A dual-base forward-looking SAR target localization method

Abstract

This invention proposes a bistatic forward-looking SAR target localization method, the implementation steps of which are: initializing the bistatic forward-looking SAR target localization scene; calculating the slant range between the receiver and the scene center, as well as the precise position of the scene center; and obtaining the target localization result. This invention decouples the bistatic slant range by establishing a system of equations based on the triangle formed by the receiver's slant range relative to the scene center at adjacent synthetic aperture center times and the flight trajectory within the synthetic aperture time interval, and the triangle formed by the transmitter's slant range relative to the scene center at the previous synthetic aperture center time, its projection, and the transmitter's height. This allows for the acquisition of the receiver's slant range relative to the scene center at the next synthetic aperture center time, avoiding the defect of amplified beamline angle error due to sine calculations, thus improving localization accuracy. Furthermore, the data used to calculate the transmitter's slant range relative to the scene center can be directly obtained from the constructed localization scene, without the need for beamline angle calculations, thus possessing universality and broadening the application scope.

Description

Technical Field

[0001] This invention belongs to the field of radar image target localization technology, and relates to a bistatic forward-looking SAR target localization method, which can be applied to high-speed platforms to accurately locate slow-moving targets in a broad background under bistatic forward-looking SAR imaging mode. Background Technology

[0002] Bistatic synthetic aperture radar (SAR) features separate transceiver platforms, offering more flexible geometric configurations than traditional monostatic SAR and overcoming the limitation of monostatic SAR's inability to perform forward-looking imaging. A bistatic forward-looking SAR system mounts the receiving and transmitting radars on separate platforms, which work collaboratively and share data link information. Through appropriate configuration and system parameter design, it can achieve two-dimensional high-resolution imaging of targets directly in front of the radar. Bistatic forward-looking SAR increases the radar's observation range, acquires scattering characteristics of targets from different azimuths, and enhances the radar's effective target identification. Simultaneously, the receiver passively receives signals, exhibiting electromagnetic silence. Furthermore, due to its spatially distributed nature, suppressive jamming targeting the transmitter often fails to affect the receiver, providing inherent anti-jamming capabilities.

[0003] Currently, typical target localization methods are based on image matching point information. This method first prepares a reference map, then registers it with real-time acquired SAR images to obtain target matching point information, thereby acquiring the target's precise coordinates. However, in many situations, such as vast ocean areas, image reference points often do not exist, and it is impossible to prepare a reference map in advance when the target is not cooperative. These factors render this method unusable. In such cases, detection and recognition algorithms can be used to determine the target's position in the image, and then combined with bistatic forward-looking SAR geometry to locate the target. Therefore, researching a bistatic forward-looking SAR target localization method has practical application significance.

[0004] For example, patent application CN 106556835 A, entitled "Target Localization Method Based on Bistatic Forward-Looking SAR Image," discloses a target localization method based on bistatic forward-looking SAR image. This method uses the geometric relationship between the receiver and transmitter and the target at the center time of two adjacent synthetic apertures to perform mathematical modeling, decouples the bistatic slant range, and combines the positional relationship between the target and the scene center in the ground distance SAR image to achieve accurate target localization. However, in the triangular relationship formed by the transmitter and the scene center, the sine operation amplifies the error caused by the beamline angle, resulting in a large error in the calculated receiver slant range, which affects the further improvement of positioning accuracy. At the same time, this method calculates the slant range of the transmitter relative to the scene center through the beamline angle of the transmitter, but the beamline angle is difficult to obtain directly in some practical applications, which limits the expansion of the application scope. Summary of the Invention

[0005] The purpose of this invention is to overcome the defects of the prior art and propose a bistatic forward-looking SAR target localization method to solve the technical problem of poor positioning accuracy in the prior art.

