Multi-frequency simultaneous cross-correlation near-field holographic measurement method for radio telescope antennas

Abstract

The present invention discloses a near-field holographic measurement method for multi-frequency simultaneous cross-correlation of a radio telescope antenna, wherein a multi-frequency comb spectrum signal is used as a transmitting source, a main receiving path and a reference receiving path are used to simultaneously receive the multi-frequency radio frequency signal, and the multi-frequency frequency domain complex value is obtained by performing FFT and cross-correlation operations on the two receiving ends, and then the multi-frequency frequency domain complex value is corrected and re-grid, and inverted one by one to the antenna aperture field to further obtain the radio telescope antenna surface error distribution diagram and surface error RMS corresponding to the multi-frequency signal, and finally averaged to obtain the final radio telescope antenna surface error distribution and RMS. The present invention measures multiple frequency signals simultaneously, averages the antenna surface error distribution diagrams corresponding to all obtained frequency points to obtain the final antenna surface error distribution diagram, and through the multi-frequency averaging process, the averaged multi-frequency reduces the measurement error caused by the multipath effect compared with the single frequency.

Description

Technical Field

[0001] The invention belongs to the technical field of telescopes, and in particular relates to a near-field holographic measurement method of multi-frequency simultaneous cross-correlation of a radio telescope antenna. Background Art

[0002] The construction of radio telescopes is developing towards a large-aperture trend. However, large-aperture antennas are often composed of dozens or even hundreds of panels. Whether during installation or regular maintenance, they need to be measured and adjusted with the help of surface shape detection technology to maintain the surface accuracy requirements of the antenna reflector. In order to obtain higher reflector surface accuracy, the technology related to antenna reflector surface shape detection is also constantly improving, and the method of large-aperture antenna surface shape detection is also gradually updated. Microwave holographic measurement is currently the most accurate measurement method in the surface shape detection technology of large-shaped antenna reflectors. The term "holographic" in radio holographic measurement mainly refers to the acquisition of the aperture field phase. The aperture field phase of an ideal parabola is equal everywhere, but the actual parabola is not exactly the same, so the surface error distribution can be further derived based on the inverted aperture field phase information. The phase correlation method is one of the microwave holographic measurement methods. It simultaneously measures the amplitude and phase of the antenna's radiation pattern (i.e., vector pattern), performs phase correlation, and then performs inverse Fourier transform on the correlated vector pattern to obtain the antenna vector aperture field. From the phase distribution of the aperture field, the surface error distribution is further obtained. The phase correlation method is divided into far-field holographic measurement and near-field holographic measurement according to the distance between the beacon source and the antenna under test.

[0003] At present, most of the near-field holographic measurement experiments use single-frequency narrowband signals, and usually use two transmission signal frequencies with a large frequency difference as the nominal frequency for measurement. This method has certain limitations. First, the frequency of the transmitting source signal is single-frequency, which requires multiple measurements. It takes a long time to obtain the results of multiple frequencies; second, the measurement results of each frequency have a certain time interval rather than being obtained at the same time. In this process, the systematic errors caused by the deformation of the antenna surface due to factors such as gravity, temperature and wind load are not eliminated, thereby introducing new errors. The more serious problem of using single-frequency measurement is that in the actual measurement process, there is often multipath interference on the path from the transmitting source to the receiver, that is, in addition to the normal transmission path signal expected from the transmitting source, there are other reflection signals from the ground and other objects around the antenna reflection surface, such as the sub-surface support rod. These reflection signals are mixed with the received signal to form interference and are received by the receiver, making the received signal no longer a "pure" signal, which affects the relevant output results of the receiver.

