RU2366973C1 - Method for detection of targets from accidental reverberation disturbances - Google Patents

Abstract

FIELD: physics, measurements.

SUBSTANCE: invention is related to echo ranging and may be used for detection of immovable and nonmobile targets of complex geometric shape (sea underwater objects, bottom or scuba divers). Method for detection of targets from accidental reverberation disturbances is based on cyclic radiation of probing pulses with high resolution by distance, reception of reflected signals, establishment of amplitude first threshold for detection of reflected signals at the background of noise disturbances, separation of enveloping reflected signals, establishment of amplitude second threshold and processing of reflected signals, besides in each of two neighboring cycles of location, one or several reflected signals are separated by exceeding of their level above the first threshold within the limits of time window comparable with duration of reflected signal from target, their matched filtration is carried out, detected for separation of enveloping signals, out of maximums of enveloping signals sequences of impulses are generated as identical by amplitude or proportional to amplitude of maximums, for instance rectangular ones, then they are memorised, mutual correlation functions are calculated between appropriate sequences of pulses memorised in two neighboring cycles of location, medium level of each calculated mutual correlation function is identified and memorised in the section of defined duration, and target is recognised, which corresponds to two reflected signals in neighboring cycles, for which maximum is observed in mutual correlation function, exceeding its medium level by value of specified second threshold.

EFFECT: improved noise immunity of recognition.

2 cl, 4 dwg

Description

The invention relates to sonar and can be used to recognize stationary and inactive targets of complex geometric shape (real marine objects, bottom or scuba divers) from random reverberation interference (volume and surface reverberation).

Known methods and devices for target recognition by sonar and radar signals from reverberation interference (see French patent No. 2524983 MKI ³ G01S 9/00, priority 10.04.82; Swedish patent No. 2404231, MKI G01S 9/00, publication 25.05.79; international application No. 86/01001, MKI ⁴ G01S 13/52, 7/23 publication 13.02.86; international application No. 86/01000, MKI ⁴ G01S 7/52, 15/87 publication 13.02.86; French application No. 2525774, MKI G01S 7/28, 15/00 publication 10/28/83; European application No. 0093057, MKI G01S 15/52; US patent No. 4195359 MKI G01S 9/66, 7/23 publication 25.03.80, and other).

In known methods, the radiation of probe pulses and the reception of reflected signals are used. Thresholds are set to detect reflected signals. In some cases, the distance from radiation to scattering reflections is measured. The spectra of the reflected signals are analyzed and reverberation interference from moving targets, which differ in significant Doppler shifts of the spectrum of the reflected signals due to the movement of the targets, is recognized.

Fixed and sedentary targets are recognized provided that the levels of reflected echo signals from the targets are significantly higher than the reverberation noise level.

In Swedish patent No. 2404231 “A method for distinguishing signals from moving targets from signals reflected by the surface of the water” (MKI G01S 9/00, publication 05/25/79) emit sound sounding pulses and receive the reflected signals. The wave spectra of the reflective excited water surface and the maximum angle of inclination of this surface relative to the emitter are measured. Determine the maximum expected change in the distance between the emitter and the surface of the water in the absence of a target. Only the moving target is recognized by the spectrum of the reflected signals, different from the spectrum of the signals scattered by the excited surface, and by the distance to the moving target, different from the distance between the emitter and the excited surface of the water.

A known method of location and detection (RF patent No. 2176401) of an underwater target in a protected marine area, the essence of which is that using focused radiation in the place of the intended location of the target in a protected marine area they emit a shock sound wave or a sequence of shock sound waves. Sound waves reflected from an underwater target along with radiated sound waves are received by a sonar receiver with a highly directional directivity characteristic. As a result of processing the received signals, the underwater target is detected and tracked. Moreover, target detection is possible if the level of the reflected signal from it significantly exceeds the level of reverberation noise.

There is a method of detecting underwater objects at the sea line in the shallow sea (RF patent No. 2161319), in which sonar pulses directed to the upper half-space are emitted from a number of points with known coordinates located at the bottom on the boundary line, and receive echo signals from these objects the same points and the position of the object are judged by the coordinates of the receiver that detected the echo signal, emit pulses at neighboring points of different frequencies at different frequencies and at all points at the same time, take an echo from each point drove the natural frequency and at least two other frequencies from neighboring emitters on each side of the line, the distance between adjacent pick-up points is chosen equal to the double depth of the sea along the boundary line, the arrival time of the echo signals is limited by the arrival time of the pulses reflected from the sea surface at each of the received at this frequency point, and the location of the detected object is specified by the coordinates of the emitters operating at the frequency of the received echo signals. They monitor the level of direct sonar pulses received from a neighboring emitter, and when this level changes, the appearance of an object near the bottom is judged. However, this method cannot be detected underwater object or scuba diver, hanging on the surface or near the surface of the sea, as well as scuba diver or underwater object stationary or walking at low speed against the background of volumetric reverb, such as fish clusters.

