CA1140246A - Digital time-delay beamformer for sonar systems - Google Patents

Abstract

ABSTRACT

A DIGITAL TIME-DELAY BEAMFORMER
FOR SONAR SYSTEMS

There is described a digital sonar receiver having a novel beamformer, own doppler nullifier (O.D.N.), and adaptive time varied gain controller. The beamformer interrogates a multi-element transducer by utilizing an interleaved sampling scheme which samples the elements in the transducer array sequentially, in such a fashion that continuous, uniformly spaced samples are obtained. The samples are subsequently converted into digital format, weighed and summed. The O.D.N. eliminates the own ship's doppler effect on the beam output signal resulting in a half beam output signal representative of the target doppler, The O.D.N. offsets the frequency of the beam output signal and a digital generated signal representative of the own ship's doppler. The O.D.N. multiplies these two offsetted signals resulting in a signal lying in each of the sum and difference frequency bands for which the signal lying in the difference frequency band corresponds to the target doppler. The adaptive time varied gain controller varies the gain of preamplifiers at the input of the sonar receiver to be inversely proportional to the momentary average reverberation level of the medium in which the receiver operates. The adaptive time varied gain control apparatus initially sets the gain of the preamplifiers to be inversely proportional to a standard reverberation curve during the first ping-cycle and subsequently updates the gain of the preamplifiers during each successive ping-cycle by computing a gain correction from the average value of a pre-determined number of beam output signals.

Description

4~

This invention relates to a digital sonar receiver com-prising a digital beamformer, a digital adaptive time varied gain controller, and a digital own doppler nulllfler (O.D.N.).
me sonar receiver receive~ sonar ~ignals in analogue ~orm and converts the sonar signal~ into digital form in the beamiormer when forming beam output 8ignal~. l~e remainder of the sonar receiver which includes the O.D.N. and adaptive time varied gain controller operate~ digitally.

In the pa~t, ~onar recelvers for the most part have pro-cessed sonar signals ln analogue format. However, more recentsonar receivers have been directed towards proce~sing the sig~als in digltal format.

Previous digital beamformer designs have been directed towards a method of forming slmultaneous beams. To form simultaneous beam~ a cylindrical array of staves had been ~lmultaneou~ly sampled at 80me initial tlme. me staves are then sampled simultaneously at later moments in tlme correspond-ing to the delay time~ introduced by the geometry of the array.
Thi8 method of simultaneou~ beamforming when implemented, require~ for each stave a sample and hold circuit and an A/D
converter. This method of beamforming h~s resulted in rather uneconon~ical uae of time a~ well as hardware.

The received sonar signal~ prior to beamforming comprise amblent noise and noise from any undesirable target~. The ll~VZ~

received sonar signal be~ides including an echo from the desired target also inclu~es noise due to reverberation. Reverberation is a function of the range or time taken for a transmitted slgnal to hit a target and ret~rn. Reverberation decreases a8 range or time increases. Eventually the noise due to rever-beration decrea~es below the ambient noise in the medium of operation which then becomes the dominant noise.

To eliminate the effect of noi~e due to re~erberatlon the gain of the preampli~ier~ ln a sonar receiver is ad~usted to be inversely proportional to an As~umed "standard" reverberation curve. The ad~ustment of the gain of the pre~mplifiers with re~pect to the "standard" reverberation curve has not been too effective in reduclng the noise due to the actual reverberation.
Large discrepencies between the ~standard~ reverberation curve and actual reverberatlon easily result from sur~ace re~lection~, back scatt~ring, and marlne life.

Subsequent to beamforming~ beam output ~ignal8 that contain information corresponding to the target are often shifted in frequency. mis frequency shift is known as the total doppler 8hift which, for the most part, i8 due to the doppler shift produced by target mo~ement and the doppler shl~t produced by the source o~ tran~misslon's movement or the o~ ~hlp's doppler.
It is necessary to eliminate the doppler effect o~ the own ship from the beam output signal 80 that the target information may be analyzed. Analogue sonar receivers eliminate the own ship's doppler by mlxlng the beam output signal wlth a generated signal 114~32~ti corre~ponding to the own ship's doppler. me signal correspond-ing the own ship's doppler may be calculated i~rom the velocity of sound in water, the shlp's ~peed and heading whlch are all known. The result of thls mixing proce~s yields a sum slgnal and a difference signal where the dif~erence signal is repre-sentative of the target information only. However, when the beam output sl~nal~ are in digital form the e~fects of noise foldin~ o~er ~rom the ne~ative frequency band may introduce error in the digltal dif~erence signal. Moreover the sum digltal signal may overlap or lie too close to the dlfference digital signal thereby preventing the economical u8e 0~ ~ilterg to ~ilter out the sum signal.

The present inventlon relate~ to a dig1tal ~onar receiver having a no~el beamformer ~or digitally beamforming output slgnal~ ~rom receiv~d ~onar si~nals, a novel own doppler nul-lifier, and an apparatus ~or varying the gain oi~ sonQr pre-ampllfiers to be inversely proportional to the momentary average reverberation level of the medium ln whlch the sonar ` receiver operates.

In accordance with one aspect of this invention there is provided a beamformer ~or forming a plurality of dig~tal beam output ~lgnal~ in a sonar receiver which ha~ a multi-element symmetrical transducer array. Each beam output signal has a d~rection which is related to at least one respective directlon-al element in the transducer array. The total numb~r o~
samples to be taken 1~ given as the product o~ the total number 24~

o~ beams M to be formed and the number sample-~ets n desired to form any beam. Due to the symme-try of the array a sample-set may consist of two samples ~aken Xrom each element symmetrically disposed on oppo~lte ~ldes of the at lea~t one directional element. The beamformer ~equentially samples the element~, ln such a ~ashion that contlnuous, unliormly spaced samples are obtalned. To ob~aln uniformly spa^ed s~mple~ the method of beamforming and beamformer apparatus s~mple each 1 element symmetrlcally di3po~ed to the left and r.~ght of the directional element ior each beam m during each consecutive sampling i~ter~al, z, ~here i is defined as an integer value greater than or equal $o O and le~s than or equal to n-l. The sa~pling interval z is defined as an lnteger O ~z ~ Z. The beamformer determines from the sampllng lnterval which element~ symmetrically dis-posed to the leit and right of th~ directional element ~r~ to be sampled. The directional element which i~ utilized to form the beam m may be given by:
m,Remainder ~ +l th where di i~ a time delay of th~ i sample to the right and left o~ beam m with respect to it~ ~ir~t sample. For any gi~en sampling interval beam m may be determined if the time delay is a i~ ction o~ the sampling interval. The time delQy ls an approximation o~ the exact time delay for the movement o~ a ~onar 81gnal through the medlum of operatio~ to impinge on the elements of the sample-~et aiter the sonar signal has lmpinged on the at least one respect~ve dlrectional element i~or the beam.
It ~hould be under~tood that for thi~ method the time delay is Z4ti, expressed as an integer number of sampllng intervals where di is chosen to be the integer number of ~ampling intervals closest to the exact time delay such th~t; the remainder of the delay when divided by the total number of sa~ple-~et~ to be taken has a different integer value for each value o~ i. The value of i may be determined from the remainder ~i/n~ which is equal to the remainder ~z/n~ . During each sampling interval the beam~ormer sums together the aample~ taken ~rom each 1 element to the right and left o~ be~m m, 80 a~ to ~orm a ~ample-set.
The be~m~ormer converts the sample-set from analo~u~ format to digital format. The beam~ormer multiplies the digital sample-set with weights. The beamformer adds each digital ~ample-~et to its corresponding part~al sum o~ sample-sets and stores thi~ partial ~um. The output of the partial ~um for each beam ~hen all n samples of the sample-set ~or beam m ha~e been ~ummed is the beam output signal correspondin~ to beam mO

