Tracking and Pointing of Target by a Bifocal Relay Mirror Spacecraft Using Attitude Control and Fast Steering Mirrors Tilting

Abstract

The present paper reports the results of numerical simulations carried out on a model of the Bifocal Relay Mirror spacecraft. This spacecraft consists of two large mechanically and optically coupled telescopes, used to redirect a laser beam from a ground-based, airborne or spacecraft based source to a distant target point on the earth or in space. The two telescopes are gimbaled and the spacecraft inertia has a large variation during the angle maneuvers needed to maintain the laser cross link. Moreover the spacecraft has very tight pointing and jitter requirements. The task of the presented simulations was to preliminarily validate and compare two different control approaches proposed for the tracking and pointing of the target of the Bifocal Relay Mirror. The attitude control system consists of reaction wheels, star trackers and rate gyros. The optical control system consists of two two-axis fast steering mirrors and two optical tracker sensors. In the first control option considered, feedforward and PD feedback are used for the spacecraft attitude control, while PID feedback is used for the optical subsystem, in order to compensate the pointing error. In the second control approach, the spacecraft and the optical control systems are integrated.