IN-FLIGHT CALIBRATION OF THE NIMBUS-7 EARTH RADIATION BUDGET (ERB) SENSORS .1. A THERMAL-MODEL FOR THE SHORTWAVE CHANNELS

Much of the early record of spectrally broadband earth radiation budge t (ERB) measurements was taken by the ERB instrument launched on the N imbus-7 spacecraft in October 1978. The wide-field-of-view (WFOV) sens ors measured the emitted and reflected radiation from November 1978 th rough January 1993, and the first nine years have been processed into a stable, long-term dataset. However, heating and cooling of the ERB e xperiment introduced thermal perturbations in the original measurement s that were only significant in the shortwave (SW) channels. These sen sors were covered by spherical filter domes to absorb incident longwav e (LW) radiation. In this paper, a thermal regression model-the therma l calibration adjustment table (CAT)-is developed to track and remove these thermal signals from the SW data. The model relies on instrument temperatures within and near the surface of the ERB instrument, and t he observed nonzero nighttime sensor readings represent the thermal si gnals. Confidence that the model is stable for daytime applications wa s gained by smoothing the solution using ridge regression and noting t he effect on the solution coefficient vector. The bias signal produced by the thermal CAT portrays the balance of instrument heating and coo ling within the Nimbus-7 variable external radiation environment. Cool ing occurs over about two-thirds of an orbit including satellite night . During the nighttime, the sensor bias change is about 17 W m(-2) (co mpare with mean daytime SW flux of about 200 W m(-2)) with little seas onal or annual fluctuation. Strong warming takes place during morning and evening twilight when direct solar radiation illuminates the WFOV sensors. This warming effectively compensates for nighttime cooling wh en the opposite thermal signature is found. Additional daytime warming occurs for satellite positions near the solar declination when the ef fects of combined LW and SW terrestrial fluxes exceed thermal cooling to space. However, this heating is influenced by the terrestrial scene and so it varies seasonally. The thermal CAT was one of two semi-inde pendent procedures, each of equal mean accuracy, developed to validate and correct for thermally induced sensor signals. The other, called t he global CAT, is described in the second paper in this series. Althou gh the thermal CAT was considered heuristically superior, the global C AT was chosen for the basic calibration work since it was thought to b e potentially more stable for the production of a consistent long-term ERB dataset.