LISA L3 gravity wave observatory: Non-linear modelling and possible DFAC methods

Recently, we witnessed the revolutionary discovery of gravitational waves (GW) by a ground-based laser interferometric observatory: a potentially game-changing observation tool in astronomy. Hence, the opportunity of setting up a space-based GW observatory, including their low-frequency spectrum not accessible from the ground, is gaining more and more support. In this framework, the Laser Interferometer Space Antenna (LISA) mission has been already selected, within the European Space Agency selection of the L3 launch opportunity. Consequently, LISA might be the first space mission scanning the sky to retrieve both polarisations of the GWs simultaneously, and to measure their source parameters in a bandwidth spanning from 10-4 to 10-1 Hz. The latest LISA mission concept, nominally lasting 4 years in Science Mode, encompasses three identical satellites, in an Earth-trailing heliocentric orbit about 50 Mkm from the Earth. What is more, the three satellites will be placed in a triangular constellation, whose three arms, averagely long 2.5 Mkm, are endowed with six optical links for laser interferometry. Laser interferometry aims to measure, with high accuracy, the distance variations among the free-flying test masses hosted in the three spacecrafts. To this purpose, each spacecraft is drag-free controlled, in order to follow its own two test masses, along each of its two interferometric axes. In this paper, we first review the general aspects of the LISA mission, including those successfully tested in the LISA Pathfinder experiment. Then, an overall non-linear model is proposed to describe the LISA constellation dynamics. Possible methodologies for the LISA Drag-Free Attitude Control System (DFACS) are finally discussed.