CN106019279A - Ionosphere space-variant effect influence determining method in spaceborne SAR orientation imaging - Google Patents

Abstract

The invention provides an ionosphere space-variant effect influence determining method in spaceborne SAR orientation imaging. The technical scheme is as follows: by taking spaceborne system parameters as input, step one, for any target point p of an observation scene, ionosphere puncture point coordinates (x<i,tn>, y<i,tn>, z<i>) of an orientation slow time point t0 are calculated; step two, for all the ionosphere puncture point coordinates (x<i,tn>, y<i,tn>, z<i>), by use of an online IRI model, an STEC(x<i,tn>, y<i,tn>, z<i>, t0) of the t0 time point is obtained; step three, an average space-variant rate delta STEC of the ionosphere STEC along an orientation is calculated; and step four, the delta STEC and a threshold are determined, and a determining result is obtained. The ionosphere space-variant effect influence determining method in the spaceborne SAR orientation imaging, brought forward by the invention, gives a determining threshold and a determining method, and is widely applied to spaceborne SAR ionosphere influence analysis and correction processing.

Description

Ionosphere space-variant effect in the imaging of satellite-borne SAR orientation affects decision method

Technical field

The invention belongs to the interleaving techniques field that space flight is combined with microwave remote sensing, particularly to a kind of satellite-borne SAR The shadow of the ionosphere space-variant effect in (Synthetic Aperture Radar, synthetic aperture radar) orientation imaging Ring decision method.

Background technology

Growing along with space technology and microwave sounding technology, important as space exploration of satellite-borne SAR Means, play the most important effect.Currently, band limits and the resolution capability of satellite-borne SAR enters one Step extension, function further enhances.Wherein, the high-resolution satellite-borne SAR being operated in low-frequency range is main sending out One of exhibition direction.The high-resolution satellite-borne SAR of low-frequency range has good penetration performance, can either find hidden Military target, be also widely used for forest biomass inverting.But, along with operating frequency reduction and point The raising of resolution, ionospheric effect is the most serious on the impact of its imaging performance.

For the satellite-borne SAR of middle low resolution, the radar beam in the synthetic aperture time is on ionosphere Inswept length is less, it is generally recognized that in the range of Gai, ionosphere approximation be constant.But it is it is true that ionospheric Form, structure and dielectric property can be different and change with locus, i.e. have space-variant effect.To low-frequency range For high-resolution satellite-borne SAR, the length that the radar beam in the synthetic aperture time is inswept on ionosphere is permissible Reaching the tens the most tens of kms of km, the most ionospheric space-variant effect can reduce satellite-borne SAR echo-signal Coherence, and then the decline of its orientation image quality may be caused.

The impact of satellite-borne SAR orientation imaging is embodied a concentrated reflection of at STEC (Slant Total by ionosphere space-variant effect Electron Content, oblique distance is to total electron amount) in change within the synthetic aperture time, the most not yet find Affect decision method accordingly.

Summary of the invention

It is an object of the invention to: propose a kind of ionosphere space-variant effect and the impact of satellite-borne SAR orientation imaging is sentenced Certainly method, can apply to satellite-borne SAR ionosphere effect analysis and correction process.

The technical scheme is that

The systematic parameter of known satellite-borne SAR includes: carrier frequency f_c, synthetic aperture time T_s, azimuth resolution ρ_a, in observation scene, the coordinate under arbitrary target points P sky coordinate system northeastward is (x₀,y₀,z₀), this point is corresponding Synthetic aperture central instant be t₀, satellite-borne SAR in orientation to slow time t_nSky, the northeast coordinate system rail in moment Road coordinate data isThe value of n be arbitrary integer and | t_n-t₀|≤T_s/2.Ionospheric height Degree is z_i。

The first step, for arbitrary target points P in observation scene, utilizes space line equation, is calculated orientation To slow time t_nThe satellite-borne SAR wave beam in moment sky, northeast coordinate system point of puncture (Ionospheric on ionosphere Penetration Point, IPP) coordinate

$x_{i,t_n}=\frac{z_i-z_0}{z_{t_n}-z_0}\&CenterDot;(x_{t_n}-x_0)+x_0$

$y_{i,t_n}=\frac{z_i-z_0}{z_{t_n}-z_0}\&CenterDot;(y_{t_n}-y_0)+y_0$

Second step, for all of point of puncture coordinateUtilize online IRI (International Reference Ionosphere, international reference ionosphere) model acquisition t₀The ionosphere in moment vertically to TEC (Total Electron Content, total electron amount) value TECRecycling following formula obtains The STEC value of space-variant:

$STEC(x_{i,t_n},y_{i,t_n},z_i,t_0)=TEC(x_{i,t_n},y_{i,t_n},z_i,t_0)\&CenterDot;\mathrm{\&gamma;}\left(t_0\right)$

Wherein, geometric transformation factor gamma (t₀) it is:

