CN113406632A - L-band spaceborne bistatic SAR signal processing method and device and storage medium - Google Patents

Abstract

The invention provides an L-band spaceborne bistatic SAR signal processing method, a device and a storage medium; the method comprises the following steps: acquiring a signal to be corrected; carrying out synchronous phase error correction on a signal to be corrected to obtain a first intermediate signal; performing pulse compression processing on the first intermediate signal to obtain a second intermediate signal; the second intermediate signal is a distance frequency domain signal; and carrying out absolute phase error correction and space-variant phase error correction on the second intermediate signal to obtain a corrected signal. The problems of defocusing and geometric deviation of the final imaging result, geometric deviation of the interference processing result and the like are solved through absolute phase error correction and space-variant phase error correction.

Description

L-band spaceborne bistatic SAR signal processing method and device and storage medium

Technical Field

The invention relates to a signal processing technology, in particular to an L-band spaceborne bistatic SAR signal processing method, a device and a storage medium.

Background

Synthetic Aperture Radar (SAR) is a microwave imaging Radar which can be installed on flight platforms such as airplanes, satellites, spacecraft and the like, observes the ground all the time and all the weather, and has certain ground surface penetration capability.

A Bi-basic Synthetic Aperture Radar (Bi-SAR) is an SAR imaging system in which a receiver and a transmitter are located on different working platforms at a certain distance in space. Because the receiving and transmitting platform is separately arranged, the Bi-SAR system has many advantages that the traditional single-base double-channel SAR does not have: first, the transceiver system is separated, and the configuration of 'one-sending-multiple-receiving' can be realized with lower hardware cost. Secondly, platforms carried by a transmitter and a receiver are various and form different bistatic imaging systems, for example, an on-orbit satellite-borne SAR is used as a transmitting source, an airborne platform forms a receiving system to form a satellite-machine bistatic SAR system, or the receiver is placed at a fixed position to form a satellite-ground one-station fixed bistatic SAR system. In addition, the double-star formation can also form a double-base SAR system, such as the German TanDEM-X system in orbit at present, and the double-star formation is used for acquiring global high-precision digital elevation information. Due to the fact that the base line configuration of the double-base system is flexible, the problems of time decoherence and atmospheric effect when the single-base SAR system conducts interference processing are solved, and a terrain elevation measurement result better than that of the single-base SAR system can be obtained.

For many military or civil applications, such as biomass inversion, disaster warning, environmental monitoring, etc., low frequency radar signals (typically less than 2GHz) with better penetration performance are often used. However, due to the existence of ionospheric effects, phenomena including phase dispersion, attenuation, faraday rotation and flicker affect the signal, so that the imaging result has problems of defocusing, geometric shift and the like. Meanwhile, because of the existence of the base line in the double-base formation configuration, the Total Electron Content (TEC) on the signal path has small null change, and the TEC null change along the vertical base line direction causes geometric deviation of the interference processing result, and in severe cases, the horizontal deviation and the vertical deviation of tens of meters exist.

Disclosure of Invention

The embodiment of the invention provides an L-band spaceborne bistatic SAR signal processing method, an L-band spaceborne bistatic SAR signal processing device and a storage medium, which are used for solving the problems of defocusing and geometric deviation of a final imaging result, geometric deviation of an interference processing result and the like.

The technical scheme of the invention is realized as follows:

the embodiment of the invention discloses an L-band spaceborne bistatic SAR signal processing method, which comprises the following steps:

acquiring a signal to be corrected;

carrying out synchronous phase error correction on the signal to be corrected to obtain a first intermediate signal;

performing pulse compression processing on the first intermediate signal to obtain a second intermediate signal; the second intermediate signal is a distance frequency domain signal;

and carrying out absolute phase error correction and space-variant phase error correction on the second intermediate signal to obtain a corrected signal.

In the foregoing solution, the absolute phase error correction specifically includes:

determining a TEC initial estimation interval corresponding to the signal to be corrected; the TEC initial estimation interval comprises an optimal TEC estimation value;

determining the optimal TEC estimation value in the TEC initial estimation interval;

compensating the optimal TEC estimated value back to the second intermediate signal, thereby completing the absolute phase error correction.

In the foregoing scheme, the determining an optimal TEC estimation value within the TEC initial estimation interval includes:

dividing the TEC initial estimation interval into a first TEC subinterval and a second TEC subinterval according to the median value of the initial interval;

respectively determining a first candidate TEC estimation value/a second candidate TEC estimation value in the first TEC subinterval/the second TEC subinterval; the first candidate TEC estimation value/the second candidate TEC estimation value respectively correspond to a first minimum image entropy/a second minimum image entropy; the first/second minimum image entropy indicates that entropy of the resulting image is minimal after compensating the first/second candidate TEC estimate values in the first/second TEC subintervals;

if the first minimum image entropy is smaller than or equal to the second minimum image entropy, taking the first candidate TEC estimated value as the optimal TEC estimated value;

and if the first minimum image entropy is larger than the second minimum image entropy, taking the second candidate TEC estimation value as the optimal TEC estimation value.

In the foregoing solution, the determining a first candidate TEC estimate value/a second candidate TEC estimate value in the first TEC subinterval/the second TEC subinterval, respectively, includes:

respectively calculating the interval median value of the first TEC subinterval/the interval median value of the second TEC subinterval;

respectively calculating a first image entropy/a second image entropy based on two endpoint values and an interval median value of the first TEC subinterval/two endpoint values and an interval median value of the second TEC subinterval; the first image entropy comprises image entropies respectively corresponding to two endpoint values and an interval median value of the first TEC subinterval; the second image entropy comprises image entropies respectively corresponding to two endpoint values and an interval median value of the second TEC subinterval;

based on the first image entropy/the second image entropy, the first minimum image entropy/the second minimum image entropy is approximated respectively, so that the first candidate TEC estimation value/the second candidate TEC estimation value corresponding to the first minimum image entropy/the second minimum image entropy is obtained.

In the foregoing solution, the calculating a first image entropy/a second image entropy based on two endpoint values and an interval median value of the first TEC subinterval/two endpoint values and an interval median value of the second TEC subinterval respectively includes:

respectively substituting the two endpoint values and the interval median value of the first TEC subinterval/the two endpoint values and the interval median value of the second TEC subinterval into an absolute phase compensation expression to obtain a corresponding intermediate compensation coefficient;

multiplying the intermediate compensation coefficients with the second intermediate signals respectively to obtain corresponding compensated intermediate frequency domain signals;

respectively carrying out inverse Fourier transform on the compensated intermediate frequency domain signals to obtain compensated intermediate time domain signals;

and respectively calculating the first image entropy/the second image entropy by using an image entropy calculation expression based on the compensated intermediate time domain signal.

In the foregoing solution, the approximating the first minimum image entropy/the second minimum image entropy based on the first image entropy/the second image entropy, respectively, so as to obtain the first candidate TEC estimate value/the second candidate TEC estimate value corresponding to the first minimum image entropy/the second minimum image entropy, includes:

if the first image entropy/second image entropy satisfies E_up≥E_downThen, there is Δ E ═ E_mid-E_downI, make TEC_up＝TEC_mid(ii) a If the first image entropy/second image entropy satisfies E_up≤E_downThen, there is Δ E ═ E_mid-E_upI, make TEC_down＝TEC_mid(ii) a Wherein, the TEC_up、TEC_midAnd TEC_downRespectively a left end point value, an interval median value and a right end point value of the first TEC subinterval/a left end point value, an interval median value and a right end point value of the second TEC subinterval; said E_up、E_midAnd E_downRespectively is as follows_up、TEC_midAnd TEC_downThe corresponding first image entropy/second image entropy;

if the delta E is greater than or equal to a preset precision threshold, repeating the determining of the first candidate TEC estimate value/the second candidate TEC estimate value in the first TEC subinterval/the second TEC subinterval, respectively, until the delta E is less than the delta EEnding iteration to obtain the E when the preset precision threshold value is met_midAs the first minimum image entropy/second minimum image entropy, will be equal to the E_midCorresponding TEC_midAs the first candidate TEC estimate/second candidate TEC estimate.

In the foregoing solution, the compensating the optimal TEC estimate back to the second intermediate signal to complete the absolute phase error correction includes:

substituting the optimal TEC estimation value into the absolute phase compensation expression to obtain a corresponding absolute phase compensation coefficient;

multiplying the second intermediate signal by the absolute phase compensation coefficient to complete the absolute phase error correction.

In the foregoing scheme, the determining the TEC initial estimation interval corresponding to the signal to be corrected includes:

acquiring a target TEC estimated value in an imaging target area by using an international reference ionosphere model; the signal to be corrected is an echo of the imaging target region;

and arranging the target TEC estimated values according to the size sequence, thereby obtaining the TEC initial estimation interval.

In the foregoing solution, the space-variant phase error correction specifically includes:

calculating to obtain a target intersection point coordinate by using a space linear equation; the target intersection point coordinate is the intersection point coordinate of the radar beam and the ionized layer;

calculating a target TEC gradient value along the direction vertical to the base line based on the target intersection point coordinate; the base line is a connecting line between a main satellite and an auxiliary satellite of the satellite-borne bistatic SAR;

compensating the target TEC gradient value back to the second intermediate signal, thereby completing the space-variant phase error correction.

