Optical Metrology Applications at TAS-I in support of Gravity and Fundamental Physics

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Optical Metrology Applications at TAS-I in support of Gravity and Fundamental Physics Template reference : 100181670S-EN

1 Optical Metrology Applications at TAS-I in support of Gravity and Fundamental Physics Template reference : S-EN Stefano Cesare, Thales Alenia Space Italia, Torino Workshop GG/GGG: state of the art and new possibilities Pisa, 12 Febbraio 2010

2 Optical metrology at TAS-I Main research projects and applications of optical metrology at TAS-I: Monitoring of the GAIA astrometric instrument stability ( ). Co-phasing of optical interferometers ( ). Satellite-to-satellite laser tracking for Next Generation Gravity Missions (2004 present). Nanobalance facility for characterization of micro-newton thrusters (2001 present). Collaborations: Page 2 Istituto Nazionale di Ricerca Metrologica Politecnico di Torino INAF Osservatorio Astronomico di Torino

3 Metrology for GAIA astrometric instrument Page 3 3 x IFR signal: Control error Distance variation [m] Time [s]

4 Co-phasing of optical interferometers Page 4 Breadboard of a two-aperture optical interferometer with its co-phasing system, for the development project of a synthetic-aperture optical telescope. Fringe Sensor Unit realized for the co-phasing of the VLTI (ESO). Operative at Cerro Paranal, Chile.

Satellite-to-satellite laser tracking for NGGM Gravimetry by satellite-to-satellite tracking Page 5 F D2 Satellite 2 Satellite 1 d D F D1 2 D 1 d = d g G + d D 2 g 1 Earth The distance variation

5 Satellite-to-satellite laser tracking for NGGM Gravimetry by satellite-to-satellite tracking Page 5 F D2 Satellite 2 Satellite 1 d D F D1 2 D 1 d = d g G + d D 2 g 1 Earth The distance variation between two satellites ( d) is measured by a laser metrology system. The distance variation between the satellites produced only by drag forces ( d D ) is measured by accelerometers. Subtracting ( d D ) from ( d) the distance variation produced by the gravity acceleration is obtained: d G = d - d D relative distance error SD [m/sqrt(hz)] nm/ Hz frequency [Hz] Requirement for the laser interferometer measurement noise (relative distance = 10 km)

6 Satellite-to-satellite laser tracking for NGGM Satellite 2 PSD 1 d L Optical metrology concept for the NGGM Satellite 1 PSD 4 pd 2 l 2 p 4 Page 6 S pd2 = 2I {1 + cos [2π(ν 1 -ν 2 )t + φ + δφ]} RR 2 PSD 3 PSD 2 angle and lateral displacement sensors amplitude modulated beam BSM telescope PSD 6 PSD 5 angle sensors f 1 c 1 q 2 ν 1 p 1 p 2 pbs 2 q 1 ν 2 p 3 l 1 RR 1 pbs 1 pd 1 δl = 1 2 δφ 2π λ c 2 S pd1 = 2I {1 + cos [2π (ν 1 -ν 2 )t + φ]} Frequency Stabilisation System Nd:YAG Laser source 1064 nm ν m ν' m ν 0 bs ν 0 ν 1 FS AM f 2 ν 2 Michelson-type heterodyne laser interferometer based on polarized beams, with chopped measurement beam to avoid spurious signals and non-linearity caused by the unbalance between the strong local beam and the weak return beam. Passive retro-reflection of the laser beam on S2: simple solution, suitable for d up to 100 km.

