim 1, wherein the transmission means transmits sonar signals.
15. A system according to claim 13 wherein the transmission means transmit radar signals.
16. A diversified transmission multichannel detection system comprising transmission means using a system of adjacent radiating elements located at the apex of a predetermined angular sector and reception means which, at the said apex, detect echoes coming from the said sector, wherein the transmission means comprise simultaneous pulse excitation means of the radiating elements assigning to each of them a signal shape permitting its identification as an element of said system, the reception means comprising a single receiver connected to several reception channels, each of the reception channels being connected to a matched filter.
17. A system according to claim 16, wherein the filter is matched to the particular configuration of the echoes from the said sector.
18. A system according to claim 16, wherein the filter is matched to the signal shape, a transmission channel formation circuit on reception being supplied by the said filter.