Range ambiguity separation method for airborne MIMO-SAR based on combined transmit array and pulse coding

Abstract

The present invention discloses a method for separating range ambiguity of airborne MIMO-SAR using a combined transmit array and pulse coding. First, a transmit signal phase coding matrix is ​​constructed, and independent phase coding is added to the transmit signal of each transmitter within each pulse repetition period. Then, the echo data of each receiver is band-pass filtered in azimuth, thereby separating the echoes of the transmit signals of different transmitters scattered by ground targets. Subsequently, the echoes containing phase coding obtained in the above steps are phase decoded. Finally, a spatial filtering steering vector matrix is ​​constructed to separate the desired signal from the range ambiguous signal. The present invention phase encodes the transmit signals of different transmitters and, by taking advantage of the high pulse repetition frequency radar system, separates the transmit signals of each transmitter in the range Doppler domain. The present invention effectively utilizes the degrees of freedom brought by the transmitter array in the MIMO radar, and can effectively separate higher-order range ambiguous signals compared to traditional multi-receiver radars.

Description

Technical Field

[0001] The present invention relates to the field of radar signal processing, and in particular to a method for separating airborne Multiple-Input Multiple-Output Synthetic Aperture Radar (MIMO-SAR) signal transmission and fuzzy signals. Background Art

[0002] Airborne Synthetic Aperture Radar (SAR) is an active microwave imaging sensor mounted on an aircraft platform. It can penetrate clouds, rain, and fog, enabling all-day, all-weather ground-to-ground imaging. Due to the limited flight altitude of airborne SAR, when the detection range is too long, the radar beam footprint on the ground becomes too long, resulting in severe range ambiguity. To avoid range ambiguity, the radar system's pulse repetition frequency (PRF) is often reduced to increase the radar sampling window's open range. However, reducing the system's PRF to increase detection range not only affects azimuth resolution, but also significantly reduces echo energy attenuation as the detection range increases. A low PRF reduces the radar system's average transmit power, significantly affecting the output signal-to-noise ratio (SNR) after azimuth coherent integration of the echo signals. As the requirements for airborne SAR applications continue to increase, low-resolution, low-SNR SAR data no longer meet conventional requirements for imaging scenes at longer distances. Therefore, when airborne SAR detects long-distance scenes, the technical difficulty of improving the azimuth resolution and average transmission power by increasing the system PRF lies in how to effectively separate the distance ambiguity signal caused by the high PRF.

[0003] Currently, numerous researchers have proposed solutions to range ambiguity, such as azimuth phase coding, which places the ambiguous signal and the desired signal in different frequency bands in the range Doppler domain. This technique then uses bandpass filtering to suppress range ambiguity. While azimuth phase coding can eliminate ambiguous signals to a certain extent, when the number of ambiguities is high, the radar system's PRF must be several or even dozens of times greater than the azimuth signal bandwidth to completely eliminate the ambiguous signals, which is difficult to achieve in engineering. Alternatively, an elevation array antenna is used to receive echo signals, and then digital beamforming (DBF) technology is used to perform spatial filtering. This creates a null in the antenna receive pattern in the direction of the ambiguous signal's arrival, thereby suppressing range ambiguity. However, this approach is limited by the aperture of the airborne SAR antenna. Due to the limited number of elevation receiving elements, the elevation antenna has insufficient degrees of freedom, resulting in ineffective ambiguity suppression.

[0004] With the development of MIMO radar technology, combining radar transmit antenna arrays with radar receive antenna arrays, coupled with transmit pulse phase encoding, theoretically allows for the creation of more equivalent degrees of freedom in elevation and pitch, effectively suppressing multiple range-ambiguous signals. In summary, developing a method and device for effectively separating range-ambiguous signals based on MIMO radar is an urgent challenge. Summary of the Invention

[0005] The present invention provides an airborne MIMO-SAR range ambiguity separation method combining a transmitting array with pulse coding, so as to solve the problem that the existing range ambiguity suppression method is insufficient.

