Flechtner F.M., Gruber Th., G?ntner A., Mandea M., Rothacher M., Sch?ne T., Wickert J. (Eds.) System Earth via Geodetic-Geophysical Space Techniques

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Near-Real-Time Provision and Usage of Global Atmospheric Data 431

provision and usage of global atmospheric data from CHAMP and GRACE) was

applied within the research programme GEOTECHNOLOGIEN “Observing of the

System Earth from Space”, funded by the German Ministry for Education and

Research in 2005. The proposal application was supported by various weather pre-

diction centers world-wide. Among them ECMWF, Met Ofﬁce and DWD directly

participated in the preparation of the research project.

The main objective of the proposed project was to develop, install and operate

the necessary complete ground infrastructure for the operational provision of GPS

RO data from CHAMP and GRACE in near-real time. These data should be opera-

tionally provided according to the timely requirements of the weather centers for the

assimilation to global numerical weather forecasts. Furthermore the project partners

DWD, ECMWF and the external user Met Ofﬁce should assimilate the provided

data to study the impact on the global forecasts. Hereby a positive impact from

the occultation data was expected. A real and operational assimilation of the near-

real-time RO data from CHAMP and GRACE at various weather centers should be

started as highlight of the project.

According to the expectations the major success of the NRT-RO project was

reached in September 2006, when the ﬁrst GPS RO data ever (chapter “Global

Atmospheric Data from CHAMP and GRACE-A: Overview and Results” by

Wickert et al., this issue) were operationally assimilated to the global weather fore-

casts from Met Ofﬁce. The extensive use especially of the CHAMP data for several

impact studies and for operational assimilation tests since 2001 was in addition one

of the major preconditions for the later begin of the operational use of COSMIC and

Metop GPS RO data to improve global forecasts.

The following book chapter overviews the main results of the interdisciplinary

research project NRT-RO. Wickert et al. (2009, chapter “Global Atmospheric Data

from CHAMP and GRACE-A: Overview and Results”, this issue) overview the

project and its main results in more detail. Michalak et al. (2009, chapter “Near-Real

Time Satellite Orbit Determination for GPS Radio Occultation with CHAMP and

GRACE”, this issue) characterize the precise orbit determination of the LEO and

GPS satellites in near-real-time. Schmidt et al. (2009, chapter by “The Operational

Processing System for GPS Radio Occultation Data from CHAMP and GRACE”,

this issue) describe the operational processing of the GPS RO data at GFZ. The use

of the GPS RO data from the NRT-RO project for improving global weather fore-

casts is described by Pingel et al. (2009, chapter by “Assimilation of CHAMP and

GRACE-A Radio Occultation Data in the GME Global Meteorological Model of

the German Weather Service”, this issue).

Acknowledgments We thank the CHAMP, GRACE and TerraSAR-X teams for their great work

to make the GPS RO data continuously available. Our project was supported by the German

Ministry for Education and Research within the GEOTECHNOLOGIEN programme (Research

project NRT-RO) and also by GFZ basic funding. This project is also signiﬁcantly aided by

ECMWF, Met Ofﬁce, Météo-France and Japan Meteorological Agency. We are very grateful for

this support.

432 J. Wickert

References

Anthes R, Bernhardt P, Chen Y, Cucurull L, Dymond K, Ector D, Healy S, Ho SP, Hunt D, Kuo

Y-H et al. (2008) The COSMIC/FORMOSAT-3 mission: Early results. Bull. Am. Met. Soc.

(March), 1–21

Anthes RA, Rocken C, Kuo Y (2000) Applications of COSMIC to meteorology and climate. Te rr.

Atmos. Ocean. Sci. 1(1), 115–156.

Buontempo C, Jupp A, Rennie M (2008) Operational NWP assimilation of GPS radio occultation

data. Atmos. Sci. Lett. 9(3), 129–133.

v Engeln A, Andres Y, Marquardt C, Sancho F (2008) GRAS status and future European radio

occultation missions, Proc. ECMWF workshop on GPS RO, 37-42, ECMWF publication,

http://www.ecmwf.int/publications/library/do/references/list/11122008 .

Foelsche U, Borsche M, Steiner AK, Gobiet A, Pirscher B, Kirchengast G, Wickert J, Schmidt

T (2007) Observing upper troposphere–lower stratosphere climate with radio occultation data

from the CHAMP satellite. Clim. Dyn., doi: 10.1007/s00382-007-0337-7.

Healy S (Spring 2007) Operational assimilation of GPS radio occultation measurements at

ECMWF. ECMWF Newslett. 111, ECMWF, 2007.