[0004] Taking ground reflection as an example, ΔR1 is the absolute optical path difference between the S main receiving path and the R reference receiving path, and ΔR2 is the additional optical path difference between the transmission path of the transmitted signal received by the S main receiving path and the other paths that reach the receiving end after ground reflection. The absolute optical path difference ΔR1 and the additional optical path difference ΔR2 are as follows: Figure 1 As shown. Because the existence of ΔR1 and ΔR2 will cause the phase of the S main receiving path and the R reference path to change after FFT correlation, which will lead to measurement errors. In theory, when the antenna under test is placed in an open field, the impact of multipath effect will be greatly reduced, and the reflection effect at this time mainly comes from ground reflection.

[0005] From the above analysis, it can be seen that since the single-frequency holographic measurement results are obtained at a certain time interval rather than simultaneously, the systematic error caused by the deformation of the antenna surface due to factors such as gravity, temperature and wind load during the entire multiple measurement process is not eliminated, and the measurement results are easily affected by the multipath effect, causing measurement errors. Summary of the invention

[0006] In view of the above problems, the present invention provides a multi-frequency simultaneous cross-correlation near-field holographic measurement method for a radio telescope antenna. In order to suppress the influence of the multipath interference effect, the present invention designs and establishes a multi-frequency simultaneous near-field holographic measurement system for a radio telescope antenna. A broadband multi-frequency signal is used to replace the traditional narrowband signal as the transmitting source signal, and multiple frequency signals are measured simultaneously. The surface error distribution maps corresponding to all the obtained frequency points are averaged to obtain the final surface error distribution map. Through the averaging processing of multiple frequencies, the multi-frequency after averaging reduces the measurement error caused by the multipath effect compared with the single frequency.

[0007] In order to achieve the above technical objectives, the technical solution adopted by the present invention is:

[0008] The multi-frequency simultaneous cross-correlation near-field holographic measurement method for radio telescope antenna adopts multi-frequency comb spectrum signal as the emission source, and adopts two receiving ends, the main receiving path and the reference receiving path, to simultaneously receive the multi-frequency signal, and obtain the multi-frequency frequency domain complex value by performing FFT and cross-correlation operations on the two receiving ends, and then correct and re-grid the multi-frequency frequency domain complex value, and invert them one by one to the telescope aperture field to further obtain the telescope antenna surface error distribution diagram and surface error RMS corresponding to the multi-frequency signal, and finally take the average to obtain the final telescope antenna surface error distribution and RMS.

[0009] In some embodiments, the distance R between the transmitting source and the receiving end satisfies: Where D is the aperture of the antenna under test, λ is the operating wavelength, and the position of the transmitting source is fixed during the measurement process.

[0010] In some of the embodiments, the reference receiving path is located at the main focus of the parabolic antenna of the radio telescope and remains stationary, and the main receiving path moves around the transmitting source in a raster scanning manner to receive the transmitting source signal.

[0011] In some of the embodiments, the specific method in which the main receiving path moves around the transmitting source in a raster scanning manner is as follows: if there are N*N matrix sampling points, a center point is first determined, and before starting the scan, the main receiving path moves to the center point for a center calibration, and after each scan of a row of sampling points, returns to the center point for a center calibration, until the scan is completed and finally returns to the center point for a center calibration, that is, the sampling points are scanned N*N times in total, and the number of center point calibrations is N+1.

[0012] In some of the embodiments, the multi-frequency comb spectrum signal emitted by the transmitting source includes M frequency points. For a certain frequency point, a set of frequency domain complex values ​​of the real part and the imaginary part is generated after the FFT cross-correlation between the main receiving path and the reference receiving path. Therefore, a total of M frequency domain complex values ​​are generated after the FFT cross-correlation between the main receiving path and the reference receiving path. The digital acquisition correlator adopts Labview's FFT kernel module, operator, custom memory, FIFO, and queue management module to obtain the frequency domain complex values ​​after the FFT cross-correlation between the main receiving path and the reference receiving path through graphical programming; the FFT cross-correlation operation of the main receiving path and the reference receiving path is performed at each raster scanning position to generate a series of complex data.

