An optimization method for ISAR short-term imaging based on target motion estimation

Abstract

The present application relates to the field of real-time image processing for ISAR radar systems and discloses an optimization method for ISAR short-term imaging based on target motion estimation, comprising the following steps: S1, determining the oblique angle between the radar line of sight and the maritime search and rescue target, as well as the direction of the line connecting the radar line of sight and the maritime search and rescue target; S2, constructing a rotational motion model of the maritime search and rescue target; S3, constructing a rotation matrix of the maritime search and rescue target; S4, obtaining the rotation components of the maritime search and rescue target; S5, establishing a radar view coordinate system; S6, synthesizing the effective rotation vector of the maritime search and rescue target; S7, establishing a time-varying characteristic of the effective rotation vector; and S8, selecting an ISAR imaging time period. The present invention selects an appropriate imaging time period through accurate target motion estimation, ensuring data acquisition during the optimal imaging period, thereby maximizing imaging quality.

Description

Technical Field

[0001] The present invention relates to the technical field of real-time image processing of an ISAR radar system, in particular to an optimization method for ISAR short-time imaging based on target motion estimation. Background Art

[0002] Accurate motion estimation of targets such as drifting objects and ships is crucial in maritime search and rescue operations. Maritime target radar imaging technology involves multiple aspects, including the extraction and separation of multi-target radar echo data, compensation for unknown target translational motion, and azimuth focusing of individual scattering points on the target. Maritime target imaging essentially involves two-dimensional reconstruction of scattering points on a non-cooperative target with an unknown, complex, and oscillating state using radar echoes from different perspectives.

[0003] Maritime search and rescue targets are non-cooperative, with unknown motion parameters. Their complex swaying behavior and non-cooperative nature make traditional ISAR imaging methods difficult to directly apply. Existing techniques typically estimate the imaging moment and image accumulation time based on Doppler center estimation from motion-compensated data. However, this approach ignores echo phase and amplitude variations caused by target rotation, resulting in an inaccurate description of the target's motion characteristics and potential errors in selecting the optimal imaging time.

[0004] Under high sea conditions, the motion of maritime search and rescue targets is even more complex. The Doppler values ​​of the echoes from scattered points on the target undergo complex variations over time, similar to the motion of a pendulum. Existing signal frequency processing methods cannot accurately determine the complex, time-varying Doppler frequency if the target is directly processed in azimuth, directly impacting imaging quality.

[0005] These issues illustrate the shortcomings of existing technologies in dealing with highly dynamic, non-cooperative maritime targets and show that there is room for improvement and refinement in this area. Summary of the Invention

[0006] In response to the shortcomings of the existing technology, the present invention provides an optimization method for ISAR short-term imaging based on target motion estimation. According to the motion characteristics of the maritime search and rescue target at sea, a motion model of the maritime search and rescue target at sea is constructed, and parameters are estimated through the model to obtain a function of the synthetic motion vector of the maritime search and rescue target changing with time. The time domain window in which the relative motion between the maritime search and rescue target and the radar is relatively stable is identified, and a higher quality image is synthesized.

[0007] To achieve the above objectives, the present invention is implemented through the following technical solutions: an optimization method for ISAR short-term imaging based on target motion estimation, comprising the following steps:

[0008] S1. Determine the oblique angle between the radar line of sight and the maritime search and rescue target as a, the line connecting the radar line of sight and the maritime search and rescue target as the RLOS direction, the center of the maritime search and rescue target is located in the fixed reference system (X, Y, Z), and the coordinate system of the maritime search and rescue target is (X s , Y s , Z s ), the rotation matrix is ​​expressed as:

[0009]

[0010] Wherein, a(t) is the real-time rotation amplitude of the maritime search and rescue target at time t;

[0011] S2. Construct the rotational motion model of the maritime search and rescue target as follows:

[0012] a(t)=<α r (t), α p (t), α y (t)>

[0013]

[0014] Among them, α r (t), α p (t), α y (t) are the real-time amplitudes of roll, pitch and yaw respectively, q r ,q p and q y are the maximum swing amplitudes of roll, pitch and yaw respectively, T r 、T p and T y are the sway periods of roll, pitch and yaw sway, respectively; and are the initial phases of roll, pitch and yaw respectively;

[0015] S3. Construct the rotation matrix of the maritime search and rescue target, which is:

[0016]

[0017] Among them, θ r ,θ p and θ y They are the rotation angles of the target in three directions at time t relative to the initial time, roll(θ r ), pitch(θ p ) and yaw(θ y ) are the rotation matrices corresponding to roll, pitch and yaw motions, respectively, then:

[0018]

[0019] in,

[0020]

[0021] S4. Obtain the rotation component of the maritime search and rescue target, and substitute the rotation motion model of the maritime search and rescue target into the radar echo signal. The original signal is expressed as:

[0022]

[0023] in, is the original signal, the scattering point coordinates of point A are (x, y, z), A a represents the amplitude of the scattering point A, The radar transmits a linear frequency modulated pulse signal. represents the fast time variable, t n represents the slow time variable, c represents the speed of light, r a (t n ) is the sum of the translational component and the rotational component, then the rotational component of the maritime search and rescue target is obtained as:

[0024] r i (t n )=sin(α(t n ))(a 11 x+a 12 y+a 13 z)+cos(α(t n ))(a 21 x+a 22 y+a 23 z) (7)

