A Two-Dimensional Ambiguity Suppression Method for Phase-Coded Multi-Beam SAR Imaging

Abstract

The present invention provides a two-dimensional ambiguity suppression method for phase-coded multi-beam SAR imaging. The method includes: obtaining initial APC waveform echoes of multiple range sub-bands to be imaged; padding zeros at intervals along the azimuth direction for the multiple initial APC waveform echoes to obtain multiple up-sampled echoes of the initial APC waveforms; separating the multiple up-sampled echoes by using azimuth DBF technology to obtain multiple intermediate APC waveform echoes; demodulating the multiple intermediate APC waveform echoes based on the coding form and the range ambiguity number of the range sub-bands to be imaged; performing pulse compression processing on the demodulated multiple intermediate APC waveform echoes and processing them by using elevation DBF technology to obtain multiple target APC waveform echoes; obtaining an unambiguous sub-region image by performing azimuth compression on any one of the target APC waveform echoes; and splicing the sub-region images of the multiple range sub-bands to be imaged to obtain an unambiguous wide-swath SAR image. This method converts range ambiguity suppression into azimuth ambiguity suppression, improves the two-dimensional ambiguity suppression ability, and increases the mapping swath width.

Description

Technical Field

[0001] The present invention relates to the technical field of electronic information technology radar, and specifically relates to a two-dimensional ambiguity suppression method for phase-coded multi-beam SAR imaging. Background Art

[0002] In the application of synthetic aperture radar (SAR) imaging, it is usually desired to obtain a larger imaging width in the elevation direction, so as to reduce the number of flights and improve the information acquisition efficiency. For traditional single-antenna SAR, due to the limitation of the minimum antenna size, it is impossible to solve the range ambiguity and Doppler ambiguity problems simultaneously. Phased array antennas have been widely used since the 1970s, evolving from analog passive arrays to unit-level digital arrays. The receiving and transmitting beams of fully digital array antennas can be formed by digital technology, introducing the radar signal processing into the digital domain stage. More degrees of freedom provide solutions for high-resolution, wide-swath imaging (HRWS) and ground moving target detection (GMTI).

[0003] In the prior art, the orthogonal waveform simultaneous multi-beam SAR system in the elevation direction transmits orthogonal waveforms to different range ambiguity sub-bands, and separates the orthogonal waveforms in the echo to achieve the purpose of suppressing range ambiguity. Generally, a relatively high pulse repetition frequency (PRF) is used to suppress range ambiguity and achieve wide-swath imaging. However, the maximum unambiguous range of this technology is limited by the pulse repetition frequency, and the ambiguity suppression ability is easily affected by the target height fluctuation, and the orthogonality of the orthogonal signals greatly affects the ambiguity suppression result. Summary of the Invention

[0004] (1) Technical Problems to be Solved

[0005] Aiming at the above deficiencies, the main purpose of the present invention is to provide a two-dimensional ambiguity suppression method for phase-coded multi-beam SAR imaging, so as to improve the two-dimensional ambiguity suppression ability of the existing wide-swath imaging method, suppress azimuth ambiguity (i.e., Doppler ambiguity) and range ambiguity simultaneously, and further improve the mapping swath.

[0006] (2) Technical Solutions

[0007] To achieve the above object, the present invention provides a two-dimensional ambiguity suppression method for phase-coded multi-beam SAR imaging, the method comprising: acquiring initial APC (azimuth phase coding) waveform echoes of a plurality of range sub-bands to be imaged; padding zeros at intervals along the azimuth direction for the plurality of initial APC waveform echoes to obtain a plurality of up-sampled echoes of the initial APC waveforms; separating and reconstructing the plurality of up-sampled echoes by using azimuth DBF (digital beamforming) technology to obtain a plurality of intermediate APC waveform echoes, wherein the intermediate APC waveform is a waveform without Doppler ambiguity; demodulating the plurality of intermediate APC waveform echoes based on the coding form and the range ambiguity number of the range sub-bands to be imaged; performing pulse compression processing on the demodulated plurality of intermediate APC waveform echoes and processing them by using elevation DBF technology to obtain a plurality of target APC waveform echoes; obtaining an unambiguous sub-region image by azimuth compression of any one of the target APC waveform echoes; and stitching the sub-region images of the plurality of range sub-bands to be imaged to obtain an unambiguous wide-swath SAR image.

