Research On Surface Accuracy Measurement Of Large Radio Telescope Using Broadband Microwave Holography

Surface accuracy measurement methodology is one of the key technologies of constructing and operating large radio telescope,and microwave holography is a main development trend of all the surface accuracy measurement methodologies.Since the narrowband microwave holographic method,which is a category of microwave holography,was not able to measure the surface accuracy of large radio telescopes operated at any elevation angle,this paper will focus on studying how to use broadband microwave to measure the surface accuracy.The research contents are as follows:Firstly,the surface accuracy measurement of 40 m radio telescope in Ground Radio Antenna System-3(GRAS-3)will be taken as the test radio telescope(the research object).Considering that in broadband microwave holography,the signal of a cosmic source is much weaker than that of a geostationary satellite,noise effects would be significant if no necessary actions was taken.Therefore,the GRAS-1 50 m telescope will be necessarily used as the reference radio telescope to increase signal-to-noise ratio(SNR).By analyzing the traditional scan process of test radio telescope,microwave propagation path delay calculation algorithm,and holographic data processing method,a new scan process is proposed.In the new scan process,each scan will pass through the aperture center of the test radio telescope.Based on the new scan process,a general train of thought about microwave propagation path delay calculation and holographic data processing are illustrated.Then,according to the general train of thought mentioned above,deep studies are carried out on the broadband microwave propagation path delay calculation.A new microwave propagation absolute path delay calibration algorithm,and a novel fractional time delay estimation algorithm are designed to solve the problem that microwave propagation path delay changes slowly due to the rotation of the radio source relative to the earth.Meanwhile,in this section,the data-processing capacity of Frequency Domain Zero-Padding method and Cross-Spectrum method are compared based on computer simulation.According to piecewise processing idea,the broadband signal is decomposed into several parts.The final time delay and phase delay are calculated based on the piecewise processing results.These works would be helpful for solving the difficulties and complexities of broadband microwave holographic data processing.Finally,an experiment is carried out at Miyun station of National Astronomical Observatories of China(NAOC)by using a set of programs of scan outline,time delay,and data decomposing,which were written before.A hardware system of measurement experiment consists of two-channel receiver,transmission cable,radio signal recorder,inclinometer is established.The surface accuracy of GRAS-3 40 m telescope obtained is about 0.42 mm,and the average error is less than 0.12 mm.The result is in accordance with that of photogrammetry,which means that the method used in this paper is feasible.The broadband microwave holographic method studied in this paper,has the advantages of high utilization of scanning points,short measurement time,and having the ability of measuring the surface accuracy of large radio telescopes operated at any elevation angle.It is expected to use the same method studied here to measure the surface accuracy of GRAS-4 70 m telescope.This paper also provides a reference for the surface measurement of other large radio telescopes.