[0025] S5. Establish the radar view coordinate system (R, H, V). The R axis is always along the RLOS direction. The H axis is perpendicular to the R axis and is located in the (X, Y) plane. The V axis is perpendicular to the (R, H) plane. The ground-grazing angle is H a is the height of the radar carrier, R a is the distance between the radar carrier and the ship, is the angle of view of the radar observing the maritime search and rescue target, that is, the angle between the RLOS projection on the ground and the longitudinal axis of the maritime search and rescue, and the angular velocities of roll, pitch, and yaw are determined as ω respectively. r 、ω p 、ω y , the rolling, pitching and yaw swing amplitudes are q r ,q p ,q y , the rolling, pitching and yaw periods are T r 、T p 、T y , roll, pitch, yaw The cosine function of the angular velocity of the viewing angle is expressed as:

[0026]

[0027] S6, synthetic maritime search and rescue target effective rotation vector ω e :

[0028]

[0029]

[0030] Where γ is ω e The angle between the V axis and the V axis represents ω e The direction of ω v is the vertical rotation component of the maritime search and rescue target, ω h The horizontal rotation component of the maritime search and rescue target;

[0031] S7. Establishing effective rotation vector ω for maritime search and rescue targets e Time-varying characteristics and vertical component ω v The characteristics of the change over time, the vertical rotation component ω v The changing trend of m is:

[0032]

[0033]

[0034] Among them, Δf(t) is the Doppler frequency difference between the head and tail scattering points of the search and rescue target, Δr is the slant range difference between the head and tail scattering points of the search and rescue target, and x s，b is the horizontal coordinate of the scattering point corresponding to the head of the search and rescue target, x s，s is the horizontal coordinate of the scattering point corresponding to the tail of the search and rescue target, is the radar viewing angle at the start of imaging;

[0035] S8. Select the IASR imaging time period to perform short-time ISAR imaging.

[0036] Preferably, the step S8 further comprises: v The steps of selecting a time period for ISAR imaging based on the trend of change in the image include:

[0037] When h ≥3ω v When m≤m min The corresponding time window is used for short-term ISAR imaging to obtain the side view, where m min is the vertical rotation component ω v The lower threshold of the change trend;

[0038] when When m≥m max The corresponding time window is used for short-term ISAR imaging to obtain a top view, where m max is the vertical rotation component ω v The upper threshold value of the change trend;

[0039] when , a mixed top and side view is obtained.

[0040] Preferably, when ω h ≥3ω v When H≥H max , H max is the maximum wave height, where H is:

[0041]

[0042] Among them, V is the wind speed at sea, D is the duration of the wind, C is the empirical coefficient, and g is the acceleration of gravity.

[0043] Preferably, the When H≤H min , H min is the minimum wave height.

[0044] Preferably, the step S8 further includes evaluating the image brightness and Doppler frequency in each time window to determine the imaging moment with the highest image contrast.

[0045] Preferably, the step of evaluating the image brightness and Doppler frequency in each time window to determine the imaging moment with the highest image contrast includes:

[0046] Dividing the translation-compensated image into azimuthally overlapping sub-apertures;

[0047] Calculate the Doppler broadening of each sub-aperture and the slope of the ship centerline to obtain the synthetic effective rotation vector ω e and its vertical component ω v Characteristics of changes over time and

[0048] Set the Doppler broadening threshold DS and adjust the Doppler broadening Δf ds =f max -f min Evaluate the Doppler broadening Δf of the image ds ;

[0049] according to and The change of is compared with the set threshold to determine the imaging moment with the maximum image contrast;

[0050] After the search imaging window is completed, the selected subaperture in azimuth is Fourier transformed to optimize the image quality.

[0051] Preferably, the Doppler broadening threshold DS is set, and the Doppler broadening Δf ds =f max -f min Evaluate the Doppler broadening Δf of the image ds The steps include:

[0052] The lowest Doppler frequency unit f where the image brightness exceeds 10 times the average value min and the highest Doppler frequency unit f max The width between them is the Doppler broadening Δf of the image ds =f max -f min , set the Doppler broadening threshold DS, in, is the lower limit of the Doppler broadening threshold, Is the upper limit of the Doppler broadening threshold, DS min Minimum Doppler broadening threshold lower limit amplitude, DS max The upper limit of the maximum Doppler broadening threshold.

[0053] Preferably, the Know The change of is compared with the set threshold to determine the imaging moment with the maximum image contrast.

[0054] when When the change amplitude is lower than the threshold ΔDS, the method further includes:

[0055] according to The magnitude of the amplitude change over time is used to determine the range of change in the slope of the vertical axis of the maritime search and rescue target, and whether the maximum modulus value during the amplitude change process is less than the set threshold constant;

[0056] If yes, then during the entire observation time T, start searching from the starting time t0 for the time t where the image contrast C(t) reaches its maximum. opt =arg max t∈T C(t), get the imaging window size ΔT opt ;

[0057] If not, the imaging is performed in the time period with the smaller Doppler broadening estimate and the larger slope of the ship axis. ε represents an arbitrarily small positive number.

[0058] Preferably, the and The change of is compared with the set threshold to determine the imaging moment with the maximum image contrast.

[0059] when When the change amplitude is higher than the threshold ΔDS, the method further includes:

[0060] according to The magnitude of the amplitude change over time is used to determine the range of change in the slope of the vertical axis of the maritime search and rescue target, and whether the maximum modulus value during the amplitude change process is less than the set threshold constant;

[0061] If yes, then during the entire observation time T, from The search starts at the moment t when the image contrast C(t) reaches its maximum opt =arg max t∈T C(t), get the imaging window size ΔT opt ;

[0062] If not, then select the time period where the slope is close to zero and the Doppler broadening is large for imaging, i.e. ε represents an arbitrarily small positive number.

