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The Gravitational Wave Experiment

Doppler Tracking - The Gravitational Wave Experiment

Doppler tracking is a technique to accurately monitor the distance to a interplanetary space probe. A radio signal is sent to the probe, and retransmitted back to the same or another antenna on ground. The transmitted and received signals are compared in phase, using an atomic clock to propagate the phase during the long (several thousands of seconds) round-trip travel time of the radio signal.

The very accurate (typically some hundred microns) measurement can be used to investigate:

the gravity field of bodies (asteroids to planets) that the spacecraft flies by
the atmosphere of planets and satellites, when the radio link happens to cross them
and, last but not least, gravitational waves, i.e. small ripples in the spacetime structure, expected from Einstein general relativity, that may cause the Earth-spacecraft distance to oscillate slightly.

Experiments to try to detect gravitational waves have been carried in 1992 and 1993 using the S/C Ulysses and several VLBI radiotelescopes, including Medicina, Noto and Kashima (Japan). Data analysis is in progress.

References:

B. Bertotti, R. Ambrosini, S.W. Asmar, J.P. Brenkle, G. Comoretto, G. Giampieri, L. Iess, A. Messeri, H.D. Wahlquist: "The Gravitational Wave Experiment", Astronomy and Astrophysics Suppl. Series 92, 431 (1992).

B. Bertotti, G. Comoretto, L. Iess: "Doppler tracking of spacecraft with multi-frequency links", Astronomy and Astrophysics, 269, 608 (1993).

B. Bertotti, R. Ambrosini, J.W. Armstrong, S.W. Asmar, G. Comoretto, G. Giampieri, L. Iess, Y. Koyama, A. Messeri, A. Vecchio, H.D. Wahlquist: "Search for gravitational wave trains with the spacecraft Ulysses", Astronomy and Astrophysics, 296, 13 (1995).

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