GB2184236A - Surveillance sonar for torpedo detection and guidance - Google Patents

Abstract

The sonar transmits frequency swept pulses each of which has a portion of relatively narrow bandwidth and relatively short duration. On reception the pulses, after being digitised at 6 are stored at 6A. The relatively short, narrow- bandwidth, portions of the received pulses are used in a processor 10 to identify "cells" having dimensions of range and velocity, in which a target, or features in which a target may be hidden, exists or exist. The output from the first processor 10 is used to feed only those parts of the stored information which relate to the identified cells from the store 6 to a second processor 18. Here the whole of the received pulses are processed to scrutinise the aforementioned cells for a target of interest. The invention is equally applicable to the processing of information concerning other variables of a target such as range and direction. The invention is also applicable to radar systems. <IMAGE>

Description

SPECIFICATION A surveillance system This invention relates to a surveillance system. It was originally conceived in relation to underwater sonar systems forthe guidance and detection of torpedoes. The invention is not however exclusively concerned with such systems and may find practical application in radars operating bythetransmission and reception of electromagnetic signals.

Existing techniquesforunderwatersonarsur- veillance are able, in theory, to indicatethevariables of range, direction and velocity of targets of interest to a great degree of accuracy. Forexample,thevari- able of range can be measured accurately using pulse compression techniques; the variables of direction and velocity can be measured accurately using extended aperture and Dopplertechniques.

In practice such accuracy may be difficu It to achieve because ofthe large amounts of data to be processed when employing pulse compression, extended aperture and Dopplertechniques. This difficulty is particularly acute where space is limited e.g. in a torpedo, missile or spacecraft.

This invention provides a surveillance system for detecting a feature of interest which can be described by at least one variable quantity, the system comprising: atransmitterfortransmitting into a space or medium surrounding the transmitter signals having different first and second components or parts; and a receiverfor receiving the signals after reflection from features in the space or medium;the receiverincluding meansfor using the first components or parts to conduct a coarse survey of possible values ofthe variable quantity and to determine a region or regions of said possiblevalues containing relatively strong reflected signal components and therefore in which the feature of interest or other features not of interest exists or exist, and for using the second components or parts to conduct a fine survey in said region or regions thereby deter mining whetherthe object ofinterest is presentthere and/orthe value ofthe said variable quantity associated with it.

The variable may be range, velocity or direction and indeed there are other possibilities such as acceleration. In one preferred form ofthe invention the quantities both of range and of velocity are surveyed since these variables are both of particularvalue in distinguishing between objects which are of interest and those which are not of interest. In another preferred form ofthe invention the quantities of range and direction are surveyed. In both cases the resultofthe coarse survey is to identify within the medium being surveyed "cells" having dimensions of range and direction in which an object of interest may exist either alone or being concealed by other objects of little interest. The fine survey serves to inspect these "cells" in detail.Because, in accordance with the invention, it is not necessary to perform a fine survey throughout all the "cells" it is practical to employ such techniques as pulse compression, extended aperture and Doppler velocity measurements without the need for an excessively large data processing system.

The aforementioned "first" components or parts ofthe signals can be short pulses (or parts of pulses) ofnarrowfrequency bandwidth which may or may not be frequency swept or otherwise be selected so thatthey do not require a great deal of processing on reception to give the coarse survey for which they are intended. The aforementioned "second" components or parts of the signals can be long, and preferably frequency swept pulses of wide frequency bandwidth or otherwise be such that sophisticated processing techniques such as synthetic apertures and pulse compression can be used in the receiverto give the required fine survey within the cells identified bythe coarse survey as being of particular interest.

The aforementioned first and second components or parts can be separate pulses separated by a time interval, can occur simultaneously or one can follow immediatelyafterthe otherwith no time spacing.

They can be separated by a frequency interval, can be within the same frequency band or can have different contiguous frequency bands. In a preferred arrangement both the first and second components or parts are defined by a composite pulse of continuously varying frequency, one part of it constitut ing the saidfirst componentor partwhilsta larger partorthewhole of it constitutesthe second component or part.

Whilstthe invention is particularly applicable to an "active" sonar or radar system it may also be usefully applied to passive systems i.e. systems which do not themselves radiate energy. In such a system there would be no transmitter but instead some means defining a signal known to emanate from a particularclassoftargets.Thus in accordancewith a second aspect ofthis invention there is provided a surveillance system for detecting a feature of interest which can be described by at least one variable quantity, the system comprising: means defining ordes- cribing signals transmitted into a space or medium surrounding the system, these signals having differentfirst and second components or parts; and a receiver for receiving the signals from the space or medium; the receiver including meansfor using the firstcomponents or partsto conducta coarse survey of possible values of the variable quantity and to determine a region or regions of said possible values containing relatively strong reflected signal components and therefore in which the feature of interest or otherfeatures not of interest exists or exist, and for using the second components or parts to conduct a fine survey in said region or regions thereby determining whetherthefeature of interest is present there and/orthe value ofthe said variable quantity associated with it.

