Ground clutter resisting method of missile-borne FMCW radar

Abstract

The invention provides an anti-ground clutter method for a missile-borne FMCW radar, and the method comprises the following steps: (1), enabling a missile-borne radar antenna to adopt a bias angle, and carrying out the bias of a half wave beam under a general condition, so as to enable the antenna to form conical scanning; (2) transmitting a group of continuous wave dot frequency signals by the missile-borne FMCW radar; (3) carrying out FFT (Fast Fourier Transform) processing; step (4), according to the FFT processing result and the Doppler frequency fv of the ground clutter, calculating the conical scanning frequency fa and the spectrum center frequency fv + fa or fv-fa of the clutter; and (5) distinguishing the ground clutter from the moving target on the ground in the frequency domain to achieve the purpose of suppressing the ground clutter. According to the method, the ground clutter can be accurately estimated, targeted filtering is realized, the ground clutter and the target are effectively distinguished, the ground clutter is effectively inhibited, accurate detection of the signal target is ensured, the signal processing effect is good, and a good effect is achieved in time engineering application.

Description

Technical Field

[0001] The invention relates to the field of radar detection technology, in particular to a method for resisting ground clutter of a missile-borne FMCW radar. Background Art

[0002] During the flight, the radar carrier will rotate at a certain rotation speed due to its unstable posture and low flight altitude. The ground objects around the target have a great influence on the detection and tracking measurement of the target. The radar received signal not only contains the echo signal from the moving target, but also the clutter signal generated by the scattering of background objects such as the ground. The existence of clutter signals will seriously affect the radar's ability to detect targets. Therefore, effective clutter suppression methods are needed in radar target detection to improve the ability to detect moving targets in clutter.

[0003] Missile-borne SOC radars face a more complex ground clutter environment than ground-based radars due to platform motion. Due to the low flight altitude and large clutter area, the clutter intensity is much greater than the target signal intensity. Due to the movement of the carrier, stationary objects such as ground objects have radial velocity relative to the radar, resulting in a Doppler shift in the center frequency of the main clutter. It is moved from the original zero frequency or near zero frequency to a certain frequency. At this time, the MTI and MTD technologies for static traditional ground radars are not applicable to missile-borne radars. Targeted filtering must be performed based on the characteristics of the clutter in the spectrum to improve the radar's detection performance. Summary of the invention

[0004] The technical problem solved by the present invention is to provide a method for resisting ground clutter of a missile-borne FMCW radar, so as to solve the problems raised in the above background technology.

[0005] The technical problem solved by the present invention is achieved by adopting the following technical solution: A method for resisting ground clutter of a missile-borne FMCW radar comprises the following steps:

[0006] Step (1). The missile-borne radar antenna adopts an offset angle, and under general conditions, offsets half of the beam so that the antenna can form a conical scan;

[0007] Step (2). The missile-borne FMCW radar transmits a set of continuous wave frequency signals, and the radar receiver receives the returned radar echo signal. After the beat signal output by the receiver is processed by FFT, the corresponding Doppler frequency is calculated to obtain the movement speed of the radar carrier, that is, the relative radial speed of the ground clutter relative to the radar carrier;

[0008] Step (3). The missile-borne FMCW radar transmits a set of LFM sawtooth waves, the radar receiver receives the returned radar echo signal, the AD collects the mixed I and Q signals, and performs FFT processing;

[0009] Step (4). According to the FFT processing results and the Doppler frequency fV of the ground clutter, the frequency fa of the conical scan and the spectral center frequency fV+fa or fV-fa of the clutter are calculated;

[0010] Step (5). Separate ground clutter from moving targets on the ground in the frequency domain to achieve the purpose of ground clutter suppression.

[0011] In the step (1), the sine wave modulation frequency is the conical scanning frequency ωα of the antenna. The conical scanning modulation will cause side frequencies to appear at both ends of the Doppler spectrum of the target, with a frequency of f d ±f α , where f d is the target Doppler frequency shift, f α is the cone scanning frequency; the Doppler center frequency of the main lobe clutter:

[0012]

[0013] Where v is the flight speed of the radar platform, σ is the angle between the radar platform velocity vector and the radar target line, and a z is the azimuth, e l It is the pitch angle. Conical scanning causes the angle at which the beam illuminates the ground to change periodically. This change causes the instantaneous center frequency of the main lobe ground clutter to move periodically.

