gravity probe b reaches the science...

Gravity Probe BReaches the

Science Phase

27 September 2004

R. Torii

The Relativity Mission ConceptThe Relativity Mission Concept

Main GP-B Systems

Gyroscope Telescope Science Instrument

Cryogenic Probe Payload Space Vehicle

• Two modes: A) Displacement, turn off active force control of rotor position “flying spacecraft around rotor” and B) Accelerometer, fire thrusters based on commanded forces on rotor

• Electrostatic system used to control position of gyroscope rotors provides sensor for both modes

• Proportional thrusters offer better control over on-off thrusters

• Gas supply for thrusters provided by boil off of liquid helium cryogen

GP-B Proportional ThrustersGP-B Proportional Thruster Schematic

Drag-Free Implementation

GP-B Launch: April 20, 2004 -- 09:57:24

Delta II Accuracy - 50%x

The Actual Orbit - Delta II

Orbit Achieved ~100 meters

From the pole

Required Final Orbit Area

Delta II Nominal Accuracy

GP-B Launch: April 20, 2004 -- 09:57:24

GP-B Set-up Highlights

l Weeks 1 - 4(a) SQUID set-up (b) telescope set-up (c) gyro suspension (d) low-T bakeout e)

first drag-free

l Weeks 5 - 8(a) ‘flux-flush’ (b) 0.3 Hz spin (c) lock on guide star (d) charge control

l Weeks 9 - 12(a) increase S/V roll rate (b) reboot flight computer (c) 3 Hz spins

l Weeks 13 - 16(a) final 60 - 80 Hz spins (b) ATC tuning (c) ‘coarse’ gyro alignment

l Weeks 17 - 19(a) final 77.5 s period roll (b) ATC tuning (c) fine (~ 5 arc-s) gyro alignment

GP-B Launch: 20 April 2004 – 09:57:24

Entered Science Phase: 27 August 2004 – 12:00:00

On-Orbit GP-B Technology Demonstrations

Electrostatic Positioning System –– 0.45 nm rms position noise

Gyroscopes –– Spin-down < 1 µHz/hourCharging < 0.3 pC/day

Charge Control System –– < 5 pC controlGSS Charge Measurement < 1 pC rmsUV Charge Discharge Rate > 0.3 pC/min

SQUID Readout –– < 3x10-5 Φ0/√Hz at 0.5 rpmBeats requirement, all SQUIDS

Magnetics –– AC attenuation ~ 1012

dc trapped flux ~ 1 µG

Telescope System –– < 34 marcsec/√Hz readout noise

Technologies Demonstrated On-Orbit by GP-B

Orbit Accuracy –– Inclination error < 0.00007 deg, (< 100m)orbit average to star < 0.004 deg

Proportional Helium Thruster –– 1 – 10 mN/thruster

Drag Free Control –– < 10 nm vehicle positionmean cross-track average < 10-11 g

GPS System –– > 95% lock ratio at all roll ratesTime transfer accuracy < 3 µsec UTC to vehicle timeNavigation accuracy < 7 m rms, < 0.7 cm/s

Superfluid Flight Dewar (2400 l ) –– Lifetime ~ 15 months, Porous plug Dynamic flow range 2-18 mg/s

Gyro #4 Analog Backup Levitation and De-levitation

GP-B Gyroscope Suspension

Position Measurement Performance

0 500 1000 1500 2000 2500 3000-6

-4

-2

0

2

4

G3 X

pos (

nm

)

seconds

Rep. position profile in science mode (not drag free), GP-B Gyro3 (VT=142,391,500)

0 0.05 0.1 0.15 0.2 0.2510

-12

10-11

10-10

10-9

10-8

Mag (

nm

)

Freq (Hz)

Single sided FFT, GP-B Gyro3 (VT=142,391,500)

Measurement noise –0.45 nm rms

Gyro position –

non drag-free gravity

gradient effects in Science Mission Mode

Noise floor

Gravity Gradient Measured by GyroscopesGyro #3, #4 Suspension Control Effort (2+ orbits)

