Brief communication: Classification of thawed/frozen topsoil state by spectral gradient methods based on SMAP and GCOM-W1 radiometric data

From 2015 to 2020, using spectral gradient radiometric methods, the possibility of frozen/thawed state identification of tundra soils was investigated based on SMAP and GCOM-W1 satellite observations of ten test sites located in the Arctic regions of Canada, Finland, Russia, and U.S.. It is shown that the spectral gradients of brightness temperature and reflectivity, measured on the frequency range from 1.4 GHz to 36.5 GHz on horizontal polarization with a determination coefficient from 0.775 to 0.834, root-mean-square-error from 6.6 days to 10.7 days, and bias from -3.4 days to +6.5 days, make it possible to identify the thawed/frozen state of the tundra soils. Spectral gradient methods have a significantly higher accuracy for identification of frozen and thawed state of tundra soils in relation to single-frequency methods based on the calculation of polarization index.