An Improved RD Imaging Method Applicable to GEOSAR

Abstract

The present invention discloses an improved RD imaging method applicable to GEOSAR, including: obtaining the slant range error caused by ionospheric refraction according to the spatio-temporal variation characteristics of the background ionosphere within the synthetic aperture time; obtaining the two-way slant range error value caused by the ionosphere based on the non "stop-go-stop" assumption; obtaining the two-dimensional echo signal expression of GEOSAR; using the series inversion method to obtain the simplified expression of the two-dimensional echo signal of GEOSAR; processing the influence of the ionosphere; compensating the influence of the high-order phase function; range compression and range migration correction; azimuth compression processing; obtaining a clear two-dimensional image of point targets. The improved RD imaging method applicable to GEOSAR provided by the present invention can achieve clear imaging of targets within a large scene range; and the method of the present invention has a small amount of calculation and is convenient to be applied in actual engineering.

Description

Technical Field

[0001] The present invention relates to the technical field of SAR imaging, and particularly relates to an improved RD imaging method applicable to GEOSAR. Background Art

[0002] Geosynchronous synthetic aperture radar (GEOSAR) has the advantages of short revisit period and strong anti-strike ability, and has broad application prospects and development potential in military and civilian fields. During the transmission of GEOSAR signals, it is inevitably affected by the ionosphere. For LEOSAR, the influence of the ionosphere on imaging quality can be ignored due to the short synthetic aperture time. However, GEOSAR generally operates in the L band. Due to the long synthetic aperture time, generally on the order of hours, the time-varying characteristics of the ionosphere will seriously affect the imaging focusing performance of this radar.

[0003] In the existing imaging methods applicable to GEOSAR, there are problems such as high algorithm complexity, long processing time, and the influence of the target slant range high-order phase error within the long synthetic aperture time on imaging quality is not considered, and the influence of path loss is not considered. Summary of the Invention

[0004] The purpose of the present invention is to provide an improved RD imaging method applicable to GEOSAR, aiming to solve the problems in the prior art such as high algorithm complexity, long processing time, and the influence of the target slant range high-order phase error within the long synthetic aperture time on imaging quality is not considered, and the influence of path loss is not considered.

[0005] To achieve the above object, the present invention provides an improved RD imaging method applicable to GEOSAR, including:

[0006] Step S1: Obtain the slant range error caused by ionospheric refraction according to the spatio-temporal variation characteristics of the background ionosphere within the synthetic aperture time;

[0007] Step S2: Based on the non-"stop-go-stop" assumption, obtain the two-way slant range error value caused by the ionosphere;

[0008] Step S3: Obtain the two-dimensional echo signal expression of GEOSAR;

[0009] Step S4: Use the series inversion method to obtain the simplified expression of the two-dimensional echo signal of GEOSAR;

[0010] Step S5: Process the influence of the ionosphere;

[0011] Step S6: Compensate the influence of the high-order phase function;

[0012] Step S7: Range compression and range migration correction;

[0013] Step S8: Azimuth compression processing;

[0014] Step S9: Obtain a clear two-dimensional image of the point target.

[0015] Preferably, in the step S1: Define that at the moment of t0 + t a , the ionospheric refractive index at the altitude h is n ion (t0 + t a , h), and the slant range error R ion (t0 + t a ) caused by the ionospheric refraction is expressed as:

[0016]

[0017] In the formula: f represents the operating frequency;

[0018] TEC is the integrated electron content;

[0019] According to the time variation characteristics of the integrated electron content, assume that the integrated electron content at the moment of t0 is TEC0, and TEC changes with the slow time t a as:

[0020]

[0021] In the formula: E n (n = 1, 2,...) is the nth derivative of TEC,

[0022] and

[0023] Preferably, in the step S2, based on the non-"stop-go-stop" assumption, the two-way slant range error RR ion (t0 + t a ) caused by the ionosphere is expressed as:

[0024]

[0025] In the formula: τ is the electromagnetic wave transmission time from the radar to the target;

[0026] The polynomial coefficients are

[0027]

[0028]

[0029]

[0030] f c is the range frequency.

