A pointing calibration method for high-orbit SAR antennas

Abstract

This invention discloses a high-orbit SAR antenna pointing calibration method. The satellite transmits beacon signals to the ground by sequentially forming four calibration beams through different feed channel combinations: two symmetrical calibration beams in the range direction and two symmetrical calibration beams in the azimuth direction. Two ground receivers receive the beacon signals, and the data received by the two ground receivers are subjected to IQ sampling, pulse compression processing, and sum-difference processing to obtain range pointing deviation measurements and azimuth pointing deviation measurements. The pointing of the high-orbit SAR antenna is then calculated to obtain the range pointing deviation and azimuth pointing deviation values, ultimately yielding the three-dimensional systematic deviation of the high-orbit SAR antenna.

Description

Technical Field

[0001] This invention belongs to the field of radar detection technology, specifically relating to a high-orbit SAR antenna pointing calibration method. Background Technology

[0002] Synthetic Aperture Radar (SAR) is an active microwave remote sensing method that, compared to traditional optical remote sensing, offers advantages such as all-weather, all-day operation and superior penetration. As SAR applications expand, higher performance requirements are being placed on it. To overcome the drawbacks of low-Earth orbit (LEO) satellite-borne SAR, such as long revisit times and small coverage areas, an effective method is to raise the orbital altitude to a low-inclination geosynchronous orbit (GEO). GEO SAR can shorten the average revisit time to only 8 hours and achieve a mapping bandwidth of up to 3000 km.

[0003] The pointing of a spaceborne SAR antenna in orbit is two-dimensional, including range and azimuth pointing. However, during the operation of a spaceborne SAR in orbit, the antenna pointing is subject to deviations due to the combined effects of various errors, leading to a decrease in SAR radiometric calibration accuracy and image quality. Therefore, the pointing accuracy of a spaceborne SAR antenna is one of the key parameters of the system, and the corresponding SAR antenna pointing calibration technology is an important part of the key technologies for radiometric calibration.

[0004] Traditional spaceborne SAR utilizes the uniformly distributed Amazon rainforest for range pointing calibration. It employs notch-beam imaging of the Amazon rainforest and measures the range pointing direction based on the sum-difference beam direction finding principle to complete the calibration. Traditional spaceborne SAR also uses a ground receiver to record the azimuth pattern and perform azimuth pointing calibration. Furthermore, receiver direction finding methods based on the amplitude-comparison monopulse principle can obtain the antenna pointing and pointing deviation at the current moment.

[0005] The above technical solutions have the following shortcomings:

[0006] (1) Traditional spaceborne SAR determines the range direction by imaging the Amazon rainforest and using the sum-difference beam direction finding principle. Due to the limitations of the orbit, the observation area of ​​high-orbit SAR may not cover the Amazon rainforest, and the distribution range of the rainforest is relatively narrow, which cannot meet the mapping bandwidth requirements of high-orbit SAR. Therefore, the traditional spaceborne SAR method cannot be used to complete the range direction calibration.

[0007] (2) High-orbit SAR operates in a geosynchronous orbit with a small inclination angle. It has curved trajectory and oblique imaging problems, which leads to severe two-dimensional coupling between range and azimuth of high-orbit SAR. In addition, there is time-varying azimuth pointing. Traditional azimuth pointing calibration methods of spaceborne SAR are difficult to apply to high-orbit SAR.

[0008] (3) The receiver direction finding method based on the amplitude-ratio single pulse principle can only obtain the antenna pointing and pointing deviation at the current moment. However, the two-dimensional pointing measurement results of the high-orbit SAR range and azimuth change with time and require dynamic calibration. Furthermore, it cannot obtain the three-dimensional systematic pointing deviation of the high-orbit SAR antenna. Summary of the Invention

[0009] In view of this, the present invention provides a high-orbit SAR antenna pointing calibration method, which can solve the problem that the existing antenna pointing calibration methods are not applicable to high-orbit SAR, and realize high-orbit SAR antenna pointing calibration.

[0010] The technical solution for implementing the present invention is as follows:

[0011] A method for pointing calibration of a high-orbit SAR antenna involves the satellite transmitting beacon signals to the ground by sequentially forming four calibration beams through different combinations of feed channels. These are two symmetrical calibration beams in the range direction and two symmetrical calibration beams in the azimuth direction. Two ground receivers receive the beacon signals, and the data received by the two ground receivers are subjected to IQ sampling, pulse compression processing, and sum-difference processing to obtain the range pointing deviation measurement value and the azimuth pointing deviation measurement value. The pointing of the high-orbit SAR antenna is then calculated to obtain the range pointing deviation value and the azimuth pointing deviation value, ultimately obtaining the three-dimensional systematic deviation of the high-orbit SAR antenna.

