In-Flight Calibration of the Nimbus-7 Earth Radiation Budget (ERB) Sensors. Part II: Short-Term Perturbations

Abstract

This paper describes the production calibration adjustment algorithms used to remove thermal perturbation and stray light noise signals from the Nimbus-7 earth radiation budget (ERB) measurements. Sunlight, both direct and scattered from the sensor baffles, contaminated the ERB measurements at satellite sunrise and sunset. The problem covered subsatellite solar zenith angles from 90° to 120° and reduced the usefulness of the longwave spectral radiation measurements. Scattered light corrections are made from 90° to 99° while orbit-by-orbit interpolation is used frown 99° to 121°. Tests indicate that in the mean the midpoint interpolation error is less than 1 W m?2 with a standard deviation of about 5 W m?2. Thermal perturbations on the total channel 12 (0.2?50 ?m) appeared to be always less than 0.3%. However, the Suprasil-W domes on the otherwise similar shortwave channels 13 and 14 in some way helped produce thermal perturbations of up to 6% or more in channel 13 (0.2?3.8 ?m and up to 3% or more in channel 14 (0.7?2.8 ?m). These perturbations arose from variations in external radiant heating during the day, night, sunrise, and sunset. In addition, the on/off cycles of the ERB and neighboring experiments produced day-to-day variations. The algorithms described here helped produce a stable 9-year-long measurement set. No thermal corrections were made in channel 12 and the obvious thermal perturbations in channels 13 and 14 were corrected. The absolute accuracy of the calibrated measurements is difficult to determine. The remaining uncertainty depends on the perturbing functions that were greater at high latitudes, near satellite sunrise and sunset, than in the Tropics. In June and July, the corrections for the daytime thermal perturbations near the North Pole may be too large by 3?5 W m?2. In general, the Nimbus-7 ERB products show good agreement with the follow-on Earth Radiation Budget Experiment (ERBE) products.