$\mathrm{\&gamma;}\left(t_0\right)=\frac{\sqrt{{(x_{i,t_0}-x_0)}^2+{(y_{i,t_0}-y_0)}^2+{(z_i-z_0)}^2}}{z_i-z_0}$

3rd step, calculate ionosphere STEC along orientation to mean space rate of change Δ STEC:

$\mathrm{\&Delta;}STEC=E\left\{\frac{STEC(x_{i,t_{n+1}},y_{i,t_{n+1}},z_i,t_0)-STEC(x_{i,t_n},y_{i,t_n},z_i,t_0)}{\sqrt{{(x_{i,t_{n+1}}-x_{i,t_n})}^2+{(y_{i,t_{n+1}}-y_{i,t_n})}^2}}\right\}$

Wherein, E{ } represent be averaging.

4th step, if following inequality is set up simultaneously,

$\left|\mathrm{\&Delta;}STEC\right|\&le;\frac{f_c^2}{2{\mathrm{Kz}}_i}\&CenterDot;{\mathrm{\&rho;}}_a$

The impact of ionosphere space-variant effect is ignored, wherein time then on the orientation imaging observing arbitrary target points P in scene K=40.28m³/s²；Otherwise, it is necessary to consider that it affects.

Use the present invention desirable following technique effect:

The present invention is directed to satellite-borne SAR orientation imaging processing, it is proposed that the impact judgement side of ionosphere space-variant effect Method, gives decision threshold and decision method, has in correction process in satellite-borne SAR ionosphere effect analysis Extensively application.

Accompanying drawing explanation

Fig. 1 is the ionosphere space-variant effect impact judgement flow process in the satellite-borne SAR orientation imaging that the present invention provides Figure；

Fig. 2 is satellite-borne SAR and ionosphere geometrical relationship schematic diagram；

Fig. 3 is that the ionosphere with spatial position change is vertically to TEC data；

Fig. 4 is ionosphere space-variant effect satellite-borne SAR orientation imaging results when exceeding decision threshold；

Fig. 5 is that ionosphere space-variant effect is not less than satellite-borne SAR orientation imaging results during decision threshold.

Detailed description of the invention

The method provided the present invention below in conjunction with accompanying drawing is described in detail.

Ionosphere space-variant effect impact judgement flow process in the satellite-borne SAR orientation imaging that Fig. 1 provides for the present invention Figure, is always divided into four steps.With Spaceborne SAR System parameter for input, the first step: appoint in observation scene Meaning impact point P, is calculated orientation to slow time t_nThe ionosphere point of puncture coordinate in momentThe Two steps: for all of ionosphere point of puncture coordinateOnline IRI model is utilized to obtain t₀Moment STEC3rd step: calculate ionosphere STEC along orientation to mean space rate of change ΔSTEC；4th step: judge the size of Δ STEC and threshold value, obtains court verdict.

Fig. 2 is satellite-borne SAR and ionospheric geometrical relationship schematic diagram.Wherein, O is the earth's core, and P is observation Arbitrary target points in scene, the fine line above figure is the running orbit of satellite-borne SAR, and thick dashed line represents and takes out As the ionosphere for thin screen, black circle IPP represents t_nThe satellite-borne SAR in moment is to the radar wave of impact point P Bundle point of puncture on ionosphere, heavy line is the point of puncture track in the synthetic aperture time, Z_iFor ionosphere Highly, span is usually 350～400km.

Ionospheric space-variant effect mainly show as STEC along orientation to spatial variations, can be characterized as Δ STEC, its unit is TECU/km.This effect can introduce orientation to linear phase error, and then causes into The orientation of picture point is to translation:

${\mathrm{\&Delta;L}}_S=\frac{K\&CenterDot;z_i}{f_c^2}\&CenterDot;\mathrm{\&Delta;}STEC$

Generally orientation is to translation Δ L_S≤ρ_a/ 2 impacts that can ignore space-variant effect, and then obtain the decision gate of Δ STEC Limit:

$\left|\mathrm{\&Delta;}STEC\right|\&le;\frac{f_c^2}{2{\mathrm{Kz}}_i}\&CenterDot;{\mathrm{\&rho;}}_a$

Fig. 3 is that the ionosphere with spatial position change utilizing online IRI model to obtain is vertically to TEC data. 6 points 0 second when observation time is 7 days 7 November in 2013, observation scope is: north latitude 13.824～22.776 Degree, east longitude 104.585～114.015 degree, layer height z_i=400km.Figure is given electricity in observation scope Absciss layer is vertically to the equal pitch contour of TEC, it can be seen that in different locus, and ionosphere is vertically to TEC Value also differs, and maximum fluctuating can reach tens TECU.In following l-G simulation test, be given with Fig. 3 Space-variant ionized layer TEC as input.

