Assessment and Mitigation of Ionospheric Residual Errors in GPS Radio Occultation Retrievals

Atmospheric soundings derived from Global Positioning System radio occultations (GPSRO) have the potential to be global climate benchmark observations of significant value to the global climate observing system. Past studies comparing refractivity and temperature retrievals from different GPSRO satellite platforms have demonstrated the very high precision of the measurements. However, such studies do not provide an upper bound on any systematic errors that might exist in the retrievals. One possibly significant error source for stratospheric retrievals comes from the Earth’s ionosphere. To remove the ionospheric contribution to the GPSRO measurements, a linear combination of the bending angles from the two GPS frequencies is typically used. However, due to raypath separation from the two GPS signals as well as higher-order frequency dependency of the refractive index, not all the ionospheric effects will be eliminated with this procedure. Uncorrected ionospheric diurnal- and solar-cycle variability could have an undesirable impact on the interpretation of stratospheric trends inferred from GPSRO. In this talk, we will provide an assessment of the ionospheric residual errors using three-dimensional raytracing through the USC/JPL Global Assimilative Ionosphere Model (GAIM) and discuss a new approach towards mitigating the ionospheric residual errors.