[0013] In some of the embodiments, during the scanning process, the main receiving path performs pitch and azimuth movements according to instructions, and the main receiving path antenna saves the grid number, pitch, horizontal angle information and received multi-frequency signal information and sends them to the processing unit to calculate the telescope surface error distribution and RMS.

[0014] The specific method for the processing unit to process and calculate the telescope surface error distribution and RMS includes the following steps:

[0015] Step 1: Scan data averaging: average the data of repeated sampling at each sampling point in the received information to obtain the real and imaginary data of M groups of N*N sampling points, and further obtain the corresponding complex number.

[0016] Step 2: Center calibration data averaging: average the center calibration data to obtain M sets of real and imaginary data with N+1 sampling points, and further obtain the corresponding complex numbers.

[0017] Step 3, fitting the center calibration data, and correcting the scan data with the fitting curve data: using the center calibration data obtained in step 2 to obtain the fitting curve data by interpolation, and then dividing the scan data processed in step 1 by the fitting curve data to obtain the corrected directivity pattern complex distribution function;

[0018] Step 4: Correct the image field rotation: that is, the telescope antenna aperture center and the rotation center do not coincide, which leads to the additional optical path and parallax effect during the antenna movement. The correction method is to use the rotation matrix to perform phase correction on the data of the complex distribution function of the directional pattern obtained in step 3.

[0019] Step 5: According to the different distances from the center of the telescope antenna reflector aperture and the center of the reference horn to the beacon source, correct the near-field phase of the data obtained in step 4;

[0020] Step 6: Perform local interpolation on all the data corrected in step 5 to obtain the amplitude and phase distribution of the near-field two-dimensional radiation pattern;

[0021] Step 7: Perform inverse Fourier transform on the amplitude and phase distribution obtained in step 6 to obtain the telescope aperture field amplitude and phase distribution;

[0022] Step 8, correcting the axial focus error of the aperture field: that is, calculating the aperture field optical path difference introduced by the axial focus error according to the known axial focus error, and adding the optical path difference into the aperture field complex distribution function obtained in step 7 to obtain the aperture field phase distribution;

[0023] Step 9, correcting the influence of the phase distribution of the telescope prime focus feed horn on the aperture field phase distribution: that is, subtracting the phase distribution of the prime focus feed horn from the aperture field phase distribution obtained in step 8 to obtain a linear phase;

[0024] Step 10: Calculate the antenna surface error distribution diagram and its RMS value corresponding to each frequency point by using the surface fitting correction constant and linear phase, and the contribution of feed source offset;

[0025] Step 11: Average the obtained telescope antenna surface error distribution diagrams corresponding to all frequency points to obtain the final antenna surface error distribution diagram, and calculate its RMS.

[0026] The beneficial effects of the present invention are:

[0027] The method of the present invention breaks through the limitation of narrowband transmission signals in traditional near-field holographic measurement, can realize the simultaneous measurement of multi-frequency signals, and can obtain multi-frequency signal data under the same conditions after one holographic measurement. The multi-frequency frequency domain complex values ​​are obtained by performing FFT and cross-correlation operations on the multi-frequency signal data, and then the multi-frequency frequency domain complex values ​​are corrected and grid-reorganized, and inverted one by one to the telescope antenna aperture field to further obtain the surface error distribution diagram and surface error RMS (root mean square) corresponding to the multi-frequency signals, and finally the average is taken to obtain the final antenna surface error distribution and RMS. The method can have a good suppression effect on the multipath effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of the additional optical path difference caused by the multipath effect;

[0029] Figure 2 It is the FFT related algorithm flow chart;

[0030] Figure 3 It is a schematic diagram of the grid scanning trajectory and sampling points of the main receiving path of the present invention;

[0031] Figure 4 This is a schematic diagram of the multi-frequency simultaneous near-field holographic measurement (prime focus installation) experiment. DETAILED DESCRIPTION

[0032] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0033] It should be understood that when used in this specification and the appended claims, the terms "include" and "comprises" indicate the presence of described features, integers, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or combinations thereof.

