Abstract
The Libya-4 desert area, located in the Great Sand Sea, is one of the most important bright desert CEOS pseudo-invariant calibration sites by its size and radiometric stability. This site is intensively used for radiometer drift monitoring, sensor intercalibration and as an absolute calibration reference based on simulated radiances traceable to the SI standard. The Libya-4 morphology is composed of oriented sand dunes shaped by dominant winds. The effects of sand dune spatial organization on the surface bidirectional reflectance factor is analyzed in this paper using Raytran, a 3D radiative transfer model. The topography is characterized with the 30 m resolution ASTER digital elevation model. Four different regions-of-interest sizes, ranging from 10 km up to 100 km, are analyzed. Results show that sand dunes generate more backscattering than forward scattering at the surface. The mean surface reflectance averaged over different viewing and illumination angles is pretty much independent of the size of the selected area, though the standard deviation differs. Sun azimuth position has an effect on the surface reflectance field, which is more pronounced for high Sun zenith angles. Such 3D azimuthal effects should be taken into account to decrease the simulated radiance uncertainty over Libya-4 below 3% for wavelengths larger than 600 nm.
Highlights
Since the seventies, satellite observations have provided a global view of the Earth, from which it has been possible to generate multi-decadal time series to support global climate monitoring [1]
Pseudo-invariant calibration sites (PICSs) play a critical role, because they are suitable for sensor stability monitoring, a prerequisite to climate data record (CDR) generation
We evaluate the azimuthal distribution of the slopes having a repose angles affecting surface bidirectional reflectance factor (BRF), i.e., lying between 15◦ and 35◦ ; Figure 4
Summary
Satellite observations have provided a global view of the Earth, from which it has been possible to generate multi-decadal time series to support global climate monitoring [1]. Cosnefroy et al [2], who performed early works to identify PICSs, selected a series of about 20 such sites located in the Sahara Desert and Arabian Peninsula. These PICSs were selected according to their spatial uniformity, low cloud cover and precipitation rate. The Committee on Earth Observation Satellites (CEOS) has further refined this list to about six sites for their good spatial and temporal stability. These sites are Mauritania-1, Mauritania-2, Algeria-3, Algeria-5, Libya-1 and Libya-4
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