In the above scheme, the target intersection point coordinates include a first intersection point coordinate of a main satellite transmitting beam and an ionosphere and a second intersection point coordinate of an auxiliary satellite receiving beam and the ionosphere; the calculating the target TEC gradient value along the vertical baseline direction based on the target intersection point coordinate comprises:

respectively obtaining a first intersection point TEC/a second intersection point TEC corresponding to the first intersection point coordinate/the second intersection point coordinate by using an international reference ionosphere model;

respectively calculating the first TEC projection amount/the second TEC projection amount of the first intersection point TEC/the second intersection point TEC along the direction vertical to the base line;

and solving the gradient value of the target TEC based on the first TEC projection amount/the second TEC projection amount.

In the foregoing solution, the compensating the target TEC gradient value back to the second intermediate signal to complete the space-variant phase error correction includes:

performing virtual index operation on the TEC gradient value to obtain a space-variant phase compensation coefficient;

and multiplying the second intermediate signal by the space-variant phase compensation coefficient, thereby completing the space-variant phase error correction.

In the foregoing solution, after performing absolute phase error correction and space-variant phase error correction on the second intermediate signal to obtain a corrected signal, the method further includes:

and carrying out imaging processing based on the corrected signal to obtain a fine focusing image.

The embodiment of the invention also provides an L-band spaceborne bistatic SAR signal processing device, which comprises:

an acquisition unit configured to acquire a signal to be corrected;

the synchronous phase error correction unit is used for carrying out synchronous phase error correction on the signal to be corrected to obtain a first intermediate signal;

the pulse compression processing unit is used for performing pulse compression processing on the first intermediate signal to obtain a second intermediate signal; the second intermediate signal is a distance frequency domain signal;

and the ionosphere effect correction unit is used for carrying out absolute phase error correction and space-variant phase error correction on the second intermediate signal to obtain a corrected signal.

In the foregoing solution, the ionospheric effect correction unit includes: determining a sub-unit and a compensation sub-unit, wherein:

the determining subunit is configured to determine an TEC initial estimation interval corresponding to the signal to be corrected; the TEC initial estimation interval comprises an optimal TEC estimation value; and determining the optimal TEC estimation value in the TEC initial estimation interval;

and the compensation subunit is configured to compensate the optimal TEC estimation value back to the second intermediate signal, so as to complete the absolute phase error correction.

In the above scheme, the determining subunit is further configured to calculate to obtain a target intersection point coordinate by using a space linear equation; the target intersection point coordinate is the intersection point coordinate of the radar beam and the ionized layer; calculating a target TEC gradient value along the direction vertical to the base line based on the target intersection point coordinate; the base line is a connecting line between a main satellite and an auxiliary satellite of the satellite-borne bistatic SAR;

the compensation subunit is further configured to compensate the target TEC gradient value back to the second intermediate signal, thereby completing the space-variant phase error correction.

The embodiment of the invention also provides an L-band spaceborne bistatic SAR signal processing device, which comprises:

a memory for storing executable instructions;

and the processor is used for realizing the L-band spaceborne bistatic SAR signal processing method in the scheme when the executable instructions stored in the memory are executed.

The embodiment of the invention also provides a storage medium, which stores executable instructions and is used for causing a processor to execute the executable instructions so as to realize the L-band spaceborne bistatic SAR signal processing method in the scheme.

Therefore, the embodiment of the invention provides an L-band spaceborne bistatic SAR signal processing method, a device and a storage medium, which can firstly carry out synchronous phase error correction on a signal to be corrected after the signal to be corrected is obtained, so as to obtain a first intermediate signal; then, performing pulse compression processing on the first intermediate signal to obtain a second intermediate signal; and finally, carrying out absolute phase error correction and space-variant phase error correction on the second intermediate signal to obtain a corrected signal. Therefore, the problems of defocusing, geometric deviation and the like of the final imaging result are solved through absolute phase error correction; through the correction of the space-variant phase error, the influence of TEC space-variant property is eliminated, and the problem of geometric deviation of an interference processing result is solved.

Drawings

Fig. 1 is a first flowchart of an L-band spaceborne bistatic SAR signal processing method according to an embodiment of the present invention;

fig. 2 is a schematic view of a satellite-ground geometric relationship of an L-band spaceborne bistatic SAR signal processing method according to an embodiment of the present invention;

fig. 3 is a flowchart ii of an L-band spaceborne bistatic SAR signal processing method according to an embodiment of the present invention;

fig. 4 is a flowchart three of an L-band spaceborne bistatic SAR signal processing method according to an embodiment of the present invention;

fig. 5 is a fourth flowchart of an L-band spaceborne bistatic SAR signal processing method according to an embodiment of the present invention;

fig. 6 is a fifth flowchart of an L-band spaceborne bistatic SAR signal processing method according to an embodiment of the present invention;

fig. 7 is a sixth flowchart of an L-band spaceborne bistatic SAR signal processing method according to an embodiment of the present invention;

fig. 8 is a seventh flowchart of an L-band spaceborne bistatic SAR signal processing method according to an embodiment of the present invention;

fig. 9 is an eighth flowchart of an L-band spaceborne bistatic SAR signal processing method according to an embodiment of the present invention;

fig. 10 is a first schematic view illustrating an effect of an L-band spaceborne bistatic SAR signal processing method according to an embodiment of the present invention;

fig. 11 is a schematic diagram illustrating an effect of an L-band spaceborne bistatic SAR signal processing method according to an embodiment of the present invention;

fig. 12A is a schematic diagram illustrating an effect of an L-band spaceborne bistatic SAR signal processing method according to an embodiment of the present invention;

fig. 12B is a schematic diagram illustrating an effect of the L-band spaceborne bistatic SAR signal processing method according to the embodiment of the present invention;

fig. 13 is a schematic diagram illustrating an effect of an L-band spaceborne bistatic SAR signal processing method according to an embodiment of the present invention;

fig. 14A is a schematic diagram illustrating an effect of an L-band spaceborne bistatic SAR signal processing method according to an embodiment of the present invention;

fig. 14B is a schematic diagram illustrating an effect of the L-band spaceborne bistatic SAR signal processing method according to the embodiment of the present invention;

fig. 15 is a schematic structural diagram of an L-band space-borne bistatic SAR signal processing apparatus according to an embodiment of the present invention;

fig. 16 is a schematic structural diagram of an L-band spaceborne bistatic SAR signal processing device according to an embodiment of the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention are further described in detail with reference to the drawings and the embodiments, the described embodiments should not be construed as limiting the present invention, and all other embodiments obtained by a person of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

In the following description, reference is made to "some embodiments" which describe a subset of all possible embodiments, but it is understood that "some embodiments" may be the same subset or different subsets of all possible embodiments, and may be combined with each other without conflict.

To the extent that similar descriptions of "first/second" appear in this patent document, the description below will be added, where reference is made to the term "first \ second \ third" merely to distinguish between similar objects and not to imply a particular ordering with respect to the objects, it being understood that "first \ second \ third" may be interchanged either in a particular order or in a sequential order as permitted, to enable embodiments of the invention described herein to be practiced in other than the order illustrated or described herein.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used herein is for the purpose of describing embodiments of the invention only and is not intended to be limiting of the invention.

The following is an explanation of some concepts involved in embodiments of the present invention:

the azimuth direction is as follows: refers to the flight direction of the aircraft;

distance direction: refers to a direction perpendicular to the direction of flight of the aircraft.

Total Electron Content (TEC): the sum of the number of electrons per square meter from the bottom of the ionosphere (about 90km height) to the top of the ionosphere (about 1000km height). TEC is an important parameter for describing the form and structure of an ionized layer, and is helpful for researching the influence of the ionized layer on electromagnetic wave propagation.

Echo signals: the SAR target signal is a signal obtained by reflecting a radar signal transmitted by the SAR after reaching a target object.

Fig. 1 is an optional schematic flow chart of an L-band spaceborne bistatic SAR signal processing method according to an embodiment of the present invention, and will be described with reference to the steps shown in fig. 1.

And S101, acquiring a signal to be corrected.

In the embodiment of the invention, an L-waveband satellite-borne bistatic SAR signal processing device (signal processing device for short) can acquire an echo signal of the Bi-SAR to be used as a signal to be corrected. Fig. 2 is a schematic diagram of an optional satellite-ground geometric relationship of the Bi-SAR system in the embodiment of the present invention, as shown in fig. 2, in the Bi-SAR system, a main satellite and an auxiliary satellite are included, where: the main satellite transmits radar signals to target objects on the ground, and the auxiliary satellite receives echo signals reflected by the radar signals. Because the main satellite transmitting beam and the auxiliary satellite receiving beam are influenced by ionosphere disturbance, absolute phase errors are brought, and the problems of defocusing, geometric deviation and the like of an imaging result occur. At the same time, there is a baseline B between the primary and secondary stars_⊥That is, the primary satellite and the secondary satellite have a certain difference in spatial position, so the TEC on the signal paths of the primary satellite transmitting beam and the secondary satellite receiving beam has space-variant property, which brings aboutThe space-variant phase error causes a problem of geometric shift of the interference processing result.