[0034] It should also be understood that the terms used in this specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. As used in the specification of the present invention and the appended claims, the singular forms "a", "an" and "the" are intended to include the plural forms unless the context clearly indicates otherwise.

[0035] In the near-field holographic measurement of radio telescopes, according to the different installation positions of the main signal path receiving horn, there are two different measurement schemes: prime focus holography and stuck focus holography. In the prime focus holographic measurement, the main signal path receiving feed is installed near the main focus position of the reflector antenna, with the opening facing the main reflector, and the reference receiving feed is installed back to back with the main feed. Take the prime focus near-field holographic measurement as an example to illustrate. Figure 4 shown.

[0036] The multi-frequency simultaneous near-field holographic measurement system mainly includes three parts: multi-frequency signal transmission, reception and cross-correlation; cross-correlation data acquisition; and multi-frequency cross-correlation data processing.

[0037] 1. Multi-frequency signal transmission, reception and cross-correlation

[0038] (1) Multi-frequency signal transmitter

[0039] The transmitting beacon source is generally placed at a height of about 27 meters from the telescope under test, and its position is fixed during the measurement process. The transmitting source signal adopts a comb spectrum signal, which is mainly composed of a local oscillator frequency synthesis generator, a fixed attenuator, an integrated transmitting module, an isolator, an adjustable attenuator, and a transmitting corrugated horn. Among them, the integrated transmitting module consists of an 8-frequency multiplier, an up-conversion mixer and an amplifier. The generation of the comb spectrum signal is realized by programming using NI's FPGA-based PXIe-5785 intermediate frequency transceiver module. The frequency range of the comb spectrum signal is 6.25MHz to 1.2GHz (frequency interval 6.25MHz), and the local oscillator signal uses 90GHz, with a total of 192 spectral lines, which are transmitted to the outside through the transmitting horn through the isolator and attenuator.

[0040] (2) Multi-frequency dual-channel receiver

[0041] The dual-channel receiver contains two receivers: the main receiver (S) and the reference receiver (R). The reference feed source is located at the main focus and does not move. The feed horn of the main receiver can be laterally offset and scanned on the focal plane. The telescope antenna scans the transmitting source within a certain range, measures the phase between the two receiving paths, and outputs the voltage amplitude ratio. A series of coordinate transformations are performed on the focal plane field sampling data obtained by the scan, and then the deformation of the parabolic antenna surface can be obtained using the Fourier transform relationship.

[0042] (3) Multi-frequency dual-channel FFT cross-correlation calculation

[0043] The main channel receiving signal and the reference channel receiving signal enter the data acquisition and real-time FFT digital correlation system at the same time. The algorithm flow chart is as follows: Figure 2 As shown. The mathematical principle of the correlation machine is the multiplication process of two complex numbers. That is, Fourier transform is performed first, and then multiplication is performed. From the mathematical theorem, it can be known that the time domain convolution corresponds to the frequency domain conjugate multiplication. For the cross-correlation result of the above signal, Y is the time domain convolution, S(t) and R(t) are the time domain signals of the main receiving path and the reference receiving path respectively. Y S is the main frequency domain signal, Y R * is the conjugate of the reference channel frequency domain signal, and then Y is accumulated according to the integration time. Ic(k) and Qc(k) are the real and imaginary parts of the complex number after the effective channel signal is extracted.

[0044] This part is implemented in the PXIe-5785 intermediate frequency transceiver module through Labview programming. The entire FFT spectrum is extracted in the host computer of the digital acquisition correlator to obtain each frequency point of 1.5625MHz+n*3.125MHz, and the real and imaginary parts of each frequency point are saved.

