Midcourse Space Experiment attitude system performance for gyro disabled operations

Results of analysis and flight tests of the Midcourse Space Experiment attitude control system using angleonly attitude sensors are presented. The premature degradation of one of the spacecraft's ring laser gyroscopes after 18 months in orbit has led to the power cycling of the remaining gyroscope package in order to maximize its useful lifetime. The gyroscopes are now disabled when the spacecraft is not collecting data. In this configuration, the attitude system has no direct measurement of the angular velocity. The attitude state is derived using angle-only attitude sensors. The current configuration uses only coarse sensors including a horizon scanner, Sun sensors, and magnetometer. A more robust configuration that incorporates the star camera into the estimation process has been developed. The new configuration is accomplished by updates to the Kalman filter's measurement model and process noise parameters. The control system gains are also updated in response to the new sensor configuration. The flight configuration is implemented via storable data structures in the attitude computer. The development of the new configuration was accomplished by analysis of ground based simulations and has been validated on-orbit by a series of flight tests. Simulation and flight test data are reported. INTRODUCTION The premature degradation of a ring laser gyroscope (RLG) aboard the Midcourse Space Experiment (MSX) spacecraft has prompted a performance analysis of the attitude system with the gyroscopes disabled. This paper presents the results of analysis and flight tests of the attitude system using combinations of angle-only sensors without the use of a rate sensor. The analysis was performed by first uncoupling the attitude determination function from the control laws in order to evaluate the attitude estimation process without the interaction of the control system. Then the steady state and dynamic response of the entire attitude system was assessed to develop new control law parameters. Analysis of different system configurations was performed using the MSX Testbed Simulator, which provides a real-time simulation capability of the spacecraft attitude and tracking systems using brassboard flight computers that execute the flight software. The attitude system configuration changes are accomplished via a number of uploadable parameters in the flight computer. A series of on-orbit tests were performed to validate the results of the analysis and define a new flight configuration for the attitude system. * This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. f Senior Analyst, Senior AIAA Member * Space Department, Group Supervisor, Mission Concepts and Analysis Group, AIAA Member § Space Department, Mission Concepts and Analysis Group, Senior AIAA Member 1 Space Department, Mission Concepts and Analysis Group, Senior AIAA Member * Strategic Systems Department, Defense and Strike Systems Evaluation Group, AIAA Member 1 American Institute of Aeronautics and Astronautics Copyright© 1998, American Institute of Aeronautics and Astronautics, Inc. Mass spectrometer UVISI WFOV and NFOV imager visible I Krypton lamp UVISI spectrographic imagers (5) Rotatable solar array 1.2kW, 120 sq. ft MLI cryostat thermal insulation Electronics section Command & Telemetry RF Sensor electronics Power system Attitude control Beacon receiver Thermal control OSDP UVISI WFOV and NFOV imager UV Space infrared imaging telescope | (SPIRIT III) Space-based visible (SBV) instrument Reference Objects (6) j Optical bench, star camera, and gyros S-band beacon receiver antennas Steerable X-band antennas (25 Mbps) S-band TT&C antennas Figure 1. MSX Spacecraft This paper identifies tradeoffs made among the parameters for attitude determination and control and the resulting pointing performance of the MSX spacecraft with the gyro disabled. Although the scope of the analysis is limited to the MSX instruments and configuration, the spacecraft sensor complement and nadir orientation of the spacecraft provides valuable flight experience to attitude system designers seeking to reduce the cost and complexity of satellite design by the elimination of rate sensors for some missions. Background The MSX mission is sponsored by the U.S. Department of Defense (DOD) Ballistic Missile Defense Organization (BMDO). The spacecraft was designed, built and is operated by The Johns Hopkins University Applied Physics Laboratory (JHU/APL). The mission objectives and instruments are fully described in Reference 1. The spacecraft was launched into a 900 km near-Sun synchronous orbit in April 1996 and has a design life of 5 years. Figure 1 shows a diagram of the spacecraft. The goal of the mission is to collect and analyze target and background phenomenology data over a large number of wavelengths for the evaluation of space based sensors for missile tracking during the midcourse phase. The spacecraft collects data on ballistic missile type targets as well as resident space objects. Phenomenology data are collected for the terrestrial, Earth-limb, and celestial backgrounds. The diverse experiment set requires a platform with a unique combination of agility and stability. The spacecraft operates in three modes, Safe, Track, and Park. Safe mode is used for initial attitude acquisition, protection and recovery from spacecraft anomalies. The implementation of Safe mode in the attitude computer allows the attitude determination process to be uncoupled from the control laws. Track mode is used during the data collection events. Park mode provides a benign thermal and power environment for the vehicle between data collection events. In Park mode the spacecraft jc-axis, along which the instrument boresights are aligned, is slightly offset from the zenith direction. The offset from zenith provides a gravity gradient torque used for momentum management. The spacecraft y-axis is pointed away from the Sun to provide thermal control. The solar arrays rotate around the spacecraft z-axis to provide power for the spacecraft. The purpose of this analysis was to provide a robust, reliable configuration for Park mode operations with the gyros disabled when high precision pointing and fast slew maneuvers are not required. The requirement to operate in this configuration, without a rate sensor, was motivated by the early degradation of one of the gyroscopes in one of two redundant RLG packages American Institute of Aeronautics and Astronautics Copyright© 1998, American Institute of Aeronautics and Astronautics, Inc. aboard the spacecraft. . MSX is only the second spacecraft to fly this type of gyroscope. An investigation by Honeywell, the gyroscopes manufacturer, identified the cause of the degradation and showed that the degradation process is driven by the total operating time. The duty cycling of the redundant RLG during Park mode by powering down the unit should at least double its useful life.