dc.description.abstract | The aim of this paper is the improvement of GPS Radio Occultation Retrieval Error of E Region Electron Density. Before the error correction was made, it was necessary to understand the feature of FORMOSAT3/COSMIC electron density profiles. On the basis of 66 month data (July 2006- December 2011), the investigation and analysis of the ionospheric E region electron density were made. In order to screen the data in harmony with the physical principle, the quality control algorithm was developed, including mean deviations, F region slope in a range of 420 and 490 km, Kp index provided by NOAA, and the fluctuations of signal-to-noise (SNR) ratio and excess phase. The global distribution and diurnal variations of COSMIC observation in E region show that there are three anomalous daytime enhancements in the magnetic latitude region ±30°–50° and magnetic equator, electron density depletion in the magnetic latitude region ±10°–30°, aural oval situated around 70° magnetic latitude.
The International Reference Ionosphere (IRI) model was selected to simulate temporal and spatial distributions of global electron densities from 80 to 2000 km in 2008. The COSMIC geometry were utilized to retrieve the electron density in each occultation events. The retrieval error were computed by comparing the retrieved electron density with IRI electron density in E and F region. The major cause of the retrieval error is ionospheric electron density which violate assumption of spherical symmetry. The result show that the simulation-retrieved electron densities seem to be significantly overestimated in the magnetic latitude regions ±30°–50° and magnetic equator, and underestimated in the magnetic latitude regions ±10°–30°, which is consistent with COSMIC observation. The retrieval error were ±35 % in the daytime and nighttime F region, ±200% in the daytime E region depended on the different latitudinal region, and greater than 200% in the nighttime E region. Besides the retrieval error of the electron density, the retrieval error of the vertical TEC was also examined. Overall, the retrieval error of the vertical TEC were ±30 %.
Despite regional discrepancies in the magnitudes of the E region electron density, the IRI model simulations can, on the whole, describe the COSMIC measurements in quality and quantity. On the basis of global ionosonde network and the IRI model, the retrieval errors of the global COSMIC-measured E region peak electron density (NmE) from July 2006 to July 2011 are examined and simulated. The COSMIC measurement and the IRI model simulation both reveal that the magnitudes of the percentage error (PE) and root mean square error (RMSE) of the relative RO retrieval errors of the NmE values are dependent on local time (LT) and geomagnetic latitude, with minimum in the early morning and at high latitudes and maximum in the afternoon and at middle latitudes. In addition, the seasonal variation of PE and RMSE values seem to be latitude dependent. After removing the IRI model-simulated GPS RO retrieval errors from the original COSMIC measurements, statistics show that the monthly residual errors remained in the corrected COSMIC-measured NmE vary in a range of -20% - 38%, which are comparable to or larger than the percentage errors of the IRI-predicted NmE fluctuating in a range of -6.5% - 20%. The RMSE after correcting are in a range of 3.4*10^4 - 6*10^4 #/cm3, which are greater than the RMSE of the IRI-predicted NmE fluctuating in a range of 1.2*10^4 - 2.3*10^4 #/cm3.
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