FR2635388A1 - Electromagnetic method and device for detection with correlation, and proximity fuse incorporating such a device - Google Patents

Abstract

The invention relates to a method and device for electromagnetic detection with correlation. The device comprises a CW radar system transmitting a microwave signal C of nominal frequency Fo. A pseudo-random reference code, of sampling time tau , is generated by a shift register 21 and the rising and falling edges of the code trigger variations of duration DELTA ts less than tau and alternately positive and negative with respect to the nominal frequency Fo, which is also the mean transmission frequency. The video signal G returned by the moving target is correlated with the said code delayed by Tm(H). The product of the correlation I passes through a Doppler filter 11, is detected 12 and is compared 13 with a threshold s; when the threshold is exceeded, an indication S is given that there is a target at a distance which is smaller than or equal to a distance L = mc tau /2, where c is the speed of light and m is a positive whole number. Application to proximity fuses.

Description

METHOD AND ELECTROMAGNETIC DEVICE
COR DETECTION SYSTEM AND PROXIMITY FUSE
COMPRISING SUCH A DEVICE
The present invention relates to a method and an electromagnetic correlation detection device, applied more particularly to the realization of active proximity rockets, of very small dimensions and weight, transported by machines, also called "missiles", moving at high speed .

 A proximity fuze shall fuse the explosive charge carried by the missile when the fixed or moving target is in a certain area in the vicinity of the target. This explosion must take place with the greatest probability, even in the presence of strong interference. The detection and trigger device must be light, compact, robust and reliable and must not disturb the aerodynamic qualities of the carrier missible.

 There are several types of detection devices, including those that provide firing after estimating distance to the target by methods exploiting either the Doppler effect or the short pulse technique.

 Devices of the first type are extremely sensitive to interference, even simple and have a high probability of parasitic triggering. Those of the second type require a very important peak power given the low form factor required by the suppression of ambiguities in distance. Such a device in addition to its size and weight, can trigger firing outside the expected area given the speed ambiguity it has.

 The correlation method makes it possible to provide an unambiguous distance or speed detection device, with a transmission peak power that is much lower than that of the preceding devices, which makes the emission more discrete. In addition, the probability of false triggering outside the area intended for firing or in the presence of significant interference is very small.

 The proximity fuzes designed so far according to this method use a phase modulation of the emitted microwave wave, either with a phase O, or with a phase 17. This modulation is controlled by a pseudo-random code. This type of modulation eliminates ambiguities and considerably increases the form factor of the sequence of pulses. At the reception, the received wave, sent back by the target, is mixed with a fraction of the unmodulated microwave wave in phase. A video-frequency signal is thus obtained, the sign of which depends on the phase O or lt of the microwave wave received after reflection on the target. The delay between the video signal and the corresponding transmitted wave depends on the distance of the target and known delays specific to the circuits. The video signal can then be correlated with the delayed transmitted code so as to eliminate the coded modulation of the video signal for a given target distance and to render the useful signal.

 In such devices the presence of an O-X phase modulator and auxiliary circuits, control and insulators, makes the realization of the microwave assembly more complex. It is the same in the reception chain given the presence of switching noise introduced by the modulator O-TZ. In addition, such a modulator O- <is difficult to achieve at very high microwave frequencies.

 The present invention aims to overcome the aforementioned drawbacks and allows for a particularly reliable detection device ensuring the firing in a perfectly defined area.

 Another object of the present invention is a detection method for producing a detection device whose peak transmission power is low, which can work at extremely high microwave frequencies, and which is compact.

 Another object of the present invention is a particularly simple and inexpensive detection device.

 Another object of the present invention is to use a modulation and a single or double correlation circuit making it possible to reconstruct from the video signal, whose average is substantially zero, a continuous signal, characteristic of a signal received from a target at a given distance from the correlation radar.

