CN103792521B - A kind of radar target back scattering analogy method that is subject to Faraday rotation variable effect - Google Patents

Abstract

The radar target back scattering analogy method that the invention provides a kind of Faraday of being subject to rotation variable effect, comprising: four width that obtain Same Scene are calibrated the complete polarization SAR complex pattern after various error effects; Obtain the polarization scattering matrix S of target; Calculate the Faraday anglec of rotation Ω that each impulse ejection moment is caused by ionized layer TEC value; Calculating is subject to the polarization scattering matrix O of Faraday rotation variable effect; Obtain the backscattering coefficient that is subject to Faraday Effect of Rotation; The backscattering coefficient that is subject to Faraday rotation variable effect is brought into SAR echo expression formula, does imaging and analyze accordingly. Adopt this method, can simulate any Faraday rotation and change the impact on backscattering coefficient and SAR imaging. And, be subject to the radar target backscattering coefficient of Faraday rotation variable effect based on existing SAR image simulation, simply effective.

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, ionizationLayer, to SAR effect of signals and SAR signal process field, particularly, relates to one and is subject to Faraday(faradThe) the radar target back scattering analogy method of rotation variable effect.

Background technology

The scattering properties of target is relevant with polarization of electromagnetic wave, and what the element in polarization scattering matrix was target is sameThe cross polarization backscattering coefficient of polarization backscattering coefficient, target, and do not have polarization of ele rotation to appointThe backscattering coefficient of a target when angle of meaning.

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

Ionosphere is the region in partial ionization state in earth atmosphere, starts one from about 50km overheadDirectly extend to about 1000km, satellite-borne SAR signal passes through ionosphere twice, and SAR signal occurs in ionosphereFaraday rotation, thus the backscattering characteristic of target is changed.

If the variation difference of synthetic aperture time internal object backscattering characteristic, can make echo-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 at this moment in sectionBe static constant, thereby in the synthetic aperture time, Faraday rotate the impact on target backscattering characteristicBe identical, i.e. Faraday rotation can be ignored the impact of SAR focal imaging. Geostationary orbit SAR(GeoSAR) orbit altitude is 36000km left and right, and the synthetic aperture of L frequency range GeoSAR is for up to severalTen minutes and even hour magnitude, ionosphere is dynamic change within the time period of hour magnitude, therebyIn the GeoSAR synthetic aperture time, Faraday rotates the difference that affects on target backscattering characteristic,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 rotationOn the impact of SAR image-forming radiation precision, and Faraday rotation changes to the research of SAR focal imaging also notSee disclosed report.

Summary of the invention

The object of the present invention is to provide a kind of radar target back scattering of the Faraday of being subject to rotation variable effectAnalogy method, this analogy method can join the rotation of Faraday arbitrarily target backscattering characteristicIn simulation, 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 following stepRapid:

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

Second step, obtain the polarization scattering matrix S of target: taking resolution cell as unit, at four width complex patternsMiddlely obtain respectively the multiple pixel value that this resolution cell is corresponding, 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 getting in four width complex patterns respectivelyThe multiple pixel value obtaining, h represents horizontal polarization, v represents vertical polarization;

The 3rd step, calculate according to formula below that each impulse ejection moment causes by ionized layer TECFaraday anglec of rotation Ω:

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

In formula, B_avFor earth's magnetic field is along the mean value on propagation path, f_cFor the carrier frequency transmitting, TEC is SARIonosphere total electron content in 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 target scattering spyThe impact of property, 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, hRepresent horizontal polarization, v represents vertical polarization;

The 5th step, basis formula below calculate and are subject to the back scattering observing of Faraday Effect of Rotation to beNumber, that is, and two backscattering coefficient P that same polarization passage observes_hh、P_vv, two cross polarization passages are seenThe backscattering coefficient P measuring_hv、P_vh：

P_hh=O_hh，P_vv=O_vv，P_vh=O_vh，P_hv=O_hv；

The 6th step, the GeoSAR that the backscattering coefficient observing obtaining in the 5th step is brought into below returnIn ripple signal expression, the echo-signal of this echo-signal expression formula statement is carried out to imaging, and to becomingDo corresponding analysis as result, in following formula, X represents respectively P_hh、P_vh、P_hv、P_vv, to analyze respectively FaradayRotation changes the polarization to horizontal polarization emission level and receives image formation, horizontal polarization transmitting vertical polarization is connectReceive image formation, vertical polarization emission level polarization is received image formation, vertical polarization is launched to the vertical utmost pointChange the impact that receives image formation:

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

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 directionFigure; f_cFor carrier frequency; γ is frequency modulation rate; R_BFor the minimum distance between target and GeoSAR; R (t_m,R_B) beMoment 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,Multiple pixel value to selected resolution cell resolution cell is around averaged, in polarization scattering matrix SCorresponding element.