It ~hould ~e understood that the sampling of the beam~
sequentially, in ~uch a ~ashion that continuous, uni~ormly spaced ~amples are obtained may be determined ~or any multi-elements symmetrical array and may be stored as a timing schedule ln a memory device. It should there~ore become apparent that a beamformer using this sampling method only re~uires one A/D converter. It should al~o be ~pparent that the sampling method result~ in an economical use of equlpment and time.

ll~VZ4ti ~3 In accordance ~,~ith another aspect of this invention there is pro~ided an adaptive time varying gain control apparatus for varying the gain of the preamplifiers of the sonar receiver during each consecutive ping cycle to be in~ersely proportional to the momentarr a~erage reverberation in the medium o~ operation.
The apparatus comprises a storage mean~ ~hich has a plurality of storage space~ for storing a stored gain. A preset means i8 provided for initially storing in these storage spaces gain step approximations of an inverse standard reverberatlon curve 10 ~here each of the gain step approximations has a respective time duration which i8 determined from the standard reverber-ation curve. ~lso included ~n the apparatus is a means ~or ~toring each time duration and for as~ociating each of the tlme durations with the storage ~pace where the time durations respective gain step approximation has been initially ~tored.
A control means i8 provided ~or varying the gain of the pre-ampl1~1ers to corre~pond to each stored ga~n during each ping-cycle whereby each stored gain i~ applied to the preampliflers for a duration which i~ equal to the time duration Qssociated 20 with the stored gains respecti~e storage space. An averaging means i8 a~so provided in the apparatus for averaging a plurallty of beam output signals ~rom a beam~ormer in the sonar receiver during each time duration. me apparatus includes a means for determlning a gain correction as a function of a re~erence signal and the average of the beam output signals during each time duration. The apparatus further includes mean~ ror computing a new gain by summing the gain correction during each time duration ~th the respective stored gain associated with 114VZ4Ç;

that time duration, Lastly, a means for storing a new gain in the stored gains respective storage space during each time duration is provided.

It should be under~tood that the gain step approximations may be uniform step approximations. Also the means for deter-mining a gain correction may be a means for computing a logarithmic ~unGt~on of the average of the be~m output signal when divided by the reference 3~gnal. The determini~g means may also include a comparing mean~ which compares the average of the beam output signals ~rith the re~erence signal to give a coded value of the gain correction. This coded ~alue may be used to acces~ a memory which ha8 ~recalculated values for the gain correction stored therein~

The adaptive time varying gain control apparatus contin-ually updates the gain of the preampllfiers during each ping-cycle by correcting during each t~me duration the gain approxi-mations of the standard reverberation curve on the subsequently stored gains when the gain approximations or the subsequently stored gains are applied to the preamplifiers. This adaptive time varled control apparatus eliminates noise due to the momentary average reverberation in the medium of operation.

In accordance with another aspect of this invention, there is provided an own doppler nulli~ier for extracting ~he own ship's doppler information from a digital beam output signal of a beamformer to obtain target doppler infor~ation. The

2~
own doppler nullifier introduces an offset frequency A to the digital beam output slgnal which offset i~ a function of the beamformer sampllng frequency. The own doppler nullifier apparatu~ and method also provides for the generation of a digital slgnal representative of the own ship's doppler infor-matlon and off~etting the digl~al signal by a frequency amount B which ls a functlon of the beamformer ~ampling frequency and off~et frequency A. The generated digital signal and the beam output ~lgnal are multiplied together to obtain a ~ignal having a difference frequency component and a sum frequ~ncy component. Both frequency components will lie in thelr re~pect-ive difference frequency and 8um i'requency bands. The dif-ference frequency band wlll have a center frequency which i~
offset by an amount A-B whereby the affect of noise due to fold-over from the negative frequency band into the difference frequency band 1~ eliminated. The own doppler nullifier pro~ides for the ~um frequency component to be filtered out leaving only the dl~ference frequency component correspondlng to the target doppler information.

The own doppler nullifier makes it possible to digltally filter the sum frequency component and difference frequency component of the mixed generated dlgital signal and digital beam output slgnal. The introduction of the offset~ shii~ts the difference frequency component away from any affect~ of noise due to fold-o~er from the negati~e frequency b~nd.
Moreover, the offsets introduced to the signals shifts the sum frequency component further ~way from the difference lZ~

frequency co~ponent to make the i~iltering of the sum frequency component from the dif~erence frequency component pos~ible.

It should be understood that the of~set frequency A may be chosen a~ the difference between the transmltted fre~uency of the sonar receiver minus the nearest low~r multiple of the beamformer sampling frequency to that transmitted irequency.
It should be under3tood that ofiset frequency B may be chosen to be equal to half the beamformer sampllng frequency le~s offset irequency A 80 as to provide the optimum result.

For a better under3tanding o~ the nature and ob~ects of the invention, reference may be had to the following detailed description taken in con~unction with the accompanylng diagrammatic drawing~ in which:

Figure 1 shows a block dlagram for a digltal sonar rece~ver;

Flgure 2 is a schematlc reprasentation of a circular transducer array ~howing a wave front implnging the trans-ducer array;

Figure 3 is a tlming diagram showing a partial sampling schedule for the digital beam~ormer;

Figure 4 ~hows a schematic diagram of the digital beamformer;

Figure 5 show~ a ~'standard" reverb0ratio~ curve, and the affect of amblent noise on such a curve (on the same sheet as Fig. 2);

Figure 6 i~ a block dlagram for the input section of the sonar receiver;

Flgure 7 18 a block diagram for the adaptive time ~aried gain controllerJ

Flgures ~A and 8B ~how the affect of noise on the di~fer~nce band be~ore and ~fter the band is shlfted by frequenoy ~mount A-B;

Figure 9 iB a block diagr~m for the own doppler nulli~1-cation apparatu~.

