Anisotropia del campo della radiazione riflessa sulle superfici naturali

1. Introduction
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Section I-Directionality in the visible and solar-infrared portion of the
electromagnetic spectrum
The remote sensing community is becoming more and more interested in the anisotropy of the
reflected radiation field over natural surfaces due to its influence on remote sensing data. The
anisotropy of the reflected radiation field is described by the Bidirectional Reflectance Distribution
Functions (BRDFs) which are equations describing how the reflectance of a surface varies with the
solar-view geometry.
Several papers have been published concerning the algorithms for the BRDF modelling (Chopping
2000a; Chopping 2000b; Lucht et al., 2000; Duchemin, 1999; Rahman et al., 1999; Verstraete et
al., 1990; Pinty et al., 1989; Hu et al., 1997), and about methods and instrumentation for measuring
the Bidirectional Reflectance Factor (BRF) (Jensen and Schill, 2000; Sandmeier, 2000; Deering et
al., 1999; Greuell and de Ruyter de Wildt, 1999; Sandmeier and Itten, 1999; Tsuchida et al., 1999).
The modelling approach has been well studied in order to normalise the off-nadir acquisition of
sensors having large swath (e.g., NOAA-AVHRR, SPOT-VEGETATION, low altitude
spectrometry imageries) or specifically designed off-nadir acquisitions (e.g., MISR, IKONOS), and
to correct the geometry of nadir-viewing sensors, such as the Landsat series, in mountainous terrain
(Greuell and de Ruyter de Wildt, 1999; Schaaf et al., 1994). The directional features of remote
sensors imply the need to add the “directional reflectance” resolution (Myneni et al., 1995) to the
basic four resolutions (i.e., spatial, spectral, radiometric and temporal) characterising the Earth
observation systems. The off-nadir normalisation allows reflectance values, band ratios, indexes
(e.g., normalised vegetation indices) and all the others parameters related to remote sensing
measurements of surfaces to be compared independently of whether they are positioned in the
centre or at the boundaries of an image. Moreover, after having modelled the BRDFs over natural
surfaces it is possible to estimate albedo as the ratio between the hemispheric fluxes, or irradiances,
of the reflected and incoming radiation. Accurate estimation of surface albedo is of critical
importance in understanding circulation models and climate dynamics
BRDF modelling is commonly based on semi-empirical models, which need an accurate
parameterisation, based on the comparison between modeled and measured values of the BRFs for
several Sun-target-sensor geometries. In order to acquire a sufficient set of BRFs it is necessary to
observe the surface from several angles of view in a short time avoiding the changes of the surface
properties. Ground bidirectional reflectance data will be important not only for parameterisation of
existing and new BRDF models but also to validate satellite related measurements acquired from
multiangular spaceborne sensors, such as MISR, MODIS and POLDER, as well as to investigate
the relationship between biogeophysical parameters and BRDF effects (Sandmeier, 2000).