[0014] The LFMCW radar transmission signal in step (2) can be expressed as:

[0015]

[0016] Where f0 is the transmitted signal frequency, is the initial phase of the oscillator, T r is the sweep period, B is the sweep bandwidth, K = B / T r is the frequency modulation slope, after the delay of the distance R between the radar and a single point target,

[0017]

[0018] t d ≤t≤T r +t d

[0019] Where K1 is the attenuation coefficient related to spatial propagation, t d =2R / c, c is the speed of light.

[0020] After differential mixing and filtering, the zero intermediate frequency echo is obtained:

[0021]

[0022] ta ≤t≤T r

[0023] Where K is the mixing gain, let Q = KK1A 2 / 2, F = Kt d , Then we have:

[0024] S IF (t) = Qcos(2πFt+θ)

[0025] Due to t d <<T r , so for any t, we have approximately:

[0026] S IF (t)=Qcos(2πFt+θ)[U(t)-U(tT r )]

[0027] Where U(t) is a step function.

[0028] The conical scanning linear frequency modulation continuous wave signal in step (2) can be expressed as

[0029] u(m,t)=(1+u ε (t))u s (t)

[0030] =Q{1+ηεcos[2πf α (mT r +t+t d )-θ0]}·exp[j(2πF(mT r +t)+θ)]

[0031] t=iT s ,i=0,1,...,N-1,m=0,...,M-1

[0032] Among them, T s is the sampling frequency, N is the number of samples in one modulation cycle, M is the number of data frames obtained by one cone scan, and M = 1 / (f a T r ).

[0033] In step (3), it is assumed that Q=1, θ=0, and leakage is not considered, thus obtaining:

[0034]

[0035] In the step (3), after the beat signal is processed by FFT, the bullet is aimed at the target, which is equivalent to the antenna electrical axis being aimed at the target. The target echo signal received by the radar is a constant amplitude signal that is not modulated by the scanning frequency of the conical scanning antenna. After FFT processing, the frequency of the target echo signal is the frequency introduced by the target echo delay.

[0036] f r and the Doppler frequency f introduced by the high-speed movement of the bullet v The ground clutter around the target received by the radar is swept by the conical scanning antenna and modulated by the scanning frequency of the conical scanning antenna. After FFT processing, the frequency of the ground clutter signal is the frequency f introduced by the target echo delay. r and the Doppler frequency f introduced by the high-speed movement of the bullet v and the scanning frequency f introduced by the high-speed rotating conical scanning antenna scanning α It can be seen that when aiming at the target and there is no ground clutter around the target, the frequency of the target echo is f r +f v ; When aiming at a target and there is ground clutter around the target, the frequency of the target echo is f r +f v ±f α , filter out f r +f v ±f α , leaving f r +f v , the ground clutter is filtered out, leaving the target at which the bullet is aimed.

[0037] Compared with the prior art, the beneficial effects of the present invention are: the present invention can accurately estimate ground clutter, realize targeted filtering, effectively distinguish ground clutter from targets, effectively suppress ground clutter, ensure accurate detection of signal targets, have good signal processing effect, and achieve good results in engineering applications of time. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a schematic diagram of the method flow of the present invention.

[0039] Figure 2 This is a block diagram of the ground clutter suppression principle of the present invention.

[0040] Figure 3 It is a schematic diagram of a sawtooth linear periodic frequency modulation signal of the present invention.

[0041] Figure 4 It is a schematic diagram of the offset beam cone scanning principle of the present invention. DETAILED DESCRIPTION

[0042] In order to make the technical means for realizing the present invention, the creative features, the objectives and effects to be achieved easy to understand, the present invention is further explained below in conjunction with specific diagrams. In the description of the present invention, it should be noted that, unless otherwise clearly stipulated and limited, the terms "installation", "connection" and "connection" should be understood in a broad sense. For example, it can be a fixed connection or a detachable connection, or an integral connection can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, or it can be internal communication between two elements.

