Defining the Correlation between Gyro Temperature and Gyro Current through Analytic Study of RXTE Mission Engineering Data

Abstract

As the Rossi X-ray Timing Explorer (RXTE) is in its fourteenth year of flight, understanding the state of health of the observatory and its components is paramount in assessing risk (and corresponding funding) in continuing/extending mission operations. This paper examines the performance of the RXTE Inertial Reference Unit (IRU), specifically Life of Mission (LOM) data of gyro motor current and temperature. Through analytic study, a relationship between gyro motor current and temperature has been identified, defined through numerical equations, and measured for accuracy. A custom tool was developed to define this relationship for any specified period. A focused discussion on IRU anomalies over the LOM examines internal and external factors for anomalies including IRU location, gyro lubricant accumulation/buildup, and seasonal variances. Through measuring the rate of change of motor current and temperature during anomalies, a resulting correlation to gyro drift was identified and calculated. Practical applications of this analysis in Flight Operations are discussed including a prediction utility for performance limit threshold violations using dynamic models updated with spacecraft telemetry. I. Introduction he Rossi X-Ray Timing Explorer (RXTE), a National Aeronautics and Space Administration (NASA) spacecraft built and operated at the Goddard Spaceflight Center (GSFC), was launched on December 30, 1995 into a near-circular earth orbit at 580 km altitude and 23 degrees inclination. RXTE is a three-axis stabilized earth orbiter with no propulsion system. Now in its 14 th year (current altitude of approximately 482 km), RXTE has far surpassed its design life (two year minimum with a goal of five years). Analysis of Life of Mission (LOM) data as well as historical Inertial Reference Unit (IRU) anomalies have allowed the RXTE Flight Operations Team (FOT) and NASA engineers to gain further insight into IRU performance over 14 years of flight time. It has been observed through trending and analysis that the correlation existing between gyro temperature and gyro drift can be defined. The goal of this study was to define this correlation with a numerical function. Raw data samples spanned specific periods over the RXTE mission life in order to yield an equation which characterizes the performance relationship between gyro temperature and current. At this stage of RXTE mission operations, it is critical to understand the performance of subsystems and components. While on-board instruments continue to provide quality science, budgetary constraints in extended mission operations require the FOT to take on a larger role in understanding and communicating spacecraft state of health. The research conducted in the formulation of this paper provides NASA with detailed analysis of the performance of the RXTE IRU, as well as a prediction utility which propagates truth models to forecast performance limit threshold violation based on telemetry. When RXTE was first launched, the mission design slewed RXTE to an average of 20 targets per day to record science. Currently, RXTE slews anywhere from 15 to 50 targets per day. Slew durations are no longer than 30 minutes and the slew rate is 6 degrees per minute. In nominal operations, RXTE stays at slew targets between slews as generally the science mission planning team will collect science at a particular target for a period of time, after which will slew to the next target to collect science. Science observation durations at a particular target vary. RXTE will stay at a particular target up to 12 hours for long science observations. RXTE will also perform raster