Special Study Group 2.161 Report Report
Probing the Atmosphere by GPS
Christian Rocken
COSMIC Program Office University Corporation for Atmospheric Research (UCAR)
Boulder, CO., 80301 USA

Introduction

It is now well established that space-based Global Positioning System (GPS) receivers can be used to profile Earth's ionosphere and neutral atmosphere with high accuracy and vertical resolution. The first proof-of-concept mission has been successfully completed and its data have been analyzed. Two follow-on missions have been launched during the first half of 1999 and a series of more ambitious missions to profile the atmosphere using the GPS limb sounding technique are planned for launch within the next 4 years. This report summarizes the dramatic progress in space-borne profiling during the last several years. We will review the most important results and accomplishments from the proof-of-concept GPS/MET mission; summarize future GPS occultation missions and challenges, and discuss applications of these data to meteorology, space weather, and climate studies.

GPS/MET

Scientists at Stanford University and the Jet Propulsion Laboratory (JPL) developed the radio occultation sounding technique for the remote sensing of planetary atmospheres. The GPS/MET (GPS/Meteorology) program was established by the University Corporation for Atmospheric Research (UCAR) in 1993, jointly with the University of Arizona and JPL, to demonstrate active limb sounding of the Earth's neutral atmosphere and ionosphere using the radio occultation technique. The demonstration system observed occulted GPS satellite signals received on a LEO satellite, MicroLab-1 (ML-1), launched April 3, 1995. From raw GPS/MET observations, vertical profiles of ray bending angle and refractivity were retrieved; from which ionospheric electron density, neutral atmospheric density, pressure, temperature, and moisture profiles were computed. The program has been exceptionally successful, having accomplished nearly all of the proof of concept goals, plus a number of additional ones. As a direct result, GPS/MET technology is now widely recognized as a potential candidate for a new, accurate global observing system in support of weather prediction, climate change research and space weather. Several groups at JPL, the University of Arizona, UCAR, the Danish Meteorological Institute (DMI), the University of Graz, the Geoforschungszentrum, Potsdam (GFZ), the Institute of Atmospheric Physics, Moscow (IAP), the Institut d'Estudies Espacials de Catalounya, the University of Kyoto, and others, developed data inversion and analysis software for the GPS/MET data. In addition to studies based on GPS/MET data, a team at Stanford University conducted detailed investigations on error sources affecting the occultation technique. Studies on the assimilation of occultation data into numerical atmospheric models were conducted primarily at the National Center for Atmospheric Research (NCAR), Florida State University (FSU), the National Center for Environmental Prediction (NCEP), and the Max Plank Institute (MPI). The participation in GPS/MET and the use of its data was much broader than originally anticipated when the special study group (SSG2.161) on atmospheric sounding with GPS was formed at the 1995 IUGG in Boulder, Colorado.

A summary of key GPS/MET results based on studies by these groups from research institutions all over the world is presented in the following table:
 
62,000 neutral atmospheric soundings were processed to level 1 (GPS phase, range and amplitude data), 11,000 processed to Level 3 (high-resolution profiles of temperature, pressure, refractivity, humidity, geopotential height) and published on the web for A/S on and A/S off time periods. Several thousand soundings were compared to data from independent operational weather analyses and other observing systems.
  • The high theoretical temperature accuracy was verified (approximately 1K) in the range from the surface to 40 km.
    		•
  • Accurate retrieval of water vapor was demonstrated with use of ancillary temperature data.
    		•
  • High vertical resolution of approximately 500 m for sensing of the tropopause and upper level fronts was demonstrated.
    		•
  • New processing techniques were developed to reduce diffraction and multipath effects and to improve the vertical resolution beyond the Fresnel zone limit. 
    		•
  • All weather (including aerosols, clouds and precipitation) sounding capability was verified.
    		•
  • Determination of accurate geopotential heights of ~10 m was demonstrated.
    		•
  • Detection of gravity waves from the middle troposphere to the stratosphere was demonstrated
    		•
  • Over 40,000 electron density profiles were processed and compared to ionosonde data
    		•
  • Techniques were developed to account for horizontal gradients in the electron density distribution
    		•
  • Accurate retrieval of vertical electron density profiles was demonstrated by ~10% level agreement with foF2 frequency data from the global ionosonde network 
    		•
  • Tomographic techniques were developed and tested to combine occultation data and ground based GPS observations to reconstruct global 4-D electron density grids.
    		•
  • GPS/MET orbit data were used in the development of the new Earth Gravitational Model EGM96 (Lemoine et al., 1998). 
    		•
  • Observing systems simulation experiments and real-data assimilation experiments have indicated a likely positive impact of GPS/MET data on model initialization and weather prediction.
    		•
  • A total of 123 data use agreements were issued for access through the GPS/MET web site, which still receives about 40,000 "hits"/month (http://cosmic.gpsmet.ucar.edu/gpsmet).
    		•
  • The data have been used extensively and internationally for science and planning of follow-on missions.
    		•
  • A large number of peer-reviewed publications have been written using the GPS/MET data.
    		•
    All of these accomplishments were achieved by scientists world-wide independently and in collaboration during the last 4 years - a truly remarkable advancement in atmospheric and occultation science. The extensive list of references at the end of this report points to detailed descriptions of this work.