The method according to this invention possesses useful technique effect:

Faraday rotation arbitrarily can be changed to the shadow joining different radar target backscattering characteristicsIn sound, 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 Faraday based on existing SAR image simulationThe radar target backscattering coefficient of rotation variable effect, simply effective.

Brief description of the drawings

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.

Detailed description of the invention

Below in conjunction with the drawings and specific embodiments to the Faraday rotation variable effect of being subject to according to the present inventionRadar target back scattering analogy method 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 anglec of rotation that synthetic aperture time internal ionization layer causes changes, i.e. the synthetic aperture timeInterior radar target backscattering coefficient changes, thereby can affect SAR focal imaging. For this reason, the present inventionFaraday rotation arbitrarily can be changed to the impact joining different radar target backscattering characteristicsIn, can analyze quantitatively Faraday rotation and change the impact on SAR focal imaging.

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

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

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

Second step, obtain the polarization scattering matrix S of target: taking resolution cell as unit, at four width complex patternsMiddlely obtain respectively the multiple pixel value that this resolution cell is corresponding, 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 getting in four width complex patterns respectivelyThe multiple pixel value obtaining, h represents horizontal polarization, v represents vertical polarization.

In practical operation, can answering the resolution cell around selected resolution cell in every width complex patternPixel value does on average, as corresponding element in polarization scattering matrix S.

The 3rd step, calculate according to formula below that each impulse ejection moment causes by ionized layer TECFaraday anglec of rotation Ω:

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

In formula, B_avFor earth's magnetic field is along the mean value on propagation path, f_cFor the carrier frequency transmitting, TEC is SARIonosphere total electron content in signal propagation path. In practice, TEC can be surveyed by ionosphere monitoring equipmentAmount obtains; If the rate of change F of the known Faraday anglec of rotation_v, certain impulse ejection moment T_nFaradayThe anglec of rotation is F_v·T_n. SAR echo-signal also will be passed through ionosphere, and the time delay of SAR echo is second-time, can recognizeFor ionosphere at this moment between section in be static, the Faraday anglec of rotation in echo moment equals impulse ejectionThe Faraday anglec of rotation in 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 target scattering spyThe impact of property, 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 and are subject to the back scattering observing of Faraday Effect of Rotation to beNumber, that is, and two backscattering coefficient P that same polarization passage observes_hh、P_vv, two cross polarization passages are seenThe backscattering coefficient P measuring_hv、P_vh：

P_hh=O_hh，P_vv=O_vv，P_vh=O_vh，P_hv=O_hv。（5）

The 6th step, bring the backscattering coefficient observing obtaining in the 5th step into GeoSAR echo belowIn signal expression, the echo-signal of this echo-signal expression formula statement is carried out to imaging, and to imagingResult is done corresponding analysis, and in following formula, X represents respectively P_hh、P_vh、P_hv、P_vv, to analyze respectively FaradayRotation changes the polarization to horizontal polarization emission level and receives image formation, horizontal polarization transmitting vertical polarization is connectReceive image formation, vertical polarization emission level polarization is received image formation, vertical polarization is launched to the vertical utmost pointChange the impact that receives image formation:

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

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 directionFigure; f_cFor carrier frequency; γ is frequency modulation rate; R_BFor the minimum distance between target and GeoSAR; R (t_m,R_B) beMoment t_mTime instantaneous oblique distance between GeoSAR and target.

Thus, according to imaging results, the electromagnetic wave can quantitative analysis synthetic aperture time internal ionization layer causingFaraday rotation changes the impact on GeoSAR focal imaging; And can be by Faraday rotation arbitrarilyChange and join in the impact of different target backscattering characteristic, be convenient to analyze the Faraday of different sizesRotation changes impact on different target backscattering characteristic, so quantitative analysis its to SAR focal imagingImpact.

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

This embodiment is taking L-band geostationary orbit SAR(GeoSAR) be example (carrier frequency 1.25GHz, LFMSignal pulsewidth 500us, bandwidth 35MHz, polarization mode is that horizontal emission level receives signal, when synthetic apertureBetween be taken as 15 minutes).

