Millimeter wave detection method based on near-field target echo characteristic representation model

Abstract

The invention discloses a millimeter wave detection method based on a near-field target echo characteristic representation model. According to the method, ground electromagnetic scattering characteristics of a detection range are analyzed, electromagnetic scattering coefficients are analyzed by using a random rough surface, a ground electromagnetic scattering model is established, a geometrical relationship model of a detector and the ground is constructed, scattering units are divided from an equal slope distance angle, and finally echoes of the scattering units are superposed. Deriving a representation model of the echo characteristics of the surface target; model construction is more rigorous, errors caused by approximately neglecting echo signals as point target echo signals are avoided, and comprehensiveness and reliability of effective information features in an echo signal model are improved; millimeter wave detection is carried out based on the model, the detection precision and efficiency are effectively improved, and the accuracy of detection result analysis is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of millimeter-wave detection, and particularly relates to a millimeter-wave detection method based on a near-field target echo characteristic characterization model. Background Art

[0002] Millimeter-wave detection technology is a technology that uses millimeter-wave band electromagnetic waves for detection and measurement. Due to its high resolution and strong penetration ability, it can accurately detect and identify targets, so it is often widely used in the detection scenarios of ground targets.

[0003] In the existing millimeter-wave detection methods, the target to be measured is often approximately regarded as a point target during the detection process, and the point target echo model is used to process and analyze the echo signal, so as to obtain the relevant characteristic information of the target. However, in the actual millimeter-wave detection situation, the detection target range is often the ground within the antenna beam irradiation range. Therefore, the target to be measured cannot be simply regarded as a point target with a single characteristic, and the actual echo signal processing also needs to be comprehensively considered. Summary of the Invention

[0004] In view of this, the present invention provides a millimeter-wave detection method based on a near-field target echo characteristic characterization model, which can improve the detection accuracy and efficiency, and improve the accuracy of the detection result analysis.

[0005] In the millimeter-wave near-field target echo signal characterization model of the present invention, with the projection P of the antenna on the ground as the center, the antenna irradiation range is divided into several scattering units from the slant range dimension; the power P of the echo signal r is the sum of the backscattering signal powers of each scattering unit, and the waveform s of the echo signal r (t) is the superposition of the round-trip propagation delay signals of the transmitted signal corresponding to the antenna slant ranges of different scattering units.

[0006] Preferably, an equal slant range division method is adopted to divide the antenna irradiation range into annular scattering units with a width of Δr.

[0007] Preferably, the scattering unit division angle and the number n of scattering units are obtained according to the following formula:

[0008]

[0009] n is an integer

[0010] where, ΔR is the range resolution of the millimeter-wave detector; H is the height of the millimeter-wave detector from the ground, ψ is the angle formed by the millimeter-wave beam center and the approximate plane of the ground, is the angle between the direction of the i-th scattering unit and the beam center direction in the plane formed by the beam center direction line and the perpendicular direction of the detector's projection to the ground, csc is the cosecant function in trigonometric functions, and θ 0.5 is the 3dB width.

[0011] Preferably, in the linear frequency modulation system, ΔR = c / 2B; where c is the speed of light and B is the modulation bandwidth of the linear frequency modulation signal.

[0012] Preferably, the echo signal power P r and the echo signal waveform expression s r (t) are respectively:

[0013]

[0014] Among them, P t is the transmission power; λ is the wavelength corresponding to the center frequency of the transmitted signal; G i is the antenna gain value of the i-th scattering unit, expressed as the product of the maximum gain value in the beam center direction and the average normalized gain coefficient; σ i is the radar cross section area of the i-th scattering unit, expressed as the product of the backscattering coefficient of this scattering unit and the unit area ΔA i ; R i is the slant range of the i-th scattering unit; U is the amplitude of the delay signal; s t is the waveform expression of the delay signal; c is the speed of light.

[0015] Preferably, the transmitted signal is a single-frequency signal, a sawtooth wave frequency modulation signal, a triangular wave modulation signal or a sine modulation signal.

[0016] The present invention also provides a millimeter wave detection method based on the near-field target echo characteristic characterization model, which transmits millimeter wave signals, constructs echo signals according to the above model, and performs signal processing based on the constructed echo signals.