[0045] 2. Cross-correlation data acquisition:

[0046] In order to obtain all the antenna surface data of the telescope, the telescope under test moves around the emission source in a raster scanning manner. A center calibration is performed before starting the scan, and after each scan line, the center calibration is performed again after returning to the center position until the scan is completed and the last center calibration is performed. The scanning trajectory of the telescope is as follows: Figure 3 The black dots indicate the sampling points, and the arrows represent the movement path of the antenna during the scanning process. (a)-(i) are the sampling points, and the trajectory is in the direction facing the telescope reflector. If the scanning grid size is an N*N matrix, the number of center calibrations is N+1. When the scanning grid size is a 45x45 matrix, it takes 33 minutes to complete a scanning measurement, and the number of calibrations is 46 times.

[0047] During the scanning process, the telescope needs to move in pitch and azimuth according to certain instructions. These instructions are issued by the telescope motion control computer, which communicates with the telescope control system through the serial port and sends the positions that the telescope will reach to the servo control module one by one through the serial port, so that the telescope can complete the scanning process. Figure 3 Scan in.

[0048] In addition, the telescope motion control computer also communicates with the digital acquisition correlator (NI system) through the serial port. The telescope's current row and column number, pitch and horizontal angle information are sent to the digital acquisition correlator computer. This information is manually defined by the developer. In practice, the communication protocol between the telescope motion control computer and the digital acquisition correlator is defined as follows:

[0049] * / RTREQ <sp> <line> <sp> <row> <sp> <adeckang> <sp> <edeckang> <sp> <counter> <sp> <cali>The protocol contains a total of 46 bytes, and the specific parsing content is as follows:

[0050]

[0051] The digital acquisition correlator uses Labview to write the data acquisition program of the host computer. This program mainly realizes three functions: (1) communicates with the antenna motion control computer through the serial port, receives the instructions sent by the telescope motion control computer, obtains the row and column number of the current telescope, the pitch and azimuth data of the current telescope and the current scanning status (row and column scanning or center calibration). (2) The FFT kernel module and operator, custom memory, FIFO, and queue management module of Labview are used to obtain the frequency domain complex values ​​after the FFT cross-correlation of the main receiving path and the reference receiving path through graphical programming. In this application, the number of spectral lines transmitted at the transmitting end is 384. For a spectral line of a certain frequency, the two-way cross-correlation generates a set of complex values ​​of real part plus imaginary part, so the data after the cross-correlation processing corresponding to the 384 spectral lines (transmitting frequency) is 384 complex values, that is, 384 real parts plus 384 imaginary parts. (3) Cross-correlation operation is performed at each raster scanning position to generate a series of complex data. Cross-correlation operations are performed on all grid scanning points, and the position information of the telescope is extracted together, and the cross-correlation data of all scans, including grid scanning and center point calibration data, are saved in the form of a txt file.

[0052] At the same time, for the convenience of data processing, the format of the generated txt file is also defined. In the normal row and column scanning state, the generated file name is scan.txt, and the file consists of multiple lines of data. Each ID corresponds to a line of data. The content of each line of data includes ID number, scanning row number, scanning column number, pitch angle, azimuth angle, and the real and imaginary data of the cross-correlation of each frequency point. The specific format is shown in the table below.

[0053]

[0054] When the scan is in the center calibration state, the generated file name is Cali.txt. Its file format is similar to the normal scan, but the row and column number of the telescope is fixed. The specific format is shown in the table below:

[0055]

[0056] 3. Processing of Multi-frequency Cross-correlation Data

[0057] The processing process mainly consists of the following steps:

[0058] Step 1: Average processing of scan data: average the data of repeated sampling at each sampling point in the scan file scan.txt to obtain 384 sets of real and imaginary data with 2025 (45*45) sampling points, and further obtain the corresponding complex numbers.

[0059] Step 2: Average processing of center calibration data: Perform the same operation on the center calibration data Cali.txt, that is, average the center calibration data to obtain 384 sets of real and imaginary data with 46 sampling points, and further obtain the corresponding complex numbers.

