Best Source for GOES Imager Scan-Mirror Emissivity Data

摘要

The Imagers carried by NOAA's Geostationary Operational Environmental Satellites observe the Earth and its atmosphere in four channels in the thermal infrared and one in the visible part of the spectrum. Because of angle-dependent anomalous absorption in the scan mirror's SiO_x coatings, the throughput of the Imagers in the infrared depends on the east-west angle of observation. If not accounted for, this effect would introduce artificial east-west gradients into the Imagers' observations of scene brightness temperature. Therefore, NOAA includes a model of the radiative processes at the scan mirror in its operational calibration. The input to the model includes a-priori values of the emissivity of the scan mirror vs east-west scan angle. The values NOAA uses operationally are estimated during post-launch testing from observations of frames of space that span the entire east-west extent of the Imagers' field of regard above and below the Earth's disk. However, we also have an alternative source of emissivity-vs-angle values -laboratory measurements made on witness samples of the scan mirrors at MIT Lincoln Laboratory. Are those emissivity values as effective as (or more effective than) the ones NOAA estimates in post-launch testing? To answer this question, we compared the results of in-orbit calibrations with the two sets of emissivity vs scan angle. Although the results depend slightly on channel and observation conditions, the values from the post-launch measurements in space are usually the better choice.