Airborne SAR long-distance high-resolution imaging method

Abstract

The invention belongs to the technical field of SAR imaging, and particularly relates to an airborne SAR long-distance high-resolution imaging method, which is a method for carrying out long-distance high-resolution SAR imaging on a ground scene by an airborne radar, and solves the problem of range ambiguity caused by cross-period sampling through alternate orthogonal waveform transceiving processing. The time delay of an echo sampling window is adjusted in real time according to the distance change of a radar and an imaging scene, the range unit migration of an echo signal is corrected, the range direction sampling data volume is reduced, an azimuth filtering downsampling method is adopted, the SAR imaging azimuth direction processing data volume is reduced, the imaging efficiency is improved, work can be carried out at a high radar pulse repetition frequency, and the imaging efficiency is improved. The method improves the radar duty ratio, reduces the requirement for the pulse width of a transmitted signal, simplifies the radar design, can achieve the long-distance high-resolution imaging of a ground scene, and reduces the demands for radar energy storage, signal processing and other hardware resources.

Description

Technical Field

[0001] The present application belongs to the technical field of SAR imaging, and specifically relates to an airborne SAR long-distance high-resolution imaging method, which is a method for airborne radar to perform long-distance high-resolution SAR imaging of ground scenes. Background Art

[0002] As an active microwave remote sensing device, synthetic aperture radar (SAR) not only has the characteristics of conventional radar in all-day, all-weather and long-range operation, but can also perform two-dimensional high-resolution imaging of ground scenes, which greatly expands the functions of conventional radar, improves the radar's information acquisition and perception capabilities, and enables radar to have the ability to reliably identify targets.

[0003] The effective range and imaging resolution are two key performance indicators of SAR imaging. A longer effective range can keep the carrier platform away from dangerous areas, improve the survivability of the carrier platform, and also increase the coverage of the beam, obtain large scene images, and improve radar reconnaissance efficiency. Higher imaging resolution is conducive to the acquisition of target detail information, improves the probability of target recognition, and achieves a detailed description of the target. However, long-range high-resolution imaging requires, on the one hand, that the radar transmit signal has a higher average power to ensure the image signal-to-noise ratio of the SAR imaging result, and on the other hand, requires a longer synthetic aperture time for coherent accumulation, which brings huge challenges to data acquisition and processing, which are specifically reflected in the following aspects:

[0004] 1) SAR waveform design is difficult. According to the SAR radar equation, the average power of the transmitted signal is positively correlated with the imaging distance and resolution. Improving the imaging distance and resolution requires a higher average power of the transmitted signal. Under the same peak power condition, a higher signal duty cycle is required. If the pulse repetition frequency remains unchanged, increasing the duty cycle will increase the pulse width of the transmitted signal, which will in turn increase the number of distance sampling points and the amount of echo data. This will increase the back-end processing pressure, and the pulse width of the transmitted signal may be too large to exceed the radar system capability and cannot be achieved. If a certain pulse width is maintained, in order to reduce the transmitted pulse width under the premise of a certain signal duty cycle, it is necessary to increase the pulse repetition frequency of the radar system to increase the duty cycle, which will cause distance ambiguity of the echo signal;

[0005] 2) SAR range signal acquisition and processing are difficult. In order to meet the azimuth resolution requirements, the acquired echo data must be able to contain all the echoes of the synthetic aperture that meet the resolution requirements. Therefore, the length of the SAR range acquisition data must cover three parts: the sampling data corresponding to the transmission pulse width, the sampling data corresponding to the scene range width, and the sampling data corresponding to the range unit migration. The synthetic aperture time of long-distance high-resolution imaging is long, and the range unit migration of the echo signal is serious. The fixed delay sampling method used by conventional radars will lead to a very high number of range sampling points, which increases the amount of echo data and signal processing operations, and poses a challenge to the system link bandwidth and computing power;

[0006] 3) The amount of azimuth accumulated data is large and imaging processing is difficult. In order to meet the average power requirements of the transmitted signal and reduce the transmit pulse width for long-distance high-resolution imaging, the radar transmits and receives signals at a higher pulse repetition frequency, resulting in a significant increase in the number of azimuth accumulated pulses within the synthetic aperture time, which greatly increases the amount of data and calculations for SAR imaging processing. Moreover, the radar pulse repetition frequency may be much higher than the Doppler bandwidth of the echo signal, and there is a large amount of sampling redundancy in the azimuth direction that is not beneficial to improving the SAR imaging results, resulting in a waste of processing resources.

