Estimation of the Residual Error due to Ionospheric Bending in Relative TEC using COSMIC-2 GPS-RO Measurements

The Total Electron Content (TEC) derived from Global Positioning System (GPS) radio occultation (RO) measurements contains residual bending errors that arise from signal refraction and dispersion in the ionosphere. Motivated by the need to ensure the accuracy of TEC retrievals, particularly under high solar activity conditions, we investigate the impact of ionospheric bending on the conventional dual-frequency TEC. This work marks a new attempt in assessing the residual bending error in TEC estimates using real RO measurements, in contrast to prior research efforts that relied primarily on simulation studies. We present an estimation method for the residual bending error based solely on measurement data, avoiding reliance on a priori knowledge of the true ionospheric state. Utilizing real RO observations from COSMIC-2, our analysis reveals that the residual bending error is largely influenced by the vertical gradient of TEC, with additional dependencies on solar activity and local time. Large residual bending errors were observed under high solar activity conditions, reaching up to 17.7 TECU in a 15-day dataset from January 2023. Residual bending errors in dual-frequency TEC were often found to exceed the 3 TECU threshold from COSMIC-2 accuracy requirements, suggesting the use of single-frequency TEC or the application of residual bending error correction as a potential alternative under certain conditions inducing high TEC gradients.