GPS RADIO OCCULTATION WITH CHAMP AND GRACE: OVERVIEW, RECENT RESULTS AND OUTLOOK TO METOP

Transcription

1 GPS RADIO OCCULTATION WITH CHAMP AND GRACE: OVERVIEW, RECENT RESULTS AND OUTLOOK TO METOP J. Wickert 1, G. Beyerle 1, S. Heise 1, T. Schmidt 1, G. Michalak 1, R. König 1, A. Helm 1, M. Rothacher 1, N. Jakowski 2, and B. Tapley 3 1 GeoForschungsZentrum Potsdam (GFZ), Germany 2 DLR Neustrelitz, Institut für Kommunikation und Navigation (IKN), Germany 3 University of Texas, Center for Space Research, U.S. Contact: jens.wickert@gfz-potsdam.de ABSTRACT The German geoscience satellite CHAMP (CHAllenging Minisatellite Payload) almost continuously provides global atmospheric measurements since early It currently generates a unique long-term set of GPS radio occultation (RO) data. Around 400,000 occultation measurements were performed as of May Currently the mission is expected to last until Data and analysis results are provided to the international scientific community and stimulated several activities for GPS RO data analysis and application in atmospheric research and weather forecasts. The data are currently in use by more than 40 research groups and are also widely used for the preparation of the GRAS SAF (GNSS Receiver for Atmospheric Sounding Satellite Application Facility). First occultation measurements from the U.S.-German GRACE mission (Gravity Recovery and Climate Experiment) became available mid 2004; a first longer GRACE occultation campaign with duration of one month was performed in January/February During this campaign a first near-real-time provision of radio occultation data from a small multi-satellite configuration (CHAMP and GRACE-A) with average delay of around 4 hours between measurement and data provision was demonstrated. We review the current status of both occultation missions. Hereby we introduce the operational infrastructure used for space-based atmosphere sounding and characterize in more detail the scientific data analysis of both, neutral gas and ionosphere, occultation measurements at GFZ and DLR. Based on validation results with independent meteorological and ionospheric data we discuss advantages and weaknesses of CHAMP s data. We also present selected scientific results, achieved by analysing the long term data set from CHAMP: climatological studies of global tropopause and gravity wave characteristics. As outlook to the Metop mission we present a research project which was proposed within the announcement of opportunity and is related to the scientific analysis of the GPS occultation data from Metop. This project can contribute to the GRAS calibration/validation activities to ensure a high quality of the derived atmospheric data products from Metop. Figure 1. Observation scenario of GPS radio occultation aboard CHAMP (from Wickert, 2002). Proceedings of the 1st EPS/MetOp RAO Workshop, May 2006, ESRIN, Frascati, Italy (ESA SP-618, August 2006)

2 1. INTRODUCTION Fig. 1 introduces the GPS Radio Occultation (GPS RO) technique. Vertical profiles of atmospheric parameters can be derived on a global scale. The main characteristics of GPS RO measurements are: allweather capability, calibration free, high accuracy and high vertical resolution. GPS RO can be used for vertical sounding of the neutral and ionised part of the atmosphere. 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 (Wickert et al., 2006), the U.S./German GRACE satellites (Wickert et al., 2005) and the 6 satellites of the COSMIC mission (launch April 14, 2006, see, e.g. Rocken et al., 2000). Metop will provide GPS occultation measurements of the GRAS instrument (Loiselet et al., 2000). 2. STATUS OF CHAMP RADIO OCCULTATIONS Figure 3. CHAMP decay scenario after the orbit uplift by 19 km end of March 2006 (light blue and right red curves for different intensity of solar flux). CHAMP (Fig. 2) reached 6th anniversary in orbit on July 15, In result of successful orbit uplift by 19 km end of March 2006 a lifetime until end 2008 can be expected (Fig. 3). The satellite and instrument status is excellent; the operation is currently funded until The (in total) third orbit manoeuvre extended the expected lifetime by about one year. Figure 4. CHAMP radio occultations During 1743 days of activation 382,617 occultations were recorded (~220 daily, total column height) as of March 14, ,811 phase delays and (~77%, green + blue) 246,654 profiles (~64%, green columns) were generated by the GFZ processing system. The CHAMP measurements form the first long-term RO data set which hopefully can be extended at least until end First climatological studies on a global scale can be performed. The data set will overlap with the RO data from Metop and other RO missions (COSMIC, GRACE). Figure 2. The 6th anniversary of CHAMP in Orbit was on July 15, By mid May the satellite was ~2,150 days in orbit and completed around 33,500 orbits around the Earth (Fig. by ASTRIUM).