[0008] In the above solution, separating and reconstructing the plurality of up-sampled echoes by using azimuth DBF technology to obtain a plurality of intermediate APC waveform echoes includes: separating the plurality of up-sampled echoes by using azimuth DBF technology to obtain a plurality of separated APC waveform echoes and corresponding multi-order Doppler ambiguity components; respectively accumulating the multi-order Doppler ambiguity components corresponding to each of the separated APC waveform echoes to obtain a plurality of intermediate APC waveform echoes without Doppler ambiguity, wherein the intermediate APC waveform echo is an APC waveform echo after partial range ambiguity suppression.

[0009] In the above solution, performing pulse compression processing on the demodulated plurality of intermediate APC waveform echoes and processing them by using elevation DBF technology to obtain a plurality of target APC waveform echoes includes: forming nulls at the ambiguity angles in the elevation direction for the plurality of intermediate APC waveform echoes after pulse compression processing to suppress range ambiguity and obtain a plurality of target APC waveform echoes.

[0010] In the above solution, the target APC waveform echo is an echo without Doppler ambiguity and without range ambiguity.

[0011] In the above solution, the method further includes: transmitting beams of a plurality of initial APC waveforms along the elevation direction to a plurality of range sub-bands to be imaged, wherein the beam of any one of the initial APC waveforms points to a corresponding range sub-band to be imaged, and each range sub-band to be imaged has a corresponding range ambiguity number.

[0012] In the above solution, the initial APC waveform is a Doppler-domain orthogonal-chirp signal, and the number of the initial APC waveforms is the same as the number of the range sub-bands to be imaged.

[0013] In the above solution, any initial APC waveform echo is the echo of the initial APC waveform corresponding to the sub-band of the distance to be imaged.

[0014] (III) Advantageous Effects

[0015] The technical solution of the embodiment of the present invention has at least the following advantageous effects:

[0016] (1) By using azimuth multi-channel DBF, range and azimuth ambiguity suppression are simultaneously achieved, and range ambiguity suppression is completed while increasing the maximum unambiguous range.

[0017] (2) After separating the APC waveform by azimuth DBF, elevation nulling DBF is used to further suppress ambiguity, improving the ambiguity suppression ability.

[0018] (3) By using the method of simultaneous multi-beam transmission in the elevation direction of the APC waveform, range ambiguity is converted into azimuth ambiguity, thus reducing the influence of range-height fluctuation on the range ambiguity suppression ability.

[0019] (4) By using the simultaneous multi-beam system in the elevation direction, the transmission power and transmission bandwidth of different range sub-bands can be flexibly controlled to achieve various range resolutions. Description of the Drawings

[0020] Figure 1 Schematically shows the flowchart of the two-dimensional ambiguity suppression method for phase-coded multi-beam SAR imaging according to an embodiment of the present invention;

[0021] Figure 2 Shows the schematic diagram of the transmitting beam and receiving beam of the initial APC waveform of the sub-band of the distance to be imaged according to an embodiment of the present invention;

[0022] Figure 3 Schematically shows the corresponding relationship diagram between the azimuth DBF angle and the multi-order Doppler ambiguous APC waveform echo according to an embodiment of the present invention;

[0023] Figure 4A Schematically shows the imaging result diagram of directly imaging the two-dimensional ambiguous scene by the range-Doppler algorithm according to an embodiment of the present invention;

[0024] Figure 4B Schematically shows the range cross-section diagram of the imaging result of directly imaging the two-dimensional ambiguous scene by the range-Doppler algorithm according to an embodiment of the present invention;

[0025] Figure 4C Schematically shows the azimuth cross-section diagram of the imaging result of directly imaging the two-dimensional ambiguous scene by the range-Doppler algorithm according to an embodiment of the present invention;

[0026] Figure 5A Schematically shows the imaging result diagram of the two-dimensional ambiguous scene after azimuth digital beamforming according to an embodiment of the present invention;