[0031] Preferably, in the step S3, the two-dimensional echo signal Ss of the GEOSAR ion (f r , t a ) has the following expression:

[0032] where: ρ r (·) and ρ a (·) are the range window function and the azimuth window function of the radar chirp signal respectively;

[0033] γ is the chirp rate of the radar chirp signal;

[0034] C is the wave velocity;

[0035] f r is the azimuth frequency;

[0036] RR(t0 + t a ) is the slant range model under ideal conditions.

[0037] Preferably, in the step S4, since f r is much smaller than f c , the expression of the two-dimensional echo signal of the GEOSAR is:

[0038] Ss ion (f r , t a ) = A i Ss(f r , t a )H ir (f r )H ia (t a )

[0039] where:

[0040]

[0041]

[0042]

[0043] A i = exp(-j2πq0).

[0044] Ss(f r , t a ) represents the range frequency - azimuth time domain expression of the ideal echo signal of the GEOSAR; H ir (f r) and H ia (t a ) respectively represent the influence of the ionosphere on the range - and azimuth - direction impulse response functions; A i represents the influence of the ionosphere on the amplitude of the echo signal,

[0045] Through Taylor series expansion, H ir (f r ) can be expanded as

[0046]

[0047] In the above formula, its first exponential term represents the influence of the ionosphere on the amplitude of the echo signal; the second exponential term reflects the range walk term caused by the ionosphere, and its image range offset is which has a more serious impact on low - frequency radar; the remaining exponential terms all cause a decrease in range resolution ability, and its peak second - order phase quantity is The peak high - order phase quantity is When the total phase error is greater than π / 4, it will lead to a decrease in range resolution ability, otherwise the influence of the ionosphere on range resolution ability can be ignored.

[0048] H ia (t a ) can be expanded and expressed as:

[0049]

[0050] In the above formula, the first exponential term is the azimuth walk term, indicating that the image azimuth offset is -λR c 40.3E1 / (f c CV s (t0)); the second and third exponential terms both cause a decrease in azimuth resolution ability, and its peak second - order phase error is The peak high - order phase error is

[0051] Preferably, in the step S5:

[0052] Apply the dual - frequency time - delay difference estimation method to obtain the TEC quantity varying with time, estimate and obtain the phase functions and According to the phase functions and for the GEOSAR echo signal Ss ion (f r ,t a ) contaminated by the ionosphere, perform phase compensation processing. The expression of the GEOSAR echo signal Ss ion2 (f r ,t a ) after phase compensation processing is:

[0053]

[0054] The GEOSAR echo signal after phase compensation processing includes an ideal echo signal Ss(f r , t a ) and the ionospheric influence residual signal H i2 (f r , t a ). Then the expression of the foregoing Ss ion2 (f r , t a ) is expressed as:

[0055] Ss ion2 (f r , t a ) = Ss(f r , t a )H i2 (f r , t a )

[0056] By azimuth Fourier transform, applying the series inversion algorithm to obtain the two-dimensional spectrum expression of the point target of the GEOSAR:

[0057]

[0058] Preferably, in the step S6:

[0059] There are high-order range and azimuth coupling terms in the two-dimensional spectrum expression of the point target of the GEOSAR, which makes the radar echo signal different from the signal form of the reference RD. Therefore, first remove the high-order coupling terms, that is, multiply by its high-order phase compensation function in the two-dimensional frequency domain, and the compensation function is

[0060]

[0061] Preferably, in the step S7:

[0062] Perform range compression processing on the echo signal, that is, multiply by the range compression function in the two-dimensional frequency domain, and the expression of the range compression function is:

[0063]

[0064] Through range migration correction, focus the image range towards the slant range R sm of the point target, and the function expression of the range migration correction is:

[0065]

[0066] Preferably, in the step S8:

[0067] The azimuth compression process is specifically multiplying in the range-time - azimuth frequency domain by an azimuth compression function, and the expression of the compression function is:

[0068]

[0069] Preferably, in step S9:

[0070] According to the autofocus algorithm, the residual ionospheric phase error in the echo signal is processed so that the processed imaging result is focused on the true target position.

