GB1605200A - Pulse radar method and apparatus - Google Patents

Description

(54) PULSE RADAR METHOD AND APPARATUS (71) We ELTRO G.m.b.H. & Co., GESELLSCHAFT FUR STRAHLUNGSTECHNIK, a German Limited Liability Company, of Schlosswolfsbrunnenweg 33-35, Heidelberg, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a pulse radar method and apparatus for locating moving objects. Such method and apparatus is used, for example, for triggering the fuse of a missile or projectile upon approach to a target. In air-to-ground applications, the fuse should detonate the missile or projectile upon ground approach at a certain altitude without its triggering operation being affected by the nature of the ground or by the presence of trees.

In air-to-air applications, a missile should be triggered by the fuse in proximity to the target only if the latter would otherwise be missed; thus a direct hit is not prevented.

In both applications the fuse should be suitable for several kinds of projectiles or missiles.

Some known electronic proximity fuses use the Döppler effect and have the disadvantage that the triggering distance cannot be set very accurately; this insufficient accuracy is also disadvantageously affected by the nature of the ground surface and by the presence of trees, roofs, and so on. Also it is in practice necessary to design a special fuse for each type of projectile or missile, since it is the latter itself which serves as the antenna and thus affects behaviour of the fuse. Finally, the known fuses are vulnerable to jamming transmitters.

An object of the invention is to overcome as far as possible these deficiencies.

According to this invention there is provided a pulse radar method in which high fre quency electromagnetic transmitted pulses and feeler impulses are used with a pre-determined delay between a transmitted pulse and a feeler impulse wherein, if the distance of an object from a transmitting and receiving system is such that received reflected pulses coincide with feeler impulses, there is, during each feeler impulse in the system a summation of the respective amplitudes of reflected pulse and feeler impulse, the values of successive summations changing in accordance with movement of the object relative to the antenna, and the changing values being used to form a low frequency signal.

The method of the invention is based on a measurement of travel time, in which an extremely short pulse of transmitted energy, after reflection from a target object, is received and processed by a receiver. For this purpose, a single antenna only can be used, which serves both as transmitting and receiving antenna.

The extremely short transmission pulses may be generated by repeated d-e surges of extremely short duration from a suitable switching device, for example a tunnel diode or a step-recovery diode. With switching times of less than 100 micro-microseconds, the occurring switching fronts will contain components which exceed the X-band and can easily be radiated and also readily received by a very small antenna.

The invention will now be described by way of example, with reference to the accompanying diagrammatic drawings, in which: Figures 1(a) and l(b) show a proximity fuse respectively in end view and side section; Figure 2 is a block diagram of apparatus of the invention; Figure 3 shows oscillograms of impulses and pulses as they occur in the method of the invention; Figure 4 shows further oscillograms of impulses; Figure 5 is a block diagram of an antijamming arrangement, in which only a single feeler impulse, selected from a transmitting impulse series, is used; Figure 6 is a block diagram of an antijamming arrangement in which two feeler impulses are used; and Figure 7 is a diagram showing the mode of operation of the arrangements of Figures 5 and 6.

The arrangement illustrated in Figures 1(a) and l(b) is a tested example of a proximity fuse with coaxial omnidirectional radiator 1, two receiving diodes 2, a tunnel diode 3 serving as an impulse generator, a capacitor 4, and a terminating resistor 5. In lieu of two receiving diodes it is also possible to use only one receiving diode. In lieu of the tunnel diode 3, a step-recovery diode can be used.

According to the block diagram shown in Fig. 2, the tunnel diode 3 (or the step-recovery diode) is caused to switch by a driver generator 6 so that at its terminals a square wave voltage (Fig. 3a) is produced. This voltage is differentiated by the capacitor 4 so that both positive and negative switching fronts S and R respectively (Fig. 3b) are radiated by the antenna 9 (Fig. 2) The negative impulse R additionally serves as a feeler or scanning pulse and switches the otherwise reverse-biased receiving diode 2. The latter charges a capacitor 7 in which a sawtooth voltage (Fig. 3c) is thus generated, the amplitude of which will vary as a function of the switching impulse amplitudes.