[0027] Figure 5B Schematically shows a range cross-section view of the two-dimensional ambiguous scene imaging result after azimuth digital beamforming according to an embodiment of the present invention;

[0028] Figure 5C Schematically shows an azimuth cross-section view of the two-dimensional ambiguous scene imaging result after azimuth digital beamforming according to an embodiment of the present invention;

[0029] Figure 6A Schematically shows a two-dimensional ambiguous scene imaging result diagram after two-dimensional digital beamforming according to an embodiment of the present invention;

[0030] Figure 6B Schematically shows a range cross-section view of the two-dimensional ambiguous scene imaging result after two-dimensional digital beamforming according to an embodiment of the present invention;

[0031] Figure 6C Schematically shows an azimuth cross-section view of the two-dimensional ambiguous scene imaging result after two-dimensional digital beamforming according to an embodiment of the present invention. Detailed implementation manners

[0032] To make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to specific embodiments and the accompanying drawings.

[0033] Figure 1 Schematically shows a flowchart of a two-dimensional ambiguity suppression method for phase-coded multi-beam SAR imaging according to an embodiment of the present invention.

[0034] Please specifically refer to Figure 1 , the specific process of the two-dimensional ambiguity suppression method for phase-coded multi-beam SAR imaging according to the embodiment of the present invention includes operations S110 to S170.

[0035] In operation S110, initial APC (azimuth phase coding) waveform echoes of multiple range sub-bands to be imaged are acquired.

[0036] In operation S120, zero-padding is performed on multiple initial APC waveform echoes along the azimuth direction to obtain multiple up-sampled initial APC waveform echoes.

[0037] In operation S130, multiple up-sampled echoes are separated and reconstructed by using azimuth DBF (digital beamforming) technology to obtain multiple intermediate APC waveform echoes, where the intermediate APC waveform is a waveform without Doppler ambiguity.

[0038] In operation S140, multiple intermediate APC waveform echoes are demodulated based on the coding form and the range ambiguity number of the range sub-bands to be imaged.

[0039] In operation S150, pulse compression processing is performed on the demodulated multiple intermediate APC waveform echoes, and elevation DBF technology is used for processing to obtain multiple target APC waveform echoes.

[0040] In operation S160, azimuth compression is performed on any one of the target APC waveform echoes to obtain an unambiguous sub-region image.

[0041] In operation S170, the sub-region images of multiple sub-bands of the distance to be imaged are stitched together to obtain an unambiguous wide-swath SAR image.

[0042] Through the embodiments of the present invention, the method is based on the azimuth phase coding (APC) waveform design and the elevation simultaneous multi-beam transmission system, and converts range ambiguity suppression into azimuth ambiguity suppression; through azimuth DBF, different APC waveforms and multi-order Doppler ambiguity components of different APC waveforms are separated, and both azimuth ambiguity and range ambiguity are suppressed; through elevation DBF, the two-dimensional ambiguity suppression ability of the wide-swath imaging method is further improved, thereby increasing the mapping swath width.

[0043] The embodiments of the present invention will be described in detail below.

[0044] In operation S110, initial APC (azimuth phase coding) waveform echoes of multiple sub-bands of the distance to be imaged are obtained.

[0045] Specifically, beams of multiple initial APC waveforms are transmitted along the elevation direction to multiple sub-bands of the distance to be imaged, wherein the beam of any one initial APC waveform points to a corresponding sub-band of the distance to be imaged, and each sub-band of the distance to be imaged has a corresponding range ambiguity number.

[0046] In the embodiments of the present invention, any one of the obtained initial APC waveform echoes is the echo of the initial APC waveform of the corresponding sub-band of the distance to be imaged. Among them, the initial APC waveform is a Doppler-domain orthogonal-linear frequency modulation signal, and the number of initial APC waveforms is the same as the number of sub-bands of the distance to be imaged.

[0047] First, the design of the initial APC (azimuth phase coding) waveform will be described.

