A method for establishing an attitude reference of a SAR satellite in motion imaging

Abstract

The application aims at SAR satellite imaging in motion, and provides a method for establishing an attitude reference for SAR satellite imaging in motion. The method for establishing the attitude reference can calculate target attitude quaternion and attitude angular velocity in an orbit coordinate system in real time in the process of satellite imaging maneuver, and ensures the accuracy of the establishment of the imaging reference in the process of imaging in motion.

Description

Technical Field

[0001] This invention relates to the field of spacecraft attitude control technology, mainly to the control technology for establishing attitude references during the maneuvering of SAR satellites. Background Technology

[0002] Compared to traditional pushbroom and agile maneuver modes, SAR (Synthetic Aperture Radar) satellite imaging mode offers significant performance improvements and a variety of novel imaging methods, but it also presents considerable difficulties and challenges in establishing satellite imaging attitude references.

[0003] The accuracy of satellite attitude control depends primarily on the establishment of the attitude reference, and the accuracy of the established imaging attitude reference directly affects the imaging quality of the SAR payload. In particular, low-Earth orbit spacecraft are more susceptible to external interference such as atmospheric drag, making them more dependent on the attitude reference for on-the-moment imaging. Even a small deviation in the attitude reference can seriously affect the imaging quality, or even lead to serious consequences such as imaging failure.

[0004] Currently, the publicly available methods for establishing attitude references for in-motion imaging are mostly at the algorithm level, and have not formed a complete process for SAR satellites to obtain the target attitude quaternions and angular velocities in the orbital coordinate system from the imaging ground target point. Summary of the Invention

[0005] The purpose of this invention is to provide a method for establishing the attitude reference of SAR satellite in motion imaging. The method includes: step S1, calculating the vector r from the geocenter to the ground target. et Step S2: Calculate the transformation matrix A between the orbital coordinate system and the geocentric inertial coordinate system. oi Step S3, using the transformation matrix A between the orbital coordinate system and the geocentric inertial coordinate system. oi Obtain the vector r from the Earth's center to the ground target. et Representation in orbital coordinate system Step S4, based on the vector r from the Earth's center to the ground target et Representation in orbital coordinate system Calculate the vector from the satellite to the ground target. And calculate the vector from the satellite to the ground target. The components r on the X-axis of the orbital coordinate system x The component r along the Y-axis in the orbital coordinate system y The component r along the Z-axis in the orbital coordinate system z Step S5, based on the vector from the satellite to the ground target The components r on the X-axis of the orbital coordinate system x The component r along the Y-axis in the orbital coordinate system y The component r along the Z-axis in the orbital coordinate system zCalculate the current target attitude quaternion q md Step S6, based on the target attitude quaternion q in the k-th frame. md (k) Calculate the attitude angular velocity ω at the k-th beat. md (k)

[0006] Preferably, step S1 includes: the vector r from the Earth's center to the ground target. et Represented as:

[0007]

[0008] Among them, [X et_g Y et_g Z et_g ] T X represents the vector of a ground target in the WGS84 coordinate system. et_g The vector components of the ground target in the WGS84 coordinate system are the X-axis and Y-axis components. et_g The vector components of the ground target in the WGS84 coordinate system are the Y-axis and Z-axis components. et_g The vector components of the ground target in the WGS84 coordinate system along the Z-axis; transformation matrix R i←e for:

[0009] R i←e =(R M ·R S ·R N ·R P ) -1

[0010] Among them, R M R is the rotation matrix for polar shift; S R is the sidereal rotation matrix; N R is the nutation rotation matrix; P This is the precession rotation matrix.

