Several potential secondary mission objectives arise from the opportunity to explore Earth for the first time with a P-band SAR system. The Biomass Secondary Objectives Assessment Study (Paillou et al., 2011) identified a variety of secondary applications and assessed whether their requirements could be accommodated within the mission specifications. In particular, three objectives are expected to benefit significantly from the long P-band wavelength, while at the same time being feasible and compatible with the Biomass mission design. The presentation will detail each science objective and provide current insights on these applications for Biomass.
1. Mapping subsurface geology
Access to freshwater resources is already a major concern: in Saharan and sub-Saharan Africa, most people do not have access to safe water supplies, and the situation is expected to get worse in the future. Geological maps are crucial for mineral and groundwater exploration, and remote sensing is an important tool in establishing such maps. However, in arid regions such as North Africa, the geology is mostly hidden under a thin layer of dry, sandy sediments. Low-frequency SAR is able to penetrate dry sediments and map the subsurface down to several metres, because of low absorption and limited volume scattering. For example, L-band SAR has proven capable of penetrating a few metres of dry, homogeneous material such as sand (McCauley et al., 1982). If the sand surface is smooth, dry and thin, the subsurface of interest will not be masked, and the measured backscatter will provide an image of the subsurface roughness and slope. This can then be turned into information that is useful for exploration and geophysical prospecting (Paillou, 2017). Aircraft campaigns have illustrated the capacity of P-band SAR to penetrate at least 4 m of dry sediment (Paillou et al., 2011). The enhanced penetration capabilities of a P-band SAR, being less sensitive to the covering sediments, will be important in groundwater exploration but will also offer a unique opportunity to reveal the hidden and still unknown past hydrological history of deserts.
2. Ice sheet applications
Large changes of the Greenland and Antarctic ice sheets have been observed over recent decades, and SAR data have shown a significant acceleration of the glacier velocities both in Greenland and in Antarctica. One way of estimating the mass balance of ice sheets is by mapping the ice velocity at a flux gate with known ice thickness. Accurate ice velocity maps are also needed when modelling the response of ice sheets to climate change. In Greenland, ice sheet velocity maps are generated on an operational basis (Solgaard et al. 2021), and the velocity fields of the Antarctic ice sheets have also been mapped (Rignot et al. 2011). The measurement accuracy, however, is 1 m/yr to 17 m/yr with the currently available data, while the histogram for the entire Antarctica peaks at 5 m/yr (Rignot et al. 2011). To achieve a high velocity sensitivity, SAR data must be acquired with a long temporal baseline, and the correlation time increases with decreasing frequency, as seen when comparing L- and C-band results (Rignot and Mouginot, 2012). P-band excels by an even longer correlation time, as deep penetration makes the radar signal interact with stable subsurface scatterers in the dry snow zone. A temporal baseline defined by BIOMASS’ 8 months global mapping cycle is not unrealistic (Dall et al. 2013). Ice applications are dependent on sufficient compensation for ionospheric scintillations, which are particularly severe at high latitudes. Without any compensation, the ionosphere is the primary error contributor at L-band, and at P-band the ionosphere may be prohibitive, because the impact of the ionospheric scintillations increases with decreasing frequency.
3. Terrain Topography under Dense Vegetation
Digital Terrain Models (DTMs) represent the elevation of the ground in the absence of vegetation, buildings and so on. These ‘bare-earth’ images are crucial in a range of applications, including ecology, forest management, water resource management, mineral exploitation, national security and scientific research. However, currently available large-scale products are more accurately described as Digital Elevation Models (DEMs) because in forested areas they differ significantly from a true DTM. At P-band, vegetation causes less attenuation, therefore Biomass can fill this major gap in our knowledge of global topography. In addition, the scattering centre of the tree-ground double bounce- signal occurs at ground level and can be isolated using polarimetry.
Over its lifetime, Biomass will produce a DTM of the terrain topography under dense vegetation, thus removing the biases in DEMs using shorter wavelengths, such as the Copernicus DEM. Biomass will also be able to exploit this new DTM for slope corrections associated with the primary objectives, allowing initial products generated with current DEMs to be reprocessed, thus refining the biomass products.
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