3 3. STATUS OF GRACE OCCULTATIONS GFZ Potsdam operates an operational infrastructure for the processing of GPS RO data (overview see Fig. 6). The GPS data (navigation and occultation) from CHAMP and GRACE are transferred to the ground segment via receiving antennas at Neustrelitz, Germany and Ny-Ålesund, Spitsbergen as part of the science data stream of both missions and transmitted to the Information System and Data Center (ISDC) at GFZ Potsdam. Together with GPS ground data from the operational global network (jointly operated by GFZ and JPL) these data form the main input for the automated processing systems for the satellite orbit and occultation data analysis (for more details, see, e.g. Wickert et al., 2004 or 2006; König et al., 2006). An overview of the automated occultation processing system is given by Fig. 7. Figure 5. GRACE had ist 4th anniversary in orbit on March 17, As of mid May 2006 it is 1480 days in orbit and completed about 22,000 revolutions (Fig. provided by ASTRIUM). First occultation measurements from GRACE (see Fig. 5 and Tapley et al., 2004) were recorded during an 25 h activation period of the BlackJack flight receiver (provided by Jet Propulsion Laboratory; JPL) during July 28/29, 2004 aboard the GRACE-B satellite (Beyerle et al., 2005; Wickert et al., 2005). A longer period of GRACE-A measurements (41 days) is available between January 12 and February 20, The GRACE-A occultations are continuously activated since May 22, OCCULTATION DATA ANALYSIS AT GFZ Figure 7. Operational occultation processing system at GFZ Potsdam. The yellow ellipses indicate software modules, which are embedded in an operational processor environment for automated data processing. Generated data products are: atmospheric excess phase data and vertical profiles of bending angles, refractivitiy, temperature and water vapor (Wickert et al., 2004; Heise et al., 2005). In near-real time are provided: excess phases, and vertical profiles of bending angle and refractivity. Figure 6. GPS radio occultation infrastructure at GFZ (Overview).

4 5. COMPARATIVE DATA ANALYSIS OF CHAMP AND GRACE-A The first longer period of GRACE-A measurements (41 days, see sect. 3) was the basis for a first comparative analysis of CHAMP and GRACE data. The operational GFZ analysis software version 006 was used for this study. Fig. 8 shows comparisons of CHAMP and GRACE-A orbit data with Satellite Laser Ranging (SLR) data. Fig. 9 contains results of comparisons of CHAMP and GRACE-A profiles with ECMWF data. CHAMP data are processed using a space-based single difference technique (Wickert et al., 2002) whereas the GRACE-A occultation measurements were analysed using a zero-difference technique (Beyerle et al., 2005). The application of the single difference technique almost completely eliminates the influence of irregularities which are introduced to the CHAMP data by periodic clock adjustments to achieve a 1μs maximum deviation from coordinate time. But a slight influence of these irregularities (which are additional noise) remains (introduced by the ionosphere correction of the reference link). This reduces the data quality at stratospheric altitudes. GRACE-A processing needs no inclusion of a reference link (ultra-stable oscillator as satellite clock), therefore the additional noise, observed in the CHAMP data, is avoided. The first analyses of larger data sets from CHAMP and GRACE-A (Fig. 9) indicate reduced bias and also RMS of the GRACE-A refractivity profiles to ECMWF above 25 km in relation to the CHAMP data. This is regarded as indication for a higher quality of the GRACE-A data in relation to CHAMP at higher altitudes. Lower troposphere bias of the 006 profiles was reduced in relation to version 005 by improved implementation of the wave optics based data analysis and more strict quality control. This slightly decreased the lower troposphere data yield in relation to the 005 product version. Figure 9. Fractional refractivity and temperature deviation of ~7,000 CHAMP (above) and ~5,500 GRACE-A (below) profiles from ECMWF (GFZ occultation software 006, use of RSO, standard processing). The comparisons show comparable results, but slight differences are observed. GRACE seems to have slightly better data quality in the stratopshere, which can be caused by the better satellite clock in relation to CHAMP. Figure 8. Rapid Science Orbit (RSO) accuracy of CHAMP (left) and GRACE-A (right) for GFZ operational data analysis (Comparison with SLR data) between September 23, 2005 and February 21, The average total 3D-RMS of 4.8 (CHAMP) and 4.5 cm (GRACE-A) is comparable.

