Developments Toward a Disciplined Timekeeping System for Lunar and Planetary Navigation

Abstract

The future of both lunar and planetary missions will involve the use of an increasing number of orbiters, landers and robotic rovers with ever expanding objectives over greater land area range. The coordination of these individual assets into a system capable of supporting complex exploration activities will necessarily require a local timekeeping system able to directly assist navigation, event synchronization and trunk communication back to Earth.The Johns Hopkins University Applied Physics Laboratory (JHU/APL) has derived a method for disciplining quartz Ultra-stable Oscillators (USO) to extend its superior frequency stability from 10 seconds to nearly 30 days of operating time. This is accomplished through a Kalman estimator that optimally removes the drift of the free running USO. The accuracy of the estimator is improved by periodically comparing the frequency of the USO to an atomic reference, typically located on the Earth, and transferring its accuracy to the timekeeping system via periodic uplink sessions. This is a method most frequently known as 'disciplining an oscillator'. JHU/APL will report on the accuracy of a disciplined USO that maintains better than ±1.5 pt seconds over 30 days and describe our roadmap toward a robust, timekeeping system capable of providing both short-term and long-term frequency stability for lunar and planetary navigation infrastructure and timekeeping.

Composite USO/CSAC timekeeping system

“…Our research into USO-based timekeeping shows that incorporating an in-situ control system for removing USO deterministic frequency drift and error bias, based on a multi-variance method with a Bayesian maximum likelihood estimator (MLE) process, should mitigate post-processing dependency and enhance local navigation autonomy. This work is a continuation of the work presented in [7] and showed, through simulation with archived USO test data, a possible 1000 fold improvement of USO timekeeping performance of < 1 μs over 30 to 70 days of autonomous operation (no ground clock update).…”

“…The composite clock architecture is entirely similar to the system described in Fig. 10 of [7]. In fact, the method for atomic frequency discrimination used in the CSAC is based on coherent population trapping, as also described in [7].…”

A low complexity smoothing algorithm for improved GPS point solutions on board LEO spacecraft

“…Accurate position and time estimates are important for a variety of smallsat missions. Examples of missions, which require high performance timekeeping, are deep space missions using one‐way ranging (Imken et al., 2017; Wallis et al., 2005; Weaver & Kantsiper, 2007), small satellite constellations functioning as sensor arrays for astronomy (Engelen et al., 2013; Rajan et al., 2015), low Earth orbit (LEO) constellations for Global Positioning System (GPS) augmentation and improved positioning (Ge et al., 2018; Reid et al., 2018), and Earth observation missions which require accurate position and time information for imagery and science data.…”