CN113534143B - Elevation space-variant error compensation method based on geosynchronous orbit synthetic aperture radar - Google Patents

Elevation space-variant error compensation method based on geosynchronous orbit synthetic aperture radar Download PDF

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CN113534143B
CN113534143B CN202110807471.3A CN202110807471A CN113534143B CN 113534143 B CN113534143 B CN 113534143B CN 202110807471 A CN202110807471 A CN 202110807471A CN 113534143 B CN113534143 B CN 113534143B
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李德鑫
常法光
董臻
张永胜
何志华
余安喜
黄洋
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National University of Defense Technology
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Abstract

The invention discloses an elevation space-variant error compensation method based on geosynchronous orbit synthetic aperture radar, which comprises the following steps of: s1, performing improved RD-ACS algorithm imaging on data acquired by the geosynthetic aperture radar system according to the known rough DEM information to obtain a rough focusing result; s2, dividing the coarse focusing image into a plurality of first sub-blocks Pi(ii) a S3, judging the first sub-block PiThe normalized amplitude at a certain pixel position is larger than the threshold g1And the amplitude of the pixel at this position is greater than the amplitudes at the surrounding positions and the first sub-block PiWhen the internal contrast is less than the threshold c1, the first sub-block P is divided into two sub-blocksiInto a plurality of second sub-blocks Pi,j(ii) a S4, judging the second sub-block Pi,jInternal contrast is less than threshold c2Then, the MD algorithm is adopted for compensation, and the second sub-block P is judgedi,jInternal contrast, if less than threshold c2Compensating by adopting a PGA algorithm; s5, for each first sub-block P according to the position in the coarse focusing resultiAnd a second sub-block Pi,jAnd carrying out image splicing. The method has wide application prospect in quality improvement and compensation of the high-resolution GEO SAR image.

