CN103792521A - Radar target back scattering simulation method influenced by Faraday rotation changes - Google Patents

Abstract

The invention provides a radar target back scattering simulation method influenced by Faraday rotation changes. The method comprises the steps of obtaining four complete polarization SAR compound images of an identical scene after various error influences are calibrated; obtaining a polarization scattering matrix S of a target; calculating a Faraday rotating angle omega caused by an ionized layer TEC value of each impulse transmission moment; calculating a polarization scattering matrix O influenced by the Faraday rotation changes; obtaining a back scattering coefficient influenced by the Faraday rotation changes; putting the back scattering coefficient influenced by the Faraday rotation changes into an SAR echo expression to carry out imaging processing and corresponding analysis. Through the adoption of the method, any influence on the back scattering coefficient and SAR imaging by the Faraday rotation changes can be simulated. In addition, radar target back scattering coefficients influenced by the Faraday rotation changes can be simulated easily and effectively based on existing SAR images.

Description

A kind of radar target back scattering analogy method that is subject to Faraday rotation variable effect

Technical field

The present invention relates to target scattering characteristics field, SAR imaging field, belong to Electromagnetic Scattering of Target, ionosphere to SAR effect of signals and SAR signal process field, particularly, relate to one and be subject to Faraday(faraday) rotation variable effect radar target back scattering analogy method.

Background technology

The scattering properties of target is relevant with polarization of electromagnetic wave, element in polarization scattering matrix is the same polarization backscattering coefficient of target, the cross polarization backscattering coefficient of target, and the backscattering coefficient of target while not having polarization of ele to rotate any one angle.

The response difference of different target to polarization of ele characteristic.

Ionosphere is the region in partial ionization state in earth atmosphere, start to extend to about 1000km from about 50km overhead, satellite-borne SAR signal passes through ionosphere twice, and ionosphere makes SAR signal that Faraday rotation occur, thereby the backscattering characteristic of target is changed.

If the variation difference of synthetic aperture time internal object backscattering characteristic, can make echoed signal decoherence, thereby SAR focal imaging is exerted an influence.

At present, the synthetic aperture time of SAR is second-time in-orbit, can think between ionosphere to be at this moment static constant in section, thereby in the synthetic aperture time, Faraday rotation is identical on the impact of target backscattering characteristic, i.e. Faraday rotation can be ignored the impact of SAR focal imaging.Geostationary orbit SAR(GeoSAR) orbit altitude be about 36000km, the synthetic aperture of L frequency range GeoSAR is for up to dozens of minutes and even hour magnitude, ionosphere is dynamic change within the time period of hour magnitude, thereby in the GeoSAR synthetic aperture time, Faraday rotates the difference that affects on target backscattering characteristic, i.e. Faraday rotation meeting exerts an influence to the focal imaging of GeoSAR.Also there is no at present GeoSAR in-orbit.

Therefore, the research on SAR impact with regard to Faraday rotation at present, mainly concentrates on Faraday and rotate the impact on SAR image-forming radiation precision, and Faraday rotation variation yet there are no disclosed report to the research of SAR focal imaging.

Summary of the invention

The object of the present invention is to provide a kind of radar target back scattering analogy method of the Faraday of being subject to rotation variable effect, this analogy method can join Faraday rotation arbitrarily in the simulation of target backscattering characteristic, can analyze quantitatively Faraday rotation and change the impact on SAR focal imaging.

The technical scheme that the present invention realizes above-mentioned purpose employing comprises:

A radar target back scattering analogy method that is subject to Faraday rotation variable effect, comprises the following steps:

The first step, obtain four width complete polarization SAR complex patterns of Same Scene, and this four width image is the image after the various error effects of calibration;

Second step, obtain the polarization scattering matrix S of target: take resolution element as unit, obtain respectively the multiple pixel value that this resolution element is corresponding in four width complex patterns, as corresponding element in polarization scattering matrix S, this polarization scattering matrix S is expressed as follows:

$S=\left[\begin{array}{cc}{S}_{\mathrm{hh}}& S_{\mathrm{hv}}\\ S_{\mathrm{vh}}& S_{\mathrm{vv}}\end{array}\right],$

In above formula, S is the matrix on 2 × 2 rank, S _hh, S _hv, S _vh, S _vvfor the multiple pixel value of obtaining in four width complex patterns respectively, h represents horizontal polarization, and v represents vertical polarization;

The 3rd step, basis formula below calculate the Faraday rotation angle Ω that each impulse ejection moment is caused by ionized layer TEC:

