CN103454637A - Terahertz inverse synthetic aperture radar imaging method based on frequency modulation step frequency - Google Patents

Abstract

The invention discloses a terahertz inverse synthetic aperture radar imaging method based on a frequency modulation step frequency to solve the problem that in the prior art, the range resolution and the direction resolution of inverse synthetic aperture radar imaging in the microwave band are not high. The method includes the steps of firstly, obtaining a terahertz pulse echo signal; secondly, conducting pulse compression; thirdly, synthesizing a broadband; fourthly, conducting windowing and correcting; fifthly, conducting distance Doppler imaging; sixthly, obtaining an imaged picture. According to the method, the characteristic that the terahertz wave has the high frequency and the broadband is combined with the frequency modulation step frequency system so that the high-resolution terahertz inverse synthetic aperture radar imaging can be achieved through the method of synthesizing the broadband.

Description

Terahertz inverse synthetic aperture radar imaging method based on the frequency modulation Step Frequency

Technical field

The invention belongs to communication technical field, further relate to a kind of Terahertz inverse synthetic aperture radar imaging method based on the frequency modulation Step Frequency in the inverse synthetic aperture radar (ISAR) technical field.The present invention adopts the signal of frequency modulation Step Frequency system and terahertz wave band to realize the high-resolution inverse synthetic aperture radar imaging, to solve the not high problem of common microwave region signal inverse synthetic aperture radar imaging resolution, obtain high-resolution inverse synthetic aperture radar imaging figure.

Background technology

THz wave is the electromagnetic wave (frequency is that 0.1THz is to 10THz) between millimeter wave and infrared light, is also the frequency range that last mankind is not yet fully cognitive and utilize.THz wave has merged the advantage of microwave and millimeter wave and infrared light, be adapted to especially the characteristic of moderate beam angle, wide system bandwidth and large Doppler shift, be more suitable for the realization of very big signal bandwidth and extremely narrow antenna beam, be more conducive to the high-resolution inverse synthetic aperture radar imaging.

Radar with respect to common microwave region, the Terahertz radar is with its High Range Resolution, super large signal bandwidth, strong penetrating power, low intercepting and capturing rate, strong anti-interference, superior anti-stealthy and penetrate plasma capable and have numerous functions that powerful technical advantage realizes the systems such as radar detection and imaging, but because himself characteristic has also been brought huge challenge to imaging algorithm.The frequency modulation stepped frequency signal adopts the subpulse of linear FM signal as stepped frequency signal, characteristics with chirp and Step Frequency pulse, therefore when obtaining High Range Resolution, can reduce the requirement to the digital signal processor instant bandwidth, the data transfer rate of raising system, guarantee the operating distance of signal simultaneously, realize that the Terahertz inverse synthetic aperture radar (ISAR) carries out the purpose of high-resolution imaging to target, so the Terahertz radar of frequency modulation Step Frequency system there is very important application prospect at the high-resolution inverse synthetic aperture radar imaging.

The patented claim " data processing method of the large bandwidth laser synthetic aperture radar image-forming of THZ level system " that Shanghai Institute of Technical Physics of the Chinese Academy of Sciences proposes (application number: 201210091702.6, publication number: the data processing method that discloses the large bandwidth laser synthetic aperture radar image-forming of a kind of THZ level system CN102636776A).At first the method adopts based on Linear Tuning laser pulse signal on a large scale, adopts homodyne coherent detection technology, balance detection technology and synthetic aperture technique, finally utilizes special data processor, calculates the two dimensional image of target.The weak point of the method is: adopt laser pulse signal, the impact that is vulnerable to weather and atmosphere during work makes laser beam distorted and shake, directly affects the operating accuracy of radar, can not reach the high-resolution effect of inverse synthetic aperture radar imaging.

The patented claim " a kind of two-dimentional terahertz imaging system and formation method thereof " that Wu Zhouling proposes (application number: 201210108140.1, publication number: disclose a kind of two-dimentional terahertz imaging system and formation method thereof CN102621070A).The method utilization two dimension terahertz imaging system is positioned over imaging object between terahertz imaging lens and Terahertz condenser lens while obtaining image, then the laser initiation terahertz light lead antenna array sent by femto-second laser produces the terahertz wave beam group, converged on terahertz wave detector by the Terahertz condenser lens again, obtain two-dimentional Terahertz image.The weak point of the method is: the method is based on optoelectronic formation method, can only carry out imaging to in-plant object, and the resolution that imaging reaches does not have utilization high against the resolution of the method imaging of synthetic aperture.