5 6. VALIDATION Figure 10. Comparison of ~215,000 CHAMP refractivity profiles with ECMWF between 10 and 30 km (example for validation studies; May 2001 Sept 2005). The Fig. shows vertical zonal means for (a) bias(champ-ecmwf) and (b) RMS. The differences confirm weaknesses of the ECMWF analyses (see text). The quality of the CHAMP data was evaluated within numerous validation studies using independent atmospheric data from meteorological analyses, radiosondes and other remote sensing satellites (e.g. Gobiet et al., 2005; Wickert et al., 2004). As an example for these validation studies Fig.10 shows a comparison of more than CHAMP refractivity profiles (GFZ product version 005) with corresponding 6 hourly analysis data from ECMWF (Gaussian grid with 0.5 x 0.5 resolution at the Equator, 60 altitude levels) between 10 and 30 km altitude. The comparison shows nearly bias-free refractivity (see Fig. 10a). The standard deviation is ~1 % and less. The deviations depend on latitude and may help to identify weaknesses of the analysis. E.g., the wavelike bias structures (CHAMP- ECMWF) in the Antarctic region (Fig. 10a) were related to known problems of ECMWF in the southern hemisphere due to the assimilation of other satellite data (AMSU, Advanced Microwave Sounding Unit, and AIRS, Atmospheric Infrared Sounder, see Gobiet et al. 2005). CHAMP data also can be used to reveal differences in the data quality of different types of radiosondes (e.g., Wickert, 2004). 7. LOWER TROPOSPHERE In contrast to the excellent data quality in the UTLS (Upper Troposphere Lower Stratosphere) region, systematic deviations between CHAMP and independent meteorological data are observed in the lower troposphere (see, e.g., Beyerle et al., 2006). The deviations were investigated in more detail within several studies. Causes of the bias are, beside multi-path signal propagation, also signal tracking errors of the GPS receiver and critical refraction, a physical limitation of the RO technique. As an example for these studies Fig indicate results of the study from Beyerle et al. (2006). The GPS receiver tracking process was included in the end-to-end simulations (Fig. 11) as base for this study (Fig. 11). Realistic atmospheric conditions were used for the signal propagation (Fig. 12). The results of this study (Fig. 13) indicate that a reduction of the observed biases and an increasing of the data yield in the lower troposphere can be reached by the application of advanced signal tracking methods (e.g., Open Loop (OL, see also Sokolovskiy et al., 2006)) or modified CL (Closed Loop) technique. Also improved signal strength due to the use of more advanced occultation antenna configurations will improve the results, as e.g., for Metop (Loiselet et al., 2000). Figure 11. Overview of the simulation procedure to investigate the lower troposphere bias of the CHAMP data. Using the inverse Full Spectrum Inversion (FSI -1 ) method the atmospheric propagation of a GPS signal based on realistic refractivity profiles (radiosonde data, see Fig. 12) is modeled. The generated signal amplitude and phase serves as input to a GPS software receiver. The receiver s output are converted to bending angle profiles using the FSI method. The simulation loop is closed by Abel-transforming the bending angle profiles into refractivity profiles.

6 Figure 12. Map of aerological soundings performed aboard research vessel Polarstern between December 1982 and June 2005 (grey dots). The study of Beyerle et al. (2006) was focused on 1992 radio sonde observations recorded between 45 S and 45 N (black dots). 58.3% of the profiles exhibit critical refraction. Figure 13. Results of the recent GFZ simulation study from Beyerle et al. (2006) to investigate the influence of the GPS signal tracking to the observed biases in RO data. The figures 13a (above), 13b (middle) and 13c (bottom) show fractional refractivity errors derived from simulations using different receiver models and related yield of the data vs. altitude m(z) for different signal strength of 40, 45 and 50 db. (a) 2-quadrant, fly wheeling (CHAMP like tracking): unable to penetrate layers of critical refraction; already above 3 km receiver induced errors, no critical refraction in that region (b) 4-quadrant, Open Loop (OL, also used for Metop) significant reduced bias and RMS in relation to the CHAMP like model, high data yield in the lower troposphere; but also (c) closed loop with reduced order of tracking loop provides comparable results in relation to OL tracking (for details see Beyerle et al., 2006).

