Two-step Structural Design of Mesh Antennas for High Beam Pointing Accuracy

A well-designed reflector surface with high beam pointing accuracy in electromagnetic performance is of practical significance to the space application of cable mesh reflector antennas. As for space requirements, cir- cular polarizations are widely used in spaceborne antennas, which usually lead to a beam shift for offset reflectors and influence the beam pointing accuracy. A two-step structural design procedure is proposed to overcome the beam squint phenomenon for high beam pointing accuracy design of circularly polarized offset cable mesh reflectors. A simple structural optimal design and an integrated structural electromagnetic optimization are combined to alleviate the beam squint effect of circular polarizations. It is imple- mented by cable pretension design and adjustment to shape the offset cable mesh surface. Besides, in order to increase the efficiency of integrated optimization, an update Broy- den-Fletcher-Goldfarb-Shanno （BFGS） Hessian matrix is employed in the optimization iteration with sequential quadratic programming. A circularly polarized offset cable mesh reflector is utilized to show the feasibility and effectiveness of the proposed procedure. A high beam pointing accuracy in order of 0.0001~ of electromagnetic performance is achieved.

Effect of the alidade thermal behavior on the pointing accuracy of a large radio telescope

The alidade’s non-uniform temperature field of a large radio telescope is very obvious under solar radiation.Estimating a radio telescope’s pointing errors,caused by the alidade deformation under solar radiation,is significant to improve the telescope’s pointing accuracy.To study the effect of the alidade thermal behavior on the pointing accuracy of a large radio telescope,a temperature experiment is first carried out in a 70-m radio telescope on a sunny day.According to the measured results,the temperature distribution rule of the alidade is summarized initially.In addition,the alidade’s temperature field is calculated by finite element thermal analysis.The simulated results are proved to be in good agreement with the experimental results.Finally,the alidade deformation under solar radiation is computed by finite element thermalstructure coupling analysis.The telescope’s pointing errors caused by alidade deformation are estimated via the alidade’s node displacements.The final results show that the effect of alidade thermal behavior on the telescope’s elevation pointing errorsΔε2+Δεr is much more than the effect on the telescope’s crosselevation pointing errorsΔε1.The maximum ofΔε2+Δεr is more than 45″,while the maximum ofΔε1 is less than 6″.This study can provide valuable references for improving the pointing accuracy of large radio telescopes.

A Reverse-Design Strategy for the Track Error of the Qi Tai Telescope Based on Pointing Accuracy

The Qi Tai Telescope(QTT),which has a 110 m aperture,is planned to be the largest scale steerable tele-scope in the world.Ideally,the telescope’s repeated pointing accuracy error should be less than 2.5 arc seconds(arcsec);thus,the telescope structure must satisfy ultra-high precision requirements.In this pur-suit,the present research envisages a reverse-design method for the track surface to reduce the difficulty of the telescope’s design and manufacture.First,the distribution characteristics of the test data for the track error were verified using the skewness coefficient and kurtosis coefficient methods.According to the distribution characteristics,the azimuth track error was simulated by a two-scale model.The error of the long period and short amplitude was characterized as large-scale and described by a trigonometric function,while the short period and high amplitude error was characterized as small-scale and simulated by a fractal function.Based on the two-scale model,effect of the error on the pointing accuracy was deduced.Subsequently,the relationship between the root mean square(RMS)of the track error and the RMS of the pointing accuracy error of the telescope was deduced.Finally,the allowable RMS value of the track error was derived from the allowable pointing accuracy errors.To validate the effectiveness of the new design method,two typical radio telescopes(the Green Bank Telescope(GBT)and the Large Millimeter Telescope(LMT))were selected as experimental examples.Through comparison,the theoretical calculated values of the pointing accuracy of the telescope were consistent with the measured values,with a maximum error of less than 10%.

Gain adaptive sliding mode controller based on interval type-Ⅱfuzzy neural network designed for attitude control for micro aircraft vehicle

Purpose–Micro aerial vehicle is nonlinear plant;it is difficult to obtain stable control for MAV attitude due to uncertainties.The purpose of this paper is to propose one robust stable control strategy for MAV to accommodate system uncertainties,variations,and external disturbances.Design/methodology/approach–First,by employing interval type-II fuzzy neural network(ITIIFNN)to approximate the nonlinearity function and uncertainty functions in the attitude angle dynamic model of micro aircraft vehicle(MAV).Then,the Lyapunov stability theorem is used to testify the asymptotic stability of the closed-loop system,the parameters of the ITIIFNN and gain of sliding mode control can be tuned on-line by adaptive laws based on Lyapunov synthesis approach,and the Lyapunov stability theorem has been used to testify the asymptotic stability of the closed-loop system.Findings–The validity of the proposed control method has been verified through real-time experiments.The experimental results show that the performance of interval type-II fuzzy neural network based gain adaptive sliding mode controller(GASMC-ITIIFNN)is significantly improved compared with conventional adaptive sliding mode controller(CASMC),type-I fuzzy neural network based sliding mode controller(GASMC-TIFNN).Practical implications–This approach has been used in one MAV,the controller works well,and which could guarantee the MAV control system with good performances under uncertainties,variations,and external disturbances.Originality/value–The main original contributions of this paper are:the proposed control scheme makes full use of the nominal model of the MAV attitude control model;the overall closed-loop control system is globally stable demonstrated by Lyapunov stable theory;the tracking error can be asymptotically attenuated to a desired small level around zero by appropriate chosen parameters and learning rates;and the MAV attitude control system based on GASMC-ITIIFNN controller can achieve favourable tracking performance than GASMC-TIFNN and CASMC.