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

The Global Positioning System/Meteorology (GPS/MET) mission has been the first experiment to use a low Earth orbiting (LEO) satellite (the MicroLab-1) to receive multi-channel Global Positioning System (GPS) carrier phase signals and demonstrate active limb sounding of the Earth's atmosphere and ionosphere by radio occultation technique. Under the assumption of spherical symmetry at the locality of the occultation, the dual-band phase data have been processed to yield ray-path bending angle profiles, which have then been used to yield profiles of refractive index via the Abel integral transform. The refractivity profiles can then, in turn, yield profiles of ionospheric electron density and other atmospheric variables such as neutral atmospheric density, pressure, and temperature in the stratosphere and upper troposphere, and water vapor in the lower troposphere with the aid of independent 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 (neglecting bending). In order to assess the accuracy of the GPS/MET ionospheric electron density retrievals, coincidences of ionosonde data with GPS/MET occultations have been examined. The retrieved electron density profiles from GPS/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° N, 121.0° E) and by Utah State University (the Bear-Lake dynasonde; 41.9° N, 111.4° W). A fuzzy classification method for the automatic identification and scaling of ionogram traces has been applied to recorded ionograms, and then bottom-side ionospheric electron density profiles are determined from true-height analysis. The comparison results show better agreement for both of the derived electron density profiles and the F2-layer critical frequency (foF2) at mid-latitude observations than at low-latitude observations. The rms foF2 differences from the GPS/MET retrievals are 0.61 MHz to the Bear-Lake dynasonde measurements and 1.62 MHz to the Chung-Li digisonde measurements.