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| Titel |
Quantifying residual ionospheric errors in GNSS radio occultation bending angles based on ensembles of profiles from end-to-end simulations |
| VerfasserIn |
C. L. Liu, G. Kirchengast, K. Zhang, R. Norman, Y. Li, S. C. Zhang, J. Fritzer, M. Schwaerz, S. Q. Wu, Z. X. Tan |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 8, no. 7 ; Nr. 8, no. 7 (2015-07-29), S.2999-3019 |
| Datensatznummer |
250116493
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| Publikation (Nr.) |
 copernicus.org/amt-8-2999-2015.pdf |
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| Zusammenfassung |
| The radio occultation (RO) technique using signals from the Global
Navigation Satellite System (GNSS), in particular from the Global
Positioning System (GPS) so far, is currently widely used to observe the
atmosphere for applications such as numerical weather prediction and global
climate monitoring. The ionosphere is a major error source in RO
measurements at stratospheric altitudes, and a linear ionospheric correction
of dual-frequency RO bending angles is commonly used to remove the
first-order ionospheric effect. However, the residual ionospheric error (RIE)
can still be significant so that it needs to be further mitigated for high-accuracy applications, especially above about 30 km altitude where the RIE
is most relevant compared to the magnitude of the neutral atmospheric
bending angle. Quantification and careful analyses for better understanding
of the RIE is therefore important for enabling benchmark-quality
stratospheric RO retrievals. Here we present such an analysis of bending
angle RIEs covering the stratosphere and mesosphere, using quasi-realistic
end-to-end simulations for a full-day ensemble of RO events. Based on the
ensemble simulations we assessed the variation of bending angle RIEs, both
biases and standard deviations, with solar activity, latitudinal region and
with or without the assumption of ionospheric spherical symmetry and
co-existing observing system errors. We find that the bending angle RIE
biases in the upper stratosphere and mesosphere, and in all latitudinal
zones from low to high latitudes, have a clear negative tendency and a
magnitude increasing with solar activity, which is in line with recent empirical
studies based on real RO data although we find smaller bias magnitudes,
deserving further study in the future. The maximum RIE biases are found at low
latitudes during daytime, where they amount to within −0.03 to −0.05 μrad, the smallest at high latitudes (0 to −0.01 μrad; quiet
space weather and winter conditions). Ionospheric spherical symmetry or
asymmetries about the RO event location have only a minor influence on RIE
biases. The RIE standard deviations are markedly increased both by
ionospheric asymmetries and increasing solar activity and amount to about
0.3 to 0.7 μrad in the upper stratosphere and mesosphere. Taking also into
account the realistic observation errors of a modern RO receiving system,
amounting globally to about 0.4 μrad (unbiased; standard deviation),
shows that the random RIEs are typically comparable to the total observing
system error. The results help to inform future RIE mitigation schemes that
will improve upon the use of the linear ionospheric correction of bending
angles and also provide explicit uncertainty estimates. |
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