[0012] Furthermore, the specific steps are as follows:

[0013] Step 1: The satellite sequentially forms four calibration beams through different feed channel combinations: two calibration beams symmetrically positioned to the left and right in the range direction, and two calibration beams symmetrically positioned to the left and right in the azimuth direction. Beacon signals are then transmitted to the ground in the order of the range left calibration beam, the range right calibration beam, the azimuth left calibration beam, and the azimuth right calibration beam.

[0014] Step 2: The two ground receivers receive the raw data of the beacon signal transmitted in Step 1 and sort the data into segments in the following order: range left calibration beam, range right calibration beam, azimuth left calibration beam, and azimuth right calibration beam.

[0015] Step 3: Perform IQ sampling and pulse compression processing on the data output by the first ground receiver in Step 2 to obtain complex signals V1, V2, V3, and V4; where V1 and V2 are the range left beacon signal and range right beacon signal, respectively, and V3 and V4 are the azimuth left beacon signal and azimuth right beacon signal, respectively.

[0016] Step 4: Perform sum-difference processing on the complex signals obtained in Step 3; perform sum-difference processing on V1 and V2 to obtain the distance pointing deviation measurement value. By performing summation and difference processing on V3 and V4, the azimuth pointing deviation measurement value is obtained.

[0017] Step 5: Repeat steps 3 and 4 for the data output from the second ground receiver in step 2 to obtain the range pointing deviation measurement value. Azimuth pointing deviation measurement value

[0018] Step 6: Calculate the high-orbit SAR antenna pointing based on the preset wavefront antenna pointing data and the acquired satellite attitude data, and input the high-orbit SAR pointing conversion matrix.

[0019] Step 7: Based on the antenna pointing direction obtained in Step 6, calculate the antenna pointing deviation value, i.e., the range pointing deviation value θ. r And azimuth pointing deviation value θ az ;

[0020] Step 8: Based on the pointing deviation value θ obtained in Step 7 r θ az and the pointing deviation measurements obtained in steps 4 and 5 By establishing a one-to-one correspondence, a system of linear equations is constructed to solve for the three-dimensional systematic deviation of the high-orbit SAR antenna.

[0021] Furthermore, step 6 specifically includes:

[0022] Preset antenna pointing is in The view from the high-orbit SAR antenna is given by the satellite attitude data as (θ). Y ,θ P ,θ R ), where θ Y Let θ be the yaw angle. P Let θ be the pitch angle. R The roll angle is taken into account. Considering the deformation of the high-orbit SAR antenna during installation, on-orbit deployment, and on-orbit operation, as well as the combined effect of antenna feed channel errors, the high-orbit SAR antenna exhibits a systematic pointing deviation, which can be regarded as a three-dimensional systematic pointing deviation of the antenna. in This is a systematic pointing deviation of the yaw axis. For pitch axis systemic pointing deviation, This is due to a systematic pointing deviation of the rolling axis.

[0023] Antenna preset direction After (θ) Y ,θ P ,θ R )and Rotate, become That is, the antenna pointing at the current moment, specifically expressed as:

[0024]

[0025] In the formula, x, y, and z are the coordinate values ​​in the satellite's flight coordinate system, specifically expressed as follows:

[0026]

[0027]

[0028]

[0029]

[0030]

[0031]

[0032] Furthermore, step 7 specifically includes:

[0033]

[0034]

[0035] Furthermore, step 8 involves constructing a system of linear equations as follows:

[0036]

[0037] Beneficial effects:

[0038] (1) The method of the present invention is based on the amplitude-ratio single pulse principle. Four calibration beams are emitted sequentially on the satellite, and two ground receivers are used to receive the beacon signals sequentially. After data processing, the three-dimensional systematic pointing deviation of the high-orbit SAR antenna can be obtained, and the pointing calibration of the high-orbit SAR antenna can be realized.

[0039] (2) The method of the present invention forms four calibration beams sequentially through a high-orbit SAR antenna and receives the calibration beams with two ground receivers respectively. It can simultaneously calibrate the range and azimuth directions of the high-orbit SAR antenna, overcoming the traditional method of separately calibrating the range and azimuth directions, and improving the calibration efficiency.