7 8. APPLICATIONS CHAMP refractivity and temperature data in the UTLS region are not affected by background temperature fields and are most accurate in this altitude region (Kursinski et al., 1997). Therefore they can be used to investigate climate change processes. CHAMP s continuous measurements initiated first long-term climatological investigations, especially of temperature, tropopause parameters, gravity wave characteristics (see, e.g, Schmidt et al., 2005, 2006). Fig. 14 and 15 show examples for these investigations. Recently also first successful impact studies using CHAMP data for the numerical weather prediction were documented (Healy et al., 2005). Stimulated by the encouraging results results, ECMWF started activities to assimilate GPS RO data (Healy and Thepaut, 2006). 9. REFLECTIONS Signal components of the CHAMP occultation data which were caused by reflections at water or ice surfaces were detected using wave-optics based data analysis (for details see Beyerle et al., 2002 or Cardellach et al., 2004). The reflected signal components contain additonal information on the reflecting surface (e.g. altimetric information or roughtness, which can be related to wave heights and consequently to sea surface winds) and the propagation medium, the atmosphere/ionosphere. Figure 14. Temperature anomalies (K) over the Equator region (4 N-4 S) from CHAMP measurements for the period May 2001-April The heavy dashed white line shows the monthly mean cold-point tropopause altitude (update from Schmidt et al., 2005). Figure 16. Temporal evolution of radio hologram power spectra derived from CHAMP occultation number 47 on February 16, Blue colors denote low power levels, colors ranging from yellow to red indicate increased power levels (for details see Beyerle et al., 2002). Figure 15. Time-latitude section of the monthly mean occurrence distribution of at least two lapse rate tropopauses in the CHAMP temperature profiles for the period May 2001-April 2006 (see also Schmidt et al., 2006). Figure 17. Outlook to a satellite based system using GPS reflections for the observation of water and ice surfaces on a global scale (altimetry, waveheigths, wind speed and direction, ice). The black dots at the Earth s surface indicate the locations of the reflection observations for the tracking of 6 GPS satellites in parallel.

8 Preliminary investigations with a ground based singlefrequency L1 receiver at GFZ (Helm et al., 2006) indicated an accuracy of the carrier phase-delay derived relative height observations with centimeter accuracy. A future satellite based GPS (extended by Galileo) reflection system (see Fig. 17) would allow for a global ocean/ice/atmosphere/ionosphere monitoring with various applications in geosciences (see, e.g., Komjathy et al., 2000). 10. NEAR-REAL TIME ACTIVITIES The operational occultation infrastructure of GFZ allows for the demonstration of near-real time provision of occultation data from CHAMP and GRACE. These activities and the related software developments are funded within a national research project of the German Ministry for Education and Research within the GEOTECHNOLOGIEN research programme (see Fig. 18). Main goals of the project are the development of software modules for the precise derivation of atmospheric data from GPS occultation measurements, the demonstration of a near-real time provision of atmospheric data products from CHAMP and GRACE and corresponding assimilation in global weather models from ECMWF, MetOffice and German Weather Service. An average delay between measurement aboard the satellites and provision of globally distributed vertical atmospheric profiles of about 2 hours will be reached. In a first stage of the project an average delay of parallel CHAMP and GRACE-A measurements of less than 4 hours was demonstrated for a period of 41 days in January/February This was the first NRT demonstration for a multi-satellite GPS RO constellation (consisting of 2 satellites). The development of the NRT orbit system with 2 h average delay is already finished and currently in process of automatization. This system together with the NRT occultation processing system will enable generation of NRT occultation data not older than 2.5 h, which is below the 3 h time line for NWP systems. Initial validation results indicate an accuracy of the LEO 3D-position of 13.6 cm and a corresponding velocity error of 0.16 mm/s. Figure 18. Title page of the Near-Real Time Radio Occultation (NRT-RO) research project proposal NRT (in German). 11. RESEARCH PROPOSAL FOR METOP GFZ submitted a research proposal within the call for Research Announcement of Opportunity (RAO) Evaluation, calibration and validation of GRAS measurements from Metop. To perform the proposed activities within the given time schedule additional funding was applied for. Since this funding cannot be provided, the amount and type of work which can be realistically carried out must be newly defined. The main goal of the originally proposed project was an active participation in the calibration/validation activities for the GRAS instrument aboard Metop. This would mean the installation and validation of a complete Metop GRAS processor to generate precise Metop orbit data using the GPS navigation measurements as well as the derivation of atmospheric excess phase data and vertical atmospheric profiles from the GRAS occultation measurements. The resulting atmospheric profiles should be validated with ECMWF and radio sonde data but also with CHAMP and GRACE measurements. Several improvements of GRAS data in relation to CHAMP are expected. The high gain antenna in combination with Open Loop tracking will allow for improved data quality and yield in the Lower Troposphere. Also stratospheric retrievals are expected to have a higher quality at higher altitudes compared to CHAMP. The increased field of view and the possibility to track also rising occultations will increase the number of daily available occultations. The missing of the ability of GRAS to record ionospheric occultation measurements to get information on the vertical electron

