CN115166656A - Estimation compensation method and device for frequency stepping SAR (synthetic aperture radar) combined ionosphere and troposphere - Google Patents

Estimation compensation method and device for frequency stepping SAR (synthetic aperture radar) combined ionosphere and troposphere Download PDF

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CN115166656A
CN115166656A CN202210792201.4A CN202210792201A CN115166656A CN 115166656 A CN115166656 A CN 115166656A CN 202210792201 A CN202210792201 A CN 202210792201A CN 115166656 A CN115166656 A CN 115166656A
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distance
frequency domain
troposphere
frequency
image
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肖枫
王新民
孙晗伟
李炳沂
董威
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Beijing Institute of Radio Measurement
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
    • G01S7/40—Means for monitoring or calibrating
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/88—Radar or analogous systems specially adapted for specific applications
    • G01S13/89—Radar or analogous systems specially adapted for specific applications for mapping or imaging
    • G01S13/90—Radar or analogous systems specially adapted for specific applications for mapping or imaging using synthetic aperture techniques, e.g. synthetic aperture radar [SAR] techniques
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
    • G01S7/41—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00 using analysis of echo signal for target characterisation; Target signature; Target cross-section

Abstract

The invention discloses a method and a device for estimating and compensating a frequency stepping SAR (synthetic aperture radar) combined ionosphere and troposphere. The method comprises the following steps: ionosphere estimation compensation is carried out based on the maximum contrast when the subband echo synthesis distance image is displayed; carrying out grid division on an imaging scene, and determining a projection distance and a delay projection distance of each grid in the imaging scene; carrying out back projection on the distance image according to the projection distance to obtain an SAR image, and determining the contrast of the SAR image; and updating the troposphere slope distance for multiple times, respectively determining an SAR image and contrast corresponding to the troposphere slope distance after each updating, and selecting an SAR image corresponding to the maximum value of the contrast. The ionosphere estimation compensation is carried out in the distance direction when the distance direction image is synthesized by the sub-band echoes, and the troposphere estimation compensation is carried out when the SAR image is generated by the distance direction image projection, namely, the ionosphere and troposphere influence is combined to carry out estimation compensation, so that the SAR image with good focus can be obtained.

Description

Estimation compensation method and device for frequency stepping SAR (synthetic aperture radar) combined ionosphere and troposphere
Technical Field
The invention relates to the technical field of synthetic aperture radars, in particular to a method and a device for estimating and compensating a frequency stepping SAR in combination with an ionosphere and a troposphere.
Background
Synthetic Aperture Radar (SAR) is a full-time and all-weather high-resolution microwave remote sensing imaging radar, and can be installed on flight platforms such as airplanes and satellites. The method has unique advantages in the aspects of environmental monitoring, marine observation, resource exploration, crop estimation, mapping, military affairs and the like, and can play a role which is difficult to play by other remote sensing means. For a satellite-borne SAR system, the imaging of the satellite-borne SAR with the resolution of the X wave band close to 0.1m is usually realized by frequency stepping, and the system works in a sliding bunching mode, so that the influence of non-ideal factors such as an ionosphere, a troposphere and the like on the sliding bunching imaging quality cannot be ignored.
The prior art solution is only applicable in the case where there is one effect (i.e. only ionosphere effect or only troposphere effect), and does not consider estimation and compensation when both have an effect on SAR imaging quality. Generally, non-ideal factors (ionosphere and troposphere) couple the influence of high-resolution frequency stepping SAR imaging, and only considering one influence cannot acquire an SAR image with good focus. How to acquire an SAR image with good focus becomes a technical problem to be solved urgently.
Disclosure of Invention
The invention aims to solve the technical problem in the prior art and provides a method and a device for estimating and compensating a frequency stepping SAR (synthetic aperture radar) combined ionosphere and troposphere.
In order to solve the technical problem, the invention provides a frequency stepping SAR combined ionosphere and troposphere estimation compensation method, which comprises the following steps: ionosphere estimation compensation is carried out based on the maximum contrast when the subband echo synthesis distance image is displayed; carrying out mesh division on an imaging scene, determining the projection distance of each mesh in the imaging scene, and determining the delay projection distance of each mesh according to the troposphere slant distance; according to the projection distance and the delay projection distance, carrying out back projection on each grid of an imaging scene on the distance direction image to obtain an SAR image, and determining the contrast of the SAR image; and updating the troposphere slope distance for multiple times, respectively determining the SAR image and the contrast corresponding to the troposphere slope distance after each updating, and selecting the SAR image corresponding to the maximum contrast value when the updating times of the troposphere slope distance meet a first preset condition.
