Dome Degradation Pattern of the ERBE Wide Field-of-View Shortwave Radiometer

摘要

The Earth Radiation Budget Satellite carried a nonscanning radiometer package for the measurement of the solar radiative flux which is reflected by the Earth and the outgoing longwave radiation from the Earth. These fluxes are measured by wide field-of-view radiometers, which are active cavity radiometers (ACRs) having apertures which are open to radiation from the Earth from one limb to the other. The reflected shortwave radiative flux is measured by an ACR which has a quartz dome over the aperture to cut out the OLR component. The instrument has operated for over 15 years, producing an unbroken data record of Earth radiation budget from one instrument. However, the during this time the dome degraded due to direct solar radiation during sun rise and sunset at the spacecraft. The degradation is taken into account by rotating the instrument every 2 weeks so as to view the Sun, which serves as a calibration target. However, the solar dosage varies over the surface of the dome, so that its degradation is not uniform. Moreover, the spacecraft operates so as to always have one side to the Sun, so that one side of the dome gets more exposure than the other. The question then arises as to what is the pattern of solar dosage and the resulting non-uniform degradation on the measurements? This paper computes the pattern of solar exposure of the dome.