Comparisons of GPS/MET retrieved ionospheric electron density and ground based ionosonde data

The Global Positioning System/Meteorology (GPS/MET) mission has been the fi rst experiment to use a low Earth orbiting (LEO) satellite (the MicroLab-1) to receive multi-channel Global Positioning System (GPS) carrier phase sig nals and demonstrate active limb sounding of the Earth's atmosphere and ion osphere by radio occultation technique. Under the assumption of spherical s ymmetry at the locality of the occultation, the dual-band phase data have b een processed to yield ray-path bending angle profiles, which have then bee n used to yield profiles of refractive index via the Abel integral transfor m. The refractivity profiles can then, in turn, yield profiles of ionospher ic electron density and other atmospheric variables such as neutral atmosph eric density, pressure, and temperature in the stratosphere and upper tropo sphere, and water vapor in the lower troposphere with the aid of independen t temperature data. To approach a near real-time process, electron density profiles can also be derived by the Abel transform through the computation of total electron content (TEC) assuming straight-line propagation (neglect ing bending). In order to assess the accuracy of the GPS/MET ionospheric el ectron density retrievals, coincidences of ionosonde data with GPS/MET occu ltations have been examined. The retrieved electron density profiles from G PS/MET TEC observations have been compared with ionogram inversion results derived from digital ionospheric sounders operated by the National Central University (the Chung-Li digisonde; 24.6 degreesN, 121.0 degreesE) and by U tah State University (the Bear-Lake dynasonde; 41.9 degreesN, 111.4 degrees W). A fuzzy classification method for the automatic identification and scal ing of ionogram traces has been applied to recorded ionograms, and then bot tom-side ionospheric electron density profiles are determined from true-hei ght analysis. The comparison results show better agreement for both of the derived electron density profiles and the F-2-layer critical frequency (f(0 )F(2)) at mid latitude observations than at low-latitude observations. The rms f(0)F(2) differences from the GPS/MET retrievals are 0.61 MHz to the Be ar-Lake dynasonde measurements and 1.62 MHz to the Chung-Li digisonde measu rements.