Non-uniform temperature distribution of the main reflector of a large radio telescope under solar radiation

The non-uniform temperature distribution of the main reflector of a large radio telescope may cause serious deformation of the main reflector, which will dramatically reduce the aperture efficiency of a radio telescope. To study the non-uniform temperature field of the main reflector of a large radio telescope, numerical calculations including thermal environment factors, the coefficients on convection and radiation, and the shadow boundary of the main reflector are first discussed. In addition, the shadow coverage and the non-uniform temperature field of the main reflector of a 70-m radio telescope under solar radiation are simulated by finite element analysis. The simulation results show that the temperature distribution of the main reflector under solar radiation is very uneven, and the maximum of the root mean square temperature is 12.3°C. To verify the simulation results, an optical camera and a thermal imaging camera are used to measure the shadow coverage and the non-uniform temperature distribution of the main reflector on a clear day. At the same time, some temperature sensors are used to measure the temperature at some points close to the main reflector on the backup structure. It has been verified that the simulation and measurement results of the shadow coverage on the main reflector are in good agreement, and the cosine similarity between the simulation and the measurement is above 90%. Despite the inevitable thermal imaging errors caused by large viewing angles, the simulated temperature field is similar to the measured temperature distribution of the main reflector to a large extent. The temperature trend measured at the test points on the backup structure close to the main reflector without direct solar radiation is consistent with the simulated temperature trend of the corresponding points on the main reflector with the solar radiation. It is credible to calculate the temperature field of the main reflector through the finite element method. This work can provide valuable references for studying the thermal deformation and the surface accuracy of the main reflector of a large radio telescope.

Deformation measurement by single spherical near-field intensity measurement for large reflector antenna

This paper presents a new method to obtain the deformation distribution on the main reflector of an antenna only by measuring the electric intensity on a spherical surface with the focal point as the center of the sphere, regardless of phase. Combining the differential geometry theory with geometric optics method, this paper has derived a deformation-intensity equation to relate the surface deformation to the intensity distribution of a spherical near-field directly. Based on the finite difference method (FDM) and Gauss-Seidel iteration, deformation has been calculated from intensity simulated by geometrical optics (GO) and physical optics (PO) methods, respectively, with relatively small errors, which prove the effectiveness of the equation proposed in this paper. By means of this method, it is possible to measure the deformation only by scanning the electric intensity of a single hemispherical near-field whose area is only about 1/15 of the aperture. The measurement only needs a plane wave at any frequency as the incident wave, which means that both the signals from the outer space satellite and the far-field artificial beacon could be used as the sources. The scanning can be realized no matter what attitude and elevation angle the antenna is in because the size and angle of the hemisphere are changeable.

Optimization of 110 m Aperture Fully Steerable Radio Telescope Prestressed Back Frame Structure Based on a Genetic Algorithm

Radio telescopes play an important role in lunar exploration projects, manned space flight projects, and navigation systems. China is constructing a giant 110 m aperture ground-based fully steerable radio telescope in Qitai County, Xinjiang Uygur Autonomous Region. In this paper, a 110 m aperture fully steerable radio telescope prestressed back frame structure is proposed and optimized to improve the reflector accuracy and to reduce the weight of the telescope. First, prestressed cables are introduced into the back frame structure, and three innovative cable layout schemes are presented. Second, for stress state analysis, the wind pressure distribution on the main reflector is explored using wind tunnel experiments. Third, some improvements in genetic algorithms for addressing computational complexity are explained. Finally, the effects of prestressed cables on the weight reduction and reflector accuracy improvement are analysed. Additionally, in order to evaluate the safety of the prestressed back frame structure, its strength has been checked, and the internal force and displacement under static conditions and in earthquakes are interpreted in detail.

Wind resistance calculation of large rotary mesh antenna and its influence on dynamic balance precision

Due to the wind resistance which acts on the main reflector of large rotary mesh antenna, the correct balancing result of satellite antenna is difficult to be gotten in the ground dynamic balancing test. In order to solve this problem, the dynamic balance method of large rotary mesh antenna which is under the influence of wind resistance in both low pressure environment and standard atmospheric pressure environment on the ground is studied. Based on the theoretical analysis and the experimental data of two-dimensional flow around circular cylinder, a new method of the large rotary mesh antenna wind resistance calculation is proposed, according to the CFD analysis of the three dimensional flow field. Through the dynamic equivalent method, the distributed wind resistance acted on the main reflector of the mesh antenna in the rotating state is equivalent to the principal vector and principal moment of the action point in each quadrant, and then transformed into the eccentric mass on the distribution plane. It provides a feasible and innovative way to estimate the influence of wind resistance on the dynamic balance accuracy of large mesh antenna, so as to compensate the wind resistance effect. Combined with the ground dynamic balancing requirements of a certain type of satellite mesh antenna, the whole finite element model of the mesh antenna is established, the simulation of ground dynamic balancing test is carried out, and the influence of wind resistance on the ground dynamic balancing results of the antenna is analyzed in this study, which provides important data for compensating the influence of wind resistance and ensuring the on-orbit balancing accuracy of the antenna.

Fault Tolerance for Active Surface System with Actuator Faults

The QiTai Radio Telescope (QTT) will be equipped with the active surface adjustment system (ASAS) to correct the main reflector deformation caused by environmental loading. In order to guarantee the stability and performance of the active surface system under fault conditions, it is necessary to adopt the fault-tolerant method when actuator faults have occurred. In this paper, a fault control method based on actuator faults weighting is proposed to solve the active surface fault control problem. According to the coordinates of the adjustable points of the panels corresponding to the faulty actuators, a new paraboloid is fitted by a weighted health matrix, and the fitting surface is taken as the target to adjust the surface shape.