NO129482B - - Google Patents

Description

Procedure for presentation of

the screen of a cathode ray tube ratio

between time and a time-dependent variable.

The present invention relates to a method for displaying on the screen of a cathode ray tube the relationship between time and a time-dependent variable.

According to DAS 1.075-02, it is known to produce several variables

using a cathode ray tube. The indicator tube is usually dark-colored to indicate directional information. There is only an increase in the illuminance to show direction markings.

With the present invention, the aim is for the complete, received echo signal to be displayed and subjected to a marking of the directional deviation in a way that the user is familiar with. The aim is thus to facilitate the interpretation of the echo information for the user and the aim is also to provide directional information in a particularly favorable way.

The method according to the invention is characterized by the fact that the time-dependent variable is superimposed by amplitude modulation on a high-frequency carrier wave with a higher frequency so that it forms an enveloping curve for the display and that the display is subjected to

in a manner known per se, a modulation of the illuminance 'in dependence on another variable.

The lighting modulation according to the invention can be carried out

in different ways, for example by the modulation of the illuminance being carried out in steps with different degrees of illuminance, which are clearly graduated in relation to each other. It is preferred that the steps in the illuminance are allocated to different levels for the second variable.

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Of particular importance is the new method for instruction in underwater sound technology. When fishing, it is common when using echo sounders from the trawl to indicate the shape of the cathode ray tube so that the echo free<a> fishes

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which is located in the area of the trawl opening. Here it is important that

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the trawl's baseline, which is detected by vertical soldering, is shown to be clearly distinguishable from the fish echoes and especially from the bottom echoes. A method of instruction is used for this, whereby the instruction of; The amplitude of the rnottaker curve occurs when the cathode ray is diverted across the time axis. This known method of instruction is subject to medi; the disadvantage, that especially in dense fish deposits close to the bottom,! no sufficient image resolution is achieved to distinguish the echo indications from the bottom, the baseline and the fishes. Also in pelagic fisheries, the echoes from the trawl's baseline and from the school of fish are often indistinguishable from each other. By using lighting modulation according to the present invention in, the resolution of such echo images can be improved significantly. If one uses the illumination modulation with illumination levels that are staggered in relation to each other, then the illumination levels are assigned different, preferably adjustable amplitude levels of the echo receiver voltage. This assignment preferably takes place like this,'

that the large amplitudes are indicated with low illuminance and

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the small amplitudes with high illuminance. This achieves that the relatively weak fish echoes are indicated with high illuminance, while the echo from the baseline is indicated with medium illuminance and the very strong bottom echo with low illuminance. Especially at the lighting jump locations, a clear distinction is made between the echoes from the various objects, especially when a lighting control takes place in the boundary area due to an amplitude saddle in this narrow area, whereby an eye-catching bright dividing line■is indicated..

However, the lighting modulation can also be carried out in significantly different ways. Thus, it is possible to subject locally limited areas of the picture screen to lighting modulation. This can happen to only expose such areas to the lighting modulation and leave the rest of the image unaffected by this, for example to be able to notice the modulation better. However, the limitation of the lighting modulation to certain areas of the picture screen can also serve to derive further information, as this selection of areas can be assigned further specific properties to further variables. Here, the lighting modulation can take place according to a specific line or surface pattern, in particular according to a line grid, whereby a particularly easily identifiable image modulation is achieved.

In the case of instructions that occur symmetrically about the tilt line, an area selection can be used, whereby the lighting modulation is effective on only one side of the tilt line. In this way, it is also possible to specify a change in size, in particular a change in sign of an independent variable when changing pages or lighting modulation. In underwater sound technology, such a form of illumination modulation is particularly suitable, in order to identify the purpose of the movement of the side direction and the zero crossing of the side direction of the illumination modulation in such a way that the side direction indicates a Doppler shift downwards correspondingly an approach to or if a Doppler shift upwards corresponding to a distancing of the soldered object from the point of view exists, while the disappearance of the illumination modulation indicates that neither an approach nor a distancing of the target exists.

If an eigen-Doppler compensation is used below in a known manner, then the advantageous possibility consists of giving the echo images from stationary and moving objects identification characteristics by means of lighting modulation, and in particular to realize that moving objects stand out image-wise from the echoes of stationary objects, since for example a U-boat moving over the seabed gives a jagged echo image, while the seabed itself, on the other hand, gives an unjagged echo image.

