Analysis of ionospheric structure influences on residual ionospheric errors in GNSS radio occultation bending angles based on ray tracing simulations
Abstract. The Global Navigation Satellite System (GNSS) radio occultation (RO)
technique is widely used to observe the atmosphere for applications such as
numerical weather prediction and global climate monitoring. The ionosphere is
a major error source to RO at upper stratospheric altitudes, and a linear
dual-frequency bending angle correction is commonly used to remove the
first-order ionospheric effect. However, the higher-order residual
ionospheric error (RIE) can still be significant, so it needs to be
further mitigated for high-accuracy applications, especially from 35 km
altitude upward, where the RIE is most relevant compared to the decreasing
magnitude of the atmospheric bending angle. In a previous study we quantified
RIEs using an ensemble of about 700 quasi-realistic end-to-end simulated RO
events, finding typical RIEs at the 0.1 to 0.5  µ rad noise level, but
were left with 26 exceptional events with anomalous RIEs at the 1 to 10  µ rad level that remained unexplained. In this study, we focused on
investigating the causes of the high RIE of these exceptional events,
employing detailed along-ray-path analyses of atmospheric and ionospheric
refractivities, impact parameter changes, and bending angles and RIEs under
asymmetric and symmetric ionospheric structures. We found that the main
causes of the high RIEs are a combination of physics-based effects – where
asymmetric ionospheric conditions play the primary role, more than the
ionization level driven by solar activity – and technical ray tracer effects
due to occasions of imperfect smoothness in ionospheric refractivity model
derivatives. We also found that along-ray impact parameter variations of more
than 10 to 20 m are possible due to ionospheric asymmetries and,
depending on prevailing horizontal refractivity gradients, are positive or
negative relative to the initial impact parameter at the GNSS transmitter.
Furthermore, mesospheric RIEs are found generally higher than
upper-stratospheric ones, likely due to being closer in tangent point heights to
the ionospheric E layer peaking near 105 km, which increases RIE
vulnerability. In the future we will further improve the along-ray modeling
system to fully isolate technical from physics-based effects and to use it
beyond this work for additional GNSS RO signal propagation studies.