It should be noted that the configuration of the Bi-SAR system can be flexible and changeable, for example, the number of satellites can be multiple, so as to realize the configuration of "one-sender multiple-receiver"; for another example, the receiving device for the echo signal may be placed at the main satellite or at a fixed position. The L-band spaceborne bistatic SAR signal processing method and the device in the embodiment of the invention can be correspondingly adjusted according to the configuration change of the Bi-SAR system, and are not limited to the above.

S102, synchronous phase error correction is carried out on the signal to be corrected, and a first intermediate signal is obtained.

In the embodiment of the invention, the signal processing device can perform synchronous phase error correction on the acquired signal to be corrected (namely the Bi-SAR echo signal), so as to obtain the first intermediate signal. For a specific method of correcting the synchronous phase error, reference may be made to the following patent documents:

[1] electronic research institute of Chinese academy of sciences, a method and a device for processing a bistatic SAR phase synchronization signal based on a carrier frequency signal, China, 202010089336.5[ P ], 20200609;

[2] institute of electronics, academy of sciences of china, phase synchronization method and apparatus, device, storage medium, china, 201710287459.8P, 20171201.

S103, performing pulse compression processing on the first intermediate signal to obtain a second intermediate signal; the second intermediate signal is a distance frequency domain signal.

In the embodiment of the present invention, the signal processing apparatus may perform pulse compression processing on the first intermediate signal subjected to the synchronous phase error correction, thereby obtaining the second intermediate signal. Wherein the second intermediate signal is a distance frequency domain signal.

In an embodiment of the present invention, the pulse compression process may be represented as follows:

wherein s (τ, η) represents the first intermediate signal; f. of_τRepresents the range-wise frequency; τ and η represent range-direction time and azimuth-direction time, respectively; k_rRepresenting a range chirp; fft (·) denotes a fourier transform operation.

And S104, carrying out absolute phase error correction and space-variant phase error correction on the second intermediate signal to obtain a corrected signal.

In the embodiment of the present invention, the signal processing apparatus may perform absolute phase error correction and space-variant phase error correction on the second intermediate signal, so that a corrected signal is obtained. The absolute phase error correction can correct absolute phase errors caused by ionosphere interference, and the space-variant phase error correction can correct space-variant phase errors caused by space position difference between the main satellite and the auxiliary satellite; the absolute phase error correction and the space variant phase error correction may be collectively referred to as ionospheric effect correction.

It should be noted that, due to the existence of the baseline between the two platforms (i.e., the primary satellite and the secondary satellite) of the bistatic SAR, the influence of the ionospheric effect on the bistatic SAR is different from that of the bistatic SAR, that is, the change of the TEC value along the vertical baseline direction will cause the Digital Elevation Model (DEM) result of the subsequent interference processing to have a shift along the horizontal and vertical directions. Therefore, the influence of the ionospheric effect on the bistatic SAR can be divided into two aspects of absolute phase error and space-variant phase error: the absolute phase error will cause the imaging process to appear defocused and the SAR image to shift; the space-variant phase error will cause the interferometric DEM results to shift. For cooperative double-basis SAR with a shorter baseline, its absolute phase error correction method can be considered to be common to single-basis SAR.

In some embodiments of the present invention, the absolute phase error correction in S104 may be implemented by S201 to S203 shown in fig. 3, which will be described in conjunction with the steps.

S201, determining a TEC initial estimation interval corresponding to a signal to be corrected; the optimal TEC estimation value is included in the TEC initial estimation interval.

In the embodiment of the present invention, the signal processing apparatus may utilize an International Reference ionosphere (International Reference ionosphere)re, IRI) model, obtaining target TEC estimated value in the imaging target area, wherein the signal to be corrected is echo of the imaging target area, and the amplitude value of each sampling point in the signal to be corrected comes from backscattering coefficient in the imaging target area; then, the obtained target TEC estimated values are arranged according to the size sequence, so that a TEC initial estimation interval [ TEC ] is obtained_up,0,TEC_down,0]. Wherein, [ TEC_up,0,TEC_down,0]The arrangement can be from big to small, and also can be from small to big.

It should be noted that the IRI model is established according to a large amount of ground observation data and ionosphere research results accumulated for many years, is the most widely used empirical ionosphere model in the world at present, and reflects the average state of the ionosphere in a statistical forecasting mode. The IRI model can be used for obtaining the TEC distribution condition of the imaging target area to obtain the initial estimation interval [ TEC ] of the TEC_up,0,TEC_down,0]The optimal TEC estimate is within this interval.

S202, determining an optimal TEC estimation value in the TEC initial estimation interval.

In the embodiment of the invention, the signal processing device determines the TEC initial estimation interval [ TEC_up,0,TEC_down,0]Then, the optimal TEC estimate may be determined as follows:

step one, dividing the TEC initial estimation interval into a first TEC subinterval and a second TEC subinterval according to the median value of the initial interval.

The signal processing device can be used for processing the signal according to the initial interval median TEC_mid,0＝(TEC_up,0+TEC_down,0) /2 initial estimation interval of TEC [ TEC_up,0,TEC_down,0]Divided into a first TEC sub-interval [ TEC_up,1,TEC_down，1]And a second TEC subinterval [ TEC ]_up,2,TEC_down,2]. Wherein, for [ TEC_up,1,TEC_down，1]There is TEC_up,1＝TEC_up,0And TEC_down，1＝TEC_mid,0(ii) a To [ TEC_up,2,TEC_down,2]There is TEC_up,2＝TEC_mid,0And TEC_down,2＝TEC_down,0。

That is to say, the TEC initial estimation interval is divided into two parts according to the median value of the initial interval, the left half part is the first TEC subinterval, and the right half part is the second TEC subinterval.

It should be noted that, when the initial estimation interval is determined by using the IRI model, it cannot be accurately guaranteed that the optimal TEC estimation value is in the vicinity of the median value of the initial estimation interval, so that a situation may occur in which the optimal TEC estimation value is too far away from the median value of the interval to be searched. Therefore, the initial estimation interval is divided into sub-intervals, and the two sub-intervals are respectively searched in a 'class dichotomy' manner, so that the optimal TEC estimation value can be searched with the maximum probability.

And step two, respectively calculating the interval median of the first TEC subinterval/the interval median of the second TEC subinterval.

The signal processing device may calculate the median TEC in the first interval of the first TEC subintervals_mid,1＝(TEC_up,1+TEC_down，1) /2 and median TEC in second interval of second TEC subintervals_mid,2＝(TEC_up,2+TEC_down，2)/2。

And step three, respectively calculating two end point values and an interval median value of the first TEC subinterval/two end point values and an interval median value of the second TEC subinterval corresponding to the first image entropy/the second image entropy.

The signal processing device may perform phase error compensation on the second intermediate signal respectively by using an interval endpoint value and an interval median value of the two TEC subintervals to obtain a compensated distance frequency domain signal.

Taking the first TEC subinterval as an example, the absolute phase compensation expression is:

wherein f is_cRepresents a carrier frequency; k represents a standard electronic parameter with a value of 40.28m³/s²。

It should be noted that, for cooperative bistatic SAR with a short baseline, the operating conditions can be setThe absolute phase error correction method is considered to be universal with the single-base SAR, so that the TECs corresponding to the transmitting beam of the main satellite and the receiving beam of the auxiliary satellite in the absolute phase compensation expression (2) are all similar to the same TEC, namely the TEC is

First, the signal processing apparatus may use the absolute phase compensation expression (2) to make

Are respectively TEC_up，1、TEC_down，1And TEC_mid，1And obtaining the corresponding intermediate compensation coefficient. Then, the intermediate compensation coefficients are respectively compensated back to the second intermediate signals, and compensated intermediate frequency domain signals are obtained:

then, the signal processing device may transform the compensated intermediate frequency domain signal into a time domain to obtain a compensated intermediate time domain signal:

where ifft (·) denotes an inverse fourier transform operation.

Similarly, for a second TEC subinterval, the same approach may be taken to obtain s_{com_up，2}(τ,η)、s_{com_down，2}(τ, η) and s_{com_mid，2}(τ,η)。

Finally, based on the compensated intermediate time domain signal, calculating the image entropy E under three TEC values of two TEC subintervals_up，1、E_down，1And E_mid，1。

The image entropy can be calculated by the following expression:

wherein, I represents an image to be calculated and corresponds to the compensated intermediate time domain signal; na and Nr are row and column labels in the signal matrix, and Na and Nr are the total number of rows and columns in the signal matrix; | · | represents a take magnitude operation. According to the calculation formula (5), under the condition that three compensation conditions of two endpoint values and an interval median value of the first TEC subinterval are calculated, the first image entropy E of the first TEC subinterval_up，1、E_down，1And E_mid，1。

Similarly, for a second TEC subinterval, the same approach may be taken to obtain a second image entropy E_up，2、E_down，2And E_mid，2。

And fourthly, based on the image minimum entropy criterion, respectively approaching the minimum image entropy in the two TEC subintervals by using a 'dichotomy-like method', so as to respectively obtain a first candidate TEC estimated value/a second candidate TEC estimated value in the two TEC subintervals.