In order to solve the above technical problem, the present invention further provides a device for estimating and compensating ionosphere and troposphere by combining frequency-stepped SAR, comprising: the distance direction image synthesis module is used for carrying out ionosphere estimation compensation based on the maximum contrast when the sub-band echo synthesis distance direction image is synthesized; the projection distance determining module is used for carrying out grid division on an imaging scene, determining the projection distance of each grid in the imaging scene and the delayed projection distance of each grid in the imaging scene caused by troposphere delay;
the back projection module is used for carrying out back projection on each grid of an imaging scene on the distance direction image according to the projection distance and the delay projection distance to obtain an SAR image and determining the contrast of the SAR image; and the SAR image selecting module is used for updating the troposphere slope distance for multiple times, respectively determining the SAR image and the contrast corresponding to the troposphere slope distance after each updating, and selecting the SAR image corresponding to the maximum contrast value when the updating times of the troposphere slope distance meet a first preset condition.
In order to solve the above technical problem, the present invention further provides a device for estimating and compensating a frequency-stepped SAR for combined ionosphere and troposphere, which includes a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the method for estimating and compensating a frequency-stepped SAR for combined ionosphere and troposphere according to the above technical solution when executing the program.
To solve the above technical problem, the present invention also provides a computer-readable storage medium, which includes instructions that, when executed on a computer, cause the computer to execute the frequency-stepped SAR combined ionosphere and troposphere estimation compensation method according to the above technical solution.
The beneficial effects of the invention are: the invention considers the ionosphere influence and the troposphere influence simultaneously, carries out ionosphere estimation compensation in the distance direction when the subband echo synthesizes the distance direction image, and carries out troposphere estimation compensation when the SAR image is generated by projecting the distance direction image, namely, the invention carries out estimation compensation by combining the ionosphere and the troposphere influence, and can obtain the SAR image with good focus.
Additional aspects of the invention and its advantages will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.
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Fig. 1 is a flowchart of a method for estimating and compensating an ionosphere and a troposphere by combining a frequency-stepped SAR according to an embodiment of the present invention;
FIG. 2 is a flow chart of a subband synthesis algorithm incorporating ionospheric phase compensation according to an embodiment of the present invention;
FIG. 3 is a geometric schematic diagram of the propagation of an spaceborne SAR signal in the troposphere;
FIG. 4 is a diagram showing the variation of the atmospheric refractive index with altitude;
FIG. 5 is a schematic diagram of the variation of the slope distance introduced by atmospheric refraction;
fig. 6 is a graph of ionospheric phase estimation results of a maximum contrast-based self-focusing algorithm according to an embodiment of the present invention;
FIG. 7 is a graph of a sliding spotlight mode surface target imaging result without error estimation compensation;
FIG. 8 is a diagram of a target imaging result of a sliding spotlight mode surface after error estimation compensation by using the method of the embodiment of the invention.
Detailed Description
The embodiments of the present disclosure are described below with specific examples, and other advantages and effects of the present disclosure will be readily apparent to those skilled in the art from the disclosure in the specification. It is to be understood that the described embodiments are merely a subset of the disclosed embodiments and not all embodiments. The disclosure may be carried into practice or applied to various other specific embodiments, and various modifications and changes may be made in the details within the description and the drawings without departing from the spirit of the disclosure. It is to be noted that the features in the following embodiments and examples may be combined with each other without conflict. All other embodiments, which can be derived by a person skilled in the art from the embodiments disclosed herein without making any creative effort, shall fall within the protection scope of the present disclosure.
It is noted that various aspects of the embodiments are described below within the scope of the appended claims. It should be apparent that the aspects described herein may be embodied in a wide variety of forms and that any specific structure and/or function described herein is merely illustrative. Based on the disclosure, one skilled in the art should appreciate that one aspect described herein may be implemented independently of any other aspects and that two or more of these aspects may be combined in various ways. For example, an apparatus may be implemented and/or a method practiced using any number of the aspects set forth herein. Additionally, such an apparatus may be implemented and/or such a method may be practiced using other structure and/or functionality in addition to one or more of the aspects set forth herein.
Fig. 1 is a flowchart of a method for estimating and compensating an ionosphere and a troposphere by combining a frequency-stepped SAR according to an embodiment of the present invention. As shown in fig. 1, the method includes:
s1, ionosphere estimation compensation is carried out based on maximum contrast when a distance map image is synthesized by sub-band echoes.
