The Relativity Mission, Gravity Probe B

Experimental Design,

Sources of Error, and Status

Mac Keiser

Snowmass 2001

July 4, 2001

GP-B Concept

ee R

R

R

Rc

GIR

Rc

GM 232

132

3

241

2

1v

Experimental Design Guidelines

Reduce Classical Torques on Each Gyroscope Less Than 0.3 mas/yr.

Measure Drift Rate of Each of Four Gyroscopes Relative to the Guide Star with an Accuracy of better than 0.3 mas/yr.

Use Optical Aberration as a Known Calibration Signal.

Using VLBI, Measure Proper Motion of Guide Star to an Accuracy of Better than 0.15 mas/yr.

Include Telemetry to Monitor Potential Experimental Errors and Commands to Change Experimental Conditions

Reduce Overall Experimental Error to Less than 0.5 mas/yr.

Gyroscope Drift Rate

Reduce Support Dependent Torques by Drag-Free Satellite Rolling Spacecraft Careful Selection of Gyroscope Orientation and Orbit Tight Manufacturing Tolerances

Reduce Support Independent Torques by Monitoring and Controlling Charge on Gyroscope Rotor Residual Magnetic Field Less Than 9 Microgauss Residual Gas Pressure Less Than 10-11 Torr Rolling Spacecraft and Orbit Selection

Gyroscopes

•Fused quartz rotor R/R<10-6

•Quartz housing R/R<10-5

•Electrostatic suspension 10-9g - 1 g

•Capacitative positioning <0.3nm at roll

•He gas spin-up >150Hz

•UV charge control <5 pC

•Low temp. bake-out <10-13 torr

•Spin to roll alignment <1arcsec

< 0.01 marcsec/yr (non supported gyro)<0.08 marcsec/yr (suspended gyro)

•London moment read-out with dc SQUIDs

•Superconducting pickup loop on gyroscope housing

London Moment Read-Out

SQUID

)(GssL e

mcB 71014.1

2

)( 3cmG ssL rr

e

mcM 383 1069.5

< 810-29 J/Hz (<50 0/Hz) 200marcsec/Hz (510-11 G/ Hz )

Superconducting Magnetic Shields

Magnetic shielding: < 510-7 G, >1012 total ac attenuation

Folded Schmidt Cassegranian

150” focal length, 5.6” diameter

All quartz construction

Potassium hydroxide bonding

Image splitting with roof prisms for quadrant read-out information

Low temperature read-out using photodiodes and silicon preamplifiers at 70K

4 telescope windows with 60% transmission in visible and IR and rf rejection

Telescope0.1 marcsec pointing precision

Quartz BlockMetrology Frame

Four Gyroscopes Mounted 8.25 cm center-to-center

Telescope Bonded to End of Quartz Block

Orientation of Gyroscope Spin Axis Relative to Measured Direction to Guide Star Determined by Difference Between Gyroscope and Telescope Readouts

Stability of Quartz Block Metrology Reference Frame at Satellite Roll Frequency Better Than 1.3 arc sec

Low Thermal Expansion Coefficient Small Variation in Temperature at

Satellite Roll Freuquency

Probe and Dewar

Probe Ultrahigh Vacuum

<10-13 Torr Achieved Using Low Temperature Bakeout

Sintered Titatanium Cryopump Provides Equivalent of 50 m2 of surface area

Dewar Holds 2300 liters of

superfluid helium Lifetime on Orbit

Greater Than 17 months

Main GP-B Systems

Gyroscope Telescope Science Instrument

Cryogenic Probe Payload Space Vehicle

Uncertainty in Proper Motionof GP-B Guide Star HR 8703 (IM PEG)

• Visible and radio star

• Magnitude mv = 5.69

• Declination= 16.84 deg (close to equator)

• Proper motion calibrated by SAO using VLBI

• Expected accuracy by 2002 is about 0.09 marcsec/yr

Preliminary HR 8703 Radio CentroidSolar System Barycentric, J2000 Coordinate System

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

32550326003265032700

Dec

linat

ion

- 16

d50'

(m

as)

28300

28350

28400

28450

28500

28550

16.9 Jan 9718.9 Jan 97

30.0 Nov 9721.9 Dec 97

27.9 Dec 97

1.8 Mar 98 12.5 Jul 98

8.4 Aug 9817.3 Sept 98

13.8 Mar 99 15.6 May 99

18.3 Sept. 99 E

15.0 Dec 91

22.4 June 93 13.2 Sept 93

24.3 July 94

10.0 Dec 99

GP-B Mission Timeline

Initial set-up and calibration

40-60 days, ~ 1,000 events

Science Mission data

13-15 months

Post Mission calibration

30 days, ~ 100 events Data rates 26 kbit/s total 15 kbit/s science data 11 kbit/s snapshots, vehicle

Experiment AccuracyOne Gyroscope

Support Dependent0.08 m as/yr

Support Independent0.01 m as/yr

G yroscope T orques0.08 m as/yr

Statistical Error0.13 m as/yr

System atic Error0.12 m as/yr

Readout Error0.17 m as/yr

Accuracy of M easuredG yroscope Drift Relative

to G uide Star0.19 m as/yr

Uncertainty inProper M otion of

G uide Star0.09 m as/yr

Overall Experim ent AccurcyOne G yroscope

0.21 m as/yr

GP-B From Now to Launch

Complete Payload Verification 10/1/01

Payload/Spacecraft Integration 10/22/01

Space Vehicle Testing 8/21/02

Launch 10/30/02

Experimental Test of General Relativity in Space

Special Relativistic Effect - Aberration of Starlight - Used as Known Calibrating Signal

Gravitation Deflection of Light by the Sun Used as Experimental Cross-Check

Precise Measurement of Motional (Lense-Thirring) Effect

Most Accurate Experimental Test of General Relativity