Optical alignment of the Global Precipitation Measurement (GPM) star trackers

The optical alignment of the star trackers on the Global Precipitation Measurement (GPM) core spacecraft at NAS A Goddard Space Flight Center (GSFC) was challenging due to the layout and structural design of the GPM Lower B us Structure (LBS) in which the star trackers are mounted as well as the presence of the star tracker shades that blocke d line-of-sight to the primary star tracker optical references. The initial solution was to negotiate minor changes in th e original LBS design to allow for the installation of a removable item of ground support equipment (GSE) that coul d be installed whenever measurements of the star tracker optical references were needed. However, this GSE could only be used to measure secondary optical reference cube faces not used by the star tracker vendor to obtain the relationsh ip information and matrix transfonnations necessary to determine star tracker alignment. Unfortunately, due to unexpectedly large orthogonality errors between the measured secondary adjacent cube faces and the lack of cube calibration data, we required a method that could be used to measure the same reference cube faces as originally meas'lfed by the vendor. We describe an alternative technique to theodolite autocollimation for measurement of an optical reference mirror pointing direction when normal incidence measurements are not possible. This technique w as used to successfully align the GPM star trackers and has been used on a number of other NASA flight projects. We also discuss alignment theory as well as a GSFC-developed theodolite data analysis package used to analyze angular metrology data.