[0060] Step 3, fitting the center calibration data, and correcting the scan data with the fitting curve data: using the center calibration data obtained in step 2 to obtain the fitting curve data by interpolation, and then dividing the scan data processed in step 1 by the fitting curve data to obtain the corrected directivity pattern complex distribution function;

[0061] Step 4: Correct the image field rotation: that is, correct the influence of the additional optical path and parallax effect caused by the misalignment between the center of the telescope antenna aperture and the rotation center during the antenna movement. The correction method is to use the rotation matrix to perform phase correction on the data of the complex distribution function of the directional pattern obtained in step 3;

[0062] Step 5: According to the different distances from the center of the telescope antenna reflector aperture and the center of the reference horn to the beacon source, correct the near-field phase of the data obtained in step 4;

[0063] Step 6: Perform local interpolation on all the data corrected in step 5 to obtain the amplitude and phase distribution of the near-field two-dimensional radiation pattern;

[0064] Step 7: Perform inverse Fourier transform on the amplitude and phase distribution obtained in step 6 to obtain the telescope aperture field amplitude and phase distribution;

[0065] Step 8, correcting the axial focus error of the aperture field: that is, calculating the aperture field optical path difference introduced by the axial focus error according to the known axial focus error, and adding the optical path difference into the aperture field complex distribution function obtained in step 7 to obtain the aperture field phase distribution;

[0066] Step 9, correcting the influence of the phase distribution of the telescope prime focus feed horn on the aperture field phase distribution: that is, subtracting the phase distribution of the prime focus feed horn from the aperture field phase distribution obtained in step 8 to obtain a linear phase;

[0067] Step 10: Calculate the antenna surface error distribution diagram and its RMS value corresponding to each frequency point by using the surface fitting correction constant, linear phase, feed source offset, etc.

[0068] Step 11: Average the antenna surface error distribution diagrams corresponding to all the obtained frequency points to obtain the final telescope antenna surface error distribution diagram, and calculate its RMS.

[0069] The above are only preferred embodiments of the present invention, and the protection scope of the present invention is not limited to the above embodiments. All technical solutions under the concept of the present invention belong to the protection scope of the present invention. It should be pointed out that for ordinary technicians in this technical field, some improvements and modifications without departing from the principle of the present invention should be regarded as the protection scope of the present invention.< / cali> < / sp> < / counter> < / sp> < / edeckang> < / sp> < / adeckang> < / sp> < / row> < / sp> < / line> < / sp>