A known method for detecting underwater objects, which is based on the irradiation of the controlled space by a set of tone-modulated signals, the carrier and modulation frequencies of which are selected in accordance with the estimated size of the objects, their position relative to the surface of the reservoir and speed (see RF patent No. 2008692). The received signal is filtered, detected, and the attenuation of the corresponding signal components is judged on the presence and size of objects. A device for detecting underwater objects includes a radiator, a power amplifier, a master oscillator, a modulator, a set of carrier frequencies, a block of modulation frequencies, a receiving antenna, an amplifier, a multi-channel frequency filter, detector, indicator. The method identifies a known object when the signal reflected from it exceeds the reverberation noise level.

A known method of detecting an invasion of an underwater object in a controlled area of a body of water (US patent No. 4319349), using sequential sonar irradiation of various zones of a controlled body of water and receiving hydroacoustic signals reflected from the desired underwater object with a sonar receiver, with subsequent determination of the location, course and speed of the object according to the parameters received signal.

A known method of detecting an invasion of an underwater object in a controlled water area of the reservoir (RF patent No. 2150123), in which using the sonar reflectors located on an elliptical surface, set the controlled region of the reservoir. In the foci of the elliptical surface have a sonar emitter and receiver. The emitter is made in the form of a circular sonar antenna, and the receiver connected to the indicator of all-round visibility, with a uniform directional characteristic. Consistently in different directions, the emitter generates pulses of acoustic energy, which for the same period of time reach the receiver, reflected from hydroacoustic reflectors. A series of pulses are formed on the circular view indicator. When an underwater object invades the monitored zone, one of the pulses on the circular viewing indicator disappears, which indicates the presence of a target. Sequential processing of the output signals allows you to determine the course and speed of the target. This method will give false positives when entering fish in a protected reservoir. A protected body of water cannot cover the surface of the sea.

As a prototype, French patent No. 2524983 “Method and device for target recognition and suppression of spurious signals in radar and sonar stations,” MKI G01S 7/66, priority from 04/10/82, is accepted.

In this method, the station-controlled zone is divided in azimuth and range into several cells. For each cell, the average power of the total (noise and reverberation) interference is estimated in an adaptive way. As a result of these estimates, a threshold is established in each cell that exceeds the level of total interference. If the level of the reflected signal in the cell exceeds the threshold, then the presence of the target is recorded. Additionally, suppression of reverberation noise and reflected signals from stationary and inactive targets that do not have Doppler shift spectra or have a small Doppler shift is provided. In this case, the speed range of sedentary targets, the reflected signals from which are suppressed together with the reverberation noise, are determined by the bandwidth of the suppression (notch) filter tuned to the width of the spectrum of the reverberation noise. Driving targets are sorted by several speed ranges depending on the magnitude of the Doppler shift of the spectra of the signals reflected from them.

This method allows you to recognize stationary and sedentary targets from reverberation interference, but only if the level of reflected signals from these targets confidently exceeds the level of reverberation interference, estimated in the absence of targets, by the value of the specified threshold.

The aim of this proposal is to increase the noise immunity of recognition of stationary and inactive targets of complex geometric shape (marine underwater objects, bottom or scuba divers) from volume and surface reverberation.

The goal is achieved by the fact that in the method of target recognition from random reverberation interference (including cyclic radiation of zoning pulses, receiving reflected signals, setting an amplitude first threshold for detecting reflected signals against the background of noise interference and processing reflected signals) emit a probe pulse with a high range resolution, in each of two adjacent locations, one or more reflected signals are distinguished by exceeding their level above the first threshold within the time strobe, commensurate with the duration of the reflected signal from the target, they are matched by filtering, envelopes are extracted, and pulse sequences of the same amplitude or proportional to the amplitude of the maxima are formed from their maxima, for example, rectangular, these sequences of pulses are recorded in RAM, then the mutual correlation functions between the corresponding sequences are calculated pulses recorded in the memory in two adjacent cycles of location, determine and remember the average level azhdoy calculated mutual correlation function for a certain duration portion and recognizing the target, corresponding to the two reflected signals in adjacent cycles, for which there is a peak greater than its average level by an amount specified by the second threshold in the mutual correlation function. When processing signals for several irradiation cycles, the calculated mutual correlation functions in neighboring cycles are averaged and the target is recognized by exceeding the maximum in the average mutual correlation function of its average level by the value of the specified second threshold.