The disappearance or appearance of lighting modulation in objects

which is stationary in the echo image, can also be used as a criterion for the correct setting of the self-Doppler compensation.

In the drawing, the invention is shown with the help of some design examples, in which: Fig. 1 shows the connection diagram for a sonar device with

lighting modulation in two lighting stages.

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Fig. 2 shows the connection diagram for a sonar device with a combination of an illumination modulation with two illumination stages and an illumination modulation with image area switching for indication of the Doppler shift.

The one in fig. 1 reproduced sonar device is described as

a fish finder. A sonar transmitter 2 and an echo receiver can be arranged on board a ship or on the trawl itself. A cathode ray tube 3 serves for guidance on board the ship - In tact with the sending of plumb impulses towards the seabed, the cathode ray or its spot of light on the screen of the cathode ray tube is guided by means of an impulse gerator ^ controlled sawtooth trigger 5 along a time axis 6 above the picture screen. To the second olate pair of the cathode ray tube, the echo receiving signal from the receiver 1 is passed over an amplifier 7,

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so that the cathode ray is assigned deflections proportional to the magnitude of the receiver voltage. In this way, the echo of objects located between the transducers 1, 2 and the seabed A, in particular one ground-

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line B which lies close above the seabed and fish C which lies in the area of the trawl opening as well as below the bottom line B, indicated.

The amplitude of the bottom echo a is many times greater than<1>the amplitude of the echo b of the baseline B and also than of the echo c from fisherman C who is in the trawl opening. The echo a from the bottom, due to the large extent of the bottom and due to the limited directional sharpness of the sound radiation, also has an even greater extent in the direction of the time axis than the echoes b and c from the bottom line B and the fish C. On! due to the large amplitude difference, it is not possible to bring the echoes a,

b and c for imaging in their natural size on the image screen. In order to indicate the fish echoes with sufficient amplitude, the amplification factor of the amplifier 7 must be set so high that the derivation for the amplitude of the bottom echo a goes well beyond the edge of the picture screen, as shown in fig.l.

In the coupling according to fig. 1 means are arranged, [by which it is achieved that the illumination intensity of the indication at the bottom echo a is much smaller than the illumination intensity of the baseline echo |b and the fish echo c indication. With this lighting jump, a clear delineation of the boundary g between the bottom echo a and the baseline echo b occurs. Be-

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the tractor can thereby more easily identify at what height the baseline is above the seabed. This is very important, on the one hand to be able to move the trawl as close as possible over the seabed in the case of shoals of fish that are close to the bottom, and on the other hand to avoid damage to the trawl when it touches the bottom. The border between

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the baseline's echo b and the fish's echo c can be clearly distinguished in the echo image on the cathode ray tube's screen by the amplitude jump between the two echo images b and c, so that an illumination modulation by tapering off the illumination intensity between these two echoes is not necessary. Equally, it would be possible to arrange a third lighting stage and correspondingly assign these three lighting stages to three amplitude stages for the input voltage. In the embodiment shown, there are only two lighting stages. The connection, which is here designed for two lighting stages, can, however, easily be changed to use for three or more lighting stages. Otherwise, the threshold value corresponding to the amplitude step in the reproduced link is steplessly adjustable, so that the lighting step can be moved to an arbitrary place, to optionally place the lighting step in the amplitude difference area between two echoes that must be specified separately from each other. Where the boundary between the echoes b and c should therefore not be easily identifiable or precisely determined, the threshold can be shifted so that not only the echo a from the bottom, but also the echo b from the baseline, is reproduced with lay, illuminance and only the echo c from the fish are reproduced with great illuminance. With such a setting, instead of the boundary g between the echoes a and b, the boundary h between the echoes b and c is markedly brought into focus by means of the lighting step, and the position of the baseline can be clearly recognized from this. By shifting the threshold, one can therefore determine the limits g and h one after the other and thus the relative position of the echo objects accurately. Of course, this does not only apply to the echo objects reproduced in the present example, but also to all echo objects that have different distances from the point of observation and that can be distinguished from each other by noticeable differences in amplitude.

It will now be described with which switching technical means the amplitude-dependent lighting tap-off, which has so far been described by effect, has been achieved in the reproduced example.

