CHAMP and SAC‐C atmospheric occultation results and intercomparisons

Abstract

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 pot...

A climatology of multiple tropopauses derived from GPS radio occultations with CHAMP and SAC‐C

“…11, 2001 and provides RO data continuously in an operational manner since mid-2001 [Wickert et al, 2005]. The amount of data from SAC-C has varied over the last years [Hajj et al, 2004]. Thus, SAC-C data are only available for this study from August -October 2001 and March-November 2002.…”

Local time influence in single‐satellite radio occultation climatologies from Sun‐synchronous and non‐Sun‐synchronous satellites

“…Foelsche et al, Observing climate with radio occultation data from the CHAMP satellite, submitted to Climate Dynamics, 2006, hereinafter referred to as Foelsche et al, submitted manuscript, 2006]. First of all, the validity of these climatologies depends on the reliability of the measurements and its retrieval [e.g., Hajj et al, 2004;Wickert et al, 2004], but it also depends on sampling time and location resulting in the sampling error (Foelsche et al, submitted manuscript, 2006). This paper focuses on the sampling error for single LEO satellites, in particular on the effect of local times at which the RO profiles have been taken.…”

On the possible use of radio occultation middle latitude electron density profiles to retrieve thermospheric parameters

“…The technique for sounding the Earth's atmosphere was demonstrated by the proof-of-concept GPS Meteorology (GPS/MET) experiment in 1995-1997 (Ware et al 1996). Following GPS/MET, additional missions, that is, the Challenging Minisatellite Payload (CHAMP; Wickert et al 2001) and the Satellite de Aplicaciones Cientificas -C (SAC-C; Hajj et al 2004), have confirmed the potential of RO sounding of the ionosphere, stratosphere, and troposphere. The FORMOSAT-3/COSMIC (Rocken et al 2000) also applies this technique, with primary scientific goal to demonstrate the value of near-real-time RO observations in operational numerical weather prediction, including space weather science and operations.…”