Combination of GPS and Satellite Altimetry Observations for Local Ionosphere Modeling Over Iran

The ionosphere is the upper part of the atmosphere that extends from 80 to 1200 km above the Earth’s surface. The existing free electrons and ions in the ionosphere layer affect the signal propagation speed such as satellite positioning and satellite altimetry signals. Regardless of the fact that Dual-frequency measurements can remove ionospheric delay effect, dual-frequency observations of the permanent GPS stations can also be utilized to produce the ionosphere maps including the vertical total electron content  (VTEC) values. For instance, the International GNSS Service (IGS) sub-centers produce daily global ionosphere maps (GIMs) using the GNSS data. The spatial resolution of GIMs in the latitude and longitude directions is 2.5 degrees and 5.0 degrees, respectively, and their temporal resolution is 2 hours. One of the IGS sub-centers, namely CODE produces the GIMs based on the spherical harmonic basis functions up to the degree and order 15. The aim of this research is to develop a local ionosphere model based on the B-spline basis functions using the combined GPS and satellite altimetry observations over Iran. Accordingly, the potentiality of the B-spline basis functions for local ionosphere modeling was studied at first. For this purpose, a local ionosphere model (LIM) was produced based on observation data from 16 Iranian permanent GPS stations and 5 IGS ones and B-spline basis functions. My assumptions in this modeling are as follows: first, the ionosphere is a thin shell that is located 450 km above the Earth’s surface, second, the smoothed code station observations obtained by Bernese 5.0 software is considered as observation vector. Third, the weight matrix elements are proportional to the satellite elevation angle.  Forth, the differential code biases (DCBs) for all satellites which are obtained from IGS precise products, are considered as known parameters in the equations. And the last assumption was that a simple cosine mapping function was used to convert the slant total electron content (STEC) to the VTEC. As a result, the comparison between the LIM and the GIM showed that the B-spline basis functions were more efficient than the spherical harmonic ones for local ionosphere modeling. Following the first result, a new LIM, which is based on the B-spline basis functions, was produced by the integration of permanent GPS station and Jason-2 satellite altimetry observations. The GPS and satellite altimetry observations were chosen from day 107 of the year 2014, according to the latest maximum solar activity. The weight matrix of the GPS and satellite altimetry observations were determined based on the least-square variance component estimation (LS-VCE) method. The results showed that the local ionosphere model derived from combination of the GPS and satellite altimetry observations were more accurate than the local ionosphere model derived from the GPS observations only, this is due to the fact that the dual-frequency radar altimetry data are the main source of the ionospheric observations at sea, where there is no GPS permanent station, and can be used to improve the GIMs and LIMs. Finally, As by-products, the DCB values for the permanent GPS receivers and the bias term between the GPS and satellite altimetry observations were determined.