GB2052910A

GB2052910A - Echo location apparatus

Info

Publication number: GB2052910A
Application number: GB8018961A
Authority: GB
Original assignee: Fried Krupp AG
Current assignee: Fried Krupp AG
Priority date: 1979-06-15
Filing date: 1980-06-10
Publication date: 1981-01-28
Also published as: NL8003201A; DE2924176A1; GB2052910B; DE2924176C2; FR2459486A1; AU5906380A; AU534768B2; FR2459486B1

Abstract

An echo location apparatus includes an image store (13) (preferably digital) and a display tube (38). During each echo location period the received signals are stored in positions determined by their location coordinates and with their correct intensities. The build-up of stored signals is such that moving located objects are represented in the display by a plurality of traces, positions occupied by such objects during previous echo location periods being possibly shown with diminished brightness. The read-out of the store is at a fast flicker-free TV scan rate. True motion shifts may be given to the scan signals, and the 'now' position of the own object may be maintained in the centre of the display. <IMAGE>

Description

SPECIFICATION Echo location apparatus The invention relates to a method of portraying echo location signals of an echo location apparatus which are deposited in an image store and indicated from this store with located objects appearing as an afterglow trace and apparatus for carrying out the method.

Such methods for producing after glow traces on an indicator in an echo location apparatus are necessary to provide traces of located objects. An observer should be able to read from the afterglow trace the movements performed by located objects, to distinguish moving from stationary objects to recognize dangers of collision arising from the directions of movement and thereby avoid manoeuvres leading to this result.

German Specification 2637935 describes an indicator for echo location signals which indicates by an afterglow trace objects which are moving in relation to the observing echo location apparatus. This apparatus uses a socalled plasma screen in the form of a point matrix which is organized in accordance with location coordinates. The indicator which portrays intensities is controlled by a calculator with cartesian coordinates derived from location coordinates and the intensities of the echo location signals are stored as brightness information on the plasma screen the individual points in the matrix being capable of assuming two conditions only, bright or dark.

Intensities are, it is true extinguished at different times by accidental or pseudo accidental extinction of individual matrix points, but always reappears when the echo location signals from stationary or moving objects are again received and stored. On the other hand old repeated intensities, which are not received afresh, gradually disappear by such extinction to give afterglow traces of diminishing intensity.

The use of a plasma screen as an image store for producing afterglow traces on the indicator of an echo location apparatus is disadvantageous because only one form of specially designed cathode ray tube specially designed for this purpose permits characterization of the behaviour of moving objects. This inhibits modular construction of an echo location apparatus from conventional standardized circuits and components for indicators.

It is therefore the object of the invention to provide an indicator for echo location signals which can utilize an image store of any desired construction, in which intensities of echo location periods can be read to produce afterglow traces for portrayal of located objects and the store content of which can be transferred to a conventional electronic image reproduction device and portrayed as grey tones corresponding to the brightness dynamic range of this device.

This object is achieved in accordance with the invention in that the intensities of the actual echo location signals are entered into the image store in accordance with their location coordinates and displayed in common with the echo location signals received and stored in previous echo location periods.

In display of the content of the image store in accordance with the method of the invention, located objects are displayed in terms of the intensities of the echo location signals, for example in terms of brightness on a cathode ray tube.

Objects which move in relation to the observing echo location apparatus are subjected to multiple reception with different location coordinates in successive echo location periods and their intensities are stored. Since the entire content of the image store is displayed, the intensities stored in successive echo location periods appear relatively displaced on the display to form traces of the objects related to the observing echo location apparatus, the position of which may for example be at the centre of the image. The traces are generally displayed as afterglow traces. The afterglow traces obtained by this method may be as long as desired being only limited by the display range of the indicating device, so that in regions of the sea where there is little traffic the traces of located objects can be displayed for a period as long as desired.For a clear display in other regions of the sea, the entire display is expunged after a prescribed number of echo location periods so that the newly received echo location signals will be able to portray the changes in the scanned panorama occurring in the interim.

Reproduction of the intensities stored in the image store takes place continuously and at higher image frequency in a time interval immediately following entry. This time interval provides for reproduction of the intensities during the echo location periods because the duration of the entry in the image store in dependence upon reception of the last possible echo signal is shorter than an echo location period.