Summary of the invention

The present invention is directed to the not high problem of inverse synthetic aperture radar imaging resolution in above-mentioned prior art, a kind of Terahertz inverse synthetic aperture radar imaging method based on based on the frequency modulation Step Frequency has been proposed, by synthetic large bandwidth, high-frequency signal, make the resolution of inverse synthetic aperture radar imaging be improved.

Realize that concrete steps of the present invention are as follows:

(1) obtain the terahertz pulse echoed signal:

The Terahertz frequency modulation stepped frequency signal of Terahertz radar receiver receiving target scattering point reflection, obtain Terahertz frequency modulation Step Frequency echoed signal, Terahertz frequency modulation Step Frequency echoed signal is comprised of Terahertz frequency modulation Step Frequency train of impulses, and every group pulse string is comprised of chirp.

(2) pulse compression:

Each Terahertz frequency modulation Step Frequency echoed signal is carried out to pulse compression, the signal after the acquisition pulse compression.

(3) synthetic large bandwidth:

3a) signal after each pulse compression in every group of Terahertz frequency modulation Step Frequency train of impulses is carried out to frequency displacement, the pulse signal after the acquisition frequency displacement;

3b) pulse signal after frequency displacement is synthesized to a large bandwidth signal according to the following formula:

$X\left(t\right)=\underset{m=1}{\overset{M}{\mathrm{\Σ}}}{y}_m\left(t\right)$

Wherein, large bandwidth signal after X (t) means to synthesize, t means that radar receiver receives the time that chirp experiences, M means the total number of chirp that every group of Terahertz frequency modulation Step Frequency train of impulses comprises, the M value is to be more than or equal to 1 positive integer, m means m chirp, the positive integer that the m span is 1～M, y _m(t) mean m the pulse signal after the signal frequency shift after pulse compression.

(4) windowing correction:

According to the following formula, the large bandwidth signal windowing correction to after synthetic obtains the revised large bandwidth signal of windowing:

Z(t)=IFFT[FFT[X(t)]×H(f)]

Wherein, Z (t) means the revised large bandwidth signal of windowing, t means that radar receiver receives the time that chirp experiences, IFFT[] mean the large bandwidth signal of windowing correction is carried out to inverse Fourier transform, FFT[] mean the large bandwidth signal after synthetic is carried out to Fourier transform, large bandwidth signal after X (t) means to synthesize, H (f) means the rectangular window of frequency domain.

(5) RANGE-DOPPLER IMAGING:

5a) to the orientation of the revised large bandwidth signal of windowing to carrying out Fourier transform, obtain Terahertz inverse synthetic aperture radar (ISAR) target scattering point orientation to full resolution pricture;

5b) to the revised large bandwidth signal of windowing the distance to carrying out pulse compression, obtain Terahertz inverse synthetic aperture radar (ISAR) target scattering point apart to full resolution pricture.

(6) obtain image:

By Terahertz inverse synthetic aperture radar (ISAR) target scattering point orientation to full resolution pricture and distance to full resolution pricture comprehensively in two-dimensional coordinate system, obtain two-dimentional high-resolution Terahertz inverse synthetic aperture radar imaging figure.

The present invention has the following advantages compared with prior art:

The first, the present invention, by selecting the signal of THz wave as inverse synthetic aperture radar imaging, has overcome the not high problem of inverse synthetic aperture radar imaging azimuth resolution in the prior art, has realized the orientation high-resolution in the inverse synthetic aperture radar imaging.

Second, adopt frequency modulation stepped frequency signal system in the present invention, by anamorphic zone, roomy mode obtains large bandwidth signal, overcome the not high problem of the little range resolution caused of inverse synthetic aperture radar imaging bandwidth in the prior art, realized that the distance in the inverse synthetic aperture radar imaging is to high-resolution.

The accompanying drawing explanation

Fig. 1 is process flow diagram of the present invention;

The model schematic diagram that Fig. 2 is target scattering point in the present invention;

The inverse synthetic aperture radar imaging figure that Fig. 3 is target scattering point in the present invention.

Embodiment

Below in conjunction with accompanying drawing, the present invention is described in further detail.