[0063] The present invention also provides an ISAR short-time imaging system based on target motion estimation, comprising a transmitting system, a receiving system, a signal processing system, and a control and data processing unit, wherein:

[0064] The transmitting system includes a radio frequency generator, a power amplifier, and a transmitting antenna. The radio frequency generator is used to generate radio frequency signals. The power amplifier is used to amplify the radio frequency signals to a level sufficient to excite the transmitting antenna. The transmitting antenna is used to convert the radio frequency signals into electromagnetic waves and transmit them to the target.

[0065] The receiving system includes a receiving antenna, a low-noise amplifier, a mixer, and an intermediate frequency amplifier. The receiving antenna is used to receive radar signals reflected from the target. The low-noise amplifier is used to amplify the received weak signal. The mixer is used to mix the received RF signal with the signal generated by the local oscillator to obtain an intermediate frequency signal. The intermediate frequency amplifier is used to amplify the intermediate frequency signal output by the mixer.

[0066] The signal processing system includes an analog-to-digital converter (ADC), a digital signal processor (DSP), and an ISAR short-time imaging module based on target motion estimation. The ADC converts analog signals into digital signals, and the DSP performs filtering, demodulation, compression, and imaging. The ISAR short-time imaging module based on target motion estimation generates high-resolution images based on received radar data and target motion parameters.

[0067] The motion compensation system includes a target motion estimation module and a radar motion compensation module. The target motion estimation module estimates the target's motion parameters, such as velocity and acceleration. The radar motion compensation module corrects the received radar data according to the motion state of the radar platform to eliminate the impact of platform motion on imaging quality.

[0068] The control and data processing unit includes a control unit and a data processing unit. The control unit is responsible for the system parameter setting, timing control and coordination, and the data processing unit performs the reception, processing, storage and transmission of radar data.

[0069] The present invention provides an optimization method for ISAR short-term imaging based on target motion estimation, which has the following beneficial effects:

[0070] 1. By accurately estimating the motion parameters of maritime search and rescue targets and constructing an actual motion model, this invention can more accurately compensate for the effects of target motion during imaging. This motion compensation mechanism significantly improves imaging accuracy and resolution, especially when processing high-speed moving targets, effectively reducing image blur and positional deviation caused by target motion.

[0071] 2. By analyzing the target's motion characteristics, the present invention can identify time windows where the target's relative motion with the radar is relatively stable. This optimization strategy ensures data acquisition during the optimal imaging period, maximizing imaging quality. This is particularly important for emergency response and search and rescue operations in dynamic ocean environments, providing clear target images at critical moments. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 A flow chart for selecting the ISAR imaging time period of the present invention;

[0073] Figure 2 This is a schematic diagram of the six-degree-of-freedom motion of a ship according to the present invention;

[0074] Figure 3 Schematic diagram of three rotations of the ship of the present invention;

[0075] Figure 4 This is a schematic diagram of the geometric relationship of ship imaging according to the present invention;

[0076] Figure 5 Schematic diagram of the optimal imaging subaperture selection process of the present invention;

[0077] Figure 6 Schematic diagram of RD imaging results of a measured data set in Example 1 of the present invention;

[0078] Figure 7 Schematic diagram of the translation compensation results of measured data in Example 1 of the present invention;

[0079] Figure 8 This is a schematic diagram of a time variation curve of the ship center slope of the measured data in Example 1 of the present invention;

[0080] Figure 9 Schematic diagram of a curve showing Doppler broadening changing with time in Example 1 of the present invention;

[0081] Figure 10 This is a schematic diagram of the optimal time selection result for imaging the measured data set in Example 1 of the present invention;

[0082] Figure 11 Schematic diagram of RD imaging results of a measured data set in Example 2 of the present invention;

[0083] Figure 12 Schematic diagram of the translation compensation results of the measured data set in Example 2 of the present invention;

[0084] Figure 13 This is a schematic diagram of a time variation curve of the ship center slope of measured data in Example 2 of the present invention;

[0085] Figure 14 Schematic diagram of a curve showing Doppler broadening changing with time in Example 2 of the present invention;

[0086] Figure 15 This is a schematic diagram of the optimal time selection results for imaging the measured data set in Example 2 of the present invention. DETAILED DESCRIPTION

[0087] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the present specification. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0088] Please see the attached Figure 1 -Attached Figure 5 The present invention provides an optimization method for ISAR short-time imaging based on target motion estimation, which aims to select a suitable imaging time period through accurate target motion estimation to achieve high-quality imaging.

[0089] Specifically, the method includes the following steps:

[0090] S1. Determine the oblique angle between the radar line of sight and the maritime search and rescue target, as well as the direction of the line connecting the radar line of sight and the maritime search and rescue target.

[0091] First, determine the line direction RLOS and the oblique angle a between the radar line of sight and the maritime search and rescue target. Assume that the center of the maritime search and rescue target is located in a fixed reference system (X, Y, Z), and the coordinate system of the target is (X s , Y s , Z s In order to accurately describe the spatial orientation of the target at a specific observation moment, a rotation matrix is ​​used to describe the real-time orientation change of the target. This rotation matrix is ​​calculated based on the rotation angle of the target relative to the radar, which mainly involves the rotation of the target relative to its motion axis, as shown below:

[0092]

[0093] Wherein, a(t) is the real-time rotation amplitude of the maritime search and rescue target at time t.

