Quantifying residual ionospheric errors in GNSS radio occultation bending angles based on ensembles of profiles from end-to-end simulations

Abstract

Abstract. The radio occultation (RO) technique using signals from the Global Navigation Satellite System (GNSS), in particular from the Global Positioning System (GPS) so far, is currently widely used to observe the atmosphere for applications such as numerical weather prediction and global climate monitoring. The ionosphere is a major error source in RO measurements at stratospheric altitudes, and a linear ionospheric correction of dual-frequency RO bending angles is commonly used to remove the first-order ionospheric effect. However, the residual ionospheric error (RIE) can still be significant so that it needs to be further mitigated for high-accuracy applications, especially above about 30 km altitude where the RIE is most relevant compared to the magnitude of the neutral atmospheric bending angle. Quantification and careful analyses for better understanding of the RIE is therefore important for enabling benchmark-quality stratospheric RO retrievals. Here we present such an analysis of bending angle RIEs covering the stratosphere and mesosphere, using quasi-realistic end-to-end simulations for a full-day ensemble of RO events. Based on the ensemble simulations we assessed the variation of bending angle RIEs, both biases and standard deviations, with solar activity, latitudinal region and with or without the assumption of ionospheric spherical symmetry and co-existing observing system errors. We find that the bending angle RIE biases in the upper stratosphere and mesosphere, and in all latitudinal zones from low to high latitudes, have a clear negative tendency and a magnitude increasing with solar activity, which is in line with recent empirical studies based on real RO data although we find smaller bias magnitudes, deserving further study in the future. The maximum RIE biases are found at low latitudes during daytime, where they amount to within −0.03 to −0.05 µrad, the smallest at high latitudes (0 to −0.01 µrad; quiet space weather and winter conditions). Ionospheric spherical symmetry or asymmetries about the RO event location have only a minor influence on RIE biases. The RIE standard deviations are markedly increased both by ionospheric asymmetries and increasing solar activity and amount to about 0.3 to 0.7 µrad in the upper stratosphere and mesosphere. Taking also into account the realistic observation errors of a modern RO receiving system, amounting globally to about 0.4 µrad (unbiased; standard deviation), shows that the random RIEs are typically comparable to the total observing system error. The results help to inform future RIE mitigation schemes that will improve upon the use of the linear ionospheric correction of bending angles and also provide explicit uncertainty estimates.

Analysis of ionospheric structure influences on residual ionospheric errors in GNSS radio occultation bending angles based on ray tracing simulations

“…Another step was the somewhat puzzling side result in our Liu et al (2015) study that the end-to-end simulations of an ensemble of about 700 RO events produced about two dozen RIE outlier profiles. The basis were 3D ray tracing simulations, where the ionospheric model NeUoG (Leitinger and Kirchengast,5 1997) was used as quasi-realistic model for large-scale 3D ionospheric structures, together with the atmospheric model MSIS-90 (Hedin, 1991) for simple but representative neutral atmosphere reference conditions.…”

“…Figure 1 illustrates the GNSS radio occultation geometry that constitutes the basis of the RO technique. The focus 5 is to schematically show essential aspects relevant to this study on along-ray ionospheric influences on RO bending angles, which deepens insight on top of our recent Liu et al (2015) study.…”

“…What remained unexplored in our Liu et al (2015) study and was also not yet explored elsewhere-but 20 is critical to be understood for further improvement of the existing RIE corrections that apply spherical symmetry (Syndergaard, 2000;Healy and Culvervell, 2015;Angling et al, 2017)-are the influences of the three-dimensional and asymmetric ionospheric structures along the GNSS-to-LEO signal paths on the RIE, in particular the conditions that may lead to anomalously high RIEs. A first step in this direction, though not focusing on bending angle RIEs, was the study by Mannucci et al (2011) which found that under ionospheric storm conditions anomalous effects can be significant.…”

“…More precisely, the RIE standard deviation of 26 profiles from the simulations exceeded a threshold value of 7 µrad within the upper stratosphere and mesosphere. These were therefore rejected from the ensemble statistics results reported by Liu et al (2015).…”

“…In our work over the recent years we have assessed the variation of bending angle RIEs (biases and standard deviations) with solar activity, latitudinal region, and with or without the assumption of ionospheric spherical symmetry and of co-existing RO observing system errors, using quasi-realistic end-to-end simulations for single RO events (Liu et al, 2013) and a full-day ensemble of RO events (Liu et al, 2015). As shown in these explanatory simulation studies, in overall agreement with the 15 empirical study of Danzer et al (2013), the RIE biases have a clear negative tendency and a bias magnitude increasing with solar activity.…”