    Below is a list of names and email addresses of the individuals that are part of the special study group and/or contributed to the scientific results summarized in this report. Many others made significant contributions and this is not a complete list.

    Anderson, David <danderson@sec.noaa.gov> Mette Mortensen MDM@dmi.dk

    Mike Watkins mmw@cobra.jpl.nasa.gov Chris Reigber reigber@gfz-potsdam.de

    Antoni Rius rius@ieec.fcr.es Michael Gorbunov gorbunov@dkrz.de

    Norbert Jakowski jakowski@nz.dlr.de Luis Kornblueh kornblueh@dkrz.de

    Giulio Ruffini ruffini@ieec.fcr.es Toshitaka Tsuda tsuda@kurasc.kyoto-u.ac.jp

    Per Hoeg hoeg@dmi.min.dk Tom Yunck Thomas.P.Yunck@jpl.nasa.gov

    Rob Kursinski erk@cobra.jpl.nasa.gov Gerry North northead@ariel.met.tamu.edu

    Alejandro Flores flores@ieec.fcr.es Sergey Sokolovskiy sergey@ucar.edu

    Bill Kuo kuo@ucar.edu Ben Herman herman@air.atmo.arizona.edu

    Bill Melbourne william.g.melbourne@jpl.nasa.gov Chris Rocken rocken@ucar.edu

    Francois Vandenbergh vandenb@ucar.edu George Hajj George.A.Hajj@jpl.nasa.gov

    Joan Alexander alexand@colorado-research.com Larry Young Lawrence.E.Young@jpl.nasa.gov

    Xiaolei Zou zou@bergeron.met.fsu.edu Rick Anthes anthes@ucar.edu

    Gottfried Kirchengast gottfried.kirchengast@kfunigraz.ac.at

    The next Missions

    The next table provides a list of mission names, their launch dates, number of daily soundings, and a comment about the most important aspect of the occultation part of the mission. Note that for most of these missions the GPS atmospheric sounding instrument is just one of many experiments and that the primary objective of these missions is generally not atmospheric sounding.
     
    Mission Name
    Launch Date
    # of daily profiles
    comment
    GPS/MET 
    April 1995
    125
    Proof of concept
    Ørsted 
    Feb 1999 
    125
    Similar GPS receiver as GPS/MET
    Sunsat 
    Feb. 1999
    125
    Similar GPS receiver as GPS/MET
    CHAMP 
    Jan. 2000
    250
    Improved GPS tracking receiver
    SAC-C
    Jan. 2000
    500
    Setting and rising occultations
    GRACE
    May 2001
    500
    		  
    COSMIC
    Nov. 2002
    4000
    Operational demonstration
    METOP
    2003
    500 - 1000
    May track GLONASS plus GPS
     
    Ørsted was successfully launched in Feb. 1999. It is managed by the DMI and carries a JPL-developed GPS receiver that is a slightly refined version of the GPS/MET instrument. The receiver uses less power than GPS/MET has better data compression and some performance enhancements. This receiver only tracks setting GPS occultations and its data quality is strongly degraded (as was the case for GPS/MET) when the GPS signals are encrypted by Anti Spoofing measures (A/S), which is presently the normal mode of operation. The main purpose of the GPS soundings collected by Ørsted is its use for climate research and to advance the occultation technology.
    Sunsat is a South African mission that is flying the same GPS receiver as Ørsted.
    CHAMP is a German mission that will fly an enhanced JPL-developed receiver, called "BlackJack". The main advantage of this next generation receiver is a 3-dB signal-to-noise ratio (SNR) improvement over the previous tracking techniques under weak signal conditions. It is expected that this receiver will surpass the GPS/Met A/S off tracking performance even when A/S is on. Thus this receiver will track the occultation signal more reliably in the lower troposphere than its predecessors.
    SAC-C is an Argentine led mission that will use the same GPS instrument as CHAMP. SAC-C will be the first mission to attempt tracking setting and rising occultations. Tracking of rising occultations is more demanding because the receiver needs to pick up faint signals from behind Earth's limb as the GPS satellite appears in the (approximate) direction of the low earth orbiter's velocity vector.
    GRACE is a U.S./German mission that will fly a JPL-developed receiver with added capability to track a Ka-band crosslink ranging signal between the two GRACE satellites and to determine precise attitude from an integrated digital star camera.
    COSMIC is Taiwan/U.S. mission that will fly 8 satellites to measure 4000 soundings each day globally. The main goal of COSMIC is to demonstrate the operational value of occultation data to numerical weather prediction and space weather monitoring. To achieve this goal the data from this mission shall be analyzed and delivered to operational and research centers worldwide within 3 hours of data collection. COSMIC plans to fly JPL-developed receivers adapted from the "BlackJack" generation instrument.
    METOP is a European operational satellite that will carry a European - developed GPS/Glonass receiver. METOP occultation data will be used for operational weather prediction.
    Applications and Challenges

    The main characteristics of space-based GPS atmospheric sounding data and their applications are summarized below.
     