[0094] By applying this rotation matrix, the relative motion of the target can be converted into a specific mathematical expression, allowing the correct geometric correction of the radar data during the image processing stage. Real-time calculation of α(t) and updating of the rotation matrix allows the system to dynamically adjust the observation parameters, ensuring that the oblique angle α is optimally aligned with the target at all times, which is crucial for improving imaging quality.

[0095] S2. Constructing a rotational motion model for maritime search and rescue targets

[0096] Next, a rotational motion model of the target is constructed to estimate parameters and obtain the motion characteristics of the target.

[0097] Since maritime targets, such as ships, are not only affected by heading changes but also experience complex three-dimensional swaying motions, including roll, pitch, and yaw, accurately simulating these motions is crucial to improving the quality of radar imaging.

[0098] A ship at sea has six degrees of freedom, including three translational degrees of freedom (surge, sway, and heave) and three rotational degrees of freedom (pitch, roll, and yaw). Figure 2 In the present invention, the main focus is on the rotational degrees of freedom, because these movements directly affect the angle changes of ISAR imaging and are the key to achieving high-quality imaging.

[0099] like Figure 2 As shown, the bow direction is the X-axis, the direction perpendicular to the bow direction and parallel to the hull plane is the Y-axis, and the direction perpendicular to the hull plane is the Z-axis. The swaying around the X-axis is defined as roll sway, the swaying around the Y-axis is defined as pitch sway, and the swaying around the Z-axis is defined as yaw sway. In order to accurately describe and predict the rotation state of the ship, the present invention defines a motion model with three rotational degrees of freedom. The motion of each degree of freedom is expressed as a sine function. Let the maximum amplitude of the roll, pitch, and yaw sway at time t be αr (t), α p (t), α y (t), then:

[0100] a(t)=<α r (t), α p (t), α y (t)>

[0101]

[0102] Among them, α r (t), α p (t), α y (t) are the real-time amplitudes of roll, pitch and yaw respectively, q r ,q p and q y are the maximum swing amplitudes of roll, pitch and yaw respectively, T r 、T p and T y are the sway periods of roll, pitch and yaw sway, respectively; and are the initial phases of roll, pitch and yaw respectively. Through this model, the specific sway state of the ship at any time can be predicted.

[0103] S3. Constructing the rotation matrix of the maritime search and rescue target

[0104] Based on the above rotation model, three independent rotation matrices are used to represent roll, pitch, and yaw motions, respectively, to construct a composite rotation matrix rot(θ r ,θ p ,θ y ), expressed as:

[0105]

[0106] Among them, θ r ,θ p and θ y They are the rotation angles of the target in three directions at time t relative to the initial time, roll(θ r ), pitch(θ p ) and yaw(θ y ) are the rotation matrices corresponding to roll, pitch, and yaw motions; each rotation matrix is ​​defined as follows:

[0107]

[0108] in,

[0109]

[0110] Calculate the element a in the result matrix 11 , a 12 ,…,a 33 It is used to accurately describe the spatial orientation and dynamic changes of the target. By applying this rotation matrix, the radar system can dynamically adjust the observation parameters to compensate for the three-dimensional motion of the target.

[0111] S4. Obtain the rotational component of the maritime search and rescue target

[0112] Substituting the above rotational motion model into the radar echo signal model, the change of the echo signal can be calculated. The original signal is expressed as:

[0113]

[0114] in, is the original signal, the scattering point coordinates of point A are (x, y, z), A a represents the amplitude of the scattering point A, The radar transmits a linear frequency modulated pulse signal. represents the fast time variable, t n represents the slow time variable, c represents the speed of light, r a (t n ) is the sum of the translational component and the rotational component. The position coordinates (x, y, z) of the scattering point are obtained by the rotation matrix to obtain the rotation component r i (t n ) can be calculated as:

[0115] r i (t n )=sin(α(t n ))(a 11 x+a 12 y+a 13 z)+cos(α(t n ))(a 21 x+a 22 y+a 23 z) (7)

[0116] Through this model, the real-time position and attitude of the target can be estimated, thereby optimizing the quality of ISAR imaging.

[0117] S5. Establish radar viewing angle coordinate system

[0118] like Figure 4 As shown, for further accurate imaging, the radar view coordinate system (R, H, V) is established, where the R axis is always along the RLOS direction, the H axis is perpendicular to the R axis and located in the (X, Y) plane, and the V axis is perpendicular to the (R, H) plane. The ground-grazing angle ψ is calculated as Among them, H ais the height of the radar carrier, R a is the distance between the radar carrier and the ship, is the angle of view of the radar observing the maritime search and rescue target, that is, the angle between the RLOS projection on the ground and the longitudinal axis of the maritime search and rescue, and the angular velocities of roll, pitch, and yaw are determined as ω respectively. r 、ω p 、ω y , the rolling, pitching and yaw swing amplitudes are q r ,q p ,q y , the rolling, pitching and yaw periods are T r 、T p 、T y , the initial phases of roll, pitch and yaw are According to the previously defined rotation model, first calculate the target's roll ω r 、Pitchω p , yaw ω y These angular velocities are periodic functions of time and can be expressed using the following cosine function:

[0119]

[0120] These parameters directly reflect the ship's rolling dynamics along the three main axes.

