Gravitational sensor for LISA and its technology demonstration mission

Abstract

We describe the current design of the European gravitational sensor (GS) for the LISA Technology Package (LTP) that, on board the mission SMART-2, aims to demonstrate geodetic motion within one order of magnitude of the anticipated LISA performance. We report also the development of a noise model used in assessing the performance and determining the feasibility of achieving the overall noise goals for the GS. This analysis includes environmental effects that will be present in the sensor. Finally, we discuss open questions regarding the GS for LTP and LISA, ground testing, and verification issues.

Capacitive sensing of test mass motion with nanometer precision over millimeter-wide sensing gaps for space-borne gravitational reference sensors

“…The amplitude of the injected signal is selected such as to produce a 0.6 V peak voltage on the TM [11]. It is useful to assign the sensing channels with specific names.…”

“…The injection bias amplitude affects directly the sensing gain. Since in the GRS this amplitude is kept low to prevent back-action forces on TM [11], the sensing noise must be reduced as much as possible. The sensitivity of the capacitive measurement is limited by the performance (the noise) of the front stage, i.e.…”

“…A symmetric design of the GRS has been selected for LPF in order to ensure stability down to 0.1 mHz and to avoid an excess of cross talk between non-science DoFs and the main science DoF [10,11]. Moreover the TM is 'suspended' with no contact to the EH.…”

“…This means that TM discharge is performed by UV lamp illumination [13] and that each TM is capacitively biased to a given voltage in order to allow for capacitive sensing of its motion. Since a number of physical effects perturbing the TM motion are inversely proportional to the gap between the TM and the EH [10][11][12], the LPF GRS has been designed to work with millimetre-wide sensing gaps, the widest sensing gaps ever implemented for a drag-free space mission [7,[14][15][16]. Biasing the TM results in a stiffness coupling, which allows the TM readout noise to leak into the TM acceleration noise.…”

GAUGE: the GrAnd Unification and Gravity Explorer

“…Modulation amplitudes of d = 4 × 10 −3 m, compatible with the LTP IS gaps, give acceleration signals between 10 −12 and 4 × 10 −11 ms −2 . An LTPlike electrostatic sensor acceleration noise of 3 × 10 −14 ms −2 / √ Hz at 1 mHz will be assumed for all sensors [53]. Then in 10 6 s we can resolve the induced accelerations to better than 1 ppm at 30 cm and 30 ppm at 1 m. Along the IRISL sensitive (axial) axis, use of the LTP laser interferometry can mitigate noise coupled from the spacecraft, leaving a residual acceleration noise of 7 × 10 −15 ms −2 / √ Hz, improving the resolution at 1 m to about 7 ppm.…”