Salient features of the proposed solutions from the known technical solutions and prototype are:

- calculation of mutual correlation functions between the corresponding sequences of pulses formulated from the maxima of the envelopes of the reflected signals compressed in the matched filter recorded in the neighboring target location cycles;

- recognition of the target corresponding to two reflected signals in adjacent location cycles for which a maximum is observed in the mutual correlation function, exceeding its average level by the value of the specified second threshold.

Thus, the claimed solution has significant differences.

The positive effect is an increase in the noise immunity of recognition of stationary and inactive targets (marine underwater objects - MPO, bottom or scuba divers) from surface and volume reverb is achieved by calculating the mutual correlation function between sequences of pulses formed from the maxima of the envelopes of the compressed signals reflected from the target at close angles location (the angle of location between the cycles of irradiation of the target changes by no more than 2-3 °). The reflected signals from the target consist of the sum of pulses, the delays between which {τ _j } correspond to the location of the most significant elements in the reflectivity of the object. With a small change in the angle of location (2-3 °), the delays between pulses {τ _j } in the reflected signals change insignificantly. These delays {τ _j } are detected directly in the form of temporal positions of the maxima in the envelope of the reflected signal when using a short probe pulse (the spatial length of which is significantly less than the length of the target) and in the envelope of the reflected signal compressed in a matched filter when using a complex probe pulse, for example, with a frequency modulation.

When using complex probe pulses in a matched filter, compression of the reflected signals is performed. When applying short tonal probe pulses instead of matched filtering, band-pass frequency filtering of the reflected signals is performed. The stability of the temporal positions of the maxima {τ _j } was proved by performing multi-alternative recognition of real underwater objects of complex geometric shape (Davydov BC The optimal decision rule for recognizing bodies of complex geometric shape by reflected hydroacoustic signals. Proceedings of the IV Far Eastern Conference "Acoustic Methods and Means of Ocean Research" AN USSR, Vladivostok 1986) The amplitudes of these maxima are less resistant to a change in the angle of location. Therefore, it is often advisable to form a sequence of pulses of the same amplitude, for example, rectangular, from the maxima of the envelope of the compressed signal. Thereby, information on the relative position of the most significant reflectors on the underwater object’s body is extracted from the reflected signal. In two signals reflected from the target’s body at close location angles {τ _i }, they almost coincide. Therefore, in the mutual correlation function calculated between successors of pulses, the time delays between which almost coincide and are equal to {τ _j }, a maximum is observed. This maximum will exceed the average level of the cross-correlation function by N times, in proportion to the number of coinciding maxima in two sequences. The number of these maxima is equal to the number of the most significant reflective elements of the object’s body. This maximum will not be formed when calculating the mutual correlation function between sequences of pulses constructed from random reverberation noise (volume and surface reverberation), determined by the scattering of probing pulses by randomly moving scatterers (for example, fish clusters, excited sea surface). For these reverberation noises, there is no stability in the temporal positions of the maxima {t _j } in adjacent location cycles. Similarly, this maximum will not be observed when calculating the cross-correlation function between sequences of pulses generated from reverberation noise and the signal reflected from the target.

To recognize stationary and inactive goals, it is required to determine the excess of the maximum in the mutual correlation function over its average level by the value of the specified second threshold. Moreover, the level of reverberation noise can be commensurate with the level of signals reflected from the target, since information about the levels of reflected signals and reverberation noise is excluded due to the formation of pulse sequences of equal amplitude. In the prototype for the recognition of stationary and inactive targets, the level of reverberation noise should be less than the level of reflected signals from the target by the value of the specified threshold.

Thus, in the claimed solution, the level of random reverberation interference (surface and volume reverb) is allowed, comparable with the level of signals reflected from the target, exceeding the allowable level of reverberation interference in the prototype.

Therefore, the noise immunity of the proposed solution is higher than that of the prototype and other known methods for the recognition of stationary and inactive targets against a background of reverberation interference.

Figure 1 shows an example of recording envelopes compressed in a matched filter of a probe pulse, reverberation noise and a signal reflected from a real target, recorded during the Ladoga tests, where

1 - envelope of the compressed probe pulse;

2 - envelope of compressed reverberation noise;

3 - envelope of the compressed reflected signal.

