High Accuracy Space Structures Monitoring by a Close-range Photogrammetric Network

The Planck mission of the European Space Agency (ESA) is designed to image the anisotropies of the Cosmic Background Radiation Field over the whole sky. To achieve this aim, sophisticated reflectors are used as part of the Planck telescope receiving system. The system consists of secondary and primary reflectors which are sections of two different ellipsoids of revolution with mean diameters of 1 and 1.6 meters. Deformations of the reflectors which influence the optical parameters and the gain of receiving signals are investigated in vacuum and at very low temperatures. For this investigation, among the various high accuracy measurement techniques, photogrammetry was selected. With respect to the photogrammetric measurements, special considerations should be taken into account in different steps of design and processing, such as determinability of additional parameters under the given network configuration, datum definition, reliability and precision issues as well as workspace limits and propagating errors from different sources. We have designed an optimal close-range photogrammetric network by heuristic simulation for the primary and secondary reflectors with a relative precision better than 1:1,000,000 to achieve the requested accuracies. A least squares best-fit ellipsoid was developed to determine the optical parameters of the reflector. In this paper we will report about our network design and the results of real measurements based on the tests executed by Alcatel Alenia Space France (AASF) under European Space Technology and Research Center (ESTEC) contract in vacuum and under very low temperatures.