GPS Sounding of the Atmosphere from Low Earth Orbit: Preliminary Results

Abstract

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.

Two-dimensional variational analysis of near-surface moisture from simulated radar refractivity-related phase change observations

“…The GPS-slant-path water vapor measurements are most effective at the mid to upper levels where slant paths intersect (Liu and Xue, 2006). At the same time, the low-orbiting-satellite-based GPS occultation data generally do not reach the surface and have poor horizontal resolution (Ware et al, 1996;Anthes et al, 2008). Mesoscale surface observational networks (e.g., Brock et al, 1995) are currently the best platform for near-surface moisture observations, but their spatial resolution may not be sufficient to resolve fine-scale structures important for convective initiation (Weckwerth et al, 2004).…”

Greenhouse gas profiling by infrared-laser and microwave occultation: retrieval algorithm and demonstration results from end-to-end simulations

“…As described by Kirchengast and Schweitzer (2011), the LMIO method can be considered as a next generation of the well established and successful GNSS-LEO radio occultation (GRO) method (Ware et al, 1996;Kursinski et al, 1997;Steiner et al, 2001;Anthes et al, 2008;Luntama et al, 2008;Steiner et al, 2009;Ho et al, 2009). LMIO and GRO share the occultation measurement principle (Phinney and Anderson, 1968; and the use of highly coherent and stable inter-satellite signals, and therefore the potential of providing accurate, long-term, consistent benchmark data with high vertical resolution and global coverage.…”

GPS radio occultation with CHAMP: First results and status of the experiment

“…The pioneering satellite mission for GPS radio occultation technique for global sounding of Earth's neutral atmosphere was the U.S. American GPS/MET experiment [Ware et al, 1996]. GPS/-MET also demonstrated the potential of GPS radio occultation technique for global sounding of Earth's ionosphere [e.g.…”