The SRTM sub-arcsecond metrology camera

To achieve the necessary interferometric baseline for the recent Shuttle Radar Topography Mission (SRTM), an outboard radar antenna was deployed using a 60 meter mast from a main radar antenna in the shuttle payload bay. The mast had dominant flexible modes around 0.1 Hz with tip displacements on the order of several centimeters in response to shuttle thrusters. Continuous knowledge of the mast tip position to submillimeter accuracy and antenna orientation to an accuracy of 100 arcseconds (1.6 sigma) was necessary to meet the fundamental SRTM requirements. The selected approach involved an optical metrology camera which tracked three LED targets located on the outboard antenna structure. The metrology camera was required to generate target centroid coordinates (in CCD frame) with a relative accuracy of 0.8 arcsec, 1 sigma. This paper provides a description of the metrology camera, the process used to prepare it for flight on SRTM, the LED target development effort, test and alignment issues, and the observed in-flight performance