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| Titel |
Errors in GNSS radio occultation data: relevance of the measurement geometry and obliquity of profiles |
| VerfasserIn |
U. Foelsche, S. Syndergaard, J. Fritzer, G. Kirchengast |
| 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 ; 4, no. 2 ; Nr. 4, no. 2 (2011-02-09), S.189-199 |
| Datensatznummer |
250001670
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| Publikation (Nr.) |
 copernicus.org/amt-4-189-2011.pdf |
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| Zusammenfassung |
| Atmospheric profiles retrieved from GNSS (Global Navigation Satellite
System) radio occultation (RO) measurements are increasingly used to
validate other measurement data. For this purpose it is important to be
aware of the characteristics of RO measurements. RO data are frequently
compared with vertical reference profiles, but the RO method does not
provide vertical scans through the atmosphere. The average elevation angle
of the tangent point trajectory (which would be 90° for a vertical scan)
is about 40° at altitudes above 70 km, decreasing to about 25° at
20 km and to less than 5° below 3 km. In an atmosphere with high
horizontal variability we can thus expect noticeable representativeness
errors if the retrieved profiles are compared with vertical reference
profiles. We have performed an end-to-end simulation study using
high-resolution analysis fields (T799L91) from the European Centre for
Medium-Range Weather Forecasts (ECMWF) to simulate a representative ensemble
of RO profiles via high-precision 3-D ray tracing. Thereby we focused on the
dependence of systematic and random errors on the measurement geometry,
specifically on the incidence angle of the RO measurement rays with respect
to the orbit plane of the receiving satellite, also termed azimuth angle,
which determines the obliquity of RO profiles. We analyzed by how much
errors are reduced if the reference profile is not taken vertical at the
mean tangent point but along the retrieved tangent point trajectory (TPT) of
the RO profile. The exact TPT can only be determined by performing ray
tracing, but our results confirm that the retrieved TPT – calculated from
observed impact parameters – is a very good approximation to the "true"
one. Systematic and random errors in RO data increase with increasing
azimuth angle, less if the TPT is properly taken in to account, since the
increasing obliquity of the RO profiles leads to an increasing sensitivity
to departures from horizontal symmetry. Up to an azimuth angle of 30°,
however, this effect is small, even if the RO profiles are assumed to be
vertical. For applications requiring highest accuracy and precision it is
advisable to exclude RO profiles with ray incidence angles beyond an azimuth
of 50°. Errors in retrieved atmospheric profiles decrease significantly,
by up to a factor of 2, if the RO data are exploited along the retrieved
TPT. The tangent point trajectory of RO profiles should therefore be
exploited whenever this is possible. |
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