Healy SB, Wickert J, Michalak G, Schmidt T, Beyerle G (2007) Combined forecast impact of

GRACE-A and CHAMP GPS radio occultation bending angle proﬁles. Atmos. Sci. Lett. 8(43–

50), doi: 10.1002/asl.149.

Healy S, Jupp A, Marquardt C (2005) Forecast impact experiment with GPS radio occultation

measurements. Geophys. Res. Lett. 32, doi: 10.1029/2004GL020806.

Kursinski ER et al. (1997) Observing Earth’s atmosphere with radio occultation measurements

using the Global Positioning System. J. Geophys. Res. 102, 23429–23465.

Rocken C, Anthes R, Exner M, Hunt D, Sokolovskiy S, Ware R, Gorbunov M, Schreiner W, Feng

D, Herman B, Kuo YH, Zou X (1997) Analysis and validation of GPS/MET data in the neutral

atmosphere. J. Geophys. Res. 102(29849–29866), doi: 10.1029/97JD02400.

Schmidt T, Wickert J, Beyerle G, Heise S (2008) Global tropopause height trends estimated from

GPS radio occultation data. Geophys. Res. Lett., doi: 10.1029/2008GL034012.

Wickert J, Michalak G, Schmidt T, Beyerle G, Cheng CZ, Healy SB, Heise S, Huang

CY, Jakowski N, Köhler W et al. (2009) GPS radio occultation: Results from CHAMP,

GRACE and FORMOSAT-3/COS MIC. Terr. Atmos. Ocean. Sci. 20, doi: 10.3319/

TAO.2007.12.26.01(F3C).

Wickert J, Schmidt T, Beyerle G, Heise S, Reigber Ch (2006) Global atmospheric sounding

with GPS radio occultation aboard CHAMP. In: Flury J, Rummel R, Reigber C, Rothacher

M, Boedecker G, Schreiber U (eds.), Observation of the Earth System from Space, Springer,

Berlin, pp. 55–77.

Wickert J, Beyerle G, König R, Heise S, Grunwaldt L, Michalak G, Reigber Ch, Schmidt T (2005)

GPS radio occultation with CHAMP and GRACE: A ﬁrst look at a new and promising satellite

conﬁguration for global atmospheric sounding. Ann. Geophysicae 23, 653–658.

Wickert J, Rothacher M (2005) Near-real-time provision and usage of global atmospheric data from

GRACE and CHAMP (NRT-RO), Group project proposal within the BMBF/DFG-Research

program GEOTECHNOLOGIEN, GFZ.

Wickert J (2002) The GPS radio occultation experiment aboard CHAMP: Algorithms, Processing

system and ﬁrst results (Germ.), GFZ Scientiﬁc Technical Report STR 02/07, Potsdam.

Global Atmospheric Data from CHAMP

and GRACE-A: Overview and Results

Jens Wickert, Georg Beyerle, Carsten Falck, Sean B. Healy, Stefan Heise,

Wolfgang Köhler, Grzegorz Michalak, Dave Ofﬁler, Detlef Pingel,

Markus Ramatschi, Markus Rothacher, and Torsten Schmidt

1 Introduction

Recently ground and space based GPS atmospheric remote sensing techniques

including the GPS Radio Occultation (GPS RO) method are more and more

accepted by the meteorological community. Brief overviews on several techniques

and applications are given, e.g., by Wickert and Jakowski (2007) or Wickert et al.

(2007). The application of GPS RO aboard Low Earth Orbiting (LEO) satellites

allows for the derivation of vertical proﬁles of atmospheric parameters on a global

scale. Main characteristics of GPS RO measurements are: all-weather capability,

calibration independence, high accuracy and high vertical resolution. A detailed

introduction to GPS RO is given, e.g., by Kursinski et al. (1997). Currently GPS

RO is continuously applied aboard the German CHAMP and the US/German

GRACE satellites (Wickert et al., 2005, 2006, 2008, 2009a, b), the 6 satellites of the

FORMOSAT-3/COSMIC mission (launched April 14, 2006; see, e.g., Anthes et al.,

2008) and aboard Metop (v. Engeln et al., 2008; Launch October 18, 2006). GPS

RO data are also available from the US-Argentinean satellite SAC-C (Satelite de

Aplicaciones Cientiﬁcas-C). Initial GPS RO results from the German TerraSAR-X

mission complete this multi-satellite RO data set.

A major application of GPS RO data is the use to improve global weather fore-

casts. A breakthrough for this application was reached in 2006, when operational

assimilation was started by Met Ofﬁce and ECMWF (e.g., Healy, 2007; Buontempo

et al., 2008). In preparation of this operational phase several investigations were

made at the weather centers to quantify the impact of the GPS RO data on the quality

of the global forecasts. For these studies mainly data of the CHAMP mission were

used (Cucurull et al., 2007; Healy and Thepaut, 2006; Healy et al., 2005, 2007).

