Spacecraft Clock Maintenance for MESSENGER Operations

Abstract

PACECRAFT exposed to dynamic fluctuations in temperature, velocity, and gravity present significant challenges for a high-precision timekeeping system. En route to Mercury, the MESSENGER spacecraft has endured a total of six planetary flybys, considerable variation in proximity to the Sun, and large velocity gradients. The probe completed 15 solar revolutions during the heliocentric cruise phase, with a minimum perihelion distance of 0.302 AU and a maximum aphelion distance of 1.07 AU. Its environment changed once again when the spacecraft was captured in the gravity well of Mercury and the planetary orbital phase of the mission began. The accuracy goal of the timekeeping system is to maintain ground knowledge of the spacecraft clock to within 100 ms. This paper summarizes MESSENGER’s complex timekeeping system and the steps that have been taken to address the mission’s unique challenges. I. Introduction The MErcury, Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, now in orbit about Mercury, uses two types of onboard oscillators: an oven-controlled crystal oscillator (OCXO) for precision timekeeping as well as an internal, coarse oscillator. Nominally, both oscillators run simultaneously, giving the mission operations team the option to select either. During certain operations, such as maneuvers that require the firing of thrusters, it is preferable to select the coarse oscillator. This precaution decreases the probability that spacecraft vibrations will trigger an anomaly with the OCXO that could potentially result in a reboot of the main processor (MP). For the majority of the mission, however, it is desirable to select the OCXO. The selected oscillator drives the clock that increments the mission elapsed time (MET), which provides a common frame of reference on which to base all events for the life of the mission. When commands are sequenced, they are executed at an assigned MET, and all information coming from the spacecraft is time-stamped with this MET. The unit of measurement for the MET is seconds, counted in terrestrial dynamical time (TDT). The formula for calculating TDT is expressed numerically as TDT = UTC + 32.184 s + (leap seconds).¹ For the ground-system component of MESSENGER’s timekeeping system, the mission operations team generates spacecraft clock data files called spacecraft clock (SCLK) kernels. One SCLK kernel is generated from each Deep Space Network support during which contact is established with the spacecraft. Telemetry and predictive ephemeris data are used to interpolate the clock drift rate and to create a linear formula to convert MET to TDT. This linear formula consists of a MET/TDT intercept and the drift rate of the oscillator (TDTRATE). These parameters are loaded to the spacecraft to support onboard calculations of TDT from MET. The spacecraft uses TDT knowledge to derive position from the onboard ephemerides. The guidance-and-control subsystem utilizes these position states to orient the spacecraft relative to Earth, the Sun, and other heavenly bodies. The SCLK kernels are also used to support ground system operations, converting time tags of telemetry data, supporting commandsequencing operations, and evaluating the performance of onboard TDT knowledge. While analyzing timekeeping data through the planetary flybys, the mission operations team observed unpredicted trends in the clock drift-rate data. These trends showed a divergence from the nominally expected drift rate. The unusual drift-rate behavior caused a greater-than-expected error in the onboard TDT knowledge, although the calculated error was still within the specified requirements. Analysis by the MESSENGER team determined that