Method for detecting underwater and surface objects using phase-keyed signals in form of m-sequence

Abstract

FIELD: hydroacoustics.SUBSTANCE: proposed invention can be used in sonar systems for detecting and determining coordinates and parameters of movement of marine objects. The method includes the formation, emission and reception of phase-shift keyed signals in the form of an M-sequence with a phase shift of 0° and 180°, as well as their processing upon reception. Detection is accomplished by calculating the cross-correlation function of the reflected and reference signals.EFFECT: increase in the range and reliability of target detection, as well as a reduction in the radius of unmasking of probing signals.1 cl, 4 dwg

Description

[0001] The proposed invention relates to the field of hydroacoustics and can be used in sonar systems for target detection, determining coordinates and movement parameters of marine objects.

[0002] The efficiency of sonars largely depends on the choice of parameters of probing signals and the methods of their processing in the receiving path.

[0003] The selection of parameters for sonar sounding signals is made depending on the purposes of their use (obtaining the maximum range of target detection against the background of noise and reverberation interference, ensuring maximum accuracy in determining the distance and bearing).

[0004] Modern sonars use long-duration probing signals to provide long-range target detection, and also use various types of signals and various processing methods to ensure high noise immunity.

[0005] Using complex phase-shift keyed signals with a large base and optimal echo signal processing in the receiving path as probing signals makes it possible to simultaneously provide:

[0006] - the ability to change the energy of probing signals in large ranges;

[0007] - high noise immunity compared to tone and frequency-modulated signals;

[0008] - high noise immunity when detecting signals against the background of reverberation interference even at low target speeds;

[0009] - low visibility of the emitted signal, which will reduce the radius of de-masking;

[0010] - reduce the negative impact on the marine biosphere;

[0011] - the ability to create a large volume of “bank” of probing signals.

[0012] One type of phase-shift keyed signal is the M-sequence signal.

[0013] The closest approach to the proposed method of detecting objects in an aquatic environment is the method of determining the location of objects located in the water by emitting acoustic energy into the aquatic environment in the form of sound pulses (parcels) and receiving (registering) the energy reflected from the object in the form of an echo signal, called hydrolocation [1, p. 192]. In this method, the direction of the detected object is determined by the position of the acoustic antenna, and the distance to it is determined by the time interval between the transmission and reception of the signal. This method of detecting underwater objects can be selected as a prototype.

[0014] The disadvantage of the prototype is low noise immunity compared to phase-shift keyed signals under conditions of the predominant influence of reverberation interference and a large unmasking radius of probing signals.

[0015] The aim of the invention is to develop a method for generating and emitting a phase-shift keyed signal in the form of an M-sequence and processing it upon reception, the technical implementation of which will improve the efficiency of sonar stations (SSS) in searching for and detecting underwater and surface objects.

[0016] The stated objective is achieved by using two groups of transmit-receive converters, one of which emits pulses for reception and transmission with a phase of 0°, and the second with a phase of 180°.

[0017] The hydroacoustic antenna consists of acoustic transducers.

[0018] The reception and transmission of hydroacoustic signals in phase is performed by one set of transducers, while the reception and transmission in antiphase is performed by another set of transducers. The transducers are arranged in a staggered pattern.

[0019] The use of identical receiving and transmitting acoustic transducers is due to the formation of a sequence of phase-shift keyed pulses, each subsequent emitted pulse has a phase shift that differs from the previous one by 180 degrees.

[0020] Thus, one group of transmit-receive converters emits pulses for reception and emission with a phase of 0°, and the second with a phase of 180°.

[0021] When synthesizing two antennas on a single antenna base, all antenna base transducers are distributed into two groups. This distribution is designed so that each transducer group synthesizes an antenna identical to the antenna synthesized by the other group.

[0022] When synthesizing antennas, it is necessary that the distance between the transducers during radiation be no more than half the wavelength.

[0023] The emitted pulses are phase-shift keyed signals in the form of an M-sequence.

[0024] An M-sequence is a sequence of binary symbols formed by a k-th degree polynomial. The length of an M-sequence is determined by the expression:

[0025] L = 2 к - 1,

[0026] where k is the number of binary elements of the M-sequence.

[0027] For example, for k=7, the length of the M-sequence is 127 bits, and for k=10, it is 1023 bits. A characteristic feature of M-sequences is their repetition periodicity after a certain number of cycles.

[0028] The structure (type) of the M-sequence is determined by the generating polynomial.