[0048] Exemplarily, according to the imaging swath width, the number Q of sub-bands of the distance to be imaged and the range ambiguity number A of the initial APC waveform echoes of each sub-band are designed. q, the Doppler ambiguity number K, the number of elements M in the elevation dimension, and the number of elements N in the azimuth dimension are used to design the azimuth phase-encoded waveforms for several sub-bands. For example, there are 5 elevation elements M, 4 range sub-bands Q, 16 azimuth elements N, and the maximum Doppler ambiguity number K is 3. It should be noted that the values of N and K only need to satisfy N≥(Q*(K + 1)). The value of the number of elevation elements M can be selected according to the number of azimuth elements N and the Doppler ambiguity number K. The best range ambiguity suppression effect will be achieved when M≥Q.

[0049] In the embodiment of the present invention, the encoded signal of the q-th range sub-band is designed as:

[0050] s q (t r , t a ) = ss q (t r ) exp(j2πt a (q - 1)f PRF / 4)

[0051] where, f PRF represents the pulse repetition frequency of the SAR, j represents the imaginary sign, Q represents the number of sub-regions, i.e., the number of waveforms, q represents the range sub-band index, q = 1, 2, 3,......Q, t a represents the azimuth slow time, t r represents the range fast time, represents the chirp rate of the q-th chirp signal, T p represents the pulse width of a single chirp signal, and rect(·) represents the rectangular window function.

[0052] Next, based on the designed multiple azimuth phase-encoded waveforms, simultaneous multi-beam transmission in the elevation direction is performed to each range sub-band to be imaged.

[0053] Specifically, the simultaneous multi-beam system in the elevation direction adopts a two-dimensional planar array in the elevation and azimuth directions. The elevation linear array transmits signals, and the entire array receives signals simultaneously. The transmission adopts the simultaneous multi-beam mode of the elevation linear array. Each beam corresponds to a waveform and points to a range sub-band, so that the echoes of each range sub-band are received within the echo window after passing through different range ambiguity times. The number of the nearest range sub-band is Q, and the expression of range ambiguity is described as:

[0054]

[0055] where, c represents the speed of light; R q is the slant range of the q-th range sub-band; A q is the range ambiguity number of the q-th range sub-band, and the difference between the ambiguity numbers of adjacent sub-bands is 1.

[0056] Among them, the expression of the transmitted waveform corresponding to the q-th range sub-band of the m-th element in the pitch direction is:

[0057]

[0058] Among them, m = 1, 2, 3,......, M, where M represents the number of pitch elements, d r represents the pitch element spacing, θ q represents the angle between the beam direction of the q-th range sub-band and the normal of the antenna array, and λ represents the wavelength.

[0059] For example, in this exemplary embodiment, M = 5, and the transmitted signal of the m-th element in the pitch direction can be expressed as:

[0060] s m，q (t r , t a ) = s q (t r , t a )exp(-j2π(m - 3)d r sinθ q / λ)

[0061] Among them, Figure 2 shows the schematic diagram of the initial APC waveform transmit beam and receive beam of the range sub-band to be imaged according to the embodiment of the present invention. As Figure 2 shown, the azimuth array is not drawn, and the transmit and receive antennas (T represents transmit, R represents receive). Q = 4 waveforms are simultaneously transmitted to different range sub-bands, and the beam angles correspond to θ 1 , θ 2 , θ 3 , θ 4 . Their echoes are received within the echo window range after passing through different range ambiguity times A q . It is expressed by the formula:

[0062]

[0063] Through the embodiment of the present invention, by adopting the APC waveform pitch-direction simultaneous multi-beam transmission method, the range ambiguity is converted into azimuth ambiguity (i.e., Doppler ambiguity), thus reducing the influence of the range-height fluctuation on the range ambiguity suppression ability. By adopting the pitch-direction simultaneous multi-beam system, the transmit power and transmit bandwidth of different range sub-bands can be flexibly controlled to achieve various range resolutions.

[0064] In operation S120, multiple initial APC waveform echoes are padded with zeros at intervals in the azimuth direction to obtain multiple initial APC waveform upsampled echoes.

[0065] In operation S130, multiple upsampled echoes are separated and reconstructed using azimuthal DBF (Digital Beam Forming) technology to obtain multiple intermediate APC waveform echoes, where the intermediate APC waveform is a non-Doppler ambiguous waveform.