[0011] Preferably, step S2 includes: step S21, obtaining the orbital inclination i, the right ascension Ω of the ascending node, and the argument of latitude u; step S22, calculating based on the orbital inclination i, the right ascension Ω of the ascending node, and the argument of latitude u:

[0012] A oi11 =-sinucosΩ-cosucosisinΩ

[0013] A oi12 = -sinusinΩ + cosucosicosΩ

[0014] A oi13 =cosusini

[0015] A oi21 =-sinisinΩ

[0016] A oi122 =sinicosΩ

[0017] A oi23 =-cosi

[0018] A oi31 =-cosucosΩ+sinucosisinΩ

[0019] A oi32 =-cosusinΩ-sinucosicosΩ

[0020] A oi33 =-sinusini

[0021] Step S23: Calculate the transformation matrix A between the orbital coordinate system and the WGS84 coordinate system. oi :

[0022]

[0023] Preferably, step S3 includes:

[0024]

[0025] The vector r from the Earth's center to the ground target et Representation in orbital coordinate system, where X et_o The components of the vector from the Earth's center to the ground target along the X-axis and Y-axis in the orbital coordinate system. et_o The components of the vector from the Earth's center to the ground target along the Y-axis and Z-axis in the orbital coordinate system. et_o The vector from the Earth's center to the ground target is represented by the Z-axis component in the orbital coordinate system.

[0026] Preferably, step S4 includes: step S41, calculating the scalar r from the satellite to the Earth's center:

[0027]

[0028] Where a is the semi-major axis of the orbit, e is the orbital eccentricity, and f is the true anomaly angle;

[0029] Step S42: Calculate the representation of the satellite-to-Geocenter vector in the orbital coordinate system.

[0030]

[0031] Step S43: Calculate the vector from the satellite to the ground target.

[0032]

[0033] Where A(q) d ) is the rotation matrix corresponding to the guided quaternion; step S44, let The vector from the satellite to the ground target is calculated. The component r on the X-axis of the orbital coordinate system x The component r along the Y-axis in the orbital coordinate system y The component r along the Z-axis in the orbital coordinate system z .

[0034] Preferably, step S5 includes: calculating the current target pose quaternion q. md :

[0035]

[0036]

[0037]

[0038]

[0039]

[0040] q md =[q md1 q md2 q md3 q md4 ] T ;

[0041] in, The current roll angle of the satellite target. θ is the instantaneous roll angle of the satellite being photographed, measured by the attitude sensor. m q represents the current elevation angle of the satellite target. rot_x Let q be the quaternion of the current target attitude obtained by rotating the vector from the satellite to the ground target along the X-axis of the orbital coordinate system. rot_y The quaternion for the current target attitude is obtained by rotating the vector from the satellite to the ground target along the Y-axis of the orbital coordinate system.

[0042] Preferably, step S6 includes: calculating the derivative of the target attitude quaternion with respect to time in the k-th frame. Obtained using the difference method:

[0043]

[0044] Where, q md(k) Let k be the target pose quaternion. Let T be the derivative of the target attitude quaternion with respect to time in the k-th frame. s q is the difference time interval.md(k-1) Given the target attitude quaternion for the (k-1)th frame in the difference method, we obtain the corresponding q. md(k) and Then, calculate the attitude angular velocity ω in the k-th frame. md (k):

[0045]

[0046] Where, ω mdx (k), ω mdy (k), ω mdz (k) represents the target attitude angular velocity components in the k-th frame of the vector from the satellite to the ground target in the X, Y, and Z axes of the orbital coordinate system.

[0047] The advantages of this invention compared to the prior art are:

[0048] First, the target attitude quaternion and attitude angular velocity in the orbital coordinate system are calculated in real time based on the imaging ground target point to ensure the accuracy of the imaging reference establishment during the imaging process.

[0049] Second, a complete process has been established for SAR satellites to establish attitude references from imaging ground target points to the orbital coordinate system. Attached Figure Description

[0050] Figure 1 This is a flowchart of a method for establishing a SAR satellite attitude reference for in-motion imaging according to the present invention. Detailed Implementation

[0051] The quality of SAR satellite imaging in motion largely depends on the establishment of an attitude reference, especially for low-Earth orbit spacecraft, which are more susceptible to external interference such as atmospheric drag. This makes them even more reliant on the attitude reference for in-motion imaging, and even small deviations in the attitude reference can severely impact image quality. By performing real-time calculation and conversion of the target's attitude quaternions and angular velocity in the orbital coordinate system from the imaged ground target point, a high-precision attitude reference for the satellite can be established.

