Equivalent velocity and squint angle calculation method for long synthetic aperture observation

Abstract

The invention discloses an equivalent speed and squint angle calculation method under long synthetic aperture observation, and aims to solve the current situation that no special full-aperture equivalent speed and squint angle calculation method with unified parameters exists under the geometric characteristics of novel ultra-long synthetic aperture time bending orbit observation in China at present. The invention provides a corresponding equivalent speed and squint angle calculation method. The orbit reference time is flexibly determined by calculating the orbit position information near the observation aperture time, and the orbit reference time interval can be changed along with the change of the orbit characteristics, so that the optimal adaptation and optimal matching to different orbits are achieved, and uniform imaging parameters can be further obtained. The method solves the problem of uniform parameter calculation caused by parameter time variation under long synthetic aperture time-varying geometry, avoids the problems of image defocusing, spectrum leakage and even influence on scattering characteristic inversion caused by errors generated by signal processing such as imaging focusing, and is suitable for the current novel SAR system.

Description

Technical Field

[0001] The invention belongs to the field of signal processing and relates to a method for calculating equivalent velocity and oblique angle under long synthetic aperture observation. Background Art

[0002] In order to meet the observation requirements of higher resolution and wider coverage of spaceborne SAR, the new SAR payload works in a higher medium- and high-orbit orbit and uses a longer synthetic aperture time to achieve coherent observation. Due to the particularity of high-orbit observation geometry and the long-term characteristics of synthetic aperture, the satellite orbit shows a large-angle bending characteristic in a single observation, which is different from the straight orbit characteristics of low-orbit.

[0003] Since the observation geometry of low-orbit satellites is relatively fixed, the effective velocity and slant angle within the synthetic aperture time can be obtained by analyzing the local slant range change and Doppler characteristics for subsequent imaging processing.

[0004] However, the curved orbit corresponding to the new SAR system in the prior art will cause its observation geometry to no longer be fixed, and the relative relationship will change all the time with the different orbital positions. Accordingly, it is impossible to obtain the overall equivalent velocity and squint angle under the full course of the synthetic aperture based on the local analysis method of the low-orbit satellite. This will cause the traditional low-orbit method of calculating the equivalent velocity and squint angle to bring very large errors to the description of the signal characteristics of the full aperture, which will in turn cause errors in signal processing such as imaging focusing, causing image defocusing and spectrum leakage, and seriously affecting the inversion of scattering characteristics and target interpretation. Summary of the invention

[0005] The purpose of the present invention is to provide a method for calculating equivalent velocity and squint angle under long synthetic aperture observation, so as to solve the problem that the calculation method in the prior art is not suitable for a new SAR system.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions to achieve the above problems:

[0007] A method for calculating equivalent velocity and slant angle under long synthetic aperture observation comprises the following steps:

[0008] Step 1, in a curved track observation under a single long synthetic aperture, determining a reference time of the curved track according to the center time of the observation aperture;

[0009] Step 2, according to the reference time of the curved track, calculate the speed in the reference interval corresponding to the reference time as the equivalent speed in the synthetic aperture time;

[0010] Step 3, determining the equivalent radar operation orbit vector according to the curved orbit reference time;

[0011] Step 4, determining a reference observation vector between the radar and the scene center according to the curved track reference time and the three-dimensional position of the scene center of the image taken by the observation aperture radar;

[0012] Step 5: Determine the equivalent slant angle based on the equivalent radar orbit vector and the reference observation vector.

[0013] The present invention also has the following features:

[0014] Further, step 1 includes the following sub-steps:

[0015] Step 11, determine the central time t of the observation aperture c ;

[0016] Step 12: Use the following formula to calculate the radar at the center time t c Three-dimensional position in WGS84 coordinate system:

[0017]

[0018] Where t represents any time, t = t c ;

[0019] Among them, the WGS84 coordinate system is a geocentric coordinate system.

[0020] Step 13, based on the position characteristics of the curved track, determine the reference time interval t1 to t2 of the curved track, and use the following formula to determine the reference time t1 and t2 of the curved track respectively:

[0021]

[0022] Among them, |·| represents the modulus value of the vector;

[0023] k represents a proportionality constant.

[0024] Furthermore, in step 13, the value of k is 16000.

[0025] Furthermore, in step 2, the equivalent velocity within the synthetic aperture time is calculated using the following formula:

[0026]

[0027] Among them, V ref Indicates equivalent speed.