[0006] To achieve the above objectives, the technical solution adopted by the present invention includes the following steps:

[0007] (1) Initialize the bistatic forward-looking SAR target localization scenario:

[0008] Initialization includes a bistatic forward-looking SAR and a target localization scenario with target P distributed in the northeast-northeast sky coordinate system Oxyz. At the initial moment, the center point of the scenario formed by the intersection of the receiver and transmitter beam centers and the ground is located at the origin O of Oxyz. The time positions of the adjacent synthetic aperture centers of the receiver and transmitter are A and A, respectively. r and B r A t and B t The center point S of the scene formed by the intersection of the beam center and the ground during the flight of the receiver and transmitter is located near point O;

[0009] (2) Calculate the receiver at B r The slant distance of the scene center point S relative to the receiver:

[0010] For the receiver at A r B r The slant distance R relative to the scene center point S ra R rb Flight trajectory within synthetic aperture time The triangle △A formed r B r S, and the transmitter in A t The slant distance R relative to the scene center point S ta R ta The projection d ta and the height H of the transmitter ta The triangle △A formed t A t The equations established by 'S are used for bibasic slant-range decoupling to obtain the receiver's position at B. r The slant distance R between the time and the scene center point S rb A t ′ indicates that the transmitter is at A t The ground projection point at that time;

[0011] (3) Calculate the precise position of the scene center point S:

[0012] via receiver at B r The slant distance R between the time and the scene center point S rbCalculate the horizontal slant distance d between the receiver and the scene center point S. rb and through d rb Calculate the precise position (x) of the scene center point S. s ,y s );

[0013] (4) Obtain the target location result:

[0014] By the precise location (x) of the scene center point S s ,y s ) Calculate the precise location (x) of target P p ,y p ).

[0015] Compared with the prior art, the present invention has the following advantages:

[0016] (1) The present invention decouples the bibase slant range by establishing a set of equations formed by the triangle formed by the slant range of the receiver relative to the center point of the scene at the adjacent synthetic aperture center time and the flight trajectory during the synthetic aperture time, and the triangle formed by the slant range of the transmitter relative to the center point of the scene at the previous synthetic aperture center time, its projection and the height of the transmitter, so as to obtain the slant range of the receiver relative to the center point of the scene at the next synthetic aperture center time. This avoids the defect of the prior art that amplifies the beam line angle error due to sine operation, and effectively improves the target positioning accuracy.

[0017] (2) The transmitter height and ground angle used in the present invention to calculate the slant distance of the transmitter relative to the center of the scene can be directly obtained from the constructed positioning scene, which avoids the defect that the beam line angle used in the prior art is difficult to obtain directly in some practical applications. It has universality and thus broadens the application scope. Attached Figure Description

[0018] Figure 1 This is a flowchart illustrating the implementation of the present invention.

[0019] Figure 2 This is a schematic diagram of the bistatic forward-looking SAR target localization scenario used in this invention.

[0020] Figure 3 This is a top-down schematic diagram of the dual-base forward-looking SAR radar platform and target localization scenario of the present invention.

[0021] Figure 4 This is a schematic diagram of the SAR image coordinate system involved in the present invention.

[0022] Figure 5 This is a top-view schematic diagram of the SAR image coordinate system of the present invention in the northeast-sky coordinate system.

[0023] Figure 6This is a schematic diagram of the motion trajectory of a dual-base platform and a target in the northeast-central coordinate system provided by an embodiment of the present invention.

[0024] Figure 7 This is a top-view schematic diagram of the motion trajectory of a dual-base platform and a target in the northeast-northeast coordinate system provided in an embodiment of the present invention.

[0025] Figure 8 This is a comparison diagram of the positioning errors of the target in the east and north directions under error conditions provided by the present invention and the prior art.

[0026] Figure 9 This is a schematic diagram of the target positioning error in the east and north directions under error conditions provided by an embodiment of the present invention. Detailed Implementation

[0027] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0028] Reference Figure 1 The present invention includes the following steps:

[0029] Step 1) Initialize the bistatic forward-looking SAR target localization scene, the structure of which is as follows: Figure 2 As shown.