[0007] At present, there are many studies on airborne SAR long-range high-resolution imaging technology, and there are rich research results in system structure, working mode, imaging algorithm, etc. However, the existing fuzzy suppression method based on multi-channel system structure faces the problems of difficult engineering implementation and complex data processing; the working mode design is mostly antenna beam scanning design, and does not involve the collection of echo signals; the imaging algorithm mainly pursues higher imaging accuracy, and does not consider the amount of data and the amount of calculation for engineering implementation. In view of this, this application is proposed. Summary of the invention

[0008] A method for long-range high-resolution imaging of airborne SAR is provided. On the basis of the existing imaging method, the radar transmission waveform, echo collection method and imaging processing method are improved to solve the problems of cross-period sampling distance ambiguity caused by high radar pulse repetition frequency and large range unit migration, high amount of range sampling data and large amount of calculation due to large number of azimuth accumulation pulses, thus laying a necessary foundation for future engineering applications.

[0009] The technical solution of this application is:

[0010] An airborne SAR long-distance high-resolution imaging method, comprising:

[0011] Step 1: Control the radar beam to illuminate the center of the imaging scene and transmit broadband orthogonal signals alternately;

[0012] Step 2: According to the distance between the scene center and the antenna phase center, the radar receiving sampling delay is calculated and adjusted in real time, and the threshold sampling is changed to receive the echo signal;

[0013] Step 3: Perform range-matched filtering on the echo signal according to the form of each transmitted broadband orthogonal signal, and correct the phase of each frequency component of the echo broadband signal to be the same;

[0014] Step 4: Compensate for the sampling delay variation in the range frequency domain to restore the range unit migration characteristics of the echo signal;

[0015] Step 5: Perform Doppler center compensation on the echo signal and set the azimuth Doppler center compensation to zero;

[0016] Step 6: Add multiple echo signals adjacent to each other in azimuth to achieve azimuth downsampling;

[0017] Step 7: Perform Doppler center inverse compensation on the echo signal to restore the Doppler center of the echo signal;

[0018] Step 8: Adjust the pulse repetition frequency used for echo signal processing, use the SAR imaging algorithm to process the echo signal, and obtain a SAR image.

[0019] According to at least one embodiment of the present application, in the above-mentioned airborne SAR long-distance high-resolution imaging method, in step 1, the radar beam is controlled to illuminate the center of the imaging scene, and broadband orthogonal signals are alternately emitted, including positive frequency modulation signals s + (t) and negative frequency modulation signal s - (t):

[0020]

[0021]

[0022] in,

[0023] rect(·) is the calculation function;

[0024] t is time;

[0025] T p is the pulse width of the transmitted signal;

[0026] j is the imaginary unit;

[0027] f c is the center frequency of the transmitted signal;

[0028] γ is the frequency modulation slope of the transmitted signal.

[0029] According to at least one embodiment of the present application, in the above-mentioned airborne SAR long-distance high-resolution imaging method, in step 2, according to the distance between the scene center and the antenna phase center, the radar receiving sampling delay is calculated and adjusted in real time to 2R N / C, where RN is the distance from the phase center of the radar antenna to the center of the scene when the sampling delay is adjusted for the Nth time; C is the speed of light;

[0030]

[0031] in,

[0032] R s is the distance from the antenna phase center to the scene center;

[0033] θ s is the SAR imaging oblique angle;

[0034] n is the number of pulses for adjusting the sampling delay interval;

[0035] v is the carrier speed;

[0036] PRF is the radar pulse repetition frequency.

[0037] According to at least one embodiment of the present application, in the above-mentioned airborne SAR long-distance high-resolution imaging method, in step 2, the method of adjusting the radar receiving sampling delay includes a continuous adjustment method and a periodic adjustment method, wherein:

[0038] When continuous adjustment is adopted, n=1;

[0039] When the periodic adjustment method is adopted, n>1.

[0040] According to at least one embodiment of the present application, in the above-mentioned airborne SAR long-range high-resolution imaging method, in step 3, according to the form of each transmitted broadband orthogonal signal, the echo signal is subjected to range-directed matched filtering, using a positive frequency modulation signal range-directed matched filtering function s + (f r ), negative frequency modulation signal distance matched filter function s - (f r );

[0041]

[0042] in,

[0043] f r is the distance frequency;

[0044] w r (·) is the range frequency domain window function.