[0028] Furthermore, in step 3, the equivalent radar orbit vector is determined using the following formula:

[0029]

[0030] in, represents the equivalent radar orbit vector;

[0031] x m ,y m ,z m They respectively represent the standard vector values ​​of the three axes in the WGS84 coordinate system.

[0032] Further, step 4 includes the following sub-steps:

[0033] Step 41, assuming that the three-dimensional position of the scene center of the image captured by the observation aperture radar is

[0034]

[0035] Step 42, determine the reference observation vector using the following formula:

[0036]

[0037] Among them, x r ,y r ,z r Denote the reference observation vectors The standard vector values ​​of the three axes in the WGS84 coordinate system.

[0038] Furthermore, in step 5, the equivalent squint angle is determined using the following formula:

[0039]

[0040] Among them, θ represents the equivalent oblique angle;

[0041] cos -1 Represents the inverse cosine function.

[0042] Compared with the prior art, the present invention has the following technical effects:

[0043] (I) The present invention provides a method for calculating the equivalent velocity and squint angle under long synthetic aperture observation, which aims at addressing the current situation in China where there is no dedicated method for calculating the full-aperture equivalent velocity and squint angle with unified parameters under the geometric characteristics of new ultra-long synthetic aperture time-bending orbit observations.

[0044] By calculating the orbital position information near the observation aperture time and flexibly determining the orbital reference time, the orbital reference time interval can be changed as the orbital characteristics change, thereby achieving the best adaptation and best matching for different orbits, and further obtaining uniform imaging parameters. While solving the problem of uniform parameter calculation caused by time-varying parameters under long synthetic aperture time-varying geometry, it avoids errors caused by signal processing such as imaging focusing, causing image defocusing and spectrum leakage, and even affecting the inversion of scattering characteristics, and is suitable for the current new SAR system.

[0045] (II) The present invention provides a calculation process for the equivalent speed and squint angle calculation method under long synthetic aperture observation based on orbit and target scene data. This process is a universal signal processing process, which can be easily implemented in a variety of processors such as DSP, FPGA, ARM, etc., and has good versatility. The analytical expressions for calculating the equivalent speed and equivalent squint angle are given. The processing scheme is simple and easy to implement, and is widely applicable. It can be used in satellites of different orbits such as low orbit, medium orbit, and high orbit. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 It is a flow chart of the calculation steps in the method for calculating equivalent velocity and oblique angle under long synthetic aperture observation according to the present invention;

[0047] Figure 2 It is a geometric diagram of the observation vector in the present invention. DETAILED DESCRIPTION

[0048] It should be noted that, unless otherwise specified, all components in the present invention are components known in the prior art.

[0049] Specific embodiments of the present invention are given below. It should be noted that the present invention is not limited to the following specific embodiments, and all equivalent changes made on the basis of the technical solution of this application fall within the protection scope of the present invention.

[0050] This embodiment proposes a method for calculating the equivalent velocity and squint angle under long synthetic aperture observation. In view of the time-varying characteristics of the observation geometry under large-angle bending of the satellite orbit under the long synthetic aperture observation geometry, it is impossible to use the fixed time calculation method of the low orbit to obtain the uniform parameters within the full aperture time. For the orbital data information, firstly, the orbital reference time is determined according to the orbital characteristics at the aperture center moment, and then the equivalent velocity, reference orbit vector and reference observation vector are calculated within the reference time interval, and then the equivalent squint angle is calculated to realize the calculation of the overall imaging parameters under the long synthetic aperture time, solving the problem of overall parameter calculation under long aperture. The processing scheme is widely applicable and can be used in satellites in different orbits such as low orbit, medium orbit and high orbit, and has wide applicability.

[0051] like Figure 1 As shown, this embodiment mainly includes five parts: orbit reference time determination, equivalent speed calculation, reference orbit radar operation vector determination, reference observation vector determination and equivalent squint angle calculation. The specific steps are as follows:

[0052] Step 1, in a curved track observation under a single long synthetic aperture, determining a reference time of the curved track according to the center time of the observation aperture;

[0053] Step 2, according to the reference time of the curved track, calculate the speed in the reference interval corresponding to the reference time as the equivalent speed in the synthetic aperture time;

[0054] Step 3, determining the equivalent radar operation orbit vector according to the curved orbit reference time;

[0055] Step 4, determining a reference observation vector between the radar and the scene center according to the curved track reference time and the three-dimensional position of the scene center of the image taken by the observation aperture radar;

[0056] Step 5: Determine the equivalent slant angle based on the equivalent radar orbit vector and the reference observation vector.

