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.
Highlights
Global navigation satellite system (GNSS) radio occultation (RO) [1,2] is an innovative atmospheric remote sensing technique, which can deliver global coverage, all-weather capability, long-term stability, traceability to the international standard (SI) of time, high vertical resolution and high accuracy of atmospheric profile retrievals
Where α1(a) and α2(a) are the two bending angles derived from the two GNSS frequency signals at the impact parameter α, and αc(a) is the resulting linearly corrected bending angle that is corrected for the f −2 term of Equation (1) but has the other higher-order terms uncorrected, as well as a dominating error related to the ray path splitting, which is proportional to f −4 [16]
In order to investigate their effects on the bending angle residual ionospheric error (RIE), the solar, ionospheric, and geomagnetic environment conditions of chosen test days will be described via solar activity F10.7 index, IGS vertical total electron content (vTEC) maps, and geomagnetic field intensity maps
Summary
Global navigation satellite system (GNSS) radio occultation (RO) [1,2] is an innovative atmospheric remote sensing technique, which can deliver global coverage, all-weather capability, long-term stability, traceability to the international standard (SI) of time, high vertical resolution and high accuracy of atmospheric profile retrievals (bending angle, refractivity, pressure, temperature, humidity profiles). The higher-order residual ionospheric errors (RIEs) after this correction are still not negligible for high-accuracy applications such as global climate change analysis and NWP [3,13,14,15] This applies especially above about 35 km altitude, i.e., from the upper stratosphere upwards, where the RIEs become increasingly relevant compared to the exponentially decreasing magnitude of the neutral atmospheric bending angle [14,16,17,18]. A follow-up study conducted a detailed along ray path analysis of atmospheric and ionospheric refractivity, impact parameter changes, accumulated bending angles and RIEs under asymmetric and symmetric ionospheric structures [26] As shown in these explanatory simulation studies, in overall agreement with the empirical study of Danzer et al [21], the RIE biases have a clear tendency to be negative and a bias magnitude increasing with solar activity as well as being affected by deviations from ionospheric spherical symmetry.
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