Fig. 4 is ionosphere space-variant effect satellite-borne SAR orientation imaging results when exceeding decision threshold.Emulation ginseng Number is set to: f_c=1.25GHz, ρ_a=4.45m, therefore the decision threshold of | Δ STEC | is 3.1×10^-3TECU/km.In intercepting space-variant ionized layer TEC value shown in Fig. 3, one piece of data is as emulation Input, analyzes this segment data and obtains Δ STEC=1.04 × 10^-2TECU/km.The present invention is utilized to make decisions Result be need consider space-variant effect.For the ease of contrast, Fig. 4 gives simultaneously and does not comprises ionosphere sky Change effect (ideal situation, dotted line represents) and comprise ionosphere space-variant effect (practical situation, solid line represents) Simulation imaging result in the case of two kinds.By Fig. 4 it is found that owing to the space-variant effect of this segment data compares Seriously, | Δ STEC | has exceeded decision threshold, and compared to ideal situation, satellite-borne SAR orientation is to imaging results Occur in that obvious picture point offsets (deviant: 7.66m).

Fig. 5 is that ionosphere space-variant effect is not less than satellite-borne SAR orientation imaging results during decision threshold.Emulation Parameter arranges the decision threshold of identical with Fig. 4, therefore | Δ STEC | and keeps constant.Additionally intercept shown in Fig. 3 In space-variant ionized layer TEC value, one piece of data is as emulation input, analyzes this segment data and obtains Δ STEC=1.5 × 10^-3TECU/km.The result utilizing the present invention to make decisions is to imitate without the concern for space-variant Should.Similarly, Fig. 5 gives simultaneously and does not comprises ionosphere space-variant effect (ideal situation, dotted line represents) With the simulation imaging result comprised in the case of ionosphere space-variant effect (practical situation, solid line represents) two kinds. By Fig. 5 it is found that owing to the space-variant effect of this segment data is less obvious, | Δ STEC | less than decision threshold, Compared to ideal situation, satellite-borne SAR orientation only has slight picture point skew (deviant: 1.09m) to imaging.

Claims (1)

1. the ionosphere space-variant effect in the imaging of satellite-borne SAR orientation affects a decision method, and SAR refers to Synthetic aperture radar, it is known that the systematic parameter of satellite-borne SAR includes: carrier frequency f_c, synthetic aperture time T_s, side Position is to resolution ρ_a；In observation scene, the coordinate under arbitrary target points P sky coordinate system northeastward is (x₀,y₀,z₀), The synthetic aperture central instant that this point is corresponding is t₀；Satellite-borne SAR in orientation to slow time t_nThe sky, northeast in moment Coordinate system orbit coordinate data areThe value of n be arbitrary integer and | t_n-t₀|≤T_s/2；Electricity The height of absciss layer is z_i；It is characterized in that, comprise the steps:

The first step, for arbitrary target points P in observation scene, utilizes space line equation, is calculated orientation To slow time t_nThe satellite-borne SAR wave beam in moment sky, northeast coordinate system point of puncture coordinate on ionosphere

$x_{i,t_n}=\frac{z_i-z_0}{z_{t_n}-z_0}\&CenterDot;(x_{t_n}-x_0)+x_0$

$y_{i,t_n}=\frac{z_i-z_0}{z_{t_n}-z_0}\&CenterDot;(y_{t_n}-y_0)+y_0$

Second step, for all of point of puncture coordinateUtilize online international reference ionosphere mould Type obtains the ionosphere in t0 moment vertically to TEC total electron amount valueRecycling Following formula obtains space-variantValue:

$STEC(x_{i,t_n},y_{i,t_n},z_i,t_0)=TEC(x_{i,t_n},y_{i,t_n},z_i,t_0)\&CenterDot;\mathrm{\&gamma;}\left(t_0\right)$

Wherein, geometric transformation factor gamma (t₀) it is:

$\mathrm{\&gamma;}\left(t_0\right)=\frac{\sqrt{{(x_{i,t_0}-x_0)}^2+{(y_{i,t_0}-y_0)}^2+{(z_i-z_0)}^2}}{z_i-z_0}$

3rd step, calculate ionosphere STEC along orientation to mean space rate of change Δ STEC:

$\mathrm{\&Delta;}STEC=E\left\{\frac{STEC(x_{i,t_{n+1}},y_{i,t_{n+1}},z_i,t_0)-STEC(x_{i,t_n},y_{i,t_n},z_i,t_0)}{\sqrt{{(x_{i,t_{n+1}}-x_{i,t_n})}^2+{(y_{i,t_{n+1}}-y_{i,t_n})}^2}}\right\}$

Wherein, E{ } represent be averaging；

4th step, if following inequality is set up,

$\left|\mathrm{\&Delta;}STEC\right|\&le;\frac{f_c^2}{2{\mathrm{Kz}}_i}\&CenterDot;{\mathrm{\&rho;}}_a$

The impact of ionosphere space-variant effect is ignored, wherein time then on the orientation imaging observing arbitrary target points P in scene K=40.28m³/s²；Otherwise, it is necessary to consider that it affects.