[0071] Compared with the prior art, the present invention has the following beneficial effects:

[0072] The improved RD imaging method applicable to GEOSAR provided by the present invention analyzes the influence of the ionosphere on the GEOSAR echo signal model, analyzes the simplified two-dimensional echo signal model of GEOSAR by using the series inversion method, and uses the improved RD imaging algorithm to achieve clear imaging of targets within a large scene range; moreover, the method of the present invention has a small amount of calculation and is convenient to be applied in actual engineering. BRIEF DESCRIPTION OF THE DRAWINGS

[0073] In order to more clearly illustrate the technical solutions of the present invention, the drawings required for description will be briefly introduced below. Obviously, the drawings in the following description are an embodiment of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts:

[0074] Figure 1 It is a schematic flowchart of an improved RD imaging method applicable to GEOSAR provided by an embodiment of the present invention;

[0075] Figure 2 It is a diagram of the daily TEC variation trend in a certain day in Beijing provided by an embodiment of the present invention;

[0076] Figure 3 It is a simulation result diagram of the embodiment provided by an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0077] The following is combined with the attached Figures 1-3The following specific implementation manners further elaborate on the improved RD imaging method for GEOSAR proposed by the present invention. According to the following description, the advantages and features of the present invention will become clearer. It should be noted that the attached drawings are in a very simplified form and use non-precise scales, only for conveniently and clearly assisting in explaining the purpose of the embodiments of the present invention. To make the purpose, features, and advantages of the present invention more obvious and understandable, please refer to the attached drawings. It should be known that the structures, scales, sizes, etc. shown in the drawings of this specification are only used to cooperate with the content disclosed in the specification for those skilled in this technology to understand and read, and are not used to limit the limiting conditions for the implementation of the present invention. Therefore, they do not have substantial technical significance. Any modification of the structure, change in the proportional relationship, or adjustment of the size, without affecting the effects that the present invention can produce and the purposes that can be achieved, should still fall within the scope covered by the technical content disclosed by the present invention.

[0078] In view of the deficiencies in the imaging methods for GEOSAR in the prior art, in order to enable the imaging method for GEOSAR to achieve clear imaging of targets within a large scene range, reduce the computational amount, and facilitate its application in actual projects, the present invention provides the following embodiments:

[0079] Embodiment conditions: According to the global ionospheric grid map provided by IGS on November 1, 2018, taking the Beijing area as an example to observe the daily variation trend of TEC, as Figure 2 shown.

[0080] As Figure 1 shown, the method proposed by the present invention performs the following steps on the above embodiment conditions:

[0081] Step S1: Obtain the slant range error caused by ionospheric refraction according to the spatio-temporal variation characteristics of the background ionosphere within the synthetic aperture time.

[0082] In the said step S1: Define that at the moment of t0 + t a , the ionospheric refractive index at the height h is n ion (t0 + t a , h), and the expression of the slant range error R ion (t0 + t a ) is:

[0083]

[0084] In formula (1): f represents the operating frequency;

[0085] TEC is the integrated electron content;

[0086] According to the time variation characteristics of the said integrated electron content, assume that the integrated electron content at the moment of t0 is TEC0, and the variation of TEC with the slow time t a is:

[0087]

[0088] Where: E n (n = 1, 2, …) is the nth derivative of TEC,

[0089] and

[0090] Step S2: Based on the non - "stop - go - stop" assumption, obtain the two - way slant - range error value caused by the ionosphere;

[0091] In the said step S2, based on the non - "stop - go - stop" assumption, obtain the two - way slant - range error RR ion (t0 + t a ) and its expression is:

[0092]

[0093] In Equation (3): τ is the electromagnetic wave propagation time from the radar transmitting electromagnetic wave to the target;

[0094] The polynomial coefficients are

[0095]

[0096]

[0097]

[0098] f c is the range frequency.