Description

Elevation space-variant error compensation method based on geosynchronous orbit synthetic aperture radar
Technical Field
The invention relates to the technical field of microwave remote sensing, in particular to an elevation space-variant error compensation method based on geosynchronous orbit synthetic aperture radar.
Background
The GEO SAR system (i.e., geosynthetic aperture radar system) is an active radar located on a geosynchronous orbit, and the ultra-long synthetic aperture time (up to hundreds or even thousands of seconds) can ensure high resolution and realize long-term investigation of a specific area; the height of the orbit is about 36000Km, so that the observation range is wide, and the method is not influenced by geographical boundary conditions and meteorological conditions, and has important application value in the aerospace field and the microwave remote sensing field.
In the GEO SAR imaging process, due to the fact that the synthetic aperture time is long, the flight path is asymmetric about a zero Doppler plane, and the observed terrain elevation introduces space variation errors in the imaging process and increases along with the increase of the terrain elevation. The elevation space-variant error is caused by the fact that the real slope course of the target is different from the slope course of the corresponding point. Due to the elliptical orbit of GEO SAR, a target at a certain elevation cannot find a corresponding point with the same skew distance course, and the doppler modulation frequency in azimuth compression is related to the second derivative of the skew distance course. Therefore, due to the existence of elevation information, the doppler frequency modulation rate is mismatched during azimuth compression, so that azimuth defocusing occurs, and the readability and subsequent application of the high-resolution satellite-borne SAR image are affected.
In order to solve the problem of elevation space-variant errors introduced by elevation in the GEO SAR image, some researchers propose to perform imaging in a sub-aperture division manner. And dividing data in the whole synthetic aperture time into a plurality of sub-apertures, so that the satellite orbit can be approximately linear in each sub-aperture, thus the elevation space-variant error can be ignored in each sub-aperture, and finally, the imaging results in each sub-aperture are spliced. But the synthetic aperture time of each sub-aperture is short, the azimuth resolution is low, and the resolution of the image and more subsequent use are influenced. And the image splicing difficulty is also high, and slight splicing errors can cause serious influence in an image domain.
Disclosure of Invention
The invention aims to provide a method for compensating elevation space-variant errors based on geosynchronous orbit synthetic aperture radar, so as to overcome the defects in the prior art.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
an elevation space-variant error compensation method based on geosynchronous orbit synthetic aperture radar comprises the following steps:
s1, performing improved RD-ACS algorithm imaging on data acquired by the geosynthetic aperture radar system according to the known rough DEM information to obtain a rough focusing result;
s2, dividing the coarse focusing image in the coarse focusing result into a plurality of first sub-blocks PiAccording to the set amplitude threshold g1And a contrast threshold c1, each of the first sub-blocks P is preliminarily detectediWhether or not unfocused objects are contained;
s3, judging the first sub-block PiThe normalized amplitude at a certain pixel position is larger than the threshold g1And the amplitude of the pixel at this position is greater than the amplitudes at the surrounding positions and the first sub-block PiWhen the internal contrast is less than the threshold c1, the first sub-block P is divided into two sub-blocksiDivided into a plurality of second sub-blocks Pi,j；
S4, judging the second sub-block Pi，jInternal contrast is less than threshold c2Then, the MD algorithm is adopted for compensation, and the second sub-block P is judgedi,jInternal contrast, if less than threshold c2Compensating by adopting a PGA algorithm;
s5, for each first sub-block P according to the position in the coarse focusing resultiAnd a second sub-block Pi,jAnd splicing the images to obtain accurately focused image data.
Further, the step S1 specifically includes the following steps:
according to original echo data acquired by a geosynthetic aperture radar system, selecting a scene central point as a reference point, and obtaining a two-dimensional frequency spectrum of the target through a series inversion and stationary phase principle;
and carrying out conjugate multiplication on the obtained target two-dimensional frequency spectrum and the two-dimensional frequency spectrum of the reference point to complete consistent phase compensation to obtain a coarse focusing result.
Further, in the step S3, the first sub-block P is divided into two sub-blocksiInto a plurality of second sub-blocks Pi,jThe method of (1) is to use the first sub-block PiNumber of inner targets, constructing a second sub-block P around the targeti,j。
Further, the MD algorithm in the step S4 is used to compensate the second-order polynomial error.
Compared with the prior art, the invention has the advantages that: the method divides the complex scene into the inclined plane and the quick-change part by dividing the complex scene. For the imaging of the inclined plane, the elevation is decomposed into two directions of distance and direction, a high-order Taylor expansion slope distance model of a target is accurately fitted, the elevation space variation is converted into a two-dimensional space variation problem of the distance and the direction, and then the RD-ACS algorithm is used for imaging, so that the two-dimensional space variation error can be effectively compensated. For the rest part, contrast detection is carried out on the area where the target is located through sub-block division, subsequent self-focusing processing is carried out on the area with poor focusing effect, and the problem of elevation space-variant error is solved. Simulation experiment results show that the invention can compensate the influence of elevation space-variant errors and effectively improve the imaging quality of the GEOSAR system. The invention has wide application prospect in the quality improvement and compensation of high-resolution GEOSAR images.
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In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the embodiments or the technical solutions in the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
FIG. 1 is a flow chart of an elevation space-variant error compensation method based on geosynchronous orbit synthetic aperture radar according to the present invention.
Fig. 2 is a schematic diagram of sub-block division in the present invention.
FIG. 3 is a graph of elevation versus altitude for three performance parameters affected by elevation space-variant in accordance with the present invention.
FIG. 4(a) is a lattice scene; FIG. 4(b) is the imaging result by the modified RD-ACS algorithm; FIG. 4(c) is an azimuthal cross-section of two points taken away from the bevel; FIG. 4(d) is the imaging result after the autofocusing process; fig. 4(e) is an azimuthal cross-sectional view of the two points after precise focusing.
Detailed Description
The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings so that the advantages and features of the present invention can be more readily understood by those skilled in the art, and the scope of the present invention will be more clearly and clearly defined.
Referring to fig. 1 and fig. 2, the present embodiment discloses an elevation space variation error compensation method based on geosynchronous orbit synthetic aperture radar, which includes the following steps:
and step S1, performing improved RD-ACS algorithm imaging on the data acquired by the geosynthetic aperture radar system according to the known rough DEM information to obtain a rough focusing result.
Firstly, a scene center point is selected as a reference point for the obtained GEO SAR original echo data, and a two-dimensional spectrum of the target can be obtained through a series inversion and stationary phase (POSP) principle. Because the satellite flight trajectory is relatively complex in the GEO SAR, a fifth-order slant range model is adopted, namely:
R(η)＝R0+k1η+k2η2+k3η3+k4η4+k5η5 (1)
wherein R is0Represents the closest distance, k, of the object1～k5Coefficients representing a five-order taylor expansion slope distance model.
And transforming the echo to a two-dimensional frequency domain, and then performing conjugate multiplication with a two-dimensional frequency spectrum of a reference point to complete consistent phase compensation. In an inclined plane scene, each order coefficient in the slant range histories of different targets is related to the positions and elevations of the targets, the elevation direction can be decomposed into distance and azimuth directions by utilizing the relation between the elevations of the targets on the inclined plane and the position coordinates of the targets, and the slant range histories of the targets can be expressed to be only related to the positions of the targets. Namely:
Figure BDA0003167137330000031
wherein etacIndicating the azimuth time, Δ k, at which the beam center passes through the targeti(ηc,r0) Indicating the space-variant coefficient, Δ k, in relation to position (distance and orientation)j,h(ηc,r0) The elevation-dependent space-variant coefficients are represented, wherein fourth-order and fifth-order coefficients are negligible compared to third-order coefficients. Delta k'i(ηc,r0) Representing the space-variant coefficients that are ultimately only position dependent. Since the first order coefficients have no influence on the imaging focusing effect, and the fourth order and fifth order coefficients are negligible, delta k'i(ηc,r0) Only second and third orders are considered, which can be expressed as:
Δk’i(ηc，r0)≈k’i,r·(r0-r0,c)+k’i,a(η0-η0,c)+k’i,ra(r0-r0,c)·(η0-η0,c)，i＝2，3 (3)
from the above analysis, the higher order model coefficients are changed with the distance direction, and therefore, Δ k is available in the range migration term and the azimuth compression termi+k’i，r·(r0-r0，c) In place of Δ kiAnd the distance space-variant compensation can be completed in the range-Doppler domain. And after the distance space-variant compensation, converting the data into a two-dimensional time domain. Because the azimuth compression phase changes with both azimuth time and azimuth frequency, the azimuth-time varying compensation cannot be performed directly in the range-doppler domain as the range-space varying compensation. Since only the second and third order terms of the time-varying term need to be considered, the function is introduced by frequency modulation scaling
Figure BDA0003167137330000041
Let η be0，c0 or (η - η)0) And expanding, the slope model is:
Figure BDA0003167137330000042
to implement an azimuthally invariant slope model, η0Should be set to zero, i.e.:
Figure BDA0003167137330000043
the coefficients in the scaling function can be obtained through the formula. After the azimuth time domain compensation is completed, data are converted to a distance Doppler domain, errors introduced by a scaling equation and azimuth time-varying azimuth compression phases are calculated through a series inversion and stationary phase principle, and the frequency domain compensation phase is as follows:
Figure BDA0003167137330000044
wherein the anchor point is:
Figure BDA0003167137330000045
multiplying the phase in the range-doppler domain completes the first step of coarse focusing.
Step S2, dividing the coarse focusing image in the coarse focusing result into a plurality of first sub-blocks PiAccording to the set amplitude threshold g1And a contrast threshold c1, each of the first sub-blocks P is preliminarily detectediWhether it contains an unfocused target.
Step S3, judging the first sub-block PiThe normalized amplitude at a certain pixel position is larger than the threshold g1And the amplitude of the pixel at the position is larger than the amplitude of the surrounding positions and the first sub-pixelBlock PiWhen the internal contrast is less than the threshold c1, it indicates that there is an unfocused target in the sub-block, as shown in equations (8) and (9), and then finds the first sub-block PiThe number of inner targets (each target having the largest amplitude around it), centered on the target, constitutes a small second sub-block P in its vicinityi,jAs shown in fig. 3. Then in the second sub-block Pi,jAnd carrying out second contrast judgment in the region.
Figure BDA0003167137330000051
Figure BDA0003167137330000052
Where J (q, k) represents the normalized amplitude at a certain pixel position, NiRepresents the first sub-block PiThe number of inner objects, representing the contrast within the sub-block.
Step S4, judging the second sub-block Pi,jInternal contrast is less than threshold c2And then, the MD algorithm is adopted for compensation, because the MD algorithm has no requirement on strong points, the overall compensation effect is better, and the second sub-block P is judgedi,jInternal contrast, if less than threshold c2And then, the focusing effect is still poor, the PGA algorithm is adopted for compensation, high-order errors can be compensated, and the MD algorithm can only compensate second-order polynomial errors.
Step S5, for each first sub-block P according to the position in the coarse focusing resultiAnd a second sub-block Pi,jAnd splicing the images to obtain the SAR image data with accurate focusing.
Table 1 below is the GEO SAR system parameters set in the simulation. Including semi-major axis, eccentricity, track tilt, carrier frequency, squint angle, pulse duration, elevation-crossing right ascension, perigee amplitude, true perigee angle, antenna size, angle of incidence, and signal bandwidth.
TABLE 1
Figure BDA0003167137330000053
Figure BDA0003167137330000061
The invention is utilized to carry out the two-step imaging processing on a lattice scene under the parameters shown in the table 1.
Fig. 2 is a schematic illustration of the sub-block division, showing the first sub-block PiTargets with several focusing differences, for the second sub-block P where each target is locatedi,jThe self-focusing processing is carried out, a curve of performance parameter main lobe broadening of a point target along with the height is given in fig. 3a, a curve of performance parameter PSLR (peak side lobe ratio) of the point target along with the height is given in fig. 3b, and a curve of performance parameter ISLR (integral side lobe ratio) of the point target along with the height is given in fig. 3c, and it can be seen that the focusing performance is worse along with the increase of the height.
FIG. 4 shows a lattice scene with two points away from the slope, and FIG. 4(a) shows a schematic diagram of the lattice scene; FIG. 4(b) is the result of coarse focusing with the improved RD-ACS algorithm; FIG. 4(c) shows an azimuthal cross-section of these two points; FIG. 4(d) shows the fine focusing result after being processed by the block MD-PGA algorithm; FIG. 4(e) is an azimuthal cross-section of two points. By the method, the GEOSAR image defocusing is obviously inhibited, and the image quality is obviously improved. Meanwhile, the method is not only useful for GEOSAR, but also useful for other high-resolution SAR with asymmetric satellite trajectories about a zero Doppler plane.
Although the embodiments of the present invention have been described with reference to the accompanying drawings, various changes or modifications may be made by the patentees within the scope of the appended claims, and within the scope of the invention, as long as they do not exceed the scope of the invention described in the claims.