[0045] According to at least one embodiment of the present application, in the above-mentioned airborne SAR long-distance high-resolution imaging method, in step 4, the sampling delay variation is compensated in the range frequency domain, specifically, each echo is compensated for a phase in the range frequency domain, and the phase compensation function is H com (m,f r);

[0046]

[0047] in,

[0048] m is the echo pulse number;

[0049] is the ceiling function.

[0050] According to at least one embodiment of the present application, in the above-mentioned airborne SAR long-range high-resolution imaging method,

[0051] Step 5: Perform Doppler center compensation on the echo signal. The compensation function is H fdc (m);

[0052]

[0053] in,

[0054] f dc is the Doppler center of the echo signal;

[0055] N a Accumulates the number of pulses for the bearing.

[0056] According to at least one embodiment of the present application, in the above-mentioned airborne SAR long-distance high-resolution imaging method, in step 6, multiple echo signals adjacent in azimuth are added, specifically:

[0057] s′(m)=s((m-1)×K+1)+s((m-1)×K+2)+…+s(m×K)…(7)

[0058] in,

[0059] s′(m) is the mth echo signal after addition, m=1,2,3,…N a / K;

[0060] s(m×K) is the m×Kth echo signal before addition;

[0061] K is the azimuth downsampling factor.

[0062] According to at least one embodiment of the present application, in the above-mentioned airborne SAR long-distance high-resolution imaging method, in step 7, the Doppler center inverse compensation is performed on the echo signal, and the compensation function is H′ fdc (m);

[0063]

[0064] According to at least one embodiment of the present application, in the above-mentioned airborne SAR long-distance high-resolution imaging method, in step eight, the pulse repetition frequency used for echo signal processing is adjusted to PRF′;

[0065] PRF′=PRF / K.

[0066] This application has at least the following beneficial technical effects:

[0067] The invention provides an airborne SAR long-distance high-resolution imaging method. By means of alternate orthogonal waveform transceiver processing, the distance ambiguity problem caused by cross-period sampling is solved. The echo sampling window delay is adjusted in real time according to the distance change between the radar and the imaging scene, the distance unit migration of the echo signal is corrected, and the amount of sampling data in the distance direction is reduced. The azimuth filtering downsampling method is adopted to reduce the amount of SAR imaging azimuth processing data, improve the imaging efficiency, operate at a higher radar pulse repetition frequency, improve the radar duty cycle, reduce the requirement for the pulse width of the transmission signal, simplify the radar design, and can reduce the demand for radar energy storage, signal processing and other hardware resources while realizing long-distance high-resolution imaging of ground scenes. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Figure 1 A schematic diagram of the airborne SAR long-range high-resolution imaging method provided in this application;

[0069] Figure 2 Schematic diagram of the working geometry of airborne SAR long-range high-resolution imaging provided for this application;

[0070] Figure 3 A schematic diagram of a radar providing for the present application that alternately transmits broadband orthogonal signals;

[0071] Figure 4 The scene simulation imaging using conventional single frequency modulation signal provided in this application;

[0072] Figure 5 The scene simulation imaging using positive and negative frequency modulation signals provided in this application;

[0073] Figure 6 A schematic diagram comparing conventional fixed threshold and variable sampling delay echo sampling provided in this application;

[0074] Figure 7 A schematic diagram of the simulation results of conventional fixed threshold sampling receiving echo provided in the present application;

[0075] Figure 8 A schematic diagram of the simulation results of the variable threshold sampling receiving echo provided by the present application;

[0076] Fig. 9This is a schematic diagram of simulation results of imaging point targets A and B using the airborne SAR long-range high-resolution imaging method provided by the present application;

[0077] Fig.10 Contour map of the imaging results of the center and edge points of the point target A scene provided in this application;

[0078] Fig.11 This is a contour map of the imaging results of the center point and edge point of the point target B scene provided in this application.

[0079] In order to better illustrate the present embodiment, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of the actual product. In addition, the drawings are only used for illustrative purposes and should not be construed as limitations on the present application. DETAILED DESCRIPTION

[0080] In order to make the technical solution and advantages of the present application clearer, the technical solution of the present application will be described in further detail in detail and in detail with reference to the accompanying drawings. It can be understood that the specific embodiments described here are only partial embodiments of the present application, which are only used to explain the present application, not to limit the present application. It should be noted that, for the convenience of description, only the parts related to the present application are shown in the accompanying drawings, and other related parts can refer to the general design.