With reference to accompanying drawing 1, concrete steps of the present invention are as follows:

Step 1, obtain the terahertz pulse echoed signal.

The Terahertz frequency modulation stepped frequency signal of Terahertz radar receiver receiving target scattering point reflection, obtain Terahertz frequency modulation Step Frequency echoed signal, Terahertz frequency modulation Step Frequency echoed signal is comprised of Terahertz frequency modulation Step Frequency train of impulses, and every group pulse string is comprised of chirp.

Utilize the mathematical model of linear FM signal to obtain the mathematical model of frequency modulation Step Frequency, mathematical model used is as follows: one group of Terahertz frequency modulation stepped frequency signal is comprised of M chirp, and wherein m chirp of emission can be written as:

$c_m\left(t\right)=\mathrm{rect}\left(\frac{t}{T}\right)e^{({\mathrm{j\&pi;<figure-callout\; id="2"\; label="kt"\; filenames="HDA0000378859990000031.png"\; state="\{\{state\}\}">kt</figure-callout>}}^2+{\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HDA0000378859990000031.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;f}}_m)}$

Wherein, c _m(t) mean m Terahertz chirp pulse signal, the positive integer that the m value is 1～M, M means the total number of chirp that every group pulse string comprises, the M value is to be more than or equal to 1 positive integer, t means that radar receiver receives the time that chirp experiences, and rect () means the time domain rectangular window, and T means the cycle of chirp, k means the frequency modulation rate of chirp, f _mthe carrier frequency that means m Terahertz chirp.

With reference to accompanying drawing 2, set up the coordinate system of radar and target scattering point, R ₀mean 0 distance to radar of target scattering dot center, the coordinate of scattering point P under x '-y ' coordinate system is (x ₀, y ₀), at the lower coordinate of x-y coordinate system (rotating coordinate system), be (x, y), true origin is R to the distance of radar ₀:

x==rcos(θ+α)=x ₀cosθ-y ₀sinθ

y==rsin(θ+α)=x ₀sinθ+y ₀cosθ

Wherein, x means the horizontal ordinate of scattering point P under x-y coordinate system (rotating coordinate system), and y means the ordinate of scattering point P under x-y coordinate system (rotating coordinate system), x ₀mean the horizontal ordinate of scattering point P under coordinate system x '-y ', y ₀mean the ordinate of scattering point P under coordinate system x '-y ', α means the angle between the horizontal ordinate of target scattering point and rotating coordinate system x-y, and θ means the angle between the horizontal ordinate of target scattering point and rotating coordinate system x '-y '.

Centered by radar, set up the coordinate system parallel with x '-y ' coordinate system, now the scattering point coordinate is (x+R ₀, y), target scattering put radar the distance be $R=\sqrt{{(x+R_0)}^2+{\mathrm{<figure-callout\; id="2"\; label="y"\; filenames="HDA0000378859990000031.png"\; state="\{\{state\}\}">y</figure-callout>}}^2}\&ap;R_0+x_0\cos \mathrm{\&theta;}-{\mathrm{<figure-callout\; id="0"\; label="y"\; filenames="HDA0000378859990000031.png"\; state="\{\{state\}\}">y</figure-callout>}}_0\sin \mathrm{\&theta;}.$ Only consider the target scattering point platform center rotating that rotates, the angle that while launching i group pulse string, the relative radar of target turns over is θ _i=ω * i * PRT, i=0,1 ..., N _a-1, N _ameaning total group of number of Terahertz frequency modulation Step Frequency train of impulses, can obtain the coordinate of target scattering point in coordinate system, is R (n) thereby try to achieve target to the distance of radar, and wherein n means n scattering point.

According to following formula, acquisition Terahertz chirp echoed signal is:

$s_r\left(t\right)=\underset{n=1}{\overset{N}{\mathrm{\&Sigma;}}}{c}_m(t-\frac{2R\left(n\right)}{c})$

Wherein, s _r(t) mean m Terahertz chirp echoed signal, t means that radar receiver receives the time that chirp experiences, and n means n target scattering point, and N means target scattering point sum, c _m(t) mean m Terahertz chirp pulse signal, R (n) means that n target scattering put the distance of radar, and c means the light velocity.

Step 2, pulse compression.

Each Terahertz frequency modulation Step Frequency echoed signal is carried out to pulse compression, the signal after the acquisition pulse compression.

Step 3, synthetic large bandwidth.

