Probing the test mass motion through optical path-length changes
The Optical Metrology System on-board LISA Pathfinder is capable of measuring the attitude and position of the two free-falling test masses (TMs) to high precision.
Using an ultra-stable optical bench and a low noise digital phasemeter, changes in optical path-lengths due to test mass motion are precisely measured and delivered to the LTP computer (Data Management Unit - DMU) which processes, in real time, the actual positions of the TMs, their actual attitudes for two angles each, and the actual distances of the TMs.
These data are continuously provided to the main control computer on LPF for use in control loops and for delivery to ground.
It is from these precise measurements that the forces and torques on the test masses can be estimated and used to develop the physcial model which describes the stray forces perturbing the test masses from their geodesic motion.