The effect of surface roughness on the reflected radiance of bare terrain : measurements, modelling and inversion studies

Bare terrain surfaces have attracted little attention in albedo and remote sensing studies despite their large representation in desert and semi-arid regions of the world. It is the aim of this thesis to investigate the interaction of solar radiation with the Earth's bare surface structure in two stages. The first stage investigates the effect of surface structure on the incoming and outgoing flux using measurements of the Bidirectional Reflectance Distribution Function (BRDF) for various surface types. The second stage developes a methodology for the extraction of roughness information from the BRDF. Theories describing the effect of facets orientation and multiple reflections on the reflected radiance field are reviewed and discussed. Two major deficiencies are identified: firstly, there is a lack of radiometric data for rocky and stony terrain; secondly there is a lack of measurements which examine the contribution of multiple reflections to the reflected radiance. A methodology was developed to measure the contribution of secondary reflection (where the first reflection occurs outside the sensor field of view) to the upwelling radiance. In agreement with published models, results showed that the contribution can be high ( 30%) in certain geometrical configurations. However, unlike other model results, the role of anisotropic reflections via the particulate nature of the surface was found to be of prime importance.A simulation model was developed and it supported the experimental findings. A study of the BRDF in relation to surface properties was conducted in arid land near Fowlers Gap Research Station, New South Wales. New apparatus was developed to take accurate Hemispherical Directions Radiance Measurements (HDRM). A digitizer for three dimensional in situ roughness measurements was also developed. More than 100 hemispherical data sets were collected for various illumination conditions and surface types: dunes, desert stony pavements and rocky terrain slopes. In general it was found that most of the surfaces exhibited an anisotropic reflection, combining a major component of backscattering with a weaker component of forward scattering. The BRDF of the different surface types in relation to their roughness properties as determined by the field digitizer is then examined. The main hypothesis in this research concerns the inverse process of determining the microstructure of the surface from the directional distribution of reflected radiance. It states that given several sets of directional reflected radiance measurements, it should be possible to derive the roughness characteristics of the surface. The question whether the BRDF has a one-to-one correspondence with the surface microstructure is then addressed. A technique was developed to simulate a wide range of microelements and their BRDF. Results from the simulation show a significant number of cases of equifinality in which a variety of microstructures exhibit the same BRDF. Analysis of these cases suggests that there are many distinctively different combinations of structural properties which can balance the effect of structure on the reflected radiance field. A comparison of field measurements with the simulated cases confirms that BRDF equifinality must be considered in remote sensing applications where there is a need to derive surface properties such as roughness, microstructure and texture from satellite data. Analysis of the microstructures which exhibited equifinality revealed that they have similarity in their roughness properties. Two parameters are then introduced for characterizing the surfaces' roughness in relation to their reflectance properties. On the basis of the simulation data a method is derived for determining the roughness parameters from reflectance. data in certain viewing angles. The assessment of the results showed a moderate to high accuracy which varied with the sun zenith angle.