[0040] (3) Based on the one-to-one correspondence between the pointing deviation value and the pointing deviation measurement value, this invention can overcome the shortcomings of existing methods that can only obtain the pointing deviation of dynamic antennas by constructing and solving a system of linear equations, and innovatively obtains the systematic error. Attached Figure Description

[0041] Figure 1This is a flowchart of the high-orbit SAR antenna pointing calibration method of the present invention.

[0042] Figure 2 The timing design diagram for beam calibration.

[0043] Figure 3 This is a schematic diagram of the ground coverage for beam calibration. Detailed Implementation

[0044] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0045] like Figure 1 As shown, the method of the present invention includes the following steps.

[0046] Step 1: The high-orbit SAR antenna sequentially forms four calibration beams through different feed channel combinations: two calibration beams symmetrically positioned to the left and right in the range direction, and two calibration beams symmetrically positioned to the left and right in the azimuth direction. Beacon signals are transmitted to the ground receiver sequentially in the order of range left, range right, azimuth left, and azimuth right. The beacon signals are broadband signals, such as linear frequency modulated (LFM) signals. The specific method for the calibration beams to transmit the beacon signals sequentially in a time-division manner is as follows... Figure 2 As shown.

[0047] Step 2: Two ground receivers receive the raw beacon signal data transmitted by the high-orbit SAR in Step 1 and perform data segmentation and sorting. The two ground receivers are positioned within the 3dB beamwidth of each calibration beam, separated along the azimuth direction, and symmetrical about the center of the calibration beam, as shown below. Figure 3 As shown; the receiver data is segmented and sorted in the following order: range left, range right, azimuth left, azimuth right, as follows: Figure 2 As shown.

[0048] Step 3: Based on the data output by the first receiver in Step 2, perform IQ sampling and pulse compression processing to obtain two complex signals V1 and V2 for range (left and right) and two complex signals V3 and V4 for azimuth (left and right). The specific expressions are as follows:

[0049] V1=gf1(θ r A+Z1

[0050] V2=gf2(θ r A+Z2

[0051] V3=gf3(θ az A+Z3

[0052] V4=gf4(θ az A+Z4

[0053] Where g is the feed channel gain; f1(θ) r ) and f2(θ rThe range pointing deviation of the high-orbit SAR antenna is θ. r At that time, the directivity functions of the range-to-left calibration beam and the range-to-right calibration beam at the first ground receiver; f3(θ) az ) and f4(θ az The azimuth pointing deviation of the high-orbit SAR antenna is θ. az At that time, the directional functions of the azimuth-left calibration beam and the azimuth-right calibration beam at the first ground receiver; A is the known complex beacon signal transmitted by the high-orbit SAR satellite; Z i (where i = 1, 2, 3, 4) represents the thermal noise of the receiving channel when the first ground receiver receives the range left, range right, azimuth left, and azimuth right calibration beams.

[0054] Step 4: Based on the complex signal obtained in Step 3, perform sum and difference processing on it using amplitude-comparison single pulses. The specific processing method is as follows:

[0055] Sum and difference processing is performed on the two complex signals V1 and V2 in the range direction to the left and right, respectively, to obtain the range direction sum signal V. Σr and range vector difference signal V Δr The expression is:

[0056] V Σr =V1+V2=g(f1(θ) r )+f2(θ r A+(Z1+Z2)

[0057] V Δr =V1-V2=g(f1(θ) r )-f2(θ r ))A+(Z1-Z2)

[0058] Further transformations can yield the following results:

[0059] V Σr =V1+V2=ηA+(Z1+Z2)

[0060] V Δr =V1-V2=ηF(θ) r A+(Z1-Z2)

[0061] In the formula, η=g(f1(θ) r )+f2(θ r ));

[0062] F(θ r For amplitude-comparison single-pulse output, the expression is:

[0063]

[0064] Based on the amplitude-ratio single pulse output F(θ)r ) and θ r The relationship can be used to derive the distance pointing deviation measurement value. In this example, the directivity function of the calibration beam is in Gaussian form. Let the angles by which the centers of the two symmetrical calibration beams deviate from the equal power axes of the two beams be ±θ. s The 3dB beamwidth of each of the four calibration beams is θ. B Then the normalized directivity functions of the range to the two calibration beams are respectively

[0065] f1(θ r )=exp(-α(θ r -θ s ) 2 / θ B 2 )=exp(β(θ r -θ s ) 2 )

[0066] f2(θ r )=exp(-α(θ r +θ s ) 2 / θ B 2 )=exp(β(θ r +θ s ) 2 )

[0067] In the formula, α=4ln2, β=α / θ B 2 All of them are constant values.