 According to the invention, the electromagnetic correlation detection method for detecting the presence of a target at a distance less than or equal to a predetermined distance from a continuous wave radar is characterized in that the frequency transmitted by the radar varies. alternatively around the nominal frequency Fo which also constitutes the average value of the transmitted frequency and in that the rising and falling edges of a pseudo-random code control said variations of the transmission frequency, positive and negative respectively relative to said mean frequency Fo, the duration ts of each variation being less than the duration of a code sample.

Other characteristics and advantages of the present invention will appear on reading the detailed description given below with reference to the appended figures which represent:
FIG. 1, the block diagram of a preferred embodiment with a simple correlation of the device according to the invention;
- Figure 2 the form of the signals at different points of the device of Figure 1 in the case of a stationary target stationary relative to the radar; and
- Figure 3, the block diagram of another embodiment of dual correlation device according to the invention.

 As already mentioned, the electromagnetic detection device must perform the firing of the associated device only when the proximity fuse constituting the assembly and carried by the missile enters the assigned distance zone L with respect to the target or what amounts to the same when a target is detected at the assigned distance L of the transceiver radar on board the missile. The explosion must take place with the greatest probability, even in the presence of jamming. To be difficult to identify by possible countermeasure devices, the detector according to the invention uses a transmitter of low power on an extremely high frequency and an aerial illuminating the target for a time which is short compared to the duration of the journey traveled The device may comprise, as in FIG. 1, two different aerials for transmission and reception or as in FIG. 3 a single aerial associated with a circulator.

 Ancillary devices known not falling within the scope of the present invention ensure the guidance of the missile.

 In the electromagnetic correlation detection device according to the invention, the continuous wave radar emits a microwave signal whose frequency undergoes variations around a nominal frequency Fo. The variations are alternately positive and negative with respect to the frequency Fo, have the same duration ts and follow an identical curve (when we consider the absolute value of the variations with respect to Fo) so that the emission frequency presents a mean value equal to the nominal frequency Fo.

 These positive or negative variations (transmission frequency greater than or less than the average frequency Fo) are controlled by the rising and falling edges respectively of a pseudo-random code of which each sample has a duration t and which has a perid (2 " -I) -with n natural number, the duration A ts of each variation then being chosen lower than this duration t so that the frequency returns to the nominal frequency Fo during the last part (of duration 5 - tts) of the sample of which the forehead triggered the frequency variation.

 Preferably, the integral of each variation curve (positive or negative with respect to Fo during a real time Ats) calculated over the duration r of the code sample corresponding to the control edge, is preferably equal to 1/2 so as to that, at the end of the time Ats, the microwave emitted emitted is out of phase with what it would have been if the frequency emitted had remained Fo.

For the sake of simplification, according to a preferred non-limiting embodiment, the variations in the transmission frequency consist of frequency jumps from F to
o
Fo - #F or Fo + #F controlled by the high level / low level or low level / high level transitions respectively of a pseudo-random code. The programs at the frequency Fo + # F or Fo - #F have a duration # ts.

The value F is chosen such that after a time t
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ can be expressed by the relation:
2 F. #ts = 1
The radar thus emits a microwave wave modulated by phase ramps from a to O or O to a time interval #ts, the start of the ramp cosncidant with a transition level high / low level or low / high level a pseudo-random code. The phase ramps are separated by phase bearings O or a depending on the level at the end of the ramp, these levels corresponding to the emission of a signal at the frequency F
o
FIG. 1 represents the block diagram of a preferred nonlimiting embodiment of the device forming the subject of the invention and corresponding to the detection of a target located at a distance less than or equal to an assigned distance d = mc # / 2, where m is a positive integer or zero less than 2 "- 1, c is the speed of light, etc. the width of a code sample.

In the transmission part, the generator 1, consisting for example of a non-limiting manner by a Gunn diode coupled to a varactor diode, delivers a microwave wave whose frequency is equal to F - t F, or Fo, or F + n F. Switching between these
oo three frequencies being controlled by a signal Vc, designated in Figure 1 by the reference B and delivered by a circuit 2. This control circuit 2 comprises a circuit 22 generating the control signal Vc whose voltage varies at the rate of a pseudo-random code A developed by the circuit 21. This circuit 21 is for example constituted by a feedback shift register, composed of n stages and controlled by the clock 3.

