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


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.


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
    April 1995
    Proof of concept
    Feb 1999 
    Similar GPS receiver as GPS/MET
    Feb. 1999
    Similar GPS receiver as GPS/MET
    Jan. 2000
    Improved GPS tracking receiver
    Jan. 2000
    Setting and rising occultations
    May 2001
    Nov. 2002
    Operational demonstration
    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.
    Limb sounding geometry complementary to ground and space nadir viewing instruments
    High Accuracy
    High vertical resolution
    Consistency of horizontal and vertical scales of observations
    All weather-not affected by clouds or precipitation
    Independent height and pressure
    Requires no first guess sounding
    Independent of radiosonde or other calibration
    No instrument drift
    No satellite-to-satellite bias
    Top and bottom side sounding
    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.


    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.

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