[0043] like Figures 1 to 4 As shown, a method for combating ground clutter of a missile-borne FMCW radar comprises the following steps:

[0044] Step (1). The missile-borne radar antenna adopts a certain offset angle and offsets half of the beam under general conditions so that the antenna can form a conical scan;

[0045] Step (2). The missile-borne FMCW radar transmits a set of continuous wave frequency signals, and the radar receiver receives the returned radar echo signal. After the beat signal output by the receiver is processed by FFT, the corresponding Doppler frequency is calculated to obtain the movement speed of the radar carrier, that is, the relative radial speed of the ground clutter relative to the radar carrier;

[0046] Step (3). The missile-borne FMCW radar transmits a set of LFM sawtooth waves, the radar receiver receives the returned radar echo signal, the AD collects the mixed I and Q signals, and performs FFT processing;

[0047] Step (4). According to the FFT processing results and the Doppler frequency fv of the ground clutter, the frequency fa of the conical scan and the spectral center frequency fv+fa or fv-fa of the clutter are calculated;

[0048] Step (5). Separate ground clutter from moving targets on the ground in the frequency domain to achieve the purpose of ground clutter suppression.

[0049] The linear frequency modulation continuous wave is combined with conical scanning. The amplitude modulation of the linear frequency modulation continuous wave by the conical scanning antenna and the Doppler frequency modulation of the linear frequency modulation continuous wave by the speed of the moving target are used. The spatial domain filtering of the conical scanning antenna and the frequency filtering of the frequency filter are used to suppress clutter.

[0050] Conical scanning can use a single offset beam to rotate to form directions of equal signal strength and determine the deviation from the target direction based on the amplitude modulation envelope of the echo.

[0051] During the conical scanning of a single beam deviating from the antenna normal by a certain angle around the normal (antenna electrical axis), the antenna rotates around the electrical axis (rotation axis). Since the direction of the antenna rotation axis is the direction of the equal signal axis, the gain of the antenna in this direction remains unchanged during the scanning process. When the antenna is aimed at the target, that is, the antenna rotation axis directly points to the target, the echo signal output by the receiver is a series of equal amplitude pulses.

[0052] If the target deviates from the direction of the equal signal axis, when the beam maximum rotates to different positions during the scanning process, the target causes the received echo signal amplitude to change accordingly. It can be proved that the output signal amplitude is approximately considered to be modulated by a sine wave, and the sine wave modulation frequency is the antenna's conical scanning frequency ω α , the modulation depth depends on the magnitude of the target deviation from the iso-signal axis.

[0053] Cone-scan modulation will cause side frequencies to appear at both ends of the Doppler spectrum of the target, with a frequency of f d ±f α , where f d is the target Doppler frequency shift, f α is the cone sweep frequency.

[0054] Doppler center frequency of main lobe clutter:

[0055]

[0056] Where v is the flight speed of the radar platform, σ is the angle between the radar platform velocity vector and the radar target line, and a z is the azimuth, e l is the pitch angle. Conical scanning causes the angle at which the beam illuminates the ground to change periodically. This change causes the instantaneous center frequency of the main lobe ground clutter to move periodically.

[0057] Ideally, the LFMCW radar transmit signal can be expressed as:

[0058]

[0059] Where f0 is the transmitted signal frequency, is the initial phase of the oscillator, T r is the sweep period, B is the sweep bandwidth, K = B / T r is the frequency modulation slope. After the delay of the distance R between the radar and a single point target

[0060]

[0061] t d ≤t≤T r +t d

[0062] Where K1 is the attenuation coefficient related to spatial propagation, t d =2R / c, c is the speed of light.

[0063] After differential mixing and filtering, the zero intermediate frequency echo is obtained:

[0064]

[0065] t d ≤t≤T r

[0066] Where K is the mixing gain. Let Q = KK1A 2 / 2, F = Kt d , Then we have:

[0067] S IF (t) = Qcos(2πFt+θ)

[0068] Due to t d <<T r , so for any t, we have approximately:

[0069] S Ir (t)=Qcos(2πFt+θ)[U(t)-U(tT r )]

[0070] Where U(t) is a step function.

