A method for obtaining virtual rotation points in sliding spotlight mode of spaceborne SAR

Abstract

The present invention proposes a method for acquiring a virtual rotation point in a sliding spotlight mode of a spaceborne SAR, which is suitable for acquiring a virtual rotation point in a sliding spotlight mode of a spaceborne SAR in ultra-high resolution conditions. The present invention segments the entire imaging time, acquires the closest point of the beam pointing vector for each time period, and then averages the positions of the closest points of the beam pointing vectors for all acquired time periods to obtain the position coordinates of the virtual rotation point. Compared to the prior art, the present invention fully considers the impact of changes in scanning rate and greatly reduces this impact, ensuring ultra-high resolution and wide-angle observation. It can meet the requirements of spaceborne SAR imaging with an ultra-high resolution better than 0.1m and a spaceborne SAR azimuth scanning angle of ±50°.

Description

Technical Field

[0001] The invention belongs to the field of signal processing and relates to a method for acquiring a virtual rotation point in a sliding beamforming mode of a spaceborne SAR. Technical Background

[0002] Due to its weather-independent, all-day, all-weather operation, spaceborne synthetic aperture radar (SAR) has become an important means of Earth observation, widely used in both military and civilian applications. To achieve high-resolution observations, spaceborne SAR typically employs a wide bandwidth in range and a sliding beam observation mode in azimuth. In the azimuth sliding beam observation mode, the azimuth beam rotates counterclockwise, always pointing to a location below the ground, known as the virtual rotation point. When performing imaging processing on spaceborne SAR sliding beam mode data, it is necessary to obtain the coordinates of the virtual rotation point to which the azimuth beam is pointing.

[0003] When the resolution is less than 0.3 m, the azimuth scanning angle of the spaceborne SAR is small, and the position of the virtual rotation point in the sliding spotlight mode can be obtained by the antenna beam scanning rate. However, when achieving ultra-high resolution observations (≤ better than 0.1 m), the azimuth scanning angle of the spaceborne SAR reaches ±10°. During the imaging time, the antenna beam scanning rate no longer remains constant, but varies to a certain extent. Therefore, the method of using the antenna beam scanning rate to obtain the position of the virtual rotation point is no longer applicable. Therefore, it is necessary to study a method for obtaining the virtual rotation point in the sliding spotlight mode of ultra-high resolution spaceborne SAR. Summary of the Invention

[0004] Considering that the azimuth scanning angle of the ultra-high-resolution spaceborne SAR sliding spotlight mode is large and the antenna beam scanning rate varies, the traditional method of using the scanning rate of the antenna beam to obtain the position coordinates of the virtual rotation point is no longer suitable. The present invention proposes a method for obtaining the virtual rotation point in the sliding spotlight mode of the spaceborne SAR.

[0005] The specific technical solutions are:

[0006] A method for acquiring a virtual rotation point in the sliding spotlight mode of a spaceborne SAR is proposed. The entire imaging time is segmented, and the closest point of the beam pointing vector in each time segment is obtained. Then, the positions of the closest points of the beam pointing vectors in all time segments are averaged to obtain the position coordinates of the virtual rotation point.

[0007] Furthermore, obtaining the closest point of the beam pointing vector at any time period includes the following specific steps:

[0008] Step 1: Calculate the instantaneous beam vector of the antenna beam in the Earth-fixed coordinate system corresponding to the start and end times of the time period respectively; the instantaneous beam vector of the antenna beam in the Earth-fixed coordinate system is obtained based on the pointing direction of the antenna beam in the body coordinate system and the attitude data of the satellite;

[0009] Step 2: Use the satellite's position in the Earth's fixed coordinate system corresponding to the start and end times of the time period And the instantaneous beam vector of the antenna beam in the earth-fixed coordinate system, obtain the closest point of the beam pointing vector during this period

[0010] Furthermore, the calculation process of the closest point of the beam pointing vector at any time period is as follows:

[0011] Let t i represents the azimuth time (i=1,…,i,i+1,…N), then t i and t i+1 Indicates the start and end time of the period. At these two times, the positions of the satellite in the Earth's fixed coordinate system are and The instantaneous beam vectors of the antenna beam in the earth-fixed coordinate system are and The closest point of the beam pointing vector in this period is for:

[0012]

[0013] in,

[0014]

[0015]

[0016] in, express arrive vector; express and The cross product of and Common perpendicular vectors; T1 and T2 are t i and t i+1 The position of the satellite in the Earth-fixed coordinate system at the moment and To common perpendicular vector Therefore, the distance and The instantaneous beam vectors are and Vectors perpendicular to each other The midpoint of these two intersections is the closest point of the beam pointing vector at adjacent moments.