 The periodicity of this code is equal to N #, where # is the duration of an increment and N is equal to 2n - 1. In such a pseudo-random code, the difference between the number of positive and negative samples of a sequence is always equal to +1 and the multiplication of the code by itself delayed restores the original code but with a new delay.

 The control signal Vc may be a pulse signal of zero average value and whose pulses, of arts width, and of level + V, or -V, have their rising or falling edge respectively, which coincides with each rising edge, or descending respectively, pseudo-random code A.

 The microwave signal C modulated by the frequency hopping F0 // F0-FetF0 // F0 + n F is emitted by the antenna 4, returned by the target and received by the receiving antenna 6.

 In the reception part 100, the received signal E, after having been mixed in a circuit 7 with a fraction of the transmitted signal C, taken by the coupler 5 contains the distance and speed information of the target.

 If the distance d between the proximity fuse and the target is such that the transmitted and received signals are not in quadrature at the mixer 7, the output signal G of the mixer is not zero.

 This signal G is first applied to an amplifier 8, either linear or limiter, providing a constant false alarm rate reception, then to the input of a correlation circuit 9 whose other input receives the pseudo-coded code. random A generated by the circuit 21 and suitably delayed by a quantity Tm and then amplified by the circuit 10 to a level # 1. The value Tm depends on the distance L = mc # / 2 assigned for the detection of the target. In the nonlimiting embodiment of FIG. 1, the delayed code of Tm is obtained by combining in an "EXCLUSIVE OR" logic circuit. 14 the reference code A and the same code delayed by m-, which is designated by A (m #) and which can be delivered by a delay circuit or as in Figure 1 by the output of the flip-flop of the register 21 corresponding to the delay mZ. The delay Tm could also be obtained using a delay circuit or simply by connecting the input of the correlation circuit 9 to the output of the latch of the register 21 corresponding to the desired delay Tm. The output I of the correlator 9 is applied to the input of a filter circuit 11 Doppler frequencies whose bandwidth corresponds to the range of speed envisaged for the targets.

 This broadband Doppler filter may also be completed by a Doppler filter bank, not shown, each corresponding to a Doppler channel and indicating the relative speed of the target. This Doppler filter circuit 11 is followed by a detection circuit 12. The detected signal having a certain amplitude is then applied to a circuit 13 where it is compared with a threshold s whose value is determined according to the noise fluctuations so to obtain a very low false alarm probability in the absence of a signal and a high probability of detection on the useful target. As soon as it exceeds this threshold that is to say when the target is at a distance d at more than the distance L chosen for the tripping of the charge, the comparison circuit 13 delivers a pulse signal S to the firing circuit (not shown) of the pyrotechnic charge carried by the missile.

In the remainder of this description, it is assumed that the distance C #
L of detection is chosen equal to, where # is the width of a code sample and c is the speed of light. The invention is then generalized for a multiple detection distance (m times) of 2
FIGS. 2A to 21 represent the signals at different points of the block diagram of FIG. 1. The indices A, B, C, D,
E, F, G, H, I refer to the points and signals indicated in Figure 1.

The index A corresponds to the pseudo-random code generated by the
circuit 21 of FIG.

The index B corresponds to the control signal Vc of the signal
issued.

The index C corresponds to the signal emitted by the transmitting antenna 4
radar, a signal whose frequency is Fo - #F, Fo or Fo + #F according to
the level -V, O or + V of the control signal Vc.

The index D corresponds to the phase #EE of the signal emitted C by
relative to the selected reference phase as that of the signal
Frequency emitted continuously at frequency F
o
The index E corresponds to the signal received by the receiving antenna
6. This is the signal sent and returned by the target at a distance of
radar. It therefore suffers a certain delay #t equal to the sum of
radar travel time to target and target to radar. Moreover, the
Transmitted frequencies are increased by the Doppler frequency.
target is driven by a relative velocity relative to the radar.