Figure 2 shows an example of target recognition, consisting of several reflectors, from random reverberation noise when using short probing pulses, where

1 - envelope of a short probe pulse in the first radiation cycle;

2 - envelope of reverberation noise and reflected signal in the first radiation cycle;

3 - a sequence of pulses formed from the maxima of the envelopes of the reverberation and reflected signals in the first radiation cycle;

4 - envelope of a short probe pulse in the second radiation cycle;

5 - envelope of reverberation noise and reflected signal in the second radiation cycle;

6 - a sequence of pulses formed from the maxima of the envelopes of the reverberation and reflected signals in the second radiation cycle;

7 - mutual correlation function between sequences of pulses, presented in a simplified form.

Figure 3 shows:

a) Histograms of the distribution of the maxima of the envelopes of the mutual correlation functions - OVKF S ^max between the echo signals and sounding pulses, as well as between the reverberation (mode No. 5 - II section) and sounding pulses.

b) Histograms of the distribution of the maxima of the sign correlation functions

r ^max for echo signals and reverberation (mode No. 5 - II section) obtained in adjacent circuit irradiation cycles.

The proposed method can be implemented by a device, a functional diagram of which is shown in figure 4.

1 - probe pulse generator,

2 - power amplifier,

3 - transceiving antenna,

4 - recognizable target

5 - amplifier receiving path,

6 - the first comparator,

7 - standby multivibrator,

8 is a key diagram,

9 - direction finder,

10 - pulse counter,

11 - amplifier limiter

12 - matched filter,

13 is a synchronization circuit,

14 - detector

15 is a second comparator,

16 is a data input device,

17 - RAM

18 is a data output device,

19 - correlator,

20 - delay line

21 - integrator

22 is a storage device,

23 is a subtractive device,

24 - the third comparator,

25 - indicator.

Figure 1 shows that in real conditions the level of the envelope of the reverberation interference after its compression in the matched filter may exceed the level of the envelope of the compressed reflected signal. In this example, a phase-shift signal was used as a probe pulse. A similar excess of the envelope levels of the compressed reverberations in the matched filter over the envelope levels of the signals reflected from the targets is observed when using complex sounding pulses with a different type of modulation (for example, with frequency modulation). In this case, the most common reverberation noise is random interference, determined by volume and surface reverberation (as in the example in FIG. 1). In this case, the temporal positions of the maxima of the envelope of the reverberation noise vary randomly from one radiation cycle to another. On the contrary, the temporal positions of the maxima {τ _j } in the envelope of the signals reflected from the target change insignificantly with a small change in the angle of target location, which takes place in neighboring cycles of target irradiation. It is proposed to use this property of targets (MPO, bottom and scuba divers) for their recognition from random reverberation interference (volume and surface reverberation). In this way, fish schools (volumetric reverb) can be recognized against the background of bottom reverb. The level of noise interference, as a rule, is significantly lower than the level of reverberation interference, which is the case in the example of FIG. 1.

Figure 2 shows the envelopes of the short probe pulses 1, 4 emitted in two adjacent target location cycles. The envelopes of the reverberation interference and reflected signals 2, 5 exceed the first threshold U _n1 , set based on the level of noise interference. First, the reverberation interference signal U _p recorded in the receiving path is shown, and then the reflected signal U _neg . From the maxima of the envelopes of the reverberation noise and the reflected signals, sequences of rectangular pulses are formed. Figure 2 shows that the delays between pulses {τ _j } in the sequences formed from the signals reflected from the target are repeated in two adjacent cycles of target irradiation. In contrast, delays between pulses {τ _j } in sequences formed from reverberation noise vary in two adjacent cycles. Mutual correlation functions (sign) between the sequences of pulses 7 are shown in figure 2 in a simplified form. The mutual correlation function between sequences of pulses formed from reverberation interference K _p does not have a pronounced maximum. In the mutual correlation function performed between the sequences of pulses formed from the reflected signals To _OTR , there is a maximum exceeding the average level of the mutual correlation function by the value of the second threshold U _n2 .

Figure 3 presents an example of the results of processing experimental data obtained during marine trials in the Black Sea. The upper graph (Fig. 3a) shows the histograms of the distribution of the envelope maximums of the mutual correlation functions - OVKF S (t) between complex probe pulses and echo signals s _e (t) from a body of complex shape, as well as between probe pulses and reverberation s _p ( t); here the probability of correct recognition of s _e (t) and s _p (t) by the maximum response of the matched filter S ^max (t) is given with an indication of the confidence interval in brackets. The lower graph (Fig.3b) shows histograms of the distribution of the maxima of the sign correlation functions r ^max ( _j ) for echo signals s _e (t) and reverberation s _p (t) obtained by the method of inter-cycle correlation processing of signals in two adjacent body irradiation cycles complex shape; here the probability of correct recognition of s _e (t) and s _p (t) by the maximum value of the sign correlation function r ^max ( _j ) is given with an indication of the confidence interval in brackets. It can be seen that the method of inter-cycle correlation signal processing can significantly increase the probability of correct recognition of echo signals from a body of complex geometric shape and reverberation noise.