Device for illumination modulation mainly consists of a measuring device for the amplitude of the receiver voltage, with which it is determined whether the amplitude lies above or below a predetermined, adjustable threshold, and of a device for switching the illumination intensity in the cathode ray tube 6 depending on the position of the amplitude above or below below the predicted amplitude threshold. For this purpose, the receiver voltage is connected via a second output of the amplifier 7 to an amplitude measuring device. The receiver voltage is rectified in an iike rectifier bridge 9, and the rectified voltage is after equalization in a capacitor 10 via a potentiometer 11 led to the base

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on an NPN transistor 12 > which together with .e-n other transistor! 13 and:

in and of themselves associated connecting elements form a :Schmitt trigger 1<*>1.with a predetermined threshold level.. In this Schmitt trigger, the common emitter resistance 15 for setting the threshold level can also be adjustable and possibly become . set . automatically with the help of a control size.

The negative potential of the rectified receiver voltage reaches the base of the potentiometer 11 and reaches the base of the transistor 12. At a corresponding height of this potential, the transistor 1 12 becomes conductive and blocks the other transistor 13. The Schmitt trigger's lH output becomes above a soening parts 19, 20 base and thus the conductivity of a transistor 18 controlled.

When the rectified receiver voltage on the potentiometer 11 exceeds the predicted voltage threshold, the transistor 12 becomes conductive and thus the transistor 13 is blocked. Above the voltage divider 19, 20, the transistor 18 then becomes conductive and the voltage on the Wehnelt cylinder, which in the blocking state of the transistor 18 across a resistor 16 is at the full height of the direct voltage 21, then drops to a low level, the size of which corresponds to the voltage divider ratio 16/17 + 18. The lighting control voltage now in the Wehnelt cylinder drops, and then proportionally with the transistor's 18 conductivity. Through the transistor's output, this conductivity is essentially changed when the Schmitt trigger's input voltage exceeds or falls below the predicted threshold level. This results in the desired gradual change of the illumination intensity, depending on the Schmitt trigger's release level. This trigger level is always below the maximum point of the bottom echo's a amplitude. If this threshold is below the amplitude of the bottom echo a, but above the amplitude of the trawl's baseline echo b, then the bottom echo a is written with low illuminance, while the echo b from the baseline and even more the echo c from the fish are written with high illuminance. By shifting the threshold by setting the potentiometer 11 or changing the common emitter resistance 15 so that already a smaller receiver voltage, which is below the amplitude

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to the echo c from the baseline, causes reconnection, then one achieves that both echoes a and b are written with low illuminance and only the echoes c from the fish are written with high illuminance.

Naturally, the device can also be designed. vice versa, so that the echoes with a large amplitude come to be indicated with a large illuminance, and those with a lower amplitude. with less illuminance, as the described mode of operation • is preferred for use when fishing.

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The degree of lighting reduction is affected by change

of the voltage divider resistor 20.

Preferably, there is arranged a setting means for the threshold level, which is connected to a graduated scale.

In fig. 2 a connection is shown, where a special type of lighting modulation serves, by soldering objects that move in relation to the point of view, to indicate this movement and its connection, that is, to design the lighting modulation as follows,

that it can be identified whether it is a relatively moving object and whether there is an approach to or distance from the observation point. When the point of observation itself moves, the frequency shifts of the echo due to the self-motion can be suppressed in a manner known per se by means of a so-called self-Doppler compensation, so that only the Doppler shift caused by the movement of other objects comes to the fore.

Such a Doppler guidance is of interest above all for horizontal echo sounding. Below, the distance to the reflective objects, as with the vertical echo sounding, according to the first embodiment, is indicated by derivation along a horizontal time axis and the echo amplitude by symmetrical transverse derivation to this time axis. The lighting modulation to reproduce the Doppler shift is designed in such a way that in the case of a Doppler shift towards higher frequencies, corresponding to an approach of the reflecting object, a lighting modulation on the side above the time axis is

shown and by a Doppler shift towards lower frequencies, corresponding to a distancing of the reflective object, by a lighting modulation of the indication below the time axis, while the lighting modulation disappears with a Doppler shift of zero. In fig. 2, echoes from such targets with different Doppler shifts are reproduced, and then an echo d from an approaching target, an echo e from a target moving away, and an echo f from a stationary target. Fig. 2 shows a connection example for a device which is suitable for producing such an instruction.