[0081] In addition, unless otherwise defined, the technical terms or scientific terms used in the description of this application shall have the usual meanings understood by those of ordinary skill in the art to which this application belongs. The words used in the description of this application to indicate orientation are only used to indicate relative directions or positional relationships. When the absolute position of the described object changes, its relative positional relationship may also change accordingly. The word "include" used in the description of this application indicates that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, but does not exclude other elements or objects.

[0082] In addition, it should be noted that, unless otherwise clearly specified and limited, the words "installation", "connection" and similar terms used in the description of this application should be understood in a broad sense. For example, the connection can be a fixed connection or a detachable connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium. Technical personnel in the field can understand its specific meaning in this application according to the specific circumstances.

[0083] A method for long-range high-resolution imaging of airborne SAR, such as Figure 1 shown.

[0084] Step 1: Control the radar beam to illuminate the center of the imaging scene and alternately transmit broadband orthogonal signals.

[0085] The working geometry of airborne radar long-range high-resolution SAR imaging is as follows: Figure 2 As shown, the height of the carrier platform is H, and it flies in a straight line at a constant speed v. The radar beam illuminates the center of the imaging scene, transmits a broadband signal that meets the imaging resolution requirements, and receives the echo.

[0086] In order to solve the distance ambiguity problem faced by long-range SAR, the radar transmits broadband orthogonal signals alternately. Taking positive and negative frequency modulation signals as an example, the transmitted signal form is as follows Figure 3 As shown, the radar alternately transmits positive frequency modulation signals s + (t) and negative frequency modulation signal s - (t), can be expressed as follows:

[0087]

[0088]

[0089] in,

[0090] T p is the pulse width of the transmitted signal;

[0091] f c is the center frequency of the transmitted signal;

[0092] γ is the frequency modulation slope of the transmitted signal;

[0093] rect(·) is the calculation function;

[0094] j is the imaginary unit;

[0095] t is time.

[0096] The scene simulation imaging using conventional single FM signal is as follows Figure 4 As shown, the scene simulation imaging using positive and negative frequency modulation signals is as follows Figure 5 As shown in the figure, by comparison, it can be seen that the distance blur energy is significantly reduced when positive and negative FM signals are used for scene imaging.

[0097] Step 2: According to the distance between the scene center and the antenna phase center, the radar receiving sampling delay is calculated and adjusted in real time, and the threshold sampling is changed to receive the echo signal.

[0098] Taking the center of the scene as reference, assuming that the distance from the phase center of the radar antenna to the center of the scene is R s , the SAR imaging oblique angle is θ s , the radar changes the sampling delay once after n pulses. According to the radar working geometry, the distance R from the radar antenna phase center to the scene center when the sampling delay is adjusted for the Nth time N for:

[0099]

[0100] in,

[0101] PRF is the radar pulse repetition frequency.

[0102] The Nth adjustment sampling delay is 2R N / C, where C is the speed of light, can eliminate the sampling points caused by the migration of the range unit and reduce the amount of sampling data in the range direction of the echo. There are two specific forms:

[0103] Continuous adjustment mode: The sampling window delay is adjusted for each echo pulse, and in this case n = 1 in equation (3). The continuous adjustment mode can minimize the impact of range unit migration, but the sampling delay needs to be adjusted for each pulse, which places high demands on the radar system.

[0104] Periodic adjustment method: the sampling window delay is adjusted once every certain number of pulses. In this case, n>1 in equation (3). The periodic adjustment method only needs to adjust the sampling delay once every certain number of pulses, which can reduce the requirements on the radar system.

[0105] Comparison between conventional fixed threshold and variable sampling delay echo sampling, such as Figure 6 As shown, (a) is the conventional fixed threshold method, (b) is the continuous adjustment method, and (c) is the periodic adjustment method.

[0106] The simulation results of receiving echo with conventional fixed threshold sampling are as follows: Figure 7 As shown in the figure, the simulation results of receiving echo with variable threshold sampling are as follows: Figure 8 As shown in the figure, due to the existence of range unit migration, the number of range sampling points required for conventional fixed threshold sampling to receive echoes is very high, reaching 36K in the simulation results. This will bring great challenges to the storage and processing of the radar system. By changing the sampling delay of the echo and changing the threshold sampling to receive the echo, most of the range unit migration is compensated during the data acquisition process, and the number of range sampling points is reduced to 16K, which can greatly reduce the amount of echo data.

