Frequency performance of the New Horizons ultra-stable oscillators nine years of continuous in-flight monitoring

The New Horizons mission has provided an unprecedented opportunity to observe the evolution two ultra-stable oscillators in deep space for over nine years. The unusual architecture of the New Horizons communications system allows for direct observation of one of the two USOs during every tracking pass. The communications system is based on a transceiver as opposed to the normal transponder. The downlink is independent of the uplink in this architecture and so the USO is solely responsible for the downlink carrier frequency. Both USOs have greatly stabilized during the nine years of flight and have recently changed the direction of the frequency drift. The history since launch in January 2006 and the recent drift history are presented and discussed.