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Figure I. View directions for AVHRR. Each polar plot represents the view zenith angle from 0° to 90° as the radius and the sun-view relative azimuth as the polar angle, (a) View directions over eleven consecutive days, (b) View directions over forty days

For many users of AVHRR data, a desirable consistency of reflectance time series can be achieved by correcting the daily observations to a standard view and illumination geometry, nadir being the obvious choice for the view direction. This is essentially an interpolation problem, since the AVHRR does indeed sometimes view the target at nadir (although cloud-free conditions will not always coincide with this ideal viewing geometry). For albedo estimation, the sampling of view directions would ideally cover the whole hemisphere represented in Figure 1. While Figure 1 illustrates the fact that AVHRR samples part of the hemisphere of view directions, albedo estimation will effectively require extrapolation to view directions that are never sampled by AVHRR.

Figure 2. Time series of AVHRR channel 1 reflectances at Tinga Tingana

O'Brien et al. (1998) developed an algorithm to correct top-of-the-atmosphere AVHRR time series to nadir viewing by fitting a parametric BRDF model to the time series within a sliding window. The algorithm was tested at semi-arid sites in Australia and performed well, judging by the consistency of the time series and the rapid response of the resulting NDVI time series to rainfall. The algorithm was also successful at predicting the radiance measured by the Japanese Geostationary Meteorological Satellite, which views the site from a direction unsampled by AVHRR. This suggests that the technique may be capable of the extrapolation in view angle required for albedo estimation.

Work is underway to further develop this BRDF fitting technique to apply to the whole of Australia, for the correction of AVHRR time series to nadir viewing and for the estimation of albedo in the AVHRR shortwave bands. The technique must be demonstrated to work for a wide variety of Australian land cover types and to work in the presence of cloud, which was largely absent for the tests at semi-arid sites described above. A variety of BRDF models will be assessed with the method, which has only been attempted so far with the model of Staylor and Suttles (1986) and Li's model for sparse canopies (Wanner et al., 1995). The criteria for ranking models, following the approach used by Hautecoeur and Leroy (1996), will include robustness for different land cover types, robustness in the presence of data gaps caused by cloudiness, and the ability to extrapolate to unmeasured view directions. It will also be of interest to determine whether the model must be fitted separately to individual pixels, or whether single model fits can be satisfactorily applied to regions of similar land cover type, giving savings in computation.

The interpolation of the AVHRR view to nadir and the angular extrapolation required for albedo will be tested with data from the Polarization and Directionality of the Earth's Reflectances (POLDER) sensor that operated on Japan's Advanced Earth Observing Satellite (ADEOS) satellite from November 1996 until June 1997. This sensor was the first to acquire global scale observations of reflectance with good angular coverage. Figure 3 shows that over a few days POLDER densely samples the hemisphere of view directions out to zenith angles of about 60°. The comparison between AVHRR and POLDER will be done in three steps:

1. for cases where the AVHRR and POLDER view directions and solar zenith angles are similar, to gauge the effect of the spectral mismatch between the bands of the two sensors;

2. for cases where the POLDER view direction is at nadir, to test the BRDF-correction of AVHRR to this standard view direction;

3. for all POLDER view directions, to test, over the largest range of view directions, the BRDF that has been fitted to AVHRR data.

Figure 3. View directions for POLDER over six consecutive days.

Figure 3. View directions for POLDER over six consecutive days.

These comparisons will be made at the top of the atmosphere, to avoid the uncertainty associated with atmospheric correction of the satellite observations in the absence of good knowledge of the aerosol and water vapour content of the atmosphere. It is assumed that if the BRDF fitting technique is shown to be robust at the top of the atmosphere then it will also perform well for surface BRDFs.

Figure 4. Locations of sites in eastern Australia for testing AVHRR algorithms for angular correction and albedo, plotted on a mosaic of AVHRR channel 2 images. Filled squares are uniform sites chosen to span a wide range of vegetation types, open squares are randomly selected sites, and diamonds are field data sites

Figure 4. Locations of sites in eastern Australia for testing AVHRR algorithms for angular correction and albedo, plotted on a mosaic of AVHRR channel 2 images. Filled squares are uniform sites chosen to span a wide range of vegetation types, open squares are randomly selected sites, and diamonds are field data sites

About seventy sites in eastern Australia have been selected at which to compare the AVHRR and POLDER data. This region has been chosen because it has good angular coverage by the AVHRR data in the archive held at CSIRO Atmospheric Research. The sites represent every class in a thirty-two class land cover map based on AUSLIG (1990), and have been chosen to be among the most uniform within their respective classes to reduce the effect of misregistration errors. Time series of AVHRR data covering at least the eight-month POLDER period are being extracted at each site. At the same time, data are being extracted at about seventy randomly selected locations to gauge the performance of the BRDF fitting algorithm at "typical" sites, and also at sites for which field measurements or airborne scanner data exist that can be used to further verify the BRDFs and albedos retrieved from AVHRR time series. Figure 4 shows all of the selected sites.

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