For the first TEC interval, first, if E_up，1≥E_down，1Then there is Δ E₁＝|E_mid，1-E_down，1I, make TEC_up，1＝TEC_mid，1(ii) a If E is_up，1≤E_down，1Then there is Δ E₁＝|E_mid，1-E_up，1I, make TEC_down，1＝TEC_mid，1。

Then, the obtained Δ E is compared₁Comparing with a preset precision threshold value if delta E₁If the precision is larger than or equal to the preset precision threshold value, repeating the second step to the fourth step until delta E₁When the precision is smaller than the preset precision threshold value, ending the iteration to obtain E_mid，1As the first minimum image entropy of the first TEC interval, E will be compared with E_mid，1Corresponding TEC_mid，1As a first candidate TEC estimate. Considering the requirement of the search times and the approximation accuracy, the preset accuracy threshold may be set to 1e^-16If a higher accuracy of the TEC estimate is required, the magnitude of the predetermined accuracy threshold may be further reduced.

Similarly, for a second TEC subinterval, the same approach may be taken to obtain a second minimum image entropy and a second candidate TEC estimate for the second TEC interval.

It should be noted that the dichotomy-like method adopted in the embodiment of the present invention continuously divides the interval in which the optimal TEC estimation value is located into two, so that two end points of the interval gradually approach the optimal value, and further obtain the optimal TEC estimation value. This is based on the idea of bisection to approximate the zero point of the function, and is therefore referred to as "dichotomy-like".

Step five, comparing the first minimum image entropy with the second minimum image entropy, and if the first minimum image entropy is smaller than or equal to the second minimum image entropy, taking the first candidate TEC estimated value as an optimal TEC estimated value; and otherwise, taking the second candidate TEC estimated value as the optimal TEC estimated value.

Namely, the optimal TEC estimated value is as follows:

it should be noted that, for the SAR image, the better the degree of focusing is, the smaller the image entropy is. Therefore, based on the image minimum entropy criterion, for all the TEC values in the estimation interval, the image compensated by the optimal TEC estimation value will have the minimum image entropy.

It can be understood that, since the optimal TEC estimation value cannot be precisely guaranteed to be near the median value of the initial estimation interval when the initial estimation interval is determined by the IRI model, a situation may occur in which the optimal TEC estimation value cannot be searched because the optimal TEC estimation value deviates too far from the median value of the interval. Therefore, the initial estimation interval is divided into sub-intervals, and the two sub-intervals are respectively subjected to binary-division-based search, so that the optimal TEC estimation value can be searched by the maximum probability.

And S203, compensating the optimal TEC estimation value back to the second intermediate signal, thereby completing absolute phase error correction.

In the embodiment of the present invention, after obtaining the optimal TEC estimation value, the signal processing apparatus may compensate the optimal TEC estimation value back to the distance frequency domain signal (i.e., the second intermediate signal) that is subjected to the pulse compression processing. Specifically, the optimal TEC estimation value is substituted into the absolute phase compensation expression (2) to obtain a corresponding absolute phase compensation coefficient, and then the second intermediate signal is multiplied by the absolute phase compensation coefficient, thereby completing absolute phase error correction:

it can be understood that, through absolute phase error correction, the optimal TEC estimation value with the minimum image entropy is obtained, and the optimal TEC estimation value is compensated back to the distance frequency domain signal by using the phase compensation expression. Thus, the problems of defocusing, geometric shift and the like of the final imaging result are solved.

In some embodiments of the present invention, the above-mentioned space-variant phase error correction in S104 may be implemented by S301 to S303 shown in fig. 4, which will be described in conjunction with the steps.

S301, calculating to obtain a target intersection point coordinate by using a space linear equation; the target intersection point coordinate is the intersection point coordinate of the radar beam and the ionosphere.

In the embodiment of the invention, the signal processing device can also carry out space-variant phase error correction. Firstly, the signal processing device can calculate and obtain target intersection point coordinates by utilizing a space linear equation, wherein the target intersection point coordinates comprise first intersection point coordinates of a main satellite transmitting beam and an ionosphere and second intersection point coordinates of an auxiliary satellite receiving beam and the ionosphere.

Specifically, the geometric relationship between the main satellite transmission, the auxiliary satellite reception and the satellite-ground of the satellite-borne bistatic SAR formation satellite is shown in fig. 2, and the signal processing device can be arranged at the azimuth η_nUnder the earth's fixed coordinate system of the moment, the coordinates of the main star are

Coordinates of the satellite are

Height of ionized layer z_i(ii) a Target point coordinate P_tarIs (x)₀,y₀,z₀) (ii) a Coordinate P of first intersection point of main satellite transmitting beam and ionized layer_TcroIs composed of

Second intersection point coordinate P of satellite receiving beam and ionized layer_RcroIs composed of

Then, the spatial linear equation is substituted:

by solving equations (8) and (9), respectively, the azimuth direction η can be calculated_nFirst intersection coordinates/second intersection coordinates of time:

thus, the target intersection coordinates are obtained.

S302, calculating a target TEC gradient value along the direction vertical to the base line based on the target intersection point coordinates; the base line is a connecting line between a main satellite and an auxiliary satellite of the spaceborne bistatic SAR.

In the embodiment of the invention, after the signal processing device obtains the coordinates of the target intersection point, the gradient value of the target TEC along the direction vertical to the baseline can be calculated, wherein the baseline is a connecting line between the main satellite and the auxiliary satellite of the satellite-borne bistatic SAR.

Specifically, the signal processing apparatus may first obtain the first intersection by using the international reference ionosphere modelPoint coordinate P_TcroThe corresponding first intersection TEC:

and second intersection point coordinate P_RcroThe corresponding second intersection TEC:

then, the signal processing device may respectively calculate a first TEC projection amount/a second TEC projection amount of the first intersection point TEC/the second intersection point TEC in the vertical baseline direction:

finally, the signal processing device may obtain a target TEC gradient value based on the first TEC projection amount/the second TEC projection amount as follows:

and S303, compensating the target TEC gradient value back to the second intermediate signal, thereby completing the correction of the space-variant phase error.

In the embodiment of the invention, after the signal processing device obtains the target TEC gradient value, the signal processing device can perform virtual index operation on the target TEC gradient value to obtain the space-variant phase compensation coefficient, and then the space-variant phase compensation coefficient is multiplied by the second intermediate signal. Thus, the target TEC gradient value is compensated back to the second intermediate signal, completing the space-variant phase error correction:

it should be noted that there is no requirement for the absolute phase error correction and the space-variant phase error correction in order, and the absolute phase compensation coefficient and the space-variant phase compensation coefficient may be first obtained separately, and then the second intermediate signal is multiplied by the absolute phase compensation coefficient and the space-variant phase compensation coefficient. This completes the absolute phase error correction and the space-variant phase error correction of the second intermediate signal, thereby obtaining the final corrected signal.

It can be understood that, through space-variant phase error correction, the TEC gradient values along the vertical baseline direction are found and compensated back into the distance frequency domain signal. Therefore, the influence of TEC null-variation is eliminated, and the problem of geometric deviation of an interference processing result is solved.

In some embodiments of the present invention, S202 shown in fig. 3 may be implemented by S401-S403 shown in fig. 5, which will be described in conjunction with the steps.

S401, dividing the TEC initial estimation interval into a first TEC subinterval and a second TEC subinterval according to the initial interval median value.

In this embodiment of the present invention, the signal processing apparatus may determine the TEC according to a median value of the initial interval_mid,0Initial estimation interval of TEC [ TEC ]_up,0,TEC_down,0]Divided into a first TEC sub-interval [ TEC_up,1,TEC_down，1]And a second TEC subinterval [ TEC ]_up,2,TEC_down,2]。

S402, respectively determining a first candidate TEC estimation value/a second candidate TEC estimation value in a first TEC sub-interval/a second TEC sub-interval; the first candidate TEC estimated value/the second candidate TEC estimated value respectively correspond to a first minimum image entropy/a second minimum image entropy; the first minimum image entropy/the second minimum image entropy indicates that the entropy of the resulting image is minimum after compensating the first candidate TEC estimate/the second candidate TEC estimate in the first TEC subinterval/the second TEC subinterval.

In this embodiment of the present invention, the signal processing device may determine the first TEC subintervals [ TEC ] respectively_up,1,TEC_down，1]And a second TEC subinterval [ TEC ]_up,2,TEC_down,2]The first candidate TEC estimate and the second candidate TEC estimate in (1). Wherein the first candidate TEC estimate corresponds to a first minimum graphLike entropy, the first minimum image entropy indicates that the entropy of the obtained image is minimum after the first candidate TEC estimate is compensated for in the first TEC subinterval; the second candidate TEC estimate corresponds to a second minimum image entropy, and the second minimum image entropy indicates that the entropy of the obtained image is minimum after the second candidate TEC estimate is compensated for in the second TEC subinterval.

S403, if the first minimum image entropy is smaller than or equal to the second minimum image entropy, taking the first candidate TEC estimation value as an optimal TEC estimation value; or if the first minimum image entropy is larger than the second minimum image entropy, taking the second candidate TEC estimated value as the optimal TEC estimated value.

In the embodiment of the present invention, the signal processing device may determine an optimal TEC estimate value among the first candidate TEC estimate value and the second candidate TEC estimate value, and if the first minimum image entropy is less than or equal to the second minimum image entropy, take the first candidate TEC estimate value as the optimal TEC estimate value; and if the first minimum image entropy is larger than the second minimum image entropy, taking the second candidate TEC estimated value as the optimal TEC estimated value.

