Gravitational Reference Technologies for Precision Navigation and Control in Space

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1 Gravitational Reference Technologies for Precision Navigation and Control in Space Karthik Balakrishnan Department of Aeronautics and Astronautics Hansen Experimental Physics Labs Stanford University

2 Outline Overview of drag free technology and applications MGRS Stanford s drag-free implementation Small satellites as a path to a full MGRS UV-LED (2013) Overview of UV-LED charge management Mission Coating selection Caging on zero-g (2013) DOSS (2014) Drag-free cubesat (2015)

Solar radiation pressure Atmospheric drag Fdrag Use an isolated proof mass inside of a

3 Fphoton Micro Meets Macro, Space Horizons 2013, Brown University Drag Free Fundamentals Spacecraft experiences many disturbance forces that cause a deviation from a geodetic orbit Solar radiation pressure Atmospheric drag Fdrag Use an isolated proof mass inside of a housing Proof mass follows geodesic isolated from non-gravitational forces Spacecraft flies itself around PM

4 Drag Free Design Considerations Other disturbances: Vehicle gravity g Proof mass at mass center of spacecraft Fine tune/measure mass-center offset of PM Image attraction of charged PM g Proof mass charge control Induced magnetic moment g Proof mass made with material of low magnetic susceptibility Shielding with MuMetal, Metglas, etc Sensor capacitive (if used) 10-8 g If used, use high bridge frequency to spectrally shift acceleration noise Sensor optical (if used) g Balance sensors Micro Meets Macro, Space Horizons 2013, Brown University

5 Drag-Free History and Applications 1 drag-free spacecraft Autonomous precision orbit determination, Aeronomy, Fundamental physics 2 drag-free spacecraft differential between two geodesics Geodesy, e.g. GRACE 3+ spacecraft Gravity-waves LISA, ~2026 GP-B, 2004 Triad I st drag-free GRACE, 2003 (non-drag-free)

MGRS System: Full Micro Meets Macro, Space Horizons 2013,

Angular Sensor Grating Displacement Sensor nanometer sensing

Andreas Zoellner Nanoradian level angular sensing picometer

range Graham Allen (alum) Proof Mass Caging UV LED Charge

launch Minimal residual velocity on release No damage to

255nm light source Charge control of proof mass and housing

6 MGRS System: Full Micro Meets Macro, Space Horizons 2013, Brown University Differential Optical Shadow Sensor Grating Angular Sensor Grating Displacement Sensor nanometer sensing for drag free signal Lower resolution, high dynamic range Andreas Zoellner Nanoradian level angular sensing picometer sensing for science signal High sensitivity, low dynamic range Graham Allen (alum) Proof Mass Caging UV LED Charge Management Full System 700 g clamping of proof mass during launch Minimal residual velocity on release No damage to proof mass surface Eric Hultgren, Chin-Yang Lui Solid state 255nm light source Charge control of proof mass and housing potential Karthik Balakrishnan 2.9 kg 70mm dia Be-Cu sphere Carbide coated sphere

7 MGRS: simplified for smallsats Micro Meets Macro, Space Horizons 2013, Brown University Drag-free error budget Spinning spherical Test Mass Housing (metrology reference)

8 Charging sources and effects The spacecraft and housing protect the proof mass from many disturbances: solar, atmospheric, etc. However, direct and secondary charging of the proof mass is still possible leading to a potential imbalance between the proof mass and housing walls Direct: High energy particles pass through the shielding and directly accumulate on either proof mass or housing Secondary: High energy particles interact with spacecraft materials, knocking off electrons which then accumulate on the proof mass or housing Approx electrons/second expected charging rate Potential imbalance leads to an electrostatic force on the proof mass

9 Charge Management Overview Micro Meets Macro, Space Horizons 2013, Brown University Positive Charge Transfer Negative Charge Transfer

10 Sphere Potential UV LED Small Sat Demonstration Micro Meets Macro, Space Horizons 2013, Brown University 2.5 Charge Amp Time (minutes) Scheduled for launch in Sept 2013

11 Proof mass coatings Want tough and robust coating on proof mass During caging, 100 g s preload on proof mass Want proof mass surface to be robust in the event it contacts housing walls Alternatives: carbide coatings Very tough, wide bandgap (close to AlGaN) Desired properties at 255 nm Sufficient QE at 255 nm (> approx. 1E-9) Reflectivity > 5% Workfunction near or lower than 4.86 ev (can be slightly higher due to Fermi Tail)

Samples immediately vacuum bagged after coating for cleanliness Sample materials: Carbides: SiC, TiC, MoC, ZrC, TaC Metals: Au

12 Coatings samples Test: carbide pellets coated on to aluminum substrates via e-beam deposition Substrate material: Al 6061-T6 machined into 1 squares Pellets: 2-4 mm diameter Samples cleaned via HF etch prior to coating, then immediately vacuum bagged for cleanliness Samples immediately vacuum bagged after coating for cleanliness Sample materials: Carbides: SiC, TiC, MoC, ZrC, TaC Metals: Au (LISA/LP-F proof mass coating), Nb (GP-B) Top row (from left): Au, Nb, Ir, SiC Bottom row (from left): TiC, MoC, ZrC, TaC SiC coated Al sphere

