Development of an attitude control system for a balloon-borne gamma ray telescope Original Research Article

We describe the attitude control system employed by the MASCO balloon-borne gamma ray telescope and present the first laboratory tests of this system. The MASCO experiment is a low energy gamma ray imaging telescope that employs a MURA coded mask. Its angular resolution is 14′ over a 13° field of view, which requires a pointing accuracy of some arcminutes in order to allow the accomplishment of the scientific goals. The attitude control system was designed to provide pointing and stabilization of a 1400 kg, 7 m-high, 2 m-wide, 2 m-deep gondola that encompasses the telescope detector system and its associated electronics. The sensors used are an electronic magnetic compass, an encoder, a two-axis solar sensor, a sun-tracker, two accelerometers, a gyroscope and three CCD stellar sensors. A GPS system is also employed. The actuators are a reaction wheel for azimuth control, an azimuth motor located in the balloon-gondola decoupling mechanism for desaturation of the reaction wheel, and a telescope elevation motor with a harmonic drive reduction. The software architecture and the operation modes are also presented.