Sonar impulses with a frequency of, for example, 30 kHz are radiated from transmitter 1. The echoes that return from reflective objects are, after reception by the underwater sound receiver 2, amplified in the amplifier 7 and applied as derivative voltages to the cathode ray tube's 3 vertical plate pairs. ,. The horizontal derivation of the cathode ray along the time axis is, as in the first embodiment, produced with a sawtooth generator 5, which with key pulses from the ouls generator 4 for the sound transmitter 1 becomes forty beats with the solder pulse broadcast.

The switching of the illuminance takes place as in the first embodiment by controlling a Schmitt trigger 14 or a potentiometer 11 sensor and the influence of the lighting control on the Wehnelt cylinder takes place directly at the Schmitt trigger's output via the switching elements 16 - 20, which are described in connection with fig. 1. Below, however, the potentiometer is not; 11, as in the first embodiment, added a rectified and equalized

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voltage which is derived from the receiver voltage, but instead the connection is designed in such a way that the potentiometer 11 is supplied with the same alternating voltage as the derivation system for the amplitude derivation on

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across the time axis. Here, however, only one half-wave reaches the potentiometer at any time, while the other is stopped, depending on whether there is a Doppler shift onpad,' or down-

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ad. To this end, the receiver alternating current after amplification in the diversion amplifier 7 is supplied to the primary side of a transformer 22, on whose secondary side one half-wave via a rectifier 2'3 and the other half-wave via a rectifier 24 are supplied to the potentiometer 11 as a control winding. Opposite a grounded center outlet 30, a voltage appears on capacitors 28 and 29 which, with regard to its polarity and time in relation to the center outlet 30, is equal to the diversion of the beam current in the cathode ray tube. It can also be designed so that the wiring circuit with the capacitor 28 and a subsequent diode 23

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comprises one half of the screen, the capacitor 29 with a 'subsequent' end diode 24 the other half of the screen and the center outlet 30, which is placed against zero potential, comprises the kip<p>line. Either the rectifier 23 or the rectifier 24 is blocked by the application of a blocking voltage for the passage of the associated half-wave, so that at any one time only one of the two half-waves arrives at the potentiometer 11 as control voltage. These blocking voltages are supplied via blocking lines 25, 26, whose potential is determined by a coupling device 27 for measurement, by the Doppler shift. !

The rectifiers 23 and 24 are connected via the capacitors 28 and 29 to the two ends of the secondary winding of the transformer 22; and to the central outlet 30 via resistors 31 and 32. The potentiometer 11, which is connected in parallel with a capacitor 33, is connected with a| page to

in the common connection between the rectifiers 23 and 24 and with,' the other side to the transformer's center outlet 30.

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In the device 27 for measuring the Doppler shift, everything becomes

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according to whether there is a Donpler displacement up or down, a transistor 34 or 35, which is connected as a direct voltage differential amplifier, is taken from a blocking to a conducting state and thereby a blocking voltage from a voltage source 36 across resistors 38 and 39 and the blocking lines 25 and 26 placed either on the rectifier 23 or on the rectifier 24.

In order to influence the conductivity of the direct voltage difference amplifier with the transistors 34 and 35 depending on the direction of the Doppler shift, the frequency of the pre-amplified receiver voltage is mixed with a fixed frequency of a reference voltage and in a connected reactance filter 43 the phase deviation which is conditioned by the frequency shift is determined and supplied to the transistors 34 and 35 which diverted direct voltage to effect control.

For this purpose, the receiver voltage from the diversion amplifier 7 is fed to a frequency mixing stage 40, which is connected to an oscillator 41. At a transmission frequency of, for example, 30 kHz and a reference frequency of 29 kHz, a differential or mixing frequency of 1000 Hz. With self-Doppler compensation, an elevation is formed for moving objects when approaching the observation point and when moving away from the observation point, a decrease in the mixing frequency with corresponding deviations from the mean frequency of 1000 Hz. The mixing frequency is first passed through a tuning circuit 42 and then reaches a low-pass filter 43 with a coil 44 and a capacitor 45. In order to

get better swing-in conditions, this low-pass filter on the input and output is connected to a resistor 46 and 47. The low-pass filter is precisely tuned to the center frequency 1000 Hz. If the intermediate frequency is equal to the mean frequency 1000 Hz, it is subjected to a full phase rotation of 360° and is transferred by a subsequent counter-phase transformer 48 to a phase discriminator 49 which is connected to the secondary side of the transformer. In front of the low-pass filter 43, a partial voltage of the intermediate frequency is supplied to a phase shifter 58 for 90° displacement, which with its voltage leading output is connected to the center outlet 50 on the secondary winding of the transformer 48.

