Document Type : Research
Surveying Department, Civil Engineering Faculty, University of Tabriz, Tabriz, Iran
Very long baseline interferometry (VLBI) has been used since the mid-1960s as a spatial geodetic tool for accurately determining coordinates on the ground, determining the Earth's rotational axis with very high accuracy and extracting important parameters related to earth. The basic principle of VLBI is measuring the time difference between the arrival time of a radio wave in two or more antennas, which is referred to as the time delay. To achieve this purpose, first the atomic clock must be used and secondly the clocks in the antennas must be synchronous. Earth orientation parameters (EOP) are a set of parameters that describe irregularities in the Earth's rotation. The VLBI method can be used to derive EOP. These parameters can be used for transformation between international terrestrial reference frame (ITRF) and celestial reference frame (ICRF) or vice versa. This transformation takes place through a sequence of rotations related to precession/nutation (NUTX, NUTY), earth rotation (DUT1) and polar motion (XPO, YPO). The geophysical effects of the Earth as well as the effects of celestial bodies such as the moon or the sun on the Earth's rotation, lead to changes in the EOP; therefore, changes in geophysical parameters of the earth can be obtained from changes in the EOP. The purpose of this study is to investigate the accuracy of the EOP after adding new observation station to the CONT14 observation network. These observation station are artificially constructed in Iran and the accuracy of EOP before and after adding new station to the network is investigated. The received wave from radio sources considered as a planar wave due to the great distance of radio sources from the stations. The wave from the quasars reaches to the antennas at different times. In this study, the CONT14 session has been used. CONT sessions are one of the most famous and important sessions in which the stations collect data continuously for two weeks. On average, the CONT sessions takes place every three years. Due to the large amount of data in these sessions, the EOP are determined with high accuracy. Due to the importance of CONT sessions, we will investigate the effect of constructing stations in Iran on the accuracy of the EOP in one of the CONT sessions, which will be added to the CONT14 observation network. Due to the high cost of constructing a VLBI observation station and to approaching reality, we will add five stations to the network in maximum case. The local network resulting from the five new stations covers the whole of Iran and the location of these five stations has been chosen arbitrarily. With analyzing the data that collected by the CONT14 session, the accuracy of the EOP is obtained. After adding new observation stations to CONT14 network and performing the new session, the collected data is processed again and the precision of the EOP is obtained. A comparison of the precision obtained in the new mode with precision obtained in CONT14 session shows the degree of improvement of EOP accuracy. In geodesy, the precision of the results is always discussed along with the results, and high-precision data are of interest to researchers because the models obtained by the data are closer to reality. The accuracy of the results can be increased by the high number of observations as well as improving the geometry of the network. In this study, the effect of constructing one or more VLBI observation stations in Iran on the precision of the EOP was investigated. By comparing the EOP precision in all possible observation networks, we came to the conclusion that if two observation stations are constructed in Ahvaz and Mashhad and add them to the CONT14 observation network, we can improve the EOP accuracy by about 10.14%.