DE2900041A1 - CORRELATION RADAR FOR NEAR DETECTION AT LOW ALTITUDE - Google Patents

Description

2300041

¹¹ Correlation radar for close-range detection at low altitudes "

The invention relates to a correlation radar system as described in the preamble of claim 1 mentioned type. Such radar systems are used in particular in distance fuses that are in a remote-controlled projectile are arranged to detonate the projectile in the vicinity of the target at a low altitude.

In this application there are difficulties because in the Remote control device, which is generally a radar for fine range measurement contains, the projectile can be mistaken for its mirror image, which is due to the reflection of the radar beams arises on the ground or the surface of the sea. The measuring accuracy of the distance measurement is therefore not satisfactory In addition, there is a great risk that the projectile will be unintentionally detonated if it comes very close to the ground.

The two functions, namely distance measurement, can be used at high altitudes of the projectile through the guidance radar and distance ignition are fulfilled independently of each other, but at a low height must an altimeter can be provided to compensate for the inaccuracies of the radar measurement and a satisfactory remote control to accomplish.

The invention is based on the object of a correlation radar system to create in which this problem is solved and which is also characterized by its low weight and compact design distinguishes what is indispensable for use with distance fuses.

In known close-range detection systems that are used in connection with Distance fuses can be used, the last-mentioned function is fulfilled by an electromagnetic detection system, that emits an electromagnetic wave that is generated by phase

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keying is modulated according to a pseudo-random code. the reflected wave is in a homodine or a heterodine receiver processed with a constant false alarm rate. This is followed by a correlation, Doppler filtering, detection and post-integration and a threshold value comparison as well as the ignition.

According to other known techniques in the same field a pain distance measurement is carried out by delay correlation between two signals with the aid of two correlation channels, which cover a common measurement range, the two correlation functions overlapping in this range and a subtraction circuit generates a discriminator curve by means of which the deviation from the center of the common area including the sign can be determined.

The solution to the above problem is by the in the characterizing Part of claim 1 solved in detail the features mentioned.

According to a feature of the invention, the radar contains three correlation channels j of which the first for operation as a distance sensor and the other two for operation as Altimeter are used. The differential output signal that is generated according to the last-mentioned mode of operation becomes sur used height-dependent frequency control of a synchronization clock.

The invention is illustrated schematically below with reference to the figures illustrated embodiments as well as explained on the basis of diagrams: It shows:

Figure 1 is a simplified block diagram of a

preferred embodiment form a kadar system for near detection according to the invention,

Figures a block diagram of a pulse radar system 2 to γ corresponding to that of Figure 1 and

explanatory diagrams for this, -6-

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FIG. 8 shows a block diagram of a further embodiment of a radar system according to the invention.

The electromagnetic detection system according to the invention will be described in connection with its application as a range detonator. It goes without saying, however, that the radar system is also suitable for other applications.

Referring to Figure 1, the radar system includes in combination a continuous or preferably a pulsed transmitter, which is switched by phase shift keying (0 -Tf) after a pseudo-random coding (or a random coding) is modulated. The other is a homodine recipient (or heterodine recipient) with a Circuit for constant false alarm (CFAR circuit from the English Constant False Alarm Receiver) provided. Furthermore is for Signal processing a correlation with the delayed modulation signal as a reference signal, a filtering of the Doppler frequencies in the evaluation spectrum under consideration, a detection and an incoherent integration. (Post-integration) over a period of time, which corresponds to the illumination of the target, a threshold value masking and an ignition trigger for the explosive charge are provided.

The parts used for this are shown in simplified form in FIG. The transmitter contains a continuously operating maximum frequency generator 1, a modulator 2, and a generator 3 for generating a phase modulation signal CO and an antenna device ^.