[0030] Initialization includes a bistatic forward-looking SAR distributed in the northeast-northeast coordinate system Oxyz and a target localization scene with target P. The positive directions of the x and y axes of the northeast-northeast coordinate system Oxyz are due east and due north, respectively, and the positive direction of the z axis is perpendicular to the Oxy plane and points to the zenith. At the initial moment, the scene center point formed by the intersection of the receiver and transmitter beam centers with the ground is located at the origin O of Oxyz. During the flight of the receiver and transmitter, due to platform jitter and other reasons, it is difficult to maintain the intersection of the beam centers with the ground at point O, and a new scene center point S is formed near point O.

[0031] The time positions of the adjacent synthetic aperture centers of the receiver and transmitter are A and B, respectively. r and B r A t and B t The slant distances of the corresponding platform locations relative to the scene center point S are R and R, respectively. ra and R rb R ta and R tb ; Receiver and transmitter in A r and A t B r and B t The sum of the slant distances between the time and the scene center point S are R and R. a R b ; Receiver at A r B rThe ground rubbing angles relative to the scene center point S are respectively In B r The height at that time is H rb The transmitter is in A t The angle of friction relative to the center point S of the scene is Height is H ta .

[0032] Step 2) Calculate the receiver's position at B. r The slant distance of the scene center point S relative to the receiver:

[0033] Receiver at A r B r The slant distance R relative to the scene center point S ra R rb Flight trajectory within synthetic aperture time The triangle △A formed r B r S has the following triangular relationship:

[0034]

[0035] Among them, D r Indicates that the receiver is A r To B r Flight distance.

[0036] This invention eliminates the need for ΔA when decoupling the bibasic slant distance. t B t The triangular relationship in S avoids the use of the beamline angle ∠SA in existing technologies. t B t and ∠SB t A t This reduces the impact of angle errors on slant range calculation and solves the problem that the method cannot be used in some practical applications because the beamline angle cannot be directly obtained. In this invention, the transmitter is used in A... t The slant distance R relative to the scene center point S ta Height H ta and mopping corners The transmitter is in A t The ground projection point at that time is denoted as A. t ′, A t Point A t Point ′ and the center point S of the scene form △A t A t The triangle relationship is as follows:

[0037]

[0038] The sending and receiving platform is in A r and At B r and B t When the slope distance is such that the following relationship holds:

[0039]

[0040] In the above formula, R a R b , D r , H ta R is a known quantity that can be obtained through inertial navigation and radar. ra R ta R rb R tb For the unknown quantity, combining the above formulas, we can obtain the receiver's value at B. r Distance R between time and scene center point S rb The calculation formula is as follows:

[0041]

[0042] Where p, b, and c are all intermediate variables, the calculation formula is:

[0043]

[0044] Step 3) Calculate the precise location of the scene center point S:

[0045] via receiver at B r The slant distance R between the time and the scene center point S rb Calculate the horizontal slant distance d between the receiver and the scene center point S. rb and through d rb Calculate the precise position (x) of the scene center point S. s ,y s );

[0046] To calculate the precise location of the scene center point S on the Oxy horizontal plane, and to simplify the structure, we will... Figure 2 Projecting the geometric configuration onto the Oxy plane, see [reference]. Figure 3 The receiver is at B r The horizontal slant distance relative to the scene center point S is d rb The beam angle is ξ. rb The azimuth angle is the horizontal angle between the beam direction and the positive x-axis; the beam direction is positive to the left of the Oxz plane. rb The beam direction is positive when it is to the left of the navigation direction; the horizontal heading is ψ. rb The direction of travel is positive to the left of the Oxz plane; the x and y coordinates are x rb y rb .

[0047] Firstly, according to Figure 2 Geometric relationship calculation d rb :

[0048]

[0049] Then according to Figure 3 Given the geometric relationships, determine the position (x) of the scene center point S. s ,y s )for:

[0050] x s =d rb ·cos(ξ rb )+x rb

[0051] y s =d rb ·sin(ξ rb )+y rb

[0052] ξ rb =θ rb +ψ rb

[0053] Where, x s y s These respectively represent the receiver at B r The x and y coordinates of the center point S of the scene at that time.