 The index F corresponds to the phase R of the received signal E.

The index G corresponds to the phase of the video signal delivered by
the mixer 7 of Figure 1, #V = #E - #R being obtained in
assigning a +1 value to a phase O video signal, the value -1 to
a video signal of phasen and the value O to a video signal of phase Ai / 2.

The index H corresponds to the identical pseudo-random code
code represented in A but delayed by a certain amount Tm according to
the distance L (L = mc 2/2) assigned for the detection of the target and
amplified to + 1.

The index I corresponds to the output signal of the correlated circuit
9 of FIG. 1, which correlates the signal illustrated on FIG.
FIG. 2G with the delayed pseudo-random code of Tm, represented
in Figure 2H.

By way of non-limiting example, the pseudo-random code on the
FIG. 2A is a 7-moment code generated by the circuit 21 which is
consisting of a three-stage shift register.

At first, we consider the particular case and very
theoretical of a stationary target stationary at a distance d by c # to the radar, with L = (m = 1).

2
So a time At = 2d / c, where c is the speed of light, flows between the moment when the signal is emitted and the moment when the antenna receives the signal sent back by the target.

If the distance d is precisely the distance L = c # chosen
2 to trigger the explosion of the pyrotechnic charge, the signal received is identical but delayed with respect to the signal emitted of an amount #t = #. It is this received signal that is illustrated in Figure 2E.

The phase #R of the received signal E is identical to the phase represented by the 2D figure of the transmitted signal C (represented by FIG. 2C) but presents for the same reason a delay = =
It is shown in Figure 2F.

 The received signal E is then mixed with a fraction of the transmitted signal.

The phase of the video signal delivered by the mixer 7 is equal to # V = # E- # R
This signal can be represented differently by giving the amplitude + 1 to the phase O and -1 video signal to the phase + + video signal. The equivalent truth table is as follows

<tb> Phase # E <SEP><SEP> of the <SEP> signal <SEP> Issued <SEP> C <SEP> - <SEP> O <SEP> O <SEP> T <SEP> t <SEP>
<tb> lere <SEP><SEP> entry of the <SEP> mixer <SEP> 7
<tb> Phase # R <SEP> of the <SEP><SEP><SEP> signal received <SEP> E <SEP> - <SEP> || <SEP> o <SEP> | <SEP>#<SEP> | <SEP> o <SEP> | <SEP>#
<tb> 2nd <SEP><SEP> entry of <SEP> mixer <SEP> 7
<tb> Phases <SEP> of the <SEP> signal <SEP> video <SEP> +1 <SEP> -1 <SEP> -1 <SEP> +1
<Tb>
The truth table above corresponds to the function "OR
EXCLUSIVE "of the transmitted signal and the received signal This result established in the more specific case of frequency hopping is also valid in the case of any variations of symmetrical duration Ats on either side of Fo.

 The average value of the output signal is substantially zero.

 This video signal is applied to the correlation circuit 9 of FIG. 1 as well as the pseudo-random code, equivalent to a near inversion, generated by the circuit 2, delayed by an amount T1 and amplified up to an amplitude + 1. The result of the combination by an "EXCLUSIVE OR" function of a pseudo-random code and the same code delayed by bt = t (width of a sample) is the same code, with a reversal, delayed by a quantity T1 = 5 r. It is represented in FIG. 2H. If it is generalized, the delayed code of Tm can be obtained by combining with an "EXCLUSIVE OR" gate the original code A and this same delayed code of m #. This is shown in FIG. 1, where the logic circuit 14 performs the "EXCLUSIVE OR" combination of the codes A and A tmt) and delivers the code A (Tm) which is then amplified before correlating the received video signal.

 Fig. 21 shows the signal from the correlation of the video signal of Fig. 2G and the equivalent pseudo-random code shown in Fig. 2H. The average value of this signal is non-zero and characterizes the presence of a target at the distance d = c # t / 2 = ct / 2.

