High altitude sky luminance measurement

Abstract

IN THE design of a star tracking system, the radiance and spectral distribution of the daytime sky radiation at bigh altitudes are of extreme importance, These factors determine the background noise level in the star tracking subsystem and, consequently, the sip-to”noise ratio and the number of navigational stars available for star sightings. Of particular importance are the characteristics of sky background radiation near the horizon and at angles close to the sun, where the sky is of relatively high intensity. Daytime star systems normally utilize photosensors with responsivity in the blue and near-visual range of the ~lec~oma~etj~ spectrum. In this range the quantum efficiency of detectors is high, resulting in high sign&to-noise ratio and rapid acquisition and tracking capability. The U~versity of Wi~nsi~ is currently under Contract No. NASw-66 from NASA to provide equipment for ultraviolet star photography in the X-15 flight research vehicle. The purpose of this experiment is to determine the configuration of equipment to be used for a later ex~~rn~t in the Orbiting Astrono~cal Observatory. Prior to the incorporation of the ultraviolet cameras in the X-15, a two-channel photoelectric photometer is being flown to measure sky radiance in the ultraviolet and nearvisual range of the spectrum. The NASA Flight Research Center, Edwards, California is supplying Nortronics, A Division of Northrop Corporation, with data from the near-visual channel of the photometer. Nortronics is funded by the USAF Ballistic Systems Division under Contract No. ~~694~291 for reduction of this data, and this paper is a report on the measurements completed to date by this joint effort of the University of Wisconsin, The National Aeronautics and Space Administration, and No~ronics,