The residual ionospheric error (RIE) in radio occultation (RO) measurement analysis is investigated through both analytic and numerical ray tracing. The study has shown that the difference between the propagation paths at different frequencies is an extremely important factor in determining the level of the RIE. Further, that horizontal ionospheric structure, such as the equatorial anomaly, can have a substantial impact. The results confirm the conclusion of previous studies that asymmetry in the ionosphere between the upward and downward legs of the propagation is a significant factor. The study has also considered the effect of traveling ionospheric disturbances upon the RIE. It has been shown that there can be dramatic changes in RIE through the full period of a traveling ionospheric disturbance (TID). Consequently, because of their frequency and unpredictability, TIDs have the potential to set a limit on the achievable accuracy of RO measurement analysis.