A method for on-orbit measurement of GEOSAR antenna patterns

Abstract

The present invention discloses an on-orbit measurement method for a GEOSAR antenna pattern, comprising the following steps: step 1, arranging calibration equipment; step 2, formulating a satellite observation plan; step 3, performing IQ digital demodulation on the signal collected by the calibrator to generate a baseband complex signal; step 4, performing pulse compression processing on the baseband complex signal generated in step 3, and extracting the peak amplitude of each pulse; step 5, reading satellite precise orbit determination parameters and satellite attitude measurement parameters, and converting the satellite orbit coordinate system E into the satellite attitude measurement system E according to the satellite attitude conversion sequence. b Convert to the satellite body coordinate system E s Then, the azimuth angle corresponding to each pulse is calculated. In step 6, the azimuth angle corresponding to each pulse calculated in step 5 is compared with the peak amplitude of each pulse extracted in step 4 to obtain the GEOSAR antenna pattern. The method of the present invention is simple in terms of ground equipment requirements and is highly practical. It is an effective and feasible method for on-orbit measurement of GEOSAR antenna patterns.

Description

Technical Field

[0001] The invention relates to an on-orbit measurement method for a GEOSAR antenna pattern, and belongs to the technical field of synthetic aperture radar (SAR) data processing. Background Art

[0002] Geosynchronous Synthetic Aperture Radar (GEOSAR) is a new type of microwave remote sensing SAR operating in geosynchronous orbit. Compared to low-Earth orbit SAR, it offers advantages such as a wider Earth observation range, shorter revisit periods, and longer continuous regional observation times. GEOSAR will fill a gap in the field of GEOSAR in my country and globally, enabling high-frequency, long-term, wide-coverage, and highly sensitive monitoring of hurricanes, waves, ocean currents, static marine targets, and coastlines. This will significantly enhance my country's capabilities for disaster prevention and mitigation, marine observation, and emergency monitoring.

[0003] Antenna pattern measurement is a key component of on-orbit testing of SAR satellite payloads. Using on-orbit test antenna patterns allows for radiometric correction of SAR images, effectively improving SAR radiometric calibration accuracy. Due to the characteristics of the GEOSAR satellite's orbit, observations of the Amazon rainforest are impossible. Furthermore, the satellite's beam footprint is slow; in normal imaging mode, the beam only slides approximately one-quarter of its 3dB beamwidth in 1800s. Therefore, traditional antenna pattern measurement methods for low-Earth orbit SAR are inapplicable, necessitating the urgent need for research on new antenna pattern measurement methods.

[0004] At present, there is no public literature on GEOSAR antenna pattern measurement at home and abroad. To address this new technical challenge, it is urgent to study a new antenna pattern measurement method suitable for GEOSAR. Summary of the Invention

[0005] The present invention proposes an on-orbit measurement method for GEOSAR antenna patterns, which can realize on-orbit measurement of GEOSAR antenna patterns and solve the technical problem that traditional low-orbit SAR antenna pattern measurement methods are not applicable to GEOSAR.

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

[0007] A method for measuring the GEOSAR antenna pattern on-orbit comprises the following steps:

[0008] Step 1: Calibration equipment layout;

[0009] Step 2: Develop a satellite observation plan. Specifically, the satellite is required to adopt a straight-on-side attitude guidance method. That is, during operation, the satellite attitude is maneuvered in real time to maintain a straight-on-side observation of the calibrator. The satellite transmits a linear frequency modulation pulse signal according to preset parameters, and the calibrator receives and stores the satellite transmission signal.

[0010] Step 3, performing IQ digital demodulation on the signal collected by the scaler to generate a baseband complex signal;

[0011] Step 4, performing pulse compression processing on the baseband complex signal generated in step 3, and extracting the peak amplitude of each pulse;

[0012] Step 5: Read the satellite precise orbit determination parameters and satellite attitude measurement parameters, and convert the satellite orbit coordinate system E into the satellite attitude measurement system according to the satellite attitude conversion sequence. b Convert to the satellite body coordinate system E s ; Calculate the relative position vector from the satellite to the calibrator based on the satellite orbit position and the calibrator position, project it along the XYZ axes in the satellite coordinate system, and then calculate the azimuth angle corresponding to each pulse;

[0013] Step 6: Compare the azimuth angle corresponding to each pulse calculated in step 5 with the peak amplitude of each pulse extracted in step 4 to obtain the GEOSAR antenna pattern.