Referring now to Fig. l, a block diagram ~or the dig~tal sonar recei~er lO i8 shown. The digital sonar recelver lO
recelve~ sonar signals ~rom a multl-element tra~sducer array corresponding stave linc 12. me stave line~ 12 apply the ~onar 8ignal8 to the input ~ection 20 of the recei~er 10.
Input section 20 reduces the dynamic range of the 8ignal~ ~
convert~ the 8ignal8 to digital and forms beam output~ for subsequent analysis. Proces~lng section 25 analy~e~ the digital beam output data and extracts target lnformation is +~hen fed to output ~ection 30 and ~ubsequently onto a ~ .

114(~Z46 display or any other ~uitable means9 for example, a recorder.

l~e input section 20 of receiver 10 comprise~ a plurality of staYes in a circular multi-element tran~ducer array (not shown) where each stave ~z connected to a pr~amplifler in preamplliier circult 32 by line 12. E~ch output o~ the preamplifier i8 connected to ~h~ input of an anti-aliasing rllter o~ ~ilter clrcuit 34. In order to reduce the large dynamic range at the lnput of recelver 10, each preampli~l~r o~ clrcult 32 ha~ a ~arlable galn which 18~ by means of an adaptive time-varied gain controller 36, ad~usted to be lnversely proportional to the momentary ~alue of the average reverberation le~el. me adJustment of the preampll*ier circuit's gain achieves reverberation back~round level normallzation. The time varied gain controller recelves in~ormatlon ~ia the beamformer 38, which sample~ the received signal~, as to the momentary reverberation level. Reduclng the dynamlc range of the sonar slgnals reduces the dlgital word length required to reprcsent the signal ln the subsequent processing.

The sonar signals once havlng their background nolse level normalized by pre~mpli~isr circuit 32 are fed to the anti-alia~ing ~ilter cir~ult 34. Circuit 34 co~prises a plurality of anti-alia~ing iilters where for each pre~mpll~ler there 18 a corresponding filter. Each anti-allas~ng filter lr.cludez a band pa~s ~ilter, with the center fr~qllency set to corre~pond to the ~requency of the transmitted 8ignal8 .

The wldth of the band of frequencie~ passed by the filter will be determined by the maximum expected doppler shift betwee~
the target echo and the transmltted signal. The ~unction of the anti-allas ~ilters i9 to ellmlnate the passing of signals in the 80 called "alias" frequency band~, introduced by the sub~equent sQmpling process. At the same tlme the ~ignal to noise ratlo is improved, because o~ the reduction in the barldwldth. A~ a r~sult o~ the reduction ln the dynamic range the sampling ~nd subsequent digital proce~sing can be performed more economically.

Beam~ormer 38 ~amples the si~nals through multlplexers 40 and converts the signal into digital format by A/D
converter and summer 42. The beamformer forms one beam for each related stave by sampling the ~taves ~ymmetrically disposed about the related stave a~ well as the related stave ltself and weighting~he sample~ before addl~g them together to ~orm the beam. Timing and control circuit 44 controls sampllng by implementlng a sampling method, which interrogates the staves in an interleaved fashion and fo~ms the beams sequent-lally.

The full beam output signal, in digital format, 1~ pa~sed through automatlc galn controller 46 o~ proce~ing section 25.
The automatic gain controller increQ~es or decrea~es the gain of each beam lndependent of the other beams after a compari30n of the actual beam output with the sverage output of a number o~ preceding ~amples ~or that beam.

The galn ad~usted beam output, is passed through own doppler nullifier device 48 which eliminate~ the effect of the own ship's doppler on the frequency of the signal.

The signal now enter~ a signal processor 50 which cor-relates the recelved signal with a plurality o~ predetermlned independently stored signals, BO called ~replicas~. The best correlation will be indicative of the ~elocity o~ the target in that beam. Such a correlation is performed for each beam in sequenc~. The output i8 sent to the output proces~or 52 Of output section 30.

A fine bearing computer 54 is included in processor Rection 25, which combines information from the automatic gain control 46 and the ~ignal prooessor 50 to more accurately compute i~rom whlch direction wlthin a beam the target infor-mation i8 received. Thi~ information i8 fed lnto the output processor 52.

The output proces~or 52 receives lnformation regarding the beam, target velocity and time since transmission, and comblnes this information with lnputs indicatlng the velocity Or sound in water, and the ship's course. From thi8 the output proces~or 52 produces a digital sienal which i8 indicative of the range, bearlng, doppler, fine bearing, and ships headlng, in a format adapted to the di~play cir--cuitry.

Figure 2 show~ planar wavefront 60 impinging on cylindrical transducer 62 containing a ~umber oi elements (staves) around its periphery, e~g. 36 sta~es at 10 angular ~eparation, deqignated Sl through S~6. Beam~ may be rormed by 3ultably combining the signal~ from a number o~ contlguous sta~es. In ~hi8 case an array of 13 contiguou~ ~ta~es i8 usad to form a beam.

The 8ignal generated in any ~ta~e of t~e array ~8 delayed with respect to the ~ignal at the point of incidence upon the tran~ducer. Thls delay may be expressed as~
d-r C-cos (0-A~ 10+6 ~S, c where: r 3 the r~dius of the tran~ducer ln feet;
c ~ velo~ity oi sound ln water in ~eet/~econd;

3 the angle between the direction of travel o~ the incident waveiront and the axis o~ the array (in degree~;
o~, the angle between the axis of the ~tave con~ldered and the axis o~ the array (in degrees).
k(& 0 are considered posltive in the clockwise directi~n,) For a transducer radius of 2 feet and a velocity of sound in water assumed at 4,890 ~eet/second, a wave~ront travelling along the axis o~ the array (0-0) will generata ~ignal8 in th~ 13 ~taves, delayed as shown in Table 1.

TABLE I
. . -- . . .. , . .... , Locat~on Stave No._ in arraY , ~ (de~rees) D~ In u3.
S7 C+6 +60204.5 S6 C+5 +50146.0 S5 C+4 ~l~o 95.7 S4 C+3 ~0 54.8 S3 C+2 +~0 24.7 S2 C+l +10 6.2 Sl C(center 0 0 S36 C-l -10 6.2 S35 C-2 -20 24.7 S34 C-3 -30 54.~
S33 C-4 -40 9~o7 S32 C-5 -50146,0 531 C-6 -60 _ ~ ___ By taking samples of the indlvldual stave ~ignals at moments ln time correspondlng to the delay time3 ~or thos~
staves~ the stave signals wlll appear in phase and can be summed to form a besm output signal.

Due to the symmetry o~ the array the wave~ront impinge~
slmultaneously on stave~ C+i and C-i of the array allowing these staves to be sa~pled at the same time, where ~ i~ an inteeer greater than or equal to 0 and less than or equal to 6. In order to form a beam it ls thus necessary only to take samples at 1+1 moments in time.

me beamformer 38 of Fig. 1 utilizes a method of inter-leaved sampling. The interleaved ~ampling method samples ,_ .

Z'~
l&
individual stave elements at dlfferent moments in t~me which approximate the delay times of Table 1. The stave elements are sequentially sampled, in such a fashiun that continuou~, uniformly 3paced ~amples are obtained.

