Determination of radio-frame position for Earth and Jupiter from Ulysses encounter tracking
Radio metric tracking data acquired from the Ulysses spacecraft about its encounter with Jupiter in February 1992 allow an accurate measurement of some components of the orbital elements describing the positions of the Earth and Jupiter with respect to extragalactic radio sources. Range and Doppler data acquired from the Earth while the spacecraft is far from any planet provide an estimate of the spacecraft trajectory relative to the orbit of the Earth. Doppler data near Jupiter provide an accurate position determination of the spacecraft with respect to Jupiter. Very Long Baseline Interferometry observations of the spacecraft with respect to the distant radio sources provide a direct measure of the spacecraft position in the radio reference frame. Combining these measurements provides a means to estimate the locations of the Earth and Jupiter in the radio reference frame. One of the three Euler angles describing the orientation of the Earth's orbit in the radio frame has been determined to an accuracy of 50 nanoradians; the result agrees with other recent determinations of this orientation. The position of Jupiter at the time of Ulysses encounter has been determined to 15 nanoradians in ecliptic latitude and longitude.