[0071] Write the above into complex form, so the conical scanning linear frequency modulated continuous wave signal can be expressed as

[0072] u(m,t)=(1+u ε (t))u s (t)

[0073] =Q{1+ηεcos[2πf α (mT r +t+t d )-θ0]}·exp[j(2πF(mT r +t)+θ)]

[0074] t=iT s ,i=0,1,...,N-1,m=0,...,M-1

[0075] Among them, T s is the sampling frequency, N is the number of samples in one modulation cycle, M is the number of data frames obtained by one cone scan, and M = 1 / (f a T r ).

[0076] Obviously, conical scanning is a mechanical scanning method, which is a "slow-varying signal" compared with linear frequency modulation continuous wave signal. Therefore, the phase effect of conical scanning on the echo of two adjacent frames can be basically ignored, and it is mainly superimposed on the echo as amplitude modulation. The effect of linear frequency modulation continuous wave on conical scanning is equivalent to equal-interval sampling, and the sampling period is T r .

[0077] Perform FFT on the above equation, assuming Q = 1, θ = 0, and ignoring leakage, and we get:

[0078]

[0079] After further FFT processing of the beat signal, the bullet aiming at the target is equivalent to the antenna electrical axis aiming at the target. The target echo signal received by the radar is an equal amplitude signal that is not modulated by the scanning frequency of the conical scanning antenna. After FFT processing, the frequency of the target echo signal is the frequency f introduced by the target echo delay (target distance). r and the Doppler frequency f introduced by the high-speed movement of the bullet v The ground clutter around the target received by the radar is swept by the conical scanning antenna and modulated by the scanning frequency of the conical scanning antenna. After FFT processing, the frequency of the ground clutter signal is the frequency f introduced by the target echo delay (target distance). r and the Doppler frequency f introduced by the high-speed movement of the bullet v and the scanning frequency f introduced by the high-speed rotating conical scanning antenna scanning α From this we can see that when aiming at a target and there is no ground clutter around the target, the frequency of the target echo is f r +f v ; When aiming at a target and there is ground clutter around the target, the frequency of the target echo is f r +f v ±f α , filter out f r +f v ±f α , leaving f r +f v , the ground clutter is filtered out, leaving the target at which the bullet is aimed.

[0080] The basis for distinguishing moving targets from fixed clutter is the difference in their speed. Due to the different Doppler frequency shifts caused by different moving speeds, the echoes of targets at different speeds can be distinguished. Fixed clutter is filtered out and the echoes of moving targets are taken out, which greatly improves the ability to detect moving targets in a clutter background and improves the radar's anti-interference ability.

[0081] The missile-borne radar emits a set of "point frequency" continuous wave signals to obtain the movement speed of the radar carrier, that is, the relative radial speed of the ground clutter relative to the radar carrier. According to the echo information of the sawtooth wave and the movement speed of the radar carrier, the frequency fa of the cone scan is calculated to determine the Doppler frequency of the ground clutter, that is, the spectral center frequency fv±fa of the clutter; the ground clutter is distinguished from the moving targets on the ground in the frequency domain to achieve the purpose of ground clutter suppression.

[0082] The present invention performs conical scanning on ground objects with antenna bias, modulates echo signals of ground clutter, transmits a group of "point frequency" continuous waves, calculates the speed of the radar carrier, and then transmits a group of sawtooth waves to process the echo signals received by the radar, obtain the conical scanning frequency, determine the center frequency of clutter suppression, suppress the frequency corresponding to the ground object, distinguish the ground target from the ground clutter, and achieve the purpose of ground clutter suppression.

[0083] The above shows and describes the basic principles and main features of the present invention and the advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, and these changes and improvements fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for combating ground clutter of missile-borne FMCW radar, characterized in that: The following steps are involved: Step (1). The missile-borne radar antenna adopts an offset angle, and under general conditions, offsets half of the beam so that the antenna can form a conical scan; Step (2). The missile-borne FMCW radar transmits a set of continuous wave frequency signals, and the radar receiver receives the returned radar echo signal. After the beat signal output by the receiver is processed by FFT, the corresponding Doppler frequency is calculated to obtain the movement speed of the radar carrier, that is, the relative radial speed of the ground clutter relative to the radar carrier; Step (3). The missile-borne FMCW radar transmits a set of LFM sawtooth waves, the radar receiver receives the returned radar echo signal, the AD collects the mixed I and Q signals, and performs FFT processing; Step (4). According to the FFT processing results and the Doppler frequency fv of the ground clutter, the frequency fa of the conical scan and the spectral center frequency fv+fa or fv-fa of the clutter are calculated; Step (5). Separate ground clutter from moving targets on the ground in the frequency domain to achieve the purpose of ground clutter suppression.