To form one beam corresponding to each stave element of Fig. 2, 7 sample-sets must be taken for each beam.
Therefore the total number of ~amples to be takell is:

Z ~ t~n ~ (36)(7) t-- 252 where M is the total nu~ber of bearns ~o be formed (equal to 36) and n is the total number of 3ample-sets (equal -to 7). Because each sample is taken at one moment in time, Z represents the total number of moments in time samples are to be taken.

The ~ampling method samples each 1 element symmetrically dlsposed to the right and lei~t of the beam m, being formed during each consecutive sampling interval z. It should be understood that o i n - 1 (or o i 6 for n = 7). Each sampling lnterval correspond~ to one moment in time of the 25 different momen-ts in time for Z.

'~he time duration of each sampling interval is chosen such that the received sonar slgnal may be adequately re-produced digitally. A total band~lidth deviation of 800 Hz for the received slgnal ~rom the transrnitted sig~nal requires a sampling frequency of at least 1,600 ~Iz to ade~uately reproduce the received signal digitally. The time duration o~ the ~ampling interval is cho~en to be a function of the first delay of 6.2 ~S from Table I. Table II sho~rs the calculation of dif~erent time durations. The ~ver~e of the time duration i~ al~o given which represents ~he ~ampling frequency. Fram Table II a sampling frequency of fs=3200 Hz i8 chosen givlng the time duration of each sampling interval as 1.24 ~S.
TABLE II

X Time Duratlon of the Sampling Frequencies Sampll ~ erval (fs~106/252.fo Hz) 1 ' 6.2 ~ ~4 2 3.1 1280 3 2.07 1920

4 1.55 2560 1.24 3200 6 1.04 3840 7 0.89 4480 8 0.78 5120 To obtain a uniform ~paced sampling rate the exact delay~ given in Table I are approximated as an integer number of sampling intervals. The approximated delays, dl~ ~rc chosen such that the quotient of the ~pproximated delay di when di~ided by the total number of sample-set~
n~7 has a different integer remainder for each ~alue of i.
Table III shows how di i~ chosen so that its remalnder i3 114~Z~6 ~ o different for eac~l value OI i~
~ BI,E Ill Tlme DelayTime Dela~ . . '`_.~ln~r of (~sec)T.~4 ~-s-o di/7 . ... , . .... . ~ .. ~ .. _ __ o o o () o 6 . 21 5 .008 5 5 2 24.67 19.~9 20 6 3 54.80 44.19 4~, 2 4 95.69 77.17 7f3 0 5 146 .lo 117.~2 l~ ~i 4 6204 . 50 16~-. 91 164 ......... ...

It should be understood from Table ~I that because di is a function of the time duration of the samplin~ interval, i may be determined for any consecutive sampling interval from t.he remainder di~7 which is equal to t}le remainder oi' z/7.
Moreover the samples taken during each consecutive sampling interval correspond to beam m wherc:
m=Remalnder ~o L~ l m--Remainder ~-7-- +3~ +1 ...
L 36 ~
where i is known for any di per Table III and di ls known for any z (because Remainder [di~ = Remainder [zJ~ ).

Referring now to Fig. 3 the timing diagr~m for the .sampling schedule may be derived by calculating for each sampling interval the sample-set to be sampled and the be~m 114V24~i to ~hich it corresponds. ~or e~arlple, from Fig. 3, during the time duration of the 25th sampli.ng interval stave elements S1.9 and s2g are sampled for beam 24. This infor~ation may be calculated from the above equations because for z=25:
Remai.nder [di/7 = Remaind.er LZ/7 ~ Renainder [25 = L;
From Table III for a re~ainder of 4, di=116 and i-5, From equation 1:
10 m = Remainder ~ 116 ~ +1 = Remainder ~3/36 ~ +1 = 23+1 Hence, stave elements m+i are sampled ~hich corresponds to S24+5 _ Sl9 and S29.

Referrlng to ~ig, 3 a repetitive sampling se~uence can be seen. At tlme intervals 0, 7, 14, etc. the first sample-?0 set for every beam is sampled. The sequence is dependentupon the remainders of di/70 The d~lay a.ssociated with the sample to be taken corresponds to the remainder of di/7. The first sample in the sequence is .~or di/7 having a remainder of 0 correspondin~ to the sample not delayed in time (i~0).
The second sample is for remainder diJ7=1 correspond~n~ +,o i.=4, the third for remalnder di/7=2 corresponding to i,3 and 50 on until ~he last sample in the sequence is i`or r~malnder di ~ correspondin~ to i=2. The delays that correspond to .. .

these samples are a function oi' the rounding of the time delays to be ~ multip~e oi~ the time duration for the sample interval.
It R~ould be underRtood that thiB sequence repeats ~t~elf after every 7 sampling lntervals because the number o~ sample-qet~ n has been cho~en equal to 7.

Figure 4 is a ~chematic diagram for the digital beamformer 38 of Flg. 1. me timing and control circuitry i8 shown within llne 70 compri~ing clock 72, counter/divlder 74 and a programmable read only memory (PROM) 7~. The timing diagram o~ Fig. 3 i8 stored in PROM 76. PROM 76 accesses multiplexers 40 o~ the beam~ormer to sa~ple input lines 82 in accordance with the interleaved sampling method of Fig. ~, Lines 82 separate to feed both multiplexers 40. There are 36 input llnes 82 ~one ~or each stave element). The two m~ltiplexers are used 80 that symmetrically di~po~ed staves may be sim-ultaneously ~ampled.

The ~onar signals entering the be~mformer 38 via input lin~s 82 have had their background noise level normalized by preampllfiers 32 and have hEd thsir bandwidth reduced 20 to 800 Hz by anti-aliasing fllter 34 tsee Fig. 1)~ Using the aamplin~ frequency o~ 3200 Hz the combined sampllng rate o~ lnput llnes 82 by multiplexer~ 40 will be (252 samples x 3200 Hz ,) 806,400 ~amples per second.

Multiplexers 40 interrogate 30nar ~ignals on input Z~
llne~ 82. The two symmetrically disposed input lines are interrogated by multiplexer~ 40 and summed by analogue summer 80 prior to A/D convertion. PROM 76 will acce~s only one multiplexer when only one ln~ut line 1~ to be sampled such as for the ~irst ample in the ~ormation o~ any one beam.
In this lnstance, the output of this one multiplexer t 5 summed with a nil outpu~ ~rom the other multiplexer re~ulting ln a sum which is lndicati~e Or the sample it~eli. The ~ummation 1~ done in analogue becau~e lt is quicker and more economical ~umming the s~etrically dispo6ed staves prior to A/D con-ver~ion. The ~nalogue ~um lenve~ summer 80 and is converted to digital formst by A/D con~erter 84.

