FR2646926A1 - METHOD FOR DETECTING AND IDENTIFYING OBJECTS FORMING TARGETS - Google Patents

Abstract

A device for detecting and identifying objects forming targets. A laser detector Sr is used, consisting of a laser transmitter LS1, AOM, TS and an optical receiver ST, O1, D1, V1, B1 produced in the form of a superheterodyne receiver, a demodulator DM1 connected to the output of the receiver optical and automatically adapting to the Doppler frequency shift conditioned by the speed of the target object, and an evaluation circuit A9 for forming a detection or identification result from the demodulated signals in the time range or in the frequency range, by comparison to models, which can be called from memory MS1, of known target objects. Application to radars.

Description

Method for detecting and identifying target objects

  The invention relates to a method

  the radar principle and used to detect and identify target objects.

  To detect and identify military targets

  In a tactical zone, various processes operating in an active manner and in a passive manner are known. As devices for putting

  of these methods, it is possible to use, for example,

  radar devices, heat-sensitive detectors and thermal imaging devices. The particular characteristics of the different devices prove to be disadvantages

  or advantages, in the case of their use

  plicated in the tactical zone, depending on a large

  number of concomitant circumstances.

  In the case of the use of radar devices,

  the Doppler frequency components of the signals of

  object objects or moving parts of these objects are evaluated, for

  spectral. Devices working in a way that

  they use, for example, the thermal radiation of the target objects and derive, from the modulation of the thermal radiation of the type provided by the rotor of a helicopter, characteristics

  clean for detection or identification.

  The object of the invention is to further improve the detection and identification of objects forming

  using a process which is based on knowledge of the

  that the drive mechanisms of all the objects used produce

  corresponding vibratory levels at their surface.

  According to the invention, the problem is solved by the use of a laser detector constituted by

  a laser transmitter comprising an optical receiver

  in the form of a superheterodyne receiver, and by a demodulator connected to the output of the optical detector, with an automatic adaptation to the Doppler frequency shift conditioned by the speed of the target object. An evaluation circuit serves to form a detection or identification result from the demodulated signals in the time range or in the frequency range, compared to

  models, which can be called from a memory, ob-

jets forming known targets.

  The characteristic vibratory spectrum, which appears

  at the surface of the objects, modulates the laser beam directed at the object,

  view of its frequency, either from the point of view of its phase.

  The reflected echo signal therefore contains a Doppler frequency component which is attributable

  exclusively to the vibrations of the surface. Being given

  that all the superficial parts of an object perform vibrations with a vibratory frequency, no matter which part is illuminated by the laser beam. As a result, compared to other processes

  known, the advantage that the detection and identification of

  are independent of the viewing angle.

  In accordance with an advantageous variant of the

  In order to carry out the method, a laser having a wavelength in the infrared range, for example a CO2 laser, is used. This wavelength is very much smaller than the vibratory amplitude at the surface of an object. Therefore a good depth of modulation is guaranteed and thus the exploitation

  even very low vibration frequencies is guaranteed.

up to less than one Hz.

  If the target object, fired by the laser beam, has its own displacement, the Doppler frequency due to the vibratory oscillations, the echo signal is superimposed on a second frequency

  Doppler dependent on speed. Automatic adaptation

  of the demodulator located in the super-receiver

  heterodyne at the frequency offset can be advantageously achieved using a PLL circuit (loop

phase locked).

  Other features and advantages of the

  will appear from the description given below_

  reference is made to the accompanying drawings, in which

  FIG. 1 represents the realization of a

  frequency modulation of the laser beam under the effect of superficial vibration of a helicopter;

  - Figure 2 represents the constitution of

  a superheterodyne optical receiver;

  - Figure 3 represents the acquisition of the

  training using a laser detector;

  FIG. 4 represents an exemplary embodiment

  of a laser detector comprising a circuit for ex-

  operation for a laser transmitter operating in continuous mode; Figures 5 and 6 show variants

  of the exploitation part of the example of

Figure 4; and

  FIG. 7 represents an exemplary embodiment

  laser detector comprising a pulsed laser emitter. Referring to Figure 1 in the case of the example of a helicopter, it will be explained that J3e is the modulation caused in a laser beam by the vibration of the cabin of the helicopter coated by

  the drive mechanism. The helicopter suspended

  -C sion is irradiated with the frequency f of the emitter la-

  s ser. The oscillation of the transmit signal is assumed to have a sinusoidal shape-and be represented by the following relationships: A = a coslL tsss where A is the amplitude at the end of the duration t, sa is the maximum amplitude, and -I = 21T f denotes the sss

  angular velocity. The signal is reflected on the heli

  coptère and arrives after an overall travel time T with the phase t in the receiver:

  For simplicity, it is assumed that the vibration of

  canic, which appears at the location of reflection on the hull of the helicopter, has a sinusoidal shape and propagates with a single frequency fH 'We can describe this oscillation by the formula: s (t) = aHsin WH t where s represents the amplitude at the end of time t, aH the maximum amplitude (excursion) and WH the angular velocity. It is further assumed that the deviation of the hull of the helicopter is precisely

  towards the receiver. As a result,

  sit T is modulated to the rhythm of the vibration. With these assumptions one can draw mathematically in a simple way the fact that the reflected laser beam comprises

a frequency modulation.

