Attitude Control System of the MASCO Balloon-Borne Gamma Ray Telescope.

Abstract

We describe the attitude control system of the MASCO experiment. The MASCO project is a balloon-borne gamma ray telescope in final phase of development at the National Institute for Space Research (INPE - Brazil). This experiment is a low energy gamma ray telescope that employs a Modified Uniformly Redundant Array (MURA) coded mask in an one-piece mask-antimask configuration. This is the first experiment to use such a mask pattern (MURA) and configuration for astrophysical purposes. The angular resolution is approximately 14' over 13 deg. field of view, which requires that the expected pointing accuracy of the attitude control system should be better than that in order to allow the accomplishment of the scientific goals. Several kinds of sensors (an electronic compass, an axis encoder, a two-axes solar sensor, two accelerometers, a gyroscope and two CCD cameras) are used by this system for redundancy and some of them achieve a precision better than 0.1 deg. The main actuators are: a reaction wheel for azimuth control, an azimuth motor located in the gondola-balloon decoupling mechanism for desaturation of the reaction wheel and a telescope elevation motor with a harmonic drive reduction. The control system software architecture consists basically of two main files, one with the main calculation routines and the other with the interface routines. The main program starts with the initialization routines and enters a loop of sequential execution of tasks. For each operation mode there is a different sequence of routines. 2.