ESTIMATION OF BRDF FROM AVHRR SHORT-WAVE CHANNELS - TESTS OVER SEMIARID AUSTRALIAN SITES

Correction of time series of Advance Very High Resolution Radiometer ( AVHRR) short-wave reflectances for directional effects is investigated for four semiarid Australian sites. Two contrasting models for the bi directional reflectance distribution function (BRDF), which satisfy th e reciprocity condition, are fitted to all cloud-free observations wit hin a sliding window centered on the current time. The window length i s adaptive, but typically is between 10 and 20 days, corresponding to one or two repeat cycles of the polar orbiting meteorological satellit es operated by the National Oceanic and Atmospheric Administration (NO AA). Observations within the window include satellite elevations rangi ng from one horizon to the other, so that the parameters of the BRDF m odels are well determined. In cloudy conditions the algorithm allows t he time window to be extended in small steps up to an upper limit in o rder to secure sufficient data to fix the parameters of the model. For the test sites examined, the algorithm appears to be robust. Once the BRDF has been estimated, times series of reflectances at the top of t he atmosphere are calculated for an observer at zenith and the Sun at fixed elevation. The corrected time series of reflectances are consist ent and reveal structure not apparent in the raw data. In particular, time series of normalized difference vegetation index (NDVI) computed from the BRDF-corrected reflectances show the rapid response of the fi tted BRDF is validated for one of the test sites by comparing reflecta nces observed by the visible channel of the Visible and Infrared Spin- Scan Radiometer (VISSR) on the Japanese Geostationary Meteorological S atellite (GMS) against reflectances predicted by the BRDF model. The c orrection between the two sets of reflectances is excellent. Because G MS samples regions of the angular domain not seen by AVHRR, the high c orrelation confirms that the BRDF is well represented by the model bas ed on AVHRR data, even for geometries unsampled by AVHRR. (C)Elsevier Science Inc., 1998.