The LISA Technology Package dynamics and control

The LISA Technology Package (LTP) to be tested on board SMART-2 mission consists conceptually of a single LISA interferometric arm contracted to approximately 35 cm length. As in LISA, the LTP control scheme must force the test masses to follow the spacecraft along all degrees of freedom not parallel to the measurement axis defined by the interferometry beam. Contrary to LISA, one of the test masses needs to be forced to follow the spacecraft also along the measurement axis, because along that axis the spacecraft can only follow a single test mass as a drag-free reference at all frequencies. The control scheme that provides the necessary test massesʹ actuation has high gain at very low frequencies (<1 mHz) to counteract dc forces. However, within the measuring band, it has the minimal necessary gain to stabilize the test masses without increasing disturbances. In addition, SMART-2, a single spacecraft mission, cannot exploit the laser wave front incoming from a distant spacecraft as the angular reference for the spacecraft attitude. Instead, the test masses are used as an angular reference at frequencies higher than ~~1 mHz, while star trackers provide de-rotation on longer time scales. In this paper we describe this articulated, 18 degrees of freedom controlled dynamical system. We show how the two LTP test masses, together with their actuation control loops, can be treated as an autonomous dynamical system. We also address the interaction between this system and the spacecraft.