Abstract
This study presents a comprehensive comparison of radar altimetry signatures at Ka-, Ku-, C-, and S-bands using SARAL, ENVISAT and Jason-2 data over the major bioclimatic zones, soil and vegetation types encountered in West-Africa, with an emphasis on the new information at Ka-band provided by the recently launched SARAL–Altika mission. Spatio-temporal variations of the radar altimetry responses were related to changes in surface roughness, land cover and soil wetness. Analysis of time series of backscattering coefficients along the West African bioclimatic gradient shows that radar echoes at nadir incidence are well correlated to soil moisture in semi-arid savannah environments. Radar altimeters are able to detect the presence of water even under a dense canopy cover at all frequencies. But only measurements at Ka-band are able to penetrate underneath the canopy of non-inundated tropical evergreen forests.
Highlights
Spaceborne radar sensors provide global observation of continental surfaces at different frequencies, resolution, incidence angles and polarizations
Our goal is to examine how the spatio-temporal variations of the surface properties, related to soil roughness, water content (i.e., surface soil moisture (SSM) or presence of open water), and vegetation cover, impact the radar responses of land surfaces in the different frequency bands
As the backscattering coefficients of the different altimetry missions are provided in dB in the Geophysical Data Records (GDRs), they were first converted into their natural values as expressed in Eqs. (1) and (2)
Summary
Spaceborne radar sensors provide global observation of continental surfaces at different frequencies, resolution, incidence angles and polarizations. Over West Africa, side-looking radar scatterometers, notably the C-band wind-scatterometer (WSC) onboard the European Remote Sensing satellites ERS-1 (launched in 1991) and ERS-2 (launched in 1995), already demonstrated a strong potential for the monitoring of surface dielectric properties (Mougin et al, 1995; Frison and Mougin, 1996a) related to soil moisture changes (Wagner et al, 1999a; Naeimi et al, 2009; Zribi et al, 2009) and vegetation dynamics (Frison and Mougin, 1996b, 1998; Hardin et al, 1996; Wagner et al, 1999b; Woodhouse and Hoekman, 2000) at a spatial resolution of. A recent study demonstrated the capability to retrieve SSM from ENVISAT RA-2 backscattering coefficients over Sahelian savannahs in the Gourma region of Mali (Fatras et al, 2012)
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