MESSENGER Onboard Timekeeping Accuracy During the First Year in Orbit at Mercury

The NASA MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission to Mercury, launched in 2004, utilizes a timekeeping system that consistently provides accurate post-processed time knowledge between the spacecraft and ground operations to within 150 µs. The main constituent of this observed error, when the round-trip light time is computed directly from the geometrical distance traveled, has been attributed to the Shapiro delay. One means that is used to monitor the accuracy of MESSENGER timekeeping is a series of in-flight tests. We typically refer to these as “Latch MET” tests because the primary feature of these tests is the latching of the onboard mission elapsed time (MET) counter value in response to specific “Latch MET” commands transmitted from a Deep Space Network station. In 2011, we increased the nominal in-orbit cadence of Latch MET tests to once per week in order to characterize the relationship between the observed MESSENGER time accuracy and the Shapiro delay. Here we provide details on the collection and analysis of these results and their relevance to operating deep-space clock systems dominated by relativistic effects.