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| Titel |
Analysis of GPS radio occultation data from the FORMOSAT-3/COSMIC and Metop/GRAS missions at CDAAC |
| VerfasserIn |
W. Schreiner, S. Sokolovskiy, D. Hunt, C. Rocken, Y.-H. Kuo |
| 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. 10 ; Nr. 4, no. 10 (2011-10-20), S.2255-2272 |
| Datensatznummer |
250002122
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| Publikation (Nr.) |
 copernicus.org/amt-4-2255-2011.pdf |
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| Zusammenfassung |
| This study investigates the noise level and mission-to-mission stability of
Global Positioning System (GPS) radio occultation (RO) neutral atmospheric
bending angle data at the UCAR COSMIC Data Analysis and Archive Center
(CDAAC). Data are used from two independently developed RO instruments
currently flying in orbit on the FORMOSAT-3/COSMIC (F3C) and Metop/GRAS
(GNSS Receiver for Atmospheric Sounding) missions. The F3C 50 Hz RO data are
post-processed with a single-difference excess atmospheric phase algorithm,
and the Metop/GRAS 50 Hz closed loop and raw sampling (down-sampled from
1000 Hz to 50 Hz) data are processed with a zero-difference algorithm. The
standard deviations of the F3C and Metop/GRAS bending angles from
climatology between 60 and 80 km altitude from June–December 2009 are
approximately 1.78 and 1.13 μrad, respectively. The F3C standard
deviation reduces significantly to 1.44 μrad when single-difference
processing uses GPS satellites on the same side of the spacecraft. The
higher noise level for F3C bending angles can be explained by additional
noise from the reference link phase data that are required with
single-difference processing. The F3C and Metop/GRAS mean bending angles
differences relative to climatology during the same six month period are
statistically significant and have values of −0.05 and −0.02 μrad,
respectively. A comparison of ~13 500 collocated F3C and Metop/GRAS
bending angle profiles over this six month period shows a similar mean
difference of ~0.02 ± 0.02 μrad between 30 and 60 km impact
heights that is marginally significant. The observed mean difference between
the F3C and Metop/GRAS bending angles of ~0.02–0.03 μrad is quite
small and illustrates the high degree of re-produceability and mission
independence of the GPS RO data at high altitudes. Collocated bending angles
between two F3C satellites from early in the mission differ on average by up
to 0.5% near the surface due to systematically lower signal-to-noise
ratio for one of the satellites. Results from F3C and Metop/GRAS differences
in the lower troposphere suggest the Metop/GRAS bending angles are
negatively biased compared to F3C with a maximum of several percents near
the surface in tropical regions. This bias is related to different tracking
depths (deeper in F3C) and data gaps in Metop/GRAS which make it impossible
to process the data from both missions in exactly the same way. |
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