$\mathrm{\Ω}=2.36\×{10}^4\frac{B_{\mathrm{av}}}{{f_c}^2}\mathrm{TEC},$

In formula, B _avfor terrestrial magnetic field is along the mean value on travel path, f _cfor the carrier frequency transmitting, TEC is the ionosphere total electron content in SAR signal propagation path;

The 4th step, basis formula below calculate the polarization scattering matrix O that is subject to Faraday rotation variable effect:

O=F·S·F，

In formula, S is the polarization scattering matrix obtaining in second step, and F represents that Faraday rotates the impact on target scattering characteristics, and its expression formula is $F=\left[\begin{array}{cc}\cos \mathrm{\Ω}& \sin \mathrm{\Ω}\\ -\sin \mathrm{\Ω}& \cos \mathrm{\Ω}\end{array}\right],$ The expression formula of O is $O=\left[\begin{array}{cc}{O}_{\mathrm{hh}}& O_{\mathrm{hv}}\\ O_{\mathrm{vh}}& O_{\mathrm{vv}}\end{array}\right],$ Wherein, h represents horizontal polarization, and v represents vertical polarization;

The 5th step, basis formula below calculate the backscattering coefficient observing that is subject to Faraday Effect of Rotation, that is, and and two backscattering coefficient P that same polarization passage observes _hh, P _vv, two backscattering coefficient P that cross polarization passage observes _hv, P _vh:

P _hh=O _hh， P _vv=O _vv， P _vh=O _vh， P _hv=O _hv；

The 6th step, the backscattering coefficient observing obtaining in the 5th step is brought in GeoSAR echoed signal expression formula below, echoed signal to this echoed signal expression formula statement is carried out imaging processing, and imaging results is done to corresponding analysis, in following formula, X represents respectively P _hh, P _vh, P _hv, P _vv, to analyze respectively the Faraday rotation variation impact that polarization receives image formation, horizontal polarization transmitting vertical polarization is received image formation, vertical polarization emission level is polarized and receives image formation, vertical polarization transmitting vertical polarization is received to image formation on horizontal polarization emission level:

$\begin{array}{c}s(t,t_m)=X\·\mathrm{rect}[t-\frac{2R(t_m,R_B)}{c}]\·w_a\left(t_m\right)\·w_r[t-\frac{2R(t_m,R_B)}{c}]\·\\ \exp \left\{j2\mathrm{\π}{f}_c\right[t-\frac{2R(t_m,R_B)}{c}]+\mathrm{j\π\γ}{[t-\frac{2R(t_m,R_B)}{c}]}^2\}\end{array}$

In above formula, j is imaginary unit, j=sqrt (1); T, t _mbe respectively distance to time, orientation to the time; C is the light velocity; w _a(t _m), be respectively orientation to antenna radiation pattern, distance to antenna radiation pattern; f _cfor carrier frequency; γ is frequency modulation rate; R _bfor the minimum distance between target and GeoSAR; R (t _m, R _b) be moment t _mtime instantaneous oblique distance between GeoSAR and target.

Preferably, in second step, in the time obtaining the polarization scattering matrix of target based on four width SAR complex patterns, the multiple pixel value of selected resolution element resolution element is around averaged, as corresponding element in polarization scattering matrix S.

The method according to this invention possesses useful technique effect:

Faraday rotation arbitrarily can be changed and joins in the impact of different radar target backscattering characteristics, be convenient to analyze different big or small Faraday rotations and change the impact on different target backscattering characteristic, and then analyze its impact on SAR focal imaging.In addition, be subject to the radar target backscattering coefficient of Faraday rotation variable effect based on existing SAR image simulation, simply effective.

Accompanying drawing explanation

Fig. 1 is according to ionospheric data TEC value schematic diagram over time in analogy method of the present invention;

Fig. 2 is the schematic flow sheet according to analogy method of the present invention.

Embodiment

Below in conjunction with the drawings and specific embodiments, the radar target back scattering analogy method of the Faraday of being subject to rotation variable effect according to the present invention is described in detail.

Fig. 1 is according to ionospheric data TEC value schematic diagram over time in analogy method of the present invention.As shown in Figure 1, within 15 minutes even longer synthetic aperture time, ionized layer TEC value changes, thereby the Faraday rotation angle that synthetic aperture time internal ionization layer causes changes, be that in the synthetic aperture time, radar target backscattering coefficient changes, thereby can affect SAR focal imaging.For this reason, the present invention can change Faraday rotation arbitrarily to join in the impact of different radar target backscattering characteristics, can analyze quantitatively Faraday rotation and change the impact on SAR focal imaging.