On certain disclosed website, obtain SAR imaging, and in this SAR imaging, obtain mountain range, ocean, plantThe polarization scattering matrix S of quilt, building_Mountain、S_Sea、S_Plant、S_Build。

Suppose interior Faraday rotation of the synthetic aperture time rate of change F of 15 minutes_vBe respectively 0.01 degree/second,When 0.05 spend/second, 0.1 degree/second, 0.2 degree/second, can know certain moment T_nThe echo-signal institute receivingThe Faraday anglec of rotation being subject to is F_v·T_n, and for all types of target, by formula (3) above, (4),(5) calculate it and be subject to accordingly the backscattering coefficient of Faraday Effect of Rotation. If known quantity is TEC,Can calculate the suffered Faraday anglec of rotation of echo-signal that certain moment receives by formula (2).

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

From showing 1-table 4: Faraday rotation changes the difference that affects on different target backscattering characteristic,Thereby on the difference that affects of SAR focal imaging. Along with the increase of synthetic aperture time, Faraday rotates changeChange more obvious on the impact of SAR focal imaging. The present invention is conceived to Faraday rotation and changes after targetTo the impact of scattering properties and SAR focal imaging, can analyze different Faraday rotation variations right simultaneouslyThe impact of different target backscattering characteristic and SAR focal imaging, simple possible.

Table 1-table 4:Faraday rotation changes different target backscattering characteristic and the shadow to SAR focal imagingRing.

The impact of spend/second on SAR imaging of table 1Faraday 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 2Faraday 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 3Faraday 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 4Faraday 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 description, is this areaTechnical staff can easily realize according to the description of this description and in conjunction with prior art, does not therefore doDescribe 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 change that can expect easilyChange or replace, within all should being encompassed in protection scope of the present invention.

Claims (2)

1. a SAR target back scattering analogy method that is subject to Faraday rotation variable effect, its feature existsIn, comprise the following steps:

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

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

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

In above formula, S is the matrix on 2 × 2 rank, S_hh、S_hv、S_vh、S_vvFor getting in four width complex patterns respectivelyThe multiple pixel value obtaining, h represents horizontal polarization, v represents vertical polarization;

The 3rd step, calculate according to formula below that each impulse ejection moment causes by ionized layer TECFaraday anglec of rotation Ω:

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

In formula, B_avFor earth's magnetic field is along the mean value on propagation path, f_cFor the carrier frequency transmitting, TEC is SARIonosphere total electron content in 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 target scattering spyThe impact of property, 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 $O=\left[\begin{array}{cc}{O}_{hh}& O_{hv}\\ O_{vh}& O_{vv}\end{array}\right],$ Wherein, hRepresent horizontal polarization, v represents vertical polarization;

The 5th step, basis formula below calculate and are subject to the back scattering observing of Faraday Effect of Rotation to beNumber, that is, and two backscattering coefficient P that same polarization passage observes_hh、P_vv, two cross polarization passages are seenThe backscattering coefficient P measuring_hv、P_vh：

P_hh＝O_hh，P_vv＝O_vv，P_vh＝O_vh，P_hv＝O_hv；

The 6th step, the SAR echo that the backscattering coefficient observing obtaining in the 5th step is brought into are below believedIn number expression formula, the echo-signal of this echo-signal expression formula statement is carried out to imaging, and imaging is tiedFruit is cooked corresponding analysis, and in following formula, X represents respectively P_hh、P_vh、P_hv、P_vv, to analyze respectively FaradayRotation changes the polarization to horizontal polarization emission level and receives image formation, horizontal polarization transmitting vertical polarization is connectReceive image formation, vertical polarization emission level polarization is received image formation, vertical polarization is launched to the vertical utmost pointChange the impact that receives image formation:

$\begin{array}{c}s\left(t,t_m\right)=X\·rect\[t-\frac{2R\left(t_m,R_B\right)}{c}\]\·w_a\left(t_m\right)\·w_r\[t-\frac{2R\left(t_m,R_B\right)}{c}\]\·\\ \exp \left\{j2{\mathrm{\πf}}_c\[t-\frac{2R\left(t_m,R_B\right)}{c}\]+j\mathrm{\π}\mathrm{\γ}{\[t-\frac{2R\left(t_m,R_B\right)}{c}\]}^2\right\}\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 directionFigure; f_cFor carrier frequency; γ is frequency modulation rate; R_BFor the minimum distance between target and SAR; R (t_m,R_B) be the momentt_mTime instantaneous oblique distance between SAR and target.

2. the SAR target back scattering mould that is subject to Faraday rotation variable effect according to claim 1Plan method, is characterized in that, in second step, loose in the polarization of obtaining target based on four width SAR complex patternsWhile penetrating matrix, the multiple pixel value of selected resolution cell resolution cell is around averaged, loose as polarizationPenetrate corresponding element in matrix S.