[0121] S6. Synthesize the effective rotation vector of the maritime search and rescue target

[0122] Using the angular velocity calculated above and the angle observed by the radar, the effective rotation vector ω of the maritime search and rescue target can be calculated and synthesized e , which is a vector obtained by combining the horizontal and vertical rotation components:

[0123]

[0124]

[0125] Where γ is ω e The angle between the V axis and the V axis represents ω e The direction of ω v is the vertical rotation component of the maritime search and rescue target, ω h It is the horizontal rotation component of the maritime search and rescue target.

[0126] S7. Establish the characteristics of the effective rotation vector changing with time

[0127] In order to identify the optimal imaging time period, the effective rotation vector ω of the target is calculated e and its vertical component ω v The time-varying characteristics give the vertical rotation component ωv The changing trend of m:

[0128]

[0129]

[0130] Among them, Δf(t) is the Doppler frequency difference between the head and tail scattering points of the search and rescue target, Δr is the slant range difference between the head and tail scattering points of the search and rescue target, and x s，b is the horizontal coordinate of the scattering point corresponding to the head of the search and rescue target, x s，s is the horizontal coordinate of the scattering point corresponding to the tail of the search and rescue target, is the radar viewing angle at the start of imaging.

[0131] S8. Select ISAR imaging time period

[0132] Based on the above analysis, we select an ISAR imaging time window for short-term ISAR imaging. In practical applications, by calculating the Doppler broadening and the slope change of the target centerline, we select the time window with the minimum Doppler change and the maximum slope change for imaging, thereby obtaining high-quality ISAR images.

[0133] Specifically, according to ω v Select the time period of ISAR imaging based on the changing trend of the image. The specific steps are as follows:

[0134] When h ≥3ω v When m≤m min The corresponding time window is used for short-term ISAR imaging to obtain the side view, where m min is the vertical rotation component ω v The lower threshold of the change trend;

[0135] when When m≥m max The corresponding time window is used for short-term ISAR imaging to obtain a top view, where m max is the vertical rotation component ω v The upper threshold value of the change trend;

[0136] when , a mixed top and side view is obtained.

[0137] like Figure 5 As shown, in order to improve the quality of the image, the selection of the time window is further refined, and the optimal imaging moment is determined by evaluating the image brightness and Doppler frequency:

[0138] 1) Divide the image after translation compensation into azimuthally overlapping sub-apertures, calculate the Doppler broadening and the slope of the ship centerline of each sub-aperture, and obtain the curve that changes with time, that is, the synthetic effective rotation vector ω e Characteristics of changes over time and its vertical component ω v Characteristics of changes over time

[0139] The lowest Doppler frequency unit f where the image brightness exceeds 10 times the average value min and the highest Doppler frequency unit f max The width between them is the Doppler broadening Δf of the image ds =f max -f min , set the Doppler broadening threshold DS, is the lower limit of the Doppler broadening threshold, Is the upper limit of the Doppler broadening threshold, DS min Minimum Doppler broadening threshold lower limit amplitude, DS max The upper limit of the maximum Doppler broadening threshold;

[0140] 2) According to The amplitude changes with time, judging the floating The relationship between the threshold ΔDS is that if the floating is lower than the threshold, Go to step 3), if the floating is higher than the threshold, Go to step 4);

[0141] 3) According to The amplitude changes with time, and the range of change of the vertical axis slope of the maritime search and rescue target is judged. If the maximum modulus value during the amplitude change is less than the set threshold constant, that is, Then, during the entire observation time T, we search for the moment t where the image contrast C(t) reaches its maximum from the starting moment t0. opt =arg max t∈T C(t), get the imaging window size ΔT opt , and go to step 5), if the maximum value during the amplitude change process is greater than the set threshold constant, that is, Then the imaging is performed in the time period when the Doppler broadening is estimated to be small and the slope of the ship axis is large. ε represents an arbitrarily small positive number, and go to step 5);

[0142] 4) According to The amplitude changes with time, and the range of change of the vertical axis slope of the maritime search and rescue target is judged. If the maximum modulus value during the amplitude change is less than the set threshold constant, that is, Then in the whole observation time T, from The search starts at the moment t when the image contrast C(t) reaches its maximum opt =argmax t∈T C(t), get the imaging window size ΔT opt , and go to step 5), if the maximum value during the amplitude change process is greater than the set threshold constant, that is, Then the imaging time period with the slope close to zero and the Doppler broadening is selected, that is, ε represents an arbitrarily small positive number, and go to step 5);

[0143] 5) Complete azimuth subaperture selection in the search imaging window and perform Fourier transform.

[0144] Through the above steps, it is possible to identify a time-domain window in which the relative motion between the target and the radar is relatively stable, and synthesize a high-quality image within this window. Through precise calculation and real-time adjustment, not only is imaging quality improved, but the ability to monitor and analyze dynamic maritime targets is also enhanced. This approach demonstrates the potential of advanced radar technology for modern maritime search and rescue, particularly in missions requiring rapid response and high-resolution imaging.

[0145] The present invention also provides an ISAR short-time imaging system based on target motion estimation, comprising: a transmitting system, a receiving system, a signal processing system, and a control and data processing unit. The system is characterized in that the transmitting system comprises a radio frequency generator, a power amplifier, and a transmitting antenna; the receiving system comprises a receiving antenna, a low-noise amplifier, a mixer, and an intermediate frequency amplifier; the signal processing system comprises an analog-to-digital converter, a digital signal processor, and an ISAR short-time imaging module based on target motion estimation; the motion compensation system comprises a target motion estimation module and a radar motion compensation module; and the control and data processing unit comprises a control unit and a data processing unit.

