For three months in 1970, NASA, Goddard Space Flight Center conducted an experiment to try and detect the motion of the Earth's pole of rotation by laser ranging to an Earth satellite. Two laser tracking stations were used, one at Goddard Space Flight Center and one near Seneca in New York State, 408 km away and on the same meridian as the Goddard station. The objective of the experiment was to precisely measure the orbital inclination of the Beacon Explore C spacecraft, on a daily basis and with a resolution of a few hours, to a few hundredths of an arc second by the simultaneous tracking of the spacecraft as it passed between the two stations. During the experiment over fifty thousand range measurements on over two hundred revolutions of the satellite were obtained and on fifty occasions the satellite was simultaneously tracked at both stations. The noise accuracy of the tracking varied between 20 and 60 cm during most of the experiment and the orbital inclination obtained from the measurements showed a systematic change of about 25 arc sec, due primarily (about 80%) to the gravitational perturbation of the orbit by the Sun and the Moon. Perturbations by solar radiation pressure and the solid-Earth tides appear to account for most of the remaining systematic changes in the inclination and from which it has been possible to derive a value of Love's number for the Earth as A preliminary determination of the motion of the pole in the meridian containing the stations by subtracting the known perturbations (including tides) from the observed perturbation suggests that an accuracy of 3 to 5 m has been obtained, over periods of about 6 hr, during the latter part of the experiment. The experiment has also demonstrated that the present laser tracking systems are able to measure the chord distance between the two tracking stations with a repeatability of 25 to 30 cm.

The present analysis of the data is directed towards an improved extraction of the gravitational perturbations and an improved determination of the Love number k2.

A full account of the experiment and its results so far will be published elsewhere.