[0017] Preferably, the transmitted signal is a sawtooth wave linear frequency modulation system signal, and the echo signal at height H is:

[0018]

[0019] Among them, n is the number of scattering units, and i represents the i-th scattering unit; P t is the transmission signal power, G0 is the maximum antenna gain value in the beam center direction; λ is the wavelength corresponding to the center frequency of the transmitted signal; is the antenna gain corresponding to the scattering unit i, is the backscattering coefficient corresponding to the scattering unit i, r i is the distance from the i-th scattering unit to the detector projection, Δr iis the width of the i-th scattering unit, R i is the slant range of the i-th scattering unit; α is the angle between the beam and the beam center direction; β is the signal frequency slope; c is the speed of light; t is the time; f c is the carrier center frequency; B is the modulation bandwidth; T m is the modulation period.

[0020] Advantageous effects:

[0021] By analyzing the ground electromagnetic scattering characteristics of the detection range, using the random rough surface to analyze the electromagnetic scattering coefficient, establishing a ground electromagnetic scattering model, constructing a geometric relationship model between the detector and the ground, dividing the scattering units from the equal slant range angle, and finally superposing the echoes of each scattering unit, the present invention derives a characterization model of the echo characteristics of the surface target; the model construction is more rigorous, avoiding the error caused by approximately ignoring the echo signal as a point target echo signal, improving the comprehensiveness and reliability of the effective information characteristics in the echo signal model; based on this model for millimeter-wave detection, the detection accuracy and efficiency are effectively improved, used to analyze the information characteristics in the ground echo, and the accuracy of the detection result analysis is improved.

[0022] The present invention stipulates the angle value standard and the number of units for dividing the scattering units, reducing a certain amount of calculation while ensuring the accuracy of the model, making the detection method more practical.

[0023] The present invention is applicable to various signal types such as sawtooth wave modulation signals, triangular wave modulation signals, sine wave modulation signals, and point frequency signals, with wide applicability.

[0024] The present invention is applicable to the detection of natural rough planes such as the ground. Description of the drawings

[0025] Figure 1 is the division of scattering units within the antenna illumination range.

[0026] Figure 2 is the connection between the angular scale and the width scale between the scattering units.

[0027] Figure 3 is the flow chart of the method of the present invention. Detailed implementation manners

[0028] The following combines the drawings and gives examples to describe the present invention in detail.

[0029] The present invention provides a millimeter-wave detection method based on a characterization model of the echo characteristics of near-field targets. First, starting from the electromagnetic scattering characteristics of rough-surface targets, a mathematical expression for a random rough surface is constructed by analyzing the statistical characteristics of the rough ground. Then, based on the electromagnetic scattering characteristics of the constructed random rough-surface target for millimeter-wave signals, the backscattering coefficient for millimeter-wave signals is derived. On this basis, the antenna illumination range of the detection signal is divided into scattering units with different scattering characteristics according to equal slant-range lines, and the relevant parameters of each scattering unit are analyzed. After coherently superimposing the echo signals of the scattering units, a final characterization model of the echo characteristics of millimeter-wave near-field targets is established. Finally, a millimeter-wave detection technology is proposed taking a signal with a sawtooth-wave linear frequency modulation system as the transmitted signal as an example.

[0030] The detailed construction process of the characterization model of the echo characteristics of millimeter-wave near-field targets in the present invention is as follows:

[0031] First, according to the statistical characteristics of the random rough surface, a random rough-surface model is established; the electromagnetic scattering characteristics and scattering coefficients of the constructed random rough ground are analyzed using electromagnetic scattering theory.

[0032] The actual ground type can generally be regarded as a rough surface, and the statistical characteristics of the random rough surface include:

[0033] 1) The height of the random rough surface is denoted as the random variable f, and the probability density function of the random variable f is denoted as p(f). The distribution of the height fluctuations of the random rough surface is characterized by p(f). In nature, the fluctuation distribution of typical ground surfaces conforms to the Gaussian distribution. Therefore, p(f) is written as:

[0034]

[0035] where σ is the standard deviation of the Gaussian distribution.