[0054] Step 4) Obtain the target location result:

[0055] By the precise location (x) of the scene center point S s ,y s ) Calculate the precise location (x) of target P p ,y p );

[0056] The image coordinate system in which the SAR image is located is as follows: Figure 4 As shown, the coordinate system is located in the Oxy plane, with the origin O1 at the scene center point S, which is located at the exact center of the SAR image. The O1x1 axis points downwards along the distance direction, and the O1y1 axis points to the right along the azimuth direction. The target is located at point P. 1p y 1p These represent the coordinates of target point P in the SAR image coordinate system along the x1 and y1 directions, respectively.

[0057] Figure 5The diagram shows a top-down view of the SAR image coordinate system within the northeast-sky coordinate system. In the diagram, ω represents the angle between the positive x1 axis of the SAR image coordinate system and the positive x-axis of the northeast-sky coordinate system. Rotation counterclockwise from the O1x1 axis to the Ox axis is positive. The precise location (x, y) of target point P can be calculated from the relationships shown in the diagram. p ,y p ):

[0058]

[0059] Where, x p y p These represent the receiver at B. r The x and y coordinates of the target point P in the Oxyz coordinate system of the northeast celestial coordinate system; a and b represent the range and azimuth resolutions, respectively.

[0060] The technical effects of the present invention will be further explained below with reference to simulation results:

[0061] 1. Experimental conditions and contents:

[0062] The simulation software used is MATLAB R2023b. In this embodiment, the radar platform moves at a constant velocity in a straight line under ideal conditions. The receiver is in forward-looking imaging mode, and the transmitter is in forward-slanting imaging mode. The target moves at a constant velocity near the center point O of the scene. During the movement of the transceiver platform, the radar beam center is always near point O, and the target is always within the scene. The bistatic forward-looking SAR platform and its motion parameters are shown in Table 1.

[0063] Table 1

[0064]

[0065]

[0066] Based on the parameters in Table 1, a dual-base model was constructed in MATLAB to obtain the motion trajectories of the dual-base platform and the target in the northeast-northeast coordinate system, as follows: Figure 6 As shown, Figure 7 for Figure 6 The top view of the model projected onto the Oxy plane. Considering that in practical engineering applications, inertial navigation data and radar data have different degrees of error, affecting positioning accuracy, radar angle measurement error, ranging error, and transceiver platform positioning error are added based on the above ideal situation. The error parameters are shown in Table 2, where the receiver platform and transmitter platform have the same error level.

[0067] Table 2

[0068] Error type Standard deviation Radar angle error / ° 0.01 Platform attitude angle error / ° 0.01 Double-base slope distance error / m 5 X-direction position error / m 1 Y-direction position error / m 1 Z-direction position error / m 5

[0069] Under the error conditions in Table 2, simulations were performed to compare the target positioning errors in the northeast and north directions in the northeast coordinate system using the present invention and existing technologies. The comparison results are as follows: Figure 8 As shown in (a) and (b), in Figure 8 The positioning error amplitude of this invention is difficult to observe directly in the middle, therefore, in Figure 8 Based on this, only the positioning error of the present invention is plotted, such as Figure 9 As shown in (a) and (b).

[0070] 2. Analysis of experimental results:

[0071] Reference Figure 8 From (a) and (b), it can be seen that, under the same error conditions, the positioning error of the existing technology reaches 10. 3 m or even 10 4 With errors on the order of m, locating a target is extremely difficult. In contrast, the method proposed in this invention has higher positioning accuracy.

