A Simple Atmospheric Delay Mapping Function for Near-Nadir Altimeters

Satellite altimeters require correction for tropospheric delay to precisely measure sea surface heights (SSHs). Wide-swath altimeters (WSAs), operating near nadir, experience a slowdown in microwave signals and slight path bending due to changes in the atmospheric refractive index. The continued fraction-based global navigation satellite system (GNSS) mapping function, originally designed for low-elevation-angle GNSS observations, inadequately addresses the challenges faced by WSAs at very high-elevation angles. In addition, it necessitates time-and-location-specific calculations based on numerical weather models (NWMs), limiting its effectiveness in offline applications. This letter introduces a novel atmospheric delay mapping function specifically developed for WSAs with incident angles below 10(degrees). Utilizing the European Centre for Medium-Range Weather Forecasts (ECMWF)'s stratified atmospheric pressure data and the ray-tracing method, this function improves accuracy by supplementing a first-order trigonometric function with remainders. This approach markedly reduces model errors from the millimeter-to-centimeter scale down to the sub-millimeter-to-millimeter scale. This new mapping function, dependent on elevation angle, allows for direct projection in the slant-range direction once the zenith delay is determined, thereby significantly simplifying its application.