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TitleThe bidirectional effect in AVHRR measurements over boreal regions
AuthorLi, Z; Cihlar, J; Zheng, X; Moreau, L; Ly, H
SourceIEEE Transactions on Geoscience and Remote Sensing (Institute of Electrical and Electronics Engineers) vol. 34, issue 6, 1996 p. 1308-1322,
Alt SeriesEarth Sciences Sector, Contribution Series 20041366
PublisherInstitute of Electrical and Electronics Engineers (IEEE)
Mediapaper; on-line; digital
File formatpdf
AreaCentral Canada; Canada
SubjectsScience and Technology; Nature and Environment; remote sensing; satellites; satellite imagery; radiometric interpretations; reflectance; vegetation; AVHRR; Normalized Difference Vegetation Index (NDVI); boreal ecosystem and atmosphere study (BOREAS)
Released1996 01 01
AbstractThe objectives of this paper are to analyze the bidirectional effects of satellite data over six land-cover types in northern regions, and to test a method for the routine correction of these effects. Analyses and corrections were carried out with both single-day and 10-day composite data obtained by the advanced very high resolution radiometer (AVHRR) from central Canada acquired in 1993/1994, in part, for the boreal ecosystem and atmosphere study (BOREAS). The model of Wu et al. [1995], developed from a separate data set collected at lower latitudes, was employed for correcting the effects. The analysis showed viewing angle dependence in AVHRR channels 1 and 2 from both single-day images and composites. Reflectances at extreme viewing angles are two to four times larger than those observed near nadir. On average, the effects introduce a variation of 30% relative to mean reflectances. Although the effects decrease in the normalized difference vegetation index (NDVI), they are nevertheless significant before the correction. Using the model of Wu et al. [1995], the BRDF-related variability is reduced by about 68% in channel 1 and 71% in channel 2. After a simple adjustment of the model coefficients, a further reduction of 4% (channel 1) and 6% (channel 2) of the BRDF-related variability was achieved for the 10/sup 6/ km/sup 2/ BOREAS region. The effectiveness of the correction with both original and refined model of Wu et al. was found to be weakly dependent on land-cover type. Corrections for coniferous, mixed wood, and cropland are better than other land-cover types (rangelands/pasture, deciduous, and transitional forests) with residual BRDF errors around 0.05 in both channels. Overall, the model (albeit simple) performs reasonably well throughout the growing season. To apply the model, only general knowledge of land-cover type is required, namely forest, cropland, grassland, and bare ground.

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