The radiation balance of Earth is best determined by measurements of Earth-emitted longwave radiation and of solar radiation reflected from the Earth using radiometers aboard spacecraft. However, the resulting radiation maps are biased by the orbit crossing times of the spacecraft, because of the diurnal cycles of the outgoing longwave radiation OLR and reflected solar radia- tion or albedo, except for geosynchronous spacecraft. The study of interannual variations is a major aspect of climate research. When researchers attempt to study interannual variations of radiation budget components, it is often necessary to use data taken from instruments in orbits with differing orbit crossing times. The question then arises as to how much of the differences are due to the measurement of radiation components at different local times from the different orbits.
The diurnal cycles of OLR and albedo have been determined by use of 5 years of data from the Earth Radiation Budget Satellite, which flew in an orbit with 57 degrees inclination and thus pre- cessed around the Earth every 72 days. A method is presented whereby with this description of the diurnal cycles one can compute as a function of latitude and longitude the bias which is asso- ciated with measurements for a given orbital inclination and Equator crossing times. The results also depend on the data processing algorithms which are used to generate the data set. For early data sets the OLR for ascending and descending nodes of the orbit were simply averaged to pro- duce a daily mean, resulting in biases. The data processing system of the Earth Radiation Budget Experiment accounted for the diurnal cycle by use of a half-sine fit time interpolation for the OLR and accounted for albedo variation by use of a directional model which was a function of solar zenith angle. The underlying assumption is that lacking information regarding variation of the scene with time of day, scene is considered to be constant. Nevertheless, this assumption is much better than earlier assumptions. Bias maps of OLR and albedo are presented for sample months of data from the Clouds and Earth Radiant Energy System (CERES) instrument aboard the Tropical Rainfall Measurement Mission spacecraft. The technique is applicable to other meteorological parameters measured by satellites.