[0017] Furthermore, the calculation process of the instantaneous beam vector of the antenna beam in the earth-fixed coordinate system is as follows:

[0018] 1) Get the pointing vector of the antenna beam in the body coordinate system

[0019] 2) Use coordinate transformation to transform the pointing vector of the antenna beam in the body coordinate system Switch to the J2000 coordinate system and obtain the instantaneous beam vector of the antenna beam in the J2000 coordinate system

[0020] 3) The instantaneous pointing vector of the antenna beam in the J2000 coordinate system Switch to the earth-fixed coordinate system and get the instantaneous beam vector of the antenna beam in the earth-fixed coordinate system

[0021] Furthermore, the pointing vector of the antenna beam in the body coordinate system is The calculation formula is as follows:

[0022]

[0023] Among them, θ AZ Indicates the azimuth angle of the antenna beam deviating from the Z axis in the XZ plane in the body coordinate system, θ EL The azimuth angle of the antenna beam deviating from the Z axis in the YZ plane in the body coordinate system, and

[0024]

[0025] Furthermore, the calculation formula of the instantaneous beam vector of the antenna beam in the J2000 coordinate system is as follows:

[0026]

[0027] Among them, M ob is the transformation matrix from the body coordinate system to the J2000 coordinate system, which is:

[0028]

[0029] Where [w,x,y,z] represents the four components of the attitude quaternion at that moment;

[0030] The instantaneous beam vector of the antenna beam in the earth-fixed coordinate system is The calculation formula is as follows:

[0031]

[0032] Among them, M wo is the transformation matrix from the Earth's J2000 coordinate system to the Earth-fixed coordinate system.

[0033] Beneficial effects

[0034] 1) This invention obtains the closest points of the beam pointing vector at adjacent moments in a segmented manner, averaging the closest points obtained for each angular segment to obtain the coordinates of the virtual rotation point. Compared to existing technologies, this invention fully considers the impact of scan rate variations and significantly reduces this effect.

[0035] 2) The present invention performs fine acquisition starting from beam pointing, and has high processing accuracy and practical application value.

[0036] 3) The present invention achieves ultra-high resolution and large-angle observation. The ultra-high resolution is better than 0.1m, and the azimuth scanning angle of the space-borne SAR reaches ±50°. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is a flow chart of a method for acquiring virtual rotation points in the sliding spotlight mode of a spaceborne SAR proposed by the present invention. DETAILED DESCRIPTION

[0038] This paper proposes a method for acquiring a virtual rotation point in the sliding spotlight mode of a spaceborne SAR. This method obtains the closest points of the beam pointing vector at adjacent moments in the entire imaging time segment by segment, then averages the closest points obtained for each angular segment to obtain the coordinates of the virtual rotation point. The specific implementation is as follows:

[0039] Let the azimuth time t i =-31.5+0.1i, where i=1,…,i,i+1,…N, N=630.

[0040] Step 1: Divide the entire imaging time into 629 segments, and calculate the instantaneous beam vector of the antenna beam in the Earth-fixed coordinate system corresponding to the start and end times of each segment;

[0041] Taking time t1 as an example, calculate the instantaneous beam vector of the antenna beam in the earth-fixed coordinate system at time t1 The process includes the following steps:

[0042] 1) Get the pointing vector of the antenna beam in the body coordinate system at time t1

[0043]

[0044] Among them, θ AZ=-0.05°,θ EL =0.22° and there is

[0045]

[0046] 2) The pointing vector of the antenna beam in the body coordinate system at time t1 Instantaneous beam vector converted to J2000 coordinate system

[0047]

[0048] Among them, M ob The transformation matrix from the body coordinate system to the J2000 coordinate system is:

[0049]

[0050] Where [w,x,y,z]=[0.46103,0.81673,0.32367,-0.12509].

[0051] 3) The pointing vector of the antenna beam in the J2000 coordinate system at time t1 is Convert to Earth-fixed coordinate system

[0052]

[0053] Among them, M wo is the transformation matrix from the Earth's J2000 coordinate system to the Earth-fixed coordinate system. It is a general formula and will not be written out here.

