Designing and prototyping “Gravitational Reference Sensors” for eLISA and LISA Pathfinder

In eLISA and LISA Pathfinder, the test masses are surrounded by electrostatic sensing and actuation devices, known collectively as the Gravitational Reference Sensor or GRS.  The GRS has three main responsibilities in the science phase of the mission:

  • Provide nm-level sensing in all degrees of freedom to allow the necessary spacecraft control
  • Provide nN-level electrostatic actuation forces on the test mass on different degrees of freedom
  • Provide the electromagnetic and thermal shield that defines an environment that limits all stray forces to the femto-Newton level

The GRS is a contribution from the Italian Space Agency.  At the University of Trento we developed the original sensor design, and we built and tested several prototypes, before collaborating with the CGS (Milano) aerospace company in their successful realization of the complete flight GRS hardware. 

The GRS design includes:

  • 2 kg test masses (TM) made from a gold-platinum alloy
  • a surrounding gold-coated electrostatic shield / electrode housing with a high thermal conductivity Mo-sapphire construction
  • integrated gold-coated electrodes used for contact free, 100 kHz capacitive sensing the TM position in 6 degrees of freedom and for audio frequency-carriers for applying near-DC electrostatic forces
  • a “vent-to-space” vacuum system to allow a final pressure below 10-11 Bar to limit forces from molecular impacts
  • a 2000 N caging system to protect the TM and GRS during launch vibrations and then release the TM into free-fall once in orbit
  • Additionally, we have consulted with colleagues in England and Switzerland as they designed, respectively the UV photoelectric discharge and sensor electronics sub-systems. 

The LPF GRS was designed for the demanding needs of the eLISA observatory, and LPF thus represents a flight verification of a crucial part of the eLISA apparatus.

Here below a gallery of selected images.

2 kg test masses (TM) made from a gold-platinum alloy.
Prototype of the capacitive sensor core of the Gravitational Reference Sensor.
The two capacitive sensors flight models, core of the Gravitational Reference Sensor for LISA Pathfinder (Courtesy of CGS S.p.A. - Compagnia Generale per lo Spazio).
LISA Pathfinder Gravitational Reference Sensor flight model (Courtesy of CGS S.p.A. - Compagnia Generale per lo Spazio).
LISA Pathfinder Gravitational Reference Sensor flight model delivery at CGS S.p.A. (3 Nov 2014): the UNITN team with the team of CGS S.p.A., Astrium GmbH and ESA that contributed to the realization of the Gravitational Reference Sensor.
LISA Pathfinder Gravitational Reference Sensor flight model delivery at CGS S.p.A. (3 Nov 2014): the UNITN team with a model of the LISA Pathfinder core. From left to right: William Joseph Weber, Rita Dolesi, Paolo Pivato, Antonella Cavalleri, Giuliana Russano, Li Liu, Mauro Hueller, Karine Frisinghelli, Andrea Cesarini, Daniele Vetrugno, Daniele Bortoluzzi and Ferran Gibert.
LISA Pathfinder Gravitational Reference Sensor flight model delivery at CGS S.p.A. (3 Nov 2014): Prof. Vitale, the Principal Investigator of LISA Pathfinder and, on the left, Prof. Paolo Collini, the Rector of the University of Trento.
LISA Pathfinder Gravitational Reference Sensor flight model delivery at CGS S.p.A. (3 Nov 2014): Prof. Roberto Battiston, the President of the Italian Space Agency (ASI).
LISA Pathfinder Gravitational Reference Sensor flight model delivery at CGS S.p.A. (3 Nov 2014): Prof. Rita Dolesi and Prof. William Joseph Weber with some prototypes of the Gravitational Reference Sensor.
LISA Pathfinder Gravitational Reference Sensor flight model delivery at CGS S.p.A. (3 Nov 2014): Dr. Mauro Hueller with some prototypes of the LISA Pathfinder test mass.