Raw gravity gradient resolution < 10-9 g

Discharge of Gyro #1

GP-B Charge Management

Discharge of Science Gyros Demonstrated

Superconducting SQUID Readout

•

A spinning superconductor develops a

magnetic “pointer” aligned with its spin axis

“SQUID” – ultra sensitive low noise magnetometer

reads angle to 1 milliarc second in 5 hours

Output of SQUID low-pass filter for caged gyros over 22 hours

)(GssLe

mcB ωωωωωωωω 71014.1

2 −−−−××××−−−−====−−−−====

SQUID Readout Noise Beats Spec(PSD (PSD ΦΦΦΦΦΦΦΦ00//√√Hz vs Hz)Hz vs Hz)

Polhode Motion

Gyro # 1

Spin Speed – 3 Hz

July 4 - 7, 2004

36-hour Polhode Period

Fpolhode = ∆ I/I cos(θ)Fspin

=> ∆ I/I < 2x10–6

Gyro #4 London Moment Readout Data

SQUIDOutput

(V)

Zero to peak ~ 100 arc/sec

Drag-Free Performance

Drag-free on

Drag-free off

Twice orbital term reduced by > 100

Suppression of Z axis gravity gradient acceleration

GP-B Telescope Pointing

Telescope Detector Signals Telescope Detector Signals Telescope Detector Signals Telescope Detector Signals

from IM Peg Divided by Rooftop Prismfrom IM Peg Divided by Rooftop Prismfrom IM Peg Divided by Rooftop Prismfrom IM Peg Divided by Rooftop Prism

-2

0

2

4

6

8

10

12

14

0 100 200 300 400 500 600

Sample SequenceSample SequenceSample SequenceSample Sequence

ST_SciSlopePX_B ST_SciSlopeMX_B

Acquiring Guide Star

Drive in time ~ 110 s

RMS pointing ~ 80 marc-s

HR 8703 (IM PEG) Guide Star Identification

IM Peg

Guide Star

HR Peg

(acquired)

NhS1

(acquired)

Palomar Star Map

Preliminary HR 8703 Positions for Peak of Radio BrightnessSolar System Barycentric, J2000 Coordinate System

(Right Ascension - 22h53m) x 15 cos(Dec) (mas)

3250032550326003265032700

De

clin

atio

n -

16

o 5

0' 2

8''

(ma

s)

250

300

350

400

450

500

550

16.9 Jan 9718.9 Jan 97

30.0 Nov 97 21.9 Dec 9727.9 Dec 97 1.8 Mar 98

12.5 Jul 98 8.4 Aug 98

17.3 Sept 98 13.8 Mar 99

15.6 May 99 19.3 Sept. 99

15.0 Dec 91

22.4 June 9313.2 Sept 93

24.3 July 94

10.0 Dec 99 15.6 May 00

7.3 Aug 00 6.1 Nov 00

7.1 Nov 00

29.5 June 0122.0 Dec 01

14.7 Apr 02

20.2 Oct 01

Very Large Array, Socorro, New Mexico

• Optical & radio binary star

• Magnitude - 5.7 (variable)

• Declination - 16.84 deg

• Proper motion measured bySAO using VLBI

The Science Mission

STEP

27 September 2004

R. Torii

25

Satellite Test of the Equivalence Principle

Dz

time

Orbiting drop tower experiment

Dz

Dz

time

F = ma mass - the receptacle of inertia

F = GMm/r2 mass - the source of gravitationNewton’s Mystery {

* More time for separation to build

* Periodic signal{

26

Space > 5 Orders of Magnitude Leap

10-18

10-16

10-14

10-12

10-10

10-8

10-6

10-4

10-2

1700 1750 1800 1850 1900 1950 2000

Newton

Bessel

Dicke

Eötvös

Adelberger , et al.

LLR

STEP

“Mission Success”

αeffect (min.)

{

DPV runaway dilaton (max.)

.

1 TeV Little String Theory

~ 5 x 10-13

100

Microscope

27

Can Gravity Be Made to Fit?