[0029] Such M-sequences can be formed by a k-bit feedback shift register (Fig. 1) and have a period of length:

[0030]

[0031] where L is the number of binary elements of the M-sequence;

[0032] k - number of bits of the forming register;

[0033] F T - clock frequency of formation.

[0034] The numbers of the register bits from which the feedback is taken are determined by the characteristic polynomial:

[0035] y(x) = x к +C k-1 X к-1 +...+ C1x+l,

[0036] where C i - these are coefficients that take the value 0 or 1.

[0037] By using different initial states of the forming register, one can obtain L M-sequences for a single polynomial, differing from each other by a cyclic shift. The autocorrelation function of an M-sequence has the form shown in Fig. 2, the magnitude of the shift, i.e., the position of the maximum at the correlator output. Thus, an ensemble of L signals (symbols) is created, each containing k binary ones.

[0038] The received signal is amplified and converted into a signal that is emitted into the water within the sonar frequency range. Thus, the transmitted signal is a pseudo-random sequence (PRS) of a modulated signal, the phase of which varies from one transmission to the next, ranging from 0° to 180°.

[0039] For example, for k=4 the number of non-repeating combinations is L= 2 4 -1 = 15. Moreover, the number of polynomials that provide such a length of non-repeating combinations is only 2 [2, p. 58].

[0040] These polynomials and the corresponding shift registers that form the M-sequences are shown in Fig. 3.

[0041] The efficiency of coordinated processing of phase-shift keyed signals in the form of an M-sequence was assessed using the technique described in GOST RV 51987-2002 and software tools implemented in the MathCad environment and in the Python programming language, under complex interference conditions:

[0042] R=F^ (-1) {F(otr)*(F{eta} ) -},

[0043] where F{⋅} and F^(-1){⋅} are the direct and inverse Fourier transforms, respectively;

[0044] eta - reference signal;

[0045] otr - signal reflected from the target.

[0046] Phase-shift keyed signals in the form of an M-sequence allow for an increase in the signal-to-noise ratio compared to the use of linear frequency modulation (LFM) and amplitude-modulated (AM) signals, which in turn leads to an increase in the accuracy of measuring the spatial coordinates of observed targets.

[0047] From the results presented in Fig. 4, it can be seen that the signal-to-noise ratio decreases with increasing noise level and target speed for all cases.

[0048] Thus, the use of phase-shift keyed signals in the form of an M-sequence makes it possible to increase the signal-to-noise ratio compared to chirp and AM signals when exposed to reverberation interference by 18%, short-term pulse interference by 16%, and long-term pulse interference by 14%, which will increase the range and reliability of target detection and significantly reduce the unmasking radius of probing signals.

[0049] This method for detecting underwater and surface objects using phase-shift keyed M-sequence signals can be used to upgrade existing active sonar systems using complex probing signals and in the construction of new sonar systems for submarines and surface ships. This detection method can be integrated into the sonar systems of unmanned surface vehicles (USVs) and unmanned underwater vehicles (UUVs) for searching and detecting underwater and surface objects, as it requires little energy.

[0050] Sources of information used to identify the detection method and compile its description:

[0051] 1. Mikhailovsky G.P. Military Encyclopedic Dictionary. Moscow, Military Publishing House, 1983, p. 192.

[0052] 2. Varakin L.E. Communication systems with noise-like signals. Moscow, Radio and Communications, 1985, p. 58.

[0053] 3. Lushankin V.I. Theory of the Fundamentals of Military Hydroacoustics. St. Petersburg, VSOK Navy, 2010.

[0054] 4. Saprykin V.A., Voloshin A.K., Rokotov S.P. Digital processing of hydroacoustic signals. St. Petersburg, VMIRE named after A.S. Popov, 1990.

[0055] 5. Saprykin V.A. Rokotov S.P. Theory of hydrolocation and digital signal processing. St. Petersburg, VMIRE named after A.S. Popov, 1988.

Claims

A method for determining the location of objects in water by emitting acoustic energy into the aquatic environment in the form of sound pulses and receiving the energy reflected from the object in the form of an echo signal, characterized in that phase-shift keyed signals in the form of an M-sequence formed by a k-bit shift register with feedback are used as the probing signal, wherein the signal is emitted by two groups of receiving and transmitting converters, one of which emits pulses with a phase of 0°, and the second with a phase of 180°, the numbers of the register digits from which the feedback is formed are determined by the characteristic polynomial y(x)=x k +C k-1 x k-1 +…+C1x+l, where C i - these are coefficients that take the value 0 or 1, the echo signal is detected by calculating the cross-correlation function of the reflected and reference probing signal.

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