Measurement of the Shapiro Time Delay Between Drag-Free Spacecraft

Abstract

In 1964 Shapiro pointed out that ? can be determined from measurements of the relativistic time delay for electromagnetic waves passing near a massive body such as the Sun. The delay for two-way measurements from Earth to a spacecraft passing behind the Sun can be more than 200 μs. Microwave range and rangerate measurements of this kind from Earth to several spacecraft have provided our best information so far on ?. Laser time-delay measurements and determinations of the deflection of laser beams near the Sun also have been proposed. A mission of this kind called Laser Astrometric Test of Relativity (LATOR) currently is being considered. Here we discuss a considerably different mission which would use dragfree spacecraft, whose orbits can be accurately determined, to measure the Shapiro time delay for laser beams passing near the Sun. One spacecraft would be located near the L1 Lagrange point, between the Earth and the Sun. The other would be launched into an orbit similar to the ones used in LATOR, with 1.5 year period and eccentricity such that three occultations by the Sun would occur within 2 years after launch. We also consider higher-order time-delay effects. In the present experiment laser signals are sent from a drag-free spacecraft at the L1 point, and transponded back by a drag-free spacecraft passing behind the Sun. A high-stability frequency standard located on the L1 spacecraft permits accurate measurement of the time delay. Both spacecraft are designed for extremely low spurious accelerations at periods out to roughly 20 days.