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INTRODUCTION and RESEARCH PROBLEMS UVOD I PROBLEMI ISTRAŽIVANJA Global Navigation Satellite Systems (GNSS), which consists of satellite technologies such as GPS (USA), GLONASS (Russia), BeiDou (China) and Galileo (EU), is the positioning and navigation system based on artificial satellites. Among them, GPS has been effectively implemented in positioning, navigation and timing applications over the last three decades. With this aspect, GPS serves several facilities such as assesment and preventing the forest and natural resources, environmental and urban planning, land use and agricultural policy, monitoring the global climate change, engineering and infrastructure services, engineering and utilization applications, e-government and personal mobile applications to the services. Moreover high precise positioning information is indispensible for various services in forestry discipline such as protection and efficient utilization of forests, forest road planning, destruction of forests, protection against or putting out of fires, inventories, insect infestation and boundary determination. In addition to this, in geomatic engineering applications, the system has been used for establishing the geodetic networks, cadastre surveys, monitoring the deformations, geographic information system (GIS), photogrammetry and Light Detection and Ranging (LIDAR) applications, where decimeter, centimeter and milimeter level accuracies are required to point positioning (Pirti, 2005; Pirti, 2008; Ocalan and Tunalioglu, 2010; Awange, 2012; Ocalan et al. 2013; Pirti et al.,2015). Traditionally, most of the professional GPS users have used relative (differential) positioning technique to provide high accuracy. However, this technique has some limitations. Minimum two or more GPS receivers should be used and the true coordinates of the reference stations should be known. Addition to this, increase of the distance between reference station (base) and rover station has reduced the position accuracy (El-Rabbany, 2006; Hoffmann-Wellenhof et al., 2008; Abd-Elazeem et al., 2011). Simultaneous observations should be made with at least two GNSS receivers: one should be occupied a reference station whose coordinates are known, and the other should be established to the point whose coordinates will be estimated. Several criteria such as; preferred survey mode (static or kinematic), observation time, the equipment used, signals and codes, data processing algorithms, infrastructure of the reference receiver/s, satellite-receiver geometry, post-processing evaluation or real time applications, provide different level accuracies for positioning. For instance, while in differential GNSS (DGNSS) technique where single-frequency code (pseudorange) observations are used, decimeter level positioning accuracy can be achieved, in real time kinematic (RTK) techniques where dual/multi-frequency carrier phase observations are used, centimeter level positioning accuracy can be derived (Rizos vd., 2012). To obtain more accurate positioning, GPS observations should be collected in relative static surveying mode, and then computed with different post-processing scientific or commercial software. However, in recent years, a method, namely PPP, has become an alternative technique to relative and differential techniques to provide high precision positioning. GPS PPP technique, which is a developed type of absolute positioning, has been implemented effectively by users. This developing process has also affected the GPS data processing and analyzing software, which provides user-friendly and free web based online positioning services developed by several organizations. These services have been used commonly by GPS users in recent years, which are significantly alternatives to the high-cost scientific and commercial software. Figure 1 represents the basic working principle of these systems, which may serve either relative positioning or PPP technique. However, the number of web based online services has been increased in recent years (Tsakiri, 2008; Alkan and Ocalan, 2013; Ocalan, 2015). As seen in Figure 1, users upload their data to the web pages of these services via internet. The results evaluated and analyzed by engine processes send back to the users via e-mail, which contains coordinates of estimated point (X, Y, Z), standard deviations (SD), root mean squares (RMS) error. Some services also send a comprehensive analyze report, which involves GPS error components and graphics beside them mentioned above. These services use high accurate data and products provided by International GNSS Service (IGS). IGS, which makes contributions for comprising and developing |