“Micro Meets Macro,” Space Horizons 2013, Brown University

Gravitational Reference Technologies for Precision Navigation and Control in Space

Karthik Balakrishnan Department of Aeronautics and Astronautics

Hansen Experimental Physics Labs

Stanford University

karthikb@stanford.edu

“Micro Meets Macro,” Space Horizons 2013, Brown University

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)

“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

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n

• Use an isolated proof mass inside of a housing – Proof mass follows geodesic – isolated

from non-gravitational forces

– Spacecraft flies itself around PM

“Micro Meets Macro,” Space Horizons 2013, Brown University

Drag Free Design Considerations

• Other disturbances: – Vehicle gravity – ̴10-11 g

• Proof mass at mass center of spacecraft

• Fine tune/measure mass-center offset of PM

– Image attraction of charged PM – ̴10-14 g • Proof mass charge control

– Induced magnetic moment – ̴10-12 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) – ̴10-18 g • Balance sensors

“Micro Meets Macro,” Space Horizons 2013, Brown University

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

Triad I 1972 1st drag-free

GRACE, 2003 (non-drag-free)

LISA, ~2026

GP-B, 2004

“Micro Meets Macro,” Space Horizons 2013, Brown University

MGRS System: Full

Grating Angular Sensor

Grating Displacement Sensor

Proof Mass Caging

UV LED Charge Management

Full System

Differential Optical Shadow Sensor

Andreas Zoellner

Eric Hultgren, Chin-Yang Lui Karthik Balakrishnan

Graham Allen (alum)

• picometer sensing for science signal • High sensitivity, low dynamic range • Nanoradian level angular sensing

• nanometer sensing for drag free signal • Lower resolution, high dynamic range

• Solid state 255nm light source • Charge control of proof mass and housing potential

• 700 g clamping of proof mass during launch • Minimal residual velocity on release • No damage to proof mass surface

• 2.9 kg 70mm dia 70-30 Be-Cu sphere • Carbide coated sphere

“Micro Meets Macro,” Space Horizons 2013, Brown University

MGRS: simplified for smallsats

Spinning

spherical Test Mass

Housing

(metrology reference)

Drag-free error budget

“Micro Meets Macro,” Space Horizons 2013, Brown University

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 50-200 electrons/second expected charging rate

• Potential imbalance leads to an electrostatic force on the proof mass

“Micro Meets Macro,” Space Horizons 2013, Brown University

Charge Management Overview

“Positive Charge Transfer” “Negative Charge Transfer”

“Micro Meets Macro,” Space Horizons 2013, Brown University

UV LED Small Sat Demonstration

Scheduled for launch in Sept 2013

10 20 30 40 50 60

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5Charge Amp

Time (minutes)

Sphere

Pote

ntial

“Micro Meets Macro,” Space Horizons 2013, Brown University

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)

“Micro Meets Macro,” Space Horizons 2013, Brown University

Coatings – samples

Top row (from left): Au, Nb, Ir, SiC Bottom row (from left): TiC, MoC, ZrC, TaC

• 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)

SiC coated Al sphere

“Micro Meets Macro,” Space Horizons 2013, Brown University

Proof mass coating measurements • Measurements:

– Quantum efficiency (λcent =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 1014 Ω Ultem tubes

• 50 µW UV incident power

• Current measured using Keithley 6485 Picoemmeter

– Reflectivity (λcent =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-07 4.4E-07 0.17 4.57

Nb 5.64E-07 2.4E-07 0.17 4.30

SiC 4.34E-07 1.4E-07 0.12 4.80

TiC 4.48E-07 1.3E-07 0.15 3.80

ZrC 3.85E-07 2.1E-07 0.11 3.70

MoC 6.82E-07 1.1E-07 0.15 4.74

TaC 6.35E-07 1.4E-07 0.13 5.0

Ir -- -- 0.6 -- QE measurement setup

“Micro Meets Macro,” Space Horizons 2013, Brown University

Differential Optical Shadow Sensor

• LED-diode Emitter-Detector pairs

– Differential common mode rejection

– Photon pressure balanced

• TM position sensor goals – ~nm/Hz1/2 position in 3 dof

– < 10–12 m/sec2 TM disturbance

“Micro Meets Macro,” Space Horizons 2013, Brown University

DOSS Sat

1nm

Previous setup (in-air)

• 2U CubeSat

• Raise Shadow Sensor TRL

• Test attitude control algorithms

• Completion: Late 2013

Courtesy A. Zoellner

“Micro Meets Macro,” Space Horizons 2013, Brown University

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

“Micro Meets Macro,” Space Horizons 2013, Brown University

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: 10–12 m/sec2Hz1/2 (for geodesy) Courtesy A. Zoellner

“Micro Meets Macro,” Space Horizons 2013, Brown University

Questions?

Construction of satellite engineering model ongoing at NASA Ames

1. B. Lange. “The Control and use of Drag-free Satellites”. PhD thesis, Stanford University, 1964.

2. D. B. DeBra and J. W. Conklin. “Measurement of drag and its cancellation”. Classical and Quantum Gravity, 28(9):094015, May 2011.

3. 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, 2009.

4. 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, 2011.

5. 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 2009.

6. 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 1995

7. 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, 2006.

8. 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, 2009

9. 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 2011.

“Micro Meets Macro,” Space Horizons 2013, Brown University

Error sources on proof mass orbit

B. Lange. The Drag Free Satellite. AIAA Journal, May 26, 1964.

“Micro Meets Macro,” Space Horizons 2013, Brown University

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

“Micro Meets Macro,” Space Horizons 2013, Brown University

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