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

Abstract

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 µrad noise level, but were left with 26 exceptional events with anomalous RIEs at the 1 to 10 µrad 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.

Sensitivity Analysis and Impact of the Kappa‐Correction of Residual Ionospheric Biases on Radio Occultation Climatologies

“…where α L1 and α L2 are the L 1 and L 2 bending angles related to the frequencies f 1 and f 2 , given at impact altitude z a . Equation 1 is a correction to the first-order ionospheric effect, neglecting higher-order ionospheric residuals that depend on the GNSS signal frequency, the Earth's magnetic field, and electron number density concentration along the signal propagation (e.g., Danzer et al, 2013;Liu et al, 2013Liu et al, , 2015Liu et al, , 2018. It produces a small negative bending angle error even for a simple spherically symmetric ionosphere, where the geomagnetic term is neglected.…”

“…The ionospheric influence is usually corrected to first order by a dual-frequency linear combination of RO bending angles (Ladreiter & Kirchengast, 1996;Vorob'ev & Krasil'nikova, 1994). Nevertheless, residual ionospheric errors (RIE) remain, leading to systematic biases in the data, increasing in relevance at altitudes above about 35 km (Danzer et al, 2013;Liu et al, 2013Liu et al, , 2015Liu et al, , 2018Mannucci et al, 2011;Syndergaard, 2000). Different approaches for higher-order ionospheric corrections exist (e.g., Hoque & Jakowski, 2008;Kedar et al, 2003;Syndergaard, 2002;Vergados & Pagiatakis, 2010, having the disadvantage of the need for additional background information, such as the electron density in the vicinity of the ray path or the geomagnetic field.…”

Performance of the Ionospheric Kappa‐Correction of Radio Occultation Profiles Under Diverse Ionization and Solar Activity Conditions

“…Since the ionospheric correction does not account for large-scale horizontal electron density gradients in the ionosphere along the GNSS signal's inbound and outbound raypaths, we also distinguish inbound/outbound-symmetric and -asymmetric conditions, in order to inspect the sensitivity of the kappa-correction also in this respect. Figure 2 illustrates the vTEC between inbound (from GNSS transmitter) and outbound (towards LEO receiver), respectively, defined as vTEC T x (inbound, x-axis) and vTEC Rx (outbound, y-axis), respectively (for discussion of detailed RO event examples and causes of such asymmetries see, e.g., Liu et al (2013Liu et al ( , 2018). For enabling classification into nearly-symmetric -5-…”

Statistically analyzing the effect of ionospheric irregularity on GNSS radio occultation atmospheric measurement

“…It means that the climate research using atmospheric RO products would be affected by the ionosphere. Some efforts have been made for RIE calibration (Danzer et al, 2013(Danzer et al, , 2015(Danzer et al, , 2020Healy and Culverwell, 2015;Angling et al, 2018;Liu et al, 2018Liu et al, , 2020Li et al, 2020). Danzer et al (2013) have analyzed the bending angle bias of CHAMP and COSMIC RO data from 2001-2011 and tried to parameterize bending angle bias versus the solar cycle to make statistical corrections.…”

“…It can reduce the systematic error variation with the solar cycle from 0.2 to 2.0 K at altitudes between 40 to 45 km. Liu et al (2018) have analyzed the ionospheric structure influences on RIE in bending angles based on ray tracing simulations and further developed a "bilocal correction approach" to calculate the RIE through an equation. This method considers both the ionospheric asymmetry effects as well as the geomagnetic effects on bending angles (Liu et al, 2020).…”