• Unification in physics – through fields (Maxwell), geometry (Einstein),

symmetries and new particles (electroweak theory)….and

now (?) supersymmetry and strings

• The problems of gravity – quantization; 10-42;

cosmological constant

Λ

(10-120!); equivalence

• Partial steps toward Grand Unification– Strings/supersymmetry in early Universe scalar-tensor theory, not Einstein’s

– Damour - Polyakov: small

Λlong range equivalence-violating dilaton

• EP violations inherent in all known GU theories

– Runaway dilaton theories

– 1 TeV Little String Theory (Antoniadis, Dimopoulos, Giveon)

STEP’s 5 orders of magnitude take physics into new theoretical territory

Gravity

Strong Nuclear Force

Weak Nuclear Force

ElectroMagnetism

⇒

⇒

(Witten)

(Damour, Piazza, Veneziano) { {η >> 10-18

up to 10-14η

~ 10-15

28

Differential Accelerometer Science Instrument

Payload Space Vehicle

Significant technology advances

from SCR/RDR 1999 to SMEX 2002

DA Package

Dewar

29

Requirement Satisfied by analysis using measured performance of

GPB SQUID and coil Key Technology Development

Ø (1972 –) Concept proven. Acceleration

sensitivity requirement met. (H.

Paik, P. Worden, S. Vitale)

Ø (1999) Coil inductance L(x) design verified.

Ø (2000) Flight coil designed, (2001) built.

Ø Manufacturing

Ø (2004) GPB SQUID readout performance

verified in polar orbit (SAA passes).

Ø Plan to develop with Strathclyde/Jena

Ø Cryogenic electronics development

Ø Verify flight SQUID position sensitivity

mass mass

SQUID

pancake coil

Magnetic

Model

Modular design

allows coil form to be processed

independently

30

Meets requirement on aligned mass motion

while preserving acceleration sensitivity Key Technology Development

Ø (1986) Concept proven (P. Worden). Bearing

performance limited by

Ø Manufacturing

Ø Trapped magnetic flux creep

Ø (1998) Distance between bearing and test masses REDUCED by factor of 10

Ø (2001) Manufactured niobium thin film

bearing meeting requirements

Ø (2001) Attached superconducting cables

Defines axis of

measurement

Aligns motion

of test masses

100 µm

31

Requirement Each test mass surrounded by

thirteen capacitance electrodes

Key Technology Development

Ø (2000) Capacitance electrode baseline

configuration by Stanford/ONERA

Ø (2001) Manufactured thin film electrodes

isolated by channels

Ø (2004) GPB UV charge control verified in

polar orbit (SAA passes)

Ø Plan to develop with Imperial College better electrostatic model and hardware

Ø Overlay magnetic coils with gold

Ø Develop dielectric to isolate gold coating

Ø Finite element model

Electric Model

FFTCAP (MIT)

MESH:

13 Plates + Mass

32

Requirement Met by optimal test

mass shape and positioning capabilityKey Technology Development

Ø Test Mass Fabrication

Ø (1993 -) Test mass design and verification by

Strathclyde.

Ø (2002) Test mass density homogeneity verification

by Birmingham.

Ø (2002) Test mass manufacturing capability by PTB

and Axsys, Inc.

Ø Test Mass Positioning Capability

Ø (2002) Capacitive position sensor by ONERA.

Ø Verify flight magnetic bearing radial force.

Ø Alignment of capacitance sensors to bearing.

Ø Axial positioning using SQUID pickup coil.

Ø Cryogenic electronics development.

FiberSuspension

x, y, z, θz

z

+ 2 axis tilt

33

STEP Technology Development

Plan 27 month plan enables

prototype Payload testingTechnology Development Goals

Ø Demonstrate maturity of STEP enabling technologies

Ø Define STEP Payload verification method

Ø Organize STEP error analysis to support Payload verification

0

1 2 3 4 5 6 7 8 9 10 11 12 13 1 4 15 16 1 7 18 19 20 21 22 2 3 2 4 25 26 27 28 29 30 31 32

0

30 00

60 00

90 00

120 00

150 00

180 00

210 00

240 00

1 2 3 4 5 6 7 8 9 10 11 1 2 13 1 4 15 16 17 18 19 20 2 1 22 2 3 2 4 25 2 6 27 28 29 30 31 32

m o n th s

cu

mu

lati

ve

fu

nd

ing

($

K)

Aerogel Manufacture Spec.

Inner Accelerometer Full Accelerometer: DA

Instrument Spec.

Dewar/Probe Spec. Dewar Build

Probe/DAP Build

Payload I&T

Large Scale Aerogel Test

Electronics Development

$3M $13M

Payload Systems Eng.

Spacecraft Systems Eng.

PDR27 Month Technology Development

Phase B

Incremental Prototyping

Engineering Hardware

gravity probe b reaches the science...

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