[0072] Reference Figure 9 As shown in (a) and (b), the positioning error at each moment does not exceed ±100m. In practical applications, as the platform flies, the illumination area of ​​the transceiver platform remains unchanged, which means that the coordinates of the scene center point S theoretically remain unchanged. Therefore, during flight, averaging the scene center position calculation results from previous moments can effectively improve the subsequent target positioning accuracy. In this embodiment, the average positioning error in the east direction is -2.27m, and the average positioning error in the north direction is 2.37m, demonstrating high positioning accuracy.

Claims

1. A bistatic forward-looking SAR target localization method, characterized in that, Includes the following steps: (1) Initialize the bistatic forward-looking SAR target localization scenario: Initialization includes distribution in the northeast celestial coordinate system The bistatic forward-looking SAR and the target are In the target localization scenario, the initial point where the intersection of the receiver and transmitter beam centers with the ground forms the scene center point is located at... The origin The time positions of the adjacent synthetic aperture centers of the receiver and transmitter are respectively and , and The center point of the scene formed by the intersection of the beam center and the ground during the flight of the receiver and transmitter. lie in Near the point; (2) Calculate the receiver's... Scene center point Relative to the slant range of the receiver: For the receiver , Time relative to the center point of the scene slant distance , Flight trajectory within synthetic aperture time The triangle formed and the transmitter in Time relative to the center point of the scene slant distance , projection and the height of the transmitter The triangle formed The established system of equations is used for bibase slant-range decoupling to obtain the receiver's... Time and the center point of the scene slant distance ,in Indicates the transmitter is in The ground projection point at that time, the expression of the system of equations is: ； in, , , All are intermediate variables. , These respectively indicate that the receiver is in and Time relative to the center point of the scene The corner of the floor, Indicates that the receiver is from arrive Flight distance, Indicates the transmitter is in Time relative to the center point of the scene The corner of the floor, , Indicates that the receiver and transmitter are in and , and Time and the center point of the scene The slope distance and; (3) Calculate the center point of the scene Precise location: via receiver Time and the center point of the scene slant distance Computation receiver and scene center point Horizontal slope distance and through Calculate the center point of the scene precise location ; (4) Obtain the target location result: Through the center point of the scene precise location Calculation target precise location .

2. The target localization method according to claim 1, characterized in that, The northeast celestial coordinate system mentioned in step (1) , among them , The positive directions of the axes are due east and due north, respectively. The positive direction of the axis is perpendicular to The plane points towards the zenith.

3. The target localization method according to claim 1, characterized in that, The receiver and scene center point mentioned in step (3) Horizontal slope distance The calculation formula is: ； in, Indicates that the receiver is at The altitude at that time.

4. The target localization method according to claim 1, characterized in that, The scene center point mentioned in step (3) precise location The calculation formula is: ； ； ； in, Indicates that the receiver is at Time relative to the center point of the scene Horizontal slope distance Indicates that the receiver is at The beam angle at that time, that is, the beam direction and The horizontal angle along the positive axis, the beam direction is in The left side of the plane is positive; Indicates that the receiver is at The radar azimuth angle at that time, with the beam direction to the left of the navigation direction being positive; Indicates that the receiver is at The horizontal heading and the direction of navigation at that time are The left side of the plane is positive; , These respectively indicate that the receiver is in time , Direction coordinates; , These respectively indicate that the receiver is in Scene center point of , Direction coordinates.

5. The target localization method according to claim 1, characterized in that, The target described in step (4) precise location The calculation formula is: ； in, , These respectively indicate that the receiver is in Target point of , Direction coordinates; Indicates the range resolution. Indicates azimuth resolution; , Representing the target points respectively Along in the SAR image coordinate system , Directional coordinates Representing the SAR image coordinate system Positive axis direction and northeast celestial coordinate system The angle between the positive axis and the axis is determined by... Axial Rotating the axis counterclockwise is positive.

6. The target localization method according to claim 5, characterized in that, The SAR image coordinate system is located in the northeast-sky coordinate system. of Plane, origin of coordinate system Center point of the scene Located at the exact center of the SAR image. The axis points downwards along the distance. The axis points to the right along the azimuth direction.

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