A pulse generated by PROM 76 on line 86 initiates the A/D conversion. me output of the A/D con~ersion i~ shown to be an 8-bit word whlch i~ multiplied by weighlng factors supplied by weight circuitry 86 to multiplier 88. me timing and control issues pulses from PROM 76 along lines 90 and 92 to initiate the multiplicatlon. me we~ghing factors are chosen with respect to whlch samples of the beam are be~ng taken. For the first sample the weighing factor will be unity. The other weighing i~actors may be a iraction le3~ thRn unity 80 as to aid in suppre3sing ~ide lobe~. The output oi the multiplier i8 ~ed into latch 94 which tran~fer~ the 8-bit ~ord representing the sampled sl~nal to digital summer 96.

Digital summer 96 adds allweighted sample~ to all other il4~2~6 ~amples o~ their re~pectlve sample-~et which when summed iorm~
a beam output ~ignal. Summer 96 utillzes R~ndom Access Memory (RAM) 98 whlch store~ the partial SUm8 0~ each beam output 3ignal to be ~ormed. Pulse~ ~nt from PROM 76 t~ along line 100 initiates the ~ummation o~ each new ~ample entering input A wlth the purtial sum of it~ sample-~et enterlng at input B.
Thi~ new p~rtlal sum A+B is then stored in RAM 98 at a~ addre~s lndicated by bus 102 from PROM 76. Bus 102 also addresses RAM 98 to bring the parti~l sum o~ the sample-set to be ~ummed ln summer 96 with each o~ ~ts new samples.

~ AM 98 store~ a partial sum for each beam to be formed.
In thls case ~6 beam~ ar~ belng ~ormed so RAM 98 must store 36 part~al ~um~ RhM 98 output~ on bus 104 digltal words representlng each i~ll beam output sign~l. Each full beam output signal is fed from the beamformer only after all 7 samples of the sample-~et for the beam output signal have been summed. At this time the RAM 98 clears that memory location whlch is used to keep the partial sum ~o that a ne~r output ~ignal can be ~ormed in that direction.

The beam~ormer 38 outputs a beam every ~7 ~amples x 1.24 ~s (the time duration of the sampllng interval3 _ 8.68 ~s.
Hence, the beamformer which samples at a rate o~ 806,400 samples per second will output approximately 115,200 full beam output signals every second.

It should be apparent that the beamformer raquires the faste~t operating hardware. Processing section 25 (Fig. l~
need not work at such a ~ast rate which makes for more economlcsl h~rdware.

~ esides the ~ull beam output ~ignals being fed into proces~lng section 25 (Fig. l) they are al80 fed back into adaptive time varled gain control (A.T.V.G.) 36 (Fig. 1).
The A.T.~.C. control 36 compensates for ~he reverberstion decay as a runctlon Or range, varlatlons in the overall level of the reverberatlon decay curve, and sudden variations in the slope o~ the decay curve (reverberation bumps). Figure

5 3hows a typical reverberation curve 106 a~ a function o~
range or time. As reverberatlon curve 106 decays lt fallR
below t~e amblent nolse 107 when then becomes the dominant noise. Reverberation i8 the energy from the transmitted pulse being re~lected back to the receiver where the reflection is caused by discrete particles and marine life res~dent ln the medium, as well as the reflection and back-scattering which occurs at the boundaries, The A,T,V,C, control ~6 reduces the large dynamlc range of the recelved sonar signals which may be in the excess of 120 dB by varying the gain of the preampli~ier 32 to be lnversely proportional to the momen-tary value of the average reverberation.

The A.T.V.G. control which i8 subsequently described has been developed maklng the as~umptions that the rever-beration level ln all directions around the ship is uni~orm, variatlons in reverberation level between successlve ping-cycles are not unacceptably large and gain control in steps il~2'~

of 1 dB i6 acceptable, Referring now to Fig. 6 the preamplifiers 110 oi pre-amplifier circuit 32 contain gain stages controllable over a range of 6~ dB in 1 dB ~teps. A binary code is ~ed into the preamplifiers 11~ along control bus 112 from T.V,G. (Tlme ~aried Gain) control 114. Initially, dur~ng the ~irst ping-cycle, the gain of preampllfiers 110 follows a fixed, pre-determined pattern, gradually lncreasing from minimum to maximum to compensate ~or an assumed "~tzndard" reverberation curve. Thls "~tandard" reverberatlon curve may be lntroduced into the T.V.G. control 114 alonginitial gain pro~ile bu~
116. me "standard" reverberation curve may corre~pond to the reverberation curve 106 cf Flg. 5. me instances at which the gain change~ are to be made have been derived ~rom the "standard~' reverberation curve and are stored in the T.V.G.
control 114.

It ~hould be understood that the period between g~in changes i~ dependent upon th~ tlme it t~kes for the re~er-berat~on to drop 1 dB. Normal reverberation close to the ship i8 large and decrease~ almost exponen~ially as the range increases. At some point in tlme the reverberation level falls below that of amblent noisa and hence need not be considered. The A.T.V.~. does not deal with the a~fect~
o~ reverberation at r~nge~ where the reverberation has fallen below the ambient noise level in the mediumO

ll~L9Z~fi Durlng the period between gain changes, a large number of full beam output~ ~rom beam~ormer output bu~ 104 are ied into gain correction computer 118 via bus input 120~ Gain correction computer 118 averages all these beam output signals.
The average i~ then compared in gain correction computer 118 with the desired full beam output signal whlch i8 introduced via reference input bu~ 122. Any discrepancies between the deslred output signal and the average is an indication that for this interval between gain changes a ga~n correctlon i~
requlred. I~ ~ G repre~ent3 the gain correction then t~e gain correction required i8S
~ G = -20 log A~/R where Av ~ average o~ the beamformer full ~eam output signals R = the desired full beam output.

For each perlod between gain changes gain correction computer 118 compute~ the v~lue of the gain correction, ~G.
me galn correction, ~G, i fed into T.V.G. control 114 along bus 124. me T.V.G. control 114 add~ the gain correction to the previou~ gain for that interval. It Qhould be understood that the galn correction may be a po~itive or a negative value.
The new gain ~alue~ obtained from the ~um are then stored in the T.V.G. control 114. During the next ping-cycle, i.e.
the maximum time between which a pulse i3 transmitted and received, the new gain values are applled to the preampliiers 110. Ihi~ process is repeated on sub~equent plng-cycles 80 that the g~tn-tlme pro~ile 1~ continually rQvised to match the current characteri~tics o~ the medium. The length Or a ll~V~46 ping-cycle may be in the order of 30 ~econd~. Becau~e the range of the target corresponds to time, different gain~ are applied to the preampliriers a~ the time of the ping-cycle increa~es.

Becau~e all the inputs are controlled ~imultaneou31y by the same control lines, beam-to-beam variation~ in normallzation wlll occur, which are removed ln a beam oriented A.G.C. tadaptive galn contrQl) stages _ (Fig. 1) iollowing the beamforMer 38.