If the distance d is greater than the distance L = c2
2 average value exceeds the set threshold s and the charge explodes triggered by the signal S delivered by the comparison circuit 12 of Figure 1.

 In the more realistic case where the target is affected by a certain speed VR relative to the receiving member, the distance d varies continuously. It is assumed that the transmitted signal is identical to that which has been generated in the case of a stationary target and which has been represented in FIG. 2C, with its phase illustrated by FIG. 2D.

The code used and the control signal VC are also those of FIGS. 2A and 2B respectively.

 The signal is sent back by the moving target. It is therefore subject to the Doppler effect. At the frequency Fo corresponds the Doppler frequency Fd, and at the frequencies Fo + F and Fo - AF correspond Fd1 and Fd2 respectively. We always have 2 # F.t5 = I on the show.

The Doppler frequency of a target animated with a VR speed by
relative to the radar emitting the frequency fE is in a known manner
2VR @E
equal to -, where c is the speed of light.

c
We therefore:
2VR Fo
Fd = c
2VR 2VR #F
Fd1 = (Fo + #F) = Fd +
cc
2VR 2VR
Fd2 = (Fo - #F) = Fd - #F.

The pseudo-random code has in a period TR an equal number n0 of rising edges and falling edges. There is therefore a number n of frequency jumps Fo // Fo + AF,
Fo // Fo - AF on transmission and (Fo + Fd) // Fo + # F + Fdl, (Fo +
Fd) // Fo - #F + Fd2 on reception.

Each broadcast, or reception, at the frequency Fo + # F, or
Fo + #F + Fdl and Fo - # F @ Fd2 respectively, last. At reception a period TR of code corresponds to a time interval nO # ts of reception at the frequency Fo + + F + Fdl, a time interval of no # ts of reception at the frequency Fo - #F +
Fd2 and a time interval (TR - 2no. # T @) receiving at the frequency Fo + Fd.

The phase rotation # D due to a Doppler frequency f @ is in the general case during the time interval AT @
#D = 2 # fD #T = 2 # x fE x #T
c
In the case described above, a phase rotation equal to:
#D = 2 # Fd (TR - 2no # ts) + no Fdl #ts + no Fd2 # ts
2VR
If we replace Fdl and Fd2 by their expression Fd + 2VR # F
c
2VR and Fd - # F respectively, terms containing # F
c cancel each other in the expression of the phase rotation # D which finally rewrites:
2VR
#D = 2 # Fd TR = 2 # x Fo x TR.

c
The phase rotation cl D due to the Doppler frequency during a period TR of pseudo-random code is therefore the same as if the frequency of the transmitted wave had been permanently Fo.

 From this point of view, the device behaves exactly like a phase-jump modulation system O -It.

The voltage obtained at the output of the correlator 9 is a recurrent signal at the Doppler frequency Fd R Fo. The presence of this
c signal is characteristic of the presence of a target at the distance c # d = L = (since this is the case) of the transceiver element.

But the principle remains the same for all the values of the assigned distance L of the radar with respect to the target which are a multiple m of c2t
2
Indeed, at each assigned distance thus defined as equal mcL to 2 & may correspond a pseudo-random equivalent code delayed by a quantity Tm with respect to the code controlling the transmission and consequently the correlation operation is feasible.

 Conversely, a pseudo-random correlation code delayed by a quantity Tm with respect to a reference code, here the transmission code, corresponds according to the known theory of pseudo-random codes to the correlation of the reference code and the code of reference delayed by an amount m Z where Z is the width of a code sample and m is an integer equal to or between 1 and 2n-1.

 Thus, at a delay Tm of the correlation code corresponds a target radar-target distance equal to mc2 the detection of a target located at this distance from the radar and possibly the explosion of the charge contained in the proximity rocket.

 FIG. 3 represents the block diagram of another embodiment of the detection device according to the invention, in which the reception part is of double correlation and allows the detection of a target situated at a distance d less than or equal to one any distance L that is not necessarily an integer multiple of 2 @ / 2 or t is the width of a code sample.

