Mercury Ion Standards

INTRODUCTION

A space-based clock with frequency stability better than 10^–14 over a several day period would enable one-way deep space navigations, where Doppler data is accumulated in a down-link only fashion. Currently, deep space navigation is implemented by measuring the Doppler frequency shift of a 2-way link from a ground station to a spacecraft (s/c) and the coherent return link. Typically, these links are maintained for 7–8 hours per s/c track, requiring full use of a 34-meter antenna in the Deep Space Network (DSN) for the time the s/c is sufficiently above the horizon. A clock with 10^–14 or better frequency stability on board a s/c could be used to navigate to the same precision as can be done with the two-way method [1]. Additionally, when more than one s/c orbit around the same planet, they can be tracked simultaneously with one antenna. Multiple s/c tracking by a single antenna can reduce antenna usage and DSN costs. The short-term performance in the small atomic clock described here, 10^–13/τ^½, can steer a s/c Ultra-Stable Oscillator (USO) reaching ~10^–15 in a few hours averaging time thereby supplying H-maser quality frequency stability in a much smaller package, 2–3 liters. Alternatively, this clock could be used to steer a 10^–12-grade quartz oscillator to exceed the typical performance of a USO beyond 100 seconds averaging and deliver 10 to 100 times improved frequency stability over that of a USO at 1-hour averaging. This is because USO quartz oscillators typically show flicker frequency noise of about 10^–13 from 1 second to longer times and show a linear frequency drift of 1–5 × 10^–10 per day [2]. There are many applications for ultra-stable clocks both in space and on the ground where a small package size is required. For example, there are severe restrictions on physical size for on-board instrumentation for deep space vehicles; total s/c mass (un-fueled) is often less than 400 kg with future trends toward even less mass. The components in a s/c radio system are 1–3 kg or less for each module. A USO is 3–4 kg, though most missions do not require one, and similarly, a Traveling Wave Tube Amplifier (TWTA) is about 2–3 kg. Ion clock technology has shown great inherent stability, reaching ~3 × 10^–16 frequency stability in a free running mode where no periodic calibrations (of, for example, Zeeman transitions to determine internal magnetic fields and their changes over time) were made to disrupt continuous clock operation [3]. The design for the small clock is based on the same approach as used in the ion clock described previously [3,4] though in a much smaller package.

Additionally, this technology has shown very small temperature coefficients, a few times 10^–15 per degree C change. These numbers are measured without any thermal shielding of the clock physics and electronic packages. Only modest thermal shielding would be required to reach 10^–15 stability. More...