9 density distribution on a global scale (see, e.g., Jakowski et al., 2002) is regarded as disadvantage. These measurements have a unique potential for ionospheric investigations (e.g. space weather) and to provide additional information for the ionosphere correction of the stratospheric refractivity and temperature retrievals. SUMMARY GPS radio occultation aboard the CHAMP and GRACE satellites was introduced, corresponding status information was given. The data analysis at GFZ was briefly overviewed and examples of recent scientific investigations were given. CHAMP generates the first long-term radio occultation data set which will overlap with the GRAS data set. Initial GRACE occultation results indicate slight improvement of the data quality in the stratosphere in relation to CHAMP. GRACE-A occultations are activated continuously since May 22, The proposed calibration/validation RAO project for Metop would help to verify and improve the data quality of the atmospheric products derived from GRAS. ACKNOWLEDGEMENTS We thank the entire CHAMP and GRACE teams for their great work to generate the occultation data. ECMWF data were provided by the German Weather Service. The near-real time activities at GFZ are supported by the German Ministry for Education and Research within the GEOTECHNOLOGIEN programme (Project NRT-RO). REFERENCES Beyerle, G., K. Hocke, J. Wickert, T. Schmidt, C. Marquardt, and C. Reigber, GPS radio occultations with CHAMP: A radio holographic analysis of GPS signal propagation in the troposphere and surface reflections, VOL. 107, NO. D24, 4802, doi: /2001jd001402, Beyerle, G., T. Schmidt, G. Michalak, S. Heise, J. Wickert, and C. Reigber, (2005a) GPS radio occultation with GRACE: Atmospheric profiling utilizing the zero difference technique, Geophys. Res. Lett., (32), L13,806, doi: /2005gl023,109. Beyerle, G., T. Schmidt, J. Wickert, S. Heise, M. Rothacher, G. König-Langlo, and K. B. Lauritsen, Observations and simulations of receiver-induced refractivity biases in GPS radio occultation, J. Geophys. Res., doi: /2005jd006673, Cardellach, E., C.O. Ao, M. de la Torre, and G.A. Hajj, Carrier phase delay altimetry with GPSreflection/occultation interferometry from low Earth orbiters, Geophys. Res. Lett., doi: /2004gl019775, Gobiet, A., U. Foelsche, A.K. Steiner, M. Borsche, G. Kirchengast, and J. Wickert, Climatological validation of stratospheric temperatures in ECMWF operational analyses with CHAMP radio occultation data, Geophys. Res. Lett.,32(12), doi: /2005gl022617, Healy, S., A. Jupp, and C. Marquardt, Forecast impact experiments with CHAMP GPS radio occultation measurements: Preliminary results, Geophys. Res. Lett., 32, L03804, doi: /2004gl020806, Healy, S.B., and J.-N. Thepaut, Assimilation experiments with CHAMP GPS radio occultation measurements, Q. J. R. Meteorol. Soc., 132, pp , Heise, S., J. Wickert, G. Beyerle, T. Schmidt, Ch. Reigber, Global monitoring of tropospheric water vapor with GPS radio occultation aboard CHAMP, Advances in Space Research, doi: /j.asr , 08/2005. Helm, A., G. Beyerle, Ch. Reigber, and M. Rothacher, Coastal altimetry at the Baltic Sea off-shore Rügen using L1 carrier phase-delay observations of reflected GPS signals at low elevation angles, Proc. Workshop on GNSS Reflections, June 14-15, ESTEC, Nordwijk, The Netherlands, Jakowski, N., A. Wehrenpfennig, S. Heise, Ch. Reigber, H. Lühr, L. Grunwaldt, and T.K. Meehan, GPS Radio Occultation Measurements of the Ionosphere from CHAMP: Early Results. Geophys. Res. Lett., 29, 1457, doi: /2001gl014364, Komjathy, A., V. U. Zavorotny, P. Axelrad, G. H. Born and J. L. Garrison, GPS signal scattering from sea surface: Wind speed retrieval using experimental data and theoretical model, Remote Sens. Environ., 73, , König, R., G. Michalak, K. Neumayer, and S. Zhu, Remarks on CHAMP Orbit Products, in: Flury, J., Rummel, R., Reigber, C., Rothacher, M., Boedecker, G., Schreiber, U. (Eds) Observation of the Earth System from Space, pp17-26, Springer Berlin Heidelberg New York, ISBN: , Kursinski et al., Observing Earth s atmosphere with radio occultation measurements using the Global Positioning System. J. Geophys. Res. 102, , 1997.