In accordance with a further feature of the method according to the invention the length of the afterglow traces is limited in that the display shows in addition to the actual echo location period only a prescribed number of preceding echo location periods. A periodic reduction of the stored intensities after this number of echo location periods results in a limitation in the length of the afterglow traces.

The number of echo location periods can be determined by partial or complete scanning of a panorama. Moreover the reduction of the stored intensities can take place continuously or in one or more increments of intensity.

If the echo location apparatus is on a moving vehicle all objects are located in relation to the position of the moving vehicle. The inten sities of the actual echo location signals are recorded in the image store in terms of their location coordinates, whereby the assumed position of the observing echo location appa ratus-which is not in general recorded as an intensity-remains unchanged in relation to the intensities received in the previous echo location periods, for example in the centre of the image. The afterglow traces therefore show the relative movements of the located objects in terms of direction and speed in relation to the locating vehicle (relative motion).Objects which move in the same direction and at the same speed as the echo location apparatus are always located with the same location coordinates and therefore shown without an afterglow trace.

According to a further feature of the invention the movement of the moving vessel carry ing the observing echo location apparatus are taken into account in that the intensities of the echo location signals of preceding echo location signals are displayed with displace ment by a travel increment together with the intensities of the actual echo location signals.

This travel increment represents the distance travelled by the observing vehicle during scanning of a panorama or partial panorama.

The displacement is opposite to the move ment of the observing vehicle. The intensities of the actual echo location signals in relation to the observing echo location apparatus are therefore displayed with their actual location coordinates while the location coordinates of the intensities of the preceding echo location periods are corrected by an appropriate travel increment and displayed as a trace adjoining the intensities of the actual echo location signals. The displayed intensities of the actual and the preceding scannings of the panorama therefore correspond to the movements of located objects in a fixed to earth reference system and depend in direction and speed on their own movements only.As the correction of the location coordinates takes place after each scanning of a panorama or partial panorama, the position of the observing echo location apparatus remains unchanged on the display and is, for example, always in the centre of the image.

The method is carried into practice in an echo location apparatus in which the echo location signal is received by an antenna or a transducer. The antenna is connected to a receiver, the outputs of which are connected to a control circuit and to an input of the image store for intensities. The capacity of the image store is advantageously determined in dependence on the number of intensities to be stored on the basis of the resoluting power pf the echo location apparatus. It is so constructed that the intensities of the echo location signals are entered in accordance with the location coordinates determined in the receiver, e.g. in accordance with azimuth and distance, by means of the control circuit. The intensities of the actually received echo loca tion signals are therefore stored without ex punging the intensities received and stored in previous echo location periods.For display of the stored intensities, an output of the image store is connected to an indicator and its inputs for image storage addresses are con nected to corresponding outputs of a read out unit which generates reproduction addresses.

Other outputs of the read out unit are con nected to the indicator over which are trans mitted control voltages for deflection elements in the indicator which are produced from the reproduction addresses.

The reproduction of stored intensities on the indicator is interrupted by entry of the intensi ties of actually received echo location signals.

In order to synchronize the reception of the echo location signals with entry of the intensi ties in the image store the receiver and the image store are connected to a control circuit.

According to a further advantageous feature of the invention the control circuit, the azimu thal reception counter of which is connected to an output of the receiver for directional information to produce reception azimuthal addresses and the control pulse generator of which is connected to the output of a radial reception counter for producing a transmitter control signal, has outputs for reception ad dresses which are connected to a multiplexer.

Two further inputs of the multiplexer are con nected to outputs of the read out unit for radial and azimuthal reproduction addressees.

The control circuit also includes a reception radial counter and a control pulse generator connected to its output which produces an entry pulse as the result of a transmitter control signal at the start of each echo loca tion period. The entry pulse switches the multiplexer, the outputs of which are con nected to address inputs of the image store, from outputs of the read out unit for reproduc tion addresses to outputs of the control circuit for reception addresses, by which the intensi ties of the actual echo location signals are entered in the image store.

In accordance with another feature of the invention the output of the image store is connected by a change over switch to the input of a damping circuit. This switch and the image store can be controlled by a read pulse from the pulse generator to interrupt the entry procedure and effect reading out and extinguishing. The read pulse is produced at each echo location period after one or more scannings of the panorama together with each radial reception address. The output of the damping circuit is connected to the input of the image store and the entry pulse effects entry of the damped intensity into its empty storage cell.

