Infrared Satellite Radiometry

Abstract

A 50.8 cm aperture infrared radiometer designed to detect infrared emission from satellite vehicles is described. The radiometer may be equipped with either cooled lead sulfide or indium antimonide detectors. Radiation modulation is accomplished by space filter type chopper discs enabling angular resolution of 1167 waves per radian to aid in background suppression. A nodding Newtonian folding mirror provides for scanning the ¼° × ¼° instantaneous field over ±2.5° in the search-acquisition mode of operation. A memory oscilloscope is used to correlate low level signals by their ordered appearance in repeated line scans. Measurements of infrared irradiance from orbiting satellites are presented. The unfiltered PbS detector irradiance levels may be explained as being largely, if not wholly, due to reflected sunlight from the satellite. Thermal radiation from 1960 ι I (Echo I) was recorded with the InSb detector with irradiance levels on the order of 2 × 10−12 watts/cm2 in the 2–6 μ region. A signal of this magnitude would indicate the surface temperature to be of the order of 500°K. It is shown that other satellites must have been significantly cooler than 500°K to escape detection due to their passive radiation only.