    Characteristic
    Meteorology
    Climate
    Ionosphere
    Limb sounding geometry complementary to ground and space nadir viewing instruments
    X
    X
    X
    High Accuracy
    X
    X
    X
    High vertical resolution
    X
    X
    X
    Consistency of horizontal and vertical scales of observations
    X
    X
    All weather-not affected by clouds or precipitation
    X
    X
    X
    Independent height and pressure
    X
    X
    Requires no first guess sounding
    X
    X
    X
    Independent of radiosonde or other calibration
    X
    X
    X
    No instrument drift
    		  
    X
    No satellite-to-satellite bias
    		  
    X
    Top and bottom side sounding
    		    
    X
    Because of these characteristics occultation data have the potential to significantly benefit meteorology, climate and ionospheric research and forecasting. In order to achieve these benefits the community has to (a) collect a large amount of high quality occultation data, and (b) develop the techniques to assimilate these observations into physical models of the atmosphere and ionosphere.

    Figure 1 GPS Sounding penetration for ~10,000 GPS/MET occultations as a function of latitude

    The upcoming missions promise to provide the needed data volume and density to demonstrate forecasting impact. In addition to more data, the data from these missions will also have to be of higher quality than what was produced by GPS/MET. Figure 1 shows the depth to which GPS/MET occultations penetrated Earth's atmosphere. It can be seen that most occultation stopped as high as 3 km above the surface and in the equatorial region hardly any soundings probed the lowest km. It is of the highest importance that future missions provide data that reliably penetrate to near the surface. This shall be achieved with a combination of improved GPS occultation instruments and higher gain antennas flown in future missions.

    New and improved algorithms will also have to be developed to deal with lower tropospheric data that will be affected by strong refractivity gradients due to water vapor.

    Summary

    The research community has made impressive gains over the last 4 years in the field of atmospheric sounding from space with GPS. New satellite missions will provide much more data in the near future and we can look forward to witnessing improvements in weather and space weather forecasting due, in part, to this sensing technique. Important challenges lie ahead to collect and correctly interpret data near the ground, and to develop optimal data assimilation techniques.

    References
    Ahmad, B., Accuracy and Resolution of Atmospheric Profiles Obtained from Radio Occultation Measurements, Ph.D. Dissertation, Stanford University, December, 1998. Also available as Scientific Report No. DPD811-1998-1, Space, Telecommunications, and Radio Sci. Laboratory, Department of Electrical Engineering, Stanford University, California, 94305-9515, October, 1998.

    Ahmad, B., and G. L. Tyler, The two-dimensional resolution kernel associated with retrieval of ionospheric and atmospheric refractivity profiles by Abelian inversion of radio occultation phase data, Radio Sci., 33, (1), 129-142, 1998.

    Ahmad, B., and G. L. Tyler, Systematic errors in atmospheric profiles obtained from Abelian inversion of radio occultation data: Effects of large-scale horizontal gradients, J. Geophys. Res., 104(D4), 3971-3992, 1999.

    Anderson, J., and G. North, Testing climate models: An approach. Bull. Amer. Met. Soc., 79, 2541-2549, 1998.

    Anthes, R., M. Exner, C. Rocken, and R. Ware, Results from the GPS/MET Experiment and Potential Applications to GEWEX, GEWEX News, 7, 1997.

    Anthes, R., W. Schreiner, S. M. Exner, D. Hunt, Y. Kuo, S. Sokolovskiy, R. Ware and X. Zou, GPS Sounding of the Atmosphere from Low Earth Orbit: Preliminary Results and Potential Impact on Numerical Weather Prediction, Augmenting the GPS Infrastructure for Earth, GPS for the Geosciences, National Academy Press, pp.114-124, 1997a.

    Belloul, B. and A. Hauchecorne, Effect of periodic horizontal gradients on the retrieval of atmospheric profiles from occultation measurement. Radio Sci., 32(2), 469-478, 1997.

    Bengtsson, L., J.R. Eyre, G. Kirchengast, A. Hauchecorne, P. Silvestrin, and P. Ingmann, Atmospheric Profiling Mission --- a candidate Earth Explorer mission (report for assessment), ESA Spec. Publ. SP-1196 (7), 58 p., ESA/ESTEC, Noordwijk, NL, 1996.

