New Higher-Order Correction of GNSS RO Bending Angles Accounting for Ionospheric Asymmetry: Evaluation of Performance and Added Value

Abstract

The residual ionospheric error (RIE) from higher-order terms in the refractive index is not negligible when using global navigation satellite system (GNSS) radio occultation (RO) data for climate and meteorology applications in the stratosphere. In this study, a new higher-order bending angle RIE correction named “Bi-local correction approach” has been implemented and evaluated, which accounts for the ray path splitting of the dual-frequency GNSS signals, the altitude of the low Earth orbit (LEO) satellite, the ionospheric inbound (GNSS to tangent point) vs. outbound (tangent point to LEO) asymmetry, and the geomagnetic field. Statistical results based on test-day ensembles of RO events show that, over the upper stratosphere and mesosphere, the order of magnitude of the mean total RIE in the bi-local correction approach is 0.01 μrad. Related to this, the so-called electron-density-squared (Ne2) and geomagnetic (BNe) terms appear to be dominant and comparable in magnitude. The BNe term takes negative or positive values, depending on the angle between the geomagnetic field vector and the direction of RO ray paths, while the Ne2 term is generally negative. We evaluated the new approach against the existing “Kappa approach” and the standard linear dual-frequency correction of bending angles and found it to perform well and in many average conditions similar to the simpler Kappa approach. On top of this, the bi-local approach can provide added value for RO missions with low LEO altitudes and for regional-scale applications, where its capacity to account for the ionospheric inbound-outbound asymmetry as well as for the geomagnetic term plays out.

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

“…We consider the chosen reanalysis datasets to fulfill this requirement in a best possible manner though still marginally. However, based on initial previous validation studies by Liu et al (2019), including SABER infrared limb sounder data, and by Danzer et al (2020), including MIPAS middle-atmosphere infrared limb sounder data, we found these other observational satellite data are not sufficiently accurate.…”

“…We further inspected co-located profiles from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) middle-atmosphere dataset, and also the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) with our RO datasets. However, these datasets were found not suitable for our analysis, as already was shown by Liu et al (2020). MIPAS faces the practical problem of having no measurements in the most interesting time frame of high solar activity (data exist only to April 2012), while SABER has the feature of a cold bias of 3 K between 20 km to 35 km, and near ±2 K between 35 km to 45 km altitude (see Innerkofler (2015)) which limits its utility as a reference dataset for the purpose of this study.…”

“…As shown by Blagoveshchensky et al (2018), sudden changes in the geomagnetic field can lead to a change in the TEC value of up to 67% of its quiet state. A possible way, to assess whether the kappa-correction accounts for such drastic changes in the geomagnetic field would be a comparison study to the bi-local correction approach, which includes the strength and the direction of the geomagnetic field (Liu et al, 2020).…”

“…All rights reserved. and strongly asymmetric cases we used the following formal definition of an ionospheric asymmetry factor, in line with Liu et al (2020), which here just serves this particular classification task:…”

“…As a complementary RO dataset we used data from the Gravity Recovery and Climate Experiment (GRACE) (e.g., Wickert et al, 2005;Beyerle et al, 2005), using the GRACE-A mission data in the same time range. For the processing we used the Wegener Center (WEGC) reference occultation processing system (rOPS) (Kirchengast et al, 2016Schwarz et al, 2017Schwarz et al, , 2018Danzer et al, 2018;Innerkofler et al, 2020;Liu et al, 2020) [V. Proschek et al, WEGC Graz, rOPS refractivity and dry-air retrieval manuscript in preparation, pers. communications, 2021].…”