And S2, carrying out grid division on the imaging scene, and determining the projection distance of each grid in the imaging scene. The size of an imaging scene is the breadth, the value is prior information, and the size of a division grid is generally 0.8 times of the resolution ratio.
S3, determining delay projection distances of each grid according to troposphere slant distances; in particular, the delayed projection distance due to tropospheric delay at each mesh can be determined from the difference between the true refraction slope and the no-refraction slope.
S4, according to the projection distance and the delay projection distance, carrying out back projection on each grid of an imaging scene on the distance direction image to obtain an SAR image, and determining the contrast of the SAR image;
and S5, updating the troposphere slope distance for multiple times, respectively determining the SAR image and the contrast corresponding to the troposphere slope distance after each updating, if the updated troposphere slope distance meets the first preset condition, returning to the S3 if the updated troposphere slope distance does not meet the first preset condition, and if the updated troposphere slope distance meets the first preset condition, selecting and outputting the SAR image corresponding to the maximum contrast value.
Wherein, the tropospheric slope distance is updated for a plurality of times by adopting a fixed interval method or a dichotomy method. When the sampling fixed interval method is used for updating, the updating times adopt preset updating times; and when the sampling bisection method is updated, determining the updating times according to the set precision.
Frequency stepping is a radar waveform that has been widely used in high resolution radars in recent years. It emits a set of pulsed signals of different carrier frequencies, where the carrier frequencies of the signals are typically linearly stepped frequencies, and high resolution range images are acquired by range image synthesis or bandwidth synthesis. Since the bandwidth of each pulse is much smaller than the total bandwidth of the composite, the instantaneous bandwidth of the system receiver only needs to meet the bandwidth of the pulse rather than the total bandwidth of the composite, which greatly reduces the instantaneous bandwidth of the system. Compared with the method of directly generating a broadband signal to realize high-resolution range profile, the frequency stepping signal has the advantages of small instantaneous bandwidth, low hardware requirement, low cost and the like, and is widely applied to a high-resolution SAR system. In addition, the sliding beam bunching mode is between the strip mode and the bunching mode, and high-resolution wide-range observation can be simultaneously realized by controlling the rotation rate and the working time of the beam. At present, the sliding beam bunching mode is widely applied to a plurality of satellite-borne SAR systems at home and abroad, such as the SAR satellites of Capella, terrasAR-X, radarsat2, RCM, GF-3 and the like.
The embodiment of the invention considers the ionosphere influence and the troposphere influence simultaneously, carries out ionosphere estimation compensation in the distance direction when the subband echo synthesizes the distance direction image, and carries out troposphere estimation compensation when the distance direction image is projected to generate the SAR image, namely, the invention carries out estimation compensation by combining the ionosphere and troposphere influence, and can obtain the SAR image with good focus.
Optionally, in an embodiment, the performing ionosphere estimation compensation based on maximum contrast when synthesizing the range map image by using subband echo includes:
s11, respectively carrying out distance Fourier transform on each sub-band echo to obtain a frequency domain signal of each sub-band echo;
s12, distance direction correction and frequency domain up-sampling processing are carried out on the frequency domain signals of the echoes of the sub-bands;
s13, performing minimum slant range phase and ionosphere phase compensation on each frequency domain signal subjected to distance direction correction and frequency domain up-sampling processing;
multiplying each frequency domain signal subjected to distance direction correction and frequency domain up-sampling processing by the compensated minimum slant distance phase and the compensated ionospheric phase;
the minimum skew phase of the compensation is as follows:
Figure BDA0003730684550000061
wherein j is an imaginary unit, N =1,2, …, N, N is the number of hopping points, Δ f is the hopping interval, R is min Is the initial sampling distance, c is the speed of light;
the compensated ionospheric phases are:
Figure BDA0003730684550000062
Figure BDA0003730684550000063
wherein the content of the first and second substances,
Figure BDA0003730684550000064
is the ionospheric phase, K is a constant, alpha is the radar down-view angle, TEC is the total electron content in the ionospheric layer, f c (n) is the carrier center frequency of the nth frequency point, f r Is the range frequency.
S14, carrying out frequency spectrum synthesis processing on each frequency domain signal subjected to phase compensation to obtain a synthesized frequency spectrum signal;
s15, performing range-direction inverse Fourier transform on the synthesized frequency spectrum signal to obtain a range-direction image, and determining the contrast of the range-direction image;
s16, updating the ionospheric phase for multiple times, respectively determining a distance direction image and a contrast corresponding to each updated ionospheric phase, and selecting the distance direction image corresponding to the maximum contrast value when the ionospheric phase updating times meet a second preset condition.
