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Satellite and Test Mass Control

LPF Technology

Satellite and Test Mass Control

Drag-free control of a 400kg spacecraft

LPF - Control Scheme

The Gravitational Reference System

LISA Pathfinder control – with forces applied to the satellite and to the TM – aims at a stable system with 18 degrees of freedom, one of which must be inertial to less than a femto-g.  In achieving this, LISA Pathfinder paves the way for similar dynamic control with LISA.

The basic control mechanism for the LISA Pathfinder science phase is illustrated in the figure and, simplifying to the X measurement axis, works as follows:

  • The satellite is controlled (using microNewton thrusters), to maintain the TM1 in a nominally centered position with respect to the satellite, based on an interferometric measurement of the TM1 displacement relative to the optical bench (o1).  This “drag-free” control loop has a bandwidth of 100 mHz.  
  • TM2 is then controlled with a weak electrostatic suspension of bandwidth of roughly 1 mHz, to follow the TM1, such as to null the readout of the differential interferometer measuring the relative displacement of the two TMs (o12).   
  • Our main science signal, which is the differential forces acting on the two TMs, is measured in the residual differential TM motion in o12, properly subtracting the known commanded actuation forces.

All other degrees of freedom will be controlled with a combination of the satellite thrusters and electrostatic suspension, using the interferometry, capacitive sensing, and satellite star trackers as readouts of the relevant displacements.