In some embodiments of the present invention, S402 shown in fig. 5 may be implemented by S4021 to S4023 shown in fig. 6, and will be described in conjunction with the steps.

S4021, respectively calculating the interval median value of the first TEC subinterval/the interval median value of the second TEC subinterval.

In the embodiment of the invention, the signal processing device determines the first TEC subinterval [ TEC ]_up,1,TEC_down，1]And a second TEC subinterval [ TEC ]_up,2,TEC_down,2]Thereafter, the median interval values of the two TEC subintervals may be calculated, respectively.

S4022, respectively calculating a first image entropy/a second image entropy corresponding to two endpoint values and an interval median value of the first TEC subinterval/two endpoint values and an interval median value of the second TEC subinterval.

In this embodiment of the present invention, the signal processing device may respectively calculate an endpoint value of the first TEC subinterval and a first image entropy E corresponding to an interval median value_up，1、E_down，1And E_mid，1(ii) a And, the endpoint value sum region of the first TEC subintervalFirst image entropy E corresponding to intermediate value_up，2、E_down，2And E_mid，2。

S4023, respectively approximating the first minimum image entropy/the second minimum image entropy based on the first image entropy/the second image entropy, so as to obtain a first candidate TEC estimation value/a second candidate TEC estimation value corresponding to the first minimum image entropy/the second minimum image entropy.

In the embodiment of the invention, a signal processing device respectively approaches a first minimum image entropy in a first TEC subinterval and a second minimum image entropy in a second TEC subinterval by using a 'dichotomy-like method' based on an image minimum entropy criterion; thus, a first candidate TEC estimation value corresponding to the first TEC subinterval and a second candidate TEC estimation value corresponding to the second TEC subinterval can be obtained.

In some embodiments of the present invention, S4022 shown in fig. 6 above may be implemented by S4024 to S4027, which will be described in conjunction with the respective steps.

S4024, respectively substituting the endpoint value and the interval median value of the first TEC subinterval/the endpoint value and the interval median value of the second TEC subinterval into the absolute phase compensation expression to obtain a corresponding intermediate compensation coefficient.

In the embodiment of the present invention, the signal processing device may respectively substitute the endpoint value and the interval median value of the first TEC subinterval/the endpoint value and the interval median value of the second TEC subinterval into the absolute phase compensation expression (2) to obtain the corresponding intermediate compensation coefficient.

And S4025, multiplying the intermediate compensation coefficients by the second intermediate signals respectively to obtain corresponding compensated intermediate frequency domain signals.

In the embodiment of the present invention, after obtaining the intermediate compensation coefficients, the signal processing apparatus may multiply the intermediate compensation coefficients with the second intermediate signals, respectively, to obtain corresponding compensated intermediate frequency domain signals.

S4026, respectively performing inverse Fourier transform on the compensated intermediate frequency domain signals to obtain compensated intermediate time domain signals.

In the embodiment of the present invention, after obtaining the compensated intermediate frequency domain signals, the signal processing apparatus may perform inverse fourier transform on the compensated intermediate frequency domain signals, respectively, to obtain compensated intermediate time domain signals.

S4027, based on the compensated intermediate time domain signal, respectively calculating a first image entropy/a second image entropy by using an image entropy calculation expression.

In the embodiment of the present invention, the signal processing apparatus may calculate the first image entropy/the second image entropy respectively by using the image entropy calculation expression (5) based on the compensated intermediate time domain signal.

In some embodiments of the present invention, S4023 shown in fig. 6 may be implemented through S4028 to S4029, which will be described in conjunction with the steps.

S4028, if the first image entropy/the second image entropy satisfies E_up≥E_downThen, there is Δ E ═ E_mid-E_downI, make TEC_up＝TEC_mid(ii) a If the first image entropy/the second image entropy satisfies E_up≤E_downThen, there is Δ E ═ E_mid-E_upI, make TEC_down＝TEC_mid(ii) a Wherein, the TEC_up、TEC_midAnd TEC_downRespectively a left end point value, an interval middle value and a right end point value of the first TEC subinterval/a left end point value, an interval middle value and a right end point value of the second TEC subinterval; e_up、E_midAnd E_downAre respectively and TEC_up、TEC_midAnd TEC_downCorresponding first image entropy/second image entropy.

In the embodiment of the invention, the signal processing device can divide E into two according to the' dichotomy class_upAnd E_downIs set as a judgment condition, corresponding delta E is obtained, and the TEC is compared_midAn assignment is made to prepare for a subsequent iteration.

S4029, if the delta E is greater than or equal to the preset precision threshold, repeatedly and respectively determining a first candidate TEC estimated value/a second candidate TEC estimated value in a first TEC subinterval/a second TEC subinterval until the delta E is smaller than the preset precision threshold, ending iteration to obtain E_midAs first minimum image entropy/second minimum image entropy, will be equal to E_midCorresponding TEC_midAs a first candidate TECEstimate/second candidate TEC estimate.

In the embodiment of the present invention, the signal processing apparatus may perform iteration again from the step of taking the interval median of the first TEC subinterval/the interval median of the second TEC subinterval when Δ E is greater than or equal to the preset accuracy threshold according to a "class bisection method", and end the iteration until Δ E is less than the preset accuracy threshold, and apply the final E_midAs first minimum image entropy/second minimum image entropy and will be compared with E_midCorresponding TEC_midAs the first candidate TEC estimate/the second candidate TEC estimate.

In some embodiments of the present invention, S403 shown in fig. 5 may be implemented by S4031-S4032, and will be described with reference to each step.

S4031, substituting the optimal TEC estimation value into an absolute phase compensation expression to obtain a corresponding absolute phase compensation coefficient.

In the embodiment of the present invention, after obtaining the optimal TEC estimation value, the signal processing device may substitute the optimal TEC estimation value into the absolute phase compensation expression (2) to obtain a corresponding absolute phase compensation coefficient.

S4032, the second intermediate signal is multiplied by the absolute phase compensation coefficient, thereby completing the absolute phase error correction.

In this embodiment of the present invention, the signal processing apparatus may multiply the second intermediate signal by the obtained absolute phase compensation coefficient, thereby completing the absolute phase error correction.

In some embodiments of the present invention, S401 shown in fig. 5 can be implemented through S4011-S4012, and will be described in conjunction with various steps.

S4011, obtaining a target TEC estimated value in an imaging target area by using an international reference ionosphere model; the signal to be corrected is the echo of the imaging target region.

In the embodiment of the invention, the signal processing device can obtain the target TEC estimated value in the imaging target area by using the international reference ionosphere model. The signal to be corrected is an echo of an imaging target area, and the amplitude value of each sampling point in the signal to be corrected is from a backscattering coefficient in the imaging target area.

S4012, arranging the target TEC estimated values according to the size sequence, thereby obtaining the TEC initial estimation interval.

In the embodiment of the invention, after the signal processing device obtains all the target TEC estimated values, the target TEC estimated values can be arranged according to the size sequence, so that the TEC initial estimation interval is obtained. The optimal TEC estimate is within the TEC initial estimate interval.

In some embodiments of the present invention, S302 shown in fig. 4 may be implemented through S3021 to S3023 shown in fig. 7, which will be described in conjunction with various steps.

S3021, respectively obtaining a first intersection point TEC/a second intersection point TEC corresponding to the first intersection point coordinate/the second intersection point coordinate by using the international reference ionosphere model.

In the embodiment of the invention, after determining the first intersection point coordinate of the main satellite transmission beam and the ionized layer and the second intersection point coordinate of the auxiliary satellite reception beam and the ionized layer, the signal processing device can respectively obtain the first intersection point TEC/the second intersection point TEC corresponding to the first intersection point coordinate/the second intersection point coordinate by using an international reference ionized layer model.

S3022, calculating first TEC projection quantity/second TEC projection quantity of the first intersection point TEC/the second intersection point TEC along the direction perpendicular to the base line respectively.

In the embodiment of the present invention, after obtaining the first intersection point TEC/the second intersection point TEC, the signal processing device may respectively calculate a first TEC projection amount/a second TEC projection amount of the first intersection point TEC/the second intersection point TEC along the vertical baseline direction.

S3023, obtaining a target TEC gradient value based on the first TEC projection amount/the second TEC projection amount.

In the embodiment of the present invention, after the signal processing device obtains the first TEC projection amount/the second TEC projection amount, the signal processing device may obtain the target TEC gradient value based on the first TEC projection amount/the second TEC projection amount.

In some embodiments of the present invention, S303 shown in fig. 4 may be implemented through S3031 to S3032, which will be described in conjunction with each step.

S3031, performing virtual index operation on the gradient value of the target TEC to obtain a space-variant phase compensation coefficient.

In the embodiment of the invention, after the signal correction device obtains the target TEC gradient value, the virtual index operation can be performed on the target TEC gradient value to obtain the space-variant phase compensation coefficient.

And S3032, multiplying the second intermediate signal by the space-variant phase compensation coefficient, thereby completing the space-variant phase error correction.

In the embodiment of the present invention, after obtaining the space-variant phase compensation coefficient, the signal processing apparatus may multiply the second intermediate signal by the space-variant phase compensation coefficient, thereby completing the space-variant phase error correction.

In some embodiments of the present invention, S105 is further included after S104 shown in fig. 1, and will be described with reference to each step.