  The modulation index m obtained satisfies the relation 4T aH m = V (s

  s denoting the wavelength of the emitted radiation.

  In general, the vibratory amplitude aH of the object is

  significantly greater than the wavelength As of the laser.

  As a result, the reflected laser beam

  is well modulated and provides a great excur-

  frequency. That is why we can expect to obtain a good signal-to-noise ratio, because the latter increases in the case of a modulation

  frequency, when the frequency deviation increases.

  If the helicopter has a relative speed, this leads to an additional Doppler shift, which must be taken into account in the preparation of

signals in the receiver.

  For the preparation of laser echo signals,

  it is necessary to use a super-heterogeneous receiver

  As can be seen from FIG. 2, the received signal consists of the modulation spectrum 8fH, for a carrier frequency f. To perform the demodulation, it is necessary to know the carrier frequency fs in the receiver, that is to say that a superheterodyne receiver is necessary. Figure 4 represents

  the constitution in principle of a superhero receiver

  optical rodyne. The LO signal of a local oscillator, which has the frequency fLO and which is injected by L

  coupling via a beam splitter

  ST and an optical system O is superimposed on the received signal having the frequency f + fH in the detector. The mixed product of the two signals, having the frequency fs - fLO + fH appears at the output of the detector D. The difference between the carrier frequency and the frequency of the local oscillator must have

  the constancy necessary for superhet reception-

  rodyne. Both frequencies should therefore be drawn from a coherent source. The mixed product appearing at the output of detector D is sent

  to an amplifier V for further processing.

  - Obtaining information is explained in

  reference to Figure 3. A laser beam from the transmitter

  The laser driver LS having the frequency fLO is frequency shifted by a value equal to an intermediate frequency fZF (in the MHz range) by the acousto-optical modulator AOM. The output signal having the frequency fLO + fZF = fs illuminates the target object ZO and is frequency modulated by the

  vibration at the surface of this ob-

  jet. The signal reflected back to the receiver con-

  holds the frequency modulation fH for a carrier frequency fLO + fZF 'At the output of a mixer stage N of the receiver, after the mixing of this frequency with the frequency fLO of the laser transmitter, the

  frequency modulation fH for the intermediate frequency

  The latter can be demodulated in the DM demodulator to provide the own vibration frequency fH An embodiment of a laser detector comprising an operating circuit for a transmitter

  Continuous mode laser is

  shown in Figure 4. The transmission frequency, produced

  in an emitter LS consisting for example in the form of a CO2 laser, is irradiated in the form

  an emission signal through a teleservice

  TS cope. The frequency offset, which is necessary for the optical receiver realized in the form of a superheterodyne receiver and which has a value equal to. an intermediate frequency ZF is carried out in an acousto-optical modulator AOM1. The MO signal is injected by coupling, together with a signal

  reflected on the object forming a target, in the receiver

  optical superheterodyne, via an ST1 beam splitter. The two signal components arrive via an optical system O1 and the input of a detector D1, the output of which delivers the modulation signal having a frequency

  average close to the intermediate frequency. This-

  gnal is amplified in the amplifier V1, is limited in the limiter D1 and is then demodulated in a

  DM1 frequency demodulator, so that the oscilla-

  vibration occurs in the bass range

  frequencies. The sum of all vibratory oscillations

  Those areas, which appear at the surface of the target object and are located within the target area illuminated by the laser, then represent the characteristic vibration spectrum. In addition to the FM demodulator, the correlator K is a component

  important part of the operating circuit. Thanks to a correlation

  lation, in the time range, with known target models, which are drawn for example from an MS1 model memory, one can determine the type

  of a target object and display it on a

A display.

  The FM demodulator, DM1, must be designed otherwise.

  their so that outside the frequency Dop-

  pler appearing under the effect of vibratory oscillations

  At the surface of a target object, an additional Doppler frequency superimposed on said Doppler frequency and dependent on the relative velocity of the target object is taken into account. In order to

  delete the action of the unwanted Doppler frequency

  rable, which depends on the relative movement of the object

  target, it is necessary to realize the demo-

  dulator so that it operates vertically

  joint rustling. From the point of view of the circuit technique, this is possible by using a PLL circuit (phase-locked loop circuit). The exploitation of the vibration spectrum can be carried out, unlike the embodiment of FIG. 4, thanks to the use of a processor producing a fast Fourier transform and mounted between the FM demodulator DM1 and the correlator K, this processor converts the low frequency vibration oscillations of the time range into the spectral range. The correlation carried out with the known models is then carried out in the frequency range

(Figure 5).