One particular way in which the invention may be performed will now be described with reference to the accompanying drawings in which: Figure lisa block diagram ofan underwater sonar surveillance system for use in a torpedo; Figure2 illustrates variations of frequency with time of a number ofdifferentwaveforms generated in a waveform generator indicated at 1 on Figure 1; Figure3A isagraphoffrequencyagainsttimeofa pulse transmitted bythe system Figure 1 and following the waveform shown at Don Figure 2; Figure3Bissimilarto Figure 3A but illustrates a graph of amplitude against time; and Figure 3C is a graph ofamplitude against time il- lustrating the output of a receiving transducer shown at 5 on Figure 1,the vertical lines on Figure 3C indicating schematicallythosetimes at which the ampli tude of the received signal is sampled by a digitiser shown at 6 on Figure 1.

Referring firstly to Figure 1 a waveform generator 1,controlled bye timing generator2 produces pulses A,B,C,D,E,F,G,B',D',F', at ultrasonicfrequencies on lines indicated by respective reference letters. The pulses are repeated at regular intervals and the characteristics of each pulse can be seen on Figures 2,3Aand 3B. Thewaveforms on lines B',D' and F' are identical to the latter parts of respective pulses B,D and F. The pulses Dare transmitted via an amplifier3 and a transducer4intothe surrounding water region to be surveyed.

Signals received back from the aforementioned region of water are detected by a receiving transducer 5 and, immediately after transmission of each pulse, the timing generator 2 causes a digitiser6tosample the received signal at times indicated schemmatic allybythevertical lines shown on Figure3C. In fact the frequency ofthe sampling times is much greater than can be shown on Figure 3C and isgreaterthan twice the bandwidth ofthe received signal.

Samples from the digitiser6 are fed to a signal store 6A, which stores all the samples taken during a period between transmitted pulses. The samples are also fed to signal channels of three correlators 7,8 and 9 in a first processor 10. Each channel contains as many samples as are taken during a time period which, as shown in Figure 2, is the duration of signals B',D' and F' i.e. T2-T3. The number of sam ples held in each channel is appropriateto thetime bandwidth product of the reference wave against which the signal is being correlated which, in the case of correlator 7, is the wave B'; in the case of correlator8 is the wave D'; and in the case ofcorrela- tor 9 is the wave F'.

The reference waveforms B',D' and F' correspond to the received signals which would be expected from receding, stationary and approaching features respectively. Therefore a high outputfrom one ofthe correlators 7,8 or9 indicates a receding,stationary or approaching feature respectively. The time of such a high output indicates the rangeofthefeature.

The outputs from the correlators 7,8 and 9 are passed through threshold detection circuits 11,12 and 1 3 which are designed to pass the outputs of the correlatorsonlywhentheyarejudged high enough to be indicative of a target or reverberations capable of concealing a target. Thethreshold detectors 11,12 and 13 thus produce outputs 1 1A,I2Aand 13Aas indictated on Figure 1.

An output on line 1 1A,12A or 13A causes a selector 1 4to feed waveforms A,B,C; C,D,E; or E,F,G onto lines 15,16 and 17 respectively.

Signals on lines 15,16 and 17 are fed to two refer ence channels ofthree correlators 18A,18B and 18C of a second processor 18 whose signal channels receive data from store 19. The data fed from the store 19to the second processor 18 is selected bythe signal on line 1 1A,12Aor 13Aand consists ofthe samples taken during the period P - + Tshown on Figure 3 which includes the length ofTofthetrans- mitted pulse i.e. t2 - t1 and a preceding period P.

Each correlator in the second processor is larger than the correlators ofthefirst processor and is able to hold samples taken during a time period such as thatshown at T. During the period P + Tall groups of successive samples occupying a period Tare correllated in turn with the contents ofthe reference channel.

A high output on a line 18A',18B' or 18C' indicates a target having a velocity defined by the reference signal A,B,C,D,E,F or G currently being fed to the associated correlator 18A,18B or 180; ata range indicated bythetime ofthat high output.

It will thus be seen that the first processor 10 produces a coarse survey within the dimensions of range and velocity and isolates a "cell" having these dim ensions and in which atargetis likely to exist. In the particular example illustrated the first processor is capable of determining velocity onlyto the extent of indicating whetherthe target is approaching, reced- ing or stationary. The second processor determines the velocity to a greater degree of accuracy and, becausethe correlators are larger, i.e. operate over a largertime bandwidth product, can also determine the time of the received pulse to a greater degree of accuracy: this giving enhanced range resolution.

In other embodiments of the invention it would be possible to em ploy th ree or more processo r for pro- cessing to signals with respective different capabilities of resolution.