[0146] The RF generator of the transmitting system generates a RF signal for transmitting radar signals, which is amplified by the power amplifier to a level sufficient to excite the transmitting antenna, and then converted into electromagnetic waves by the transmitting antenna and transmitted to the target. The receiving antenna of the receiving system receives the radar signal reflected from the target, amplifies the received weak signal by the low-noise amplifier and transmits it to the mixer, mixes the received RF signal with the signal generated by the local oscillator to obtain an intermediate frequency signal, and then transmits it to the intermediate frequency amplifier to amplify the intermediate frequency signal output by the mixer. The signal processing system transmits the output intermediate frequency signal to the analog-to-digital converter to convert the analog signal into a digital signal, and the digital signal processor completes filtering, Demodulation, compression and imaging processing, the ISAR short-time imaging module based on target motion estimation uses the ISAR short-time imaging method based on target motion estimation as described above to process the received radar data to generate a high-resolution image of the target. The motion compensation system estimates the motion parameters of the target itself, such as speed and acceleration, through the target motion estimation module. The radar motion compensation module corrects the received radar data according to the motion state of the radar platform to eliminate the impact of platform motion on imaging quality. The control unit in the control and data processing unit is responsible for the system parameter setting, timing control and coordination, and the data processing unit performs the reception, processing, storage and transmission of radar data.

[0147] Example 1:

[0148] This embodiment demonstrates, through a detailed process, how to use an optimization method for ISAR short-term imaging based on target motion estimation to improve the radar imaging quality of moving targets at sea.

[0149] 1. Dataset Description

[0150] This example uses a 10-second measured data set of ISAR echo signals from a ship target. After range compression, this example captures sub-images with lengths of 1024 in both azimuth and range directions, corresponding to a time duration of 0.56 seconds. Figure 6 The results of direct range-Doppler imaging of radar echo signals are presented.

[0151] 2. Translational compensation

[0152] In order to improve the imaging quality, the following translation compensation processing was performed on the data:

[0153] Range Alignment: Envelope alignment is performed using the adjacent envelope correlation method. Specifically, the previous echo's envelope is used as a reference, and the current echo's envelope is shifted in the range dimension. The correlation coefficient between the current echo and the reference envelope is calculated for different shift values, and the position with the largest correlation coefficient is selected as the position of the current echo after alignment.

[0154] Phase compensation: After distance alignment is completed, phase compensation is performed to further improve imaging clarity.

[0155] The result of translation compensation is as follows Figure 7 As shown, it includes the Doppler domain signal and ISAR echo signal before and after range alignment, as well as the ship image after phase compensation.

[0156] 3. Parameter calculation and imaging window selection

[0157] According to the formula in the above embodiment, the hull centerline slope and Doppler broadening are calculated, and the time variation curve is plotted, as shown in FIG. Figure 8 and Figure 9 shown.

[0158] The specific steps are as follows:

[0159] Calculating the hull centerline slope

[0160] 1) Establish the radar line of sight coordinate system and obtain relevant parameters.

[0161] 2) Calculate the hull centerline slope using the following formula:

[0162]

[0163] Among them, ω h and ω v are the horizontal rotation component and vertical rotation component of the target, respectively, which are calculated using the following formula:

[0164]

[0165]

[0166] Among them, ω r ,ω p ,ω y are the angular velocities of roll, pitch, and yaw respectively; and ψ are the radar observation angle and ground grazing angle; v a is the speed of the radar carrier; R t is the distance between the radar and the target.

[0167] Calculating Doppler broadening

[0168] The Doppler spread m is obtained by calculating the Doppler frequency difference Δf(t) and slant range difference Δr of the target head and tail scattering points:

[0169]

[0170]

[0171] Among them, xs，b and x s，s are the horizontal coordinates of the scattering points at the head and tail of the target, respectively; is the radar viewing angle at the start of imaging.

[0172] Select imaging time period

[0173] According to the above calculation results, the imaging time period is selected. The specific steps are as follows:

[0174] 1) Calculate the fluctuation of Doppler broadening. If it is less than 20% and the slope changes significantly, select the time interval where the slope maximum value is located.

[0175] 2) Select 0.15 seconds as the start time and 0.5 seconds as the end time.

[0176] 3) According to the above method, the optimal imaging window time is searched, and the optimal imaging window length is 636.

[0177] 4. Optimal imaging results

[0178] Select the best imaging sub-aperture for Fourier transform and get the final imaging result as follows: Figure 10 This result demonstrates the effectiveness and superiority of the optimized ISAR imaging method in practical applications.

[0179] This example demonstrates the application of an optimization method for ISAR short-term imaging based on target motion estimation to real data. Through precise translational compensation and parameter optimization, the imaging quality is significantly improved, verifying the practicality and effectiveness of the method.

[0180] Example 2:

[0181] This embodiment demonstrates, through another detailed process, how to improve the radar imaging quality of moving targets at sea by using the optimization method of ISAR short-term imaging based on target motion estimation.

[0182] 1. Dataset Description

[0183] The measured data set used in this embodiment contains a 6-second ISAR echo signal of a ship target. After range compression processing, this embodiment intercepts data with a length of 1024 in both azimuth and range directions. The direct range-Doppler imaging results of the radar echo signal are shown as follows: Figure 11 shown.

[0184] 2. Translational compensation processing

[0185] In order to improve the imaging quality, the same translation compensation process as in Example 1 was performed.

[0186] The result of translation compensation is as follows Figure 12As shown, the ship image includes the image after range alignment and phase compensation.

