CHAMP and SAC-C atmospheric occultation results and intercomparisons

By George Hajj, Chi O. Ao, Byron A. Iijima, D. Kuang, E. Kursinski, Anthony J. Mannucci, T. Meehan, L. Romans, Manuel de la Torre Juarez, T. Yunck. 2004

370 citations recorded by the source corpus.

[1] The German Challenging Minisatellite Payload (CHAMP) and Argentine Satelite de Aplicaciones Cientificas-C (SAC-C) Earth science missions, launched in 2000, carry a new generation of Global Positioning System (GPS) receivers for radio occultation sounding of the ionosphere and neutral atmosphere. Though the occultation concept for obtaining profiles of atmospheric temperature, pressure, and moisture was proven in 1995 with GPS/MET, concurrent measurements from CHAMP and SAC-C present the first opportunity for a preliminary evaluation of three central claims: (1) GPS soundings are effectively free of instrumental bias and drift; (2) individual temperature profiles are accurate to <0.5 K between ∼5 and 20 km; and (3) averaged profiles for climate studies can be accurate to <0.1 K. These properties imply that a weak climate trend can be monitored and detected in less than a decade and studied by different instruments at different times with no external calibration. While this detection cannot by itself tell us the source of the climate change, whether natural and anthropogenic, this detection is a prerequisite to answer the more difficult problem of understanding the cause of change. In this paper, these three claims are evaluated by comparing nearby CHAMP and SAC-C profiles. Of nearly 130,000 profiles examined, 212 pairs occurring within 30 min and 200 km of one another were found. Profile pairs agree to <0.86 K (68% confidence interval) and to within 0.1 K in the mean between 5 and 15 km altitude, after removing the expected variability of the atmosphere. If the errors in CHAMP and SAC-C are assumed to be uncorrelated, this implies that individual profiles are precise to <0.6 K between 5 and 15 km. Individual comparisons show closest agreement near the tropopause and display finer resolution than and substantially different temperatures from numerical weather model analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF). Comparisons between CHAMP and SAC-C largely indicate precision; however, several features observed in common, especially near the tropopause, tend also to indicate accuracy. Limitations of previous experiments (e.g., GPS/MET) in probing the lower troposphere have significantly improved with CHAMP and SAC-C, with the majority of profiles (60%) descending to the lowest 0.5 km. This is expected to increase to 90–95% with future system improvements. However, the N-bias problem encountered in GPS/MET is also present in CHAMP and SAC-C, and it is expected to be much reduced once open loop tracking is implemented. Examples are selected to illustrate lower tropospheric sensing, including detection of the planetary boundary layer height. For the first time, such performance is achieved with GPS Antispoofing encryption on. Daily occultations currently number ∼350–400; this is expected to reach over 1000 in the near future, rivaling the number of semidaily radiosonde launches. With several new missions in planning, this may increase tenfold in the next 3–8 years, making GPS sounding a potentially significant input to numerical weather prediction and climate research.

Subfields: Aerospace Engineering, Astronomy and Astrophysics, Oceanography

View Paper: https://www.semanticscholar.org/paper/f22b4aff6acb905305f4932b9e5a2e3bacd21fce

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Aerospace Engineering

In Astronomy and Astrophysics

In Oceanography

Citations in the corpus

The Radio Occultation Experiment aboard CHAMP: Operational Data Analysis and Validation of Vertical Atmospheric Profiles (2004, methodology): “CHAMP, together with the U.S.-Argentinean SAC-C satellite, launched on November 21, 2000, succeeds GPS/MET (see e.g. Hajj et al., 2003).”
The Extratropical Tropopause Inversion Layer: Global Observations with GPS Data, and a Radiative Forcing Mechanism (2007, background): “While the overall structure of the extratropical tropopause is tightly coupled to baroclinic wave dynamics (e.g., Haynes et al. 2001; Schneider 2004), our work suggests that radiative heating may play an important and perhaps dominant role for governing near tropopause temperature structure…”
Tropospheric water vapor, convection, and climate (2010, background): “More recently, several satellites have been launched to make occultations through the atmosphere and obtain vertical profiles tomographically up to roughly the −20°C level [Hajj et al., 2004; Wickert et al., 2009].”
Exploring earth's atmosphere with radio occultation: contributions to weather, climate and space weather (2011, positive): “The success of the proof-of-concept mission GPS/MET, which produced only a small number of soundings each day, led to several successful additional missions, i.e. CHAMP (CHAllenging Minisatellite Payload, Wickert et al., 2001, 2004), GRACE (Wickert et al., 2009) and SAC-C (Satellite de Aplicaciones ”
Kelvin wave variability near the equatorial tropopause observed in GPS radio occultation measurements (2005, methodology): “The temperature retrievals were processed by the University Corporation for Atmospheric Research (UCAR), but results are very similar to independent retrievals performed at the Jet Propulsion Laboratory (as used by Hajj et al. [2004] ) and those from the GeoForschungs Zentrum in Potsdam, Germany [W”