GPS SOUNDING OF THE ATMOSPHERE FROM LOW-EARTH-ORBIT - PRELIMINARY-RESULTS

This paper provides an overview of the methodology of and describes pr eliminary results from an experiment called GPS/MET (Global Positionin g System/Meteorology), in which temperature soundings are obtained fro m a low Earth-orbiting satellite using the radio occultation technique . Launched into a circular orbit of about 750-km altitude and 70 degre es 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 (refr acted) by the atmosphere before they reach the receiver, causing a del ay in the dual-frequency carrier phase observations sensed by the rece iver. During this occultation, GPS limb sounding measurements are obta ined from which vertical profiles of atmospheric refractivity can be c omputed. 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 clima te research. Because of the dependence of refractivity on both tempera ture and water vapor, it is generally impossible to compute both varia bles from a refractivity sounding. However, if either temperature or w ater vapor is known from independent measurements or from model predic tions, the other variable may be calculated. In portions of the atmosp here where moisture effects are negligible (typically above 5-7 km), t emperature may be estimated directly from refractivity. This paper com pares a representative sample of 11 temperature profiles derived from GPS/MET soundings (assuming a dry atmosphere) with nearby radiosonde a nd high-resolution balloon soundings and the operational gridded analy sis 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 Ex periment was available for comparison. These comparisons show that acc urate vertical temperature profiles may be obtained using the GPS limb sounding technique from approximately 40 km to about 5-7 km in altitu de where moisture effects are negligible. Temperatures in this region usually agree within 2 degrees 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 degrees C at the tropopause, degrading to about 1 degrees 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 in itial temperature and pressure assumptions in this region and initial ionospheric refraction assumptions. In the lower troposphere, the erro rs appear to be associated with multipath effects caused by large grad ients in refractivity primarily due to water vapor distribution.