GNSS Radio Occultation

During the last decade the satellite-based GPS radio occultation technique was established as a method for sounding the Earth's atmposphere that led to several new scientific findings [1]. In addition, radio occultation data were used by weather service centres for the assimilation in weather models that finally improved daily weather forecasts [2].

The radio occultation (RO) technique is a precise vertical sounding method of the atmosphere (so-called limb sounding) on a global scale. GPS receiver (in future also from other navigation systems) onboard low-Earth orbiting (LEO) satellites (less than about 1000 km) measure the phase and amplitude variation from one or more GPS satellites in high temporal resolution (e.g. 50 Hz) during a rising or setting of a GPS satellite as seen from the LEO (occultation). With the measured GPS signals influenced by the atmposphere and the knowledge of the precise orbit positions the bending of the ray pathes can be estimated which is the basis for the calculation of refractivity and temperature profiles in the altitude range up to 35-40 km. In addition in the lower troposphere water vapour information and from above 60 km to the flight level of the satellite electron densities can be derived [3].

Because of the measuring method and the characteristics of the RO technique as global availability, weather independance of the measurements (GPS signals are in the L band wit hfrequencies L1=1,57542 GHz and L2=1,2276 GHz), the high vertical resolution (about 1 km in the stratosphere and about 100 m in the tropopsphere) and long-term stability of the measurements the RO data are in particular suitable for atmospheric and climate research. With one LEO satellite about 150 to 200 global distributed atmospheric profiles are available daily.

Work at GFZ

The derivation of atmospheric parameter, the development and improvement of the analysis methods are main research fields at GFZ since the CHAMP mission started in 2000 [4]. During the last years several radio occultation missions followed. Currently GFZ process the RO data from GRACE, TerraSAR-X, and Tandem-X in near-real time and the results are provided to weather service centres [5]. In addition all GFZ radio occultation data are regularly post-processed with free data access for scientific purposes.

The joint Taiwan-USA six-satellite FORMOSAT-3/COSMIC mission started in 2006 provided about 2500 radio occultations daily over several years. Nowadays other satellite missions like the European Metop-A and Metop-B satellites contribute continuously to the RO data base and several other will follow in future (e.g., GRACE-FO, Atmosat).

The RO data are also used at GFZ for scientific investigations related to the temperature variability in the upper troposphere and lower stratosphere, tropopause dynamics (climate monitoring), the analysis of gravity waves and the occurrence of ionospheric disturbances (sporadic E) all on a global scale [6] [7] [8] [9].

Members of the working group GNSS radio occultation contribute to several projects (GISELA, MuSE, StRATEGy), programs (SPARC ATC group, Reklim), and are involved in the realization of future RO missions (GRACE-FO, Atmosat).

The work of the GNSS radio occultation group at GFZ is integrated in the Helmholtz research field "Earth and Environment" in the program "Atmosphere and Climate" in close cooperation with the KIT and FZ Jülich.

[1] Anthes, R. A. (2011): Exploring Earth's atmosphere with radio occultation: contributions to weather, climate and space weather. Atmos. Meas. Tech., 4, 1077-1103, doi:10.5194/amt-4-1077-2011.

[2] Cardinali, C. and S. Healy (2014): Impact of GPS radio occultation measurements in the ECMWF system using adjoint-based diagnostics. Q. J. R. Meteorol. Soc. 140: 2315–2320, doi:10.1002/qj.2300.

[3] Kursinski, E. R., G. A. Hajj, J. T. Schofield, R. P. Linfield, and K. R. Hardy (1997): Observing Earth's atmosphere with radio occultation measurements using the Global Positioning System. J. Geophys. Res., 102(D19), 23,429-23,465.

[4] Wickert, J., Ch. Reigber, G. Beyerle, R. König, Ch. Marquardt, T. Schmidt, L. Grunwaldt, R. Galas, T. K. Meehan, W. G. Melbourne, K. Hocke (2001): Atmosphere sounding by GPS radio occultation: First results from CHAMP. Geophys. Res. Lett., 28(17), 3263-3266.

[5] Schmidt, T., J. Wickert, G. Beyerle, R. König, R. Galas,C. Reigber (2005): The CHAMP atmospheric processing system for radio occultation measurements. In: Reigber C., H. Lühr, P. Schwintzer, J. Wickert (Eds): Earth Observation with CHAMP, Springer, Berlin, ISBN 3-540-22804-7,597–602.

[6] Schmidt, T., J. Wickert, A. Haser (2010): Variability of the upper troposphere and lower stratosphere observed with GPS radio occultation bending angles and temperatures. Adv. Space Res., 46(2), 150-161, doi:10.1016/j.asr.2010.01.021.

[7] Schmidt, T., P. Alexander, A. de la Torre (2016): Stratospheric gravity wave momentum flux from radio occultations. J. Geophys. Res. Atmos., 121, doi:10.1002/2015JD024135.

[8] Arras, C., J. Wickert, G. Beyerle, S. Heise, T. Schmidt, C. Jacobi (2008): A global climatology of ionospheric irregularities derived from GPS radio occultation, Geophys. Res. Lett., 35, L14809, doi:10.1029/2008GL034158.

[9] Heise, S. J. Wickert, G. Beyerle, T. Schmidt, Ch. Reigber (2006): Global monitoring of tropospheric water vapor with GPSradio occultation aboard CHAMPS. Adv. Space Res., 37, 2222–2227, doi:10.1016/j.asr.2005.06.066.