Active Surface Compensation for Large Radio Telescope Antennas

Hindawi International Journal of Antennas and Propagation Volume 2018, Article ID 3903412, 17 pages https://doi.org/10.1155/2018/3903412 Review Article Active Surface Compensation for Large Radio Telescope Antennas 1,2 1 1 3 3 1 2 Congsi Wang , Haihua Li , Kang Ying, Qian Xu, Na Wang, Baoyan Duan, Wei Gao, 4 5 Lan Xiao, and Yuhu Duan 1Key Laboratory of Electronic Equipment Structure Design, Ministry of Education, Xidian University, Xi’an 710071, China 2School of Civil and Environmental Engineering, University of New South Wales, Sydney, NSW 2052, Australia 3XinJiang Astronomical Observatory, China Academy of Sciences, Urumqi 830011, China 4Huawei Technologies Co. Ltd., Shenzhen 518129, China 5China Electronics Technology Group Corporation No. 39 Research Institute, Xi’an 710065, China Correspondence should be addressed to Congsi Wang; [email protected] and Haihua Li; [email protected] Received 21 September 2017; Revised 22 December 2017; Accepted 8 February 2018; Published 29 March 2018 Academic Editor: Giorgio Montisci Copyright © 2018 Congsi Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. With the development of radio telescope antennas with large apertures, high gain, and wide frequency bands, compensation methods, such as mechanical or electronic compensation, are obviously essential to ensure the electrical performance of antennas that work in complex environments. Since traditional compensation methods can only adjust antenna pointing but not the surface accuracy, which are limited for obtaining high surface precision and aperture efficiency, active surface adjustment has become an indispensable tool in this field. Therefore, the development process of electrical performance compensation methods for radio telescope antennas is introduced. Further, a series of analyses of the five key technologies of active surface adjustment is presented. Then, four typical large antennas that have been designed with active main reflector technology are presented and compared. Finally, future research directions and suggestions for reflector antenna compensation methods based on active surface adjustment are presented. 1. Introduction large, medium, and small sizes of ground-based radio telescope antennas should be redefined. An antenna whose aper- A radio telescope is a precise instrument designed to mainly ture is smaller than ten meters in diameter can be classified as receive radio waves from celestial bodies, which can radiate small, an antenna whose aperture is from ten meters to thirty electromagnetic waves. There are two primary components meters in diameter can be classified as medium-sized, an that make up a radio telescope: the antenna and the receiving antenna whose aperture is from thirty meters to one hundred system, in which a large aperture antenna is its main compo- meters in diameter can be classified as large, and an antenna nent [1, 2]. Each significant advancement in radio telescope whose aperture is larger than one hundred meters in diame- antennas has been a milestone in the development of astron- ter can be classified as ultra-large. So far, more than twenty omy, without exception. As the most commonly used radio large and medium-sized radio telescopes have been built telescope antennas, reflector antennas [3, 4] can easily imple- around the globe in various forms [7–9], including fixed ment a large aperture and narrow beam, with high resolution single-aperture spherical radio telescopes, non fully steerable and high sensitivity; hence, they are widely used in the fields standard parabolic radio telescopes, fully steerable antenna- of radio astronomy, radar, communication, and space explo- shaped radio telescopes, and telescope arrays. A large radio ration [5, 6]. In the past, the aperture of a large ground-based telescope (hereinafter referred to as a “large antenna”) oper- radio telescope antenna was usually greater than or equal to ating in a complex environment is affected by gravity, tem- ten meters. With the rapid development of modern science perature, wind, and other factors, which lead to structure and technology, the antenna design and manufacture and distortion. Further, there are random errors related to manu- feeding technology have been greatly upgraded; hence, the facture, installation, and so on. These factors together result 2 International Journal of Antennas and Propagation in reflector surface deformation of the antenna, which reflector is not adjustable, the surface accuracy of the reflector degrades surface accuracy and electric performance through cannot be adjusted using only traditional compensation gain loss, pointing error, and sidelobe degradation [10–17]. methods. The traditional compensation methods first calcu- Therefore, it is necessary to adjust the antenna structure late the best fitting surface after the deformation of the main and shape to compensate for the loss of electric performance. reflector and then adjust the azimuth and elevation of the For this purpose, the paper discusses the development reflector to, respectively, move the main reflector to the best process of electric performance compensation methods for fit surface and the subreflector to the matching position of radio telescopes, presents the characteristics of large antennas the best fit surface. In light of the changes, traditional com- with the application of the active surface adjustment, summa- pensation methods, such as moving the main reflector and rizes the key active adjustment technologies, compares typi- subreflector to the matching positions of the best-fit parabo- cal large antennas using active adjustment for the antenna loid, are ineffective. The surface shape of the main reflector main reflectors, and eventually discusses the structure design must be adjusted, which is to say that the antenna electric program and research proposal for active surface compensa- performance will be compensated by realizing active adjust- tion of the QiTai Telescope (QTT) antenna in Xinjiang, ment of the main reflector. As shown in Figure 1, to further China. The presented content provides a technical reference improve antenna surface accuracy, a variety of surface shape for performance assurance and improvement of ultra-large control methods are proposed in the design and manufacture radio telescopes. period of large antennas [22–25], such as rigid design, homology design, radome or package handling, and opti- 2. Development History of Performance mum presetting angle installation. Up to now, all large anten- Compensation for Radio Telescope Antennas nas in the gigahertz band worldwide have been integrated with rigid design, structural homologous design, and the According to the classical Ruze formula [18], antenna techni- active surface adjustment to ensure that the antenna surface cians thought that the most critical issue was to ensure the accuracy reaches the millimeter level. Therefore, active sur- surface accuracy for satisfactory electric performance of radio face design is the trend in antenna construction, which is also telescope antenna. Futhermore, under situation of the same an important technique to realize high steering accuracy. antenna efficiency, it is becoming more and more difficult for large-aperture high-frequency antennas to achieve an 3. Active Surface Compensation acceptable machined surface on account of the extremely Technology System demanding surface accuracy requirements. Later, to reduce the difficulty of panel processing, antenna During observation periods, an antenna is influenced by designers adopted not only rigid materials for antenna fabri- complex environmental factors. With changes in the anten- cation, but also precise antenna structure design strategies, na’s azimuth and elevation attitude, the antenna surface such as “homologous design” [19, 20], to improve the main topography changes in real time. It is imperative to adopt surface deformation at various elevations and ensure electric active adjustment for large antennas operating at high fre- performance. For large antennas in operation, the frequently quencies [26, 27]. used methods for compensating electric performance include Figure 2 presents the realization process of the active sur- mechanical and electronic compensation techniques, among face adjustment of large antennas. As a large antenna is other approaches [21]. Mechanical compensation techniques observing, its surface is measured in real time by modern sur- include main reflector, subreflector, and deformable plate face detection technology. Then, the actual surface deforma- compensation. Electronic compensation mostly refers to tion information is transferred to the main control computer, feed array compensation. A comparative analysis of various which can provide the panel adjustment amount through a compensation methods shows that the main reflector com- certain mathematical algorithm. Next, the control network pensation method is the best option, which can provide the of the active surface control system will adjust actuators in best integrated performance. The subreflector compensa- various positions to make mechanical motions by remote tion method can generate good results when the structural control. Thus, the position of each reflector panel can be fine distortion is not very grave, and deformable plate compen- adjusted, and eventually the surface shape of the antenna will sation can improve antenna performance to

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