[0036] 2) The roughness of the rough surface can be represented by the root-mean-square height σ h . For a three-dimensional surface rough surface, is the second-order central moment of the rough-surface height f. The rough surface is discretely sampled at the sampling intervals (Δx, Δy) to obtain f i , and the of each sampling point is calculated as follows:

[0037]

[0038] where N is the number of sampling points, is the average value of the sampling heights.

[0039] 3) The correlation degree between any two points separated by a distance R on a random rough surface is denoted as the correlation function G(R). For a random ground surface with a typical Gaussian distribution of undulations, the correlation function is expressed as: where l is the correlation length. The normalized autocorrelation function can be obtained as:

[0040]

[0041] E[.] represents the mathematical expectation. The normalized autocorrelation function ρ(R) gradually decreases as the distance R between two points increases. When R = 0, ρ(R) = 1, and when R → ∞, ρ(R) → 0. The correlation length l is defined as the value of R when ρ(R) drops to 1 / e. By performing a Fourier transform on the autocorrelation function of the rough surface, the power spectral density function W(k) of the random variable f can be obtained:

[0042]

[0043] where j is the imaginary unit and k is the spatial beam vector characterizing the scale characteristics of the rough surface.

[0044] For a typical random rough surface with a Gaussian distribution, the power spectral density W(k) is

[0045]

[0046] Based on the above statistical characteristics, a mathematical model of the rough surface is established. Let the sampling step sizes be Δx and Δy, and the rough surface scale be L x ×L y , and the height z = f(x n , y m ) at the coordinate (x n, y m ) is expressed as:

[0047]

[0048]

[0049] where W(k x,p , k y,q ) is the power spectral density function of the two-dimensional Gaussian rough surface, k x,p = 2πp / L x ; k y,q = 2πq / L y , l x and l y are the correlation lengths in the horizontal and vertical axis directions respectively, L x and L y are the sample lengths in the horizontal and vertical axis directions respectively, x n = nΔx, n = 1, 2..., N, xm = mΔy, where m = 1, 2,... M are the projection values of the rough surface in the horizontal and vertical directions respectively, N and M are the number of sampling points in the x and y directions respectively, and p and q are the spatial wave number indices.

[0050] After that, the small slope approximation method is adopted to analyze the electromagnetic scattering characteristics of the random rough ground surface, and the derivation process is as follows:

[0051] When a plane electromagnetic wave is incident on the random rough surface h(r), r is the position coordinate r = (x, y). The incident wave is expressed as:

[0052]

[0053] where is the horizontal component of the incident wave, is the vertical component of the incident wave, k1 = ω / c is the incident wave number, ω is the angular frequency, c is the speed of light. z is the vertical direction coordinate. The scattered field is expressed as the superposition of plane waves in the upper half space:

[0054]

[0055] where is the horizontal component of the scattered wave, is the vertical component of the scattered wave, is the scattering amplitude of the small slope approximation. According to the invariance of the scattering amplitude in the small slope approximation, the scattering amplitude of the small slope approximation is expressed as:

[0056]

[0057] where is the first-order coefficient matrix depending on the incident wave and the scattering polarization state. The first-order scattering coefficient of the small slope approximation can be obtained by the following formula:

[0058]

[0059] After derivation, the bistatic scattering coefficient of the small slope approximation can be obtained as:

[0060]

[0061] where ρ(0) is the height fluctuation variance of the rough surface, ρ(r) is the correlation function of the rough surface, and B pq (k, k0) in the HH polarization and VV polarization expressions are:

[0062]

[0063] For millimeter-wave detection technology with the same transceiver position, the main concern is the backscattering situation, that is, the scattering direction is conjugate symmetric to the incident direction. Set the coordinate system so that the incident plane is exactly the xoz plane, and the incident azimuth angle is zero at this time. Combining the above formula, the backscattering coefficient represented by the incident wave polar angle θ i can be obtained as follows:

[0064]

[0065] Based on obtaining the scattering coefficient of the rough ground surface, the ground echo signal model is studied. Divide the antenna illumination range into several scattering units. The parameter differences of each scattering unit result in different response characteristics to the incident electromagnetic wave. The total echo signal power P r is expressed as the sum of the backscattering signal powers of each scattering unit. The echo signal waveform s r (t) is expressed as the superposition of the two-way propagation delay signals of the transmitted signal corresponding to the antenna slant ranges of different scattering units.

