• eLISA
• gravitational reference system
• inertial reference system
• interferometer
• LISA
• Mechanism
• optical encoder
• opto-mechanical design
• stepper motor
• topography

One of the Airbus DS proposals for the different payloads concepts for LISA mission theorized
the use of a completely full drag free GRS (Gravitational Reference System). This
solution uses a spherical proof mass which is isolated from any external spurious accelerations
but gravity. An optical read out system tracks the motion of the centre of mass and
supplies data to a DRS (Disturbance Reduction System). The DRS makes the spacecraft
structure move around the proof mass which is following a geodesy motion.
This innovative system requires an accurate, on ground, topography map of the sphere
surface with an accuracy of picometers.
This thesis studies and proposes improvements for an existent tool, developed by Airbus
DS in Friedrichshafen, capable of measuring single circumference topography, with
nanometres accuracy, of a spherical proof mass via two high symmetric heterodyne interferometers.
The principal efforts of this work is addressed to the Opto-mechanical design of elements
and mechanisms in order to supply the necessary hardware which will enable the complete
2D topography of the sphere.
The designed mechanism rotates the sphere, via friction forces, in order to supplies several
circumference topographies spaced by 17 mrad each. The system is automatized using
a stepper motor and an optical incremental encoder controlled by LabView interfaces.
The proposed idea is then tested, in order to demonstrate the feasibility of this solution
which is planned to supply a sphere rotation accuracy in the order of milli radiant.
An important feature introduced by the new machine set-up is the use of a rotating ring
mirror as reference for the interferometric measurements. This idea introduces in the
interferometer outcome a double topography, at sub nanometer level accuracy, from
which the final proof mass topography will be extracted.