If the intermediate frequency is equal to the mean frequency, then the voltages that appear on rectifiers 51 and 52 in the symmetrically constructed wiring circuits of the phase discriminator 49 are equal, and the norm voltage on the discriminator is zero. If the intermediate frequency deviates from the mean frequency of 1000 Hz, a phase shift is formed in the low-pass filter which is dependent on the size of the frequency deviation and which acts so that the voltage across it of capacitors and ohmic

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resistors consisting of bridge branches 53 and 54 in the phase discriminator 49 come out of equilibrium. Across protection resistors 55 and J57, the voltage at the output of the phase discriminator reaches the base of the transistors 34 and 35, which in the previously described manner work|as a direct voltage difference amplifier. The voltages that appear on the collector working resistors 38 and 39 are, as stated above, supplied to the rectifiers 23 and 24 via the lines 25 and 26 as blocking voltages. If, for example, the transistor 35 is blocked, then in relation to the center socket 30 of the transformer 22, almost all of the negative voltage from

the voltage source 36 active. Thus, the rectifier 23 is supplied with such a negative bias that positive half-waves that come across the capacitor 28 are not allowed through the diode 23.

This has the further consequence that the Schmitt trigger 14 which is connected to the potentiometer 11 as described under fig. 1, does not connect and consequently only the receiving signal on one screen half is brightly indicated.

When the transistor 35 is blocked, the transistor 34 is conductive and its low collector potential reaches across the blocking line 26 to the diode 24. The diode 24 is then conductive for positive half-waves, so that the Schmitt trigger 14 above the potentiometer 11 when the threshold level is exceeded is connected and produces a drawing with reduced illuminance on the second half of the screen.

If the Doppler shift is zero, exactly the same voltage appears on the base of the transistors 34 and 35. These two transistors

are then both conducting, and an indifferent switching condition occurs. For this reason, either a plot with high or a plot with low illuminance appears on both sides of the time axis. A possible doubt about the presence of Doppelfor displacement zero can easily be removed by the threshold of the potentiometer 11 being shifted in such a way that the Schmitt trigger 14 must be switched on. If there is still no different illuminance on the two sides of the time axis, it can only be because there is a zero Doppler shift.

In order to compensate for possible shifts of the switching time from the zero line, a further adjustable timing link can be switched on, for example in front of the potentiometer 11.

The lighting modulation can be switched off, for example, by turning down the potentiometer 11, in order to be able to drive the cathode ray tube in the usual way.

The lighting modulation according to the invention can further serve to indicate the deviation of the soldered target and the cause of the deviation

sign, referred to a reference direction, for exemplary vertical

ning, by assigning different lighting modulation, especially by unilateral modulation with side change. Below, a phase difference is derived from the deviation of the receiver direction in relation to the reference direction by means of a per se known receiver base with two receivers or two receiver groups that are at a distance from each other, which produces the control voltage via a phase discriminator,

for example the control voltage on the potentiometer 11, for the lighting modulation.

A side switch for the illumination modulation, which is particularly suitable for indicating, the Doppler shift and the bearing direction, respectively the deviation of the target to one side or the other, can on

in a particularly advantageous way is realized by creating an alternating voltage that is proportional to the receiver voltage and this is fed in a manner known per se to the cathode ray tube's transverse derivation

system, and that further of an alternating voltage which with regard to. phase and polarity match the derivation voltage, a control voltage for the illuminance is derived in such a way that to a

each time separate control voltages which are assigned to the two current directions of the alternating voltage come into effect over two current paths (23, 24),

and that the sign of the Doppler shift or another measurement quantity

change to be indicated when switching sides of the lighting modulation, of-

a blocking voltage is conducted for one or the other current path.

