Full Polarimetric GNSS-R Assessment of the Freeze and Thaw States of the Terrestrial Cryosphere

The shift between frozen and thawed conditions on theEarth's surface influences climate, hydrology, and ecology. Aprimary objective of the Soil Moisture Active Passive (SMAP)mission is to estimate the surface binary freeze/thaw (F/T) state forthe northern hemisphere above 45° N latitude with a classificationaccuracy of 80% at 3-km spatial resolution and 2-day intervals.The objective was to be accomplished by the SMAP L-band radarmeasurements. In July 2015 the radar transmitter suffered ananomaly that prevented it from nominal operation. The missionpartially met the goal exceeding the targeted 80% accuracy but at36 km resolution. After the radar anomaly, the SMAP missionswitched the radar receiver band pass filter frequency to GPS L2c,acting as a full-polarimetric Global Navigation Satellite System –Reflectometry (GNSS-R) receiver, known as SMAP-Reflectometer(SMAP-R). This work focusses on using SMAP-R signals toclassify the F/T state over the Northern latitudes, within thelimitations of the dataset. An algorithm based on the seasonalthreshold approach that was originally envisioned for the SMAPradar is applied to the SMAP-R data (i.e., bistatic radar). Then thealgorithm is evolved using a Random Forest algorithm to aidthreshold selection from the discriminator built in the seasonalthreshold approach. This algorithm is applied for years 2016 to2022 over the Northern Hemisphere terrestrial cryosphere andshows an F/T classification accuracy agreement better than 97%with respect to the classification of the official SMAP F/TRadiometry product, proving the potential of polarimetric GNSSRto derive F/T