    Bertiger, W., and S.C. Wu, Single Frequency GPS Orbit Determination for Low Earth Orbiters, paper presented at the National Technical Meeting, Inst. Of Nav., Santa Monica, Calif., Jan., 1996.

    Businger, S., S. Chiswell, M. Bevis, J. Duan, R. Anthes, C. Rocken, R. Ware, M. Exner, T. VanHove, and F. Solheim, The promise of GPS in atmospheric monitoring, Bull. Amer. Met. Soc., 77, 5-18, 1996.

    Dymond, K., S.E. Thonnard, R.P. McCoy, and R.J. Thomas, An optical remote sensing technique for determining nighttime F region electron density, Radio Sci., 32(5), Sept., 1997.

    ESA, 1996: Atmospheric Profiling Mission. ESA SP-1196(7). The Nine Candidate Earth Explorer Missions. European Space Agency, ESA Publications Division, Noordwijk, The Netherlands, 58.

    Feng, D., B. Herman, M. Exner, B. Schreiner, R. Anthes, and R. Ware, Space-borne GPS remote sensing for atmospheric research, Proceedings from Synthetic Aperture Radar and Passive Microwave Sensing, 2584, 448-455, 1995.

    Fjeldbo, G., A. J. Kliore, and V. L. Von Eshelman, The neutral atmosphere of Venus as studied with the Mariner V radio occultation experiments, Astron. J., 76, 123-140, 1971.

    Flores A., G.Ruffini, A.Rius, Tropospheric tomography using GPS slant delays, submitted to Geophys. Res. Lett., 1998.

    Foelsche, U., and G. Kirchengast, Space-Time resolution conditions for atmospheric imaging involving GNSS occultation: A quantitative study, IMG/UoG Techn. Rep. for ESA/ESTEC No. 3/'97, 33 p., Inst. Meteorol. Geophys., Univ. Graz, Austria, 1997.

    Gorbunov, M. E., Solution of inverse problems of remote atmospheric refractometry on limb paths, Izv Acad. Sci. USSR: Atmos. Oceanic Phys., Engl. Transl., 26, 86-91, 1990.

    Gorbunov, M., and S. Sokolovskiy, Remote sensing of refractivity from space for global observations of atmospheric parameters, Rep. 119, 58 pp., Max Planck Inst. for Meteorol., Hamburg, Germany, 1993.

    Gorbunov, M. E., A. S. Gurvich, and L. Bengtsson, Advanced algorithms of inversion of GPS/MET satellite data and their application to reconstruction of temperature and humidity, Rep. 211, 40 p., Max Planck Inst. for Meteorol., Hamburg 1996.

    Gorbunov, M.E., S. V. Sokolovsky and L. Bengtsson,: Space refractive tomography of the atmosphere: Modeling of direct and inverse problems. Rep. 210, 59, Max Planck Inst for Meteorol., Hamburg, 1996a.

    Gorbunov, M.E., and A.S.Gurvich, Microlab-1 Experiment: Multipath effects in the lower troposphere, J. Geophys. Res., 103, (D12), 13819-13826, 1998.
    Gurvich, A. S., and S. V. Sokolovskiy, Reconstruction of a pressure field by remote refractometry from space, Izvestya, Atm. Ocean. Phys., 21, 7-13, 1983.
    Hajj, G. A., and L. J. Romans, Ionospheric electron density profiles obtained with the Global Positioning System: Results from the GPS/MET experiment, Radio Sci., 33(1), 175-190, 1998.

    Hajj, G. A., R. Ibanez-Meir, E. R. Kursinski, and L. J. Romans, Imaging the ionosphere with the Global Positioning System, Int. J. Imaging Syst. Technol., 5, 174-184, 1994.

    Hardy, K. R., G. A. Hajj, E. R. Kursinski, and R. Ibanez-Meier, Accuracies of atmospheric profiles obtained from GPS occultations, in Proceedings of the ION GPS-93 Conference, 1545-1556, Inst. of Navig., Alexandria, Va., 1993.

    Hernandez-Pajares, M., J. M. Juan, J. Sanz, J. G. Sole, Global observation of the ionospheric electronic response to solar events using ground and LEO GPS data, J. Geophys. Res. 103(A9), 20789-20796, 1998.

    Ho, C. M., B. D. Wilson, A. J. Mannucci, U. J. Lindqwister, and D. N. Yuan, A comparative study of ionospheric total electron content measurements using global ionospheric maps of GPS, TOPEX radar, and the Bent model, Radio Sci., 32(4), 1499-1512, 1997.
    Hocke K., Inversion of GPS meteorology data, Ann. Geophys., 15, 443-450, 1997.
    Hocke K., G. Kirchengast, A. Steiner: Ionospheric correction and inversion of GNSS occultation data: problems and solutions, IMG/UoG Technical Report for ESA/ESTEC-No. 2/1997, Graz, Austria, 1997.