And S105, carrying out imaging processing based on the corrected signals to obtain a fine focusing image.

In the embodiment of the invention, after the space-variant phase error correction device obtains the corrected signal, imaging processing can be carried out based on the corrected signal to obtain a fine focusing image. Compared with an image which is not corrected by an ionospheric effect, the fine focusing image has better focusing performance and higher image quality.

Fig. 8 is an optional flowchart of an L-band spaceborne bistatic SAR signal processing method according to an embodiment of the present invention, which will be described with reference to the steps shown in fig. 8.

S501, synchronous phase error correction is carried out on the Bi-SAR echo to obtain a first intermediate signal.

In the embodiment of the present invention, the signal processing device may first perform synchronous phase error correction on the Bi-SAR echo to obtain the first intermediate signal.

S502, performing range-direction pulse compression on the first intermediate signal to obtain a second intermediate signal.

In the embodiment of the present invention, the signal processing device may perform range-wise pulse compression on the first intermediate signal (i.e., the Bi-SAR echo subjected to synchronous phase error correction) to obtain the second intermediate signal.

S503, absolute phase error correction and space-variant phase error correction are carried out on the second intermediate signal, and a corrected signal is obtained.

In this embodiment of the present invention, the signal processing apparatus may perform absolute phase error correction and space-variant phase error correction on the second intermediate signal (i.e., the distance frequency domain signal after the distance-to-pulse compression) by using the IRI model, the orbit parameter, and the DEM, so as to obtain a corrected signal. The absolute phase error correction and the space-variant phase error correction may be collectively referred to as ionospheric effect correction.

And S504, imaging the corrected signals to obtain a fine focusing image.

In the embodiment of the invention, the signal processing device can perform imaging processing on the corrected signals subjected to synchronous phase error correction and ionosphere effect correction to obtain the fine focusing image.

In some embodiments of the present invention, S503 shown in fig. 8 may be implemented by S601-S609 shown in fig. 9, which will be described in conjunction with the steps.

S601, determining a TEC initial estimation interval, dividing TEC subintervals, and determining a median value of the subintervals.

In the embodiment of the invention, the signal processing device can obtain all target TEC estimated values in the imaging target area by utilizing the IRI model to determine the TEC initial estimation interval [ TEC [ ]_up,0,TEC_down,0]. And find the initial upper limit TEC_up,0Initial lower limit TEC_down,0And initial median TEC_mid,0To determine a sub-interval [ TEC ]_up,1,TEC_down，1]And the subinterval is two [ TEC_up,2,TEC_down,2]Wherein, for the first sub-interval, there is TEC_up,1＝TEC_up,0And TEC_down，1＝TEC_mid,0(ii) a For the second sub-interval, there is TEC_up,2＝TEC_mid,0And TEC_down,2＝TEC_down,0. Then, the median value of the first subinterval and the median value of the second subinterval are obtained.

And S602, respectively performing phase error compensation on the second intermediate signal by using the sub-interval upper limit, the sub-interval lower limit and the sub-interval median.

In this embodiment of the present invention, the signal processing apparatus may perform phase error compensation on the range-wise FFT (i.e., the second intermediate signal) by using three TEC values (i.e., an upper sub-interval limit, a lower sub-interval limit, and a median sub-interval value) of the first sub-interval and the second sub-interval, respectively.

S603, performing distance-to-IFFT operation on the compensated second intermediate signal to obtain a time domain signal of the second intermediate signal, and then respectively calculating the first image entropy/the second image entropy.

In the embodiment of the present invention, the signal processing device may perform a distance-wise IFFT operation to obtain a time domain signal of the second intermediate signal, and then calculate the image entropies corresponding to the three compensation signals in the subinterval one and the subinterval two, respectively. Namely, first image entropy: e_up，1、E_down，1、E_mid，1And second image entropy: e_up，2、E_down，2、E_mid，2。

S604, based on the first image entropy/the second image entropy, approximating a first minimum image entropy/a second minimum image entropy.

In the embodiment of the present invention, the signal processing apparatus may perform iteration by using the obtained image entropy and a "dichotomy-like method" to approximate a first minimum image entropy in the subinterval one and a second minimum image entropy in the subinterval two.

Specifically, j is 1 or 2, and corresponds to the first subinterval and the second subinterval, respectively. If E is_up，j≥E_down，jThen there is Δ E_j＝|E_mid，j-E_down，jI, make TEC_up，j＝TEC_mid，j(ii) a If E is_up，j≤E_down，jThen there is Δ E_j＝|E_mid，j-E_up，jI, make TEC_down，j＝TEC_mid，j. Then, the obtained value is compared with a preset precision threshold value if delta E_jIf the precision is larger than or equal to the preset precision threshold value, repeatedly starting to take the median value of the subintervals and repeatedly performing S602-S604 until delta E_jWhen the minimum image entropy is smaller than the preset precision threshold value, ending the iteration to obtain the minimum image entropy E_mid，1And E_mid，2。

S605, determining the optimal TEC estimation value from the first minimum image entropy/the second minimum image entropy.

In the embodiment of the invention, the signal processing device can adopt the minimum image entropy E_mid，1And E_mid，2The smaller one, and then determines the corresponding TEC value as the optimal TEC estimate TEC_esti。

And S606, calculating the intersection point coordinates of the beams and the ionized layer.

In the embodiment of the invention, the signal processing device can respectively calculate the first intersection point coordinate of the main satellite transmitting beam and the ionosphere and the second intersection point coordinate of the auxiliary satellite receiving beam and the ionosphere by utilizing the IRI model, the orbit parameter and the DEM.

S607, calculating the TECx of each azimuth moment based on the intersection point coordinates.

In the embodiment of the present invention, the signal processing device may calculate the projection amount TECx of the TEC along the vertical baseline direction at the intersection coordinate at each azimuth time based on the intersection coordinate.

S608 calculates Δ TECx at each azimuth time based on TECx at each azimuth time.

In the embodiment of the present invention, the signal processing apparatus may calculate the TEC gradient value Δ TECx in the vertical baseline direction at each azimuth time based on TECx.

And S609, performing phase error compensation based on the optimal TEC estimated value and the delta TECx of each azimuth moment.

In the embodiment of the invention, the signal processing device obtains the optimal TEC estimated value TEC_estiAnd TEC gradient value Δ TECx along the vertical baseline direction at each azimuth time, can be based on TEC_estiAnd Δ TECx performs phase error compensation on the second intermediate signal, thus completing ionospheric effect correction.

Fig. 10 to fig. 14 are schematic diagrams illustrating effects of the L-band spaceborne bistatic SAR signal processing method according to the embodiment of the present invention, and the effects of the embodiment of the present invention will be described with reference to the diagrams.

Fig. 10 is a simulation scene of a satellite-borne bistatic SAR point target under a specific parameter, and as shown in fig. 10, imaging quality evaluation is performed on a scene edge point P to obtain a contour outline and a two-dimensional cross-sectional view of the point P.

Fig. 11 is an imaging result diagram of a scene edge point P when the ionosphere effect correction is not performed by using the L-band spaceborne bistatic SAR signal processing method in the embodiment of the present invention; fig. 12A and 12B are two-dimensional cross-sectional views of the scene edge points P corresponding to fig. 11. As shown in fig. 11, 12A and 12B, when no ionospheric effect correction is performed, significant defocusing occurs in the imaging result, which affects the imaging quality.

Fig. 13 is an imaging result diagram of a scene edge point P after ionosphere effect correction is performed by using the L-band spaceborne bistatic SAR signal processing method in the embodiment of the present invention; fig. 14A and 14B are two-dimensional cross-sectional views of the scene edge points P corresponding to fig. 13. As shown in fig. 13, 14A and 14B, after the ionospheric effect correction is performed, the focusing performance of the imaging result is significantly improved, and the imaging quality is significantly improved.

Fig. 15 is an alternative structural schematic diagram of an L-band space-borne bistatic SAR signal processing apparatus according to an embodiment of the present invention. As shown in fig. 15, an embodiment of the present invention further provides a signal processing apparatus 800, including: an acquisition unit 804, a synchronous phase error correction unit 805, a pulse compression processing unit 806, an ionospheric effect correction unit 807, wherein:

an obtaining unit 804, configured to obtain a signal to be corrected;

a synchronous phase error correction unit 805, configured to perform synchronous phase error correction on the signal to be corrected, so as to obtain a first intermediate signal;

a pulse compression processing unit 806, configured to perform pulse compression processing on the first intermediate signal to obtain a second intermediate signal; the second intermediate signal is a distance frequency domain signal;

an ionospheric effect correction unit 807, configured to perform absolute phase error correction and space-variant phase error correction on the second intermediate signal to obtain a corrected signal.

In some embodiments of the present invention, the ionospheric effect correction unit 807 further comprises: a determination subunit 808 and a compensation subunit 809, wherein:

the determining subunit 808 is configured to determine a TEC initial estimation interval corresponding to the signal to be corrected; the TEC initial estimation interval comprises an optimal TEC estimation value; and determining the optimal TEC estimation value in the TEC initial estimation interval;

the compensation subunit 809 is configured to compensate the optimal TEC estimate back to the second intermediate signal, thereby completing the absolute phase error correction.