Measurements: Proof mass coating measurements Quantum efficiency (λ cent =255 nm) Measured twice: 2 weeks after coating, and 16 months after coating Used an integrating sphere with 10 V bias between

13 Measurements: Proof mass coating measurements Quantum efficiency (λ cent =255 nm) Measured twice: 2 weeks after coating, and 16 months after coating Used an integrating sphere with 10 V bias between coated sample and sphere Samples isolated from ground via Ω Ultem tubes 50 µw UV incident power Current measured using Keithley 6485 Picoemmeter Reflectivity (λ cent =255 nm, θ=45 ) Used Newport 918D head connected to Newport 1931-C power meter Material QE (2 wk) QE (16 mos) R (255 nm) φ (ev)* Au 3.40E E Nb 5.64E E SiC 4.34E E TiC 4.48E E ZrC 3.85E E MoC 6.82E E TaC 6.35E E QE measurement setup Ir

rejection Photon pressure balanced TM position

14 Differential Optical Shadow Sensor LED-diode Emitter-Detector pairs Differential common mode rejection Photon pressure balanced TM position sensor goals ~nm/hz 1/2 position in 3 dof < m/sec 2 TM disturbance

15 DOSS Sat 2U CubeSat Raise Shadow Sensor TRL Test attitude control algorithms Completion: Late 2013 Previous setup (in-air) 1nm Courtesy A. Zoellner

16 Proof Mass Caging System Clamp proof mass during launch with >200 N force 13.5:1 gear ratio Will be tested on NASA Zero-G flight at the end of April Courtesy E. Hultgren

17 Drag Free Cubesat 3U CubeSat Full MGRS demo Completion: mid 2015 Research goals: Drag-free control algorithm On-orbit performance evaluation of MGRS Performance goal: m/sec 2 Hz 1/2 (for geodesy) Courtesy A. Zoellner

18 Questions? 1. B. Lange. The Control and use of Drag-free Satellites. PhD thesis, Stanford University, D. B. DeBra and J. W. Conklin. Measurement of drag and its cancellation. Classical and Quantum Gravity, 28(9):094015, May Ke-Xun Sun, Saps Buchman, Robert Byer, Dan DeBra, John Goebel, Graham Allen, John W Conklin, Domenico Gerardi, Sei Higuchi, Nick Leindecker, Patrick Lu, Aaron Swank, Edgar Torres, and Martin Trittler. Modular gravitational reference sensor development. Journal of Physics: Conference Series, 154:012026, K.-X. Sun, A. Alfauwaz, M. Alrufaydah, H. Altwaijry, K. Balakrishnan, S. Buchman, R. L. Byer, J. W. Conklin, D. B. DeBra, J. Goebel, E. Hultgren, and A. Zoellner. Modular Gravitational Reference Sensor (MGRS) Technology Development. In Proceedings of the 8th International LISA Symposium, Journal of Physics Conference Series, T J Sumner, DNA Shaul, M O Schulte, S Waschke, D Hollington, and H Araujo. LISA and LISA Pathfinder charging. Classical and Quantum Gravity, 26(9):094006, May S. Buchman, T. Quinn, G. M. Keiser, D. Gill, and T. J. Sumner. Charge measurement and control for the Gravity Probe B gyroscopes. Review of Scientific Instruments, 66:120{129, January Ke-Xun Sun, Brett Allard, Saps Buchman, Scott Williams, and Robert L Byer. LED deep UV source for charge management of gravitational reference sensors. Classical and Quantum Gravity, 23(8):S141{S150, Ke-Xun Sun, Nick Leindecker, Sei Higuchi, John Goebel, Sasha Buchman, and Robert L Byer. UV LED operation lifetime and radiation hardness qualification for space flights. Journal of Physics: Conference Series, 154:012028, K. Balakrishnan, E. Hultgren, J. Goebel, and K.-X. Sun. Space Qualification Test Results of Deep UV LEDs for AC Charge Management. In 11th Spacecraft Charging Technology Conference, poster presentation, September Construction of satellite engineering model ongoing at NASA Ames

19 Error sources on proof mass orbit Micro Meets Macro, Space Horizons 2013, Brown University B. Lange. The Drag Free Satellite. AIAA Journal, May 26, 1964.

20 Coating adhesion test results - carbides Adhesion test procedure Scotch tape pull test Performed at two different speeds, once on each half of the sample (1) Tape put down, pressed firmly onto coating, and then slowly pulled off (2) Tape put down, pressed firmly onto coating, and then yanked off quickly All carbides adhered well, with no pulloff during the test

Coating adhesion test results gold (E-beam) Same adhesion test

dep Gold coating significant section peeled off Not a

place, with no chance for contacting walls, and no caging or

21 Coating adhesion test results gold (E-beam) Same adhesion test was performed with the gold sample which was coated by e- beam dep Gold coating significant section peeled off Not a significant issue for UV LED sat the proof mass is clamped in place, with no chance for contacting walls, and no caging or uncaging Issue for any future drag-free missions using a free-floating proof mass

UV LED charge control of an electrically isolated proof mass at 255 nm

UV LED charge control of an electrically isolated proof mass at 255 nm Karthik Balakrishnan Department of Aeronautics and Astronautics Hansen Experimental Physics Labs Stanford University karthikb@stanford.edu