HIGH ACCURACY DRAG-FREE CONTROL FOR THE MICROSCOPE AND LISA MISSIONS

摘要

The recent development of high-accuracy drag-free techniques has been made possible by the emergence of new ultra-sensitive sensors and accurate actuators, and thus several space missions for Science requiring very low level of non-gravitational accelerations are now envisaged by space agencies. The extremely high level of accuracy required by this type of scientific missions require a specific design process for the Drag-Free and Attitude Control System (DFACS). Astrium SAS has been in charge of the DFACS definition and design for numerous drag-free projects, with the STEP and GOCE Phase A studies in the late nineties, and more recently with the Phase A study of the ESA LISA cornerstone mission, and with the phase A of the μScope (Microscope) mission, initiated by CNES and ONERA. This paper will present the common DFACS design methodology used for these two missions, the resulting design and the performances demonstrated through simulations. μScope is a fundamental physics mission derived from the original STEP mission concept (testing the Equivalence Principle with an accuracy of 10~(15)) proposed to be flown by CNES as part of the "Myriade" microsatellite programme. The μScope DFACS is based on the technology of capacitive inertial sensors, designed for this mission as cylindrical differential accelerometers, and on accurate proportional thrusters, the FEEP (Field Emission Electric Propulsion) thrusters. The main DFACS performance is specified as a spectrum level- 3.10~(-10) m/s~2/Hz~(1/2) -in a very narrow frequency band around the frequency of possible violation of the Equivalence Principle (F_(EP), equal to 7.9 10~(-4) Hz). The feasibility of the DFACS design proposed by Astrium SAS was consolidated thanks to detailed time simulations, allowing the verification of the performances of the design in representative conditions, including all types of disturbing noises and inter-axes coupling. The NASA-ESA joint mission LISA - dedicated to the detection in space of gravitational waves through laser interferometry between three satellites separated by 5 10~6 km - sets quite challenging requirements on the translation & attitude control of each spacecraft. Indeed, the scientific objective, interferometer arm length measurement with an accuracy of 20 pm/Hz~(1/2) over the MBW (Measurement Bandwidth, from 0.1 mHz to 0.1 Hz) translates into spacecraft pointing and stability needs (30 nrad & 8 nrad/Hz~(1/2)) orders of magnitude better than current high pointing accuracy systems. The LISA mission shares with μScope the same actuators, FEEP thrusters, and the same type of inertial sensor technology, although utilised here as proofmasses rather than accelerometers. The drag-free control needs are quite unique, since the spacecraft shall track the two isolated test masses supporting the interferometer reference mirrors with an accuracy of 2.5 nm/Hz~(1/2) in the MBW. The DFACS involves 19 degrees of freedom (6 for the S/C and 6 for each test mass plus the angle between the two telescopes), resulting in a quite complex control architecture with intricate loops. A SISO (Single-Input/Single-Output) design approach was however shown to be adequate, and validated through a detailed simulation of the 2D problem that allows capturing coupling effects while reducing the number of DOFs to 10.