    Hoeg, P., A. Hauchecorne, G. Kirchengast, S. Syndergaard, B. Belloul, R. Leitinger, and W. Rothleitner, Derivation of atmospheric properties using a radio occultation technique, Sci. Rep., 95-4, Danish Meteorol. Inst., 1996.

    Howe, B. M., K. Runciman, and J. A. Secan, Tomography of the ionosphere: Four-dimensional simulations, Radio Sci., 33(1), 109-128, 1998.

    Hurrell, J.W. and K.E. Trenberth, Spurious trends in satellite MSU temperature from merging different satellite records. Nature, 386(3),1997.

    Hurrell, J.W. and K.E. Trenberth, Difficulties in obtaining reliable temperature trends: Reconciling the surface and satellite microwave sounding unit records. J. Climate, 11(5), pp. 945–967, 1998.

    Climate change 1995. The science of climate change. Contribution of WGI to the second assessment report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK, 572 pp., IPCC,1996.

    Jakowski N., J. Wickert , K. Hocke , Ch. Reigber, Ch. Förste, R. König, Atmosphere/Ionosphere sounding onboard CHAMP, paper presented at 1998 Western Pacific Geophysics Meeting, Taipei/Taiwan, 21-24 July 1998, Session G42B, Abstr. publ. in EOS Suppl., Transactions, AGU, 79(24), W20, 1998.

    Jakowski N., Scientific Goals and Objectives – Ionosphere, Proc. IGS-GFZ-JPL Workshop on Low Earth Orbiter Missions, Potsdam, Germany , March 9-11,1999.

    Juan, J.M., M. Hernandez-Pajares, A. Rius, J. Sanz, A two-layer model of the ionosphere using Global Positioning System data, Geophys. Res. Lett., 24(4), 393-396, 1997.

    Karayel, E.T. and D. P. Hinson, Sub-Fresnel vertical resolution in atmospheric profiles from radio occultation, Radio Sci. 32(2), 411-423, 1997.

    Kirchengast, G., End-to-end GNSS occultation performance simulator functionality definition, IMG/UoG Techn. Rep. for ESA/ESTEC No. 1/'96, 25 p., Inst. Meteorol. Geophys., Univ. Graz, Austria, 1996.

    Kirchengast, G., End-to-end GNSS occultation performance simulator overview and exemplary applications, Wissenschaftl. Ber. No. 2/1998, 142 p., Inst. Meteorol. Geophys., Univ. Graz, Austria, 1998.

    Kirchengast, G., und H.-P. Ladreiter, Das Potential der Radio-Okkultationstechnik basierend auf GPS/GLONASS-Signalen zur Bestimmung fundamentaler atmosphaerischer Parameter, Kleinheubacher Ber., 39, 677-686, 1996.

    Kirchengast, G., M. Gorbunov, N. Jakowski, L. Kornblueh, U. Mallow, A. Rius, C. Rocken, M. Rothacher, G. Schmidtke, M. Sust, J. Ward, and A. Wernik, ACLISCOPE --- Atmosphere and Climate Sensors Constellation Performance Explorer (ESA Earth Explorer Opportunity Missions proposal), Wissenschaftl. Ber. No. 4/1998, 60 p., Inst. Meteorol. Geophys., Univ. Graz, Austria, 1998.

    Kirchengast, G., and J. Ramsauer, Impact of an atmospheric profiling mission on NWP: Assessment of mission scenarios and definition of OSSE study test missions, IMG/UoG Techn. Rep. for ESA/ESTEC No. 3/1999, 16 p., Inst. Meteorol. Geophys., Univ. Graz, Austria, 1999.

    Kuo, Y.-H., X. Zou, and W. Huang, The impact of Global Positioning System data on the prediction of an extratropical cyclone: an observing system simulation experiment, J. Dyn. Atmos. Oceans, 27, 439–470, 1997.

    Kuo, Y.-H., X. Zou, and Y.-R. Guo, Variational assimiliation of precipitable water using a nonhydrostatic mesoscale adjoint model. Part I: moisture retrieval and sensitivity experiments., Mon. Wea. Rev., 124, 122-147, Jan. 1996.

    Kuo, Y.-H., X. Zou, F. Vandenberghe, W. Huang and R. Anthes, Assimilation of GPS/MET data for Numerical Weather Prediction. Terrestrial, Atmospheric and Oceanic Sciences, (this issue), 1999.

    Kursinski, E. R., G. A. Hajj, K. R. Hardy, L. J. Romans and J. T. Schofield, Observing tropospheric water-vapor by radio occultation using GPS, Geophys. Res. Lett. 22, 2365-2368, 1995.