[0068] Considering θ r and θ s Satisfy |θ r |<|θ s | << 1, for f1(θ) r ) and f2(θ r Performing a first-order Taylor series expansion, we obtain...

[0069]

[0070] Because |θ r |<|θ s |<<1, can be further simplified to

[0071]

[0072] It can be known that θ r With F(θ) r Under certain conditions, it can be approximated as a linear relationship, from F(θ) r θ can be derived from )r The estimated value, i.e., the distance pointing deviation measurement value.

[0073] Sum and difference processing is performed on the left and right azimuth beacon signals to obtain the azimuth sum signal V. Σaz and azimuth difference signal V Δaz The expression is:

[0074] V Σaz =V3+V4=g(f3(θ) az )+f4(θ az ))A+(Z3+Z4)

[0075] V Δaz =V3-V4=g(f3(θ) az )-f4(θ az ))A+(Z3-Z4)

[0076] Further transformations can yield the following results:

[0077] V Σaz =V3+V4=μA+(Z3+Z4)

[0078] V Δaz =V3-V4=μF(θ) az A+(Z3-Z4)

[0079] In the formula, μ=g(f3(θ) az )+f4(θ az ));

[0080] F(θ az For amplitude-comparison single-pulse output, the expression is:

[0081]

[0082] Based on the amplitude-ratio single pulse output F(θ) az ) and θ az The relationship can be used to derive the azimuth pointing deviation measurement value. As stated above, the normalized directivity functions of the two calibration beams in the azimuth direction are:

[0083] f3(θ az )=exp(-α(θ az -θ s ) 2 / θ B 2 )=exp(β(θ az -θ s ) 2 )

[0084] f4(θaz )=exp(-α(θ az +θ s ) 2 / θ B 2 )=exp(β(θ az +θ s ) 2 )

[0085] In the formula, α=4ln2, β=α / θ B 2 All of them are constant values.

[0086] Considering θ az and θ s Satisfy |θ az |<|θ s | << 1, for f3(θ) az ) and f4(θ az Performing a first-order Taylor series expansion, we obtain...

[0087]

[0088] Because |θ az |<|θ s |<<1, can be further simplified to

[0089]

[0090] It can be known that θ az With F(θ) az Under certain conditions, it can be approximated as a linear relationship, from F(θ) az θ can be derived from ) az The estimated value, i.e., the distance pointing deviation measurement value.

[0091] Step 5: Repeat the data processing steps 3 and 4 for the data output from the second ground receiver in Step 2 to obtain another set of range pointing deviation measurements. Azimuth pointing deviation measurement value

[0092] Step 6: Calculate the antenna pointing at the current moment for the high-orbit SAR based on the preset antenna pointing data and satellite attitude data. In this example, the preset antenna pointing is... in The viewpoint is from the high-orbit SAR antenna; the satellite attitude data is (θ). Y ,θ P ,θ R ), where θ Y Let θ be the yaw angle. P Let θ be the pitch angle. RThe roll angle is considered. Taking into account the deformation of the high-orbit SAR antenna during installation, on-orbit deployment, and on-orbit operation, as well as the combined effect of antenna feed channel errors, a systematic pointing deviation exists in the high-orbit SAR antenna. This can be regarded as a three-dimensional systematic pointing deviation of the antenna. in This is a systematic pointing deviation of the yaw axis. For pitch axis systemic pointing deviation, This is a systematic pointing deviation of the rolling axis.

[0093] Antenna preset direction After (θ) Y ,θ P ,θ R )and Rotate, become That is, the antenna pointing at the current moment, specifically expressed as:

[0094]

[0095] In the formula, x, y, and z are the coordinate values ​​in the satellite's flight coordinate system, specifically expressed as follows:

[0096]

[0097]

[0098]

[0099]

[0100]

[0101]

[0102] Step 7: Based on the antenna pointing at the current moment obtained in Step 6 Calculate the antenna range pointing deviation θ r and azimuth pointing deviation θ az The specific expression is:

[0103]

[0104]

[0105] Step 8: Based on the pointing deviation value θ obtained in Step 7 r θ az and the pointing deviation measurements obtained in steps 4 and 5 A one-to-one correspondence is established to construct a system of equations:

[0106]

[0107] Solving the system of equations yields the three-dimensional systematic pointing deviation of the antenna. That is, the high-orbit SAR antenna is pointed at the calibration target, and the high-orbit SAR antenna pointing calibration is completed.

