Abstract
The article of the freeze/thaw dynamic of high-latitude Earth surfaces is extremely important and informative for monitoring the carbon cycle, the climate change, and the security of infrastructures. Current methodologies mainly rely on the use of active and passive microwave sensors, while very few efforts have been devoted to the assessment of the potential of observations based on signals of opportunity. This article aims at assessing the performance of spaceborne Global Navigation Satellite System Reflectometry (GNSS-R) for high-spatial and high-temporal resolution monitoring of the Earth-surface freeze/thaw state. To this aim, reflectivity values derived from the TechDemoSat-1 (TDS-1) data have been collected and elaborated, and thus compared against the soil moisture active passive (SMAP) freeze/thaw information. Shallow subsurface soil temperature values recorded by a network of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> stations have been considered as well. Even if an extensive and timeliness cross availability of both types of experimental data is limited by the spatial coverage and density of TDS-1 observations, the proposed analysis clearly indicates a significant seasonal cycle in the calibrated reflectivity. This opens new perspectives for the bistatic L-band high-resolution satellite monitoring of the freeze/thaw state, as well as to support the development of next-generation of GNSS-R satellite missions designed to provide enhanced performance and improved temporal and spatial coverage over high latitude areas.
Highlights
AND BACKGROUNDT HE freeze/thaw (FT) state of the ground describes the seasonal landscape transition between predominantly frozen and thawed conditions [1]–[5]
The potential of spaceborne Global Navigation Satellite System Reflectometry (GNSS-R) for the soil freeze/thaw monitoring at high latitudes was investigated in this article
TDS-1 data were compared against the soil moisture active passive (SMAP) L2 freeze/thaw fraction and the SMAP L3 freeze/thaw state in Siberia, within homogeneous land cover classes
Summary
T HE freeze/thaw (FT) state of the ground describes the seasonal landscape transition between predominantly frozen and thawed conditions [1]–[5]. Passive microwave radiometers are sensitive to FT changes, primarily because of soil permittivity variations, which can become significantly smaller in case of frozen soil This causes an increase in the surface emissivity, which affects the brightness temperature measured at the receiving antenna. We carry on an assessment of the impact of the FT dynamic on the TDS-1 signal, including land-cover information of the ROI generating the GNSS reflections In this respect, the proposed analysis should be considered complementary, but somehow more realistic with respect to the one proposed in [28], where the spatio-temporal sampling of SMAP observations is determined by the high-gain conically scanning antenna, in contrast with the low-gain nonscanning pattern of the TDS-1 GNSS-R mission [24].
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