Investigating the accuracy of point location obtained by real-time measurement methods using continuously operating GPS reference stations: Kocaeli /Cayirova instance

Work has been done to ensure that the coordinate information generated by using global positioning systems has the accuracy and speed required in engineering projects and as a result of these studies the concept of stationary reference station has emerged. The concept of stationary reference station is an improved version of the RTK (Real-Time Kinematic) measurement method. Stationary reference stations record this data by making continuous satellite observations. The instant correction values are calculated by comparing the reference coordinates of the stations with the coordinates obtained from the instant measurements. GPS receivers capable of receiving instant correction values via the internet and apply the correction value to the measurements and display as corrected coordinate information to the user. The determination of the reference coordinates of the stations at high accuracy has a great importance that will affect the accuracy of the correction value to be calculated. It is claimed, consept of using network based RTK systems provides more accurate coordinate elements according to single base RTK stations. Network based RTK systems provide a interpolated solution using users' receivers location. The system calculates an error model to create a virtual reference station nearby the user's receiver so user can get more accurate coordinate elements using these corrections. Furthermore, there are many advantages like atmospheric modelling, tectonic plate movement observations. But the major problem is, if one of the reference stations have failed or provided a false interpolation value there is no way to understand the quality of correction data by users. Single RTK stations are capable of modeling short range distances up to 20 km but also have some disadvantages. Receivers over 20 km distance from base station can cause a false error modeling that provides false coordinate elements. In our country, TUSAGA-Aktif (Turkish National Fixed GPS Network – CORS-TR) system was established with the same working principle. With the system turned on, the need for local control points has been steadily reduced, and the use of GPS receivers has increased rapidly in everyday measurement activities. During the installation phase of the TUSAGA-Aktif (CORS-TR) system, 3-stage performance tests were carried out at distances of 60 km, 90 km and 120 km, and it was decided that the stations to be installed at an average distance of 80-100 km were sufficient for the design of the network. Due to some interruptions in the use of the TUSAGA-Active network in the fieldwork carried out by Çayırova Municipality, it is thought that the network needs to be tightened and the system infrastructure strengthened so that higher point location accuracy can be obtained. In 2015, T.C. Çayırova Municipality has decided to establish a continuesly operating GPS reference station which is independent of the TUSAGA-Aktif (CORS-TR) system which can only serve the region. The project was completed in February 2016 and system tests were carried out. The station named as COVA. COVA reference station is operating with single base RTK capability. TUSAGA-Aktif and IGS stations were used in order to determine the reference coordinates of the COVA station precisely. The IGS station and the COVA station's 260-day data for 2016 were evaluated and the data for the same time period with 6 TUSAGA-Aktif (CORS-TR) stations were evaluated and the results were examined. In order to analyze the reliability of the data obtained from TUSAGA-Active system, ISTN reference station was adjusted from other five TUSAGA-Active stations. ISTN station coordinates and annual velocities published by the HGK in reference coordinate system (ITRF96 epoch 2005.0) were used to compare the adjusted coordinates and the shifted coordinates to the measurement epoch. The results show that the TUSAGA-Active system works smoothly at the time interval that the measurements are made and there is no problem with stations velocity elements determined by General Command of Mapping (HGK). The evaluation processes of COVA reference station and the static measurements for the 16 points were calculated using GAMIT / GLOBK software, which is an academic GPS evaluation software developed by M.I.T. (Massachusetts Institute of Technology). Commercial GPS processing programs are unable to handle error modeling over 100 km baselines. Time series analysis, annual velocity calculations, daily horizon distribution graphs of stations were automatically produced. By examining the graphics, problems that would affect the measurement accuracy, like multipath effect or etc. can easily be distinguished. The COVA stations annual velocity was calculated using a 260-day data set. Beyond that, General Command of Mapping uses GAMIT/GLOBK for analyses of TUSAGA-Aktif system. 2 years of time series required for more accurate annual velecity calculations. But there were not enough data during this process. In order to obtain reliable velocity elements, COVA data has to be calculated, at least in a year time interval. Çayırova region is also effected by North Anatolian Fault and there is no TUSAGA-Aktif system station closer than 60 km. It has been decided that a static measurement campaign should be organized so that the accuracy analysis of the correction values issued by the COVA station can be made. At the Çayırova county and the surrounding area, 16 points were found, and a five-hour static session was held on 22-23-25 October 2016. The measurement epoch coordinates of points (2016.81 epoch) were obtained by evaluating the result obtained from the static session. The velocity elements for 16 points were calculated and shifted to the ITRF96 2005.0 epoch coordinates using 3 TUSAGA-Aktif (CORS-TR) stations (SLEE, TUBI, IZMT), which are the points for country reference system using Large Scale Map and Map Information Production Regulation's equations (Chapter 5.3.1). For the analysis of the correction values issued by the COVA station, RTK measurements were carried out by selecting 4 points of the 16 points calculated from the ITRF96 2005.0 reference coordinates by static measurements. In these operations, the receiver is mounted on control points. The receiver is connected to both the TUSAGA-Active system and COVA station. Coordinates were measured in 10 epochs also 50 epochs and the averages were recorded. The operations were carried out for 3 days in the same weather conditions and constellation of satellite array. The data obtained from the RTK method and the data obtained from the static measurements were compared. It is aimed to obtain a consistent result for the accuracy analysis of the correction data generated by reference stations' continuous observations.