COMPARISON OF AIRBORNE AND SURFACE SPECTRAL BIDIRECTIONAL REFLECTANCEFACTORS, SPECTRAL HEMISPHERICAL REFLECTANCE AND SPECTRAL VEGETATION INDEXES

Six sets of airborne advanced solid-state array spectroradiometer (ASA S) and ground modular multiband radiometer (MMR) remotely sensed bidir ectional measurements acquired over one First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment site in 198 7 and 1989 were compared for the following parameters: bidirectional s pectral reflectance factor, spectral hemispherical reflectance, simple ratio, and normalized difference vegetation index. ASAS at-sensor rad iances were atmospherically corrected and converted to surface reflect ance factors. Selected MMR bands were simulated with ASAS data, and he mispherical reflectance for both ground and airborne data sets was est imated using data collected only in or close to the solar principal pl ane. The shapes of the reflectance factor response curves (as a functi on of view zenith angle) were strongly affected by solar zenith angle and compared well between instruments. Off-nadir anisotropy in reflect ance factors was comparable between sensors, while actual values of re flectance factors differed 2-35% relative between instruments in the g reen channel and 0-38% relative in the red channel. For the data set g iving the overall closest agreement, ASAS reflectance factors differed from MMR values by 5-17% relative (0.3-1.8% absolute) in the green an d 5-10% relative (0.2-0.5% absolute) in the red. These differences sho wed no correlation with solar zenith angle. Some of the differences we re arbitrarily introduced by the 5 degrees offset in view zenith angle s (except nadir) between the two instruments. Other differences were c aused by the discrepancy in solar zenith angle for some of the sets, v ariable deviations from the solar principal plane, hotspot and associa ted shadowing effects not consistently recorded by both instruments, a nd sampled ground area variations. Estimates of hemispherical reflecta nce compared very well between sensors, with differences of only 3-14% relative (0.3-1.6% absolute) in the green and 11-27% relative (0.8-2. 4% absolute) in the red. For the ASAS data, using atmospherically corr ected reflectance factors (instead of at-sensor radiances) significant ly increased the values of the spectral vegetation indices (SVIs). Off -nadir anisotropy for the SVIs derived from both sensors' data sets wa s less than that observed for the bidirectional reflectance factors, a nd NDVI off-nadir deviations were much less than those of the simple r atio (SR). Large differences in the values of SR and NDVI between sens ors indicate SVIs calculated from broadband (MMR) versus narrowband (A SAS) data are not comparable.