[0014] Furthermore, in step 3, the IQ digital demodulation method includes the following sub-steps:

[0015] Step 31: Use the following formula to obtain a complex signal from the signal collected by the scaler:

[0016] x(τ)=cos{2πf0τ+φ(τ)}

[0017] I(τ)=x(τ)*cos(2πf0τ)

[0018] Q(τ)=x(τ)*sin(2πf0τ)

[0019] X(τ)=I(τ)+jQ(τ)

[0020] Where x(τ) represents the signal collected by the scaler, I(τ) and Q(τ) represent the I-channel and Q-channel signals respectively, f0 is the signal center frequency, τ represents time, φ(τ) represents the phase of the linear frequency modulation signal, and X(τ) represents the complex signal synthesized by I(τ) and Q(τ);

[0021] Step 32: low-pass filter the complex signal X(τ) obtained in step 31 to obtain a baseband complex signal X B (τ).

[0022] Furthermore, in step 32, the low-pass filtering operation is as follows: Fourier transform is performed on the complex signal X(τ) to obtain a signal X(f)=FFT{X(τ)}; a signal within the frequency range [-B / 2, B / 2] is selected and inverse Fourier transform is performed to obtain a baseband complex signal X B (τ)=IFFT{X(f),-B / 2≤f≤B / 2}, where B is the bandwidth of the complex signal X(τ).

[0023] Furthermore, in step 4, the pulse compression processing method operates as follows:

[0024] Step 41: construct a matched filter and compare it with the baseband complex signal X generated in step 3. B (τ) performs convolution operation to obtain the pulse pressure signal;

[0025] Step 42: extract the peak amplitude of each pulse according to the GEOSAR transmit pulse repetition interval PRI.

[0026] Furthermore, in step 5, it is assumed that the satellite attitude transition sequence is: yaw θ Y ->Pitch θ P ->Roll θ R ,but:

[0027] E s =A bs E b

[0028] in,

[0029] Furthermore, in step 5, the calculation formula for the azimuth angle is:

[0030]

[0031] The beneficial effects of the present invention compared with the prior art are:

[0032] In response to the technical difficulty that traditional low-orbit SAR antenna pattern measurement methods cannot be applied to GEOSAR, the present invention proposes an on-orbit measurement method for antenna patterns suitable for GEOSAR. This method realizes antenna beam scanning through real-time maneuvering of the satellite platform attitude. The satellite transmits a linear frequency modulation (LFM) pulse signal according to preset parameters. The ground calibrator receives the satellite signal and combines the satellite orbit measurement parameters and attitude measurement parameters to realize antenna pattern measurement. This method has simple requirements for ground equipment and strong practicality. It is an effective and feasible on-orbit measurement method for GEOSAR antenna patterns, which is of great significance for improving GEOSAR pattern measurement and radiation accuracy. The results of the satellite on-orbit pattern test show that this method is an effective and feasible on-orbit measurement method for GEOSAR antenna patterns. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a data processing flow chart of the method of the present invention;

[0034] Figure 2 Schematic diagram of a satellite adopting a positive side-view attitude guidance method in an embodiment of the present invention;

[0035] Figure 3 Schematic diagram of a satellite orbit coordinate system according to an embodiment of the present invention;

[0036] Figure 4 2 is a schematic diagram of the satellite body coordinate system according to an embodiment of the present invention;

[0037] Figure 5 The data processing result received by the scaler in the embodiment of the present invention;

[0038] Figure 6 The on-orbit pattern measurement results and ground test results in an embodiment of the present invention;

[0039] Figure 7 It is the error between the on-orbit pattern measurement result and the ground test result in the embodiment of the present invention. DETAILED DESCRIPTION

[0040] like Figure 1 As shown, the on-orbit measurement method of the GEOSAR antenna pattern provided by the present invention includes the following steps:

[0041] Step 1: Calibration equipment deployment. Specifically, select an open, flat area to deploy the calibrator. The deployment area must have an elevation angle of at least 15 degrees and be free of obstructions such as terrain, buildings, and vegetation to prevent the calibrator from receiving satellite signals.