The exact computation of ~ G ls a complicated a~d time consumlng process ior ~he g~in correction computer because o~
the logarithmic function. The gain correotion co~puter of this application useY a ~imple algorithm. For any numb~r o~
dB'~ required rOr gain correction, the ratio Av/R can be expressed as a simple binary fractlon, with su~iclently clo~e approximation. Eor exa~ple, if the gain correction (~G) i8 to be -8 dB, then 20 log Av/R-B, thus A~/R=2.511 or Av/R
would approximately equal 2.5. It would then iollow that Av/R = 2.5 Av 3 2.5 R
A-R = 1.5 R
~or R chosen to be unity.
Av-l - 1.5 The conver3e o~ this would apply and i~ Av-1~1.5 a ~ai~
correction of -8 dB would be required. Table IV ~hows a binary code which has been generated ~or gain decrease~.

~l~VZ46 TABLE IV
. . - . ~ .. . .......... ...
~G (d~) Ratio Av/R Difference Av-R
-20 log Av/R . ~ ~ - ~
. Exact ~raction Fractlon 3iDary Code 1.122 1 1/8 1/8 0000.001 2 1.259 1 1/4 1/4 00~0.010 3 1.412 1 3/8 3/8 0000.011 4 1.58~ 1 1/2 1/2 0000.100 1.778 1 3/4 3/4 oooo.llo

6 1.995 2 1 OoOl.ooo

7 2.239 2 1/4 1 1/4 000~.010

8 2.512 2 l/2 1 l/2 o~ol.loo

9 2.818 2 3/4 1 3/4 oool.llo 3.162 3 l/8 2 1/8 oolo~ool 11 3.548 3 1~2 2 l/2 oo~ oo 12 3.981 4 3 0~11.000 13 4.467 4 l/2 3 1/2 0011.100 14 5.012 5 4 oloo.ooo 5.623 5 5/8 4 5/8 0100~101 16 6,310 6 1/4 5 1/4 0101.010 17 7.079 ? 1/8 6 1/8 0110.001 18 7.943 8 7 0111.000 19 8.913 8 7/8 7 7/8 0111,111

10.000 10 9 1001.000 21 11.22 11 1/4 lo 1/4 lolo.olo 22 12.59 12 ~/8 11 5/8 loll.lol 23 14.13 14 1/8 13 1/8 1101.0~1 24 15.85 15 7/8 14 7/8 1110, It should become apparent from Table IV that for any number of dB18 the gain correction may be obtained irom a read only memoI~ which may be addre~sed by the binary code obtained ~rom the difference of Av-R.

li4~4 Figure 7 i8 a block dlagram showing the A.T.~.G. control 36 ln greater detail. Beam output signals from the beamformer are ~ed back via bus 120 to averager 126. Averager 126 accu-mulates a large sample oi beam ou~puts and averages them once every period ~et~/een gain changes. It should be understood that during one scan ~6 beam outputs are iormed. Because it ha~ been found that the minimum period for gain change is about 20 mS 32 beam output signals may be added for 32 8cans.
The ~our beams aft of the ship are excluded due to high propellor nolse. Hence ~or each period between gain change~
averager 126 sums 32 beam output ~ignals 32 times and di~ides thi~ summation by 1024~ Divlslon in the case oi~ a digital signal only requires ~hifting. The average is fed into com-parator 128 along bus 130. Comparator 128 sub~tracts desired value R on bus 1~2 from the average value. Comparator 128 output~ the ~ign value and dlf~erence value on respective buses 134 and 136 to line ~elector 138. The line selector 138 uses the sign bit to determlne if the gain should be increased or decreased. Ii the galn should be increa~ed line select 138 acces~es read only memory (ROM) 140 by lncrease bus 124 caus1ng ROM 140 to output the gain correction incr~ase at the addres~ location determined by the difference value. I~
the gain ~hould be decreased line 3elect 138 accesses ROM
140 by decreas~ bus 144 to output the gain correction decreases from ROM 140 at the address location determined by the dii-ference value.

ROM 140 i~eeds the gain correction along bUB 124 and into ~1~0~46 ~1 adder 148. Adder 148 take the gain correction valu~ and adds it to the previous ~ain stored in Random Acces~ Memory (RA~I) 150 which enters adder 148 via bus 152. The ~um represent~
the new ~ain and i~ outputted on bus 154 to the line selector 156. The line selsctor inputs the new gain into the R~ 150 via bu~ 158. Durlng the initial pi~g-cycle the a~sumed re-verberatlon curve lnformation and initial gain is deposited ln ~AM 150 and line ~elector 156 by way of bu~ 160. RAM 150 output~ the gain to the galn decoder 16 along bus 152. The gain decoder 161 code~ the n~w gain and output~ the coded gain on plurality of llne~ 162 o~ control channel 112 whi.ch adjusts the gain of the preampllflers ~or each period betwee~ galn changes. Hence ~he T.V.G. control 114 computes the new gain ~hile ~imultaneously using the previou~ gain to control the preamplifier~.

Now referring agai~ to Fig. 1 the fir~t ~tage o~ proces-~ing section 25 is the automatic gain control tAGC) stage 46.
me AGC controller 46 increase~ or decreases the gai~ of each full beam output slgnal lndependent o~ the other beam output ~ignals a~ter a compari~on of the actual beam output ~ignal's galn wlth the average ga~n of all the beam signals oi' that beam. me AGC controller 46 eliminates any beam-to-beam variation~ ln normallzation which may occur irom the A.T~V.G.
control 36. The beam output signal ~rom AGC 46 is ~ed lnto the own doppler nullifier (O.D.N.) and iine bearin~ computer 54.

Although the ~unction oi~ the O.D.N. 48 (Fig. 1) ha8 114VZ4~

been prevlou~ly described to reduce the full beam output s~gnal to hal~ by taking away the own ship's doppler effect on the slgnal, some of the prlnciples o~ the O.D.N. should now be dlscussed.

Sonar echo signals, the signal3 to be detected, are generally received at a ~requency which dif~ers ~rom the transmitted rrequency. The dif~erence i~ known a~ the doppler effect which i8 due ~or the most part to the relati~e motion o~ the target with re~pect to the ~hip.

In order to extract the target doppler from the total doppler it i8 necessary to remo~e the doppler ef~ect of the own ~hip's motion. The O.D.N. involves the mixing of the received beam formation's frequency with a local frequency whlch corresponds to the ~hip'~ doppler followed by filter-lng out the sum frequencies obtained when the ~requencies are mlxed.

In order to reconstruct the original target in~ormatlon from the ~onar slgnal~ enterlng the receiver the sampllng rate of the recelver must be at least twice the hlghest signal ~requency present. HoweYer, because the recelver o~ the pre-sent applicatlon lncorporates antl-aliasing ~ilters ~t the input o~ the receiver, the sampllng frequency i~ reduced to at least twlce the band o~ ~requencles pa~sed through the anti-aliasing ~ilters. It should be understood that the desired ~requency band may be "folded" into the baseba~d.

,_ 3Z4f~

The so called folding frequencie~ occur at multiples Or half the sampling frequency. Signals occurring at multlples out- -side the desired band will also be ~olded into the baseband.
Such frequencies are called "alia~es". me purpose of the anti-allasing filters is to restrict the frequency range to the desired band and thu~ suppres~ signals ln any other band.
The mirror image of the baseband extending from 0 Hz lnto the negati~e irequency region cannot be eliminated.