Wherein, the tropospheric slope distance is updated for a plurality of times by adopting a fixed interval method or a dichotomy method. When the sampling fixed interval method is used for updating, the updating times adopt preset updating times; and when the sampling bisection method is updated, determining the updating times according to the set precision.
The embodiment of the invention realizes efficient ionospheric phase estimation on the frequency stepping SAR.
Optionally, in an embodiment, the separately performing distance-direction correction and frequency-domain upsampling processing on the frequency-domain signal of each subband echo includes:
and S121, respectively carrying out matched filtering and time shift processing on the frequency domain signals of the sub-band echoes.
Specifically, multiplying the frequency domain signal of each subband echo by a window function, a matched filtering function and a time shifting function;
the window function is
Figure BDA0003730684550000071
The matched filter function is
Figure BDA0003730684550000072
The time shift function is
Figure BDA0003730684550000073
Wherein f is r Is the distance frequency, Δ f is the hop interval, j is the imaginary unit, K r Is the distance direction chirp slope, T p Is the pulse width, t d (N) is the delay of the nth subband signal relative to the first subband signal, N =1,2, …, N being the number of hop-points.
And S122, performing frequency domain up-sampling on each frequency domain signal subjected to matched filtering and time shifting processing. Specifically, frequency domain up-sampling may be performed by zero or difference padding.
And S123, performing inverse distance Fourier transform on each frequency domain signal subjected to frequency domain up-sampling.
S124, frequency shift processing is performed on each frequency domain signal subjected to the inverse distance fourier transform processing.
Specifically, each frequency domain signal subjected to the inverse distance fourier transform is multiplied by a linear phase in the time domain, where the linear phase is exp (j · pi · f) shift (n)·t r )；
Wherein j is an imaginary unit, f shift (N) = (N- (N + 1)/2) · Δ f, N =1,2, …, N, N is the number of hopping points, Δ f is the hopping interval, t r Is distance versus time.
And S125, performing distance Fourier transform on each frequency domain signal subjected to frequency shift processing to obtain each frequency domain signal subjected to distance correction and frequency domain up-sampling processing.
The invention will now be described in detail with reference to a specific embodiment shown in figure 2. The method for estimating and compensating the ionosphere and the troposphere by combining the frequency stepping SAR comprises the following steps of:
step one, performing distance direction FFT on the sub-band echo data to obtain a frequency domain signal of the nth frequency point. Specifically, the sub-band echo expression is shown as formula (1):
Figure BDA0003730684550000074
wherein j is an imaginary unit, t r Is the distance to time, T p Is the pulse width, t 0 ＝2·(R-R min )/c，R min Is the initial sampling distance, R is the target to radar slant distance, c is the speed of light, t d (n) is the delay of the nth subband signal relative to the first subband signal, where t d (1)＝0。K r Is the distance to chirp slope, f c (n) is the carrier center frequency of the nth frequency point, and f is set 0 For the first sub-band carrier frequency, for the nth frequency point, the carrier frequency is as shown in formula (2):
f c (n)＝f 0 +(n-1)·Δf (2)
wherein, Δ f is the frequency hopping interval which is the difference between the carrier center frequencies of two frequency points,
Figure BDA0003730684550000085
is a time domain representation of the additional phase of the electromagnetic wave after it has passed through the ionosphere.
The frequency domain signal of the nth frequency point is as shown in formula (3):
Figure BDA0003730684550000081
where rect (. Cndot.) is a gate function, f r Is the distance to frequency, B is the bandwidth,
Figure BDA0003730684550000086
for the additional phase of the electromagnetic wave after passing through the ionized layer, the expression is shown as formula (4):
Figure BDA0003730684550000082
alpha is the radar down viewing angle, K =40.28m 3 /s 2 Is constant, c is the speed of light, TEC is the total electron content in the ionosphere, in the unit of TECU, where 1TECU =10 16 Per m 3 . It should be noted that the difference between the echoes of different sub-bands is that n is different, that is, the frequency points of the echoes of the two sub-bands are different, and f is different c (n) and t d In the subsequent processing, n =1 is processed on the left side of fig. 2, and n =2 is processed on the right side.
And step two, performing matched filtering and time shifting on each sub-band echo frequency domain signal, namely multiplying the sub-band echo frequency domain signal by the formula (5) to obtain a signal shown as the formula (6).
Figure BDA0003730684550000083
Figure BDA0003730684550000084
The last phase is the ionospheric phase offset. In the absence of ionospheric phase offset, the last term of the equation is constant 1.