  In another embodiment of FIG.

  6, the cepstrum of vibratory oscillations is obtained

  by a double Fourier transform between the demodulator output and the K1 correlator input, in which case a logarithmic

  performed in the Log floor between the first and second

  Fourier transformations. The identification of the target object is then performed in the same way as in the case of the preceding examples, by

correlation and display.

  An exemplary embodiment, shown on the

  Figure 7, a laser detector works with a

  pulsed laser beam. The constitution of the circuit point of view therefore corresponds to that of a pulse Doppler radar. For the formation of the frequency

  intermediate in the detector D2, we send the signal -

  of beat, which is injected by coupling at the same

  while the signal received at the receiver input op-

  tick, directly from a continuous laser. In addition, the continuous laser feeds a laser amplifier LV via an acousto-optical modulator AOM2. A central generating station TZ including

  a cadence generator produces the pulse frequency

  which delivers both the clock frequency and

  ge for a ZFO intermediate frequency oscillator, the clock frequency for a remote gate set device, and the clock frequency for a HV / HF high voltage / high frequency source. The clock frequency delivered to a floor of

  ST switching corresponds to the recursion frequency

  pulses of the emitted laser beam. Fre -

  quence of the oscillator, which is manipulated in the floor

  switching speed at the rate of the recursion frequency.

  pulses, shifts the laser frequency, of a value equal to the intermediate frequency ZF, in the acousto-optic modulator AOM2. The LV laser amplifier, which amplifies the offset laser frequency, is pulsed further at the rate of the recurrence frequency

  impulses, with high frequency or exci-

  high voltage. The width of the pulses is greater than that of the laser pulse sent by

  the acousto-optical modulator AOM2. As a result,

  coupled laser pulse in the amplification

  LV laser is already in an excited environment and

  immediately triggers an amplified laser pulse.

  The processing of echo signals in the

  of the laser detector differs from that involved in the described embodiments.

  previously, as a result of the use of a

  ETF comprising a set of distance gates and disposed at the output of the limiter B2. As a result of using the ETB circuit having a set of distance gates, each gate in which the signal received from a target object is located is

  transmitted directly to the DM2 modulator.

Claims (11)

  1. Working device. according to the radar principle and for detecting and identifying target objects, characterized by the use of a laser detector (Sr), consisting of a laser transmitter (LS1, AOM, TS) and an optical receiver (ST). , O1,   D1, Vl, B1) produced as a super-charged receiver   heterodyne, by a demodulator (DM1) connected to the output of the optical receiver and realizing an adaptation   automatic Doppler frequency shift condition-   born by the speed of the target object, and by   an operating circuit (A9) serving to form a   detection or identification result from the demodulated signals in the time range or in the frequency range, in comparison with models, which can be called from a memory (MS1), objects forming known targets.   2. Device for detecting and identifying   target objects according to claim 1,   characterized in that the Doppler echo signal, formed by the sum of all vibratory oscillations at the surface of the target object in the illumination range of the laser beam, is operated in the superheterodyne receiver and in the the circuit operating cost (A% 9).   Device according to Claim 1 or 2, characterized in that the demodulator (DM) is   carried out in accordance with the principle of the phase rusting.   4. Device according to any one of the   1 to 3, characterized in that the local oscillator has a high frequency stability for the production of the intermediate frequency (ZF)   in the superheterodyne receiver.   5. Device according to any one of the   from 1 to 4, characterized by the fact that   (LS) of the laser detector (Sr) operates according to the continuous mode.   6. Device - according to any of the   2 to 4, characterized by the fact that the   (LS) of the laser detector (Sr) operates under the form of a pulsed transmitter.   7. Device according to any one of the   Claims 1 to 6, characterized by the fact that   director is realized in the form of a glass laser to CO2.   8. Device according to any one of the   i. 1 to 7, characterized in that the offset of the laser frequency, of a value equal to the intermediate frequency (ZS), is achieved by means of   an acousto-optic modulator (AOM).   9. Device according to any one of the   claims 1 to 8, characterized in that the identification   tification of the target object is achieved by correlation with known target models, in the time range.   10. Apparatus according to any one of the   Claims 1 to 9, characterized in that the demodulated oscillations of the signal are converted into   the spectral range by means of a real-time processor (FFT).   reading a fast Fourier transform, and that correlating with target object models   known is then carried out in the range of quences.   11. Device according to claim 10, ca.   characterized by the fact that downstream of the demodulator (DM)   connected to a first processor (FFT1)   reading a fast Fourier transform, a logarithmic logging circuit (Log) and a second processor (FFT2) realizing a Fourier transform   fast, whose output spectrum is evaluated by cor-   relationship with known models, with a view to identifying fication.