In some embodiments of the present invention, the determining subunit 808 is further configured to divide the TEC initial estimation interval into a first TEC subinterval and a second TEC subinterval according to a median value of an initial interval; and determining a first candidate TEC estimate value/a second candidate TEC estimate value in the first TEC subinterval/the second TEC subinterval, respectively; the first candidate TEC estimation value/the second candidate TEC estimation value respectively correspond to a first minimum image entropy/a second minimum image entropy; the first/second minimum image entropy indicates that entropy of the resulting image is minimal after compensating the first/second candidate TEC estimate values in the first/second TEC subintervals; and if the first minimum image entropy is smaller than or equal to the second minimum image entropy, taking the first candidate TEC estimation value as the optimal TEC estimation value; and if the first minimum image entropy is larger than the second minimum image entropy, taking the second candidate TEC estimation value as the optimal TEC estimation value.

In some embodiments of the present invention, the determining subunit 808 is further configured to calculate an interval median value of the first TEC subinterval/an interval median value of the second TEC subinterval, respectively; respectively calculating a first image entropy/a second image entropy based on two endpoint values and an interval median value of the first TEC subinterval/two endpoint values and an interval median value of the second TEC subinterval; the first image entropy comprises image entropies respectively corresponding to two endpoint values and an interval median value of the first TEC subinterval; the second image entropy comprises image entropies respectively corresponding to two endpoint values and an interval median value of the second TEC subinterval; and approximating the first minimum image entropy/the second minimum image entropy respectively based on the first image entropy/the second image entropy, thereby obtaining the first candidate TEC estimate value/the second candidate TEC estimate value corresponding to the first minimum image entropy/the second minimum image entropy.

In some embodiments of the present invention, the determining subunit 808 is further configured to substitute two endpoint values and an interval median value of the first TEC subinterval/an endpoint value and an interval median value of the second TEC subinterval into the absolute phase compensation expression, respectively, to obtain a corresponding intermediate compensation coefficient; multiplying the intermediate compensation coefficients with the second intermediate signals respectively to obtain corresponding compensated intermediate frequency domain signals; respectively carrying out inverse Fourier transform on the compensated intermediate frequency domain signals to obtain compensated intermediate time domain signals; and respectively calculating the first image entropy/the second image entropy by using an image entropy calculation expression based on the compensated intermediate time domain signal.

In some embodiments of the invention, the determining subunit 808 is further configured to determine if the first image entropy/the second image entropy satisfies E_up≥E_downThen, there is Δ E ═ E_mid-E_downI, make TEC_up＝TEC_mid(ii) a If the first image entropy/second image entropy satisfies E_up≤E_downThen, there is Δ E ═ E_mid-E_upI, make TEC_down＝TEC_mid(ii) a Wherein, the TEC_up、TEC_midAnd TEC_downRespectively a left end point value, an interval median value and a right end point value of the first TEC subinterval/a left end point value, an interval median value and a right end point value of the second TEC subinterval; said E_up、E_midAnd E_downRespectively is as follows_up、TEC_midAnd TEC_downThe corresponding first image entropy/second image entropy; and if the delta E is larger than or equal to a preset precision threshold, repeating the determination of the first candidate TEC estimation value/the second candidate TEC estimation value in the first TEC subinterval/the second TEC subinterval respectively until the delta E is smaller than the preset precision threshold, ending the iteration, and obtaining the E_midAs the first minimum image entropy/second minimum image entropy, will be equal to the E_midCorresponding TEC_midAs the first candidate TEC estimate/second candidate TEC estimate.

In some embodiments of the present invention, the compensation subunit 809 is further configured to substitute the optimal TEC estimation value into the absolute phase compensation expression to obtain a corresponding absolute phase compensation coefficient; and multiplying the second intermediate signal by the absolute phase compensation coefficient, thereby completing the absolute phase error correction.

In some embodiments of the present invention, the determining subunit 808 is further configured to obtain a target TEC estimate in the imaging target region by using an international reference ionosphere model; the signal to be corrected is an echo of the imaging target region; and arranging the target TEC estimated values according to the size sequence, thereby obtaining the TEC initial estimation interval.

In some embodiments of the present invention, the determining subunit 808 is further configured to calculate to obtain coordinates of the target intersection point by using a space linear equation; the target intersection point coordinate is the intersection point coordinate of the radar beam and the ionized layer; calculating a target TEC gradient value along the direction vertical to the base line based on the target intersection point coordinate; the base line is a connecting line between a main satellite and an auxiliary satellite of the satellite-borne bistatic SAR;

the compensation subunit 809 is further configured to compensate the target TEC gradient value back to the second intermediate signal, thereby completing the space-variant phase error correction.

In some embodiments of the present invention, the determining subunit 808 is further configured to obtain, by using an international reference ionosphere model, a first intersection TEC/a second intersection TEC corresponding to the first intersection coordinate/the second intersection coordinate respectively; respectively calculating the first TEC projection amount/the second TEC projection amount of the first intersection point TEC/the second intersection point TEC along the direction vertical to the base line; and solving the target TEC gradient value based on the first TEC projection amount/the second TEC projection amount.

In some embodiments of the present invention, the compensation subunit 809 is further configured to perform a virtual index operation on the target TEC gradient value to obtain a space-variant phase compensation coefficient; and multiplying the second intermediate signal by the space-variant phase compensation coefficient, thereby completing the space-variant phase error correction.

In some embodiments of the present invention, the signal processing apparatus 800 further comprises: an imaging processing unit 810, wherein:

the imaging processing unit 810 is configured to perform imaging processing based on the corrected signal to obtain a fine focus image.

It should be noted that fig. 16 is an optional schematic structural diagram of the correction apparatus according to the embodiment of the present invention, and as shown in fig. 16, the hardware entities of the signal processing apparatus 800 include: a processor 801, a communication interface 802, and a memory 803, wherein:

the processor 801 generally controls the overall operation of the signal processing apparatus 800.

The communication interface 802 may enable the signal processing apparatus 800 to communicate with other apparatuses or devices via a network.

The Memory 803 is configured to store instructions and applications executable by the processor 801, and may also buffer data (e.g., image data, audio data, voice communication data, and video communication data) to be processed or already processed by each module in the processor 801 and the signal processing apparatus 800, and may be implemented by a FLASH Memory (FLASH) or a Random Access Memory (RAM).

It should be noted that, in the embodiment of the present invention, if the method for detecting malicious behavior is implemented in the form of a software functional module, and is sold or used as a standalone product, it may also be stored in a computer-readable storage medium. Based on such understanding, the technical solutions of the embodiments of the present invention may be substantially implemented or portions thereof that contribute to the related art may be embodied in the form of a software product, which is stored in a storage medium and includes several instructions for enabling the signal processing apparatus 800 (which may be a personal computer, a server, or a network device) to execute all or part of the methods described in the embodiments of the present invention. And the aforementioned storage medium includes: various media capable of storing program codes, such as a usb disk, a removable hard disk, a Read Only Memory (ROM), a magnetic disk, or an optical disk. Thus, embodiments of the invention are not limited to any specific combination of hardware and software.

Correspondingly, an embodiment of the present invention provides a computer-readable storage medium, on which a computer program is stored, where the computer program, when executed by a processor, implements the steps in the method corresponding to the L-band spaceborne bistatic SAR signal processing apparatus.

Here, it should be noted that: the above description of the storage medium and device embodiments is similar to the description of the method embodiments above, with similar advantageous effects as the method embodiments. For technical details not disclosed in the embodiments of the storage medium and the apparatus according to the invention, reference is made to the description of the embodiments of the method according to the invention.

It should be noted that, in this document, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising an … …" does not exclude the presence of other like elements in a process, method, article, or apparatus that comprises the element.

In the embodiments provided in the present invention, it should be understood that the disclosed apparatus and method may be implemented in other ways. The above-described device embodiments are merely illustrative, for example, the division of the unit is only a logical functional division, and there may be other division ways in actual implementation, such as: multiple units or components may be combined, or may be integrated into another system, or some features may be omitted, or not implemented. In addition, the coupling, direct coupling or communication connection between the components shown or discussed may be through some interfaces, and the indirect coupling or communication connection between the devices or units may be electrical, mechanical or other forms.

The units described as separate parts may or may not be physically separate, and parts displayed as units may or may not be physical units; can be located in one place or 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.

In addition, all the functional units in the embodiments of the present invention may be integrated into one processing unit, or each unit may be separately regarded as one unit, or two or more units may be integrated into one unit; the integrated unit can be realized in a form of hardware, or in a form of hardware plus a software functional unit.

The above description is only an embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art can easily conceive of changes or substitutions within the technical scope of the present invention, and all such changes or substitutions are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the appended claims.

Claims (17)

1. An L-band spaceborne bistatic SAR signal processing method is characterized by comprising the following steps:

acquiring a signal to be corrected;

carrying out synchronous phase error correction on the signal to be corrected to obtain a first intermediate signal;

performing pulse compression processing on the first intermediate signal to obtain a second intermediate signal; the second intermediate signal is a distance frequency domain signal;

and carrying out absolute phase error correction and space-variant phase error correction on the second intermediate signal to obtain a corrected signal.