 The transmission part uses the method, according to the invention, of modulation by variations of the emission frequency around the nominal frequency Fo as described above. For the same reasons of simplification of FIG. 1, the transmission frequency is modulated by frequency jumps FollFoF and Fol / Fo -ssF.

 The circuits providing identical functions for the embodiments of FIGS. 1 and 3 are designated by the same references.

 For the transmission part, there is therefore in FIG. 3 a circuit 2 which makes it possible to elaborate a control signal Vc of the frequency of the microwave signal generated by the circuit 1 constituted, for example, in a nonlimiting manner by a Gunn diode coupled to a diode. varactor. The circuit 2 also consists of a logic circuit 21 which, driven by a clock 3 of frequency H 'produces a pseudo-random reference code A. As in the case of FIG. 1, the control signal Vc can be a pulse signal of zero average value whose pulses of width # ts and amplitude + V, or -V, have their rising or falling edge respectively, which coincides with the rising or falling edge respectively, of the pseudo-random reference code Generated by the circuit 21.

The signal Vc controls the frequency jumps Fo // Fo + #F and
Fo // Fo - t F of the microwave signal generated by the circuit 1.

Each emission at frequencies Fo + AF has a duration less than. The transmission frequencies Fo -tF, Fo, Fo + tF correspond respectively to the levels - V, O + V of the control signal Vc.

 The device shown in FIG. 3 comprises only an antenna 40 for transmission and reception, associated with a duplexer 50 which may be constituted by a ferrite circulator whose direction of flow is indicated by an arrow. The duplexer 50 comprises an input / output channel 51 to the antenna 40, an input channel 52 receiving the microwave signal delivered by the circuit 1 and an output channel 53 applying the signal received by the antenna 40, after having been sent back by the target at the entrance of the reception circuit 100.

The device of Figure 3 is double correlated. Its receiving part 100 comprises a first subset 60 which is fed by the signal received by the antenna 40 and transmitted by the circulator 50 on its output 53 and which consists of a microwave detector associated with a first selective amplifier video frequency. A second subset 70 comprises a first multiplier receiving for a first correlation the reference pseudo-random code A and the output signal of the first subset 60, followed by a capacitor and a second video amplifier. The output of said second amplifier feeds a third subset 90 constituted by a second multiplier which receives on its second input the pseudo-random code delivered by the logic circuit 21 and delayed by a duration pTo where p is a positive real number or zero. logic 21 is, as already mentioned, preferably constituted by an n-stage shift register and delivers a reference pseudo-random code
A period (2n - 1) @For any assigned distance pc #
L = with p any positive real number, the code equivalent to the signal received from the target situated at this distance L = is the
2 reference code delayed by pr, A (pr).

The actual positive or zero number 2 can also be considered as the sum of an integer part P and a decimal part k
oThe code A (p) can therefore be obtained, according to the nonlimiting embodiment of FIG. 3, from the reference code A delayed by Po #, A (Po #), delivered by the output adequate shift register 21, and assigned an additional delay k # less than r introduced by a circuit 80.

This code A (pZ) delayed by pu with respect to the code A is, after amplification, applied to the input of the second multiplier contained in the third subset 90. It will be recalled that this code A (pr) corresponds to the signal received by the antenna 40 after being sent back by the target located at the distance L = p c2
2 (real positive) assigned for the explosion of the load.

 The subassembly 90 also comprises a Doppler filter circuit whose frequency band corresponds to the domain of the relative missile / target speeds.

 As in the embodiment of FIG. 1, a detecting circuit with a post-integration cell 12 and a comparison circuit 13 of the input signal with an adjustable threshold according to the noise level complete the double-correlated receiver 100. .

 As in the single correlation receiver of FIG. 1, exceeding the threshold s by the detected signal produces a signal S which triggers the pyrotechnic charge of which the carrier missile is equipped.

 The transmitting part of this embodiment (FIG 3) is identical to that of FIG 1 and operates according to the same principle.

 The dual-correlated reception portion 100 of FIG. 3 operates as follows.

