Damping stochastic noise in the Lisa drag-free control system

The Laser Interferometer Space Antenna (LISA) is a proposed joint mission for NASA and ESA. It is envisaged that the interferometer will be able to detect for the first time the tiny distortions in fabric of space-time predicted in Einstein's general relativity and known as gravitational waves. One way in which gravitational waves manifest themselves is as small fluctuations in the spatial dimensions of the order of 10 -21, so small the detection is only possible with extremely sensitive instruments and in an environment where disturbances are kept to an acceptably low level. Such an environment is provided in the vacuum of deep space, far away from the Earth and its influence. In order to detect the waves, test-masses, which act as the end points of the interferometer arms, must be made to move to an unprecedented accuracy without drag-like forces acting upon them. This requires a concept known as drag-free control. The LISA drag-free control system involves the removal of accelerative disturbances to better than 10 -15 m s -2 Hz -1/2. At this level of noise removal, stochastic noise sources, such as noise in the position sensors, need to be taken into account. A dynamical model of the LISA system with a single degree of freedom has been developed. It will be shown that the stochastic noise sources feed into the forced vibrational frequencies of the system making it unstable unless damping is applied. It will be shown that required system performance can be achieved with a specified amount of damping and filtering. © 1999 Lister Science.