    Kursinski, E. R., G.A. Hajj, W. I. Bertiger, S. S. Leroy, T. K. Meehan, L. J. Romans, J. T. Schofield, D. J. McCleese, W. G. Melbourne, C. L. Thournton, T. P. Yunck, J. R. Eyre, R. N. Nagatani, Initial results of radio occultation observations of earth’s atmosphere using the global positioning system, Science, 271, 1107-1110, 1996.

    Kursinski, E.R., G.A. Hajj, J.T. Schofield, R.P. Linfield and K.R. Hardy, Observing the Earth’s atmosphere with radio occultation measurements using the Global Positioning System, J. Geophys. Res, 102(D19), 23429-23465, 1997.

    Kursinski, E. R. and G. A. Hajj, An examination of water vapor derived from global positioning system occultation observations during June-July 1995, Part I: Zonal Means, J. Geophys. Res., in press, 1998.

    Ladreiter, H.-P., and G. Kirchengast, GPS/GLONASS sensing of the neutral atmosphere: Model independent correction of ionospheric influences, Radio Sci, 31, 877-891, 1996.

    Leitinger, R., and G. Kirchengast, Inversion of the plasma signal in GNSS occultations --- simulation studies and sample results, Acta Geod. Geophy. Hung., 32, 379-394, 1997.

    Leitinger, R., H.-P. Ladreiter, and G. Kirchengast, Ionosphere tomography with data from satellite reception of Global Navigation Satellite System signals and ground reception of Navy Navigation Satellite System signals, Radio Sci., 32(4), 1657-1669, 1997.

    Lemoine, F.G., C. Kenyon, J.K. Factor, R.G Trimmer, N.K. Pavlis, D.S. Cinn, C.M. Cox, S.M. Klosko, S.B. luthke, M.H. Torrence, Y.M. Wang, R.G. Willamson, E.C. Pavlis, R.H. Rapp, and T.R. Olsen, The development of the joint NASA GSFC and the National Imagery and Mapping Agency (NIMA) Geopotential Model EGM96, NASA/TP-1998-206861, 1998.

    Leroy, S., The measurement of geopotential heights by GPS radio occultation, J. Geophys. Res., 102, 6971-6986, 1997.

    Leroy, S. S., Detecting climate signals: Some Bayesian aspects, J. Clim., 11, 640-651, 1998.

    Leroy, S.S., Optimal detection of global warming using temperature profiles: A methodology, J. Climate, in press, 1998.

    Lindall, G.F., J.R. Lyons, D.N. Sweetnam, V.R. Eshleman, D.P. Hinson, G.L. Tyler, The atmosphere of uranus: results of radio occultation measurements with voyager 2, J. Geophys. Res., 92, 14987-15001, 1987.

    Lindzen, R.S. and M. Fox-Rabinovitz, Consistent vertical and horizontal resolution. Mon. Wea. Rev., 117, 2575-2583, 1989.

    Lindzen, R.S., Some coolness concerning global warming, Bull. Amer. Met. Soc. 71, 288, 1990.

    Meehl, G.A., W. Collins, B. Boville, J.T. Kiehl, T.M.L Wigley, and J.M. Arblaster, Factors affecting climate sensitivity and climate change in the NCAR Climate System Model. J. Climate, in preparation, 1998

    Melbourne, W., E. Davis, C. Duncan, G. Hajj, K. Hardy, E. Kursinski, T. Meehan, L. Young, T. P. Yunck, The application of space borne GPS to atmospheric limb sounding and global change monitoring, JPL Publ., 94-18, 147 pp., 1994.

    Melbourne W.G, E.S. Davis, C.B. Dunkan, G.A. Hajj, K.R. Hardy, E.R. Kursinski, T.K. Meehan, L.E. Young, T.P. Yunck, The application of spaceborne GPS to atmospheric limb sounding and global change monitoring, JPL Publ., 94-18, NASA, JPL, Californian Institute of Technology, Pasadena, CA, 1994.

    Melbourne, W. G., T. P. Yunck, L. E. Young, B. H. Hager, G. F. Lindal, C. H. Liu, and G. H. Born, GPS geoscience instrument for EOS and Space Station. JPL proposal to NASA AO OSSA-1-88, 81, 1988.

    Melbourne, W.G., Sensing atmospheric and ionospheric boundaries in GPS radio occultation observations from a low Earth orbiter, Part 1, JPL Publication 99-5, Jet Propulsion Laboratory, Pasadena, CA, 1998.

    Melbourne, W.G., Sensing atmospheric and ionospheric boundaries in GPS radio occultation observations from a low Earth orbiter, Part 2, JPL Publication 99-5, Jet Propulsion Laboratory, Pasadena, CA, 1999.

    Mortensen D.M., and P.Hoeg, Inversion of GPS Radio Occultation Measurements Using Fresnel Diffraction Theory, Geophys. Res. Lett., 25(13), 2441-2444, 1998.