[0066] Figure 3 Schematically shows the correspondence diagram between the azimuthal DBF angle and the multi-order Doppler ambiguous APC waveform echoes according to an embodiment of the present invention.

[0067] In an embodiment of the present invention, azimuthal digital beam forming is implemented in the range-Doppler domain. After upsampling with zero-padding at intervals along the azimuth direction for multiple initial APC waveform echoes, azimuthal DBF (Digital Beam Forming) technology is used to separate and reconstruct the upsampled echoes under multi-order Doppler ambiguity.

[0068] Specifically, the Doppler ambiguity number of each initial APC waveform is K, and the number of azimuth array elements is N. Zero-padding is performed at intervals in the azimuth of the echoes received by each azimuth array element, and upsampled to (K + 1) times the pulse repetition frequency.

[0069] The multiple upsampled echoes are separated using azimuthal DBF technology to obtain multiple separated APC waveform echoes and the corresponding multi-order Doppler ambiguity components of the separated APC waveform echoes; the multi-order Doppler ambiguity components corresponding to each separated APC waveform echo are respectively accumulated to obtain multiple reconstructed non-Doppler ambiguous intermediate APC waveform echoes, where the intermediate APC waveform echo is the APC waveform echo after partial range ambiguity suppression.

[0070] Among them, to achieve sufficient null directions, the condition N ≥ (Q * (K + 1)) needs to be satisfied. According to the formula:

[0071]

[0072] where, f a represents the Doppler frequency, v represents the radar moving speed, as Figure 3 shown, δ represents the azimuth incoming wave direction; δ q，k represents the azimuth incoming wave direction of the k-th order Doppler ambiguity component of waveform q; Δf q，k , k = 0, 1,..., K represents the Doppler shift of the k-th order Doppler ambiguity component of the q-th waveform. To simultaneously separate the azimuth phase-coded waveform and the multi-order Doppler ambiguity components of each waveform, azimuthal DBF needs to be performed on the upsampled echoes at each Doppler frequency unit, with the beam pointed to the required Doppler ambiguity component of the required waveform, and nulls are achieved at other Doppler ambiguity components and other waveforms. Under multi-order Doppler ambiguity, the azimuthal DBF steering vector corresponding to the k-th order Doppler ambiguity component of waveform q is:

[0073]

[0074] where d a represents the azimuth element size, and δ q，k represents the azimuth incident direction of the k-th Doppler ambiguity component of waveform q, and {·} T represents matrix transpose operation. The expression of the azimuth DBF weight is:

[0075] w q，k = D -1 L q，k

[0076] where {·} -1 represents matrix inversion or pseudo-inversion operation;

[0077] D = [a 1，0 , a 2，0 ,..., a Q，0 , a 1，1 ,..., a q，k ,..., a Q，K T ;

[0078] L q，k = [0, 0,..., 1,..., 0] T represents a zero vector with the number of elements Q*(K + 1), and only the ((q - 1)*(K + 1)+k + 1)-th element is 1. After upsampling the signals received by each azimuth element and performing azimuth DBF in the range-Doppler domain, it is expressed as:

[0079]

[0080] where S q represents the q-th intermediate APC waveform echo after separation and reconstruction;

[0081] t′ a represents the upsampled azimuth slow time; s′ n , n = 1, 2,..., N represents the echo after zero-padding and upsampling of the azimuth interval received by the n-th azimuth element; FFT ta (·) represents the azimuth slow-time Fourier transform; IFFT ta (·) represents the azimuth slow-time inverse Fourier transform.

[0082] For example, in the exemplary embodiment of the present invention, there are N = 16 receiving channels in the azimuth, each waveform has K = 3 Doppler ambiguity components, and the azimuth interval of the echo is zero-padded and upsampled by 4 times f PRF and digital beamforming is implemented in the range-Doppler domain according to the formula:

[0083] ​

[0084] Among them, Δf q，k , where k = 0, 1, ..., 3 represents the Doppler shift of the k-th order Doppler ambiguity component of the q-th waveform.

