Grazing Global Navigation Satellite System Reflectometry (GNSS-R) altimetry, while promising for precise surface height retrieval, faces challenges from tropospheric refraction. Beyond path delay errors, tropospheric refraction can cause an asymmetry in the incident and reflected signal paths due to tropospheric gradients and the asymmetric observation geometry. The impact of this refraction asymmetry on grazing GNSS-R altimetry remains unclear. This study systematically quantifies this impact and characterizes the resulting errors in grazing observations. We propose a new surface height retrieval model that accounts for refraction asymmetry and applies to both spaceborne and airborne scenarios. Through a 30-day global GNSS-R simulation, in conjunction with European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data and a ray tracing technique, our results show that decimeter-level systematic bistatic delay biases can be observed in airborne scenarios (3.5 km), increasing exponentially with decreasing elevation angles. In contrast, these errors are millimeter-level in spaceborne scenarios (530 km). A comparison with the traditional height retrieval model reveals that uncorrected refraction asymmetry in airborne observations can introduce decimeter- to meter-level altimetric errors at extremely low elevation angles (2–7 degrees). This finding highlights the critical importance of accounting for the effects of refraction asymmetry on airborne grazing GNSS-R altimetry, especially for continuous coherent observations covering a wide range of low elevation angles.
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