Ocean surface retracking in tropical cyclones with the KaIA airborne radar altimeter

KaIA is an airborne Ka-band radar altimeter capable of centimetric range resolution and near real-time significant wave height retrievals. Beginning in 2019, KaIA has been installed on National Oceanic and Atmospheric Administration (NOAA) WP-3D Hurricane Hunter aircraft to collect data in Atlantic tropical cyclones during the hurricane season, and extratropical cyclones during the winter storm season. This article details recent retracker algorithm innovations that address specific difficulties with airborne altimetry in extreme weather. Our two most significant contributions to retracker algorithm development are: 1) a higher-order expansion of the classic Brown (1977) altimetry waveform to accommodate off-nadir pointing angles up to 3.25∘; 2) a GPS stabilization algorithm to enable along-track averaging while aircraft altitude is changing unpredictably. Comparisons against coincident measurements and modeled data are presented to validate algorithm improvements and document KaIA's performance throughout the 2021-2023 hurricane seasons. We measure less than 0.1 m bias in significant wave height retrievals relative to coincident satellite altimeters.