~he doppler effect o~ the beam output signal may vary from a minimum of ft-ftd to a m~ximum ~ ft~td~ where ~t repre~ent~ the transmitted frequency and ftd represents the total doppler shlft.

If the ~a~pling ~requency is cho~en for the minimum recei~ed rrequency to be a multiple oi the sampling frequency, as ls the case in the receiver of this application, the received frequency of the beam output signal wlll always iall in the band o~ (a)(fs) to ~ fs, (where a i3 an integer and fs the sampling ~requency). The sampllng frequency will be ~olded into the baseband from 0 to fs/2. The frequency o~
the recelved beam output signals 19 then:
e ido + ftd. ~lhere fdO is the ~requency of zero total doppler and ~td is the total doppler shi~t which may ha~e poslti~e a~ well as negat ve Yalue3.

As lndloated earller the maximum doppler shi~t of the ~ 2 4 ~

frequency about the transmitted frequency wa~ cho~en to be 400 Hz. The value of maximum doppler wa~ chosen by assuming that the maximum ~hlp ~peed and target speed were each 40 knot~. Given a conversion factor o~ 5 Hz/knt, the maximum speed of 80 knts becomes a doppler ~hlft of 400 Hz (~ 80 knt 5 Hz/knt)~

If thi~ low rrequency recQived beam output ~ignal, fe~
i5 mixed wlth a frequency proportlonal to the own shlp'~
doppler ta known variable), the resulting difference component will no longer contain the o~n ship 1 8 doppler. Thl8 result-ant frequency is proportional to the target doppler.

During the mlxing proces~ within the O.D.N. device the followlng ~hould be considersd:

1. In addltion to the dif~erence frequency the output of the mixer will co~tain the sum frequency, which i8 to be filtered out.

2. Sincs the d~ppler i8 variable, the sum and differen¢e frequencie~ will each requlre a frequency band. The~e fre-quency band~ mu3t be separated su~flciently so that the above filtering in the above paragraph may be done.

3. m e effective noi~e bandwidth of the anti-alia3ing filter may be a~umed to be not more than 1~5 times the pas~band. This mean~ that the noise bandwidth of the di~-114t335 i'erence signal will be considerably larger than the band requlred i'or the actual ~ignal l'requencies. In order to obtain a maximum S/N enhancement the center of the difference fre-quency band must be shifted to the right, so that the portion of the noise band in the negative frequency region, when folded over, fall~ outslde the desired difference band. Thls 18 clearly shown in Figs. 8A and 8B. Flgure 8A ~hows the ef~ect fold over of noise on the de~ired difference frequency band 170.
The filter response i8 designated as 172 having a noise band 174. A~ shown in Fig. 8A the noise band in the negative frequency reglon 176 folds over into the desired band as indicated by shaded are~ 17B. This ~olding affect effectively increases the nolse in the desired difference frequency band 170. Referring now to Fig. 8B shifting the center frequency of the difference band 170 to the right by desired amou~t 180 wil result in the folded over noi~e from the negati~e frequency reglon, a3 indicated by ~haded area 178, ~alling outside of the de~lred difference frequency band 170.

4. The requirement~ ~f paragraph~ ~ and 3 above may be satisfied by lntroducing an "offset" in the relation between the beam output signal frequency and sampling fre-quency~ and also in the local oscillator l'requency.

The off~et in the beam output signal frequency entering the O~D.N. devlce may be Aef~ned a~:
A - (transmitted frequency) - (nearest lower multiple of sampling frequency with re-spect to the tran~mitted frequency) 114VZ9~

and the local o~cillator frequency i~ defined by:
fm = B + own ship's doppl~r~ where B is the offset in the oscillator ~requency.

It should be understood that the optimum solution is obtained when:
A + B ~ fs/2 and the center of the difference band (A-B) is located as shown in Flg. 8B, Referring now to Flg. 9 there i~ ~hown an O.D.N. devlce 10 which takes into consideration the frequency techniques previou~ly mentloned in the mlxlng operation.

A read only memory (ROM) 182 is shown addreQsed by three buses 184, 186 and 188. ROM 182 outputs a value K on bus 190 ~hich corre~ponds to the value o~:
K , ft COS~, where ft = transmitted frequency, CO ~ = beam angle relative to the bo~ o~ the ship, and c ~ the velocity of sound in water (Y.O.S.I.~i.).
The ROM 182 i~ used to ~tore predetermined values of K for dlfferent values of ft, c and COS~, thereby elimlnating lengthly computatlon times. The valve of ~ read from ROM
182 corresponds to the information on the three Qddress bu~es. Address bus 184 i8 indicatlve of the tran~mltting frequency which i8 a known variable. Address bus 186 i~ in-dlcative of the V.O.S,I,W. for wh~ch certain values have been calculated. Lastly, address bus 188 is indicatlve of the beam angle wlth respect to the bow of the ship,~ . Because +~he direction of the beam being proce~sed is known, a signal indi-cative of the be~m may address ROM 182. In summary, ROM 182 output~ a value K which represents a predetermlned computation for ~elected known input values of the transmittlng ~requency, bea~ angle and V.O~S.I.W.

The value K i~ m~ltiplied wlth the own ~hip's speed, a ~nown variable U inputted on bus 192 to multiplier 194. The result of the multipllcatlon is outputted on bu~ 196 and oor-responds to the doppler eifect of the ship, fO~, where:
fos ~ U x ft cas~

The own s~lip's doppler, fO~, ls then added in adder 200 ~ith offset B introduced on bus 198. The adder 200 may perform the addition in 2~ 8 compliment arithmetic so that the ~utput on bus 202 is fm~B+fo~. The plus or minuS 8ign are indicative of the ship movlng forward or backward, re~pectively~ At this point (bus 202) the O.D.N. device has determined the proper value for the own ship's doppler effect and offset this Yalue by B to give the offset in the local oscillator ~requency, fm.

The O.D.N. ha~ now to produce ior each beam output signal a string of samples which repre~ent the functlon:
COS(2 ~r~ fm b ~t), where t = l/f ... (the sampling frequency, the interval between samples oi the ~ame beam)) and b = is a succession of integer value~ 1, 2, 3 etc.

1~4~24Çi 3~
This cosine value is to be multiplied with the beam information to obtain the desired difference band.

It should be understood that 2 ~r. ~t is a con~tant which lea~es the quantity fm b proportional to the required angle COS(2 ~r- ~m b ~t). It should be under tood that the requlred angle may be determined by the accumulation of ~alues for ~m above for each succe~sive beam output signal by using a Random Access Memory.

The accumulation is done in angle accumulator or adder 2040 Each successive value of ~ is added to the sum o~ the prior values stored in RAM 206. me proper c-orrelation between fm and is correspondlng accumulation is governed by beam address bus input Z08 which address~ RAM 206.

