In permafrost regions, active layer thickness (ALT) observations measure the effects of climate change and predict hydrologic and elemental cycling. Often, ALT is measured through direct ground-based measurements. Recently, synthetic aperture radar (SAR) measurements from airborne platforms have emerged as a method for observing seasonal thaw subsidence, soil moisture, and ALT in permafrost regions. This study validates airborne SAR-derived ALT estimates in three regions of Alaska, USA using calibrated ground penetrating radar (GPR) geophysical data. The remotely sensed ALT estimates matched the field observations within uncertainty for 79% of locations. The average uncertainty for the GPR-derived ALT validation dataset was 0.14 m while the average uncertainty for the SAR-derived ALT in pixels coincident with GPR data was 0.19 m. In the region near Utqiaġvik, the remotely sensed ALT appeared slightly larger than field observations while in the Yukon-Kuskokwim Delta region, the remotely sensed ALT appeared slightly smaller than field observations. In the northern foothills of the Brooks Range, near Toolik Lake, there was minimal bias between the field data and remotely sensed estimates. These findings suggest that airborne SAR-derived ALT estimates compare well with in situ probing and GPR, making SAR an effective tool to monitor permafrost measurements.

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In permafrost regions, the active layer is the top soil layer subject to seasonal freeze and thaw. The active layer is differentiated from the seasonal frost zone in temperate regions by the presence of underlying water-impermeable permafrost that maintains the active layer soils in a cold and wet condition and reduces decomposition rate (Woo, 2012). As a result, a large amount of organic carbon has accumulated in the active layer and underlying permafrost since the Pleistocene (Ping et al., 2015). The swift rise in air temperature in the northern circumpolar region has enhanced active layer thawing, which subsequently promotes the decomposition of soil organic matter (OM) and the release of carbon in the form of carbon dioxide or methane into the atmosphere (Schaefer et al., 2014). This positive feedback can threaten the already vulnerable permafrost carbon cycle and it is therefore crucial to have a means to monitor active layer thaw progression and permafrost degradation across the northern circumpolar region.Since 2015, National Aeronautics and Space Administration (NASA)'s Arctic-Boreal Vulnerability Experiment (ABoVE) has initiated a decade-long research campaign aiming to better understand the vulnerability and resilience of social-ecological systems to the permafrost environment in Alaska and western Canada. As an effort to link field-based studies with satellite remote sensing data, ABoVE has conducted a series of airborne campaigns starting in 2017 to acquire synthetic aperture radar (SAR), LiDAR, and imaging spectrometer observations (Miller et al., 2019). The flight transects span major vegetation (tundra to boreal forest), permafrost (continuous to sporadic), and climate (polar Arctic to boreal) gradients, and encompass a diverse range of disturbance conditions, including thermokarst, human infrastructure developments, and wildfire burns. NASA/Jet Propulsion Laboratory (JPL)'s Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) was deployed to fly its Land P-band radars two times each during the thaw season of 2017. The L-and P-band SAR data are sensitive to