[0099] Step S3: Obtain the two - dimensional echo signal expression of GEOSAR.

[0100] In the said step S3, the two - dimensional echo signal Ss ion (f r , t a ) and its expression is:

[0101]

[0102] In Equation (4): ρ r (·) and ρ a (·) are the range window function and azimuth window function of the radar linear frequency - modulated signal respectively;

[0103] γ is the frequency modulation rate of the radar linear frequency - modulated signal;

[0104] C is the wave speed;

[0105] f ris the azimuth frequency;

[0106] RR(t0 + t a ) is the slant range model under ideal conditions.

[0107] Step S4: Use the series inversion method to obtain the simplified expression of the two-dimensional echo signal of the GEOSAR;

[0108] In the said step S4, since f r is much smaller than f c , the expression of the two-dimensional echo signal of the GEOSAR is:

[0109] Ss ion (f r , t a ) = A i Ss(f r , t a )H ir (f r )H ia (t a ) (5)

[0110] In the formula:

[0111]

[0112]

[0113]

[0114] A i = exp(-j2πq0).

[0115] Ss(f r , t a ) represents the expression of the ideal echo signal of the GEOSAR in the range frequency-azimuth time domain; H ir (f r ) and H ia (t a ) respectively represent the influence of the ionosphere on the range and azimuth impulse response functions; A i represents the influence of the ionosphere on the amplitude of the echo signal, and the subscript "i" in H ir (f r ) and H ia (t a ) represents the ionosphere, "a" represents the direction dimension, and "r" represents the range dimension.

[0116] Through Taylor series expansion, H ir (f r ) can be expanded as

[0117]

[0118]

[0119] In the above formula (6), the first exponential term represents the influence of the ionosphere on the amplitude of the echo signal; the second exponential term reflects the range walk term caused by the ionosphere, and its image range offset is It has a relatively serious impact on low-frequency radars; the remaining exponential terms all cause a decrease in range resolution ability, and its peak quadratic phase quantity is The peak high-order phase quantity is When the total phase error is greater than π / 4, it will lead to a decrease in range resolution ability, otherwise the influence of the ionosphere on range resolution ability can be ignored.

[0120] H ia (t a ) can be expanded as:

[0121]

[0122] In the above formula (7), the first exponential term is the azimuth walk term, indicating that the image azimuth offset is -λR c 40.3E1 / (f c CV s (t0)); the second and third exponential terms both cause a decrease in azimuth resolution ability, and its peak quadratic phase error is The peak high-order phase error is

[0123] Step S5: Process the ionospheric influence;

[0124] In the said step S5:

[0125] Apply the dual-frequency time-delay difference estimation method to obtain the TEC quantity varying with time, and estimate to obtain the phase function and According to the said phase function and For the GEOSAR echo signal Ss ion (f r ,t a ) contaminated by the ionosphere, perform phase compensation processing, and the expression of the GEOSAR echo signal Ss ion2 (f r ,t a ) after phase compensation processing is:

[0126]

[0127] The GEOSAR echo signal after phase compensation processing includes the ideal echo signal Ss(fr ,t a ) and the ionospheric influence residual signal H i2 (f r ,t a ), then the aforementioned Ss ion2 (f r ,t a ) is expressed as:

[0128] Ss ion2 (f r ,t a ) = Ss(f r ,t a )H i2 (f r ,t a ) (9)

[0129] Through azimuth Fourier transform and applying the series inversion algorithm, the two-dimensional spectrum expression of the point target of the GEOSAR is obtained:

[0130]

[0131] Step S6: Compensate for the influence of the high-order phase function;

[0132] In the aforementioned step S6:

[0133] There are high-order range and azimuth coupling terms in the two-dimensional spectrum expression of the point target of the GEOSAR, which makes the radar echo signal different from the signal form of the reference RD. Therefore, first remove the high-order range and azimuth coupling terms, that is, multiply by its high-order phase compensation function in the two-dimensional frequency domain, and this compensation function is

[0134]

