NATIONAL REPORT OF THE FEDERAL REPUBLIC OF ... - IAG Office

Introduction
GNSS Based Sounding of the Atmosphere/Ionosphere
During the last decade GNSS based techniques for ground
and space based atmospheric/ionospheric sounding were
established as the forth pillar of the classic geodetic trinity
Earth rotation, geo-kinematics and gravity&geoid
(RUMMEL, 2003). The atmospheric refraction, error source
for the majority of geodetic applications, is used as
measurement signal. Atmospheric properties, as, e.g.,
globally distributed vertical profiles of refractivity, temperature,
water vapor and electron density can be derived from
space based techniques. Ground based measurements,
provided by global and regional networks, allow for the
derivation of vertically or along the line-of-sight (slant)
integrated water vapor or electron density (Total Electron
Content, TEC) content on a global and also regional scale
(see, e.g., WICKERT and GENDT, 2006; JAKOWSKI, 2005a,b).
Space based techniques
Within the reporting period fell the begin of the era of a
multi-satellite LEO (Low Earth Orbiter) constellation for
precise atmospheric sounding on a global scale using the
innovative GPS Radio Occultation (GPS RO) technique.
In addition to the German CHAMP (CHAllenging Minisatellite
Payload) satellite (e.g., REIGBER et al., 2005;
WICKERT et al., 2006a, 2004a), which provides almost
continuously data since 2001, data from several additional
missions became available in 2006. GPS RO aboard the
U.S.-German GRACE-A satellite (Gravity Recovery And
Climate Experiment) was continuously activated on May
22, 2006 (BEYERLE et al., 2005; WICKERT et al., 2006b,
2005). The six satellites of the U.S.-Taiwan COSMIC/
Formosat-3 (Constellation Observing System for Meteorology,
Ionosphere and Climate) were successfully launched
on April 15, 2007 and will provide about 2,500 globally
distributed profiles per day (e.g., WICKERT et al., 2007).
The European operational weather satellite METOP was
launched on Oct. 19, 2006 and the GRAS (GNSS Receiver
for Atmospheric Sounding) was switched to occultation
mode for the first time on Oct. 27. 40 measurements were
recorded during one revolution, each lasting ~100 min.
This LEO configuration (as of April 2007) will be extended
soon by the German TerraSAR-X, which is foreseen to be
launched in May 2007 with a Russian Dnepr-1 from
Baikonur. Several activities in other countries led to the
realization of additional occultation missions, as, e.g.,
OCEANSAT (India), KOMPSAT-5 (South Korea) and
TANDEM-X (Germany). The application of GPS RO
J. WICKERT 1 , N. JAKOWSKI 2
1 Jens Wickert: GeoForschungsZentrum Potsdam ,(GFZ), Telegrafenberg, D - 14473 Potsdam, Department 1 – Geodesy and Remote
Sensing, Germany; Tel. +49 - 331 - 288-1758, e-mail wickert@gfz-potsdam.de, www.gfz-potsdam.de/atmo
2 Norbert Jakowski: Deutsches Zentrum für Luft- und Raumfahrt, Institut für Kommunikation und Navigation, Aussenstelle Neustrelitz,
Kalkhorstweg 53, D - 17235 Neustrelitz, Germany, Tel. +49 - 3981 - 480-151, e-mail norbert.jakowski@dlr.de
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aboard several satellites multiplies the potential of the
innovative atmospheric sounding technique for several
applications in atmospheric research, weather forecast and
climate change related studies.
A highlight of these applications is the use of GPS RO data
to improve global weather forecasts (e.g., HEALY et al.,
2007). Hereby a breakthrough was reached. Data from the
German CHAMP and U.S.-German GRACE-A satellites
(GFZ analyses) were assimilated as the first GPS radio
occultation measurements operationally in 2006 to improve
global weather forecasts at the U.K. MetOffice, the European
Centre for Medium-Range Weather Forecasts
(ECMWF) and the Japan Meteorological Agency (JMA).
Currently the data are assimilated in parallel with
COSMIC/Formosat-3 data.
The CHAMP data set (started in 2001 and covers as of 2007
already 6 years), including analysis results is made available
by GFZ for the international scientific community. The data
were and are the basis for the preparation of several occultation
missions, the improvement of analysis algorithms (e.g.
BEYERLE et al., 2006), and are used for several atmospheric
investigations. Examples for such investigations are, e.g.,
climatological studies (FOELSCHE et al., 2005) or characterization
of global tropopause (SCHMIDT et al., 2006, 2005,
2004) and gravity wave (DE LATORRE et al., 2006) properties.
The CHAMP data set is also used to derive the global
distribution of water vapor (e.g., HEISE et al., 2005a) or to
reveal weaknesses of meteorological analyses or radiosonde
measurements (e.g., GOBIET et al., 2005; WICKERT, 2004).
GPS RO has also been further developed for monitoring
the ionosphere (e.g. JAKOWSKI, 2005a,b). To estimate
resolution and accuracy of the electron density profiles from
CHAMP under different geophysical conditions, validation
work for ionospheric retrievals was continued and
supported by the European COST 271 action (e.g.
JAKOWSKI et al., 2005b; STOLLE et al., 2004). Comparative
studies with ionospheric 3D models such as IRI (JAKOWSKI
and TSYBULYA, 2004) and NeQuick (JAKOWSKI et al.,
2005c) were made, demonstrating that the RO measurements
can effectively be used for validation and/or development
of these models. The data may also effectively be
applied for tomographic 3D reconstructions of the ionospheric
plasma density distribution as shown by STOLLE
et al. (2005). Key parameters of the ionospheric profiles
such as the scale height can easily be derived from RO data
for modeling or reconstruction (STANKOV and JAKOWSKI,
2005, 2006a,b). First approaches were developed for