2. The L-band spaceborne bistatic SAR signal processing method according to claim 1, wherein said absolute phase error correction specifically comprises:

determining a TEC initial estimation interval corresponding to the signal to be corrected; the TEC initial estimation interval comprises an optimal TEC estimation value;

determining the optimal TEC estimation value in the TEC initial estimation interval;

compensating the optimal TEC estimated value back to the second intermediate signal, thereby completing the absolute phase error correction.

3. The method for processing an L-band spaceborne bistatic SAR signal according to claim 2, wherein said determining the optimal TEC estimation value within the TEC initial estimation interval comprises:

dividing the TEC initial estimation interval into a first TEC subinterval and a second TEC subinterval according to the median value of the initial interval;

respectively determining a first candidate TEC estimation value/a second candidate TEC estimation value in the first TEC subinterval/the second TEC subinterval; the first candidate TEC estimation value/the second candidate TEC estimation value respectively correspond to a first minimum image entropy/a second minimum image entropy; the first/second minimum image entropy indicates that entropy of the resulting image is minimal after compensating the first/second candidate TEC estimate values in the first/second TEC subintervals;

if the first minimum image entropy is smaller than or equal to the second minimum image entropy, taking the first candidate TEC estimated value as the optimal TEC estimated value;

and if the first minimum image entropy is larger than the second minimum image entropy, taking the second candidate TEC estimation value as the optimal TEC estimation value.

4. The method of claim 3, wherein said determining the first/second candidate TEC estimates in the first/second TEC subintervals, respectively, comprises:

respectively calculating the interval median value of the first TEC subinterval/the interval median value of the second TEC subinterval;

respectively calculating a first image entropy/a second image entropy based on two endpoint values and an interval median value of the first TEC subinterval/two endpoint values and an interval median value of the second TEC subinterval; the first image entropy comprises image entropies respectively corresponding to two endpoint values and an interval median value of the first TEC subinterval; the second image entropy comprises image entropies respectively corresponding to two endpoint values and an interval median value of the second TEC subinterval;

based on the first image entropy/the second image entropy, the first minimum image entropy/the second minimum image entropy is approximated respectively, so that the first candidate TEC estimation value/the second candidate TEC estimation value corresponding to the first minimum image entropy/the second minimum image entropy is obtained.

5. The L-band spaceborne bistatic SAR signal processing method according to claim 4, wherein the calculating the first image entropy/the second image entropy based on the two endpoint values and the interval median of the first TEC subinterval/the two endpoint values and the interval median of the second TEC subinterval respectively comprises:

respectively substituting the two endpoint values and the interval median value of the first TEC subinterval/the two endpoint values and the interval median value of the second TEC subinterval into an absolute phase compensation expression to obtain a corresponding intermediate compensation coefficient;

multiplying the intermediate compensation coefficients with the second intermediate signals respectively to obtain corresponding compensated intermediate frequency domain signals;

respectively carrying out inverse Fourier transform on the compensated intermediate frequency domain signals to obtain compensated intermediate time domain signals;

and respectively calculating the first image entropy/the second image entropy by using an image entropy calculation expression based on the compensated intermediate time domain signal.

6. The L-band space-borne bistatic SAR signal processing method according to claim 4 or 5, wherein the approximating the first/second minimum image entropy based on the first/second image entropy, respectively, to obtain the first/second candidate TEC estimation values corresponding to the first/second minimum image entropy comprises:

if the first image entropy/second image entropy satisfies E_up≥E_downThen, there is Δ E ═ E_mid-E_downI, make TEC_up＝TEC_mid(ii) a If the first image entropy/second image entropy satisfies E_up≤E_downThen, there is Δ E ═ E_mid-E_upI, make TEC_down＝TEC_mid(ii) a Wherein, the TEC_up、TEC_midAnd TEC_downRespectively a left end point value, an interval median value and a right end point value of the first TEC subinterval/a left end point value, an interval median value and a right end point value of the second TEC subinterval; said E_up、E_midAnd E_downRespectively is as follows_up、TEC_midAnd TEC_downThe corresponding first image entropy/second image entropy;

if the delta E is larger than or equal to a preset precision threshold value, repeating the determination of the first candidate TEC estimation value/the second candidate TEC estimation value in the first TEC subinterval/the second TEC subinterval respectively until the delta E is smaller than the preset precision threshold value, ending the iteration, and obtaining the E_midAs the first minimum image entropy/second minimum image entropy, will be equal to the E_midCorresponding TEC_midAs the first candidate TEC estimate/second candidate TEC estimate.

7. The L-band spaceborne bistatic SAR signal processing method according to claim 5 or 6, wherein the compensating the optimal TEC estimation value back to the second intermediate signal to complete the absolute phase error correction comprises:

substituting the optimal TEC estimation value into the absolute phase compensation expression to obtain a corresponding absolute phase compensation coefficient;

multiplying the second intermediate signal by the absolute phase compensation coefficient to complete the absolute phase error correction.

8. The L-band spaceborne bistatic SAR signal processing method according to any one of claims 2 to 7, wherein the determining the TEC initial estimation interval corresponding to the signal to be corrected comprises:

acquiring a target TEC estimated value in an imaging target area by using an international reference ionosphere model; the signal to be corrected is an echo of the imaging target region;

and arranging the target TEC estimated values according to the size sequence, thereby obtaining the TEC initial estimation interval.

9. The L-band spaceborne bistatic SAR signal processing method according to any of claims 1 to 8, wherein said space-variant phase error correction specifically comprises:

calculating to obtain a target intersection point coordinate by using a space linear equation; the target intersection point coordinate is the intersection point coordinate of the radar beam and the ionized layer;

calculating a target TEC gradient value along the direction vertical to the base line based on the target intersection point coordinate; the base line is a connecting line between a main satellite and an auxiliary satellite of the satellite-borne bistatic SAR;

compensating the target TEC gradient value back to the second intermediate signal, thereby completing the space-variant phase error correction.

10. The L-band spaceborne bistatic SAR signal processing method according to claim 9, wherein said target intersection coordinates comprise a first intersection coordinate of a primary satellite transmission beam and ionosphere and a second intersection coordinate of a secondary satellite reception beam and ionosphere; the calculating the target TEC gradient value along the vertical baseline direction based on the target intersection point coordinate comprises:

respectively obtaining a first intersection point TEC/a second intersection point TEC corresponding to the first intersection point coordinate/the second intersection point coordinate by using an international reference ionosphere model;

respectively calculating the first TEC projection amount/the second TEC projection amount of the first intersection point TEC/the second intersection point TEC along the direction vertical to the base line;

and solving the gradient value of the target TEC based on the first TEC projection amount/the second TEC projection amount.

11. The L-band space-borne bistatic SAR signal processing method according to claim 9 or 10, wherein said compensating said target TEC gradient value back into said second intermediate signal to complete said space-variant phase error correction comprises:

performing virtual index operation on the target TEC gradient value to obtain a space-variant phase compensation coefficient;

and multiplying the second intermediate signal by the space-variant phase compensation coefficient, thereby completing the space-variant phase error correction.

12. The method for processing an L-band space-borne bistatic SAR signal according to any of claims 1 to 11, wherein said performing absolute phase error correction and space-variant phase error correction on said second intermediate signal, after obtaining a corrected signal, further comprises:

and carrying out imaging processing based on the corrected signal to obtain a fine focusing image.

13. An L-band spaceborne bistatic SAR signal processing device is characterized by comprising:

an acquisition unit configured to acquire a signal to be corrected;

the synchronous phase error correction unit is used for carrying out synchronous phase error correction on the signal to be corrected to obtain a first intermediate signal;

the pulse compression processing unit is used for performing pulse compression processing on the first intermediate signal to obtain a second intermediate signal; the second intermediate signal is a distance frequency domain signal;

and the ionosphere effect correction unit is used for carrying out absolute phase error correction and space-variant phase error correction on the second intermediate signal to obtain a corrected signal.

14. The L-band spaceborne bistatic SAR signal processing device according to claim 13, wherein said ionospheric effect correction unit comprises: determining a sub-unit and a compensation sub-unit, wherein:

the determining subunit is configured to determine an TEC initial estimation interval corresponding to the signal to be corrected; the TEC initial estimation interval comprises an optimal TEC estimation value; and determining the optimal TEC estimation value in the TEC initial estimation interval;

and the compensation subunit is configured to compensate the optimal TEC estimation value back to the second intermediate signal, so as to complete the absolute phase error correction.

15. The L-band spaceborne bistatic SAR signal processing device according to claim 13 or 14,

the determining subunit is further configured to calculate to obtain a target intersection point coordinate by using a space linear equation; the target intersection point coordinate is the intersection point coordinate of the radar beam and the ionized layer; calculating a target TEC gradient value along the direction vertical to the base line based on the target intersection point coordinate; the base line is a connecting line between a main satellite and an auxiliary satellite of the satellite-borne bistatic SAR;

the compensation subunit is further configured to compensate the target TEC gradient value back to the second intermediate signal, thereby completing the space-variant phase error correction.

16. An L-band spaceborne bistatic SAR signal processing device is characterized by comprising:

a memory for storing executable instructions;

a processor for implementing the method of any one of claims 1 to 12 when executing executable instructions stored in the memory.

17. A storage medium having stored thereon executable instructions for causing a processor to perform the method of any one of claims 1 to 12 when executed.

Images