[0107] Step 3: According to the form of each transmitted broadband orthogonal signal, the echo signal is subjected to range matched filtering processing to correct the phases of each frequency component of the echo broadband signal to be the same.

[0108] When a signal with a known waveform is matched and filtered, its impulse response is the conjugate inversion of the waveform. Matched filtering can be quickly implemented in the distance frequency domain by multiplication. Taking positive and negative frequency modulation signals as an example, the distance-direction matched filter function s of the positive frequency modulation signal is + (f r ), negative frequency modulation signal distance matched filter function s - (f r ), which is expressed as follows:

[0109]

[0110] in,

[0111] f r is the distance frequency;

[0112] w r (·) is the range frequency domain window function.

[0113] Step 4: Compensate for the sampling delay variation in the range frequency domain and restore the range unit migration characteristics of the echo signal.

[0114] Since the sampling delay is adjusted during the echo signal acquisition process, the range unit migration of the echo signal is different from that of the conventional SAR echo signal. In order to use the existing mature imaging algorithm to process the echo data, the range unit migration characteristics of the data need to be restored before imaging processing.

[0115] The adjustment of sampling delay is equivalent to introducing additional delay to the echo data. According to the Fourier transform property, the time domain delay corresponds to the first phase in the frequency domain. Therefore, before imaging processing, each echo can be compensated for the first phase in the range frequency domain. The phase compensation function H is com (m,f r )as follows:

[0116]

[0117] in,

[0118] m is the echo pulse number;

[0119] is the ceiling function.

[0120] The above phase compensation can be performed simultaneously in the process of distance matching filtering and realized by fast multiplication without introducing additional computational complexity.

[0121] Step 5: Perform Doppler center compensation on the echo signal and set the azimuth Doppler center compensation to zero.

[0122] The PRF of a long-range SAR imaging system may be much higher than the Doppler bandwidth of the echo signal, and there is a large sampling redundancy in the azimuth direction. In order to reduce the amount of data processed in the azimuth direction, downsampling can be performed. Adding multiple adjacent pulses is a feasible method, which is equivalent to filtering the echo signal first and then downsampling it.

[0123] In order to reduce the impact on the echo signal energy and Doppler bandwidth, it is first necessary to perform Doppler center compensation on the echo signal and compensate the azimuth Doppler center to zero. The compensation function H fdc (m) are as follows:

[0124]

[0125] in,

[0126] f dc is the Doppler center of the echo signal;

[0127] N a Accumulates the number of pulses for the bearing.

[0128] Step 6: Add multiple echo signals adjacent to each other in azimuth to achieve azimuth downsampling.

[0129] Adding multiple pulse echoes adjacent to each other in azimuth can realize azimuth downsampling and reduce the amount of data for SAR imaging processing. The addition process can be expressed as:

[0130] s′(m)=s((m-1)×K+1)+s((m-1)×K+2)+…+s(m×K)…(7)

[0131] in,

[0132] s′(m) is the mth echo signal after addition, m=1,2,3,…N a / K;

[0133] s(m×K) is the m×Kth echo signal before addition;

[0134] K is the azimuth downsampling factor.

[0135] Step 7: Perform Doppler center inverse compensation on the echo signal to restore the Doppler center of the echo signal.

[0136] In order to ensure that the data characteristics after azimuth downsampling are the same as before downsampling, it is necessary to perform Doppler center inverse compensation on the echo signal after azimuth downsampling. The Doppler center compensated in step 5 is inversely compensated back to the signal. The compensation function H′ fdc (m) are as follows:

[0137]

[0138] Step 8: Adjust the pulse repetition frequency PRF′=PRF / K used for echo signal processing, use the SAR imaging algorithm to process the echo signal, perform SAR imaging, and obtain a long-distance high-resolution SAR image.

[0139] Using the airborne SAR long-distance high-resolution imaging method disclosed in the above embodiment, point targets A and B are imaged. The simulation results are as follows: Fig. 9 As shown, Fig.10 , Fig.11The contour maps are the imaging results of the center and edge points of the scene A and B of point targets. It can be seen that the imaging is well focused and the focusing quality does not decrease due to the introduction of sampling delay changes and azimuth filtering downsampling.

[0140] The various embodiments in the specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other. In the absence of conflict, the embodiments in this application and the technical features in the embodiments can be combined with each other to obtain new embodiments.

[0141] So far, the technical solution of the present application has been described in conjunction with the preferred embodiments shown in the accompanying drawings. Those skilled in the art should understand that the scope of protection of the present application is obviously not limited to these specific embodiments. Without departing from the principles of the present application, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will fall within the scope of protection of the present application.