The transmitting antenna is continuous from the receiving antenna in the case Emission separated, while a common antenna can preferably be used in the case of pulsating emission. Of the Receiver comprises a demodulator connected to antenna 4 5, for example a mixer which receives part of the continuous ultra-high frequency wave via a coupler 6. the Circuit 5 supplies a video signal containing the distance information contains and determines the Doppler frequencies of the reflected

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end goal. Reception with a constant false alarm rate takes place in a circuit 7 which contains a video frequency amplifier (or an intermediate frequency amplifier in the case of heterodin reception) and a limiter which limits the received signals to a certain level before the correlation. The limiting level is chosen so that the noise is suppressed even in the absence of a useful signal and in the case of low noise (thermal inherent noise of the receiver). The output video signal SV arrives at the input of the ignition channel, namely to a correlator or a multiplier circuit 8, which additionally receives the signal C1, which corresponds to the modulation signal CO, but is delayed with respect to this. The generator 3 contains delay circuits which are designed so that the correlation of the ignition channel corresponds to a precise distance in an assigned area. The product of the signals obtained on the output side reaches an amplifier and a Doppler filter 9, which is connected in series with a detector 10 and a circuit for incoherent integration (post-integration). Downstream of this is a threshold value comparison circuit 11 and an ignition device 12. If various predetermined conditions for the approach between the projectile and the target are met, a trigger signal is generated.

The circuits used preferably consist of miniaturized ones Solid-state circuits that are characterized by great reliability and compactness. The continuous emission and coherent evaluation of the Doppler effect reduce the power to be transmitted. This results in an increased resolving power.

In order to determine the distance, the transmitted must be continuous Wflle modulates and the received echo signals through correlation be evaluated with a delayed response signal compared to the modulation signal. To a very fine distance resolution To obtain, the correlation function must have temporally very narrow and widely spaced peaks, whereby Ambiguities in the determination of the distance can be avoided. The correlation function must be used between these maxima or peaks

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assume a value as small as possible. Binary periodic pseudorandom coding provides a useful correlation function and can easily be generated with a shift register.

Two correlation channels are required for altimeter operation, which, as shown below, can be obtained by that the ignition channel and an additional second channel are used. According to a preferred embodiment, two separate channels are used which, like the ignition channel, have a correlator 18a (18b) in series with a Doppler filter 19a (19b) and an envelope detector 20a (20b).

The correlators 18a and 18b each receive a reference signal C2 and C3 which, like the signal C1, represent a repetition of the modulation signal CO, but with a greater delay than the signal C1. The delays are such that the correlation functions overlap (FIGS. 3f to i and FIG. 4). The output signals S2 and S3 of the two correlation channels reach a subtraction circuit 15 'such as a differential amplifier which ^{supplies a signal S4 (FIG. 1} Id), the amplitude of which depends on the distance from the center D4 of the common measuring range D2 and D3. This error signal is fed to an integrator 16 in order to obtain a specific integration duration. In the case of altimeter operation, the integration time must be greater than in the case of distance detonator operation, for example 10 to 100 ms instead of 1 ms. The integrated signal S5 controls the frequency of a clock generator 13> which is used to synchronize the coding. The clock frequency of the signal S6 thus contains the information about the altitude and can be used for altitude correction and to improve the steering at low altitude. For this purpose a remote control device 25 is provided which contains a distance measuring device 26, for example a radar system, a computer 27 and a remote control transmitter / receiver 28. A transceiver 30 is arranged on the floor. The output signal S7 prevents ignition in the event of incorrect detection if the projectile is still too far away from the target. The output signal S8 controls the directional vanes, which are indicated by the block 31.

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The circuit works as follows: when the altitude determined over distance by the radar 26 and the computer 27 decreases and one falls below the predetermined minimum value, can from the frequency of the signal S6, which is transmitted to the ground station 25, with Accuracy the height of the plug path can be obtained. In response to this, plug path correction signals S8 are sent to the guiding wings transfer. After the flight path has been stabilized at a low altitude, the signal S7 is used to operate the distance fuse for the Restored the final phase of the approach to the low-altitude target.

According to a preferred embodiment, the emission takes place in the form of a Doppler pulse, that is, the continuous ultra-high frequency wave is amplitude modulated j, which is used for transmission and reception one and the same antenna can be used, a homodine reception being provided in order to simplify the installation.