And step three, zero filling is carried out on each frequency domain signal subjected to matched filtering and time shifting processing, and frequency domain up-sampling is realized. The frequency domain up-sampling can also be performed by using a difference method.
And step four, performing distance-oriented IFFT operation on each frequency domain signal subjected to frequency domain up-sampling.
And step five, frequency shifting is carried out on each frequency domain signal subjected to the IFFT operation after the distance is passed, namely, a linear phase is multiplied in a time domain, wherein the linear phase is shown as the formula (7):
exp(j·π·f shift (n)·t) (7)
wherein f is shift (N) = (N- (N + 1)/2) · Δ f, N is the number of hopping points.
Step six, performing distance direction FFT operation on each frequency domain signal subjected to frequency shift processing, and obtaining result data as shown in formula (8):
Figure BDA0003730684550000091
wherein the content of the first and second substances,
Figure BDA0003730684550000092
is the ionospheric phase;
Figure BDA0003730684550000093
step seven, carrying out R on the formula (8) min And phase term and ionospheric phase compensation, namely multiplying the formula (8) by the compensated minimum slope distance phase (10) and the compensated ionospheric phase (11), and obtaining a calculation result as shown in a formula (12).
Figure BDA0003730684550000094
Figure BDA0003730684550000095
Figure BDA0003730684550000101
Step eight, performing frequency spectrum synthesis processing on each frequency domain signal subjected to phase compensation to obtain a synthesized frequency spectrum signal as shown in formula (13):
Figure BDA0003730684550000102
and step nine, performing distance-oriented IFFT operation on the synthesized frequency spectrum signal to obtain a distance-oriented image signal.
Step ten, evaluating the contrast of the distance oriented image signals, updating the ionosphere phase (namely updating the formula (9)) by adopting a fixed interval method or a dichotomy, repeatedly executing the step seven, the step eight and the step nine, calculating the distance oriented images for a plurality of times, evaluating the contrast of the distance oriented images, selecting the distance oriented images corresponding to the maximum value of the contrast, and finishing the ionosphere estimation and compensation at the moment.
Step eleven, dividing the imaging scene into grids, and calculating the projection distance R (t) of each grid in the imaging scene a ) (no refraction slant range), the size of the imaging scene is the breadth, the value is prior information, and the size of the division grid is generally 0.8 times of the resolution.
Step twelve, calculating the delay projection distance caused by troposphere delay at each grid in the imaging scene, as shown in fig. 3, the actual electromagnetic wave transmission route has a certain bend caused by atmospheric refraction. By ray tracing, the true refractive index slope is given by equation (14).
Figure BDA0003730684550000103
Wherein，n 0 Is the ground atmosphere refractive index, and n (h) is the atmospheric refractive index at the altitude ground h. As shown in fig. 3, the slope distance without refraction is calculated by the cosine law using the incident angle γ, the orbit height H, and the earth radius RE, that is, the slope distance without refraction is calculated according to equation (15).
Figure BDA0003730684550000104
For the mode of operation where there is beam rotation, its angle of incidence γ (t) a ) And the geocentric angle beta (t) a ) Is the orientation time variable. The real refraction slope distance course and the refraction-free slope distance at each moment can be calculated according to the formula (14) and the formula (15), so that the slope distance difference delta R (t) in the SAR working period is obtained a )＝R 0 (t a )-R(t a ) This is the delayed projection distance of the scene positions due to tropospheric delay.
Sixthly, carrying out backward projection on each grid of the scene according to the obtained projection distance of each grid of the scene and the projection distance of each grid of the scene caused by troposphere delay to obtain an SAR image;
fourteenth, evaluating the contrast of the SAR image obtained in the thirteenth step, updating the troposphere slope distance (namely, an updating formula (14)) by adopting a fixed interval method or a dichotomy, recalculating and executing the twelfth step and the thirteenth step to obtain the SAR image and evaluate the image contrast, selecting an image corresponding to the maximum value of the contrast, finishing the estimation and compensation of the troposphere and obtaining the SAR image with good focus.
It should be noted that the initial values of the iterative ionosphere and troposphere may be based on empirical values or a priori information.