Within the scope of the invention, many modifications and

other executions are carried out. Thus, for example, it is possible to

measure the speed while changing the oscillator frequency until, when the Doppler indication on the cathode ray tube disappears, it is determined that

between receiver frequency and comparison frequency. On a scale which is arranged in connection with the controllable oscillator's 4l setting means, the speed can then be read. In this way, for example, it is possible with a pivotable swing base to measure the vessel's own speed, either through the water, allowing the transducer to radiate obliquely towards the water surface, or the speed in relation to the bottom,

while directing the transducer obliquely towards the bottom, or by operating speed

heat, as the transducer is now directed across. In addition, in this way it is also possible to measure the sea level or the uplift to which the ship is subjected due to the sea current, by correcting the swing

is perpendicular to the seabed.

.A marked separation in relation to normal instruction is possible

achieved in particular by the illumination modulation occurring according to a specific line grid or surface pattern, in particular according to a line grid. Mon

can connect such a line grid with the one in the second embodiment

example according to fig. 2 described half-sided lighting control and use it to design the instructions for Dopp.l erf orsky venning even more markedly. However, you can also use the raster to introduce additional information, for example an information about bearing

the direction next to the Doppler shift, with an appearance of the raster being taken as the criterion for the target being ahead. In case of deviation from this position towards Tøyre or the left, the grid must then

wither.

Claims (15)

1. Method of display on the screen of a cathode ray tube ray tube the relationship between time and a time-dependent variable, characterized in that the time-dependent variable is superimposed by amplitude modulation on a high-frequency carrier wave with a higher frequency so that it forms an enveloping curve for the display and that the display is now subjected in a manner known per se to a modulation of the illuminance in dependence on another variable. 2. Method in accordance with claim 1, characterized in that the modulation of the illuminance is carried out in steps with different degrees of illuminance, which are clearly graded in relation to each other. 3. Method in accordance with claim 2, characterized in that the steps in the illuminance are allocated to different levels for the second variable. 4. Method in accordance with claim 2, used for display by echo sounding where the amplitude of the echo signals from reflective objects is indicated by deflection of the electron beam across a time axis, characterized in that the steps in the illuminance are allocated to different amplitude levels of the echo signals. 5- Method in accordance with claim 1 or 2, characterized in that local, limited areas of the display are subjected to modulation of the illuminance. 6. Method - in accordance with claim 5> characterized in that the modulation of the illuminance is limited to a smooth pattern of lines or areas in the display. 7. method in accordance with claim 5 or 6, characterized in that the display is symmetrical in relation to the time axis and the modulation of the illuminance is effective only on one side of the movement line at a certain time. 8. Method in accordance with claim 7, characterized in that the variation in the second variable is indicated by a change in the side of the time axis on which the modulation of the illuminance is carried out. 9. Method in accordance with claim 7 or 8, characterized in that the exceeding of a time-dependent variable above a predetermined value is indicated by the modulation of the illuminance disappearing. 10. Method in accordance with one of claims 7 to 9, characterized in that the modulation of the illuminance on opposite sides of the time axis serves to indicate respectively the increase and decrease of Doonler displacement. 11. Method in accordance with claim 10, characterized in that the disappearance of the modulation of the illuminance or change of the page to an echo display of static objects is used as a criterion for the correct setting of the own Doppler compensation. 12. Method in accordance with claim 1, characterized in that the deviation of a measured target from a specific direction and the direction of the deviation is indicated by the assigned difference in the modulation of the illumination intensity. 13. Method in accordance with claim 12, characterized in that a control voltage representing Doonler displacement is used to perform the modulation of the illuminance over a phase discriminator, 14-. 0. Method in accordance with claim 7, used for echo sounding, characterized in that an alternating voltage that is proportional to a received fkko voltage is generated and supplied as a deflection voltage to the deflection plates of the cathode ray tube across the time axis, a voltage for the modulation of the illuminance being derived from a alternating voltage which in phase and polarity coincides with the deflection voltage in such a way that the separate control voltages which are assigned to the two current directions of the alternating voltage come into effect over two current paths and as a blocking voltage for one or the other current path is derived from the sign of the Doppler shift so that the side of the display to be subjected to modulation of the illuminance is determined. 15. Method in accordance with claim 14, characterized in that a transformer is used to form the display, which is connected to a circuit which provides transverse deflection of the electron beam, the two halves of the transformer's secondary winding being short-circuited with resistors and the ends to the secondary winding is connected across rectifiers in the same way to a common point which is connected to the cathode ray tube's control of the illuminance and as the current paths of the rectifiers are connected to control lines which are bubbled to a discriminator output and apply reversal voltage-'' to one or the other rectifier path.