These data were operationally available already since 2003 with an average delay

of about 5 h between measurement and corresponding provision of atmospheric

J. Wickert (B)

Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences,

Department 1: Geodesy and Remote Sensing, Telegrafenberg, 14473 Potsdam, Germany

e-mail: wickert@gfz-potsdam.de

433

F. Flechtner et al. (eds.), System Earth via Geodetic-Geophysical Space Techniques,

Advanced Technologies in Earth Sciences, DOI 10.1007/978-3-642-10228-8_38,



Springer-Verlag Berlin Heidelberg 2010

434 J. Wickert et al.

Fig. 1 Title page of t he Near-Real Time Radio Occultation (NRT-RO) research project proposal

(in German). The project was performed from August 2005 until August 2008

data products (globally distributed vertical bending angle and refractivity proﬁles;

Wickert et al., 2004).

The NRT-RO research project (see Fig. 1) was mainly initiated to reduce this

delay for CHAMP and GRACE below 2 h, as r equired for the operational assimila-

tion of GPS RO data to numerical weather forecasts. We brieﬂy summarize results

of this project here.

2 Status of CHAMP and GRACE Radio Occultations

CHAMP is currently expected to reach its 10th anniversary in orbit on July 15,

2010. GPS RO data are available nearly continuously from May 2001 until October

2008 (Fig. 2). The measurements from CHAMP form the ﬁrst long-term RO

Global Atmospheric Data from CHAMP and GRACE-A 435

Fig. 2 Number of available monthly vertical atmospheric proﬁles, derived from CHAMP and

GRACE-A GPS occultation measurements (GFZ processing) as of November 2008. For short peri-

ods (mid 2001 and 2002) also GFZ processing results from the US-Argentinean satellite SAC-C

are available

data set, which currently also is the base for initial climatological investigations

(e.g., Schmidt et al., 2008; Ho et al., 2009).

The GRACE satellites (e.g., Tapley et al., 2004) had 7th anniversary in orbit on

March 17, 2009. GPS RO data from GRACE-A are continuously available since

May 2006 (Fig. 2).

3 Near-Real Time Occultation Infrastructure

and Data Analysis at GFZ

GFZ operates an operational infrastructure for the processing of GPS RO data

(overview see Fig. 3). The GPS data (navigation and occultation) from CHAMP

and GRACE are transferred to the ground via receiving antennas at Ny-Ålesund,

Spitsbergen. Together with GPS data from the operational global GPS ground net-

work (jointly operated by GFZ and JPL) these satellite observations form the main

input for the automated processing systems to derive precise satellite orbit and atmo-

spheric data (for more details, see, e.g., Wickert et al., 2009a, b and Michalak et al.,

2007). For processing of the OpenLoop (Sokolovskiy, 2001) tracking data (e.g.,

436 J. Wickert et al.

Fig. 3 GPS radio occultation infrastructure at GFZ (overview, slightly modiﬁed from Wickert

et al., 2008)

TerraSAR-X), information on the navigation bit modulation of the GPS signals is

required. These data is provided by selected stations of the globally distributed GFZ

ground network (Beyerle et al., 2008).

The atmospheric standard data products are: atmospheric excess phases and ver-

tical proﬁles of bending angles, refractivity, temperature and water vapor (Wickert

et al., 2009a, b). In parallel to the standard data processing a near-real time provision

of atmospheric data (excess phases and vertical proﬁles of bending angle, refrac-

tivity and dry temperature) was developed and demonstrated within the NRT-RO

research project. The NRT data are currently provided continuously with average

delay between measurement aboard the satellites and provision of correspond-

ing analysis results at GFZ of less than 2 h (Fig. 4). Key -preconditions for this

result is the successful work related to the installation of NRT-orbit and occul-

tation processing chains, which are described in detail by Michalak et al. (2009,

chapter “Near-Real Time Satellite Orbit Determination for GPS Radio Occultation

with CHAMP and GRACE”, this issue) and Schmidt et al. (2009, chapter “The

Operational Processing System for GPS Radio Occultation Data from CHAMP and

GRACE”, this issue).

4 Monitoring and Assimilation of CHAMP and GRACE

DatatoGlobalWeatherModels

The provision of NRT atmospheric data products from CHAMP and GRACE within

the NRT-RO research project (see Fig. 4) stimulated the pioneering use of GPS-RO

for numerical weather prediction, which was started during the course of the project.