[0066]

[0067] Among them, G i , σ i , R i represent the antenna gain coefficient, scattering cross-sectional area, and slant range of the i-th scattering unit respectively. P t represents the transmission power, and λ represents the wavelength corresponding to the center frequency of the transmitted signal.

[0068] Divide the ground scattering units in the slant range dimension, and use the integral method in the angle dimension. Considering the radiation intensity within the antenna - 3dB beamwidth θ 0.5 , when the detector detects at the incident angle ψ, the projection of the antenna illumination range on the ground surface is approximately an elliptical area. According to the equal slant range division method, with the projection P of the antenna on the ground as the center, divide the antenna illumination range into annular scattering units with a width of Δr. As Figure 1 shown.

[0069] Each point within the illumination range can be expressed in polar coordinate form (r, α) with the antenna projection P as the origin. For the same scattering unit, the distances r from each point inside to the projection P are the same, and the slant ranges R to the antenna are also the same. Therefore, the two-way propagation delay of the millimeter wave to each point within the unit is the same, and it can be regarded as a point target on the distance scale; at the same time, it can be known from the trigonometric relationship that the incident angles of the incident wave with respect to each point within the scattering unit are the same, and under the condition of unchanged ground conditions, they have the same backscattering coefficient σ ° .

[0070] Let be the angle between the direction of the i-th scattering unit in the OPQ plane and the beam center direction, then the annular width of the i-th scattering unit and The relationship can be expressed as:

[0071]

[0072] Such as Figure 2 shown.

[0073] Under the chirp modulation system, the range resolution is ΔR = c / 2B. Let the width of the scattering unit be less than the range resolution, that is, Δr i < ΔR, and at the same time make the 3dB width θ 0.5 be an integer multiple of, and obtain the angular division criterion of the scattering unit value standard and the number of scattering units n:

[0074]

[0075] n is an integer

[0076] The antenna radiation intensity is different at different antenna illumination positions, and a Gaussian function approximation model is adopted:

[0077]

[0078] Based on the above function model and formula parameters, a ground echo model based on the superposition method can be established. The time-domain waveform and power magnitude of the echo signal can be expressed as the superposition of different scattering units. Therefore, the premise of establishing the ground echo model lies in calculating the response characteristics of each scattering unit to the incident millimeter-wave signal. According to the radar principle, the echo power of the scattering unit can be expressed as

[0079]

[0080] where σ i is the radar cross section of the i-th scattering unit, which can be expressed as the product of the backscattering coefficient and the unit area ΔA i :

[0081]

[0082] G i is the antenna gain value of the i-th scattering unit. For the annular scattering unit in the present invention, the function values corresponding to the antenna directions at different positions are different. Therefore, it is necessary to calculate the average radiation intensity within the scattering unit. Express G i as the product of the maximum gain value at the beam center and the average normalized gain coefficient

[0083]

[0084] The independent variable of the antenna gain G is the beam center direction angle θ. During the calculation, an independent variable conversion is required to obtain the integral gain with α as the independent variable. From the geometric relationship,

[0085] XQ = r 2 + H 2 cot 2 (ψ) - 2rHcot(ψ)cosα

[0086] = R 2 + H 2 csc 2 (ψ) - 2RHcsc(ψ)cosθ

[0087] From the above equation, θ can be expressed as an explicit function of α:

[0088]

[0089] where

[0090]

[0091] Combining the above equations, the echo power of the scattering unit can be expressed as:

[0092]

[0093] The above equation is the simple expression for the backscattering signal power of a single scattering unit. For the time-domain characteristics of the echo signal, the backscattering echo of the i-th scattering unit has a time delay of τ = 2R i / c:

[0094]

[0095] For the sawtooth wave modulation signal, where B is the modulation bandwidth, R i is the slant range from the i-th scattering unit to the antenna, T m is the modulation period, and f c is the carrier center frequency.

[0096] The transmitted signal frequency can be expressed as:

[0097]

[0098] The phase of the transmitted signal is the integral of the frequency with respect to time, expressed as

[0099]

[0100] Therefore, the echo signal of the scattering unit can be expressed as

[0101]

[0102] By superimposing the scattering units within the scattering range according to the above formulas, the expression of the echo signal at height H is obtained:

[0103]

[0104] So far, based on the near-field target echo characteristic characterization model, an expression of the millimeter-wave echo signal with a sawtooth wave linear frequency modulation system signal as the transmitted signal is obtained, and an accurate detection method for millimeter-wave signal surface targets is realized.