[0006] To achieve the above object, the present invention provides the following solutions:

[0007] A method and device for separating range ambiguity of an airborne MIMO-SAR using a combined transmit array and pulse coding, the method comprising:

[0008] Construct a transmission signal phase coding matrix, and add independent phase coding to the transmission signal of each transmitter within each pulse repetition interval (PRI);

[0009] Perform bandpass filtering on the echo data of each receiver in azimuth direction, thereby separating the echoes of the signals transmitted by different transmitters after being scattered by ground targets;

[0010] Perform range pulse compression and vectorization on the separated echoes, and then decode the phase code in the pulse compressed data;

[0011] Construct a spatial filtering steering vector matrix to separate the desired signal from the range ambiguity signal;

[0012] Preferably, the construction of the transmission signal phase coding matrix, in each PRI, adding independent phase coding to the transmission signal of each transmitter, specifically includes:

[0013] Consider a MIMO radar with M transmitters and N receivers. Both the transmitting array and the receiving array are uniform linear arrays arranged along the pitch direction, and each array element is omnidirectional, homogeneous and independent of each other.

[0014] Construct a transmission signal phase coding matrix. In each PRI, independent phase coding is added to the linear frequency modulation signals sent by M transmitters. The phase coding matrix is ​​specifically expressed as:

[0015]

[0016] Among them, c i is a column vector of length M; the subscript i represents the i-th PRI; M is the number of transmitters; [·] T is the transpose operation.

[0017] The present invention uses the linear frequency modulation signal as the reference signal, and the transmission signal of the mth transmitter in the i-th PRI is The expression can be defined as:

[0018]

[0019] i={1,2,3,…},m={1,2,3,…,M}

[0020] Among them, rect(·) is the rectangular window function; c i,m Indicates c i The mth element in; τ is the distance to the fast time; K r is the frequency modulation; T p is the pulse width; f0 is the carrier frequency.

[0021] Preferably, the performing of bandpass filtering on the echo data of each receiver in the azimuth direction, thereby separating the echoes of the signals transmitted by different transmitters after being scattered by ground targets, specifically includes:

[0022] For the echo data received by each receiver, perform azimuth Fourier transform (FFT), then perform azimuth bandpass filtering, and finally perform azimuth inverse FFT operation to obtain the separated echo. Then the nth receiver receives the echo data s after the waveform sent by the mth transmitter is scattered by the ground. n,m (k,τ) can be obtained by the following formula:

[0023]

[0024]

[0025] Where, subscripts n and m represent the nth receiver and mth transmitter respectively; subscript k represents the kth sampling time window; FFT_a{·} represents the azimuth FFT operation; IFFT_a{·} represents the azimuth inverse FFT operation; f a is the azimuth frequency axis; s n (k,τ) represents the received echo data of the nth receiver; s n,m (k,τ) represents the n Separate the echo of the waveform sent by the mth transmitter after being scattered by the ground from (k,τ).

[0026] Preferably, performing range-direction pulse compression and vectorization on the separated echoes, and then performing phase decoding on the phase encoding in the pulse-compressed data, specifically includes:

[0027] For the echo data s n,m (k,τ) performs range pulse compression to obtain pulse pressure echo data s n,m ′(k,τ), and then construct the pulse pressure data vector s′(k,τ). The specific formula is:

[0028]

[0029] The pulse pressure data vector s′(k,τ) is decoded in the azimuth direction. It can be obtained by the following formula:

[0030]

[0031] in, represents the Kronecker product; ⊙ represents the Hadamard product; [·] * represents the conjugate operation; 1 N Represents an N-dimensional column vector.

[0032] Decoded pulse pressure data vector The specific expression is:

[0033]

[0034] in, Represents the pulse pressure echo data s n,m ′(k,τ) is the result after decoding.

[0035] Preferably, the step of constructing a spatial filtering steering vector matrix to separate the desired signal from the range ambiguity signal specifically includes:

[0036] According to the information of the expected signal’s arrival angle, the range-ambiguous signal’s arrival angle and array configuration, a spatial filtering and steering vector matrix is ​​constructed. Let the distance between the transmitting array elements be d T , the receiving array element spacing is d R , then the transmitting array direction matrix A T (θ) and the receiving array direction matrix A R (θ) can be defined as:

[0037]

[0038]

[0039] Where θ is the incoming wave direction, representing the angle at which the signal enters the array; λ is the wavelength.