In a device that implements the proposed method (figure 4), the generator 1 on command from the synchronization circuit 13 generates probing pulses with high resolution in range (short or complex probing pulses). These pulses are amplified in a power amplifier 2 and emitted by an antenna 3. The signals reflected from the target 4 and reverberation noise are amplified in the amplifier of the receiving path 5. If the received signals exceed the first threshold, which is set based on the noise interference level, the first comparator 6 is activated and turns on a waiting multivibrator 7, which generates a pulse of a temporary strobe in duration commensurate with the duration of the expected reflected signal. This pulse opens the key circuit 8 and passes the signals from the output of the amplifier 5 to the input of the amplifier-limiter 11, normalizing these signals in amplitude. After normalization, these signals are compressed in a matched filter 12 (when using complex probe pulses) or pass through a band-pass filter (which is consistent for tonal probe pulses). From the output of the matched filter 12, the signals are fed to the detector 14, in which the envelopes of the received signals are highlighted. From the maxima of these envelopes, sequences of rectangular pulses are formed using the second comparator 15. These sequences of pulses through the input device 16 are recorded in RAM 17 by commands from the synchronization circuit 13. The values of bearings on the target, measured for each of the received signals from using the direction finder 9. The direction finder measures the bearing by two reflected signals arriving at its input from two phase centers of the antenna through two amplifiers enclosed in an antenna unit Ithel receiving channel 5. By triggering pulse from timing circuit 13 is switched pulse counter 10, which measures the distance to the target by counting pulses between the probe (or its triggering pulse) and the reflected signal. The value of the ranges measured by the pulse counter 10 for the corresponding reflected signals is also stored in the operative memory 17. After recording all the indicated information in the memory, the synchronization circuit 13 instructs the probe pulse generator 1 to emit the next probe pulse. In the next cycle of irradiation of the target, the reflected signals and reverberation noise are processed in the same sequence and the similar information is written into the main memory 17. Then, through the data output device 18, according to the command from the synchronization circuit 13, the corresponding pulse sequences recorded in two adjacent cycles are read and received to the correlator 19, where the mutual correlation functions are calculated. The average levels of the calculated cross-correlation functions are determined in the front section with a duration of no more than one third of its duration using the integrator 21, stored in the storage device 22 and fed to the subtractor 23. In the subtractor, the difference between the current values of the cross-correlation function and its average level is calculated. The current values of the mutual correlation function are supplied to the subtractor via the delay line 20. From the output of the subtractor, the difference value is supplied to the third comparator 24 and compared with the second threshold value. When the threshold value is exceeded, the third comparator 24 is triggered. The output signal of the comparator 24 enters the RAM 17 and gives permission to read from the memory of the bearings to the target and the distances to the target, measured for two reflected signals in adjacent location cycles that caused the appearance of the indicated maximum in the mutual correlation functions. These values of bearings and ranges for two adjacent cycles are displayed or recorded using indicator 25.

The proposed method allows to increase the noise immunity of recognizing targets of complex shape (MPO, bottom and scuba divers) from volume and surface reverberation. It cannot be used for target recognition against the background of bottom reverb (for example, underwater rocks). This method is resistant to possible distortion of the reflected signals due to the multipath propagation of signals in the sea.

Claims (2)

1. A method for recognizing targets from random reverberation noise, based on the cyclic radiation of probing pulses with high resolution in range, receiving reflected signals, setting the amplitude first threshold for detecting reflected signals against noise, extracting envelopes of reflected signals, setting the amplitude second threshold and processing reflected signals, characterized in that in each of two adjacent cycles of location, one or more reflected signals are distinguished by exceeding them equal to the first threshold within the time strobe, commensurate with the duration of the reflected signal from the target, they are matched for filtering, detected to extract envelopes of signals, from the maxima of the envelopes of the signals, pulse sequences identical in amplitude or proportional to the amplitudes, for example, rectangular ones, are stored, calculate the mutual correlation functions between the corresponding sequences of pulses stored in two adjacent cycles of location, determine storing each calculated average cross-correlation function for a certain length portion and recognize target reflected signals corresponding to two neighboring loops, which function in mutual correlation maximum is observed exceeding its average level by an amount specified by the second threshold. 2. The method according to claim 1, characterized in that when processing the signals for several irradiation cycles, the calculated mutual correlation functions in the neighboring cycles are averaged and the target is recognized by exceeding the maximum in the average mutual correlation function of its average level by the value of the specified second threshold.

Images