A comparison circuit, the inputs of which are connected to the receiver and to the damping circuit and the output of which is connected to the image store, effects entry into the image store of the larger of the intensities applied to its inputs. The intensities of the actual echo location signals are therefore entered at their full value and the intensities of previously received echo location periods are re-entered on the image store in damped form. The adjoining intensities displayed on the indicator with different degrees of damping produce the afterglow traces.

When the afterglow traces representing two moving objects intersect, the greater intensity at this point is entered in the image store. In the display of the content of the image store intersecting afterglow traces appear in which the actual intensities are emphasized by uninterrupted greater intensities.

According to a further feature of the invention the image store addresses are modified in accordance with the individual movement of the echo location apparatus. To achieve this the address inputs of the image store are preceded by adders. The adders are connected to the outputs of the multiplexer and to an own movement evaluator for address correction values. The own movement evaluator is connected to a course indicator and to a speed indicator and also to the control circuit in order to pass to the adders after one or more scannings of the panorama address corrections derived from distance increments for the reception and reproduction addresses, when distance increments are determined between two image pulses in correspondence with path increments corresponding to the course and speed of the echo location apparatus.

Such an apparatus operates on the basis of identification of the storage cells in the image store in such a way that the reception and reproduction addresses according to which the image store is entered or read are displaced in relation to fixed image storing addresses and so take account of the movements of the echo location apparatus. With a prescribed image store with fixed addressing this has the result that an image of the scanned panorama is displaced over the image store and parts of the image overstep the boundaries of the store. If the image store is precisely as large as the image of the scanned panorama, the parts of the image which overstep the boundaries of the store can be applied to the opposite regions of the image store.This results in opposite two edges of an image of the scanned panorama lying in juxtaposition and, for example, intensities at the lower edge of the image must be expunged in a region of the image store when the upper edge of the image is displaced into this region.

This apparatus also includes a decoding circuit having an input for a pulse from the own movement evaluator and which is connected to an input of the image store for expunction signals. The decoding circuit when activated by the pulse produces expunction signals for storage cells whose reproduction addresses are determined by boundaries of the periphery of the store to prevent portrayal of intensities which due to the own movement overstep the boundaries of the periphery of the store.

The method according to the invention may also be carried out with advantage with an image store based on cartesian coordinates. In such a case the image store may be preceded by a coordinate transformer which transforms the image storage addresses prepared in terms of polar coordinates. If, in addition, reproduction and display are organised on the basis or cartesian coordinates, for example on a television monitor (on the line image basis) the coordinate transformer transforms only the reception addresses which are normally present in polar coordinates in echo location apparatus.

According to another feature of the invention for storage of intensities in an image store the address inputs of the store are connected to an own movement calculator, which generates image storage addresses and read pulses for reading out the intensities. The own movement calculator is connected to a course indicator, a speed indicator and the control circuit and is so controlled by the image pulse from the latter that after scanning of the panorama or a partial panorama path increments corresponding to course and speed are converted between image pulses into distance increments in the opposite direction.

The image storage addresses are corrected by the own movement calculator by the address corrections obtained from these distance increments and switches to enter the intensities in the image store.

If an echo location apparatus according to the invention is utilized to display echo location periods on a sonar basis it is advantageous to double the image store to render possible rapid display undisturbed by the slow reception. Storage and damping of intensities of previously received echo location periods may then be effected by transfer of the intensities from a first image store provided for reception only to a second image store.

An echo location apparatus with display in accordance with the invention has the advantage of displaying located objects as afterglow traces of freely selectable length on a conventional electronic image producing device. It is also of considerable advantage that the brightness of the afterglow traces can be reduced in prescribed stages or in increments of intensity dependent on the age of the intensity, i.e. the number of echo location periods since the storage of the intensity, and that the actual position of the located object is situated at the position of maximum intensity. A cathode ray tube phosphor screen with an extremely short afterglow can be used when, for example, its luminous colour is advantageous for particular purposes.