 The leak of the circulator 50 (between the input 52 and the output 53), which is therefore the attenuated transmitted signal, polarizes the microwave detector of the subassembly 60. This detector carries the product between the leak and the received signal.

 In the video-frequency amplifier of the subset 60, the product of the signal received with the reference code A and a spurious signal due to the residual amplitude modulation of the phase modulator of the transmission part are found. This spurious signal is identical to the reference pseudo-random code A.

 The first correlator (at the video frequency) constituted by the first multiplier circuit contained in the subset 70, performs the correlation of the reference pseudo-random code A, which is applied to its input, with the composite signal constituted by the sum of the code pseudo-random reference A and the product of the same reference code A with the received signal. It delivers a signal corresponding to the sum of a DC voltage (from the correlation of the reference code A by itself) and the signal received himself.

 A high-pass filter which may be a simple capacitor (contained in sub-assembly 70 but not shown) eliminates the DC voltage. All that remains is the signal itself which is then amplified and undergoes a second correlation in subassembly 90 with the pseudo-random code A (delayed pn of put (pu real number positive or zero) with respect to the pseudo-random code. random reference A and corresponding to the received signal The exceeding of the threshold s by the received signal detected and correlated corresponds to a radar-target distance less than or equal to this assigned distance L = pic / 2 and triggers the explosive charge.

 As already mentioned, the established results and the circuits used for the particular case of jumps of the Fo / 1 Fo + F and Fo // Fo -dF emission frequency are analogous for more complex variations of the frequency emission for a time tts around the nominal frequency Fo.

 The present invention is mainly applicable to proximity fuzes in which one seeks to work at very high frequencies up to the millimeter wave. In this case, for obvious reasons of efficiency, the purpose of such rockets being to destroy, the explosion must occur near the target and preferably at a distance assigned L = par / 2 where E is a real number positive or zero, less than n.

 Thus, a method and an electromagnetic detection device with single or double correlation, usable in high microwave ranges and detecting a target located at pc # a distance at most equal to radar, have been described.

 2

Claims (15)

