Joint Milli-Arcsecond Pathfinder Survey (JMAPS) Fine Attitude Determination Approach

Introduction: The Joint Milli-Arcsecond Pathfinder Survey 1 (JMAPS) is a Department of the Navy space astrometry mission to conduct an all-sky, bright star astrometric and spectrophotometric survey. The primary goal of the mission is to update the bright star catalog currently used by Department of Defense, NASA, and commercial star tracker manufacturers for spacecraft attitude determination. Secondary goals include the development and flight demonstration of an advanced detector such as the H4RG-10 CMOS-Hybrid detector/readout sensor. The JMAPS spacecraft configuration and instrument specifications are shown in Fig. 1. To support the mission star catalog accuracy of 1 milli-arcsecond (mas) in position, 1 mas/year in proper motion, and 1 mas in parallax, the instrument line-of-sight must maintain its pointing stability of 50 mas (1 σ) during observation. This stringent requirement is met in part by using the instrument as a fine attitude determination star tracker to produce an unprecedented 10 mas (1 σ) attitude determination accuracy at a 5 Hz rate. The approach developed at NRL to accomplish this objective is presented. JMAPS Fine Attitude Determination: Instrument fine attitude determination starts with the current spacecraft attitude estimates determined by the Kalman filter that combines star tracker and gyro measurements and estimates the attitude at 5 as (1 σ) accuracy. Using the spacecraft attitude and the onboard star catalog, guide stars in the instrument field of view (FOV) are identified. Guide star windows are then placed around the expected star locations as shown in Fig. 2. Typically, a 64 pixel by 64 pixel guide star window will be placed for an initial acquisition. The size of the guide star window decreases as the instrument fine attitude becomes available. Fine attitude determination is performed by comparing the measured centroid locations of the guide stars to their expected locations at the centers of the windows. Thanks to the small size of the guide star window, which is less than 64 pixels or 32 as, the process of refining attitude estimates through the instrument measurements is considered a small angle attitude update with respect to a reference attitude. This permits application of small angle attitude determination methods in addition to well-known, general-purpose algorithms such as TRIAD, QUEST, and q-method. 2 Using the small angle assumption, a new computationally efficient algorithm 3 was developed based on the homogeneous transformation (HT) that has typically been used to describe robot manipulator kinematics. Simulations and Results: The performance of the …