Comprise the following steps according to the radar target back scattering analogy method of the Faraday of being subject to rotation variable effect of the present invention:

The first step, obtain four width complete polarization SAR complex patterns of Same Scene, and this four width image is the image after the various error effects of calibration.

In practical operation, the four width complete polarization SAR complex patterns of calibrating after various error effects are existing SAR image products, can be obtained by disclosed SAR image.

Second step, obtain the polarization scattering matrix S of target: take resolution element as unit, obtain respectively the multiple pixel value that this resolution element is corresponding in four width complex patterns, as corresponding element in polarization scattering matrix S, this polarization scattering matrix S is expressed as follows:

$S=\left[\begin{array}{cc}{S}_{\mathrm{hh}}& S_{\mathrm{hv}}\\ S_{\mathrm{vh}}& S_{\mathrm{vv}}\end{array}\right]---\left(1\right)$

In above formula, S is the matrix on 2 × 2 rank, S _hh, S _hv, S _vh, S _vvfor the multiple pixel value of obtaining in four width complex patterns respectively, h represents horizontal polarization, and v represents vertical polarization.

In practical operation, the multiple pixel value of the resolution element around selected resolution element in every width complex pattern can be done on average, as corresponding element in polarization scattering matrix S.

The 3rd step, basis formula below calculate the Faraday rotation angle Ω that each impulse ejection moment is caused by ionized layer TEC:

$\mathrm{\Ω}=2.36\×{10}^4\frac{B_{\mathrm{av}}}{{f_c}^2}\mathrm{TEC}---\left(2\right)$

In formula, B _avfor terrestrial magnetic field is along the mean value on travel path, f _cfor the carrier frequency transmitting, TEC is the ionosphere total electron content in SAR signal propagation path.In practice, TEC can be measured and be obtained by ionosphere monitoring equipment; If the rate of change F of known Faraday rotation angle _v, certain impulse ejection moment T _nfaraday rotation angle be F _vt _n.SAR echoed signal also will be passed through ionosphere, and the time delay of SAR echo is second-time, can think between ionosphere to be at this moment static in section, and the Faraday rotation angle in echo moment equals the Faraday rotation angle in impulse ejection moment.

The 4th step, basis formula below calculate the polarization scattering matrix O that is subject to Faraday rotation variable effect:

O=F·S·F （3）

In formula, S is the polarization scattering matrix obtaining in second step, and F represents that Faraday rotates the impact on target scattering characteristics, and its expression formula is:

$F=\left[\begin{array}{cc}\cos \mathrm{\Ω}& \sin \mathrm{\Ω}\\ -\sin \mathrm{\Ω}& \cos \mathrm{\Ω}\end{array}\right]---\left(4\right)$

The expression formula of O is $O=\left[\begin{array}{cc}{O}_{\mathrm{hh}}& O_{\mathrm{hv}}\\ O_{\mathrm{vh}}& O_{\mathrm{vv}}\end{array}\right],$ H represents horizontal polarization, and v represents vertical polarization.

The 5th step, basis formula below calculate the backscattering coefficient observing that is subject to Faraday Effect of Rotation, that is, and and two backscattering coefficient P that same polarization passage observes _hh, P _vv, two backscattering coefficient P that cross polarization passage observes _hv, P _vh:

P _hh=O _hh， P _vv=O _vv， P _vh=O _vh， P _hv=O _hv。（5）

The 6th step, the backscattering coefficient observing obtaining in the 5th step is brought in GeoSAR echoed signal expression formula below, echoed signal to this echoed signal expression formula statement is carried out imaging processing, and imaging results is done to corresponding analysis, in following formula, X represents respectively P _hh, P _vh, P _hv, P _vv, to analyze respectively the Faraday rotation variation impact that polarization receives image formation, horizontal polarization transmitting vertical polarization is received image formation, vertical polarization emission level is polarized and receives image formation, vertical polarization transmitting vertical polarization is received to image formation on horizontal polarization emission level:

$\begin{array}{c}s(t,t_m)=X\·\mathrm{rect}[t-\frac{2R(t_m,R_B)}{c}]\·w_a\left(t_m\right)\·w_r[t-\frac{2R(t_m,R_B)}{c}]\·\\ \exp \left\{j2\mathrm{\π}{f}_c\right[t-\frac{2R(t_m,R_B)}{c}]+\mathrm{j\π\γ}{[t-\frac{2R(t_m,R_B)}{c}]}^2\}\end{array}---\left(6\right)$

In above formula, j is imaginary unit, j=sqrt (1); T, t _mbe respectively distance to time, orientation to the time; C is the light velocity; w _a(t _m),

be respectively orientation to antenna radiation pattern, distance to antenna radiation pattern; f _cfor carrier frequency; γ is frequency modulation rate; R _bfor the minimum distance between target and GeoSAR; R (t _m, R _b) be moment t _mtime instantaneous oblique distance between GeoSAR and target.