[0042] Step 2: Develop a satellite observation plan. Specifically, the satellite is required to use a direct-to-side attitude guidance method, which means that during operation, the satellite attitude is maneuvered in real time to maintain a direct-to-side observation of the calibrator. The satellite transmits linear frequency modulation (LFM) pulse signals according to preset parameters, and the calibrator receives and stores the satellite's transmitted signals.

[0043] Step 3, performing IQ digital demodulation on the signal collected by the scaler to generate a baseband complex signal;

[0044] The IQ digital demodulation method includes the following steps:

[0045] Step 31: Use the following formula to obtain a complex signal from the signal collected by the scaler:

[0046] x(τ)=cos{2πf0τ+φ(τ)}

[0047] I(τ)=x(τ)*cos(2πf0τ)

[0048] Q(τ)=x(τ)*sin(2πf0τ)

[0049] X(τ)=I(τ)+jQ(τ)

[0050] Where x(τ) represents the signal collected by the scaler, I(τ) and Q(τ) represent the I-channel and Q-channel signals, respectively, f0 is the signal center frequency, τ represents time, φ(τ) represents the phase of the linear frequency modulation signal, and X(τ) represents the complex signal synthesized by I(τ) and Q(τ).

[0051] Step 32: low-pass filter the complex signal X(τ) obtained in step 31 to obtain a baseband complex signal X B (τ).

[0052] Specifically, the low-pass filtering process is as follows: perform Fourier transform (FFT) on the complex signal X(τ) to obtain the signal X(f) = FFT{X(τ)}; select the signal within the frequency range [-B / 2, B / 2] and perform inverse Fourier transform to obtain the baseband complex signal X B (τ)=IFFT{X(f),-B / 2≤f≤B / 2}, where B is the bandwidth of the complex signal X(τ).

[0053] Step 4: The baseband complex signal X generated in step 3 is B (τ) Perform pulse compression processing and extract the peak amplitude of each pulse;

[0054] Specifically, the pulse compression processing method operates as follows:

[0055] Step 41: construct a matched filter and compare it with the baseband complex signal X generated in step 3. B (τ) performs convolution operation to obtain the pulse pressure signal; in step 41, the specific formula is as follows:

[0056] h(τ)=X B * (-τ)

[0057]

[0058] Among them, h(τ) is the matched filter; s out (τ) post-pulse pressure signal; X B (τ) is the baseband complex signal;

[0059] Step 42, extracting the peak amplitude of each pulse according to the GEOSAR transmit pulse repetition interval PRI;

[0060] s max (i)=max{abs(s out(n))},(i-1)*Fs*PRI+1≤n≤i*Fs*PRI,n=1,2,3…

[0061] Where i represents the number of pulses, Fs represents the sampling frequency of the scaler, abs() represents taking the absolute value, and max{} represents taking the maximum value; s out (n) is s out The discrete form of (τ).

[0062] Step 5: Read the satellite precise orbit determination parameters and satellite attitude measurement parameters, and convert the satellite orbit coordinate system E into the satellite attitude measurement system according to the satellite attitude conversion sequence. b Convert to the satellite body coordinate system E s .

[0063] Among them, it is assumed that the satellite attitude transition sequence is: yaw θ Y ->Pitch θ P ->Roll θ R ,but:

[0064] E s =A bs E b

[0065] in,

[0066] According to the satellite orbital position [R x ,R y ,R z ] and the scaler position [T x ,T y ,T z ]Calculate the relative position vector from the satellite to the calibrator Will Projection is generated along the XYZ axis in the satellite body coordinate system Right now Then calculate the azimuth angle corresponding to each pulse:

[0067]

[0068] Step 6: Compare the azimuth angle corresponding to each pulse calculated in step 5 with the peak amplitude of each pulse extracted in step 4 to obtain the GEOSAR antenna pattern.

