Precision Pointing Improvements for the Lunar Reconnaissance Orbiter Camera

Abstract

The Lunar Reconnaissance Orbiter (LRO) was launched in 2009 and, with its seven science instruments, continues to make remarkable discoveries of the Moon. LRO is in a slightly elliptical, polar lunar orbit and nominally maintains a nadir orientation. In early 2018, the Miniature Inertial Measurement Unit (MIMU) was powered off following a fairly rapid decline in the laser intensity on the X axis. An algorithm combining star tracker quaternion differentiated rates and integrated control torque commands in a complementary filter, now provides the onboard rate estimate, replacing the MIMU. One of the science instruments, the LRO Camera(LROC) system, contains a Wide Angle Camera (WAC) and two Narrow Angle Cameras (NAC). The NACs suffer from larger attitude drifts during imaging of the Moon for stereo image processing, resulting from pointing stability degradation of the star tracker derived rates. The controller gains and structural filter were redesigned to attenuate the new disturbances while also maintaining adequate stability and modal suppression. This paper documents the development, testing, and operational implementation of the revised control system parameters, specifically targeted for use during LROC imaging. The new parameters improved pointing performance for LROC.