the gravitational reference system (GRS)

The Gravitational Reference System: free-falling test masses for measuring gravitational wave tidal forces

LISA measures gravitational waves as a tidal deformation on a constellation of free-falling geodesic reference test masses, which produces a differential acceleration that is measured with laser interferometry.

The Italian hardware contribution, built by industry under the scientific guidance of the Trento PI team, to LISA is the free-falling geodesic reference test mass and the hardware that surrounds it, known collectively as the “gravitational reference system” or GRS. There are two GRS in each of the three LISA spacecraft.

The GRS includes:

The test mass itself, a 46 mm cube of roughly 2 kg of Gold-Platinum.
A surrounding “electrode housing” that serves as an electrostatic shield, a capacitive position sensor for 6-degree-of-freedom control at the nanometer level, and an electrostatic force actuator for applying nanoNewton-level forces to align the TM to the spacecraft and optical measurement system.
A vacuum chamber and accompanying vent line to space, limiting residual pressure to the 10-8 mBar level.
A UV-light discharge system, exploiting the photoelectric effect to neutralize the TM from the accumulation of charge by cosmic rays
Gravitational balance masses, to compensate the ‘’self-gravity’’ from the mass distribution of the surrounding instrument elements, particularly the nearby optical bench and telescope.
A launch-lock mechanism to hold the TM against vibrations exceeding 10g during launch.
A TM “grabbing, positioning, and release mechanism” (GPRM), used to delicately release the TM into free-fall on-orbit, with residual velocities small enough – order 10 m/s – to allow capture by the applied electrostatic control forces.

Stray forces on the TM, originating in interactions with the surrounding GRS hardware and environment, limit the LISA sensitivity at low frequencies. The GRS is thus critical to the LISA science return for the most massive gravitational wave sources.

Our work concentrates on the design and analysis of the GRS hardware, and on the strategies for verifying the needed performance, including extensive small force test campaigns in our lab in Trento. This work centers on quantifying, by analysis and measurement, all the possible force disturbances relevant to achieving free-falling test masses with a purity at the “femto-meter/second squared” – 10-15 m/s2 – level.

We work closely with the Italian industrial “prime contractor” – OHB Italia in Milano – in addition to the groups responsible for the GRS electronics (ETH Zurich in Switzerland) and the GRS UV light source (University of Florida for NASA). We also work closely to the other leading groups for the LISA instrument, including the Albert Einstein Institute (Max PIanck Hannover), UKATC Edinburgh, APC Paris and IEEC Barcellona.

The development of GRS technology for LISA Pathfinder and LISA will open new scientific possibilities for future gravitational wave missions – in the frequency band from microHz to Hz – and in geodesy, where the introduction of laser interferometry has made test mass acceleration noise the limiting factor at all frequencies “useful” for the study of the Earth’s spatial and temporal gravitational field variations.

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