On-orbit structural dynamic performance of a large-diameter antenna
An analysis of the structural vibrational performance of a 15-meter-diameter antenna subject to representative onboard dynamic disturbances is presented. Antenna performance parameter limits are defined for the root-mean-square surface roughness, pointing error, and defocus. The antenna concept is described, and the generation of the finite-element model is explained. A subreflector scanning scenario which represents an onboard disturbance to the system is modeled with orthogonal excitation functions. A modal analysis performed on the antenna in both free-flying and platform-mounted configurations is discussed. The resulting mode shapes and natural frequencies are then input to a forced-response analysis, which is performed for each configuration with the onboard scanning disturbance. The impact of the resulting dynamic distortions on the antenna performance parameters (i.e., errors) is assessed. The distortions in the surface are determined to contribute to all three errors, the displacement of the subreflector adds to the pointing error and defocus, and the displacement of the vertex is a component of the defocus.
Based on the results of the analysis, the reflector support structure and feed mast of the low-frequency microwave radiometer as designed are capable of maintaining their shapes within specifications for the assumed on-orbit disturbance. This is particularly true for the platform-mounted configuration which exhibits errors within their respective limits by at least an order of magnitude. This is also true for the free-flyer configuration surface roughness and defocus; however, the free-flyer configuration maximum pointing error is significantly closer to, yet still within, its specified limit.