Basically apparatus according to the invention can be based on analogue or digital circuitry but having regard to the present state of the art and the cost of component parts integrated digital circuitry is to be preferred. A digital image store permits relatively simple transformation of polar coordinates to cartesian coordinates and vice versa and allows reception and display of the echo location signals in different coordinate systems. If, as is usual in radar and sonar installations, the echo location signal is characterized in terms of azimuth and distance it is readily possible by suitable coordinate transformation to display by the line image procedure, for example on a television monitor.

As, in addition, the scanning, i.e. the reading in of the image store, and the display can take place at different image frequency, the use of a sufficiently high image frequency for the display avoids the otherwise disturbing continuous diminution in brightness during the scanning of the panorama.

By the display according to the invention of located objects as readily recognizable afterglow traces, the observer at the echo location apparatus can interpret unambiguously and without misunderstanding the movements of the objects in relation to the observer. When the observing echo location apparatus is on a moving vehicle all objects are shown as afterglow traces, the relative movements and direction of movement of which correspond to their movements in relation to the echo location apparatus.

The further development of the invention has the further advantage that the display takes place in a system fixed in relation to the earth in which the locating vessel is situated in the centre of the image. By taking account of the own movement in the display, afterglow traces of moving objects are obtained which correspond to the path of movement of the located objects in this fixed reference system. Stationary objects become still more clearly apparent because they produce no afterglow traces. The position of the observing echo location apparatus is always in the centre of the image and does not wander out of the display.

Embodiments of the invention will now be described with reference to the drawings, in which: Figure 1 is a symbolic portrayal of echo location signals of three successive panorama scannings of a moving echo location apparatus in a reference system fixed to the earth; Figure 2 is a symbolic portrayal of echo location signals of three successive panorama scannings in a reference system related to a moving echo location apparatus; Figure 3 is a symbolic portrayal of echo location signals of three successive panorama scannings of a moving echo location apparatus in a reference system fixed to the earth, in which the observing apparatus is always shown in the centre of the image; Figure 4 is a circuit diagram of an echo location apparatus providing a display in accordance with Fig. 2;; Figure 5 is a circuit diagram of an echo location apparatus providing a display in accordance with Fig. 3; and Figure 6 is a symbolic portrayal of echo location signals from three successive panorama scannings stored in an image store and characterization of the storage cells by image store address (BAx, BAy) and read out or reproduction addresses (AWx, AWy).

For a situation in which observations are made by an echo location apparatus on a moving vehicle it is assumed that a fixed object and a moving object are located. The locating vessel is moving during scannings of the panorama at a constant speed towards the north and the located vehicle is moving at twice this speed towards the east. The display of the echo location apparatus is oriented to the north.

Figs. 1 to 3 show different modes of display of the echo location signals by image sequences, consisting of three successive images, la, lb, lc; 2a, 2b, 2cand 3a, 3b, 3c which illustrate symbolically the development of the observed situation in time sequence and each of which represents a panorama scanning. The images 1 a, 2a and 3a show the same initial situation at time to. The images 1 b, 2b and 3b show the different situation at time t, = to + dt and the images 1 c, 2c and 3 c show the later situation at time t2 = to + 2dt, each situation being separated by the same time increment dt.

A square and a triangle designate respectively the intensities of an echo location signal from a fixed and from a moving object, whereas as the circle indicates the position of a vehicle on which the observing echo location apparatus is situated. Symbols (circle, triangle and square) of full size represent the intensities of the echo location signals received during the actual panorama scanning.

Smaller symbols represent the intensities of signals received during previous echo location periods.

The image sequence in Fig. 1 shows the display of echo location signals in a reference system fixed to the earth (True Motion Display). The situation in image 1 b has developed from that in image 1 a in that the vehicles have travelled at time t1 a distance determined by their speed. The position of the fixed object represented by the square remains unaltered. The positions of the moving vehicles at time to as shown in image 1 a are indicated in image 1 b by a smaller triangle and circle. In image 1 C the vehicles are shown to have travelled at time t2 a further distance determined by their speed and their positions at previous scannings are indicated by smaller symbols.With this mode of portrayal the stationary object does not change in position but moving objects move across the display in correspondence with their course and the successive symbols describe a trace representing the movement of the vehicle.

