Measurement of the Gravitational Time Delay Using Drag-Free Spacecraft and an Optical Clock

Details Measurement of the Gravitational Time Delay Using Drag-Free Spacecraft and an Optical Clock Measurement of the Gravitational Time Delay Using Drag-Free Spacecraft and an Optical Clock

May 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009iau...261.1904a&link_type=abstract

American Astronomical Society, IAU Symposium #261. Relativity in Fundamental Astronomy: Dynamics, Reference Frames, and Data Ana

Astronomy and Astrophysics

Astronomy

Scientific paper

Much improved accuracy in measurement of changes in the gravitational time delay of electromagnetic waves passing by the sun may be achieved with two drag-free spacecraft, one with a stable clock and laser transmitter and one with an high-stability transponder. We consider one spacecraft near the Earth-Sun L-1 point with an advanced optical clock, and the transponder on a second satellite, which has a 2 year period orbit, with eccentricity e=0.37. Then superior conjunctions will occur at aphelion 1, 3, and 5 years after launch of the second spacecraft. The measurements can be made using carrier phase comparisons on the laser beam that would be sent to the distant spacecraft and then transponded back. With the recent development of clocks based on optical transitions in cooled and trapped ions or atoms, it appears that a noise spectral amplitude of about
5x10-15/sqrt(Hz) at frequencies down to at least 1 microhertz can be achieved in space-borne clocks. An attractive candidate is a clock based on a single laser-cooled Yb+ trapped ion, with a transition wavelength of 435 nm. Clocks based on trapped Sr+ and Al+ ions and Sr atoms in an optical lattice also have shown close to the desired performance at frequencies above 10-4 Hz. The other main requirement is that both spacecraft be drag-free at a level of 1x10-13 m/s2/sqrt(Hz) at frequencies down to at least 1 microhertz. The corresponding requirement for the LISA gravitational wave mission is 3x10-15 m/s2/sqrt(Hz) at frequencies down to 10-4 Hz, and Gravitational Reference Sensors have been developed to meet this goal. They will be tested in the LISA Pathfinder mission, planned by ESA for flight in 2011. The requirements to extend the performance to longer times are mainly thermal. The achievable accuracy for determining the PPN parameter gamma appears to be about 1x10-8.

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