[0040] Assume that the desired imaging area is numbered 0, and the direction of the range gate signal in this area that changes with the fast time τ is denoted by θ 0,τ , then the spatial filtering steering vector matrix for the desired signal is Can be defined as:

[0041]

[0042] Assume that there are P fuzzy areas in the direction of the track, and the directions of these signals are Θ P,τ =[θ -1,τ ,…,θ -P,τ ]. Then the spatial filtering guidance vector matrix for the fuzzy signal towards the track direction is Can be defined as:

[0043]

[0044] Assume that there are Q fuzzy areas away from the track direction, and the directions of these signals are Θ Q,τ =[θ 1,τ ,…,θ Q,τ ]. Then the spatial filtering guidance vector matrix for the fuzzy signal far away from the track direction is Can be defined as:

[0045]

[0046] The sum of the imaging area and the blurred area should satisfy P+Q+1≤M×N. The final spatial filtering guidance vector matrix V Θ,τ Can be defined as:

[0047]

[0048] Using spatial filtering to guide the vector matrix V Θ,τ The decoded pulse pressure data vector Perform a weighted solution to separate the desired signal from the range ambiguity signal. In the echo received in the kth sampling time window, the separation of the desired signal from the ambiguity signal can be achieved through the following process:

[0049]

[0050] in,[·] H Represents the conjugate transpose operation.

[0051] Represents the separated signal vector, and the specific expression is:

[0052]

[0053] in, A signal representing the incoming wave direction θ. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0055] Figure 1 This is a flowchart of an embodiment of the present invention. DETAILED DESCRIPTION

[0056] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0057] The present invention provides an airborne MIMO-SAR range ambiguity separation method and device combining a transmitting array with pulse coding, so as to solve the problem that the existing range ambiguity suppression method is insufficient.

[0058] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0059] like Figure 1 As shown, this embodiment discloses a method for separating range ambiguity of airborne MIMO-SAR using a combined transmit array and pulse coding, the method comprising:

[0060] Step S1: construct a transmission signal phase coding matrix, and add independent phase coding to the transmission signal of each transmitter in each PRI;

[0061] Step S2, bandpass filtering the echo data of each receiver in azimuth, thereby separating the echoes of the signals transmitted by different transmitters after being scattered by ground targets;

[0062] Step S3, performing range-direction pulse compression and vectorization on the echo separated in step S2, and then decoding the phase encoding in the pulse compression data;

[0063] Step S4, constructing a spatial filtering steering vector matrix to separate the desired signal from the range ambiguity signal;

[0064] Specifically, in step S1, the construction of the transmission signal phase coding matrix, in each PRI, adds independent phase coding to the transmission signal of each transmitter, specifically includes:

[0065] S1.1 Consider a MIMO radar with M transmitters and N receivers. Both the transmitting and receiving arrays are uniform linear arrays arranged in elevation, and each element is omnidirectional, homogeneous, and independent. Construct a phase coding matrix for the transmitted signal. Within each PRI, independent phase coding is applied to the linear frequency modulation signals emitted by the M transmitters. The phase coding matrix for the i-th PRI is specifically expressed as:

[0066]

[0067] Among them, c i is a column vector of length M; the subscript i represents the i-th PRI; M is the number of transmitters; [·] T is the transpose operation;

[0068] S1.2 uses the linear frequency modulation signal as the reference signal, and the mth transmitter transmits the signal of the i-th PRI. The expression is:

[0069]

[0070] i={1,2,3,…},m={1,2,3,…,M}

[0071] Among them, rect(·) is the rectangular window function; c i,m Indicates c i The mth element in; τ is the distance to the fast time; K r is the frequency modulation; T p is the pulse width; f0 is the carrier frequency;

[0072] Specifically, in step S2, bandpass filtering is performed on the echo data of each receiver in the azimuth direction to separate the echoes of the signals transmitted by different transmitters after being scattered by ground targets, which specifically includes:

[0073] For the echo data received by each receiver, perform azimuth Fourier transform (FFT), then perform azimuth bandpass filtering, and finally perform azimuth inverse FFT operation to obtain the separated echo. Then the nth receiver receives the echo data s after the waveform sent by the mth transmitter is scattered by the ground. n,m (k,τ) can be obtained by the following formula:

[0074]

[0075]

[0076] Where, subscripts n and m represent the nth receiver and mth transmitter respectively; subscript k represents the kth sampling time window; FFT_a{·} represents the azimuth FFT operation; IFFT_a{·} represents the azimuth inverse FFT operation; f a is the azimuth frequency axis; s n (k,τ) represents the received echo data of the nth receiver; s n,m (k,τ) represents the number of n Separate the echo of the waveform sent by the mth transmitter after being scattered by the ground from (k, τ);

[0077] Specifically, in step S3, the separated echoes obtained in step S2 are pulse compressed and vectorized in the range direction, and then the phase encoding in the pulse compressed data is phase decoded, which specifically includes:

[0078] S3.1 The echo data s n,m (k,τ) performs range pulse compression to obtain pulse pressure echo data s n,m ′(k,τ).