The images 2a, 2 b and 2coin Fig. 2 show the display of an echo location system on a moving vehicle in which the coordinates of all objects are related to their actual position (Relative Motion Display). The position of the observing echo location apparatus (circle) is marked at the centre of each of images 2a, 2b and 2 c. However the fixed object (square) moves in relation to this position in the opposite direction. By comparison with images 1 c and 2c it is apparent that the diminished intensity symbols of previous panorama scannings yield traces in image 2cwhich are obtained by vectorial subtraction of a movement vector of the observing echo location apparatus from movement vectors of the objects. The fixed object provides a single trace.

The image sequence in Fig. 3 shows the display of an echo location apparatus in which the position of the observing vehicle always appears at the centre while the objects move across the display in accordance with their true courses. The situation in image 3b has developed from that in image 3a, the large triangle and the large square showing the actual positions of the located objects and the smaller triangle and square their positions at the previous panorama scanning but displayed in accordance with the movement of the locating vessel in the opposite direction. The distance travelled by the observing echo location apparatus due to its own speed during the time increment dt is exactly represented by this displacement.Comparing images 1 C and 3c it will be seen that the fixed object and the observer produce no trace while the moving object (triangle) produces a trace indicating its course. This mode of display is to be preferred in comparison with image 2c in which the fixed object produces a trace and in comparison with Fig. 1 in which on continuance of the image sequence the observer moves out of the display, because fixed objects produce no afterglow trace, the true courses of moving objects are shown and the observing echo location apparatus remains in the centre.

Fig. 4 is a block circuit diagram of an echo location apparatus according to the invention.

Each echo location signal picked up by an antenna 1 is fed to a receiver 2, which produces at one output signals Es representing the intensity of the received signal and at another output corresponding directional information RI. The echo signal Es is entered through an analogue-digital converter 3 in a pre-store 4, which can be constituted by a shift register. Sampling pulses Al and storage pulse SP for the A/D converter 3 and the prestore 4 are generated by a control circuit 5, which includes a reception radial counter 7 controlled by a pulse generator 6. The counter 7 also produces at its output a radial reception address RA corresponding to the radial location coordinates and a transmitter control signal SS, which controls the transmitter 8.

The signals SS are also fed to a control pulse generator 9. The directional information RI in the echo location signal determined in the receiver 2 is converted by an azimuthal accommodation circuit 10 into a pulse sequence, the pulses of which represent angular increments, which is fed to a reception azimuthal counter 11, at the output of which appear azimuthal reception addresses AA.

The radial reception addresses RA produced by the counter 7 and the azimuthal reception addresses AA produced by the counter 11 are transmitted via a multiplexer 1 2 as image store addresses BA to an image store 13, the content of storage cells in which is determined by the azimuthal and radial resolution of the echo location apparatus. As the multiplexer 12, in order to achieve continuous reproduction passes reproduction addresses, RW, AW to the image store 13, it is caused at the start of each echo location period to transmit the reception addresses RA, AA instead of the reproduction addresses RW, AW as a writing pulse SI which is shorter than the duration of the echo location period.

The pulse generator 9 provides together with the transmitter control signal SS the writing pulse SI which controls the storage operation in the image store 1 3 in conjunction with the multiplexer 12. In addition the pulse generator 9 compares the number of previously stored echo location periods with a predetermined number and, upon coincidence, produces a read out pulse Ll which is also passed to the image store 1 3. This number corresponds, for example, to the number of echo location periods required for complete scanning of a panorama.

The pre-store 4 stores the intensities of the echo location signals for a direction allotted to the image store addresses BA and these are read out from the pre-store 4 during the entry operation, which has priority over the reproduction, and entered in the image store 1 3.

The reproduction is carried out at a considerably higher image change frequency than the reception. For control of the reproduction pulses are generated in a read out unit 20 by a pulse generator 21 at a higher frequency than that of the pulses generated by the pulse generator 6 and these pulses produce in an azimuthal reproduction counter 22 an azimuthal reproduction address AW which is fed to a multiplexer 1 2. Another output signal of the counter 22 is fed to a filter 23 and to a radial reproduction counter 24. The counter 24 produces the radial reproduction addresses RW, which are fed to the multiplexer 1 2 and produce by means of a signal transformer 25 a saw tooth voltage. A sinusoidal output signal from the filter 23 and the output signal of the signal transformer 25 are multiplied in a multiplexer 26.To control an indicator 30, deflective signals are provided at the outputs 31, 32 of the read out unit 20 which are displaced in phase by 90 by a phase shifter 33. The deflection signals are amplified by amplifiers 34, 35 and applied to deflection coils 36, 37 of a display tube 38. This mode of connection of the counters 22 and 24 produces on the display a stepped spiral. The intensities are read out from the image store 1 3 on the basis of radial and azimuthal reproduction addresses RW and AW supplied to it by the multiplexer 1 2. The digitalized intensities are converted by a D/A converter 40 into brightness signals which are applied to control of the brightness of the display tube 38 by an amplifier 41.