    Mortensen, M. D., R. P. Linfield and E. R. Kursinski, Resolution approaching 100m for GPS occultations of the Earth's atmosphere, IEEE Trans. on Geoscience and Remote Sensing, 1998.

    North, G. R., and M. J. Stevens, Detecting climate signals in the surface temperature record, J. Climate, 11, 563-577, 1998.

    NRC, Global environmental change-Research pathways for the next decade. National Academy Press, Washington, D.C., 69, 1998.

    Pavelyev, A., K. Hocke, O. Yakovlev, N. Jakowski, J. Wickert, S. Matugov, A. Kucherjavenkov, A. Wehrenpfennig, A. Zakharov: Radioholographic method for atmosphere sounding using small satellites, for earth observation: Digest of 2nd IAA Symposium, ed. by H.P. Roser et al., Wissenschaft und Technik Verlag, Berlin, 199-202, 1999.

    Pavelyev, A.G., On possibility of radio holographic investigation at radio communication link satellite-to-satellite, Radioteknika i elektronica 43, 939-944 (in Russian), 1998.

    Phinney, R. A., and D. L. Anderson, On the radio occultation method for studying planetary atmospheres, J. Geophys. Res., 73(5), 1819-1827, 1968.

    Ramsauer, J., and G. Kirchengast, Impact of an atmospheric profiling mission on NWP. Review of sampling requirements and mission scenario study, IMG/UoG Techn. Rep. for ESA/ESTEC No. 1/1999, 70 p., Inst. Meteorol. Geophys., Univ. Graz, Austria, 1999.

    Reigber, Ch., Bock, R., Förste, Ch., Grunwaldt, L., Jakowski, N., Lühr, H., Schwintzer, P., Tilgner, C., CHAMP Phase B - Executive Summary, Scientific Technical Report STR96/13, GeoForschungsZentrum Potsdam,1996.

    Reigber Ch., R. Galas , R. König , N. Jakowski , J.Wickert , A.Wehrenpfennig, GFZ and DLR contribution to GPS ground network supporting CHAMP mission, Towards an integrated global geodetic observing system, Proc. Internat. Symp. of IAG, 5-9 October 1998, München.

    Rind, D., Latitudinal temperature gradients and climate change, J. Geophys. Res., 103(D6), 5943-5971, 1998.

    Rind, D., Just add water vapor, Science, 281, 1152, 1998.

    Rius, A., G. Ruffini, and L. Cucurull, Improving the vertical resolution of ionospheric tomography with GPS occultations, Geophys. Res. Lett., 24(18), 2291-2294, 1997.

    Rius, A., G. Ruffini, and A. Romeo, Analysis of ionospheric electron density distribution from GPS/MET occultations, IEEE Trans. on Geoscience and Remote Sensing, 36(2), Mar 1998.

    Rocken, C., R. Anthes, M. Exner, D. Hunt, S. Sokolovskiy, R. Ware, M. Gorbunov, W. Schreiner, D. Feng, B. Herman, Y. Kuo, and X. Zou, Analysis and validation of GPS/MET data in the neutral atmosphere, J. Geophys. Res, 102(D25), 29849-29866, 1997.

    Rocken, C, et al., COSMIC system overview Terrestrial, Atmospheric and Oceanic Sciences, 1999.(submitted).

    Ruffini, G., Flores, A., Rius, A., GPS tomography of the Ionospheric Electron content with a correlation functional, IEEE Transactions on Geoscience and Remote Sensing, 36(1), January 1998.

    Ruffini, G., E. Cardellach, A. Flores, L. Cucurull and A. Rius, Ionospheric calibration of radar altimeters using GPS tomography, Geophys. Res. Lett., 25(20), 1998.

    Ruffini, G., Cucurull, L., Flores, A., Rius, A., A PIM-Aided Kalman Filter for GPS tomography of the ionospheric eletron content, Phys. Chem. Earth (C), 24(4), 365-369, 1999. (http://xxx.unizar.es/abs/physics/9807026)

    Salby, M.L. and P. Callaghan; Sampling error in climate properties derived from satellite measurements: Consequences of undersampled diurnal variability. J.. Climate, 10, 18-36, 1997.

    Schreiner, W.S.., D.C. Hunt, C. Rocken, S. Sokolovskiy, Precise GPS data processing for the GPS/MET radio occultation mission at UCAR, Proc. Of the Institute of Navigation- Navigation 2000, Long Beach, CA, pp. 103-112, Jan 21-23, 1998.

    Schreiner, W.S., S. Sokolovskiy, C. Rocken, R. Ware, Analysis and validation of GPS/MET radio occultation data in the ionosphere, accepted in Radio Sci., 1998.

