The Cassini planetary mission to Saturn and Titan requires accurate pointing and complex maneuvers of the spacecraft, which is comprised of a Saturn orbiter and Titan probe. An attitude control design is described to achieve the many capabilities of Cassini including thrust vector control, precision pointing and stability control fc)r remote sciences, tracking of a probe released to the Titan atmosphere, and radar mapping of the Titan surface. Additional constraints for control design have been imposed by a high percentage of liquid propellant (60.% at launch) and lowly damped flexible spacecraft appendages. Computer simulation results of attitude determination, pointing and maneuver controls are provided to demonstrate the design capability. I 1.2 Spacecraft Configuration and Modeling 1 Cassini is a flexible spacecraft containing four structural booms and three propellant tanks. The deployed Cassini spacecraft with the Huygens Titan probe is shown in Fig. 1. The four attached booms are a magnetometer boom and thr~e radio and plasma wave. science booms. The middle structure of the spacecraft is the propulsion module consisting of two elliptical tanks for bipropellant. At the bottom of the propulsion module is the lower equipment module, which supports three radio-isotope thermoelectric generators, four reaction wheels, a probe relay antenna, and two articulable (two axis) main engines for large trajectory corrections and Saturn orbit insertion. At launch, the spacecraft total mass is approximately 5570 Kg and inertia is [8970, 9230, 3830] Kg-mz. Towards the end of the Saturn tour phase with the probe released, the mass will be reduced to 2700 Kg and inertia to [6510, 5290, 3478] Kg-m2" The spacecraft is modeled by a flexible core structure but with rigid appendages (propellant masses, main engine gimbals, and reaction wheels). The equations describing the flexible core body are generated from Nastran model data. Due to the absence of articulable rigid element at the end of the flexible booms, no modal reduction process of any significance is necessary. The magnetometer boom has a fundamental frequency at 0.65 Hz and damping between 0.2 to 1.0 %. The three radio and plasma wave antennas are much lower in mass and inertia, and have a frequency of 0.13 Hz or higher and a damping of 0.2 %. At the beginning of the mission, the spacecraft propellant is heavier than the spacecraft dry mass. The spacecraft has two bipropellant tanks and one monopropellant tank. The bipropellants, burned by the main engine and has total mass of 3000 Kg, are monomethyl hydrazine (MMH) and nitrogen tetroxide (NTO). The tanks are cylindrical with hemispheric end domes, and "stacked" along the spacecraft Z axis. The monopropellant is hydrazine and burned by the attitude control thrusters. Its mass is 132 Kg. The tank is spherical and off the Z axis. The bipropellant tanks contain an 8-vane propellant management device. The spacecraft is in "low-g" environment when the spacecraft is limit cycling or during thruster maneuvers. When the spacecra...

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For segmented large mirrors to be used effectively for astronomy they must be actively aligned and controlled to extreme levels of precision. We consider the figure control problem for a spaceborne far-IR telescope, the Precision Segmented Reflector Project Focus Moderate Mission Telescope. We propose a two-stage approach. A figure initialization controller is used to achieve initial phasing and alignment of the telescope using an imaging science detector. A figure maintenance controller keeps the telescope aligned during normal operation using a laser metrology optical truss sensor system. We show that performance of any figure control system is subject to limits on the controllability of the wavefront. Maintenance controllers are additionally limited by considerations of the observability of the wavefront from the maintenance sensors. We show preliminary results for the figure initialization controller. We present a "Wavefront Compensation" method for figure maintenance control that minimizes wavefront errors due to misalignment errors.

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