[0057] Furthermore, step 1 includes the following sub-steps:

[0058] Step 11, determine the central time t of the observation aperture c ; The center moment is the middle moment from the beginning to the end of the observation.

[0059] Step 12: Use the following formula to calculate the radar at the center time t c The three-dimensional position in the WGS84 coordinate system:

[0060]

[0061] Where t represents any time, t = t c ;

[0062] Step 13, based on the position characteristics of the curved track, determine the reference time interval t1 to t2 of the curved track, and use the following formula to determine the reference time t1 and t2 of the curved track respectively:

[0063]

[0064] Among them, |·| represents the modulus value of the vector;

[0065] k represents a proportionality constant.

[0066] Specifically, in step 13, the value of k is 16000.

[0067] Furthermore, in step 2, the equivalent velocity within the synthetic aperture time is calculated using the following formula:

[0068]

[0069] Among them, V ref Indicates equivalent speed.

[0070] Furthermore, in step 3, the equivalent radar orbit vector is determined using the following formula:

[0071]

[0072] in, represents the equivalent radar orbit vector;

[0073] x m ,y m ,z m They respectively represent the standard vector values ​​of the three axes in the WGS84 coordinate system.

[0074] Further, step 4 includes the following sub-steps:

[0075] Step 41, assuming that the three-dimensional position of the scene center of the image captured by the observation aperture radar is

[0076]

[0077] Step 42, determine the reference observation vector using the following formula:

[0078]

[0079] Among them, x r ,y r ,z r Denote the reference observation vectors The standard vector values ​​of the three axes in the WGS84 coordinate system.

[0080] Furthermore, in step 5, the equivalent squint angle is determined using the following formula:

[0081]

[0082] Among them, θ represents the equivalent oblique angle;

[0083] cos -1 Represents the inverse cosine function.

Claims

1. A method for calculating equivalent velocity and squint angle under long synthetic aperture observation, characterized in that: The steps include: Step 1, in a curved track observation under a single long synthetic aperture, determining a reference time of the curved track according to the center time of the observation aperture; Step 2, according to the reference time of the curved track, calculate the speed in the reference interval corresponding to the reference time as the equivalent speed in the synthetic aperture time; Step 3, determining the equivalent radar operation orbit vector according to the curved orbit reference time; Step 4, determining a reference observation vector between the radar and the scene center according to the curved track reference time and the three-dimensional position of the scene center of the image taken by the observation aperture radar; Step 5: Determine the equivalent slant angle based on the equivalent radar orbit vector and the reference observation vector.

2. The method for calculating equivalent velocity and squint angle under long synthetic aperture observation according to claim 1, characterized in that: Step 1 includes the following sub-steps: Step 11, determine the central time t of the observation aperture c ; Step 12: Use the following formula to calculate the radar at the center time t c Three-dimensional position in WGS84 coordinate system: Where t represents any time, t = t c ; Among them, the WGS84 coordinate system is a geocentric coordinate system; Step 13, based on the position characteristics of the curved track, determine the reference time interval t1 to t2 of the curved track, and use the following formula to determine the reference time t1 and t2 of the curved track respectively: Among them, |·| represents the modulus value of the vector; k represents a proportionality constant.

3. The method for calculating equivalent velocity and squint angle under long synthetic aperture observation according to claim 2, characterized in that: In step 13, the value of k is 16000.

4. The method for calculating equivalent velocity and squint angle under long synthetic aperture observation according to claim 1, characterized in that: In step 2, the equivalent velocity within the synthetic aperture time is calculated using the following formula: Among them, V ref Indicates equivalent speed.

5. The method for calculating equivalent velocity and squint angle under long synthetic aperture observation according to claim 2, characterized in that: In step 3, the equivalent radar orbit vector is determined using the following formula: in, represents the equivalent radar orbit vector; x m ,y m ,z m They respectively represent the standard vector values ​​of the three axes in the WGS84 coordinate system.

6. The method for calculating equivalent velocity and squint angle under long synthetic aperture observation according to claim 5, characterized in that: Step 4 includes the following sub-steps: Step 41, assuming that the three-dimensional position of the scene center of the image captured by the observation aperture radar is Step 42, determine the reference observation vector using the following formula: Among them, x r ,y r ,z r Denote the reference observation vectors The standard vector values ​​of the three axes in the WGS84 coordinate system.

7. The method for calculating equivalent velocity and squint angle under long synthetic aperture observation according to claim 6, characterized in that: In step 5, determine the equivalent squint angle using the following formula: Among them, θ represents the equivalent oblique angle; cos -1 Represents the inverse cosine function.

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