Thus, according to imaging results, the electromagnetic wave Faraday rotation can quantitative test synthetic aperture time internal ionization layer causing changes the impact on GeoSAR focal imaging; And Faraday rotation arbitrarily can be changed and joins in the impact of different target backscattering characteristic, the Faraday rotation of being convenient to analyze different sizes changes the impact on different target backscattering characteristic, and then its impact on SAR focal imaging of quantitative test.

Below, in conjunction with the processing case verification advantage of the present invention of measured data.

This embodiment is take L-band geostationary orbit SAR(GeoSAR) be example (bandwidth 35MHz, polarization mode is that horizontal emission level receives signal, the synthetic aperture time is taken as 15 minutes for carrier frequency 1.25GHz, LFM signal pulsewidth 500us).

On certain disclosed website, obtain SAR imaging, and in this SAR imaging, obtain the polarization scattering matrix S of mountain range, ocean, vegetation, building _mountain, S _sea, S _plant, S _build.

Suppose interior Faraday rotation of the synthetic aperture time rate of change F of 15 minutes _vwhile being respectively spend/second 0.01 degree/second, 0.05,0.1 degree/second, 0.2 degree/second, can know certain moment T _nthe suffered Faraday rotation angle of echoed signal receiving is F _vt _n, and for all types of target, calculate it and be subject to accordingly the backscattering coefficient of Faraday Effect of Rotation by formula (3), (4), (5) above.If known quantity is TEC, can calculate the suffered Faraday rotation angle of echoed signal that certain moment receives by formula (2).

Table 1-table 4 provided in synthetic aperture time of 15 minutes Faraday rotation rate of change and has been respectively 0.01 degree/second, 0.05 degree/second, 0.1 degree/second, when 0.2 degree/second, the peak sidelobe ratio index of the SAR imaging of all types of target.

From showing 1-table 4: Faraday rotation changes the difference that affects on different target backscattering characteristic, thus the difference that affects on SAR focal imaging.Along with the increase of synthetic aperture time, Faraday rotation changes more obvious on the impact of SAR focal imaging.The present invention is conceived to Faraday rotation and changes the impact on target backscattering characteristic and SAR focal imaging, can analyze different Faraday rotations simultaneously and change the impact on different target backscattering characteristic and SAR focal imaging, simple possible.

Table 1-table 4:Faraday rotation changes different target backscattering characteristic and the impact on SAR focal imaging.

The impact of spend/second on SAR imaging of table 1 Faraday rotation rate of change 0.01

Slope was 0.01 degree/second	Mountain range	Ocean	Vegetation	Building
					Peak sidelobe ratio (dB)	-13.6092	-13.6590	-13.4425	-13.7417

The impact of spend/second on SAR imaging of table 2 Faraday rotation rate of change 0.05

Slope was 0.05 degree/second	Mountain range	Ocean	Vegetation	Building
					Peak sidelobe ratio (dB)	-11.1355	-12.6953	-12.0126	-13.7412

The impact of spend/second on SAR imaging of table 3 Faraday rotation rate of change 0.1

Slope was 0.1 degree/second	Mountain range	Ocean	Vegetation	Building
					Peak sidelobe ratio (dB)	-7.8608	-11.6966	-10.2462	-13.7305

The impact of spend/second on SAR imaging of table 4 Faraday rotation rate of change 0.2

Slope was 0.2 degree/second	Mountain range	Ocean	Vegetation	Building
					Peak sidelobe ratio (dB)	-5.2143	-5.4526	-3.8570	-13.7301

Those skilled in the art can understand, and unspecified content in this instructions is that those skilled in the art can easily realize according to the description of this instructions and in conjunction with prior art, does not therefore describe in detail.

The foregoing is only the preferred embodiments of the present invention; but protection scope of the present invention is not limited to this; any be familiar with those skilled in the art the present invention disclose technical scope in, the variation that can expect easily or replacement, within all should being encompassed in protection scope of the present invention.