7 Satellite-to-satellite laser tracking for NGGM Page 7 Optical metrology arrangement on the satellites Accelerometers Active optical bench Interferometer core (laser emission & interferometry) Angular/lateral metrology Passive optical bench (laser retro-reflection) Beam Steering Mechanism Angular metrology Retroreflector Satellite 1 Satellite 2 The non-gravitational accelerations of the satellite COM can be measured by electrostratic accelerometers like those used on GOCE. All rights reserved, 2/26/2008, Thales Alenia Space

Spectral density of the distance variation measurement error obtained during the tests and compared to the

8 Satellite-to-satellite laser tracking for NGGM Interferometer BB test Page 8 Laser interferometer breadboard prepared for the intrinsic noise test (measurement of a constant distance). Spectral density of the distance variation measurement error obtained during the tests and compared to the requirement. In order to achieve the specified measurement performance over a distance of 10 km, the laser frequency shall have a relative stability δν/ν Hz -1/2.

9 Satellite-to-satellite laser tracking for NGGM Interferometer BB test Page 9 Laser interferometer breadboard under the functional test over a long distance (~90 m) with a moving target. The effectiveness of the measurement beam chopping scheme was successfully verified in this test.

Satellite-to-satellite laser tracking for NGGM Page 10 Test of the

Beam Steering Mechanism Angle/lateral Displacement Metrology Open-loop

Lateral displacement steps (from ±50 µm to ±5 mm) measured by the

25 mm (over the largest steps) Max. measurement noise: 14 µm 1σ.

10 Satellite-to-satellite laser tracking for NGGM Page 10 Test of the laser beam pointing control system BB. Beam Steering Mechanism Angle/lateral Displacement Metrology Open-loop test of the Lateral Displacement Metrology. Lateral displacement steps (from ±50 µm to ±5 mm) measured by the optical metrology at 10 Hz. Max. measurement error: 0.25 mm (over the largest steps) Max. measurement noise: 14 µm 1σ. Closed-loop test of the laser beam pointing control system (BSM driven by the Lateral Displacement Metrology measurements). Laser beam pointing stability results.

Nanobalance Page 11 Thrust Stand optical head alignment device Nanobalance

chamber pump group beam trap vacuometer horizontality control stage optical

System The Nanobalance is a complete test facility for the direct measurement

11 Nanobalance Page 11 Thrust Stand optical head alignment device Nanobalance Thrust Stand thermistor(s) tiltmeter optical cavity thruster under test vacuum chamber pump group beam trap vacuometer horizontality control stage optical fiber pneumatic support metrology system optical bench Monitoring and Control System The Nanobalance is a complete test facility for the direct measurement of the force provided by a micro-thruster along its thrust axis, developed by TAS-I for ESA.

12 Nanobalance Page 12 elastic joint ZERODUR Athermic spacer elastic joint elastic joint ZERODUR Athermic spacer elastic joint Micro-Thruster under terst Dummy Micro- Thruster Micro-Thruster under terst Dummy Micro - Thruster Thrust F Laser, v L Laser, v L + δv L Actiive Tilting Plate Fabry-Pérot Cavity, L Passive Tilting Plate Actiive Tilting Plate Passive Tilting Plate Fabry-Pérot Cavity, L -δl A micro-thruster force F = 0.1 µn induces a distance variation between the tilting plates δl 14 pm, corresponding to frequency variation of the laser locked to the Fabry-Perot cavity δν L 40 khz.

13 Nanobalance Page NB background noise - PSD NB Resolution Applied VC force Force Wide band meas. filter Narrow band meas. filter Force [µn/ Hz] PSD [µn/ Hz] Force [µn] Frequency [Hz] Frequency [Hz] Power Spectral density of the measurement force noise after postprocessing for removing the lowfrequency drift effect Time [s] Measured thrust corresponding to the smallest applied force steps with a voice coil actuator.

14 Nanobalance Page 14 Test of a cold-gas thruster Applications: GAIA, Microscope (backup) Test of a FEEP thruster Applications: LISA PF, Microscope, GG Test of a mini-rit Applications: NGGM, GG (backup)

Direct Thrust and Thrust Noise Measurements on the LISA Pathfinder Field Emission Thruster

Direct Thrust and Thrust Noise Measurements on the LISA Pathfinder Field Emission Thruster IEPC-9-8 Presented at the st International Electric Propulsion Conference, University of Michigan Ann Arbor, Michigan