[0079] S3.2 The pulse pressure echo data s n,m ′(k,τ) constructs the pulse pressure data vector s′(k,τ), the specific formula is:

[0080]

[0081] S3.3 decodes the pulse pressure data vector s′(k,τ) in the azimuth direction. It can be obtained by the following formula:

[0082]

[0083] in, represents the Kronecker product; ⊙ represents the Hadamard product; [·] * represents the conjugate operation; 1 N represents an N-dimensional column vector, c k represents the phase encoding matrix used when the kth pulse is transmitted;

[0084] Decoded pulse pressure data vector The specific expression is:

[0085]

[0086] in, Represents the pulse pressure echo data s n,m ′(k,τ) is the result after decoding.

[0087] Specifically, in step S4, the construction of the spatial filtering steering vector matrix to separate the desired signal from the range ambiguity signal specifically includes:

[0088] S4.1 constructs a spatial filtering steering vector matrix based on information such as the arrival angle of the desired signal, the arrival angle of the range-ambiguous signal, and the array configuration.

[0089] Let the distance between the transmitting array elements be d T , the receiving array element spacing is d R , then the transmitting array direction matrix A T (θ) and the receiving array direction matrix A R (θ) can be defined as:

[0090]

[0091]

[0092] Where θ is the incoming wave direction, representing the angle at which the signal enters the array; λ is the wavelength.

[0093] Assume that the desired imaging area is numbered 0, and the direction of the range gate signal in this area that changes with the fast time τ is denoted by θ 0,τ , then the spatial filtering steering vector matrix for the desired signal is Can be defined as:

[0094]

[0095] Assume that there are P fuzzy areas in the direction of the track, and the directions of these signals are Θ P,τ =[θ -1,τ ,…,θ -P,τ ]. Then the spatial filtering guidance vector matrix for the fuzzy signal towards the track direction is Can be defined as:

[0096]

[0097] Assume that there are Q fuzzy areas away from the track direction, and the directions of these signals are Θ Q,τ =[θ 1,τ ,…,θ Q,τ ]. Then the spatial filtering guidance vector matrix for the fuzzy signal far away from the track direction is Can be defined as:

[0098]

[0099] The sum of the imaging area and the blurred area should satisfy P+Q+1≤M×N. The final spatial filtering guidance vector matrix V Θ,τ Can be defined as:

[0100]

[0101] S4.2 Using spatial filtering to guide the vector matrix V Θ,τ The decoded pulse pressure data vector Perform a weighted solution to separate the desired signal from the range ambiguity signal. In the echo received in the kth sampling time window, the separation of the desired signal from the ambiguity signal can be achieved through the following process:

[0102]

[0103] in,[·] H represents the conjugate transpose.

[0104] Represents the separated signal vector, and the specific expression is:

[0105]

[0106] in, A signal representing the incoming wave direction θ.

Claims

1. A range ambiguity separation method for airborne MIMO-SAR using a combined transmit array and pulse coding, characterized in that: The method comprises: Step 1: Construct a transmission signal phase coding matrix, and add independent phase coding to the transmission signal of each transmitter within each pulse repetition interval PRI; Step 2: Perform azimuth Fourier transform on the echo data of each receiver, then perform azimuth bandpass filtering, and finally perform azimuth inverse FFT operation to separate the echoes of the signals transmitted by different transmitters after being scattered by ground targets; Step 3: Perform range pulse compression and vectorization on the echo separated in step 2, and then decode the phase code in the pulse compressed data; Step 4: Construct a spatial filtering steering vector matrix to separate the desired signal from the range ambiguity signal.

2. The method for separating range ambiguity of airborne MIMO-SAR using a combined transmit array and pulse coding according to claim 1, wherein: In step 1, the transmission signal phase coding matrix is ​​constructed, and the specific formula is: Among them, c i is a column vector of length M; the subscript i represents the i-th PRI; M is the number of transmitters; [·] T is the transpose operation.

