Analysis of inversion errors of ionospheric radio occultation

Abstract

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.

Long-term comparison of the ionospheric F2 layer electron density peak derived from ionosonde data and Formosat-3/COSMIC occultations

“…Schreiner et al (1999) compared GPS/MET NmF2 with ionosondes and determined variations at the magnitude of 26%, i.e., similar to the results obtained here. Schreiner et al (1999) additionally found an insignificant bias of few percentages which was later confirmed by Wu et al (2009) and various other authors. In the case of hmF2, mean differences in the range of À7 km and 7.5 km with an outlier of À22 km at the Pole during night are determined.…”

“…The strong correlation between ionosondes with NmF2 supports the outcome of Lei et al (2007) where 276 co-located value pairs between 31 globally distributed ionosondes and F-3/C measurement are analyzed and a correlation coefficient of 0.85 was found. Furthermore, a relative accuracy level of around 20% for NmF2 was determined by Hajj & Romans (1998), Tsai et al (2001), andWu et al (2009). The slightly worse accuracy level resulting from this study is probably caused by the use of automatically detected ionogram peaks contained in the SPIDR database.…”

“…For instance, Wu et al (2009) Lei et al (2007) evaluated correlations of maximum electron densities between F-3/C EDPs and ionosondes at the early stage of the constellation in July 2006. Krankowski et al (2011) validated F2 peak parameters of F-3/C measurements in the European sector during 2008 andYue et al (2010) studied the retrieval accuracy by means of synthetic data driven by background information of the NeQuick model to determine absolute and relative accuracies for NmF2 and hmF2.…”

Ionospheric electron density observed by FORMOSAT-3/COSMIC over the European region and validated by ionosonde data

“…The measurement errors include the carrier phase errors of GPS signals and the orbital errors. A study of the error sources by Wu et al (2009) showed that measurement errors are negligible; the approximation of straight-line propagation induces errors at the height of the F1 layer under the solar maximum condition. The main source of inversion error is the spherical symmetry approximation.…”

“…The correlation coefficient was 0.85. Wu et al (2009) compared the COSMIC-derived NmF2 values with the measurements of 62 ionosondes; data corresponding to ionosondes were provided by SPIDR. An analysis of globally distributed data showed that the standard deviation of the relative differences in NmF2 values was 20.7% with a mean of −3.2%.…”