[0085] Please continue to refer to Figure 3 , Figure 3 which schematically shows the correspondence diagram between the azimuth DBF angle and the multi-order Doppler ambiguity APC waveform echo according to the embodiment of the present invention. Each Doppler frequency contains the multi-order Doppler ambiguity components of each waveform, and the Doppler domain spectrum of each APC waveform echo can be regarded as the spectrum shift of the unmodulated waveform within [-2f PRF , 2f PRF .

[0086] Under multi-order Doppler ambiguity, the azimuth DBF steering vector corresponding to the k-th order Doppler ambiguity component of waveform q is:

[0087]

[0088] The expression of the azimuth DBF weight is:

[0089] w q，k = D -1 L q，k

[0090] where D = [a 1，0 , a 2，0 ,..., a 4，0 , a 1，1 ,..., a q，k ,..., a 4，3 . T . After the signals received by each azimuth array element are upsampled and passed through the azimuth DBF in the range-Doppler domain, it is expressed as:

[0091]

[0092] Through the embodiment of the present invention, range and azimuth ambiguity suppression are simultaneously achieved through azimuth multi-channel DBF, and range ambiguity suppression is completed while increasing the maximum unambiguous range.

[0093] In operation S140, based on the coding form and the range ambiguity number of the sub-band to be imaged, multiple intermediate APC waveform echoes are demodulated.

[0094] In the embodiment of the present invention, coding waveform demodulation refers to using azimuth DBF (Digital Beam Forming) technology for separation and reconstruction. After obtaining multiple intermediate APC waveform echoes, demodulation is required, and the demodulation expression is:

[0095]

[0096] where S′q Represents the q-th intermediate APC waveform echo without Doppler ambiguity after demodulation.

[0097] For example, in an embodiment of the present invention, Q = 4, and the decoding expression for the waveform q is:

[0098]

[0099] In operation S150, pulse compression processing is performed on the demodulated multiple intermediate APC waveform echoes, and elevation DBF technology is used for processing to obtain multiple target APC waveform echoes.

[0100] Specifically, pulse compression processing is performed on the demodulated multiple intermediate APC waveform echoes, and elevation DBF technology is used for processing to form a null in the ambiguity direction and further suppress range ambiguity to obtain multiple target APC waveform echoes, where the target APC waveform echo is an echo without Doppler ambiguity and without range ambiguity. When the number of elevation array elements M ≥ Q, sufficient suppression points can be achieved to obtain the best suppression effect. The range direction pulse compression expression is:

[0101] Sr′ q (t r , t′ a ) = IFFT tr (FFT tr (S′ q (t r , t′ a )) * H q )

[0102] FFT tr (·) represents the fast-time Fourier transform in the range direction; IFFT tr (·) represents the fast-time inverse Fourier transform in the range direction; H q represents the matched filter corresponding to the q-th waveform; Sr′ q represents the range direction pulse compression result of S′ q . The elevation DBF steering vector of the waveform q is:

[0103]

[0104] The expression for the elevation DBF weights of each range sub-band is:

[0105]

[0106] where D r = [ar 1 , ar 2 ,..., ar q ,..., ar Q T ; L​q = [0, 0, ..., 1, ..., 0] T represents a zero vector with Q elements, where only the q-th element is 1. After pulse compression, the signal is expressed as follows after elevation DBF:

[0107] SR′ q (t r , t′ a ) = [Sr′ 1，q (t r , t′ a ), Sr′ 2，q (t r , t′ a ),..., Sr′ M，q (t r , t′ a )]wr q

[0108] where SR′ q represents the q-th target APC waveform echo after elevation DBF, and Sr′ m，q represents the q-th waveform echo after pulse compression in the m-th row of elevation array elements.

[0109] For example, when M = 5, the elevation steering vector of waveform q is:

[0110]

[0111] The expressions for the elevation DBF weights of each range sub-band are:

[0112]

[0113] where D r = [ar 1 , ar 2 , ar 3 , ar 4 T ;

[0114] After pulse compression, the signal is expressed as follows after elevation DBF:

[0115] SR′ q (t r , t′ a ) = [Sr′ 1，q (t r , t′ a ), Sr′ 2，q (t r , t′ a ),..., Sr′ 5，q (t r , t′ a )]wr​q

[0116] Through the embodiments of the present invention, after separating the APC waveform by azimuth DBF, the elevation nulling DBF is further used to suppress the ambiguity, improving the ambiguity suppression ability.

