Evaluation of Iran Ionosphere Model Based on GPS Data Processing

Abstract

The ionosphere is the ionized region of the atmosphere which is situated between 80 and 1200 km. Ionospheric delay is the major resource of error in GNSS positioning, Therefore knowledge of the ionospheric behavior is an important factor in this field. Total Electron Content (TEC) values may be considered as a key parameter to monitor the behavior of the ionospheric medium. Nowadays, continuous GNSS observations can provide an efficient tool to monitor timely ionospheric irregularities. Many scientists have investigated global ionospheric models on the basis of different observations data. For example, IGS Ionosphere Working Group produced daily TEC maps for user services from GNSS data. In this paper, we intend to utilize dual frequency GPS observations provided by Iranian Permanent GNSS Network (IPGN) to calculate TEC maps in Iran. For this purpose, data of 43 IPGN stations and about 180 IGS stations were processed with Bernese GPS software. This process was based on the use of spherical harmonics expansion up to degree and order 15 like the global one, to provide a model of TEC. In the meantime of using GPS data to calculate TEC maps, other resource of errors in GPS positioning such as satellite and receiver clock biases, tropospheric error and multipath error must be either removed, or at least significantly reduced. For this purpose, we used the geometry free linear combinations of pseudo ranges and carrier phases. For reducing the noise level of pseudo range observations we used the carrier phase smoothed pseudo range data as well. The processing method consists of several steps; code smoothing with phase observations, estimation of Differential Code Biases (DCBs), estimation of spherical harmonic coefficients and generation of TEC maps. Before code smoothing, the phase observations were pre- processed to remove the cycle slips. The used model assumes that the whole free electrons are concentrated on a thin spherical layer to an altitude varying between 250 and 450km. We chose the altitude equals to 450km in this paper. The obtained results show that the maximal TEC value measured over Iran is about 22 TECU, this value corresponds to the noon period (midday), where the sun is close to the zenith. The minimal TEC value varied around 5 TECU, it corresponds to the midnight period, and such values were obtained for the day of Jun 22, 2009. Iranian Ionosphere Model (IRIM) was created and compared with the different solutions delivered by the several IGS Ionosphere Associate Analysis Centers (IAACs) which are CODE, ESA, JPL and UPC. Despite different IAACs use various approaches, they provide TEC maps with resolution of 2 hours, 5^◦ and 2.5^◦ in UT, longitude and latitude respectively. In order to compare our obtained results with different IAACs TEC maps, we chose TEHN station from IPGN stations to generate and display TEC profiles. The differences between the various models are less than 6 TECU. The IRIM results had minimum differences with CODE TEC maps which both use spherical harmonics as their basic functions. The remained differences caused by the fact that when CODE TEC maps are estimated, the data from IPGN stations are not used. Calculated TEC values were thereafter applied to correct and improve the quality of the single frequency solutions in absolute and relative positioning modes. It is noted that ionosphere free (L₃) solution results was considered as the reference solution. In absolute mode, we received the considerable improvements in horizontal and vertical components by using the IRIM instead of IGS models. In relative mode the comparison between the corrected L₁ and L₃ solutions showed that ignoring the ionospheric effects causes network contraction. Furthermore, the corrected L₁ solution results using IRIM rather than IGS models were closer to the L₃ solution results. Moreover, for baselines up to several hundreds of kilometers, deviations were better than 10cm in horizontal component.