The embodiment of the invention also carries out simulation experiments to verify the correctness of the estimation compensation algorithm of the ionized layer and the troposphere, simulates the target data of a small scene surface, the size of the scene is 1km, and the working time of the radar is 19s. The simulation is based on the high-resolution sliding spotlight mode spaceborne SAR parameters shown in the table 1, and the sub-band synthesis simulation parameters are shown in the table 2. And simulating two sub-bands to synthesize a large-bandwidth signal of 3GHz, and setting the ionized layer TEC as a constant value of 100TECU in the simulation. The current downward visual angle is 30deg, the bandwidth is 3GHz, the slope distance error corresponding to the first-order phase is 1m, and certain influence is exerted on positioning; the second order phase is 1.6 pi, which causes the defocusing phenomenon to occur. Therefore, for an X-band 0.1m resolution SAR signal, the influence of the ionosphere on SAR imaging needs to be considered.
TABLE 1
Figure BDA0003730684550000111
Figure BDA0003730684550000121
TABLE 2
Figure BDA0003730684550000122
In the simulation of troposphere and the influence of troposphere on imaging, a Hopfield model is used to obtain the variation of refraction coefficient N with height, and the local meteorological parameter is P 0 =1Pa, RH =14%, T =10 ℃ and h 0 Fig. 4 shows the variation of the refractive index N with altitude for 100m, and it can be seen that the atmospheric refractive index decreases with increasing altitude and falls to almost 0 at 40km altitude. The result of the change of the slant range difference introduced by atmospheric refraction along with the azimuth time is shown in fig. 5, and it can be seen that in the radar working period, the slant range difference introduced by atmospheric refraction can reach 4.5m, the relative change is also 0.5m, which is larger than the resolution, so the influence of atmospheric refraction on migration correction and azimuth focusing must be considered.
The ionospheric phase estimation is performed by using a self-focusing algorithm based on the maximum contrast, the self-focusing algorithm is a fixed interval method, the ionospheric phase estimation step length is set to be 10TECU, the estimation times are set to be 15 times, the position is estimated to be 0-150 TECU, and the change of the estimation result is shown in FIG. 6. It can be seen that at position 11, 100TECU, the contrast of the echo after pulse compression is maximum. The refraction coefficient of the earth surface can be obtained through iteration based on the same maximum contrast, the self-focusing algorithm is a dichotomy, the initial value of the refraction coefficient estimation of the troposphere is 0-500, the estimated refraction coefficient is 296.875, the iteration times are 5 times, the refraction coefficient estimation precision is 10, and the final refraction coefficient estimation precision is 6.875.
And comparing and compensating the target imaging result of the ionosphere and troposphere front and back sliding bunching mode surface. FIG. 7 illustrates the effect of surface target focusing in the presence of ionospheric and tropospheric errors, which can be seen to cause two-dimensional defocusing (primarily azimuthal defocusing, including range-wise defocusing); fig. 8 illustrates a two-dimensional contour map of the target after focusing by using the self-focusing algorithm, and it can be seen that the target does not have two-dimensional defocus after being corrected by using the self-focusing algorithm, which verifies the effectiveness of the high-resolution sliding bunching combined ionosphere and troposphere estimation compensation algorithm.
The embodiment of the invention also provides a device for estimating and compensating the ionosphere and the troposphere by combining the frequency stepping SAR, which comprises: the SAR image processing system comprises a distance image synthesis module, a projection distance determination module, a back projection module and an SAR image selection module.
The distance image synthesis module is used for carrying out ionosphere estimation compensation based on the maximum contrast when the distance image is synthesized by the sub-band echo; the projection distance determining module is used for carrying out grid division on an imaging scene, determining the projection distance of each grid in the imaging scene, and determining the delay projection distance of each grid according to the troposphere slant distance; the back projection module is used for carrying out back projection on each grid of an imaging scene on the distance direction image according to the projection distance and the delay projection distance to obtain an SAR image and determining the contrast of the SAR image; the SAR image selecting module is used for updating the troposphere slope distance for multiple times, respectively determining the SAR image and the contrast corresponding to the troposphere slope distance after each updating, and selecting the SAR image corresponding to the maximum contrast value when the updating times of the troposphere slope distance meet a first preset condition.
The embodiment of the invention also provides a device for estimating and compensating the ionosphere and the troposphere by combining the frequency-stepping SAR, which comprises a memory, a processor and a computer program which is stored on the memory and can run on the processor, wherein the processor executes the method for estimating and compensating the ionosphere and the troposphere by combining the frequency-stepping SAR provided by the embodiment.
Embodiments of the present invention further provide a computer-readable storage medium, which includes instructions, when the instructions are executed on a computer, cause the computer to execute the method for compensating the estimation of the ionosphere and the troposphere by combining frequency-stepped SAR and troposphere provided in the foregoing embodiments.
It is clear to those skilled in the art that, for convenience and brevity of description, the specific working processes of the above-described apparatuses and units may refer to the corresponding processes in the foregoing method embodiments, and are not described herein again.