[0117] In operation S160, by performing azimuth compression on the echo of any target APC waveform, a non-ambiguous sub-region image is obtained.

[0118] In operation S170, the sub-region images of multiple sub-bands of the distance to be imaged are stitched together to obtain a non-ambiguous wide-swath SAR image.

[0119] In the embodiments of the present invention, azimuth focusing of each distance sub-band is achieved by performing azimuth compression on the echo of any target APC waveform, and a non-ambiguous sub-region image is obtained. The azimuth focusing algorithms include the range-Doppler (RD) algorithm, the chirp scaling (CS) algorithm, and the back-projection (BP) algorithm. The sub-region images of multiple sub-bands of the distance to be imaged are stitched together to obtain a non-ambiguous wide-swath SAR image.

[0120] In the embodiments of the present invention, the effectiveness of the two-dimensional ambiguity suppression method for phase-coded multi-beam SAR imaging in this embodiment is also verified by simulation data.

[0121] Specifically, the system parameters are set as follows: bandwidth 150 MHz, carrier frequency 9.6 GHz, flight altitude 1000 km, pulse repetition frequency (PRF) 470 Hz, flight speed 7.4 km / s, and the maximum unambiguous range corresponding to the PRF is 319 km. Four APC waveforms (spaced PRF / 4 apart from each other in the Doppler domain) irradiate four adjacent distance sub-bands respectively. Taking the nearest distance sub-band as a reference, the relative distance ambiguity numbers of the four sub-bands are 0, 1, 2, and 3 respectively. Nine stationary point targets are set in each distance sub-band.

[0122] Figure 4A Schematically shows the imaging result diagram of directly imaging a two-dimensional ambiguous scene by the range-Doppler algorithm according to the embodiments of the present invention. Figure 4B Schematically shows the range cross-section diagram of the imaging result of directly imaging a two-dimensional ambiguous scene by the range-Doppler algorithm according to the embodiments of the present invention. Figure 4C Schematically shows the azimuth cross-section diagram of the imaging result of directly imaging a two-dimensional ambiguous scene by the range-Doppler algorithm according to the embodiments of the present invention.

[0123] As Figure 4A 、 Figure 4B And Figure 4C Shown, it is the imaging of the two-dimensional ambiguous echo directly by the imaging algorithm (range-Doppler algorithm) in the prior art. Figure 4A Is the imaging result diagram of directly imaging a two-dimensional ambiguous scene by the range-Doppler algorithm with the nearest distance sub-band as the desired region.Figure 4B and Figure 4C are respectively the range cross-section map and azimuth cross-section map of the imaging result of the two-dimensional ambiguous scene directly by the range-Doppler algorithm with the nearest sub-band as the desired region. It can be seen that the traditional imaging method cannot image two-dimensional ambiguous data, and both azimuth ambiguity and range ambiguity will occur simultaneously, and wide-swath non-ambiguous imaging cannot be achieved.

[0124] Figure 5A Schematically shows the imaging result diagram of the two-dimensional ambiguous scene after azimuth digital beamforming according to an embodiment of the present invention. Figure 5B Schematically shows the range cross-section map of the imaging result of the two-dimensional ambiguous scene after azimuth digital beamforming according to an embodiment of the present invention. Figure 5C Schematically shows the azimuth cross-section map of the imaging result of the two-dimensional ambiguous scene after azimuth digital beamforming according to an embodiment of the present invention.

[0125] As Figure 5A , Figure 5B and Figure 5C shown, it can be seen that the simultaneous multi-beam transmission APC waveform system in the elevation dimension converts range ambiguity into azimuth ambiguity, separates and reconstructs the intermediate APC waveform echoes corresponding to each range sub-band through azimuth DBF, and simultaneously suppresses azimuth ambiguity and a large amount of range ambiguity.

