On the Sampling Problem in Radiation Budget Studies

The radiation budget is the difference of downwelling and upwelling flux densities at a reference surface, whereas the quantity measured by narrow field of view instruments is the directional radiance. Insufficient knowledge of its angular anisotropy causes errors in the derived radiation budget. A new formulation is presented of the constraints in sampling the angular directions of the hemisphere over a mesoscale target area, introduced by scan patterns and by orbit repeat cycles. Comparison by simulation of line scanner, conical scanner (CSR), and Nimbus-7 ERB scanner in orbits with short and long repeat cycle reveals the latter orbit and the CSR as best suited to sample all directional classes.