As a further feature of the invention a changeover switch 42 is provided between the image store 1 3 and the D/A converter 40 and this switch has a second output connected to a damping circuit 43. The switch 42 is changed over by the read pulse Ll at the time when the reception addresses RA, AA are applied to the image store 1 3 as image store addresses, BA. The read pulse Ll enables reading out of already stored intensities from the image store 1 3 which are passed by the switch 42 to the damping circuit 43. In addition, the image store 1 3 is preceded by a comparison circuit 44, one input of which is connected to the pre-store 4 and the other to the damping circuit 43.The circuit 44 compares the intensities offered by the pre-store 4 in correspondence with reception addresses RA, AA with the intensities at the output of the damping circuit 43 and transmits the greater intensity to the image store 1 3 in order, in the case of intersecting traces from moving objects, to take account of the greater intensity of the actual echo location signal.

The read pulse Ll is always delivered after a predetermined number of echo location periods, which corresponds to complete scanning of the panorama, and enables stored intensities to be read from the image store 1 3 immediately before entry of the intensities of actual echo location periods which are entered in the pre-store 4. These read pulses Ll effect damping of all echo location signals giving a complete image but damping of all successive images is not necessary. The damping of the intensities results in limitation of the afterglow traces. The display of the contents of the image store by the unit 30 corresponds to that shown symbolically in Fig. 2, in which for example the image 2e results from damping the intensities.The length of the afterglow traces is determined by the amount of damping in dependence upon the number of scannings of a panorama considered for each change in damping.

An advantageous further development of the circuit of Fig. 4 is shown in Fig. 5 which is intended for the type of display shown in Fig. 3. The circuit includes an x/y image store 50 which is based on cartesian coordinates so that its lines represent ordinate addresses and its columns abscissca addresses.

The number of storage cells in the image store 50 corresponds to the resolution of the echo location apparatus. The position of the apparatus characterizing the origin of the reproduction and reception addresses is at the start of a location operation at a storage cell in the centre of the image store 50. The apparatus is located on a moving vehicle, the own motion of which is evaluated and taken into consideration in the display of the echo location signals. When the moving vessel carrying the echo location apparatus has moved through an increment of distance that can be resolved, its position is displaced from one storage cell to an adjoining cell.

The multiplexer 1 2 is followed by a coordinate converter 51 which transforms polar coordinates into cartesian coordinates and this is connected to an abscissca adder 52 and an ordinate adder 53. To take the own movement of the vessel into account, an own movement unit 55 includes a course indicator 56 and a speed indicator 57 which are connected to an own movement evaluator 58 which receives an image pulse BI from the control circuit 5. An image pulse BI is produced by the pulse generator 9 after that number of echo location periods which corresponds to complete scanning of a panorama.

The evaluator 58 provides at its output an abscissa and/or an ordinate pulse whenever the vehicle has moved through a distance increment during scanning of the panorama.

The abscissa and ordinate pulses are counted additively or subtractively according to the course of in respective counters 59 and 60 and applied as address corrections AK to the adders 52 and 53, at the output of which appear corrected image storage addresses BAx, BAy on the basis of which the image store 50 receives entries or is read out.

As the result of correction of the image storing addresses BAx, BAy on the basis of the own movement of the echo location apparatus, the origin of the reception addresses RA, AA and the reproduction addresses RW, AW and the basic coordinate system are displaced in relation to the image storage addresses BAx, BAy determined by the technical design of the image store 50.

Edge coordinates of the scannings of the panorama which correspond to the limits of the range of the echo location system therefore overstep the fixed image storage ad dresses BAx, BAy characterizing the storage cells. These edge coordinates correspond to reception addresses RA, AA and reproduction addresses RW, AW and must therefore be allotted to the fixed image store addresses BAx, BAy of the storage cells at the opposite edge.