An LF (low frequency) amplifier is indicated at 8, with an LF signal output 10. In the absence of a target reflection, there occurs at the capacitor 7 the sawtooth voltage (Fig. 3c), whose basic frequency is the impulse frequency (Fig. 3a).

The signal shown by Figure 3d represents the received pulse reflections from the target of the impulses Sand R. By addition of the amplitudes of both voltages (Figures 3b and 3d), the sawtooth amplitude (Figure 3e) will become either greater or smaller than it is without a reflected signal being received, so that, with constant distance of the fuse from the target, a constant dc variation (the difference between c) and e)) will occur at the charging capacitor 7. Thus, if a target is located ahead of the fuse a distance corresponding to a signal travel time equal to the time interval between the first and the second transmitted pulses S, R, the first pulse S, when reflected from the target, will coincide in the receiver with the feeler impulse R which has at that moment switched the receiving diode 2.The negative voltage of Figure 3e is less than that of Figure 3c by the amount of the positive peak voltage of the reflected signal S in Figure 3d.

The signal of Figure 3d could vary. For example one of the downwardly directed peaks of the reflected pulse S could coincide with the feeler impulse R and in this case the sawtooth voltage of Figure 3e would be negatively in creased, compared with Figure 3c, by the negative voltage of Figure 3d. In any event the peaks of the sawtooth signal of Figure 3e always lie in a line which is parallel to the abs cissa, provided that the distance between target and fuse remains constant.However, if that distance varies (that is, if the target moves relative to the fuse) then the travel time "fuse target - fuse" will vary, and instead of the re flected signal of Figure 3d there will be a reflected signal as indicated by Figure 3f, which represents a reflected signal whose pulses are constantly changing position in relation to the feeler impulses R, so that each impulse R co- incides with a different portion of the reflected signal of Figure 3f. Thus with varying fuse target distance (as compared with constant fuse - target distance) a varying sawtooth signal as shown in Figure 3g is obtained, because now signal 3f is added to 3b, instead of 3d being added to 3b.In other words, the sawtooth voltage at the charging capacitor 7 will undergo gradual change of its peak amplitude, which change corresponds to the position of the received pulses (Figure 3f). Accordingly, an LF image (dotted curve in Figure 3g) of the reflected, received HF pulses is presented to the input of the LF amplifier 8. No signal resulting from the transmitted feeler pulse R will be processed at the receiver, because at the moment of its return from the target the receiving diode 2 is switched off. It will be recalled that the diode 2 is always reverse-biassed and therefore noncon- ducting, except when switched by the impulses R.

Only that value of the reflected signal of Figure 3d or Figure 3f which coincides in the receiver circuit with the impulse R, will affect the sawtooth signal of Figure 3c. Thus in the steady state case exemplified by Figures 3d and 3e, the peak positive element of the reflected signal S always coincides with the negative sawtooth peak of Figure 3c, to produce the less negative sawtooth peaks of Figure 3e. On the other hand, in the varying state case exemplified by Figures 3f and 3g, those elements of the reflected signal S of Figure 3f which coincide in the receiver circuit with the impulses R, gradually change, so that the negative sawtooth signal of Figure 3c is modified by the gradual change, and the sawtooth signal of Figure 3g is produced.

Since only the values of the reflected pulses (in Figures 3d or 3f) during the feeler impulses R are effective in the receiver, the reflections of the transmitted pulses R in Figures 3d or 3f are without effect on the sawtooth signal of Figure 3c, and thus do not have any effect on the signals of Figures 3e or 3g. Also seen in Figures 3d and 3f are the minor peaks caused by secondary swings of the reflected pulses of S and R.

To prevent any other target-echo pulse, reflected from a greater distance, being caused to appear by a subsequent transmitted feeler pulse, a pause P is inserted between successive transmission pulses (Figures 3a and 3b), which pause is sufficiently long to make the distance, from which such a targetecho pulse would have to originate, so great that the pulse would be too greatly attenuated to be receivable.

