The flight performance of the Galileo Orbiter USO

Radiometric tracking Doppler measurements of the signal transmitted by the Galileo spacecraft using an ultrastable oscillator (USO) as a frequency reference have been acquired by antennas of the NASA Deep Space Network between Dec. 1989 and Nov. 1991. The USO is a dual oven-controlled oscillator with an AT-cut quartz crystal (SiO₁ ) resonator. The output frequency of 19.124980 MHz is multiplied by 120 to produce the transmitted signal frequency (~2.29 GHz). The average Allan deviations at 1-s and 10-s correlate well with spacecraft range, and fall as τ^-1. This is consistent with white PM noise dominating at these times scales due to the low SNR´s expected from using the low gain antennas. The Allan deviations at 100-s and 1000-s agree with pre-flight USO stability results at these time scales. The Galileo USO appears to be healthy and functioning normally