[0187] 3. Parameter calculation and imaging window selection

[0188] Using the measured data, the hull centerline slope and Doppler broadening are calculated using the same method as in Example 1, and a time variation curve is plotted, as shown in FIG. Figure 13 and Figure 14 shown.

[0189] The fluctuation of Doppler broadening is calculated. If it is higher than 20% and the slope changes slightly, a time interval with a large Doppler broadening and a slope close to zero is selected.

[0190] Select 0.18 seconds as the start time and 0.47 seconds as the end time.

[0191] The optimal imaging window time is searched according to the above method, and the optimal imaging window length is 527.

[0192] 4. Optimal imaging results

[0193] Select the best imaging sub-aperture for Fourier transform and the final imaging result is as follows: Figure 15 As shown, it shows clear ship outlines and structural details.

[0194] This example demonstrates the effective application of an ISAR short-term imaging optimization method based on target motion estimation to a ship target. Through precise translational compensation and parameter optimization, high-quality imaging of complex dynamic targets is achieved, demonstrating the method's practicality and effectiveness. This approach can significantly improve target recognition and surveillance in critical applications such as maritime search and rescue.

[0195] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. An optimization method for ISAR short-term imaging based on target motion estimation, characterized in that: The following steps are involved: S1. Determine the oblique angle between the radar line of sight and the maritime search and rescue target as a, the line connecting the radar line of sight and the maritime search and rescue target as the RLOS direction, the center of the maritime search and rescue target is located in the fixed reference system (X, Y, Z), and the coordinate system of the maritime search and rescue target is (X s ,Y s ,Z s ), the rotation matrix is ​​expressed as: Wherein, a(t) is the real-time rotation amplitude of the maritime search and rescue target at time t; S2. Construct the rotational motion model of the maritime search and rescue target as follows: Among them, α r (t), α p (t), α y (t) are the real-time amplitudes of roll, pitch and yaw respectively, q r ,q p and q y are the maximum swing amplitudes of roll, pitch and yaw respectively, T r 、T p and T y are the sway periods of roll, pitch and yaw sway, respectively; and are the initial phases of roll, pitch and yaw respectively; S3. Construct the rotation matrix of the maritime search and rescue target, which is: Among them, θ r ,θ p and θ y They are the rotation angles of the target in three directions at time t relative to the initial time, roll(θ r ), pitch(θ p ) and yaw(θ y ) are the rotation matrices corresponding to roll, pitch and yaw motions, respectively, then: in, S4. Obtain the rotation component of the maritime search and rescue target, and substitute the rotation motion model of the maritime search and rescue target into the radar echo signal. The original signal is expressed as: in, is the original signal, the scattering point coordinates of point A are (x, y, z), A a represents the amplitude of the scattering point A, The radar transmits a linear frequency modulated pulse signal. represents the fast time variable, t n represents the slow time variable, c represents the speed of light, r a (t n ) is the sum of the translational component and the rotational component, then the rotational component of the maritime search and rescue target is obtained as: r i (t n )=sin(α(t n ))(to 11 x+a 12 and+a 13 z)+cos(α(t n ))(to 21 x+a 22 and+a 23 z) (7) S5. Establish the radar view coordinate system (R, H, V). The R axis is always along the RLOS direction. The H axis is perpendicular to the R axis and located in the (X, Y) plane. The V axis is perpendicular to the (R, H) plane. The ground-grazing angle is H a is the height of the radar carrier, R a is the distance between the radar carrier and the ship, is the angle of view of the radar observing the maritime search and rescue target, that is, the angle between the RLOS projection on the ground and the longitudinal axis of the maritime search and rescue, and the angular velocities of roll, pitch, and yaw are determined as ω respectively. r 、ω p 、ω y , the rolling, pitching and yaw swing amplitudes are q r ,q p ,q y , the rolling, pitching and yaw periods are T r 、T p 、T y , the initial phases of roll, pitch and yaw are The cosine function of the angular velocity of the viewing angle is expressed as: S6, synthetic maritime search and rescue target effective rotation vector ω e : Where γ is ω e The angle between the V axis and the V axis represents ω e The direction of ω v is the vertical rotation component of the maritime search and rescue target, ω h The horizontal rotation component of the maritime search and rescue target; S7. Establishing effective rotation vector ω for maritime search and rescue targets e Time-varying characteristics and vertical component ω v The characteristics of the change over time, the vertical rotation component ω v The changing trend of m is: Among them, Δf(t) is the Doppler frequency difference between the head and tail scattering points of the search and rescue target, Δr is the slant range difference between the head and tail scattering points of the search and rescue target, and x s,b is the horizontal coordinate of the scattering point corresponding to the head of the search and rescue target, x s,s is the horizontal coordinate of the scattering point corresponding to the tail of the search and rescue target, is the radar viewing angle at the start of imaging; S8, according to ω v The IASR imaging time period is selected based on the changing trend of , and short-term ISAR imaging is performed.

2. The optimization method for ISAR short-term imaging based on target motion estimation according to claim 1, characterized in that: The step S8 also includes the following steps: v The steps of selecting a time period for ISAR imaging based on the trend of change in the image include: When h ≥3ω v When m≤m min The corresponding time window is used for short-term ISAR imaging to obtain the side view, where m min is the vertical rotation component ω v The lower threshold of the change trend; when When m≥m max The corresponding time window is used for short-term ISAR imaging to obtain a top view, where m max is the vertical rotation component ω v The upper threshold value of the change trend; when , a mixed top and side view is obtained.