10 Loiselet, M., N. Stricker, Y. Menard, and J. Luntama, GRAS MetOps GPS based atmospheric sounder, ESA Bulletin, May, 102, 38 44, Rocken, C., Y.-H. Kuo, W. Schreiner, D. Hunt, S. Sokolovskiy, and C. McCormick, COSMIC system description, Terrestrial, Atmospheric and Oceanic Sciences, 11, 21 52, Schmidt, T., S. Heise, J. Wickert, G. Beyerle, and Ch. Reigber, GPS radio occultation with CHAMP and SAC- C: global monitoring of thermal tropopause parameters, Atmospheric Chemistry and Physics, 5, , Schmidt, T., G. Beyerle, S. Heise, J. Wickert, and M. Rothacher, A climatology of multiple tropopauses derived from GPS radio occultations with CHAMP and SAC-C, Geophys. Res. Lett., 33, L04808, doi: /2005gl024600, Sokolovskiy, S., C. Rocken, D. Hunt, W. Schreiner, J. Johnson, D. Masters, and S. Esterhuizen, GPS profiling of the lower troposphere from space: Inversion and demodulation of the open-loop radio occultation signals, Geophys. Res. Lett., 33, L14816, doi: /2006gl026112, Tapley B, Bettadpur S, Watkins M, Reigber C., The gravity recovery and climate experiment: Mission, overiew and early results. Geophys. Res. Lett., 31: L09607, doi: /2004gl019920, Wickert, J., The GPS radio occultation experiment aboard CHAMP: Algorithms, processing system and first results (in German), Scientific Technical Report STR 02/07, GFZ Potsdam, Wickert, J., G. Beyerle, G.A. Hajj, V. Schwieger, and Ch. Reigber, GPS radio occultation with CHAMP: Atmospheric profiling utilizing the space-based single difference technique, Geophys. Res. Lett., 29(8), /2001GL013982, 2002 Wickert, J., Comparison of vertical refractivity and temperature profiles from CHAMP with radiosonde measurements, Scientific Technical Report STR 04/19, GFZ Potsdam, Wickert, J., T. Schmidt, G. Beyerle, R. König, Ch. Reigber, and N. Jakowski, (2004) The radio occultation experiment aboard CHAMP: Operational data analysis and validation of vertical atmospheric profiles", J. Met. Soc. Japan, 82(1B), Wickert, J., G. Beyerle, R. König, S. Heise, L. Grunwaldt, G. Michalak, Ch. Reigber, T. Schmidt, GPS radio occultation with CHAMP and GRACE: A first look at a new and promising satellite configuration for global atmospheric sounding, Annales Geophysicae, Vol. 23, , Wickert, J., G. Beyerle, T. Schmidt, S.B. Healy, S. Heise, R. König, G. Michalak, and M. Rothacher:, GPS based atmospheric sounding with CHAMP: Results achieved after four years, Proc. EUMETSAT Met. Sat. Conference, Dubrovnik, Croatia, September, pp 39-47, 2006.