The frequency content of the received HF pulse, transposed to the LF range, is dependent, in addition to the carrier frequency of the transmission pulses, also upon the velocity of relative movement between fuse and target. Thus, by choice of LF filter, the fuse can be made selective in relation to speed. As described above, the fuse is only sensitive at one particular distance, because the reflected pulses can only be received at those moments in time which are determined by the fixed delay (Fig. 3(a)).

This fixed or pre-determined delay can be adjusted by adjusting the duration of the individual pulses in Fig. 3(a) which will of course adjust the time between S and R in Fig. 3(b). There are cases, however, where it is necessary to extend the sensitivity of the fuse over a range of distances. For this purpose, it is necessary to transmit several pairs of pulses in close succession, each pair comprises pulses S and R, see Fig. 4(b) which corresponds to Fig. 3(b).

(Fig. 4(a) corresponds to Fig. 3(a)). In this manner, the fuse can be made sensitive for various distances.

As antennae for the fuse of the invention, there can be used a coaxial omnidirectional radiator antenna or an exponential guiding strip antenna. In any cases, the antenna must have a pronounced wide-band characteristic since the frequency content of the transmission pulses ranges from 3 to 10 Gigacycles.

A coaxial omnidirectional radiator has the advantage of being rotationally symmetrical, and it assures, especially for utilization of the fuse in air-to-air applications, a desirable radiation characteristic (hemispherical, with a blind cone in the flight direction) for making possible a direct hit in case the missile or projectile moves straight into the target.

In order to reduce further the susceptibility to jamming radiation (amplitude modulated continuous-dash jamming signal) there can be, as shown in Fig. 1 , two receiving diodes 2 ar ranged in the receiver at such a distance from each other that between them there will result a travel-time differential corresponding to onehalf wavelength of the frequency to be re ceived. In this case, two LF images of the re ceived HF signal are obtained with a 180-degree phase shift between them.

If both are mixed subtractively, a signal of double amplitude is obtained, whereas all LF modulation voltages from jamming transmitters will compensate each other completely, since they will appear with identical phasing at both receiving diodes. All HF voltages, for which the preset travel time differential will not corre spond to 1/2 wavelength, will at least partially subtract from each other.

The method disclosed above makes it poss ible to measure with accuracy distances amounting to a few centimetres only. The method is based on the measurement of travel time of extremely short bursts of transmission gener ated by d-e surges of extremely short duration and of high intensity from a suitable switching element such as, for example, a tunnel diode or a step recovery diode coupled to a wide-band antenna at its base point.

With switching intervals of less than 100 micro-microseconds, the switching fronts created contain frequency contents which exceed the X-band and can be emitted and received easily by a small antenna. According to what has been disclosed above, a single antenna can be used for transmitting and receiving, and the impulses generated by a transmission pulse generator of the kind outlined above are utilized both for the transmitted pulses and for the feeler impulses for switching a receiving diode and for providing a feeling or scanning process for each received pulse, in which the necessary time delay, between the received reflection and the feeler impulse, is obtained from interval to interval out of the movement, relative to the antenna, of the object to be located.

The feeler impulse which switches the receiving diode will scan from interval to interval a different amplitude value of the received reflected pulse, so that the sawtooth voltage generated at the charging capacitor connected to the receiving diode is subjected to gradual changes of its amplitude, which correspond to the time incidence of the received reflected pulses in relation to the feeler impulses.

Thus, at the output 10 there is a low frequency image of the received high-frequency pulse, suitable for further evaluation.

As already explained, such a method can serve for example, to trigger automatically the fuse of a projectile or missile at the moment of its approach within a predetermined distance of a target.

The above method offers important advantages with regard to the reliability of the proximity fuse, especially on account of the fact that the distance from the target at which the fuse is to be triggered, can be set very accurately, which is not possible with proximity or distance fuses operating for example on the CW Döppler radar system. The present method permits, with transmission pulse wavelengths of 15 cm., measurement of distances in air with a tolerance better than i 5 cm., and the nature of the target surface or of the earth's surface does not have any influence at all, as long as the target reflected pulse has sufficient amplitude.