[0108] In summary, the above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for pointing and calibrating a high-orbit SAR antenna, characterized in that, The satellite transmits beacon signals to the ground by forming four calibration beams in sequence through different feed channel combinations. These are two calibration beams symmetrically arranged in the range direction and two calibration beams symmetrically arranged in the azimuth direction. Two ground receivers receive the beacon signals and perform IQ sampling, pulse compression processing, and sum-difference processing on the data received by the two ground receivers to obtain the range pointing deviation measurement value and the azimuth pointing deviation measurement value. The pointing of the high-orbit SAR antenna is calculated to obtain the range pointing deviation value and the azimuth pointing deviation value, and finally the three-dimensional systematic deviation of the high-orbit SAR antenna is obtained. The specific steps are as follows: Step 1: The satellite sequentially forms four calibration beams through different feed channel combinations: two calibration beams symmetrically positioned to the left and right in the range direction, and two calibration beams symmetrically positioned to the left and right in the azimuth direction. Beacon signals are then transmitted to the ground in the order of the range left calibration beam, the range right calibration beam, the azimuth left calibration beam, and the azimuth right calibration beam. Step 2: The two ground receivers receive the raw data of the beacon signal transmitted in Step 1 and sort the data into segments in the following order: range left calibration beam, range right calibration beam, azimuth left calibration beam, and azimuth right calibration beam. Step 3: Perform IQ sampling and pulse compression processing on the data output from the first ground receiver in Step 2 to obtain a complex signal. ;in and These are the range-to-left beacon signal and the range-to-right beacon signal, respectively. and These are the left-facing beacon signal and the right-facing beacon signal, respectively. Step 4: Perform sum and difference processing on the complex signal obtained in Step 3; and After performing summation and difference processing, the distance pointing deviation measurement value is obtained. ;right After performing summation and difference processing, the azimuth pointing deviation measurement value is obtained. ; Step 5: Repeat steps 3 and 4 for the data output from the second ground receiver in step 2 to obtain the range pointing deviation measurement value. Azimuth pointing deviation measurement value ; Step 6: Calculate the high-orbit SAR antenna pointing based on the preset wavefront antenna pointing data and the acquired satellite attitude data, and input the high-orbit SAR pointing conversion matrix. Step 7: Based on the antenna pointing direction obtained in Step 6, calculate the antenna pointing deviation value, i.e., the range pointing deviation value. and azimuth pointing deviation value ; Step 8: Based on the pointing deviation value obtained in Step 7 , and the pointing deviation measurements obtained in steps 4 and 5 , , , By establishing a one-to-one correspondence, a system of linear equations is constructed to solve for the three-dimensional systematic deviation of the high-orbit SAR antenna. , , .

2. The high-orbit SAR antenna pointing calibration method as described in claim 1, characterized in that, Step 6 specifically involves: Preset antenna pointing is ,in The view from the high-orbit SAR antenna is as follows: satellite attitude data is ,in Yaw angle The pitch angle, The roll angle is taken into account. Considering the deformation of the high-orbit SAR antenna during installation, on-orbit deployment, and on-orbit operation, as well as the combined effect of antenna feed channel errors, the high-orbit SAR antenna exhibits a systematic pointing deviation, which can be regarded as a three-dimensional systematic pointing deviation of the antenna. ,in This is a systematic pointing deviation of the yaw axis. For pitch axis systemic pointing deviation, This is due to a systematic pointing deviation of the rolling axis. Antenna preset direction go through and Rotate, become That is, the antenna pointing at the current moment, specifically expressed as: In the formula, x, y, and z are the coordinate values ​​in the satellite's flight coordinate system, specifically expressed as follows: 。 3. The high-orbit SAR antenna pointing calibration method as described in claim 2, characterized in that, Step 7 specifically includes: 。 4. A high-orbit SAR antenna pointing calibration method as described in claim 1 or 3, characterized in that, Step 8 involves constructing a system of linear equations as follows: 。

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