EVALUATING THE DESIGN OF AN EARTH RADIATION BUDGET INSTRUMENT WITH SYSTEM SIMULATIONS .1. INSTANTANEOUS ESTIMATES

A set of system simulations has been performed to evaluate candidate s canner designs for an Earth Radiation Budget Instrument (ERBI) for the Earth Observing System (EOS) of the late 1990s. Five different instru ments are considered: 1) the Active Cavity Array (ACA), 2) the Clouds and Earth's Radiant Energy System-Instrument (CERES-I), 3) the Conical ly Scanning Radiometer (CSR), (4) the Earth Radiation Budget Experimen t Cross-Track Scanner (ERBE), and 5) the Nimbus-7 Biaxial Scanner (N7) . Errors in instantaneous, top-of-the-atmosphere (TOA) satellite flux estimates are assumed to arise from two measurement problems: the samp ling of space over a given geographic domain, and sampling in angle ab out a given spatial location. In the limit where angular sampling erro rs vanish [due to the application of correct angular dependence models (ADMs) during inversion], the accuracy of each scanner design is dete rmined by the instrument's ability to map the TOA radiance field in a uniform manner. In this regard, the instruments containing a cross-tra ck scanning component (CERES-I and ERBE) do best. As errors in ADMs ar e encountered, cross-track instruments incur angular sampling errors m ore rapidly than biaxial instruments (N7, ACA, and CSR) and eventually overtake the biaxial designs in their total error amounts. A latitude bias (north-south error gradient) in the ADM error of cross-track ins truments also exists. This would be objectionable when ADM errors are systematic over large areas of the globe. For instantaneous errors, ho wever, cross-track scanners outperform biaxial or conical scanners for 2.5-degrees latitude x 2.5-degrees longitude target areas, providing that the ADM error is less than or equal to 30%. A key issue is the am ount of systematic ADM error (departures from the mean models) that is present at the 2.5-degrees resolution of the ERBE target areas. If th is error is less than 30%, then the CERES-I, ERBE, and CSR, in order o f increasing error, provide the most accurate instantaneous flux estim ates, within 2-3 W m-2 of each other in reflected shortwave flux. The magnitude of this error is near the 10 W m-2 accuracy requirement of t he user community. Longwave flux errors have been found to have the sa me space and time characteristics as errors in shortwave radiation, bu t only about 25% as large.