3. The method for separating range ambiguity of airborne MIMO-SAR using a combined transmit array and pulse coding according to claim 1, wherein: In step 1, within each PRI, independent phase coding is added to the transmission signal of each transmitter. The specific transmission signal expression is: i={1,2,3,…},m={1,2,3,…,M} in, represents the transmitted signal of the mth transmitter at the i-th PRI; rect(·) is the rectangular window function; c i,m Represents a column vector c of length M i The mth element in M; M is the number of transmitters; τ is the range-to-speed time; K r is the frequency modulation; T p is the pulse width; f0 is the carrier frequency.

4. The method for separating range ambiguity of airborne MIMO-SAR using a combined transmit array and pulse coding according to claim 1, wherein: In step 2, the echo data of each receiver is band-pass filtered in azimuth, thereby separating the echoes of the signals transmitted by different transmitters after being scattered by ground targets. The specific steps are as follows: For the echo data received by each receiver, perform azimuth Fourier transform FFT, then perform azimuth bandpass filtering, and finally perform azimuth inverse FFT operation to obtain the separated echo; then the nth receiver receives the echo data s after the waveform sent by the mth transmitter is scattered by the ground n,m (k,τ) can be obtained by the following formula: Wherein, subscripts n and m represent the nth receiver and mth transmitter respectively; M is the number of transmitters; N is the number of receivers; subscript k represents the kth sampling time window; FFT_a{·} represents the azimuth FFT operation; IFFT_a{·} represents the azimuth inverse FFT operation; f a is the azimuth frequency axis; s n (k,τ) represents the received echo data of the nth receiver; s n,m (k,τ) represents the number of n Separate the echo of the waveform sent by the mth transmitter after being scattered by the ground from (k,τ).

5. The method for separating range ambiguity of airborne MIMO-SAR using a combined transmit array and pulse coding according to claim 4, wherein: In step 3, the echo separated in step 2 is pulse compressed and quantized in the range direction, and then the phase encoding in the pulse compressed data is decoded. The specific operations are as follows: Step 31: The echo data s n,m (k,τ) performs range pulse compression to obtain pulse compression echo data s n,m ′(k,τ); Step 32: Process the pulse compression echo data s n,m ′(k,τ) constructs the pulse compression data vector s′(k,τ), the specific formula is: Step 32: Decode the phase code in the pulse compressed data, specifically including: The pulse compression data vector s′(k,τ) is decoded in the azimuth direction, and the decoded pulse compression data vector It can be obtained by the following formula: in, represents the Kronecker product; ⊙ represents the Hadamard product; [·] * represents the conjugate operation; 1 N represents an N-dimensional column vector; c k represents the phase encoding matrix used when the kth pulse is transmitted; Decoded pulse compression data vector The specific expression is: in, Representatives n,m The result after decoding ′(k,τ).

6. The method for separating range ambiguity of airborne MIMO-SAR using a combined transmit array and pulse coding according to claim 5, wherein: In step 4, the spatial domain filtering steering vector matrix is ​​constructed, and the specific process is as follows: Transmit array direction matrix A T (θ) and the receiving array direction matrix A R (θ) is defined as: Among them, d T represents the distance between transmitting array elements; d R represents the spacing between the receiving array elements; θ is the direction of the incoming wave, representing the angle at which the signal enters the array; λ is the wavelength; Assume that the desired imaging area is numbered 0, and the direction of the range gate signal in this area that changes with the fast time τ is denoted by θ 0,τ , then the spatial filtering steering vector matrix for the desired signal is is defined as: Assume that there are P fuzzy areas in the direction of the track, and the directions of these signals are Θ P,τ =[θ -1,τ ,…,θ -P,τ ]; then the spatial filtering guidance vector matrix for the fuzzy signal towards the track direction is is defined as: Assume that there are Q fuzzy areas away from the track direction, and the directions of these signals are Θ Q,τ =[θ 1,τ ,…,θ Q,τ ]; the spatial filtering guidance vector matrix for the fuzzy signal far away from the track direction is is defined as: The sum of the imaging area and the blurred area should satisfy P+Q+1≤M×N. The final spatial filtering guidance vector matrix V Θ,τ is defined as:

7. The method for separating range ambiguity of airborne MIMO-SAR using a combined transmit array and pulse coding according to claim 6, wherein: In step 4, the separation of the desired signal and the range ambiguity signal is performed as follows: in,[·] H represents the conjugate transpose operation; Represents the separated signal vector, and the specific expression is: in, A signal representing the incoming wave direction θ.

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