Claims

1. An airborne SAR long-distance high-resolution imaging method, characterized in that: include: Step 1: Control the radar beam to illuminate the center of the imaging scene and transmit broadband orthogonal signals alternately; Step 2: According to the distance between the scene center and the antenna phase center, the radar receiving sampling delay is calculated and adjusted in real time, and the threshold sampling is changed to receive the echo signal; Step 3: Perform range-matched filtering on the echo signal according to the form of each transmitted broadband orthogonal signal, and correct the phase of each frequency component of the echo broadband signal to be the same; Step 4: Compensate for the sampling delay variation in the range frequency domain to restore the range unit migration characteristics of the echo signal; Step 5: Perform Doppler center compensation on the echo signal and set the azimuth Doppler center compensation to zero; Step 6: Add multiple echo signals adjacent to each other in azimuth to achieve azimuth downsampling; Step 7: Perform Doppler center inverse compensation on the echo signal to restore the Doppler center of the echo signal; Step 8: Adjust the pulse repetition frequency used for echo signal processing, use the SAR imaging algorithm to process the echo signal, and obtain a SAR image.

2. The airborne SAR long-distance high-resolution imaging method according to claim 1, characterized in that: In step 1, the radar beam is controlled to illuminate the center of the imaging scene, and broadband orthogonal signals are transmitted alternately, including positive frequency modulation signals s + (t) and negative frequency modulation signal s - (t): in, rect(·) is the calculation function; t is time; T p is the pulse width of the transmitted signal; j is the imaginary unit; f c is the center frequency of the transmitted signal; γ is the frequency modulation slope of the transmitted signal.

3. The airborne SAR long-distance high-resolution imaging method according to claim 2, characterized in that: In step 2, the radar receiving sampling delay is calculated and adjusted to 2R in real time according to the distance between the scene center and the antenna phase center. N / C, where R N is the distance from the phase center of the radar antenna to the center of the scene when the sampling delay is adjusted for the Nth time; C is the speed of light; in, R s is the distance from the antenna phase center to the scene center; θ s is the SAR imaging oblique angle; n is the number of pulses for adjusting the sampling delay interval; v is the carrier speed; PRF is the radar pulse repetition frequency.

4. The airborne SAR long-distance high-resolution imaging method according to claim 3, characterized in that: In step 2, the method of adjusting the radar receiving sampling delay includes a continuous adjustment method and a periodic adjustment method, wherein: When continuous adjustment is adopted, n=1; When the periodic adjustment method is adopted, n>1.

5. The airborne SAR long-distance high-resolution imaging method according to claim 4, characterized in that: In step 3, according to the form of each transmitted broadband orthogonal signal, the echo signal is processed by range matching filtering, using the positive frequency modulation signal range matching filter function s + (f r ), negative frequency modulation signal distance matched filter function s - (f r ); in, f r is the distance frequency; w r (·) is the range frequency domain window function.

6. The airborne SAR long-distance high-resolution imaging method according to claim 5, characterized in that: In step 4, the sampling delay variation is compensated in the range frequency domain. Specifically, each echo is compensated for its phase once in the range frequency domain. The phase compensation function is H com (m,f r ); in, m is the echo pulse number; is the ceiling function.

7. The airborne SAR long-distance high-resolution imaging method according to claim 6, characterized in that: Step 5: Perform Doppler center compensation on the echo signal. The compensation function is H fdc (m); in, f dc is the Doppler center of the echo signal; N a Accumulates the number of pulses for the bearing.

8. The airborne SAR long-distance high-resolution imaging method according to claim 7, characterized in that: In step 6, multiple echo signals adjacent to each other in azimuth are added together, specifically: s′(m)=s((m-1)×K+1)+s((m-1)×K+2)+…+s(m×K)…(7) in, s′(m) is the mth echo signal after addition, m=1,2,3,…N a / K; s(m×K) is the m×Kth echo signal before addition; K is the azimuth downsampling factor.

9. The airborne SAR long-distance high-resolution imaging method according to claim 8, characterized in that: In step 7, the Doppler center inverse compensation is performed on the echo signal, and the compensation function is H′ fdc (m); 10. The airborne SAR long-distance high-resolution imaging method according to claim 9, characterized in that: In step eight, the pulse repetition frequency used for echo signal processing is adjusted to PRF′; PRF′=PRF / K.

Images