[0052] like Figure 1 As shown, the present invention provides a method for establishing a SAR satellite attitude reference in motion. For the real-time establishment of the attitude reference for SAR satellite in motion, when the satellite receives an imaged ground target (meeting condition 1), the method is executed; if the aforementioned condition is not met, the system continues to wait until condition 1 is met.

[0053] The specific steps of this method are as follows:

[0054] Step S1: Calculate the vector r from the Earth's center to the ground target. et :

[0055] The purpose is to transform the target in the WGS84 (World Geodetic System 1984) coordinate system to the geocentric inertial coordinate system.

[0056] Specifically, the vector r from the Earth's center to the ground target et Represented as:

[0057]

[0058] Where [X] et_g Y et_g Z et_g ] T Let X be the vector representation of the ground target in the WGS84 coordinate system, where X et_g Y et_g Z et_g , which are the components of the ground target's vector in the WGS84 coordinate system along the X, Y, and Z axes, respectively, and can be directly measured.

[0059] Transformation matrix R i←e For calculations, please refer to the orbital calculation formula: R i←e =(R M ·R S ·R N ·R P ) -1 .

[0060] Among them, R M R is the rotation matrix for polar shift, taking the identity matrix, and is dimensionless; S R is a dimensionless rotation matrix for sidereal time; N R is a nutation rotation matrix, dimensionless; P It is a precession rotation matrix, dimensionless.

[0061] Step S2: Calculate the transformation matrix A between the orbital coordinate system and the geocentric inertial coordinate system. oi :

[0062] Specifically, it is necessary to obtain instantaneous orbital elements. Orbital elements (or orbital parameters) are parameters necessary to determine the orbit of a celestial body or spacecraft moving in its Keplerian orbit under the influence of Newton's laws of motion and Newton's law of universal gravitation.

[0063] The number of orbital elements that need to be determined in step S2 is:

[0064] Orbital inclination i: The angle of inclination of the satellite's orbital plane to the equatorial plane; the angle measured counterclockwise from the equatorial plane to the planet's orbital plane at the ascending node;

[0065] Right ascension of the ascending node Ω: the angular distance between the ascending node of the satellite orbit and the vernal equinox;

[0066] Latitude argument u: the sum of the true anomaly and the perigee argument.

[0067] After obtaining the aforementioned orbital elements, calculate the transformation matrix A between the orbital coordinate system and the geocentric inertial coordinate system. oi :

[0068]

[0069] in,

[0070] A oi11 =-sinucosΩ-cosucosisinΩ

[0071] A oi12 = -sinusinΩ + cosucosicosΩ

[0072] A oi13 =cosusini

[0073] A oi21 =-sinisinΩ

[0074] A oi122 =sinicosΩ

[0075] A oi23 =-cosi

[0076] A oi31 =-cosucosΩ+sinucosisinΩ

[0077] A oi32 =-cosusinΩ-sinucosicosΩ

[0078] A oi33 =-sinusini.

[0079] Step S3, convert the vector r from the Earth's center to the ground target from step S1... et Represented in the orbital coordinate system

[0080]

[0081] Right now The vector r from the Earth's center to the ground target et Representation in orbital coordinate system, where X et_o Y et_o Z et_o , , respectively, are the components of the vector from the Earth's center to the ground target along the X, Y, and Z axes of the orbital coordinate system.

[0082] Step S4: Calculate the vector from the satellite to the ground target. pass Calculate r x r y r z Where r x r y r z , , respectively, are the components of the vector from the satellite to the ground target in the X-axis, Y-axis, and Z-axis of the orbital coordinate system.

[0083] The specific calculations are as follows: First, calculate r, where r is the scalar distance from the satellite to the Earth's center:

[0084]

[0085] Where a is the semi-major axis of the orbit, e is the orbital eccentricity, and f is the true anomaly angle, all of which can be obtained by measurement;

[0086] The second step is to calculate the representation of the satellite-to-Geocenter vector in the orbital coordinate system based on the scalar r from the satellite to the Earth's center.