2. The method for combating ground clutter of a missile-borne FMCW radar according to claim 1, characterized in that: In the step (1), the sine wave modulation frequency is the conical scanning frequency ω of the antenna. α , cone scanning modulation will cause side frequencies to appear at both ends of the Doppler spectrum of the target, with a frequency of f d ±f α , where f d is the target Doppler frequency shift, f α is the cone scanning frequency; the Doppler center frequency of the main lobe clutter: Where v is the flight speed of the radar platform, σ is the angle between the radar platform velocity vector and the radar target line, and a z is the azimuth, e l It is the pitch angle. Conical scanning causes the angle at which the beam illuminates the ground to change periodically. This change causes the instantaneous center frequency of the main lobe ground clutter to move periodically.

3. The method for combating ground clutter of a missile-borne FMCW radar according to claim 1, characterized in that: The LFMCW radar transmission signal in step (2) can be expressed as: Where f0 is the frequency of the transmitted signal, is the initial phase of the oscillator, T r is the sweep period, B is the sweep bandwidth, K = B / T r is the frequency modulation slope, after the delay of the distance R between the radar and a single point target, t d ≤t≤T r +t d Where K1 is the attenuation coefficient related to spatial propagation, t d =2R / c, c is the speed of light; After differential mixing and filtering, the zero intermediate frequency echo is obtained: t d ≤t≤T r Where K is the mixing gain, let Q = KK1A 2 / 2, F = Kt d , Then we have: S IF (t)=Qcos(2πFt+θ) Due to t d < <T r , so for any t, we have approximately: S IF (t)=Qcos(2πFt+θ)[U(t)-U(t-T r )] Where U(t) is a step function.

4. The method for combating ground clutter of a missile-borne FMCW radar according to claim 1, characterized in that: The conical scanning linear frequency modulation continuous wave signal in step (2) can be expressed as u(m,t)=(1+u ε (t))u s (t)=Q{1+ηεcos[2πf α (mT r +t+t d )-θ0]}·exp[j(2πF(mT r +t)+θ)] t=iT s ,i=0,1,...,N-1,m=0,...,M-1 Among them, T s is the sampling frequency, N is the number of samples in one modulation cycle, M is the number of data frames obtained by one cone scan, and M = 1 / (f a T r ).

5. The method for combating ground clutter of a missile-borne FMCW radar according to claim 1, characterized in that: In step (3), it is assumed that Q=1, θ=0, and leakage is not considered, thus obtaining:

6. The method for combating ground clutter of a missile-borne FMCW radar according to claim 1, characterized in that: In the step (3), after the beat signal is processed by FFT, the bullet is aimed at the target, which is equivalent to the antenna electrical axis being aimed at the target. The target echo signal received by the radar is a constant amplitude signal that is not modulated by the scanning frequency of the conical scanning antenna. After FFT processing, the frequency of the target echo signal is the frequency f introduced by the target echo delay. r and the Doppler frequency f introduced by the high-speed movement of the bullet v The ground clutter around the target received by the radar is swept by the conical scanning antenna and modulated by the scanning frequency of the conical scanning antenna. After FFT processing, the frequency of the ground clutter signal is the frequency f introduced by the target echo delay. r and the Doppler frequency f introduced by the high-speed movement of the bullet ν and the scanning frequency f introduced by the high-speed rotating conical scanning antenna scanning α It can be seen that when aiming at the target and there is no ground clutter around the target, the frequency of the target echo is f r +f v ; When aiming at a target and there is ground clutter around the target, the frequency of the target echo is f r +f v ±f α , filter out f r +f ν ±f α , leaving f r +f v , the ground clutter is filtered out, leaving the target at which the bullet is aimed.

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