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. The Global Navigation Satellite System (GNSS) radio occultation (RO)technique is widely used to observe the atmosphere for applications such asnumerical weather prediction and global climate monitoring. The ionosphere isa major error source to RO at upper stratospheric altitudes, and a lineardual-frequency bending angle correction is commonly used to remove thefirst-order ionospheric effect. However, the higher-order residualionospheric error (RIE) can still be significant, so it needs to befurther mitigated for high-accuracy applications, especially from 35 kmaltitude upward, where the RIE is most relevant compared to the decreasingmagnitude of the atmospheric bending angle. In a previous study we quantifiedRIEs using an ensemble of about 700 quasi-realistic end-to-end simulated ROevents, finding typical RIEs at the 0.1 to 0.5  µ rad noise level, butwere left with 26 exceptional events with anomalous RIEs at the 1 to 10  µ rad level that remained unexplained. In this study, we focused oninvestigating the causes of the high RIE of these exceptional events,employing detailed along-ray-path analyses of atmospheric and ionosphericrefractivities, impact parameter changes, and bending angles and RIEs underasymmetric and symmetric ionospheric structures. We found that the maincauses of the high RIEs are a combination of physics-based effects – whereasymmetric ionospheric conditions play the primary role, more than theionization level driven by solar activity – and technical ray tracer effectsdue to occasions of imperfect smoothness in ionospheric refractivity modelderivatives. We also found that along-ray impact parameter variations of morethan 10 to 20 m are possible due to ionospheric asymmetries and,depending on prevailing horizontal refractivity gradients, are positive ornegative relative to the initial impact parameter at the GNSS transmitter.Furthermore, mesospheric RIEs are found generally higher thanupper-stratospheric ones, likely due to being closer in tangent point heights tothe ionospheric E layer peaking near 105 km, which increases RIEvulnerability. In the future we will further improve the along-ray modelingsystem to fully isolate technical from physics-based effects and to use itbeyond this work for additional GNSS RO signal propagation studies.

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