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

Abstract

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 bottomside 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 F 2 -layer critical frequency ( f o F 2 ) at midlatitude observations than at low-latitude observations. The rms f o F 2 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.

Long-term comparison of the ionospheric F2 layer electron density peak derived from ionosonde data and Formosat-3/COSMIC occultations

“…Hajj & Romans (1998) considered, for instance, the GPS single-frequency signal bending in the ionosphere and used the Abel inversion to derive EDPs to perform a validation between GPS/MET profiles with ionosonde measurements during low solar cycle conditions with a bending angle <0.01°. It was concluded that the accuracy of NmF2 yields around 20% of its absolute level, which was later confirmed by Tsai et al (2001). Furthermore, to estimate the absolute precision of the classical Abel inversion, Schreiner et al (2007) studied the accuracy of N e by co-located F-3/C EDPs and obtained a root-mean-square (RMS) difference of approximately 10 9 el/m 3 , a value which indicates already, that rough discrepancies cannot be ruled out by using the classical approach.…”

Ionospheric electron density observed by FORMOSAT-3/COSMIC over the European region and validated by ionosonde data

“…Derivation of the ionospheric electron density from RO measurements is described in more detail by Tsai et al (2001). At CDAAC, the ionospheric profiles are also retrieved by the Abel inversion from TEC along LEO-GPS rays.…”

Wave activity at ionospheric heights above the Andes Mountains detected from FORMOSAT‐3/COSMIC GPS radio occultation data

“…Nevertheless [e.g., Tsai et al, 2001], retrieved profiles from RO data frequently depict negative or positive bias or even negative N e values below altitudes around 100 km. These errors are due to (1) the considerable spatial extension of the averaging in the retrieval process around the tangent point at ionospheric heights (≈ 1000 km) and (2) the spherical inhomogeneity of N e , expected at low and midlatitude regions (in the lower neutral atmosphere the averaging around the tangent point in the T retrieval process is considerably lower).…”