Global Atmospheric Data from CHAMP and GRACE-A 437

Fig. 4 GFZ provides near-real time occultation data (bending angle and refractivity proﬁles in

BUFR) from CHAMP and GRACE-A. The ﬁgure shows a recent monitoring plot of the time delay

between GRACE-A occultation measurements aboard the satellite and availability of correspond-

ing analysis results at NRT ftp-server at GFZ. About 100% of the data is available within 3 h. Data

are provided since February 2007 in BUFR format and are currently distributed also via the Global

Telecommunication System (GTS)

Various weather forecast centres started the operational assimilation of GPS RO

data from COSMIC, CHAMP and GRACE (beginning within September 2006).

The initial operational assimilation of GPS RO data was started at Met Ofﬁce with

refractivity proﬁles from CHAMP and GRACE on September 26, 2006 (Buontempo

et al., 2008), at ECMWF with RO data from CHAMP, GRACE and COSMIC on

December 12, 2006 (Healy, 2007) and at NOAA with COSMIC RO data on May 1,

2007 (Cucurull, 2007). The Japanese Meteorological Agency (JMA) used CHAMP

data for the ﬁrst time operationally on March 22, 2007 (Shoji et al., 2008) and

Météo-France started with CHAMP, GRACE and COSMIC on September 5, 2007

(Poli et al., 2008). The assimilation related activities at the Deutscher Wetterdienst

(DWD) are described by Pingel et al. (2009, chapter “Assimilation of CHAMP and

GRACE-A Radio Occultation Data in the GME Global Meteorological Model of

the German Weather Service”, this issue, 2007).

Met Ofﬁce as part of the GRAS SAF (GNSS Receiver for Atmospheric Sounding

Satellite Application Facility) operates an operational quality monitoring of GPS RO

438 J. Wickert et al.

data from several missions. Monitoring results are available via internet (http://garf.

grassaf.org/monitoring/).

5 GPS Radio Occultation with TerraSAR-X

The German TerraSAR-X satellite was launched on June 15, 2007 with a Dnepr-1

launch vehicle from Aerodrome Baikonur (Russia). The main science instrument

aboard is a new generation X-band radar (9.65 GHz) for Earth observation with

up to 1–2 m resolution (spotlight mode). GFZ (together with University Texas) is

operating an IGOR GPS receiver for precise orbit determination and occultation

measurements.

The occultation experiment is still in the commissioning phase, but initial RO

results already indicate a promising data quality. In contrast to CHAMP and

GRACE, TerraSAR-X applies the OpenLoop (Sokolovskiy, 2001) signal tracking

technique in the lower troposphere, which allows an improvement of the data qual-

ity compared to the Closed Loop tracking applied aboard CHAMP and GRACE-A.

Figure 5 shows 2 water vapour proﬁles from TerraSAR-X, recorded on October 1,

Fig. 5 Examples for initial GPS RO results from TerraSAR-X: Vertical water vapour proﬁles

(TSX, solid line), compared with corresponding ECMWF analyses (dotted line). For details

see text

Global Atmospheric Data from CHAMP and GRACE-A 439

2008. The OpenLoop data were analysed according to Beyerle et al. (2006) using a

GFZ navigation bit data stream (Beyerle et al., 2008) and applying the Direct Water

Vapor Pressure (DWVP) technique, introduced by Heise et al. (2008). Both proﬁles

reach the last kilometre above the Earth’s surface and show a good data quality,

indicated by comparison with corresponding ECMWF data.

6 Summary

The joint research project of GFZ, DWD, ECMWF, and Met Ofﬁce signiﬁcantly

contributed to the pioneering begin of the operational GPS RO data use to improve

global numerical weather forecasts, which was started in 2006. Global atmospheric

data from CHAMP and GRACE are provided with average latency of less than 2 h

between measurements aboard the satellites and provision of corresponding anal-

ysis results at GFZ ftp-server. Usually 100% of the data is available within 3 h.

The use of the operational ground infrastructure of GFZ is precondition to reach

this result. Key-components are a Polar Satellite Receiving Antenna, located at Ny

Ålesund (Spitsbergen), a globally distributed GPS ground network and NRT-orbit

and occultation processing systems.

Several weather centres started operational assimilation of CHAMP/GRACE

atmospheric data (Met Ofﬁce, ECMWF, JMA, NCAR, Météo-France) during the

NRT-RO project course.

Acknowledgments We thank the CHAMP, GRACE and TerraSAR-X teams for their great

work to guarantee the availability of GPS occultation data. The near-real time activities at

GFZ and DWD are supported by the German Ministry for Education and Research within the

GEOTECHNOLOGIEN programme (Research project NRT-RO) and also by GFZ. This project is

also signiﬁcantly aided by ECMWF, Met Ofﬁce, Météo-France and Japan Meteorological Agency.

We are grateful for this support.

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