In the several embodiments provided in the present application, it should be understood that the disclosed apparatus and method may be implemented in other ways. For example, the above-described apparatus embodiments are merely illustrative, and for example, a division of a unit is merely a logical division, and an actual implementation may have another division, for example, a plurality of units or components may be combined or integrated into another system, or some features may be omitted, or not executed.
Units described as separate parts may or may not be physically separate, and parts displayed as units may or may not be physical units, may be located in one place, or may be distributed on a plurality of network units. Some or all of the units can be selected according to actual needs to achieve the purpose of the solution of the embodiment of the present invention.
In addition, functional units in the embodiments of the present invention may be integrated into one processing unit, or each unit may exist alone physically, or two or more units are integrated into one unit. The integrated unit may be implemented in the form of hardware, or may also be implemented in the form of a software functional unit.
The integrated unit, if implemented in the form of a software functional unit and sold or used as a stand-alone product, may be stored in a computer readable storage medium. Based on such understanding, the technical solution of the present invention essentially or partially contributes to the prior art, or all or part of the technical solution can be embodied in the form of a software product stored in a storage medium and including instructions for causing a computer device (which may be a personal computer, a server, or a network device) to execute all or part of the steps of the method according to the embodiments of the present invention. And the aforementioned storage medium includes: a U-disk, a removable hard disk, a Read-Only Memory (ROM), a Random Access Memory (RAM), a magnetic disk or an optical disk, and other various media capable of storing program codes.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.

Claims (10)

1. A frequency stepping SAR combined ionosphere and troposphere estimation compensation method is characterized by comprising the following steps:
ionosphere estimation compensation is carried out based on the maximum contrast when the subband echo synthesis distance image is displayed;
carrying out mesh division on an imaging scene, determining the projection distance of each mesh in the imaging scene, and determining the delay projection distance of each mesh according to the troposphere slant distance;
according to the projection distance and the delay projection distance, carrying out back projection on each grid of an imaging scene on the distance direction image to obtain an SAR image, and determining the contrast of the SAR image;
and updating the troposphere slope distance for multiple times, respectively determining the SAR image and the contrast corresponding to the troposphere slope distance after each updating, and selecting the SAR image corresponding to the maximum contrast value when the updating times of the troposphere slope distance meet a first preset condition.
2. The method of claim 1, wherein the ionospheric estimation compensation based on maximum contrast for the subband-echo synthesis range oriented images comprises:
respectively carrying out distance Fourier transform on each sub-band echo to obtain a frequency domain signal of each sub-band echo;
distance direction correction and frequency domain up-sampling processing are carried out on the frequency domain signals of the sub-band echoes;
performing minimum slant range phase and ionospheric phase compensation on each frequency domain signal subjected to distance direction correction and frequency domain up-sampling processing;
carrying out frequency spectrum synthesis processing on each frequency domain signal subjected to phase compensation to obtain a synthesized frequency spectrum signal;
performing distance-direction inverse Fourier transform on the synthesized frequency spectrum signal to obtain a distance-direction image, and determining the contrast of the distance-direction image;
and updating the ionospheric phase for multiple times, respectively determining a distance direction image and a contrast corresponding to each updated ionospheric phase, and selecting a distance direction image corresponding to the maximum contrast value when the ionospheric phase updating times meet a second preset condition.
3. The method according to claim 2, wherein the distance-direction correcting and frequency-domain up-sampling processing is performed on the frequency-domain signal of each sub-band echo separately, and comprises:
respectively carrying out matched filtering and time shifting processing on the frequency domain signals of the echoes of each sub-band;
performing frequency domain up-sampling on each frequency domain signal subjected to matched filtering and time shifting;
performing inverse Fourier transform on each frequency domain signal subjected to frequency domain up-sampling;
performing frequency shift processing on each frequency domain signal subjected to the distance inverse Fourier transform processing;
and performing distance Fourier transform on each frequency domain signal subjected to frequency shift processing to obtain each frequency domain signal subjected to distance correction and frequency domain up-sampling processing.
4. The method of claim 3, wherein the performing matched filtering and time shifting on the frequency domain signals of the sub-band echoes respectively comprises:
multiplying the frequency domain signal of each sub-band echo by a window function, a matched filtering function and a time shifting function;
the window function is
Figure FDA0003730684540000021
The matched filter function is
Figure FDA0003730684540000022
The time shift function is
Figure FDA0003730684540000023
Wherein f is r Is the distance frequency,. DELTA.f is the hop interval,. J is the imaginary unit, K r Is the distance to chirp slope, T p Is the pulse width, t d (N) is the delay of the nth subband signal relative to the first subband signal, N =1,2, …, N being the number of hop-points.