[0126] Figure 6A Schematically shows the imaging result diagram of the two-dimensional ambiguous scene after two-dimensional digital beamforming according to an embodiment of the present invention; Figure 6B Schematically shows the range cross-section map of the imaging result of the two-dimensional ambiguous scene after two-dimensional digital beamforming according to an embodiment of the present invention, and Figure 6C Schematically shows the azimuth cross-section map of the imaging result of the two-dimensional ambiguous scene after two-dimensional digital beamforming according to an embodiment of the present invention.

[0127] As Figure 6A , Figure 6B and Figure 6C shown, it can be seen that on the basis of the azimuth digital beamforming method, the digital beamforming in the elevation direction further suppresses the residual range ambiguity and realizes wide-swath imaging.

[0128] In the above specific embodiments, the purpose, technical solution and beneficial effects of the present invention are further described in detail. It should be understood that the above are only specific embodiments of the present invention and are not used to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A two-dimensional ambiguity suppression method for phase-coded multi-beam SAR imaging, characterized in that: The method comprises: Acquire initial APC waveform echoes of multiple range sub-bands to be imaged; Performing zero padding on the plurality of the initial APC waveform echoes along the azimuth direction to obtain the plurality of the initial APC waveform up-sampled echoes; Separating and reconstructing the multiple upsampled echoes by using the azimuth DBF technique to obtain multiple intermediate APC waveform echoes, wherein the intermediate APC waveform is a Doppler-ambiguity-free waveform; Demodulating the plurality of intermediate APC waveform echoes based on the coding form and the range ambiguity number of the range sub-band to be imaged; Performing pulse compression processing on the demodulated multiple intermediate APC waveform echoes, and processing them using the pitch DBF technology to obtain multiple target APC waveform echoes; Obtaining an unambiguous sub-region image by azimuthally compressing any of the target APC waveform echoes; splicing a plurality of sub-region images of the range sub-band to be imaged to obtain a wide-width SAR image without blur; The method of separating and reconstructing the multiple upsampled echoes by using the azimuth DBF technique to obtain multiple intermediate APC waveform echoes includes: Separating the multiple upsampled echoes by using the azimuth DBF technology to obtain multiple separated APC waveform echoes and multi-order Doppler fuzzy components corresponding to the separated APC waveform echoes; The multi-order Doppler ambiguity components corresponding to each of the separated APC waveform echoes are accumulated respectively to obtain a plurality of intermediate APC waveform echoes without Doppler ambiguity, wherein the intermediate APC waveform echo is the APC waveform echo after partial range ambiguity suppression.

2. The two-dimensional blur suppression method for phase-coded multi-beam SAR imaging according to claim 1, characterized in that: The pulse compression processing is performed on the demodulated multiple intermediate APC waveform echoes, and the pitch DBF technology is used to process the multiple target APC waveform echoes, including: The plurality of intermediate APC waveform echoes after the pulse compression processing are respectively made to form nulls at the ambiguity angles in the pitch direction to suppress the range ambiguity and obtain a plurality of target APC waveform echoes.

3. The two-dimensional blur suppression method for phase-coded multi-beam SAR imaging according to claim 2, characterized in that: The target APC waveform echo is an echo without Doppler ambiguity and distance ambiguity.

4. The two-dimensional blur suppression method for phase-coded multi-beam SAR imaging according to claim 1, characterized in that: The method further comprises: A plurality of initial APC waveform beams are transmitted along the pitch direction to a plurality of range sub-bands to be imaged, wherein any of the initial APC waveform beams points to a corresponding range sub-band to be imaged, wherein each of the range sub-bands to be imaged has a corresponding range ambiguity number.

5. The two-dimensional blur suppression method for phase-coded multi-beam SAR imaging according to claim 1 or 4, characterized in that: The initial APC waveform is a Doppler domain orthogonal-linear frequency modulation signal, and the number of the initial APC waveform is the same as the number of range sub-bands to be imaged.

6. The two-dimensional blur suppression method for phase-coded multi-beam SAR imaging according to claim 5, characterized in that: Any of the initial APC waveform echoes is an echo of the initial APC waveform corresponding to the distance sub-band to be imaged.

Images