[0054] Similarly, obtain the pointing vector of the antenna beam in the J2000 coordinate system at time t2 Convert to Earth-fixed coordinate system

[0055]

[0056] Step 2: Using the satellite's position in the Earth-fixed coordinate system The instantaneous beam vector of the antenna beam in the earth-fixed coordinate system is used to obtain the closest point of the beam pointing vector in each time period

[0057] Taking the first period as an example, the position of the satellite in the Earth's fixed coordinate system corresponding to the start and end times of the first period is used. And the instantaneous beam vector of the antenna beam in the earth-fixed coordinate system, obtain the closest point of the beam pointing vector in the first period

[0058] The positions of the satellite in the Earth-fixed coordinate system at time t1 and t2 are:

[0059]

[0060]

[0061] The instantaneous beam vector of the combined antenna beam in the earth-fixed coordinate system and The closest point of the beam pointing vector at adjacent moments is for:

[0062]

[0063] Among them, T1=-1.0089703, T2=-1.0089711.

[0064] Step 3: Average the nearest points of the beam pointing vectors for all time periods to obtain the position coordinates of the virtual rotation point

Claims

1. A method for acquiring virtual rotation points in a spaceborne SAR sliding spotlight mode, characterized by: The entire imaging time is segmented, and the closest point of the beam pointing vector in each period is obtained. Then, the positions of the closest points of the beam pointing vectors in all periods are averaged to obtain the position coordinates of the virtual rotation point. Obtaining the closest point of the beam pointing vector at any time period includes the following specific steps: Step 1: Calculate the instantaneous beam vector of the antenna beam in the Earth-fixed coordinate system corresponding to the start and end times of the time period respectively; the instantaneous beam vector of the antenna beam in the Earth-fixed coordinate system is obtained based on the pointing direction of the antenna beam in the body coordinate system and the attitude data of the satellite; Step 2: Use the satellite's position in the Earth's fixed coordinate system corresponding to the start and end times of the time period And the instantaneous beam vector of the antenna beam in the earth-fixed coordinate system, obtain the closest point of the beam pointing vector during this period 2. The method for acquiring a virtual rotation point in a sliding spotlight mode of a spaceborne SAR according to claim 1, characterized in that: The calculation process of the closest point of the beam pointing vector at any time period is as follows: Let t i represents the azimuth time (i=1,…,i,i+1,…N), then t i and t i+1 Indicates the start and end time of the period. At these two times, the positions of the satellite in the Earth's fixed coordinate system are and The instantaneous beam vectors of the antenna beam in the earth-fixed coordinate system are and The closest point of the beam pointing vector in this period is for: in, in, for arrive vector; for and The cross product of and Common perpendicular vectors; T1 and T2 are t i and t i+1 The position of the satellite in the Earth-fixed coordinate system at the moment and To common perpendicular vector Therefore, the distance and The instantaneous beam vectors are and Vectors perpendicular to each other The midpoint of these two intersections is the closest point of the beam pointing vector at adjacent moments.

3. The method for acquiring a virtual rotation point in a sliding spotlight mode of a spaceborne SAR according to claim 1, characterized in that: The calculation process of the instantaneous beam vector of the antenna beam in the earth-fixed coordinate system is as follows: 1) Get the pointing vector of the antenna beam in the body coordinate system 2) Use coordinate transformation to transform the pointing vector of the antenna beam in the body coordinate system Switch to the J2000 coordinate system and obtain the instantaneous beam vector of the antenna beam in the J2000 coordinate system 3) The instantaneous pointing vector of the antenna beam in the J2000 coordinate system Switch to the earth-fixed coordinate system and get the instantaneous beam vector of the antenna beam in the earth-fixed coordinate system 4. The method for acquiring a virtual rotation point in a sliding spotlight mode of a spaceborne SAR according to claim 3, wherein: The pointing vector of the antenna beam in the body coordinate system The calculation formula is as follows: Among them, θ AZ Indicates the azimuth angle of the antenna beam deviating from the Z axis in the XZ plane in the body coordinate system, θ EL The azimuth angle of the antenna beam deviating from the Z axis in the YZ plane in the body coordinate system, and 5. The method for acquiring a virtual rotation point in a sliding spotlight mode of a spaceborne SAR according to claim 3, wherein: The calculation formula of the instantaneous beam vector of the antenna beam in the J2000 coordinate system is as follows: Among them, M ob is the transformation matrix from the body coordinate system to the J2000 coordinate system, which is: Where [w,x,y,z] represents the four components of the attitude quaternion at that moment; The instantaneous beam vector of the antenna beam in the earth-fixed coordinate system is The calculation formula is as follows: Among them, M wo is the transformation matrix from the Earth's J2000 coordinate system to the Earth-fixed coordinate system.

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