5. The method according to claim 3, wherein the frequency shift processing on each frequency domain signal subjected to the inverse distance fourier transform processing comprises:
multiplying each frequency domain signal subjected to the inverse distance Fourier transform by a linear phase in a time domain, wherein the linear phase is exp (j & pi & f) shift (n)·t r )；
Wherein j is an imaginary unit, f shift (N) = (N- (N + 1)/2) · Δ f, N =1,2, …, N, N is the number of hopping points, Δ f is the hopping interval, t r Is distance versus time.
6. The method of claim 2, wherein the performing the minimum slope phase and ionospheric phase compensation on each frequency domain signal that has undergone distance direction correction and frequency domain up-sampling comprises:
multiplying each frequency domain signal subjected to distance direction correction and frequency domain up-sampling treatment by the compensated minimum slant distance phase and the compensated ionospheric phase;
minimum skew phase of the compensationIs located at
Figure FDA0003730684540000031
Figure FDA0003730684540000032
Wherein j is an imaginary unit, N =1,2, …, N, N is the number of hopping points, Δ f is the hopping interval, R is min Is the initial sampling distance, c is the speed of light;
the compensated ionospheric phase is
Figure FDA0003730684540000033
Figure FDA0003730684540000034
Wherein the content of the first and second substances,
Figure FDA0003730684540000035
wherein the content of the first and second substances,
Figure FDA0003730684540000036
is the ionospheric phase, K is a constant, alpha is the radar down-view angle, TEC is the total electron content in the ionospheric layer, f c (n) is the carrier center frequency of the nth frequency point, f r Is the range frequency.
7. The method of any one of claims 1 to 6, wherein the plurality of updates to the tropospheric slope distance and the plurality of updates to the ionospheric phase each employ a fixed interval method or a dichotomy method; when the sampling fixed interval method is used for updating, the updating times adopt preset updating times; and when the sampling bisection method is updated, determining the updating times according to the set precision.
8. An estimation compensation device for a frequency-stepping SAR combined ionosphere and troposphere, comprising:
the distance direction image synthesis module is used for carrying out ionosphere estimation compensation based on the maximum contrast when the distance direction image is synthesized by the sub-band echo;
the projection distance determining module is used for carrying out grid division on an imaging scene, determining the projection distance of each grid in the imaging scene, and determining the delay projection distance of each grid according to the troposphere slant distance;
the back projection module is used for carrying out back projection on each grid of an imaging scene on the distance direction image according to the projection distance and the delay projection distance to obtain an SAR image and determining the contrast of the SAR image;
and the SAR image selecting module is used for updating the troposphere slope distance for multiple times, respectively determining the SAR image and the contrast corresponding to the troposphere slope distance after each updating, and selecting the SAR image corresponding to the maximum contrast value when the updating times of the troposphere slope distance meet a first preset condition.
9. The apparatus of claim 8, wherein the distance-wise image synthesis module is specifically configured to: respectively carrying out distance Fourier transform on each sub-band echo to obtain a frequency domain signal of each sub-band echo; distance direction correction and frequency domain up-sampling processing are carried out on the frequency domain signals of the sub-band echoes; performing minimum slant range phase and ionospheric phase compensation on each frequency domain signal subjected to distance direction correction and frequency domain up-sampling processing; carrying out frequency spectrum synthesis processing on each frequency domain signal subjected to phase compensation to obtain a synthesized frequency spectrum signal; performing distance-direction inverse Fourier transform on the synthesized frequency spectrum signal to obtain a distance-direction image, and determining the contrast of the distance-direction image; and updating the ionospheric phase for multiple times, respectively determining a distance direction image and a contrast corresponding to each updated ionospheric phase, and selecting a distance direction image corresponding to the maximum contrast value when the ionospheric phase updating times meet a second preset condition.
10. An apparatus for frequency-stepped SAR-based joint ionosphere and troposphere estimation compensation, comprising a memory, a processor and a computer program stored on said memory and executable on said processor, wherein said processor when executing said program implements a method for frequency-stepped SAR-based joint ionosphere and troposphere estimation compensation according to any one of claims 1 to 7.
CN202210792201.4A 2022-07-05 2022-07-05 Estimation compensation method and device for frequency stepping SAR (synthetic aperture radar) combined ionosphere and troposphere Pending CN115166656A (en)

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