A method for calculating equivalent velocity and squint angle under long synthetic aperture observation

Abstract

The application discloses a kind of equivalent velocity and oblique angle calculation method under long synthetic aperture observation, for the current domestic new type ultra-long synthetic aperture time curved track observation geometry characteristic without the present situation of special unified parameter full-aperture equivalent velocity and oblique angle calculation method, corresponding equivalent velocity and oblique angle calculation method is provided.Through calculating the track position information near observation aperture time, the track reference time is flexibly determined, the track reference time interval can be changed with the change of track characteristics, so as to achieve the best adaptation and best matching of different orbits, and further obtain uniform imaging parameters.The problem of parameter time-varying caused by long synthetic aperture time-varying geometry is solved, and the error caused by imaging focusing and other signal processing is avoided, which causes image defocusing and spectrum leakage, and even affects the problem of scattering characteristic inversion, which is suitable for the current new SAR system.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of signal processing, and relates to a method for calculating equivalent velocity and squint angle under long synthetic aperture observation. BACKGROUND

[0002] In order to meet the observation requirements of higher resolution and wider coverage of spaceborne SAR, a new type of SAR load works in a higher medium-high orbit, and a longer synthetic aperture time is adopted to realize coherent observation. Due to the particularity of high-orbit observation geometry and the long-time characteristic of synthetic aperture, the satellite orbit presents a large-angle bending characteristic in a single observation, which is different from the linear orbit characteristic of low-orbit.

[0003] Due to the relatively fixed observation geometry of low-orbit satellites, the effective velocity and squint angle in the synthetic aperture time can be obtained through local slant range change and Doppler characteristic analysis, which is used for subsequent imaging processing.

[0004] However, the bending orbit of the new type of SAR system in the prior art causes the observation geometry to be no longer fixed, and the relative relationship changes with the change of the orbit position. Accordingly, the overall equivalent velocity and squint angle in the synthetic aperture full course cannot be obtained according to the local analysis method of the low-orbit satellite. Thus, the traditional low-orbit calculation method of equivalent velocity and squint angle will bring a very large error to the description of the signal characteristics of the full aperture, and further cause errors in imaging focusing and other signal processing, resulting in image defocusing and spectrum leakage, and seriously affecting the scattering characteristic inversion and target interpretation. SUMMARY

[0005] The purpose of the application 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 applicable to the new type of SAR system.

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

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

[0008] Step 1, in the bending orbit observation in a single long synthetic aperture, the reference time of the bending orbit is determined according to the center time of the observation aperture of the observation;

[0009] Step 2, the velocity in the reference interval corresponding to the reference time of the bending orbit is calculated as the equivalent velocity in the synthetic aperture time according to the reference time of the bending orbit;

[0010] Step 3, the equivalent radar orbit vector is determined according to the reference time of the bending orbit;

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

[0012] Step 5, the equivalent radar running orbit vector and the reference observation vector are used to determine the equivalent squint angle.

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

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

[0015] Step 11, determine the center time t c of the aperture according to the time of the observation;

[0016] Step 12, calculate the three-dimensional position of the radar at the center time t c in the WGS84 coordinate system using the following formula:

[0017]

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

[0019] Wherein, the WGS84 coordinate system is the geocentric coordinate system.

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

[0021]

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

[0023] K represents a proportional constant.

[0024] Further, in step 13, k is 16000.

[0025] Further, in step 2, the equivalent velocity in the time of the aperture is calculated using the following formula:

[0026]

[0027] Wherein, V ref represents the equivalent velocity.

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

[0029]

[0030] Wherein, represents an equivalent radar orbit vector;

[0031] x m ,y m ,z m respectively represent three-axis direction standard vector values in the WGS84 coordinate system.

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

[0033] Step 41, set the three-dimensional position of the scene center of the image taken by the current observation aperture radar as

[0034]

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

[0036]

[0037] wherein x r ,y r ,z r respectively represent three-axis direction standard vector values in the WGS84 coordinate system.

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

[0039]

[0040] wherein θ represents the equivalent squint angle.

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

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

[0043] (I) The equivalent velocity and squint angle calculation method for long synthetic aperture observation of the present application provides a corresponding equivalent velocity and squint angle calculation method in view of the current domestic situation that there is no unified parameter for full aperture equivalent velocity and squint angle calculation method under the observation geometric characteristics of new type ultra-long synthetic aperture time curved orbit.

[0044] ​By calculating the orbit position information near the observation aperture time, the orbit reference time is flexibly determined, the orbit reference time interval can change with the orbit characteristics, thereby achieving the best adaptation and best matching for different orbits, and the homogenization imaging parameters can be further obtained. The problem of homogenization parameter calculation caused by time-varying parameters under long synthetic aperture time-varying geometry is solved, at the same time, the error caused by imaging focusing and other signal processing is avoided, the problems of image defocusing and spectrum leakage, and even the influence on scattering characteristic inversion are avoided, and the method is suitable for the current new SAR system.

[0045] (II) The application aims at a long synthetic aperture observation equivalent velocity and squint angle calculation method, which gives a calculation step flow based on orbit and target scene data. The flow is a general signal processing flow, which can be easily realized in various processors such as DSP, FPGA and ARM, and has good universality. Analytical expressions for calculating the equivalent velocity and the equivalent squint angle are given, the processing scheme is simple and easy to implement, and is widely applicable and can be used in low-orbit, medium-orbit and high-orbit satellites. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 is a calculation step flowchart in the long synthetic aperture observation equivalent velocity and squint angle calculation method of the application;

[0047] Figure 2 is an observation vector geometry diagram in the application. DETAILED DESCRIPTION

[0048] It should be noted that all components in the application, if not specifically stated, are all known components in the prior art.

