GRAVITATIONAL REDSHIFT SPACE-PROBE EXPERIMENT
GP-A Final Report
Contract NAS8-27969
SUMMARY
On June 18, 1976, a Scout D rocket was launched from Wallops Island, Virginia, carrying an atomic hydrogen maser oscillator system as the payload. The frequency of signals from the oscillator was monitored on the ground at Merritt Island, Florida, by using two hydrogen masers as comparison oscillators. The first-order doppler shift in the signals was eliminated by a go-return transponder link to the payload, and the resulting data, representing the relativistic shifts, were recovered and recorded. The objective of this experiment was to measure directly the effect of the gravitational potential on the frequency of an atomic hydrogen maser assuming it to be a "proper" clock.
In the experiment, a gravitational effect amounting to some 4.5 parts in 1010 was measured with an oscillator having a stability, better than I part in 1014. Therefore,
to make the best possible use of the oscillator, we must account for all frequency shifts at the 2 to 5 X 10 to the - 15 level in of/f in the system, and this, of course, includes :,ll the phase variations that can cause such a shift to appear.
This report presents the experiment, a description of the data now available and the manner in which they were processed, and the results.
We wish to emphasize that this experiment was conducted by a large number of people and involved many different organizations. The program was managed by the George C. Marshall Space Flight Center of the National Aeronautics and Space Administration (NASA), which was also responsible for designing and assembling the payload structure and testing it before launch. NASA's Goddard Space Flight Center performed the task of tracking and data acquisition, and NASA's Langley Research Center and the Vought Corporation were responsible for the vehicle. We gratefully acknowledge the assistant of the Jet Propulsion Laboratory in the solution of the probe's trajectory.
The Smitsonian Astrophysical Observatory provided the scientific and technical leadership and constructed the probe and ground clocks and the doppler cancellation system.
1. INTRODUCTION
The test of the gravitational redshift predicted by Albert Einstein in 1907 had its beginnings with the development of atomic clocks in the 1950s; at that time, J. R. Zacharias' group at the Massachusetts Institute of Technology began working on cesium-beam clocks with an eye toward conducting an experiment between a clock placed on a mountain top and one in a valley to detect the predicted gravitational redshift. In a sense, Zacharias is a godfather of our present experiment. Its other godfather is N. F. Ramsey (1956), who discusses redshift measurement in Molecular Beams; more importantly, however, it was Ramsey who invented the hydrogen maser, in 1959.
The redshift test itself was first performed by R.V. Pound (Pound and Rebka, 1960) using a 75-:t elevator shaft. His now classic 1% test of the gravitational redshift was performed in 1960 by using the then recently discovered Mossbauer effect. It is interesting to note that this terrestrial test was made over a height approximately the same as that of the Scout rocket used in the present experiment and discussed below. The test by Pound and his coworkers, S.A. Rebka, Jr., and J. L. Snider, temporarily wiped out any further zeal to conduct a redshift experiment with clocks. To improve on their results would require far better clocks and enormously greater gravitational-potential differences between them.
However, the space program, In fact, did evolve methods to obtain larger gravitational-potential differences, and the atomic hydrogen maser was invented in 1959. This new type of oscillator was developed into a stable clock that could resolve time differences at the 10-14 level. With the entire effect of the earth's gravity (consisting....
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Inside:
TABLE OF CONTENTS
SUMMARY
INTRODUCTION
THE EQUIVALENCE PRINCIPLE ........................ 5
THE GRAVITATIONAL REDSHIFT AS A TEST OF THE EQUIVALENCE
PRINCIPLE .................... ...... ............ 11
THE BASIC CONCEPT OF THE GRAVITATIONAL REDSHIFT SPACE-
BORNE EXPERIMENT .................................. 15
THE DOPPLER 'CANCELLATION SYSTEM ..................... 21
PHASE STABILITY CONSIDERATIONS IN THE PROBE AND GROUND
STATION ANTENNA SYSTEMS.......... ... ....... ....... 31
PREDICTIONS MADE FROM PELATIVITY THEORY .............. 35
PROBE TRACKING REQUIREMENTS ........................ 43
REDSHIFT DATA ACQUISITION ........ ............. .. 47
THE DATA-REDUCTION METHOD FOR THE REDSHIFT EXPERIMENT. 51
THE SPACE PROBE MASER OSCILLATOR ...... .............. 55
EXPERIMENT OPERATIONS ... ...... .................. 61
TRACKING DATA AND TRAJECTORY SOLUTIONS .............. 0 73
CALIBRATIONDATA ........ .... ............... .... 81
EXPERIMENT RESULTS .............. .... ... .... .... 85
CONCLUSIONS ....... ... .... ........ .. 93
REFERENCES AND BIBLIOGRAPHY...... .... ......... 97
APENDICES
I. THE NASA/JPL TRAJECTORY SOLUTION
II. MASER FREQUENCY VARIATION CORRECTIONS SYSTEM PHASE
VARIATION CORRECTIONS
III. DOPPLER CANCELLING SYSTEM PERFORMANCE STATISTICS
Item Details: Book/Report - "Imaged"
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GRAVITATIONAL REDSHIFT SPACE-PROBE EXPERIMENT
GP-A Project Final Report
Contract NAS8-27969
Principal Investigator: Dr. R. F. C. Vessot
Coinvestigator: Dr. M.W. Levine
(NASA-CR--161409) GRAVITATIONAL REDSHIFT N80-19999 SPACE-PROBE EXPERIMENT Final Report
(Smithsonian Astrophysical Observatory)
147 p HC A07/MFA01 CSCL 03B Unclas G3/90 47559
Prepared for
National Aeronautics and Space Administration
George C. Marskil Space Flight Center
Huntsville, Alabama 35812
December 1979
Smithsonian Institution
Astrophysical Observatory
Cambridge, Massachusetts 02138
The Smithsonian Astrophysical Observatory and the Harvard College Observatory are members of the
Center for Astrophysics
Keywords: EXPERIMENT DESIGN, GRAVITATIONAL EFFECTS, RED SHIFT, ROCKET-BORNE INSTRUMENTS, SPACEBORNE EXPERIMENTS, ATOMIC CLOCKS, DATA REDUCTION, DOPPLER EFFECT, MASERS, RELATIVITY, SCOUT LAUNCH VEHICLE