[0069] Example

[0070] First, the calibrator is placed in an open and flat area (in this embodiment, the calibrator is placed on a flat grassland), and then a satellite observation plan is formulated. The satellite transmits a linear frequency modulation (LFM) pulse signal according to the preset parameters, and the ground calibrator receives and stores the satellite transmission signal. Figure 2 and Figure 3shown.

[0071] The data used in this embodiment is the on-orbit pattern test data of the Land Exploration No. 4 01 satellite.

[0072] The data received by the calibrator is demodulated digitally to generate a baseband complex signal, which is then pulse compressed and the peak amplitude of each pulse is extracted. Finally, the satellite precise orbit determination parameters and satellite attitude measurement parameters are read, and the azimuth angle corresponding to each pulse is calculated to obtain the GEOSAR antenna pattern. The results are as follows: Figure 5 、 Figure 6 and Figure 7 shown.

[0073] The measurement results show that the antenna pattern results measured by the on-orbit measurement method for GEOSAR antenna patterns proposed in the present invention are consistent with the ground test results, with an error of less than 0.1 dB, thus verifying the effectiveness and feasibility of the on-orbit measurement method for GEOSAR antenna patterns proposed in the present invention, and the method can be applied to on-orbit measurement of GEOSAR antenna patterns.

[0074] The method of the present invention can be used for on-orbit measurement of GEOSAR antenna patterns in any frequency band.

[0075] The contents not described in detail in the specification of the present invention belong to the common knowledge of professionals in this field.

Claims

1. A method for on-orbit measurement of GEOSAR antenna patterns, characterized in that: The specific steps include: Step 1: Calibration equipment layout; Step 2: Develop a satellite observation plan. Specifically, the satellite is required to adopt a straight-on-side attitude guidance method. That is, during operation, the satellite attitude is maneuvered in real time to maintain a straight-on-side observation of the calibrator. The satellite transmits a linear frequency modulation pulse signal according to preset parameters, and the calibrator receives and stores the satellite transmission signal. Step 3, performing IQ digital demodulation on the signal collected by the scaler to generate a baseband complex signal; Step 4, performing pulse compression processing on the baseband complex signal generated in step 3, and extracting the peak amplitude of each pulse; Step 5: Read the satellite precise orbit determination parameters and satellite attitude measurement parameters, and convert the satellite orbit coordinate system into the satellite attitude measurement system in the order of satellite attitude conversion. Convert to the satellite body coordinate system ; The relative position vector from the satellite to the calibrator is calculated based on the satellite orbit position and the calibrator position, and is projected along the X, Y, and Z axes in the satellite coordinate system. The azimuth angle corresponding to each pulse is then calculated. Step 6: Compare the azimuth angle corresponding to each pulse calculated in step 5 with the peak amplitude of each pulse extracted in step 4 to obtain the GEOSAR antenna pattern.

2. The on-orbit measurement method for GEOSAR antenna patterns according to claim 1, wherein: In step 3, the IQ digital demodulation method includes the following sub-steps: Step 31: Use the following formula to obtain a complex signal from the signal collected by the scaler: in, represents the signal collected by the scaler, 、 Represents I-channel and Q-channel signals respectively, is the signal center frequency, Indicates time, represents the phase of the linear frequency modulation signal, express and synthesize complex signals; Step 32: The complex signal obtained in step 31 is Perform low-pass filtering to obtain the baseband complex signal .

3. The on-orbit measurement method for GEOSAR antenna patterns according to claim 2, wherein: In step 32, the low-pass filtering process is performed as follows: Perform Fourier transform to get the signal ; Select frequency range Perform inverse Fourier transform on the signal in the ,in Complex signal bandwidth.

4. The on-orbit measurement method for GEOSAR antenna patterns according to claim 3, wherein: In step 4, the pulse compression processing method operates as follows: Step 41, construct a matched filter and compare it with the baseband complex signal generated in step 3 Perform convolution operation to obtain the pulse pressure signal; Step 42: extract the peak amplitude of each pulse according to the GEOSAR transmit pulse repetition interval PRI.

5. The on-orbit measurement method for GEOSAR antenna patterns according to claim 4, wherein: In step 5, assume that the satellite attitude transition sequence is: yaw ->Pitch ->Scroll ,but: in, .

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