[0087] Specifically:

[0088]

[0089] The third step is to calculate the vector distance from the satellite to the ground target.

[0090]

[0091] A(q d ) is the rotation matrix corresponding to the guided quaternion;

[0092] Step 4: Order The calculation yields r x r y r z .

[0093] Step S5: Calculate the current target pose quaternion q. md :

[0094]

[0095]

[0096]

[0097]

[0098]

[0099] q md=[q md1 q md2 q md3 q md4 ] T

[0100] in, The current roll angle of the satellite target. θ is the instantaneous roll angle of the satellite being photographed, measured by the attitude sensor. m q represents the current elevation angle of the satellite target. rot_x Let q be the quaternion of the current target attitude obtained by rotating the vector from the satellite to the ground target along the X-axis of the orbital coordinate system. rot_y The quaternion for the current target attitude is obtained by rotating the vector from the satellite to the ground target along the Y-axis of the orbital coordinate system.

[0101] Step S6, calculate the k-th attitude angular velocity ω md (k):

[0102] The target's k-th attitude angular velocity ω is obtained by applying the following inverse kinematic equation to the quaternion. md (k), k≥2, and are integers;

[0103] First calculate Obtained using the difference method:

[0104]

[0105] Where, q md(k) Let k be the target pose quaternion. Let T be the derivative of the target attitude quaternion with respect to time in the k-th frame. s q is the difference time interval. md(k-1) Let q be the target attitude quaternion for the (k-1)th frame in the difference method. md(k) q md(k-1) All of them are q in step S5 md The calculated data.

[0106] Get the corresponding q md(k) and Then, calculate the attitude angular velocity ω in the k-th frame. md (k):

[0107]

[0108] Where, ω mdx (k), ω mdy (k), ω mdz (k) represents the target attitude angular velocity components in the k-th frame of the vector from the satellite to the ground target in the X, Y, and Z axes of the orbital coordinate system.

[0109] While the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the invention. Any person skilled in the art can make possible variations and modifications to the technical solutions of the present invention using the disclosed methods and techniques without departing from the spirit and scope of the invention. Therefore, any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention, without departing from the content of the technical solutions of the present invention, shall fall within the protection scope of the present invention. Where there is no conflict, the embodiments of this application and the technical features thereof can be combined with each other.

[0110] The contents not described in detail in this specification are common knowledge to those skilled in the art.

Claims

1. A method for establishing attitude reference for SAR satellite imaging in motion, characterized in that, The method includes: Step S1: Calculate the vector r from the Earth's center to the ground target. et ; Step S2: Calculate the transformation matrix A between the orbital coordinate system and the geocentric inertial coordinate system. oi ; Step S3, using the transformation matrix A between the orbital coordinate system and the geocentric inertial coordinate system. oi Obtain the vector r from the Earth's center to the ground target. et Representation in orbital coordinate system Step S4, based on the vector r from the Earth's center to the ground target et Representation in orbital coordinate system Calculate the vector from the satellite to the ground target. And calculate the vector from the satellite to the ground target. The components r on the X-axis of the orbital coordinate system x The component r along the Y-axis in the orbital coordinate system y The component r along the Z-axis in the orbital coordinate system z ; Step S5, based on the satellite-to-ground target vector The components r on the X-axis of the orbital coordinate system x The component r along the Y-axis in the orbital coordinate system y The component r along the Z-axis in the orbital coordinate system z Calculate the current target attitude quaternion q md ; Step S6, based on the target attitude quaternion q in the k-th frame. md(k) Calculate the attitude angular velocity ω in the k-th frame. md (k).