    Schutz, B., P. Abusali, C. Schroeder, B. Tapley, M. Exner, R. McCloskey, R. Carpenter, M. Cooke, S. McDonald, N. Combs, C. Duncan, C. Dunn, and T. Meehan, GPS tracking experiment of a free-flyer deployed from space shuttle, Proceedings of GPS-95, Institute of Navigation, 1995.

    Steiner, A.K., High resolution sounding of key climate variables using the radio occultation technique (Ph.D. thesis), Wissenschaftl. Ber. 3, 170 p., Inst. Meteorol. Geophys., Univ. Graz, Austria, 1998.

    Steiner, A.K., G. Kirchengast, and H.-P. Ladreiter, Inversion, error analysis, and validation of GPS/MET occultation data, Ann. Geophys., 17, 122-138, 1999.

    Steiner, A.K., and G. Kirchengast, Gravity wave spectra from GPS/MET occultation observations, J. Atmos. Oceanic Technology, 16, in press, 1999.

    Steurer, P., Six Second Upper Air Data, TD-9948. National Climatic Data Center, Asheville, NC, 17 p, February 1996.

    Stevens, M.J., Optimal climate signal detection in four dimensions, J. Geophys. Res. - Atmos., 104, D4, pp 4089-4099, Feb., 1999.

    Syndergaard, S., Modeling the impact of the Earth’s oblateness on the retrieval of temperature and pressure profiles from limb sounding, J. of Atmos. Solar-Terres. Phys., 60, 171-180, 1998.

    Tsuda, T., Nishida N., Rocken C. and R. Ware, A global morphology of gravity wave activity in the stratosphere revealed by the GPS occultation data (GPS/MET), J. Geophys. Res. (submitted) 1998a .

    Tsuda, T., Nishida N., Rocken C. and R. Ware, Verification of temperature profiles obtained by the GPS occultation technique (GPS/MET) with radiosonde and LIDAR measurements in tropical and subtropical regions, J. Geophys. Res. (submitted), 1998b.

    Vorob’ev, V.V., and T. G. Krasil’nikova, Estimation of the accuracy of the atmospheric refractive index recovery from Dopler shift measurements at frequencies used in the NAVSTAR system, Phys. Atmos. Oceans, 29, 602-609, 1994.

    Ware, R., GPS Sounding of Earth's Atmosphere, GPS World, 3, 56- 57, 1992.

    Ware, R., M. Exner, B. Herman, W. Kuo, and T. Meehan, Active atmospheric limb sounding with an orbiting GPS receiver, Trans. Amer. Geophys. Union, EOS, 74, 336, 1993.

    Ware, R., The University Navstar Consortium, Global Positioning for Geosciences Research, IGS Special Topics and New Directions, Workshop Proceedings, Potsdam, 40-48, 1995.

    Ware, R., and S. Businger, Global positioning for geosciences Research, EOS, 76, 187, 1995.

    Ware, R., M. Exner, D. Feng, M. Gorbunov, K. Hardy, B. Herman, Y. Kuo, T. Meehan, W. Melbourne, C. Rocken, W. Schreiner, S. Sokolovskiy, F. Solheim, X. Zou, R. Anthes, S. Businger, and K. Trenberth, GPS sounding of the atmosphere from low earth orbit: Preliminary results, Bull. Amer. Met. Soc., 77, 19-40, 1996.

    WCRP, Stratospheric Processes and Their Role in Climate. World Climate Research Programme Report 105, World Meteorological Organization, Geneva, 131 pp., 1998.

    Yuan, L., R. A. Anthes, R. H. Ware, and W. Bonner, Using the Global Positioning System in atmospheric sciences and for detecting climate change, Proc. 2nd International Conference on East Asia and Western Pacific Meteorology and Climate, 1992,

    Yuan, L., R. A. Anthes, R. H. Ware, C. Rocken, W. Bonner, M. Bevis, and S. Businger, Sensing climate change using the Global Positioning System, J. of Geophys. Res., 98(14), 925- 14,937, 1993.

    Zou, X. , Y.-H. Kuo, and Y.-R. Guo, Assimilation of atmospheric radio refractivity using a nonhydrostatic mesoscale model, Mon. Wea. Rev., 123, 2229-2249, 1995.

    Zou, X., F. Vandenberghe, B. Wang, M.E. Gorbunov, Y.-H. Kuo, S. Sokolovskiy, J.C. Chang, J.G. Sela, and R.A. Anthes, Direct assimilation of GPS/MET refraction angle measurements: Part I: Concept and results of raytracing, J. Geophys. Res, accepted for pub. Sept., 1998.

    Zou, X., B. Wang, F. Vandenberghe, M.E. Gorbunov, Y.-H. Kuo, J.C. Chang, J.G. Sela, and R.A. Anthes, Direct assimilation of GPS/MET refraction angle measurements: Part II Adjoint of raytracing and results of variational analysis, J. Geophys. Res, accepted for pub. Sept., 1998.