 A method of electromagnetic correlation detection for detecting the presence of a target at a distance less than or equal to a predetermined distance from a continuous wave radar, characterized in that the frequency transmitted by the radar varies alternately around the nominal frequency Fo which is also the average value of the transmitted frequency and in that the rising and falling edges of a pseudo-random code control said variations of the transmission frequency, positive and negative respectively with respect to said average frequency Fo, the duration tts of each variation being less than the durations of a code sample.  2. A detection method according to claim 1, characterized in that, for each rising or falling edge of the pseudo-random code, calculated over the duration t: of the sample starting with said edge, the integral of the corresponding curve of variation of the frequency emitted with respect to the nominal frequency Fo is close to 1/2.  3. Detection method according to any one of claims 1 and 2, characterized in that at the reception, the signal returned by the target is successively:  - mixed with a part of the transmitted signal  - amplified;  correlated by the pseudo-random code corresponding to the "OR EXCLUS1F" combination of the reference pseudo-random code and of the same reference code delayed by an integer multiple of the sample duration;  - filtered to eliminate frequencies other than frequencies Doppler corresponding to the considered range of relative velocities of the target with respect to the radar;  - detected; then  - compared to a threshold for, in case of exceeding said threshold, give rise to a pulse signal.  4. Detection method according to any one of claims 1 and 2, characterized in that, on reception, the signal returned by the target is successively  - mixed with a part of the transmitted signal;  - amplified;  -correlated with the pseudo-random reference code;  filtered in a high-pass filter;  correlated with the delayed pseudo-random code of any quantity;  - filtered to eliminate frequencies other than frequencies Doppler corresponding to the considered range of relative velocities of the target with respect to the radar;  - detected; then  - compared to a threshold for, in case of exceeding said threshold, give rise to a pulse signal.  5. Device for the electromagnetic detection of the presence of a target at a distance d less than or equal to a predetermined distance L of a continuous wave radar whose transmission frequency can vary on either side of the nominal frequency Fo during a time ts, characterized in that the transmission part comprises control means (2) of the variations of the transmission frequency, positive and negative alternately with respect to the nominal frequency Fo and equal in absolute value, according to the fronts amounts and descendants respectively of a pseudo-random period reference code (2fil - 1) Z, being the width of a sample greater than the duration t ts of each positive or negative variation and n being a natural number greater than 1 .  6. Detection device according to claim 5, characterized in that for each rising or falling edge of the calculated pseudo-random code, over the duration C of the sample beginning with said edge, the integral of the curve of variation of the transmitted frequency. relative to the nominal frequency Fo is close to 1/2.  7. Detection device according to claim 5 or 6, characterized in that, for a distance L equal to put / 2 with p a positive integer or zero, c the speed of the light and r the sample width, the Party reception includes:  - Mixing means (7) of the transmitted signal and the signal, returned by the target, assigned a Doppler frequency, and received by the radar antenna, which deliver a video frequency signal  means for correlating (9) the video-frequency signal with the delayed reference pseudo-random code of a quantity Tp which depends on the predetermined distance L and which is introduced by delay means;  filtering means (11) for the Doppler frequency of the correlation product;  detection means (12) of the signal delivered by the filtering means; and  means for comparing (13) the signal detected at the output of the detection means (12) with a predetermined threshold s and delivering a presence pulse signal S when the threshold s is exceeded.  8. Detection device according to claim 7, characterized in that the delay means applying a delay Tp to the pseudo-random reference code controlling the frequency of the transmitted signal comprise:  a delay circuit of the reference code of a quantity p; and  a logic circuit effecting the combination by an "EXCLUSIVE OR" function of the reference code and the delayed reference code of  9. Detection device according to claim 5 or 6, characterized in that, for a distance L equal to pcr / 2 with p a positive or zero real number, c the speed of the light and r the sample width, the Party reception includes:  -mixer means (60) of the transmitted signal and the signal, returned by the target, assigned a Doppler frequency, and received by the radar antenna, mixing means which deliver a video frequency signal;  first correlation means (70) of the video frequency signal, delivered by the mixing means (60), with the reference pseudo-random code;  filtering means for eliminating the DC component of the correlation product delivered by the first correlation means  second correlation means (90) of the filtered correlation product with the reference pseudo-random code delayed by p by delay means;  second filtering means (11) passing the Doppler frequency of the signal delivered by the second correlation means (90)  detection means (12) of the signal delivered by the second filtering means (11); and  means for comparing (13) the detected signal, at the output of the detection means (12), with a pre-determined threshold s and delivering a pulse signal S of presence when the threshold s is exceeded.  10. Detection device according to any one of claims 7 and 9, characterized in that the number p is less than 2n-1.  11. Detection device according to claim 5, characterized in that the control means (2) of the frequency switching of the transmitted signal comprise:  a circuit (21) controlled by a clock (3) and delivering the pseudo-random reference code; and  a circuit (22) generating, from said pseudorandom reference code, a signal having a level, relative to the level O, of variations of duration ts constituting a replica of the variations of the transmission frequency with respect to the nominal frequency Fo and triggered, for the positive and negative levels respectively, by the rising and falling edges of the pseudo-random code, the signal thus generated controlling the corresponding variations in the transmission frequency.  12. Detection device according to claim 11, characterized in that the circuit (21) generating the reference pseudo-random code is an n-stage shift register.  13. Detection device according to claim 5, characterized in that the signal emitted by the radar is generated by a diode Gunn coupled to a Varactor diode which is controlled by the control circuit (2).  14. Proximity firing comprising a continuous wave radar transmitting an adjustable frequency signal, a pyrotechnic charge and a detection device according to any one of claims 5 to 13, characterized in that the explosion of the charge is triggered by the signal S delivered by the detection device in the case of the presence of a target at a distance less than or equal to a predetermined distance L of the radar.  15. proximity fuse according to claim 14, characterized in that the predetermined distance L is equal to pic / 2, with P a positive number or zero, less than n.