Figures 2 and 7 show a preferred embodiment of one electromagnetic detection system according to the invention. The same elements as in FIG Figure 1 is provided with the same reference numerals. The modulator is divided into a phase modulator 2a and an amplitude modulator 2 B. The amplitude modulation signals AO and the phase modulation signals CO are shown by way of example in Figures 3a and 3f. The elements 31 and 32 represent decouplers that Prevent reflections when the modulator 2b and the circuit 33 are closed. The circuit 33 is a transmit / receive switch (TR circuit), which protects the receiver from the transmitted signals. The circuit 33 is activated by the signals Al + A2 (Figure 3b) controlled, the successive the range window signal Al for the distance ignition channel and the distance window signal A2 for the height measuring channel (Figure 3d) contains. The output signal of the modulator is transmitted to the common antenna 4 a hybrid coupler of the type of a circulator with 3 channels 34, the third receiving branch with the transmit / receive switch 33 is connected. For video amplification there is a linear amplifier 7a and a limiting circuit 7b with constant False alarm ratio (CPAR circuit) provided. the

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Circuits 35a and 35b are range gate circuits, respectively can be controlled by the signals A1 and A2. In this way, the receiver is protected against the transmitter interfering signals through the circuits 33 and 35 and protected by the decouplers 31 and 32.

The amplitude modulation in the circuit 2b is determined so that a large form factor is obtained. The advantages of amplitude modulation can be seen in the fact that a common antenna can be used for sending and receiving and that echoes from the nearby ground can be suppressed when operating the distance fuse at a low altitude.

During the phase modulation in the circuit 2a, a phase becomes 0 and a phase TT for each bit value 0 or 1 of the pseudo-random code (Figure 3e) generated. Coding introduces the first ambiguity of distance pushed out sufficiently far and a large form factor for the transmitted pulses is made possible.

The peak power to be emitted is low and can be applied by a solid-state source 1. The phase of generator 1 is maintained from one pulse to the next. The detection is coherent and the shape of the signal corresponds to the limiter in the receiver. Since the receiver's noise limitation is carried out before the correlation, the false alarm rate is constant, which is an indispensable prerequisite for use as a distance ignition device. The transmitted signals are similar in both operating modes, i.e. in distance ignition mode and in altitude measurement mode. The detection method is the same in both cases and comprises a correlation between the transmitted, time-delayed signal and the received signal, which has experienced a delay and a Doppler shift. The common circuits are therefore numerous and contain the antenna 4, the ultra-high frequency chain consisting of the circuits 1, 2, 5, 6, 31, 32, 33, 3 ¹ I, the generator 3, the video frequency receiver 7 and the power supplies (not shown) .

The complete equipment used to measure the height

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mental elements are largely the same as those circuits which are located in the ignition channel.

This has the following advantages: reduction in volume and cost, increase in reliability, reduction the power consumed, and therefore the possibility of installation in small storeys where these properties are essential. Through this, that the altitude information S6 is transmitted to the ground station, the steering is improved at low altitude.

In stand-by mode, the continuous wave supplied by the source is phase-modulated by the signal CO (phase O or K ) according to a binary code C (Figure 3e) and amplitude-modulated by the pulses of duration To ₃ , which are delayed by a period T - η Τ, where T is the duration of one bit of code C. The pseudo-random code C is generated by a shift register with ρ flip-flops, which is controlled by a clock signal with the period T. The code period is MT ", where: N = 2 ^P. The first ambiguity of the range of the transmission code CO (FIG. 3f) occurs at the range (C equals the speed of light), where N and η are integers and are selected relatively prime. The value ρ is chosen so that the ambiguity is pushed out far enough.

The opening time of the transmit / receive switch 33 is ^{shifted by "7} ^ o + ε, which is slightly longer than the duration Tf o of the pulses in the transmit modulator 2. This theoretically creates a blind area corresponding to distances between 0 and EC / 2 In practice, the received signal is very strong at these short distances and it is sufficient if a little of this signal passes through the gates of the receiver in order to trigger the ignition.

The received signal that is delayed and a Doppler shift is mixed with part of the continuous wave. This mixture is homodin, so that at the output video frequency signals be won.

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The mixer 5 includes the linear amplifier 7a and the range gate circuit 35a, which completes the action of the switch 33.

The correlator 8 receives the received signal and the transmitted shifted code C1 (Figure 3g), which compared to the signal CO by To + ε is delayed. It can consist of two analog gate circuits, which record the bipolar video signal of the circuit 7 and which are each controlled by the signal Cl and its complement CT will. The output signals of the gate circuits are fed to a differential amplifier. At the output of the correlator the Coding effect suppressed. What remains is the modulation due to the Doppler effect, which is sampled by the pulses. A circuit 10 is connected downstream of the Doppler filter 9, which composed of a full-wave rectifier and an integrator, for example an RC element. The time constant of the integrator is equal to the time it takes the target to traverse the detection area. The noise is through the limiter normalized to a constant value. The threshold of the comparator 11 is set so that a certain probability for False alarms is not exceeded.