[0105] In summary, the above are only the preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A millimeter-wave near-field target echo signal characterization model, characterized in that, Taking the projection P of the antenna on the ground as the center, the antenna illumination range is divided into several scattering units from the slant range dimension; the power P of the echo signal r is the sum of the backscattering signal powers of each scattering unit, and the waveform s r (t) of the echo signal is the superposition of the two-way propagation delay signals of the transmitted signal corresponding to the antenna slant ranges of different scattering units.

2. The model according to claim 1, characterized in that, Using the equal slant range division method, the antenna illumination range is divided into annular scattering units with a width of Δr.

3. The model according to claim 2, wherein Scattering unit division angle and the number of scattering units n is obtained according to the following formula: n is an integer where ΔR is the distance resolution of the millimeter-wave detector; H is the height of the millimeter-wave detector from the ground, ψ is the angle formed by the millimeter-wave beam center and the approximate ground plane, is the angle between the direction of the i-th scattering unit and the beam center direction in the plane formed by the beam center direction line and the perpendicular direction of the detector's projection on the ground, csc is the cosecant function in trigonometric functions, θ 0.5 is the 3dB width.

4. The model according to claim 3, wherein Under the linear frequency modulation system, ΔR = c / 2B; where c is the speed of light and B is the modulation bandwidth of the linear frequency modulation signal.

5. The model according to claim 1, characterized in that, Echo signal power P r and the waveform expression s r (t) are respectively as follows: Among them, P t is the transmit power; λ is the wavelength corresponding to the center frequency of the transmitted signal; G i is the antenna gain value of the i-th scattering unit, expressed as the product of the maximum gain value at the beam center and the average normalized gain coefficient; σ i is the radar cross section of the i-th scattering unit, expressed as the product of the backscattering coefficient of this scattering unit and the cell area ΔA i ; R i is the slant range of the i-th scattering unit; U is the amplitude of the time-delay signal; s t is the waveform expression of the time-delay signal; c is the speed of light.

6. The model according to any one of claims 1 to 5, characterized in that The backscattering coefficients of each point in the scattering unit are: where \(k_1 = \omega / c\) is the incident wave number, \(\omega\) is the angular frequency, and \(c\) is the speed of light; \(\theta\) i is the angle of incidence; \(B\) pq is the element of the first-order coefficient matrix; \(\rho(0)\) is the height fluctuation variance of the target surface; \(\rho(r)\) is the correlation function of the target surface; \(r\) is the position coordinate; \(k_0\) is the horizontal component of the incident wave; \(\psi\) is the angle formed by the millimeter wave beam center and the ground plane, and \(\theta_i\) is the angle between the direction of the \(i\)-th scattering unit and the beam center direction in the plane formed by the beam center direction line and the perpendicular direction from the detector to the ground projection.

7. The model according to claim 1, wherein The transmitted signal is a single-frequency signal, a sawtooth frequency modulation signal, a triangular wave modulation signal, or a sine modulation signal.

8. A millimeter-wave detection method based on a near-field target echo characteristic characterization model, characterized in that, Transmit a millimeter-wave signal, construct an echo signal according to any one of claims 1 to 7, and perform signal processing based on the constructed echo signal.

9. The method according to claim 8, wherein The transmitted signal is a sawtooth linear frequency modulation system signal, and the echo signal at height H is: Among them, n is the number of scattering units, and i represents the i-th scattering unit; P t is the transmission signal power, G0 is the maximum antenna gain value in the beam center direction; λ is the wavelength corresponding to the center frequency of the transmission signal; is the antenna gain corresponding to the scattering unit i, is the backscattering coefficient corresponding to the scattering unit i, r i is the distance from the i-th scattering unit to the detector projection, Δr i is the width of the i-th scattering unit, R i is the slant range of the i-th scattering unit; α is the angle between the beam and the beam center direction; β is the signal frequency slope; c is the speed of light; t is the time; f c is the carrier center frequency; B is the modulation bandwidth; T m is the modulation period.

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