A Method of Estimating Residual Bending Error in GNSS-RO Absolute TEC

Ionospheric refraction and dispersion effects induce residual bending error in the Total Electron Content (TEC) derived from Global Navigation Satellite System (GNSS) radio occultation (RO) measurements. This study proposes a practical method for the direct evaluation of residual bending error in the conventional absolute TEC retrieval. We derive the expression for the residual bending error in absolute TEC, by accounting for refraction residuals in both the carrier phase and pseudorange measurements. Two error components are identified: the altitudinally varying residual bending error from carrier phase TEC, and a constant bias from phase-to-pseudorange leveling. These errors are quantitatively assessed and analyzed by utilizing real observational data from COSMIC-2. Our analysis reveals that the residual bending error is predominantly influenced by vertical TEC gradients and can exceed the level of 10 TEC units (TECU). The findings underscore the need for estimating and correcting residual bending errors to improve the accuracy of absolute TEC retrievals, particularly under high solar activity conditions.