In addition, it is possible to provide the proximity fuse with speed selectivity and, if necessary, to sensitize it for a number of triggering distances, and even for a continuous range of distances.

The field of application also requires the possibility of neutralising jamming activities.

Since the receiving diode in the receiving system is shut-off by reverse biassing, and only turned on by a feeler impulse for the targetecho pulse, jamming of the fuse by a continuous-dash microwave transmitter is practically impossible.

Amplitude-modulated continuous jamming signals can, as already explained, be eliminated by the provision in the receiving system of two receiving diodes mounted at such a distance from each other that a travel time difference corresponding to one half wavelength of the frequency of the useful signal to be received will result. In this case, two LF images of the received HF signal are obtained, which are in 1800 phase opposition. Subtractive mixing of the two LF signals produces a signal of double amplitude. On the other hand, all LF modulation voltages originating from jamming transmitters will be eliminated, since they will appear in phase at both receiving diodes and be nullified by the subtraction that follows. All HF voltages, for which the travel time difference will not amount to 1/2 wavelength, must result in at least partial subtraction.

The present invention also has the object of virtually excluding the danger of interference from phase and frequently-modulated jamming signals.

The method of the invention can create two LF images of a received HF signal in such a manner that two receiving diodes, as a result of a predetermined travel-time distance between them, are subsequently switched by one or more feeler impulses selected from a transmission impulse series and the received HF signal is fed to the receiving diodes according to the rhythm of their switched conditions.

The time difference between the switching points of the two receiving diodes results from the order of magnitude of the lengths of the transmitting impulses. Consequently, this has the effect that at no time will there be LF signals present simultaneously at both LF outputs of the receiving system if a genuine target echo pulse is being received, but that a well defined time difference will be present between the two LF images. Such a time difference, however, will be absent when a wobbled jamming signal is being received, of which, on account of its phase and frequency variations, LF images will also be obtained. The LF signals occurring in this case are always in phase, so that they can be easily eliminated by subtractive mixing.

Thus it is possible, without difficulty, to utilize the ascertained phase difference between the two LF signals as the unmistakable identification of genuine targetecho pulses.

According to Fig. 5, a tunnel or step-recovery diode 3 is caused to switch by a drivergenerator 6 so that a rectangular-wave voltage is produced at its terminals. By a capacitor 4 connected in series with a terminating resistor 5, this voltage is differentiated so that within one pulse sequence interval a pair of transmitting impulses is obtained, of which one has a positive and the other a negative switching front.

Both transmitting impulses are then radiated with a travel time difference (determined by the duration of the rectangular-wave pulse), from an exponential strip guiding antenna 9.

Simultaneously, the negative impulse is used as a scanning or feeler impulse in the following manner: First of all the scanning or feeling impulse switches the receiving diode 2, through which it charges the charging capacitor 7 to its maximum value. Shortly thereafter, the feeler impulse also reaches an additional receiving diode 12, so that this diode is also switched and its associated capacitor 13 charged.

The arrangement of the two receiving diodes 2 and 12 is such that there will result a travel time difference for the arrival of the feeler im pulse. While this occurs, the receiving diodes 2 and 12 will show identical polarity concerning the feeler impulse. Each charging capacitor 7, 13 is connected to an LF amplifier 8 and 14, from each of which signal outputs 10 and 15 are connected to an electronic evaluator 11.

The block diagram of Fig. 6 differs from that of Fig. 5 in that it provides for use of two feeler impulses and in that the arrangement of the receiving diodes 2 and 12 has been modif ied as a consequence thereof.

In the impulse generating system with the driver generator 6 and the transmitting diode arrangement 3, a rectangular-wave voltage with two rectangles of different lengths is formed, with the block 3' containing two tunnel or two step-recovery diodes. By differentiating these rectangular impulses with the capacitor 4, a transmitting impulse sequence is generated, in which, within the pulse sequence interval, three transmitting impulses are produced.

This transmitting impulse sequence can be seen in Fig. 7. A first positive impulse 31 is followed by a negative impulse 32 as well as by an additional positive impulse 33. The time interval between the impulses 32 and 33 is smaller than the one between the impulses 31 and 32.

