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ŠUMARSKI LIST 3-4/2016 str. 55     <-- 55 -->        PDF

resources, multipurpose cadastre, e-government and personal mobile applications (Ocalan and Tunalioglu, 2010; Ocalan et al. 2013).
GNSS works 24 hours a day, in any weather conditions, anywhere in the world, and provides precise positioning information where satellite visibility is available. Although there have been some surveying constrains originated from limited satellite visibility and signal interruptions, position information with different accuracy levels may be achieved in terms of GNSS survey methods used. In GNSS technology, traditionally relative and differential positioning methods are used to achieve precise positions (Rizos, 2012; Alkan, 2013; Ocalan, 2015).
In all GNSS techniques based on relative and differential positioning principle, simultaneous observations made from one or more reference stations with known coordinates are required. In other words, simultaneous observations should be made with at least two GNSS receivers: one should occupy a reference station whose coordinates are known, and the other should be established at a point whose coordinates will be estimated. Several criteria, such as the 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 (Hoffmann-Wellenhof et al., 2008; Rizos, 2012).
For instance, while in differential GNSS (DGNSS) technique, where single-frequency code (pseudorange) observations are used, decimetre level positioning accuracy can be achieved, in real time kinematic (RTK) techniques, where dual/multi-frequency carrier phase observations are used, centimetre level positioning accuracy can be derived. From this aspect, single-base RTK and Network-RTK techniques have become indispensable for users in real time geodetic studies (Rizos, 2012).
This situation has enabled the establishment of so-called CORS networks by several governments, organizations and companies to support RTK users, and the development of different mathematical correction models of Network-RTK technique. In developed and developing countries, widespread use of these networks provides significant contributions and advantages to all RTK users in terms of criteria such as time, cost and accuracy. Bringing a new concept to GNSS applications, CORS networks, which broadcast the correction data to all users in real time by data transmission and communication equipment, have been generally operated at national or local scale. Today, this technique which handles mathematical correction models such as VRS (Virtual Reference Station), MAC (Master Auxiliary Concept) and FKP (Flächen-Korrektur-Parameter) and then estimates high-accurate positioning information (centimetre-level), can provide solutions under the forested area as well (Ocalan and Tunalioglu, 2010; Rizos, 2012).
However, in some geodetic and geodynamic studies, deformation analysis where millimetre level accuracy is needed the post-processing static GNSS relative survey method is still applied.
2.2.1 Virtual Reference Station (VRS) Model – Virtualna referentna stanica (VRS)
Using CORS for real time kinematic applications, one of the correction models, namely VRS technique, uses a complex filter state model to determine virtual reference station dataset which is located near the rover (Hoffmann-Wellenhof et al., 2008; Wübbena, 2001; Wanninger, 2003). It has become a common procedure in which more than 95 % of the Network-RTK installations use this technique to transmit the correction stream in standardized formats RTCM 2.3, RTCM 3.0, RTCM3.1 or CMR/CMR+ from the server to the field user that is supported by all geodetic receiver manufacturers (Landau, 2003). Moreover, the VRS technique uses the latest model for all error sources that can continuously adjust the correction transition for each rover position (Cannon, 2001; Landau, 2002). In every second the correction values are updated, then connection to the system by a rover provides immediate product transmission after connection. After connection, bidirectional communication should be enhanced by the VRS method, which can be supported by GSM, GPRS and cell phone-based data transmission methods (Wanninger, 2002; Retscher, 2002). Nowadays, bidirectional communication is used worldwide by users with a ratio of more than 99 % in Network-RTK installations for accessing and user accounting.
Materijali i metode
3.1 Study Environment – Područje istraživanja
We established four temporary test stations to compute the coordinates of the point under the forested area, located at the Yildiz Technical University, Campus of Davutpaşa, Istanbul, Turkey. The characteristic property of the area of interest is that it is located at the border of the forested area and open sky. GNSS receiver observations made in forestry are affected by data distortion and signal losses and this negatively affects precision and accuracy measurements. To avoid this, RTK GNSS surveys have been conducted at the open area side of the border aimed at providing observations accurately to enhance the communications of the GNSS receiver with satellites from two points. In addition, terrestrial surveys have been conducted simultaneously by using the other two points. The point whose coordinates are being calculated was established at the forested area side, and terrestrial surveys and static GNSS surveys were also