[0135] Step S7: Range compression and range migration correction;

[0136] In the aforementioned step S7:

[0137] Perform range compression processing on the echo signal, that is, multiply by the range compression function in the two-dimensional frequency domain, and the expression of the range compression function is:

[0138]

[0139] Through range migration correction, focus the image range towards the slant range R of the point target sm , and the function expression of the range migration correction is:

[0140]

[0141] Step S8: Azimuth compression processing;

[0142] In the said step S8:

[0143] The azimuth compression processing is specifically multiplying the range-time - azimuth frequency domain by an azimuth compression function, and the expression of the compression function is:

[0144]

[0145] As Figure 3 shown in, Figure (a) represents the range-direction pulse function, and Figure (b) is the azimuth-direction pulse response function.

[0146] Step S9: Obtain a clear two-dimensional image of the point target.

[0147] In the said step S9:

[0148] According to the traditional autofocus algorithm, process the residual ionospheric phase error in the echo signal so that the processed imaging result focuses on the real target position.

[0149] In summary, the provided improved RD imaging method applicable to GEOSAR analyzes the influence of the ionosphere on the GEOSAR echo signal model, uses the series inversion method to analyze the simplified two-dimensional echo signal model of GEOSAR, and uses the improved RD imaging algorithm to achieve clear imaging of the target within a large scene range; moreover, the method of this embodiment has a small computational amount, reduces the processing time, and is convenient to be applied in practical engineering.

[0150] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device. Without more limitations, the element defined by the statement "including a..." does not exclude the existence of additional identical elements in the process, method, article or device including the said element.

[0151] It should be noted that the devices and methods disclosed in the embodiments of this article can also be implemented in other ways. The device embodiments described above are merely illustrative. For example, the flowcharts and block diagrams in the accompanying drawings show the possible architectures, functions, and operations of devices, methods, and computer program products according to multiple embodiments of this article. In this regard, each block in the flowchart or block diagram may represent a module, program, or part of code that contains one or more executable instructions for implementing the specified logical function. It should also be noted that in some alternative implementations, the functions marked in the blocks may occur in a different order than that marked in the accompanying drawings. For example, two consecutive blocks can actually be executed substantially in parallel, and they can sometimes be executed in the reverse order, depending on the functions involved. It should also be noted that each block in the block diagram and / or flowchart, as well as combinations of blocks in the block diagram and / or flowchart, can be implemented by a dedicated hardware-based system for performing the specified functions or actions, or can be implemented by a combination of dedicated hardware and computer instructions.

[0152] Although the content of the present invention has been introduced in detail through the above preferred embodiments, it should be recognized that the above description should not be considered as a limitation of the present invention. After those skilled in the art have read the above content, various modifications and alternatives to the present invention will be obvious. Therefore, the protection scope of the present invention should be defined by the appended claims.

Claims

1. An improved RD imaging method applicable to GEOSAR, characterized in that, Including: Step S1: Obtain the slant range error caused by ionospheric refraction according to the spatio-temporal variation characteristics of the background ionosphere within the synthetic aperture time. Step S2: Based on the non-"stop-go-stop" assumption, obtain the two-way slant range error value caused by the ionosphere. Step S3: Obtain the two-dimensional echo signal expression of GEOSAR. Step S4: Use the series inversion method to obtain the simplified two-dimensional echo signal expression of GEOSAR. Step S5: Process the influence of the ionosphere, specifically: The TEC quantity varying with time is obtained by applying the dual-frequency time-delay difference estimation method, and the phase function is estimated and According to the phase function and For the GEOSAR echo signal Ss ion (f r ,t a ) contaminated by the ionosphere, phase compensation processing is performed. The GEOSAR echo signal Ss ion2 (f r ,t a ) after the phase compensation processing has the following expression: Among them, t a represents the slow time; f r is the azimuth frequency; Ss ion (f r , t a ) represents the two-dimensional echo signal of GEOSAR; The GEOSAR echo signal after phase compensation processing includes an ideal echo signal Ss(f r , t a ) and the ionospheric influence residual signal H i2 (f r , t a ). If so, the expression of the foregoing Ss ion2 (f r , t a ) is expressed as: Ss ion2 (f r ,t a ) = Ss(f r ,t a )H i2 (f r ,t a ) Through azimuth Fourier transform, apply the series inversion algorithm to obtain the two-dimensional spectrum expression of the point target of GEOSAR: where ρ r (·) and ρ a (·) are the range window function and azimuth window function of the radar chirp signal, respectively; Step S6: Compensate the influence of the high-order phase function, specifically: The two-dimensional frequency spectrum expression of the point target of GEOSAR has high-order range and azimuth coupling terms. Removing the high-order range and azimuth coupling terms means multiplying by its high-order phase compensation function in the two-dimensional frequency domain. The phase compensation function H1(f r ,f a ) is expressed as: Step S7: Range compression and range migration correction. Step S8: Azimuth compression processing. Step S9: Obtain a clear two-dimensional image of the point target.