The first step, carry out frequency displacement to the signal after each pulse compression in every group of Terahertz frequency modulation Step Frequency train of impulses, the pulse signal after the acquisition frequency displacement.Described frequency displacement realizes according to the following formula:

$y_m\left(t\right)=s_m\left(t\right)e^{\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HDA0000378859990000031.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;\&Delta;}{f}_{m}t}$

Wherein, y _m(t) mean m the pulse signal after frequency displacement, m means m chirp, the positive integer that the m span is 1～M, M means the total number of chirp that every group of Terahertz frequency modulation Step Frequency train of impulses comprises, the M value is to be more than or equal to 1 positive integer, and t means that radar receiver receives the time that chirp experiences; s _m(t) mean m the signal after pulse compression, Δ f _mthe frequency shift amount that means m the signal after pulse compression.

Second step, synthesize a large bandwidth signal according to the following formula by the pulse signal after frequency displacement:

$X\left(t\right)=\underset{m=1}{\overset{M}{\mathrm{\&Sigma;}}}{y}_m\left(t\right)$

Wherein, large bandwidth signal after X (t) means to synthesize, t means that radar receiver receives the time that chirp experiences, M means the total number of chirp that every group of Terahertz frequency modulation Step Frequency train of impulses comprises, the M value is to be more than or equal to 1 positive integer, m means m chirp, the positive integer that the m span is 1～M, y _m(t) mean m the pulse signal after the signal frequency shift after pulse compression.

Step 4, the windowing correction.

According to the following formula, the large bandwidth signal windowing correction to after synthetic obtains the revised large bandwidth signal of windowing:

Z(t)=IFFT[FFT[X(t)]×H(f)]

Wherein, Z (t) means the revised large bandwidth signal of windowing, t means that radar receiver receives the time that chirp experiences, IFFT[] mean the large bandwidth signal of windowing correction is carried out to inverse Fourier transform, FFT[] mean the large bandwidth signal after synthetic is carried out to Fourier transform, large bandwidth signal after X (t) means to synthesize, H (f) means the rectangular window of frequency domain, fairly perfect signal during through windowing revised large bandwidth signal, filtered out the burr that corresponding clutter and signal produce.

Step 5, RANGE-DOPPLER IMAGING.

By the orientation of synthetic large bandwidth signal to carrying out the FFT processing, obtain orientation to full resolution pricture; By the distance of synthetic large bandwidth signal to carrying out process of pulse-compression, obtain distance to full resolution pricture.

Step 6, obtain image.

By Terahertz inverse synthetic aperture radar (ISAR) target scattering point orientation to full resolution pricture and distance to full resolution pricture comprehensively in two-dimensional coordinate system, obtain two-dimentional high-resolution Terahertz inverse synthetic aperture radar imaging figure.

Effect of the present invention can further illustrate by following emulation.

1. simulated conditions:

Emulation of the present invention is to carry out under the software environment of MATLAB R2012a, and the translation of target scattering point compensates, only considers the target platform center rotating that rotates.

2. emulation content:

The parameter of Radar Target Scatter point: the distance that two target scatterings are put radar is respectively R ₁=2498m and R ₂=2501m, target scattering dot center is to the distance R of radar ₀=2500m, target rotation angle θ=1 ⁰, rotating speed of target ω=0.04rad/s.The signal parameter of launching: linear frequency modulation rate K _r=5 * 10 ¹³, the burst blocks number that launch Terahertz frequency modulation stepped frequency signal is N _a=256, every a string M that comprises _r=10 subpulses.The parameter of radar ISAR imaging system: range resolution is

azimuth resolution is light velocity c=3 * 10 ⁸m/s.

Inverse synthetic aperture radar (ISAR) receives two kinds of signals: the first is the centre frequency 120GHz of frequency modulation stepped frequency signal, signal total bandwidth 5GHz, and Equations of The Second Kind is the centre frequency 30GHz of linear FM signal, signal bandwidth 1GHz, relatively simulation result.

3. analysis of simulation result:

With reference to accompanying drawing 3, simulation imaging result of the present invention is compared to analysis, wherein, and accompanying drawing 3(a) be that prior art adopts linear FM signal, the linear FM signal centre frequency is 30GHz, bandwidth is that 1GHz does not adopt the synthetic imaging effect figure of bandwidth.Accompanying drawing 3(b) be the imaging effect figure of the inventive method, adopt the frequency modulation stepped frequency signal of Terahertz, frequency modulation stepped frequency signal centre frequency is 120GHz, and bandwidth is the imaging effect figure that 5GHz adopts synthetic large bandwidth.