3. The optimization method for ISAR short-time imaging based on target motion estimation according to claim 2, characterized in that: When ω h ≥3ω v When H≥H max , H max is the maximum wave height, where H is: Among them, V is the wind speed at sea, D is the duration of the wind, C is the empirical coefficient, and g is the acceleration of gravity.

4. The optimization method for ISAR short-term imaging based on target motion estimation according to claim 1, characterized in that: When When H≤H min , H min is the minimum wave height.

5. The optimization method for ISAR short-term imaging based on target motion estimation according to claim 1, characterized in that: The step S8 further includes evaluating the image brightness and Doppler frequency in each time window to determine the imaging moment with the highest image contrast.

6. The optimization method for ISAR short-time imaging based on target motion estimation according to claim 5, characterized in that: The step of evaluating the image brightness and Doppler frequency in each time window to determine the imaging moment with the highest image contrast comprises: Dividing the translation-compensated image into azimuthally overlapping sub-apertures; Calculate the Doppler broadening of each sub-aperture and the slope of the ship centerline to obtain the synthetic effective rotation vector ω e and its vertical component ω v Characteristics of changes over time and Set the Doppler broadening threshold DS and adjust the Doppler broadening Δf ds =f max -f min Evaluate the Doppler broadening Δf of the image ds ; according to and The change of is compared with the set threshold to determine the imaging moment with the maximum image contrast; After the search imaging window is completed, the selected subaperture in azimuth is Fourier transformed to optimize the image quality.

7. The optimization method for ISAR short-time imaging based on target motion estimation according to claim 6, characterized in that: The Doppler broadening threshold DS is set, and the Doppler broadening Δf ds =f max -f min Evaluate the Doppler broadening Δf of the image ds The steps include: The lowest Doppler frequency unit f where the image brightness exceeds 10 times the average value min and the highest Doppler frequency unit f max The width between them is the Doppler broadening Δf of the image ds =f max -f min , set the Doppler broadening threshold DS, in, is the lower limit of the Doppler broadening threshold, Is the upper limit of the Doppler broadening threshold, DS min Minimum Doppler broadening threshold lower limit amplitude, DS max The upper limit of the maximum Doppler broadening threshold.

8. The optimization method for ISAR short-time imaging based on target motion estimation according to claim 6, characterized in that: The basis and The change of is compared with the set threshold to determine the imaging moment with the maximum image contrast. when When the change amplitude is lower than the threshold ΔDS, the method further includes: according to The magnitude of the amplitude change over time is used to determine the range of change in the slope of the vertical axis of the maritime search and rescue target, and whether the maximum modulus value during the amplitude change process is less than the set threshold constant; If yes, then during the entire observation time T, start searching from the starting time t0 for the time t where the image contrast C(t) reaches its maximum. opt =arg max t∈T C(t), get the imaging window size ΔT opt ; If not, the imaging is performed in the time period with the smaller Doppler broadening estimate and the larger slope of the ship axis. ε represents an arbitrarily small positive number.

9. The optimization method for ISAR short-time imaging based on target motion estimation according to claim 6, characterized in that: The basis and The change of is compared with the set threshold to determine the imaging moment with the maximum image contrast. when When the change amplitude is higher than the threshold ΔDS, the method further includes: according to The magnitude of the amplitude change over time is used to determine the range of change in the slope of the vertical axis of the maritime search and rescue target, and whether the maximum modulus value during the amplitude change process is less than the set threshold constant; If yes, then during the entire observation time T, from The search starts at the moment t when the image contrast C(t) reaches its maximum opt =arg max t∈T C(t), get the imaging window size ΔT opt ; If not, then select the time period where the slope is close to zero and the Doppler broadening is large for imaging, i.e. ε represents an arbitrarily small positive number.

10. An ISAR short-time imaging system based on target motion estimation, used to implement the method according to any one of claims 1 to 9, characterized in that: It includes a transmitting system, a receiving system, a signal processing system and a control and data processing unit, wherein: The transmitting system includes a radio frequency generator, a power amplifier, and a transmitting antenna. The radio frequency generator is used to generate radio frequency signals. The power amplifier is used to amplify the radio frequency signals to a level sufficient to excite the transmitting antenna. The transmitting antenna is used to convert the radio frequency signals into electromagnetic waves and transmit them to the target. The receiving system includes a receiving antenna, a low-noise amplifier, a mixer, and an intermediate frequency amplifier. The receiving antenna is used to receive radar signals reflected from the target. The low-noise amplifier is used to amplify the received weak signal. The mixer is used to mix the received RF signal with the signal generated by the local oscillator to obtain an intermediate frequency signal. The intermediate frequency amplifier is used to amplify the intermediate frequency signal output by the mixer. The signal processing system includes an analog-to-digital converter (ADC), a digital signal processor (DSP), and an ISAR short-time imaging module based on target motion estimation. The ADC converts analog signals into digital signals, and the DSP performs filtering, demodulation, compression, and imaging. The ISAR short-time imaging module based on target motion estimation generates high-resolution images based on received radar data and target motion parameters. The motion compensation system includes a target motion estimation module and a radar motion compensation module. The target motion estimation module estimates the target's motion parameters, such as velocity and acceleration. The radar motion compensation module corrects the received radar data according to the motion state of the radar platform to eliminate the impact of platform motion on imaging quality. The control and data processing unit includes a control unit and a data processing unit. The control unit is responsible for the system parameter setting, timing control and coordination, and the data processing unit performs the reception, processing, storage and transmission of radar data.

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