Whereas all three impulses 31,32 and 33 are being radiated via the antenna, only the two impulses 32 and 33 are used simultaneously as feeler impulses for switching the receiver diodes 2 and 12 respectively. The first positive transmitting impulse 31, however, is suppressed by a reversebiasing impulse imparted to the receiving diode 12, so that impulse 31 cannot act as a feeler impulse nor prematurely switch the receiving diode.

As already mentioned, the two impulses 32 and 33, which serve as feeler impulses, follow each other at an interval of, for example, 0.001 microseconds. For this reason, the arrangement of the two receiving diodes 2 and 12 is such that, for the individual feeler impulses, identical travel time will elapse for their arrival at a receiving diode, and the receiving diodes 2 and 12 will possess opposite polarities corresponding to their feeler impulses. Consequently, the receiving diodes 2 and 12 will be switched one after the other as a function of the time lag of, for example,0.001 microseconds between the feeler impulses 32 and 33.

The further operation in both the systems represented by the block diagrams of Fig. 5 and Fig. 6 is virtually identical since in both cases successive switching of the receiving diodes is obtained. A received HF signal can thus be imparted to the receiving diodes 2 and 12 according to the rhythm of their switched conditions.

How this occurs can be explained with reference to the diagram of Fig. 7, which is based on the circuit of Fig. 6.

First will be discussed the case where no HF signal is received by the antenna 9, which serves both for transmitting and for receiving.

Corresponding to the switching rhythm of the receiving diodes 2 and 12, the charging capacitors 7 and 13 are being charged periodically as shown in Figs. 7d and 7e. The charges of these capacitors will gradually run down through the leakage resistances of the receiving diodes 2 and 12 until, at the inception of new intervals, new charges will be created. Thus, two sawtooth voltages are produced at the charging capacitors 7 and 13, one negative (Fig.

7d) and the other positive (Fig. 7e).

The phase difference between the two sawtooth voltages is determined by the time difference between the feeler impulses 32 and 33.

The maximum amplitudes of the sawtooth voltages depend in each case on the amplitudes of the feeler impulses 32 and 33. In the circuit of Fig. 5, two similar sawtooth voltages would also result, with the difference that they would both be negative.

Since it is being assumed that no HF signal is being received (because, for instance, there exists neither a target reflection nor a jamming signal) the amplitudes of the sawtooth voltages will remain constant and therefore no LF signals can be obtained from the charging capacitors 7 and 13.

If now it is assumed that a target enters the range of the antenna 9 of the proximity fuse, then, in accordance with the sequence of the transmitting impulses, three target-echo pulses 34,35 and 36 (Fig. 7b) will be produced of which, however, in each case only the first target-echo pulse 34 will be important to the receiver. The target-echo pulses 35 and 36 are not used since, at the moment of their arrival, the receiving diodes 2 and 12 will not be switched.

Up to the moment at which a predetermined "fuse-to-target" distance is attained, the receiving diodes are also not switched for the targetecho pulse 34. This predetermined distance is defined in the present example in that it corresponds to a travel time equal to the time lag between the first transmitting impulse 31 and the third transmitting impulse 33.

As soon as this postulated distance has been reached, the target-echo pulse 34 and the feeler impulse 33, which latter switches the receiving diode 12, will both simultaneously arrive at the receiving diode 12. On account of the relative movement of the fuse relative to the target, the travel time of the transmitted pulses will now gradually diminish, so that regularly small time shifts between the periodically recurring feeler impulse 33 and the target-echo pulse 34 will result. This means, that the target-echo pulse 34 will not always strike the identical point of the feeler impulse 33 and that the latter, from interval to interval, will scan a different amplitude value of the targetecho pulse 34 received.

The sawtooth voltage at the charging capacitor 13 thus undergoes gradual changes of its peak amplitude on account of the addition of its amplitude to the momentary amplitudes of the target-echo pulse 34 received, as shown in Fig. 7g. Such changes will occur in the present example altogether for the duration of five transmitting intervals, i.e. until the moment is reached at which the incoming target-echo pulse will again encounter the receiving diode 12 in the not switched state. For the following intervals, the amplitude of the sawtooth voltage will then again remain constant, as also seen in Fig. 7g.