Contrast accompanying drawing 3(a) and accompanying drawing 3(b), the inventive method is due to the resolution of the resulting inverse synthetic aperture radar imaging figure of signal that adopts high-frequency and synthetic large bandwidth, comprise range resolution and azimuthal resolution, all than bandwidth, the imaging resolution of little traditional microwave wave band is high, thereby has been proved correctness and the validity of the inventive method by Fig. 3 (b) imaging effect.

Claims (2)

1. the Terahertz inverse synthetic aperture radar imaging method based on the frequency modulation Step Frequency, comprise the steps:

(1) obtain the terahertz pulse echoed signal:

The Terahertz frequency modulation stepped frequency signal of Terahertz radar receiver receiving target scattering point reflection, obtain Terahertz frequency modulation Step Frequency echoed signal, Terahertz frequency modulation Step Frequency echoed signal is comprised of Terahertz frequency modulation Step Frequency train of impulses, and every group pulse string is comprised of chirp.

(2) pulse compression:

Each Terahertz frequency modulation Step Frequency echoed signal is carried out to pulse compression, the signal after the acquisition pulse compression;

(3) synthetic large bandwidth:

3a) signal after each pulse compression in every group of Terahertz frequency modulation Step Frequency train of impulses is carried out to frequency displacement, the pulse signal after the acquisition frequency displacement;

3b) pulse signal after frequency displacement is synthesized to a large bandwidth signal according to the following formula:

$X\left(t\right)=\underset{m=1}{\overset{M}{\mathrm{\&Sigma;}}}{y}_m\left(t\right)$

Wherein, large bandwidth signal after X (t) means to synthesize, t means that radar receiver receives the time that chirp experiences, M means the total number of chirp that every group of Terahertz frequency modulation Step Frequency train of impulses comprises, the M value is to be more than or equal to 1 positive integer, m means m chirp, the positive integer that the m span is 1～M, y _m(t) mean m the pulse signal after the signal frequency shift after pulse compression;

(4) windowing correction:

According to the following formula, the large bandwidth signal windowing correction to after synthetic obtains the revised large bandwidth signal of windowing:

Z(t)=IFFT[FFT[X(t)]×H(f)]

Wherein, Z (t) means the revised large bandwidth signal of windowing, t means that radar receiver receives the time that chirp experiences, IFFT[] mean the large bandwidth signal of windowing correction is carried out to inverse Fourier transform, FFT[] mean the large bandwidth signal after synthetic is carried out to Fourier transform, large bandwidth signal after X (t) means to synthesize, H (f) means the rectangular window of frequency domain;

(5) RANGE-DOPPLER IMAGING:

5a) to the orientation of the revised large bandwidth signal of windowing to carrying out Fourier transform, obtain Terahertz inverse synthetic aperture radar (ISAR) target scattering point orientation to full resolution pricture;

5b) to the revised large bandwidth signal of windowing the distance to carrying out pulse compression, obtain Terahertz inverse synthetic aperture radar (ISAR) target scattering point apart to full resolution pricture;

(6) obtain image:

By Terahertz inverse synthetic aperture radar (ISAR) target scattering point orientation to full resolution pricture and distance to full resolution pricture comprehensively in two-dimensional coordinate system, obtain two-dimentional high-resolution Terahertz inverse synthetic aperture radar imaging figure.

2. the Terahertz inverse synthetic aperture radar imaging method based on the frequency modulation Step Frequency according to claim 1, is characterized in that step 3a) described in frequency displacement realize according to the following formula:

$y_m\left(t\right)=s_m\left(t\right)e^{j2\mathrm{\&pi;\&Delta;}{f}_{m}t}$

Wherein, y _m(t) mean m the pulse signal after frequency displacement, m means m chirp, the positive integer that the m span is 1～M, M means the total number of chirp that every group of Terahertz frequency modulation Step Frequency train of impulses comprises, the M value is to be more than or equal to 1 positive integer, and t means that radar receiver receives the time that chirp experiences; s _m(t) mean m the signal after pulse compression, Δ f _mthe frequency shift amount that means m the signal after pulse compression.

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