Claims (4)

1. The elevation space-variant error compensation method based on the geosynchronous orbit synthetic aperture radar is characterized by comprising the following steps of:
s1, performing improved RD-ACS algorithm imaging on data acquired by the geosynthetic aperture radar system according to the known rough DEM information to obtain a rough focusing result;
s2, dividing the coarse focusing image in the coarse focusing result into a plurality of first sub-blocks PiAccording to the set amplitude threshold g1And a contrast threshold c1, each of the first sub-blocks P is preliminarily detectediWhether or not unfocused objects are contained;
s3, judging the first sub-block PiThe normalized amplitude at a certain pixel position is larger than the threshold g1And the amplitude of the pixel at this position is greater than the amplitudes at the surrounding positions and the first sub-block PiWhen the internal contrast is less than the threshold c1, the first sub-block P is divided into two sub-blocksiInto a plurality of second sub-blocks Pi,j；
S4, judging the second sub-block Pi，jInternal contrast is less than threshold c2Then, the MD algorithm is adopted for compensation, and the second sub-block P is judgedi,jInternal contrast, if less than threshold c2Compensating by adopting a PGA algorithm;
s5, for each first sub-block P according to the position in the coarse focusing resultiAnd a second sub-block Pi,jAnd splicing the images to obtain accurately focused image data.
2. The elevation space-variant error compensation method based on geosynchronous orbit synthetic aperture radar of claim 1, wherein the step S1 specifically comprises the following steps:
according to original echo data acquired by a geosynthetic aperture radar system, selecting a scene central point as a reference point, and obtaining a two-dimensional frequency spectrum of the target through a series inversion and stationary phase principle;
and carrying out conjugate multiplication on the obtained target two-dimensional frequency spectrum and the two-dimensional frequency spectrum of the reference point to complete consistent phase compensation to obtain a coarse focusing result.
3. The geosynchronous orbit-based joint of claim 1The elevation space-variant error compensation method of the aperture-forming radar is characterized in that the first sub-block P is processed in step S3iInto a plurality of second sub-blocks Pi,jThe method of (1) is to use the first sub-block PiNumber of inner targets, constructing a second sub-block P around the targeti,j。
4. The geosynchronous orbit synthetic aperture radar-based elevation space-variant error compensation method according to claim 2, wherein the MD algorithm in the step S4 is used for compensating a second-order polynomial error.
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