The image sequence in Fig. 6 shows the effect of the build up of the content of the image store 50. The three images 6a, 6b and 6e show the image store 50. Lines and columns of the image store 50 are characterized by distiguishable addresses. The numerals 1 to 11 designate the fixed storage addresses BAx, BAy allotted to the storage cells of the image store 50, while the numerals - 5 to + 5 correspond to reproduction or reception addresses AWx, AWy. The symbols represent intensities as already explained with reference to Fig. 3.

In the images 6a, 6band 6cthe circle representing the position of the observing echo location apparatus is marked as the origin of the reception or reproduction addresses AWx = 0 and AWy = O. The intensities of located objects (large symbols) are entered in the image store 50 on the basis of this origin as can be seen from a comparison with Fig. 1. The fixed object (square) is always entered at the same image storage address BAx = 9 and BAy = 3 so that no smaller symbol is shown. In the case of moving objects intensities of echo location signals from previous echo location periods remain in the same storage cells and are shown by symbols.Image 6b differs from image 6a in that the reception and reproduction addresses AWy designating the lines are displaced in relation to the fixed image storage addresses BAy by an address value corresponding to an increment of distance which has been traversed by a vehicle carrying an echo location apparatus. The reception or reproduction address AWy = + 5 which is missing at the upper edge of the image has been joined on at the lower edge. Accordingly, as will be seen by comparing images 6band 6c, there exists between the lines AWy = - 5 and AWy = + 5 a displaceable horizontal image zone which is represented by a dash line.

Stored intensities which are crossed by this image zone must be expunged so that they are not shown in a false line or column. The fixed object (square) in image 6cfor example having reception and reproduction addresses AWx = + 3, AWy = - 5 would, upon further own movement of the vessel, be designated by the coordinates + 3, + 5.

Similar considerations apply to alteration in the column addresses AWx which result in a vertical image limit.

A display according to image 3e is obtained from the image 6c in that the content of the storage cells are read out in relation to the origin of the addresses AWx, AWy for exam ple continuously starting from the line AWy = - 5 up to the line AWy = + 5 and displayed. The lines AWy = + 4 and AWy = + 5 are then added in the correct order at the top edge of the image.

For expunction of the intensities at the image limits the circuit of Fig. 5 includes a decoding circuit 61 connected to the outputs of the coordinate transformer 51. This circuit is activated by a pulse LP from the own movement evaluator 58 and delivers with those reproduction address AWx, AWy which are determined by the image limits a signal LS for extinguishing the relevant cells in the image store 50. In the case shown in Fig. 6 they are the reproduction addresses AWx in the line AWy = 5.

Claims

1. A method of portraying echo location signals of an echo location apparatus which are deposited in an image storage and indicated from this store with located objects appearing as an afterglow trace, characterised in that the intensities of the actual echo location signals are entered into the image store in accordance with their location characteristics and displayed in common with the echo location signals received and stored in previous echo location periods.

2. A method according to claim 1, characterised in that after a number of echo location periods required for complete scanning of a panorama or part of a panorama the echo location signals received and stored during previous echo location periods are displayed displaced by an amount corresponding to the distance travelled meanwhile by the echo location apparatus as the result of its own movement.

3. A method according to claim 1 or claim 2, characterised in that the afterglow traces can be limited in dependence upon a predetermined number of echo location periods.

4. A method according to claim 1 or 2, characterised in that the stored intensities are reduced in dependence upon a predetermined number of echo location periods.

5. A method according to claim 4, characterised in that the intensities are reduced by one or more increments.

6. A method according to claim 4 or 5, characterised in that the number of echo location periods is a multiple of the number required for complete scanning of a panorama or part of a panorama.

7. Apparatus for carrying -out the method according to claim 1, characterised in that an output of the receiver of an echo location system is connected to an image store, the storage capacity of which corresponds at least to the resolution of the system, in that a control circuit for synchronizing the reception of the echo location signals with entry of their intensities in the image store in accordance with the location coordinates determining their direction of reception is connected to the receiver and to the image store, and to an indicator, thereby the read out unit produces control voltages for deflecting elements of the indicator and controls the image store to portray the intensities on the indicator.

