The COSMIC radio occultation mission represents a revolution in atmospheric sounding from space, with precise, accurate, and all-weather global observations useful for weather, climate, and space weather research and operations. GPS Signal GPS Satellite

This paper provides an overview of the methodology of and describes preliminary results from an experiment called GPS/MET (Global Positioning System/Meteorology), in which temperature soundings are obtained from a low Earthorbiting satellite using the radio occultation technique. Launched into a circular orbit of about 750-km altitude and 70° inclination on 3 April 1995, a small research satellite, MicroLab 1, carried a laptop-sized radio receiver. Each time this receiver rises and sets relative to the 24 operational GPS satellites, the GPS radio waves transect successive layers of the atmosphere and are bent (refracted) by the atmosphere before they reach the receiver, causing a delay in the dualfrequency carrier phase observations sensed by the receiver. During this occultation, GPS limb sounding measurements are obtained from which vertical profiles of atmospheric refractivity can be computed. The refractivity is a function of pressure, temperature, and water vapor and thus provides information on these variables that has the potential to be useful in weather prediction and weather and climate research. Because of the dependence of refractivity on both temperature and water vapor, it is generally impossible to compute both variables from a refractivity sounding. However, if either temperature or water vapor is known from independent measurements or from model predictions, the other variable may be calculated. In portions of the atmosphere where moisture effects are negligible (typically above 5-7 km), temperature may be estimated directly from refractivity. This paper compares a representative sample of 11 temperature profiles derived from GPS/MET soundings (assuming a dry atmosphere) with nearby radiosonde and high-resolution balloon soundings and the operational gridded analysis of the National Centers for Environmental Prediction (formerly the National Meteorological Center). One GPS/MET profile was obtained at a location where a temperature profile from the Halogen Occultation Experiment was available for comparison. These comparisons show that accurate vertical temperature profiles may be obtained using the GPS limb sounding technique from approximately 40 km to about 5-7 km in altitude where moisture effects are negligible. Temperatures in this region usually agree within 2°C with the independent sources of data. The GPS/MET temperature profiles show vertical resolution of about 1 km and resolve the location and minimum temperature of the tropopause very well. Theoretical temperature accuracy is better than 0.5°C at the tropopause, degrading to about 1°C at 40-km altitude. Above 40 km and below 5 km, these preliminary temperature retrievals show difficulties. In the upper atmosphere, the errors result from initial temperature and pressure assumptions in this region and initial ionospheric refraction assumptions. In the lower troposphere, the errors appear to be associated with multipath effects caused by large gradients in refractivity primarily due to water vapor distribution.

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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...

el; of Silurian soils as a result of pedoturbat i o~~, effectively mcreasing the average depth of soil CO, proiluction.Our results (Table 1) ~m p l y that atmospheric CO-, decllneil 1~y a factor of 10 from the Late S i h r i a n to the Early Permian, closely follow~ng (Fig. 4 ) a decline precllctecl hi; theoretical carbon lnais balance models (1). T h e largest decrease, hetween the Late Sil~lrian a11il Late Devonian. coincides with a of rapid evolution and diversificatlon of the terrestrial ecosystem (18).Estimates of atmospheric C02 levels from separated, time-equivalent ~-7aleosols are consistent, suggertlng that a coherent record of changing atlnospheric chem~stry is yreserl-eii In the ancient soil recorJ. REFERENCES AND NOTES1 R A Berner Science 261, 68 (1 993) At?? J SCI 294 56 (1 994) 2 T J Crowley and G R North, Paleoc!~~rato!ogy (Oxford UI?I\/ Press, Oxford, 1991) 3 R A Berner and R Ralswell, Geochim. Cosmochlm. Acta 47. 855 11983) L R. I