Claims

1. A near-field holographic measurement method for multi-frequency simultaneous cross-correlation of radio telescope antennas, characterized by: Using multi-frequency comb spectrum signals as the transmitting source, the multi-frequency RF signals are received simultaneously through the two receiving ends of the main receiving path and the reference receiving path. The multi-frequency frequency domain complex values ​​are obtained by performing FFT and cross-correlation operations on the two receiving ends. The multi-frequency frequency domain complex values ​​are then corrected and re-gridged, and inverted one by one to the telescope aperture field to further obtain the antenna surface error distribution map and surface error RMS corresponding to the multi-frequency signal. Finally, the average is taken to obtain the final radio telescope antenna surface error distribution and RMS. The reference receiving path is located at the main focus of the parabola antenna panel and does not move, and the main receiving path moves around the transmitting source in a raster scanning manner to receive the transmitting source signal; The specific method of the main receiving path moving around the transmitting source in a raster scanning manner is as follows: if there are N*N matrix sampling points, a center point is first determined, and before starting scanning, the main receiving path moves to the center point for a center calibration, and returns to the center point for a center calibration after each scanning of a row of sampling points, until the scanning is completed and the center point is finally returned to the center point for a center calibration, that is, the sampling points are scanned N*N times in total, and the number of center point calibrations is N+1; The multi-frequency comb spectrum signal emitted by the transmitting source includes M frequency points; The specific method for the processing unit to process and calculate the telescope antenna surface error distribution and RMS includes the following steps: Step 1, scanning data averaging processing: averaging the data of repeated sampling of each sampling point in the received information to obtain real and imaginary data of M groups of sampling points with N*N points, and further obtain the corresponding complex number; Step 2: Center calibration data averaging: average the center calibration data to obtain M groups of real and imaginary data with N+1 sampling points, and further obtain the corresponding complex numbers; Step 3, fitting the center calibration data, and correcting the scan data with the fitting curve data: using the center calibration data obtained in step 2 to obtain the fitting curve data by interpolation, and then dividing the scan data processed in step 1 by the fitting curve data to obtain the corrected directivity pattern complex distribution function; Step 4: Correct the image field rotation: that is, the telescope antenna aperture center and the rotation center do not coincide, which leads to the additional optical path and parallax effect during the antenna movement. The correction method is to use the rotation matrix to perform phase correction on the data of the complex distribution function of the directional pattern obtained in step 3. Step 5: According to the different distances from the center of the telescope antenna reflector aperture and the center of the reference horn to the beacon source, correct the near-field phase of the data obtained in step 4; Step 6: Perform local interpolation on all the data corrected in step 5 to obtain the amplitude and phase distribution of the near-field two-dimensional radiation pattern; Step 7: Perform inverse Fourier transform on the amplitude and phase distribution obtained in step 6 to obtain the telescope antenna aperture field amplitude and phase distribution; Step 8, correcting the axial focus error of the aperture field: that is, calculating the aperture field optical path difference introduced by the axial focus error according to the known axial focus error, and adding the optical path difference into the aperture field complex distribution function obtained in step 7 to obtain the aperture field phase distribution; Step 9, correcting the influence of the phase distribution of the prime focus feed horn of the measured telescope antenna on the aperture field phase distribution: that is, subtracting the phase distribution of the prime focus feed horn from the aperture field phase distribution obtained in step 8 to obtain a linear phase; Step 10: Calculate the antenna surface error distribution diagram and its RMS value corresponding to each frequency point by using the surface fitting correction constant and linear phase, and the contribution of feed source offset; Step 11: Average the obtained telescope antenna surface error distribution diagrams corresponding to all frequency points to obtain the final antenna surface error distribution diagram, and calculate its RMS.

2. The radio telescope antenna multi-frequency simultaneous cross-correlation near-field holographic measurement method according to claim 1 is characterized by: The distance R between the transmitting source and the receiving end satisfies: , where D is the aperture of the antenna under test, λ is the operating wavelength, and the position of the transmitting source is fixed during the measurement process.

3. The radio telescope antenna multi-frequency simultaneous cross-correlation near-field holographic measurement method according to claim 1 is characterized by: For a certain frequency point, after the FFT cross-correlation between the main receiving path and the reference receiving path, a set of frequency domain complex values ​​of real part and imaginary part is generated. Therefore, after the FFT cross-correlation between the main receiving path and the reference receiving path, a total of M frequency domain complex values ​​are generated. The digital acquisition correlator uses the FFT kernel module, operator, custom memory, FIFO, and queue management module of Labview to obtain the frequency domain complex values ​​after the FFT cross-correlation between the main receiving path and the reference receiving path through graphical programming; At each raster scan position, an FFT cross-correlation operation is performed between the main receiving path and the reference receiving path to generate a series of complex data.

4. The radio telescope antenna multi-frequency simultaneous cross-correlation near-field holographic measurement method according to claim 3 is characterized by: During the scanning process, the main receiving path performs pitch and azimuth movements according to the instructions. The main receiving path antenna saves the grid number, pitch, horizontal angle information and received multi-frequency signal information and sends them to the processing unit to calculate the surface error distribution and RMS of the radio telescope antenna under test.

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