2. The method for establishing a SAR satellite attitude reference for in-motion imaging according to claim 1, characterized in that, Step S1 includes: The vector r from the Earth's center to the ground target et Represented as: Among them, [X et_g Y et_g Z et_g ] T X represents the vector of a ground target in the WGS84 coordinate system. et_g The vector components of the ground target in the WGS84 coordinate system are the X-axis and Y-axis components. et_g The vector components of the ground target in the WGS84 coordinate system are the Y-axis and Z-axis components. et_g The vector component of the ground target in the WGS84 coordinate system along the Z-axis; Transformation matrix R i←e for: R i←e =(R M ·R S ·R N ·R P ) -1 Among them, R M R is the rotation matrix for polar shift; S R is the sidereal rotation matrix; N R is the nutation rotation matrix; P This is the precession rotation matrix.

3. The method for establishing a SAR satellite attitude reference for in-motion imaging according to claim 2, characterized in that, Step S2 includes: Step S21: Obtain the orbital inclination i, the right ascension of the ascending node Ω, and the argument of latitude u; Step S22: Based on the orbital inclination i, the right ascension of the ascending node Ω, and the argument of latitude u, calculate: A oi11 =-sinucosΩ-cosucosisinΩ A oi12 =-sinusinΩ+cosucosicosΩ A oi13 =cosusini A oi21 =-sinisinΩ TO oi122 =sinicosΩ TO oi23 =-like this A oi31 =-cosucosΩ+sinucosisinΩ A oi32 =-cosusinΩ-sinucosicosΩ TO oi33 =-sinus Step S23: Calculate the transformation matrix A between the orbital coordinate system and the WGS84 coordinate system. oi :

4. The method for establishing a SAR satellite attitude reference for dynamic imaging according to claim 3, characterized in that, Step S3 includes: The vector r from the Earth's center to the ground target et Representation in orbital coordinate system, where X et_o The components of the vector from the Earth's center to the ground target along the X-axis and Y-axis in the orbital coordinate system. et_o The vector from the Earth's center to the ground target is represented by its components along the Y-axis and Z-axis in the orbital coordinate system. et_o The vector from the Earth's center to the ground target is represented by the Z-axis component in the orbital coordinate system.

5. The method for establishing a SAR satellite attitude reference for in-motion imaging according to claim 4, characterized in that, Step S4 includes: Step S41, calculate the scalar r from the satellite to the Earth's center: Where a is the semi-major axis of the orbit, e is the orbital eccentricity, and f is the true anomaly angle; Step S42: Calculate the representation of the satellite-to-Geocenter vector in the orbital coordinate system. Step S43: Calculate the vector from the satellite to the ground target. Where A(q) d ) is the rotation matrix corresponding to the guided quaternion; Step S44, let The vector from the satellite to the ground target is calculated. The component r on the X-axis of the orbital coordinate system x The component r along the Y-axis in the orbital coordinate system y The component r along the Z-axis in the orbital coordinate system z .

6. The method for establishing a SAR satellite attitude reference for in-motion imaging according to claim 5, characterized in that, Step S5 includes: Calculate the current target attitude quaternion q md : in, The current roll angle of the satellite target. θ is the instantaneous roll angle of the satellite being photographed, measured by the attitude sensor. m q represents the current elevation angle of the satellite target. rot_x Let q be the quaternion of the current target attitude obtained by rotating the vector from the satellite to the ground target along the X-axis of the orbital coordinate system. rot_y The quaternion for the current target attitude is obtained by rotating the vector from the satellite to the ground target along the Y-axis of the orbital coordinate system.

7. The method for establishing a SAR satellite attitude reference for in-motion imaging according to claim 6, characterized in that, Step S6 includes: Calculate the time derivative of the target attitude quaternion in the k-th frame. Obtained using the difference method: Where, q md(k) Let k be the target pose quaternion. Let T be the derivative of the target attitude quaternion with respect to time in the k-th frame. s q is the difference time interval. md(k-1) Given the target attitude quaternion for the (k-1)th frame in the difference method, we obtain the corresponding q. md(k) and Then, calculate the attitude angular velocity ω in the k-th frame. md (k): Where, ω mdx (k), ω mdy (k), ω mdz (k) represents the target attitude angular velocity components in the k-th frame of the vector from the satellite to the ground target in the X, Y, and Z axes of the orbital coordinate system.

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