[0049] The following gives a specific embodiment of the application, it should be noted that the application is not limited to the following specific embodiments, and any equivalent transformation based on the technical solutions of the application falls within the protection scope of the application.

[0050] The embodiment proposes a long synthetic aperture observation equivalent velocity and squint angle calculation method, which aims at the characteristics of observation geometry time-varying of satellite orbit presenting large-angle bending under long synthetic aperture observation geometry, and the current situation that the fixed time calculation method of equivalent velocity and squint angle cannot be used to obtain homogenization parameters in the whole aperture time. For orbit data information, first, the orbit reference time is determined according to the orbit characteristics at the aperture center time, then the equivalent velocity, the reference orbit vector and the reference observation vector are calculated in the reference time interval, and then the equivalent squint angle is calculated, the whole imaging parameter calculation under long synthetic aperture time is realized, and the whole parameter calculation problem under long aperture is solved. The processing scheme is widely applicable and can be used in low-orbit, medium-orbit and high-orbit satellites, and has wide applicability.

[0051] As Figure 1 shown in the figure, the embodiment mainly includes five parts: orbit reference time determination, equivalent speed calculation, reference orbit radar running vector determination, reference observation vector determination and equivalent squint angle calculation, and the specific steps are as follows:

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

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

[0054] Step 3, according to the reference time of the curved orbit, the equivalent radar running orbit vector is determined;

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

[0056] Step 5, according to the equivalent radar running orbit vector and the reference observation vector, the equivalent squint angle is determined.

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

[0058] Step 11, determine the center time t c of the observation aperture according to the center time t c of the observation aperture, using the following formula:

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

[0060]

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

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

[0063]

[0064] Where |·| represents the modulus value of the vector;

[0065] k represents a proportional constant.

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

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

[0068]

[0069] wherein V ref represents the equivalent velocity.

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

[0071]

[0072] wherein represents the equivalent radar running orbit vector.

[0073] x m ,y m ,z m respectively represent the standard vector values in the three-axis directions under the WGS84 coordinate system.

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

[0075] Step 41, let the three-dimensional position of the scene center of the image taken by the radar in the observation aperture be

[0076]

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

[0078]

[0079] wherein x r ,y r ,z r respectively represent the standard vector values in the three-axis directions under the WGS84 coordinate system.

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

[0081]

[0082] wherein θ represents the equivalent squint 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 method comprises the following steps: Step 1, in the curved orbit observation under a single long synthetic aperture, according to the center time of the synthetic aperture, the reference time of the curved orbit is determined; Step 2, according to the reference time of the curved orbit, the velocity in the reference interval corresponding to the reference time is calculated as the equivalent velocity in the synthetic aperture time; Step 3, according to the reference time of the curved orbit, the equivalent radar running orbit vector is determined; Step 4, according to the reference time of the curved orbit and the three-dimensional position of the scene center of the image obtained by the synthetic aperture radar, the reference observation vector between the radar and the scene center is determined; Step 5, according to the equivalent radar running orbit vector and the reference observation vector, the equivalent squint angle is determined.

2. The method of claim 1, wherein the equivalent velocity and squint angle are calculated for a long synthetic aperture observation. Step 1 comprises the following sub-steps: Step 11, determining the center time t of the synthetic aperture according to the time of the next synthetic aperture c ; Step 12, calculate the radar position at the center time t using the following formula c Three-dimensional position in WGS84 coordinate system: wherein t represents any one time instant, t = t c ; Wherein, the WGS84 coordinate system is the geocentric coordinate system; Step 13, based on the position characteristics of the curved orbit, the curved orbit reference time interval t1-t2 is determined, and the reference time t1 and t2 of the curved orbit are determined respectively using the following formula: Wherein, |·| represents the modulus value of the vector; K represents the proportional coefficient constant.

3. The method of claim 2, wherein the equivalent velocity and squint angle are calculated for a long synthetic aperture observation. In step 13, k is 16000.

4. The method of claim 2, wherein the equivalent velocity and squint angle are calculated for a long synthetic aperture observation. In step 2, the equivalent velocity in the synthetic aperture time is calculated using the following formula: where V ref represents the equivalent velocity.

5. The method of claim 4, wherein, In step 3, the equivalent radar running orbit vector is determined using the following formula: wherein, represents the equivalent radar orbital vector; x m y m z m respectively represent the standard vector values of three-axis directions under the WGS84 coordinate system.

6. The method of claim 5, wherein, Step 4 comprises the following sub-steps: Step 41, let the three-dimensional position of the scene center of the image taken by the synthetic aperture radar this time be Step 42, the reference observation vector is determined using the following formula: wherein x r , y r , z r represent the reference observation vector The standard vector values in the three-axis direction under the WGS84 coordinate system.

7. The method of claim 6, wherein the equivalent velocity and squint angle are calculated for long synthetic aperture observations. In step 5, the equivalent squint angle is determined using the following formula: Wherein, θ represents the equivalent squint angle; cos -1 denotes the inverse cosine function.

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