A technique was developed to eliminate growth of clock drift error by representing the onboard planetary rotation model in terms of spacecraft clock seconds, rather than in terms of standard SI seconds.

Onboard models of planetary motion are traditionally expressed in terms of Ephemeris Time, which uses SI units for time duration (seconds). But all times on the spacecraft are expressed in terms of spacecraft clock time (sclk seconds). As these diverge over time, attitude estimation error grows and celestial pointing error grows.

On Sol 647, the Mars Science Laboratory (MSL) updated the onboard model of planetary motion to not only synchronize with the spacecraft clock at a specific time to eliminate accumulated error, but also updated the rate of planetary rotation - by expressing it in radians per spacecraft clock second - to match the spacecraft clock rate. This effectively eliminates the effect of clock drift on attitude estimation and celestial pointing, and potentially eliminates the need for future re-synchronizations. This is possible on MSL because the rate of spacecraft clock drift on the surface has been effectively constant. It is also possible because the onboard planetary motion model is expressed as parameters that can be updated during operations.

This software is available for commercial licensing. Please contact Michael J. Vallan at This email address is being protected from spambots. You need JavaScript enabled to view it.. Refer to NPO-49658.


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This article first appeared in the July, 2020 issue of Tech Briefs Magazine.

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