Thus, at the input to the LF amplifier 14 following the charging capacitor 13, a first lowfrequency image of the received targetecho pulse received is formed.

The generation of the second low-frequency image of the target-echo pulse 34 can only occur a certain time after the completion of the first feeling or sampling operation because, during the entire period of time during which the target-echo pulse 34 is being scanned at the periodically switched receiving diode 12 and also during an additional lapse of time as can be seen from the voltage curves of Figs. 7f and 7g, the receiving diode 2 is not switched for the targetecho pulse 34 and the amplitude of the sawtooth voltage at the charging capacitor 7 remains con stant (Fig. 7f).

The second sampling operation will only be initiated when the targetecho pulse 34 and the feeler impulse 32 arrive simultaneously at the receiving diode 2 so that the latter is switched and can be traversed by the target-echo pulse 34. The scanning of the target-echo pulse then proceeds in the same manner as previously described for the first scanning operation and, at the input of the LF amplifier 8, (which follows the charging capacitor 7), a second LF signal will be obtained, which represents another image of the targetecho pulse 34 received (Fig.

7f).

There will now be considered the case where there exists a wobbled jamming signal lying in the Gigacycle range and which is received by the antenna 9. Such a jamming signal, in which phase and frequency shifts make a scanning operation possible, is represented in Fig. 7c. This jamming signal is equally active when the receiving diode 2 is switched and when, shortly after, the receiving diode 12 is switched. Consequently, two low-frequency images of the HF jamming signal will be produced at the charging capacitors 7 and 13, which images will have identical phasing (Fig. 7h and Fig. 7i). Subtractive mixing of the two LF signals will cause their erasure, so that the proximity fuse cannot be affected by the jamming signal.Any asymmetry of the two LF channels would only show up as a de component in the subtraction.

The phase differences between two LF signals, which differences are the criteria for detecting a true target, for correct triggering of the fuse, and for elimination of jamming signals, are evaluated in the electronic evaluation unit 11, which is connected to the LF signal outputs 10 and 15. A signal output 16 leads to the fuse mechanism.

It should also be mentioned that, in the diagrams of Fig. 7 a linear time scale was purposely not chosen for the sake of clarity. Actually, the pauses between the individual pulse sequence intervals (according to Fig. 7a and Fig. 7b) are much longer than shown. The broken-line representation of the joining lines between each last pulse of a preceding, and each first pulse of a following, interval indicates this.

The pauses actually inserted between the individual pulse sequence intervals according to Fig. 7a are chosen to be so long that any poss- ible target-echo signal would have to originate from so great a distance that it could be expected to be so weakened that it could not be received. This prevents an image of a target-echo pulse, returning from a farther distance, being formed at random by the feeler impulse of a later interval.

WHAT WE CLAIM IS: 1.A pulse radar method in which high frequency electromagnetic transmitted pulses and feeler impulses are used with a pre-determined delay between a transmitted pulse and a feeler impulse wherein, if the distance of an object from a transmitting and receiving system is such that received reflected pulses coincide with feeler impulses, there is, during each feeler impulse in the system a summation of the respective amplitudes of reflected pulse and feeler impulse, the values of successive summations changing in accordance with movement of the object relative to the antenna, and the changing values being used to form a low frequency signal.

2. Pulse radar method according to Claim 1, wherein the low frequency signal is subjected to selection in accordance with the relative approach velocity of an antenna and an object, low-frequency filters being used in the receiving system.

3. Apparatus when used in carrying out the method according to Claim 1 or Claim 2, wherein a receiving system contains a receiving diode periodically switched by the feeler impulses from an impulse generator.

4. Apparatus according to Claim 3, wherein two receiving diodes are switched by feeler impulses, which diodes are spaced a distance such that there results between them a time difference corresponding to half a wavelength of a frequency to be received, and the output voltages of which are subtracted from each other.