In the system of the present applicatlon, thirty-six beam output signals are formed. Hence, adder 204 and RAM
206 keep a running value of the accumulated angle, ~ , for each beam output ~ignal. The value o~ the accumulated angle is given as:
~= b fm whlch i3 outputted on bus 210 into a cosine ROM 212. For each v~lue of the accumulated angle there is a corresponding predetermined value of the function COS(2 ~ . fm ~t) stored in the memory of co~ine ROM 212.
For the value of the accumulated angle addressing COS ROM 212 on bus 210 the correspondlng cosine function is outputted on bus 214.

Z4Çi The functlon outputted on bus 214, COS(2 1r- im b ~t) may be expressed, for the purposes o~ simplifi.cation~ a~
COS(~m) = COS(B~fo6).

The beam output signal information enters the O.D.N.
along bus 216. 1~e beam output ~lgnal in~ormation h~.s a doppler of COS(ftd) where ftd is the total doppler a~feot.
It should be ~nderstood that ~td=fo8+ftg ~ tg to the target doppler. me beam output signa~ informat~on i8 added in adder 218 with offset A inpu+ted on bus 220. It should be recalled that of~set A correspond~ to a frequency value equl~alent to the transmltted ~requency les~ the nearest lower multiple o~ the ~ampling frequency to the transmltted irequcncy. The output of adder 218 corresponds to COS(~td+A) 3 COS(A+(ftg+io~)) which is outputted on bus 222. It should be understood that this i8 not an exact representation of the beam output 3ignal in*orma~ion but an approximation thereof which is used herein ~ ~impll~y th~ ex~l~n~tion of the oper-ation o~ the O.D.N.

I~e O.D.N. has at this point calculated the o~n 3hip doppler e~i?ect and introduced the offsets. The next 8tep in the processing o~ the information i8 to multiply the information on buses 214 and 222 in multiplier 224.

The output o~ multiplier 224 on bus 225 given as:
COS(A+i?tg+fos) COS(B+~os~
(For simplicity only positi~e signs are used.) = 1/2 COS(A+B~ftg+2foS) ~ 1/2 COS((A-B) ~ (ftg+fo~) fo3) s 1/2 COS(A+B+ftg~2fos) + 1/2 COS((A-B) + f~g).

me first term is the sum term which lies in sum ilre~uency band while the seco~d term i~ the target information which lies in the dl~ference frequency band. The A~B within the paren-thesis of the ~econd term iB the offset of the center ~requency of the difference frequency band ~see Flg. 8B).

BU~ 226 i8 connected to a band pa~s filter (not shown) which fllters out the lnformatlon correspo~ding to the target doppler, in this ~ituation COS(ftg). Thls half beam output ~ignal repre~entation of the target doppler informatlon leaves O.D.N. 48 to enter correlator 50 (Fig. 1~. It should be understood that all timing and control of the hardware in Fig. 9 may be controlled by timing and control circuitry 44 of Fig. 1.

Referring once again to Fig. 1, the signal proce6sor 50 i5 shown having a correlator 51 and a spectrum le~eler 53.

The principle behind the correlator is that the best correlation between the target ini`orm~tion and set values in the correlator will result in the detection of the target speed away ~rom or toward the ship.

For the purposes of the recei~er of the present application 114~2 16 correlates are used. Each correlator ~ill co~er a range of 25 Hz, Recalling the conversion ~actor o~ 5 Hz/knt permits for speed in~ormation on the target to be given withln 5 knt~.

The output of the correlators goes through spectrum leveler 53 which aids ln reducing the noise level further.
The timlng and control of the signal processor 50 is regllated by timing and control circuitry 44.

The fine bearing computer 54, by using informat~on from correlator 51 and automatic gain control 46, can compute hoiJ
~ar to the le~t or right of the stave element ~or which the beam output signal i8 centered the target i8 positioned. The staves el~ments are 10 apart bu~ the fine bearlng computer can increa~e the bearing accuracy to about 1 to 1 1/2.

The output section 30 consists of an output proces~or 52.
From the 8ignal processor target speed is ~ed into proce sor 52~ The stave about ~1hich the beam output ~i~nal is centered and fine bearing is fed through fine bearing computer 54 into processor 52. 'l'he ~.O.S.I.W. and ships course are also fed into the proce~sor 52, Proce~sor 52 output~ in digltal format information to ~he display corresponding to:
1. Target range, 2. Target bearlng with respect to true n~rth, 3, Doppler (target speed), 4. Fine bearing, and Sh~ps heading.

~1~1324~i In summary a complete sonar receiver has been described which subsequent digital beamforming analy~es the received ~onar signals digitally.

-

Claims (6)

What I claim is:

1. A method of extracting own ship's doppler in-formation from a digital beam output signal of a beamformer to obtain target doppler information, the method comprising the steps of:
A) introducing an offset frequency "A" to the digital beam output signal which is a function of the beamformer sampl-ing frequency;
B) generating a digital signal representative of the own ship's doppler information and offsetting the digital signal by frequency amount "B" which is a function of the beamformer sampling frequency and offset frequency "A";
C) multiplying the generated digital signal with the beam output signal to obtain a signal having a difference frequency component and a sum frequency component which both lie in their respective difference frequency band and sum frequency band whereby the difference frequency band has a center frequency offset by amount A-B which eliminates the effect of noise due to fold over from the negative frequency band into the difference frequency band; and D) filtering out the sum frequency component to leave the difference frequency component representative of said target doppler information.

2. A method according to claim 1, wherein the offset frequency "A" is given by:
A = ft - Mfs wherein ft is the transmitted frequency, and Mfs is the nearest lower multiple of the beamformer sampling frequency to the transmitted frequency.

3. A method according to claim 2, wherein offset "B"
is chosen so that where fs is the beamformer sampling frequency.

4. An apparatus for extracting own ship's doppler in-formation from a digital beam output signal of a beamformer to obtain target doppler information, said apparatus com-prising:
A) an adder for introducting an offset frequency "A"
to the digital beam output signal where offset frequency "A"
is a function of the beamformer sampling frequency, B) means for generating a digital signal representative of the own ship's doppler information and means for offsetting the generated digital signal by frequency amount "B" which is a function of the beamformer sampling frequency and offset "A";
C) a multiplier for a digitally multiplying the beam output signal with the generated digital signal to obtain a signal having difference frequency component and a sum fre-quency component which both lie in their respective difference and sum frequency bands where the difference frequency band is offset in frequency by amount A-B which eliminates the effect of noise due to fold over from the negative frequency band into the difference frequency band; and D) a digital filter for filtering out the sum frequency component to leave the difference frequency component repre-sentative of the target doppler information.

5. The apparatus of claim 4, wherein the offset fre-quency "A" is the difference between transmitted frequency and the nearest lower multiple of the beamformer sampling frequency to the transmitted frequency.

6. The apparatus of claim 5 wherein the means for generating a digital signal representative of the own ship's doppler information comprise a multiplier and a read only memory.