The High Altitude Balloon Experiment (HABE) is being developed by the U.S. Air Force Research Laboratory, Space Vehicle Directorate at Kirtland Air Force Base, to investigate technologies needed to perform acquisition, tracking, and pointing (ATP) functions against boosting missiles in near-space environments. HABE is designed to demonstrate ATP sequence steps that start with acquisition of a missile plume, transition through passive IR tracking of the plume, and handover to precision tracking, which employs an active laser illuminator and imaging camera to image and track the missile nose. The Inertial Pseudo Star Reference Unit provides inertially stabilized line-of-sights (LOSs) for the illuminator laser, active fine track camera, and the marker scoring. The latter serves to measure and score the payload's pointing performance. The payload will be operated and carried aloft under a large, scientific balloon. The engagement parameters and timelines for the HABE ATP payload are consistent with scenarios encountered in space-based missile defense applications. In HABE experiments, target missiles will pass at ranges from 50 to 200 km. The performance goals of the ATP payload's LOS stabilization and marker laser pointing are required to exceed 1 microradian RMS or better in jitter, drift, and accuracy (two-axis, one sigma metrics), a requirement which stresses testing capabilities.