Estimation of the dynamics of frequency drift in mature ultra-stable oscillators: A study based on the in-flight performance from New Horizons

Since launch on Jan 19th, 2006, the frequencies of the two ultra-stable oscillators (USOs A & B) on-board the New Horizons spacecraft have been determined using the application of non-coherent Doppler radiometric data. The two resulting histories are approaching nearly ten years of operating behavior while undergoing the voyage through the Solar system, now approaching 210 million km beyond Pluto. During the majority of this in-flight time, USO A has shown a monotonically negative drift in frequency starting at launch at -3.8 E-11/day. This negative drift rate has reduced over time in a nearly logarithmic manner to its current state where the drift has either become indistinguishable from noise, or is in the process of reversing. The possible reversal in drift direction would be unexpected in a highly mature oscillator, particularly in deep space almost void of external perturbations. Recently, the method of optimal stopping has been explored as a detector for the change in highly precise clock behavior. The method has been shown to have high accuracy in locating jumps, and changes in linear drift rates within processes dominated by random walk. This paper will report on the application of the optimal stopping method toward the estimation of dynamic drift change in the recent epoch of USO A data.