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Snow thickness on sea ice is a largely undersampled parameter yet of importance for the sea ice mass balance and for satellite-based sea ice thickness estimates and thus our general understanding of global ice volume change. Traditional direct thickness measurements with meter sticks can provide accurate but only spot information, referred to as "needles" due to their pinpoint focus and information, while airborne and satellite remote sensing snow products, referred to as "the haystack," have large uncertainties due to their scale. We demonstrate the remarkable accuracy and applicability of ground-penetrating radar (GPR) snow thickness measurements by comparing them with in situ meter stick data from two field campaigns to Antarctica in late winter/early spring. The efficiency and millimeter-to-centimeter accuracy of GPR enables practitioners to acquire extensive, semiregional data with the potential to upscale needles to the haystack and to potentially calibrate satellite remote sensing products that we confirm to derive roughly 30% of the in situ thickness. We find the radar wave propagation velocity in snow to be rather constant (+/- 6%), encouraging regional snow thickness surveys. Snow thinner than 10 cm is under the detection limit with the off-the-shelf GPR setup utilized in our study.

Plain Language Summary Snow on sea ice, especially on Antarctic sea ice, plays a significant role in climate analysis due to its contribution to the mass and volume balance of the cryosphere. The thickness of snow on sea ice is not known in full detail as it is hard to derive from satellite data. Based on an extensive data set from two Antarctic winter/spring expeditions, we show the efficiency and accuracy of ground-penetrating radar to map snow thickness on a semiregional scale. Such surveys could potentially be extended to larger scales and contribute to satellite snow thickness algorithm calibration schemes.