8. Apparatus according to claim 7, characterised in that the control circuit includes a reception azimuth counter, which is connected to a directional output of the receiver and forms from directional information (RI) from the receiver azimuthal reception addresses (AA), and a pulse generator followed by a reception radial counter for generating a trans mitter control signal (SS) and radial reception addresses (RA), whereby the addresses (RA, AA) form image store addresses (BA) for entry and are applied to address outputs of the control circuit, in that the read out unit includes a repitition frequency generator with following reproduction azimuth counter and reproduction radial counter for production of azimuthal and radial representation addresses (AW, RW), whereby the addresses (AW, RW) form image store addresses (BA) for reading out which appear at address outputs of the read out unit, in that a multiplexer has its input connected to the address outputs of the control circuit and of the read out unit and its output connected to address inputs of the image store, in that the multiplexer is switchable from the read out unit to the control circuit at the start of each echo location period by a recording pulse (SI) by which the image store is controllable, whereby the control pulse (SI) is produced on a pulse generator which follows the radial reception counter and has a duration less than that of the echo location period.

9. Apparatus according to claim 8, for carrying out the method according to claim 4, characterised in that a reversing switch is provided between the image storage and the indicator and has a second output connected to a damping circuit, in that the reversing switch, and the image store together with each radial reception address are controllable by a read out pulse (LI) from the pulse generator to interrupt entry, read out and expungement, whereby the reversing switch cuts off the output of the image store from the indicator and connects it to the input of the damping circuit, the output of which is connected to the input of the image store.

10. Apparatus according to claim 9, characterised by the provision between the receiver and the image store of a comparison circuit, having a second input connected to the output of the damping circuit, whereby the comparison circuit passes the greater of the two intensities to the image store.

11. Apparatus according to claim 7 or 8, characterised in that the image store addresses (BA) are polar coordinates, whereby scanning of a panorama in each echo locating period a line of storage cells is provided in the image store corresponding in number to the resolvable distance increments, in that in each line the intensities of the echo location signals can be stored in dependence on their transit times, in that the read out unit produces reproduction addresses (AW, RW) for reading out the intensities of all echo location periods for each increment of distance and a control voltage for the deflecting elements to produce a circular display in accordance with the distance increments.

1 2. Apparatus according to claim 7, characterised in that the image store addresses are cartesian coordinates, whereby lines and columns in the image store correspond to ordinates or abscisscae, in that a coordinate converter is provided between the control circuit and reading unit and the image store which, for each distance increment r of the direction f produces as image store addresses (BAx, BAy) abscissca and ordinate coordinates x = r cos f and y = r sin +.

1 3. Apparatus according to one or more of claims 7 to 1 2 for carrying the method according to claim 2, characterised in that abscissca and ordinate adders are connected to the address inputs of the image store and to an own movement unit by which address corrections (AK) are added to the reception and reproduction addresses and in that the own movement unit is controlled by image pulses (BI) from the control circuit after scanning of a panorama or partial panorama and establishes between two image pulses (BI) distance increments corresponding to the course and speed of the path increment performed by the echo location system and delivers address corrections (AK) in the coordinates of the image store addresses (BA) to the adders, whereby the position of the echo location system is collated with the origin of the reception and reproduction addresses.

14. Apparatus according to claim 13, characterised by a decoding circuit, in which can be entered reproduction addresses which characterize limits of the capacity of the image store and having inputs which are connected to the address outputs of the read out unit and another input controllable by a pulse (LP) from the own movement unit upon alteration of the correction addresses (AK), the output of the decoding circuit being connected to an expunction input of the image store and providing an expunction signal (LS) when the reproduction addresses of the read out unit are identical with those set in the decoding unit.

1 5. Apparatus according to claims 7 to 12, for carrying out the method according to claim 2, characterised in that the address inputs of the image store for storing intensities are connected with an own movement calculator which generates image storage ad dresses and read out pulses for reading out the intensities, and in that the own movement calculator is controlled by image pulses of the control unit after scanning of a panorama or a partial panorama and determines between image pulses path increments in correspondence with course and speed but in the opposite direction to the movement of the echo location system, corrects the image storage addresses by address corrections corresponding to those path increments and provides switching for entry in the image store.