Characterizing Near-Nadir Ka-Band SAR Backscatter from Wet Surfaces and
Diverse Land Covers
Abstract
The forthcoming Surface Water and Ocean Topography (SWOT) satellite and
AirSWOT airborne instrument are the first imaging radar-altimeters
designed with near-nadir, 35.75 GHz Ka-band InSAR for mapping
terrestrial water storage variability. Remotely sensed surface water
extents are crucial for assessing such variability, but are confounded
by emergent and inundated vegetation along shorelines. However, because
SWOT-like measurements are novel, there remains some uncertainty in the
ability to detect certain land and water classes. We study the
likelihood of misclassification between 15 land cover types and develop
the Ka-band Phenomenology Scattering (KaPS) scattering model to simulate
changes to radar backscatter as a result of changing surface water
fraction and roughness. Using a separability metric, we find that water
is five times more distinct compared with dry land classes, but has the
potential to be confused with littoral zone and wet soil cover types.
The KaPS scattering model simulates AirSWOT backscatter for incidence
angles 1-27°, identifying the conditions under which open water is
likely to be confused with littoral zone and wet soil cover types. A
comparison of KaPS simulated backscatter with AirSWOT observed
backscatter shows good overall agreement across the 15 classes (median
r2=0.76). KaPS characterization of the sensitivity of
near-nadir, Ka-band SAR to small changes in both wet area fraction and
surface roughness enables more nuanced classification of inundation
area. These results provide additional confidence in the ability of SWOT
to classify water inundation extent, and open the door for novel
hydrological and ecological applications of future Ka-band SAR missions.