The correlation triangle (Figure 4) is around the distance value Dl centered:

_D1 =

The width of the triangle base is:

2Δ0 = T ₀ C.

The correlation function shows residues in the intervals (DK-AD, DK + AD), where:

DK β KntC / 2 + (To + ε) C / 2,. where K is an integer.

The transmit / receive switch 33 suppresses the signals received outside of these intervals.

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FIG. 5 shows an example of a radiation diagram of the antenna 4 in a section through a vertical plane which runs through the longitudinal axis Z of the floor. The diagram is rotationally symmetrical about the Z axis. The operational ranges of Zünddetektors and altimeter are shown hatched and in accordance with the response function of the ignition channel, which has the form of the distance Dl (Figure 4a) centered triangle _s and the response functions of the Höhenmeßkanäle, the shape of the centered about the distance D4 curve S4 (Figure 4d) have.

The difference signal S4 (Figure ¹ Jd) has a zero for the

Distance D4 = -. After going through the integrator 16

this signal S5 controls the period T of the clock. The height measuring loop determines the value for t at which the signal S4 a Has zero. Since the relationship between D4 and linear is corresponds to a height value a single value of the clock frequency.

According to a further feature of the system, the pulse duration T o of the transmission pulses can be changed essentially linearly as a function of the height. In this way it is possible to determine the maximum detection distance for the ignition channel and to steer the projectile at a low height. FIG. 6 shows the change in the correlation function S1 as a function of the distance for an output value Xo (curve SIa) and for a lower value To (curve SIb). The maximum distance goes from dma to dmb "

FIG. 7 shows an exemplary embodiment for a signal generator. The clock generator 13 contains a voltage-controlled oscillator. The control voltage is supplied by the integrated signal S5. The oscillator 13 generates a signal with the period "; Him ist a ring counter 4l with η stages connected downstream, which generates a signal with the period nT. This arrives at a monostable multivibrator 42 j the one pulse of duration TO every period nT generated. After amplification in amplifier 43, this signal represents represents the amplitude modulation signal Ao. The output signal of the oscillator represents the signal S6, which is used for remote transmission of the Altitude information is determined, and the signal also arrives at one Code generator 44, which a pseudo-random code for control

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a shift register generated. The further signals CO, C1, C2, C3 and A1, A2 are generated with the aid of monostable multivibrators 45, 46, 47 and a delay circuit 48 which introduces a delay £. Furthermore, logical AND and OR circuits are used to generate these signals. The circuits 49 and 50 are amplifiers. The signals SlO, SIl, S12 are obtained at the flip-flops of the shift register of the code generator 44. The phase difference between the signal SIl and the signal SIO is chosen so that it corresponds to that of the part of the ring counter used. The same goes for S12. During operation with variable pulse width To , the signal S5 is compared in the comparator 51 with a threshold value, and the output signal of the comparator controls the monostable multivibrators 42, 45, 46, 47, whereby a corresponding linear change is achieved.

FIG. 8 shows, similar to FIG. 2, a block diagram of an embodiment variant, in which only two correlation channels are provided. According to this variant, the distance detonator operation and the altimeter operation and also. the operating mode with variable pulse width T * ο not carried out at the same time will. The circuit 30 supplies a signal S9 for switching between the operating modes for distance ignition and altitude measurement, by which the output signal A1 instead of the output signal A2 and the output signal Cl in place of the output signal C2 is switched, the output signal C3 being generated in the altitude measurement mode. Corresponding changeover switches are in the signal generator 3 available.

When the signal S9 is received, which is sent by the remote control to the Changeover switch for switching between distance detonator operation and height measurement operation (or vice versa), consisting of flip-flops and Gate circuits are delivered, the circuit works as follows:

- In the case of distance detonator operation, the clock generator is set with the period T " blocks J for a duration equal to this period f, the transceiver switch 33 and the distance gate circuit 35 are opened and the ignition circuit 12 is enabled,

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- in height measurement mode, with a variable clock period fof the clock generator the opening times of the circuits 33 and 35 are doubled and the ignition circuit for the distance detonator operation is blocked.