In some circumstances it is possiblethatthefirst processor conducting the coarse survey might simultaneously identify two or more "cells" having identical rangevalues but different velocity values. In the system shown in Figure 1 this would result in simult aneous outputsfrom the first processor on at least two of lines 1 1A, 12A and 13A. An alternative embodiment of the invention, adapted to handle such situations, includes time multiplexing means for feeding such simultaneous outputs from the first processor in turn to the selector 14. Alternatively a pluralityofsecond processors could be included in which case the selector 14 would have to be designed so that, on receipt of simultaneous signals at two or more of its inputs, itwould feed appropriate reference signals ABC, ODE or EFG to respective second processors.

In another embodiment ofthe invention it would be possibleto employ a single processor and a suitable time multiplexing arrangementtoallocateitat different times to the functions ofthe first and second processors described with reference to Figure 1. Also, or alternatively, the functions ofthe individual correllatorsoftheoreach processorcould be performed by a single correllatorand a suitable time multiplexing arrangement.

Claims (12)

1. Asurveillancesystemfordetecting a feature of interestwhichcan be described by at least one variable quantity, the system comprising: a transmitter fortransmitting into a space or medium surrounding the transmitter signals having differentfirstand second components or parts; and a receiver for receiving the signals after reflection from features in the space or medium; the receiver including means for using thefirst components or partsto conducta coarse survey of possible values ofthe variable quantity and to determine a region or regions of said possible values containing relatively strong reflected signal components and therefore in which the feature of interest or otherfeatures not of interest ex- ists or exist, and for using the second components or parts to conduct a fine survey in said region orregions thereby determining whetherthe object of in teresa its presentthere and/orthe value of the said variable quantity associated with it.

2. Asurveillance system according to claim 1 in whichthevariable quantity is range.

3. A surveillance system according to claim 1 in which the variable quantity is velocity.

4. Asurveillance system according to claim 1 in which the variable quantity is direction.

5. Asurveillance system according to claim 1 in which the variable quantity is acceleration.

6. Asurveillancesystem according to at leasttwo oof claims 2,3,4, and 5.

7. Asurveillance system according to any pre- ceding claim in which the first component part has a shorter duration and smaller frequency bandwidth than the second component part and in which at least the second receiving means includes a pulse compressor.

8. Asurveillance system according to any pre- ceding claim in which the second component part in cludesthefirstcomponent part.

9. A surveillance system according to any preceding claim wherein the signals are mechanical waves.

10. A surveillance system according to claim 9 wherein the signals are sonic or ultrasonic signals.

11. A surveillance system according to any pre ceding claim wherein the first or second components or parts are defined by a composite pulse of continuously varying frequency one part of it constitut ing the said first component or part whilst a larger part orthewhole of it constitutes the second compo mentor part.

12. Asurveillance system for detecting afeature of interestwhich can be described by at least one variable quantity, the system comprising: means defining or describing signals transmitted into a space or medium surrounding the system, these signals each having different first and second components or parts; and a receiver for receiving the signals from the space or medium; the receiver including means for using only the first components or parts to conduct a coarse survey of possible values of the variable quantity and to determine a region or regions of said possible values containing relatively strong received signal components and therefore in which the featu re of i nterest o r other featu res notofinterestex- ists or exist, and for using the second components or parts to conduct afine survey confined tothesaid region or regions thereby determining whether the feature of interest is present there and/or the value of the said variable quantity associated with it.

12. A surveillance system for detecting a feature of interest which can be described by at least one variable quantity, the system comprising: means defining or describing signals transmitted into a space or medium surrounding the system, these signals having different first and second components or parts; and a receiver for receiving the signals from the space or medium; the receiver including means for using thefirstcomponents or parts to conducta coarse survey of possible values of the variable quantity and to determine a region or regions of said possible values containing relatively strong reflected signal components and therefore in which the feature of interest or otherfeatures not of interest ex- ists or exist, and for using the second components or parts to conduct a fine survey in said region orre gions thereby determining whetherthe.feature of interest is presentthere and/orthe value of the said variable quantity associated with it.

13. Asurveillance system according to any preceding claim in which the receiving means includes means for correllating the received signals with first reference waveforms thereby conducting the coarse survey, means for storing the received signals whilst they are being correllated with the first reference waveforms, and means for correllating stored signals belonging to the region or regions identified by the coarse survey, with additional reference waveforms thereby conducting the fine survey.

14. A surveillance system according to claim 1, substantially as described with reference to the accompanying drawings and substantially as illustrated therein.

Amendments to the claims have been filed, and have the following effect: (b) New ortextually amended claims have been filed asfollows:

1. A surveillance system for detecting a feature of interest which can be described by at least one variable quantity,the system comprising: a transmitter fortransmitting into a space or medium surrounding the transmitter signals each having different first and second components or parts; and a receiverfor receiving the signals after reflection from features in the space or medium; the receiverincluding means for using only the first components or parts to conduct a coarse survey of possible values of the variable quantity and to determine a region or regions of said possible values containing relatively strong reflected signal components and therefore in which the featu re of i nterest or other featu res not of interest exists or exist, and for using the second components or parts to conduct a fine su rvey confined to the said region or regions thereby determining whetherthe objectofinterestis presentthereandlorthevalueof the said variable quantity associated with it.