5. Pulse radar method according to Claim 1 or Claim 2, wherein two low-frequency reproductions of a received high-frequency pulse are produced, two receiving diodes being provided which are switched successively in accordance with a given time interval by one or more feeler impulses selected from one feeler impulse sequence, the received high-frequency pulse being applied to the two receiving diodes in the rhythm of their switched states, and the output signals of the receiving diodes being compared for detecting or suppressing spurious signals.

6. Apparatus for carrying out the method according to Claim 5, wherein, with the use of only a single feeler impulse, the arrangement of two receiving diodes is such that there results between them a time difference, caused by their spatial locations, for reception of the feeler impulse, the diodes having the same polarity relative to the feeler impulse.

7. Apparatus for carrying out the method according to Claim 5, wherein, with the use of two feeler impulses following each other at a given interval of time and having different polarities, the arrangement of two receiving diodes is such that there results between them the same time differences for the individual feeler impulses, the diodes having opposite polarities corresponding to the feeler impulses.

8. Pulse radar method substantially as herein described, with reference to the accompanying drawings - 9. Pulse radar apparatus substantially as herein described, with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   unit 11, which is connected to the LF signal outputs 10 and 15. A signal output 16 leads to the fuse mechanism.

   It should also be mentioned that, in the diagrams of Fig. 7 a linear time scale was purposely not chosen for the sake of clarity. Actually, the pauses between the individual pulse sequence intervals (according to Fig. 7a and Fig. 7b) are much longer than shown. The broken-line representation of the joining lines between each last pulse of a preceding, and each first pulse of a following, interval indicates this.

   The pauses actually inserted between the individual pulse sequence intervals according to Fig. 7a are chosen to be so long that any poss- ible target-echo signal would have to originate from so great a distance that it could be expected to be so weakened that it could not be received. This prevents an image of a target-echo pulse, returning from a farther distance, being formed at random by the feeler impulse of a later interval.

   WHAT WE CLAIM IS: 1.A pulse radar method in which high frequency electromagnetic transmitted pulses and feeler impulses are used with a pre-determined delay between a transmitted pulse and a feeler impulse wherein, if the distance of an object from a transmitting and receiving system is such that received reflected pulses coincide with feeler impulses, there is, during each feeler impulse in the system a summation of the respective amplitudes of reflected pulse and feeler impulse, the values of successive summations changing in accordance with movement of the object relative to the antenna, and the changing values being used to form a low frequency signal.
2. 2. Pulse radar method according to Claim 1, wherein the low frequency signal is subjected to selection in accordance with the relative approach velocity of an antenna and an object, low-frequency filters being used in the receiving system.
3. 3. Apparatus when used in carrying out the method according to Claim 1 or Claim 2, wherein a receiving system contains a receiving diode periodically switched by the feeler impulses from an impulse generator.
4. 4. Apparatus according to Claim 3, wherein two receiving diodes are switched by feeler impulses, which diodes are spaced a distance such that there results between them a time difference corresponding to half a wavelength of a frequency to be received, and the output voltages of which are subtracted from each other.
5. 5. Pulse radar method according to Claim 1 or Claim 2, wherein two low-frequency reproductions of a received high-frequency pulse are produced, two receiving diodes being provided which are switched successively in accordance with a given time interval by one or more feeler impulses selected from one feeler impulse sequence, the received high-frequency pulse being applied to the two receiving diodes in the rhythm of their switched states, and the output signals of the receiving diodes being compared for detecting or suppressing spurious signals.
6. 6. Apparatus for carrying out the method according to Claim 5, wherein, with the use of only a single feeler impulse, the arrangement of two receiving diodes is such that there results between them a time difference, caused by their spatial locations, for reception of the feeler impulse, the diodes having the same polarity relative to the feeler impulse.
7. 7. Apparatus for carrying out the method according to Claim 5, wherein, with the use of two feeler impulses following each other at a given interval of time and having different polarities, the arrangement of two receiving diodes is such that there results between them the same time differences for the individual feeler impulses, the diodes having opposite polarities corresponding to the feeler impulses.
8. 8. Pulse radar method substantially as herein described, with reference to the accompanying drawings -
9. 9. Pulse radar apparatus substantially as herein described, with reference to the accompanying drawings.