The retrieved electron density profile of ionospheric radio occultation (RO) simulation data can be compared with the background model value during the simulation and the inversion error can be obtained exactly. This paper studies the inversion error of ionospheric RO through simulation. The sources of the inversion errors are analyzed. The impacts of measurement errors, such as the errors in phase measurements and satellite orbits, are very small and can be neglected. The approximation of straightline propagation introduces errors at the height of the F1 layer under solar maximum condition. The spherical symmetry approximation of the electron density distribution is found to be the main source of the inversion error. The statistical results reveal some characteristics of the inversion errors. (1) The relative error increases with enhanced solar activity. (2) It is larger in winter than in equinox season, and it is smallest in summer.(3) For all seasons, it is smaller at middle latitude than at other latitudes. (4) For all seasons and geomagnetic latitudes, it is smaller at daytime than at other times. The NmF2 of the ROs from COSMIC are compared with the measurements of ionosondes, and the relative differences show the same dependencies on season, geomagnetic latitude and local time, as the relative errors of the simulated ionospheric ROs.

[1] The approximation of spherical symmetry of electron density is the main error source in the inversion of ionospheric radio occultation (IRO). The study in this paper shows that only even terms of the asymmetry contribute to the inversion error. An asymmetry factor for IRO is defined, and it is approximately equal to the relative error of the inversion result. The characteristics of the asymmetry factor are studied and are shown in the following: (1) the absolute value of asymmetry factor (AVAF) of a zonal occultation is much smaller than that of the meridian one; (2) higher solar activity corresponds to larger AVAF; (3) AVAF is largest in winter, second largest in equinox season, and smallest in summer; and (4) generally, AVAF at middle geomagnetic latitudes is smaller than at other latitudes, and it is large at the magnetic equator, the crest region, and the middle-latitude trough. The asymmetry factor is applied to correct the retrieved electron density, and the inversion error is greatly reduced.