2. The improved RD imaging method applicable to GEOSAR according to claim 1, characterized in that, In the step S1: Define that at the moment \(t_0 + t\) a , the ionospheric refractive index at the height \(h\) is \(n\) ion \((t_0 + t\) a , h)\), and the slant range error \(R\) ion \((t_0 + t\) a ) is expressed as: In the formula: f represents the operating frequency; TEC represents the integrated electron content; ion represents the ionosphere; According to the time-varying characteristics of the integrated electron content, assuming that the integrated electron content at time t0 is TEC0, the change of TEC with slow time t a is as follows: where: E n (n = 1, 2, …) is the nth derivative of TEC, and 3. The improved RD imaging method applicable to GEOSAR according to claim 2, characterized in that, In the step S2, based on the non-"stop-go-stop" assumption, the two-way slant range error RR caused by the ionosphere is obtained. ion (t0 + t a ) is expressed as: In the formula: τ is the electromagnetic wave transmission time from the radar transmitting electromagnetic wave to the target; The polynomial coefficients are f c is the range frequency.

4. The improved RD imaging method applicable to GEOSAR according to claim 3, characterized in that, In the step S3, the two-dimensional echo signal Ss of the GEOSAR ion (f r ,t a ) is expressed as: where: ρ r (·) and ρ a (·) are the range window function and azimuth window function of the radar chirp signal, respectively; γ is the chirp rate of the radar linear frequency modulation signal; C is the wave speed; f r is the azimuth frequency; RR(t0 + t a ) is the slant range model under ideal conditions.

5. The improved RD imaging method applicable to GEOSAR according to claim 4, wherein In the step S4, since f r is much smaller than f c , the two-dimensional echo signal expression of the GEOSAR is as follows: Ss ion (f r ,t a ) = A i Ss(f r ,t a )H ir (f r )H ia (t a ) In the formula: A i = exp(-j2πq0), Ss(f r ,t a ) represents the range-frequency - azimuth-time domain expression of the ideal echo signal of the said GEOSAR; H ir (f r ) and H ia (t a ) respectively represent the influence of the ionosphere on the impulse response functions in the range and azimuth directions; A i Indicates the influence of the ionosphere on the amplitude of the echo signal.

6. The improved RD imaging method applicable to GEOSAR according to claim 5, characterized in that, In the said Step S7: Perform range compression processing on the echo signal, that is, multiply by the range compression function in the two-dimensional frequency domain, and the expression of the range compression function is: Through range migration correction, the range direction of the image is focused on the slant range R of the point target sm , and the functional expression of the range migration correction is as follows:

7. The improved RD imaging method applicable to GEOSAR according to claim 6, wherein In the said Step S8: The azimuth compression processing is specifically to multiply by the azimuth compression function in the range-time - azimuth frequency domain, and the expression of the compression function is:

8. An improved RD imaging method applicable to GEOSAR according to claim 7, characterized in that, In the said Step S9: According to the autofocus algorithm, process the residual ionospheric phase error in the echo signal so that the imaging result after processing focuses on the real target position.

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