The orbital velocity of the projectile is known and consequently also the Doppler frequency of the ground echoes. This Doppler frequency is maximum at low altitude »

It is therefore necessary to switch over the Doppler filter of the distance ignition channel during altitude measurement. This is done by a control signal SIO _s which is transmitted by the remote controller and passes to the filter 9, which contains a switchable Doppler filter.

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Claims (5)

DIETRICH LEWINSKY HEINZOOACHIM HUBER PURE PRIETSCH MÖNCHEN 21 January 2nd 1979 GOTTHARDSTR. 81 10.773-V / Ni Thomson-CSP, Bl. Haussmann 173, P - 75008 Paris (France) Patent claims: (1.) Correlation radar for close-range detection at low altitudes, especially for distance detonators, with a transmitter that is modulated by phase shift keying according to a pseudo-random code, with a receiver that feeds two correlation channels in parallel, in which the respective correlation circuit has an amplifier Doppler filtering and an envelope detector is connected downstream, with a generator which generates modulation signals and reference signals for the correlation circuits and contains a code generation circuit controlled by the clock signal of the synchronization oscillator, the correlation channels being designed so that their correlation functions overlap in a common measurement range, and the outputs of the two correlation channels are connected to a differential amplifier in order to generate a discrimination curve centered in the measuring range, the synchronization oscillator also being controlled by the output signal of the differential amplifier according to its integra tion in an integrator is frequency-controlled, characterized in that the receiver, connected in series, contains means for receiving with a constant false alarm rate, means for correlation, means for Doppler filtering, means for detection and post-integration and means for threshold value comparison in order to allow near detection in a different measuring range allow, which is closer to the radar transmitter than the common measuring range for determining the height above the ground. 2300041 2. Radar according to claim 1, characterized in that the means connected in series have a third correlation channel represent. 3. Radar according to claim 1, characterized in that the means connected in series have one of the two correlation channels represent that the output signal of the circuit (10) for the detection and post-integration of said signal simultaneously the differential amplifier (15) and the threshold comparator (11) feeds that the signal generator contains changeover switches through which the synchronization oscillator is on with a fixed frequency for distance detonator operation and to operate with variable frequency for height measurement operation. 4. Radar according to one of claims 1 to 3, characterized in that that there is a remote control transceiver (30) for the remote transmission of the altitude information (S6) and for the reception of control signals, in particular for receiving a control signal (S8) for altitude correction and for receiving a signal (S7), that prevents the triggering of the ignition device (12) and that the radar with a distance measuring circuit (26), a Computer (27) and a remote control transmitter-receiver (28) interacts, which the control signals (S8, S7) based on generate the altitude information. 5. Radar according to one of claims 1 to 4, characterized in that that the transmission takes place in pulse form (pulse duration f o), whereby a common antenna is provided for transmission and reception can be that the transmitter comprises a generator (1) for generating a continuous ultra-high frequency wave and modulation means, a phase modulator (2a), the phase modulation according to the pseudo random code (C), and contain an amplitude modulator (2b) which an amplitude modulation after a periodic pulse train (AO) carries out that the signal generator (3) the modulation signals and reference signals generated by repetition of the phase modulation signal (CO) with a time delay. 909828/0866 Radar according to one of Claims 2, 4 and 5, characterized in that that the integrated output signal (S5) in parallel on the one hand to the oscillator and on the other hand to a threshold value comparison circuit (Threshold value Sl) is supplied, the output signal for the height-dependent control of the pulse duration (To) and hence used to change the correlation detection ranges. Radar according to claims 3, 4 and 5, characterized in that the Doppler filter (9) of the first near detection channel contains at least two filter circuits (9a, 9b) connected in parallel with a switch which optionally connects one of the two filter circuits to the input in order to control them to be switched on in height measuring mode, the switchover being carried out remotely by means of a remote control signal (SLO). Radar according to one of Claims H, 5 or 7 and according to Claim 3, for use in a projectile with distance ignition at a low altitude, characterized in that the remote control, in altitude measuring mode, receives the information about the clock period (S6) and navigation control commands for setting the steering wing transmits and that the remote control arranged on board the projectile contains the switching signals for the operating mode and in particular a control signal (S7) to prevent ignition during the height measurement operation. -H- 909828/0860