Claims (2)

1. a radar target back scattering analogy method that is subject to Faraday rotation variable effect, is characterized in that, comprises the following steps:

The first step, obtain four width complete polarization SAR complex patterns of Same Scene, and this four width image is the image after the various error effects of calibration;

Second step, obtain the polarization scattering matrix S of target: take resolution element as unit, obtain respectively the multiple pixel value that this resolution element is corresponding in four width complex patterns, as corresponding element in polarization scattering matrix S, this polarization scattering matrix S is expressed as follows:

$S=\left[\begin{array}{cc}{S}_{\mathrm{hh}}& S_{\mathrm{hv}}\\ S_{\mathrm{vh}}& S_{\mathrm{vv}}\end{array}\right],$

In above formula, S is the matrix on 2 × 2 rank, S _hh, S _hv, S _vh, S _vvfor the multiple pixel value of obtaining in four width complex patterns respectively, h represents horizontal polarization, and v represents vertical polarization;

The 3rd step, basis formula below calculate the Faraday rotation angle Ω that each impulse ejection moment is caused by ionized layer TEC:

$\mathrm{\Ω}=2.36\×{10}^4\frac{B_{\mathrm{av}}}{{f_c}^2}\mathrm{TEC},$

In formula, B _avfor terrestrial magnetic field is along the mean value on travel path, f _cfor the carrier frequency transmitting, TEC is the ionosphere total electron content in SAR signal propagation path;

The 4th step, basis formula below calculate the polarization scattering matrix O that is subject to Faraday rotation variable effect:

O=F·S·F，

In formula, S is the polarization scattering matrix obtaining in second step, and F represents that Faraday rotates the impact on target scattering characteristics, and its expression formula is $F=\left[\begin{array}{cc}\cos \mathrm{\Ω}& \sin \mathrm{\Ω}\\ -\sin \mathrm{\Ω}& \cos \mathrm{\Ω}\end{array}\right],$ The expression formula of O is $O=\left[\begin{array}{cc}{O}_{\mathrm{hh}}& O_{\mathrm{hv}}\\ O_{\mathrm{vh}}& O_{\mathrm{vv}}\end{array}\right],$ Wherein, h represents horizontal polarization, and v represents vertical polarization;

The 5th step, basis formula below calculate the backscattering coefficient observing that is subject to Faraday Effect of Rotation, that is, and and two backscattering coefficient P that same polarization passage observes _hh, P _vv, two backscattering coefficient P that cross polarization passage observes _hv, P _vh:

P _hh=O _hh， P _vv=O _vv， P _vh=O _vh， P _hv=O _hv；

The 6th step, the backscattering coefficient observing obtaining in the 5th step is brought in GeoSAR echoed signal expression formula below, echoed signal to this echoed signal expression formula statement is carried out imaging processing, and imaging results is done to corresponding analysis, in following formula, X represents respectively P _hh, P _vh, P _hv, P _vv, to analyze respectively the Faraday rotation variation impact that polarization receives image formation, horizontal polarization transmitting vertical polarization is received image formation, vertical polarization emission level is polarized and receives image formation, vertical polarization transmitting vertical polarization is received to image formation on horizontal polarization emission level:

$\begin{array}{c}s(t,t_m)=X\·\mathrm{rect}[t-\frac{2R(t_m,R_B)}{c}]\·w_a\left(t_m\right)\·w_r[t-\frac{2R(t_m,R_B)}{c}]\·\\ \exp \left\{j2\mathrm{\π}{f}_c\right[t-\frac{2R(t_m,R_B)}{c}]+\mathrm{j\π\γ}{[t-\frac{2R(t_m,R_B)}{c}]}^2\}\end{array}$

In above formula, j is imaginary unit, j=sqrt (1); T, t _mbe respectively distance to time, orientation to the time; C is the light velocity; w _a(t _m),

be respectively orientation to antenna radiation pattern, distance to antenna radiation pattern; f _cfor carrier frequency; γ is frequency modulation rate; R _bfor the minimum distance between target and GeoSAR; R (t _m, R _b) be moment t _mtime instantaneous oblique distance between GeoSAR and target.

2. the radar target back scattering analogy method that is subject to Faraday rotation variable effect according to claim 1, it is characterized in that, in second step, in the time obtaining the polarization scattering matrix of target based on four width SAR complex patterns, multiple pixel value to selected resolution element resolution element is around averaged, as corresponding element in polarization scattering matrix S.

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