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ŠUMARSKI LIST 7-8/2021 str. 66     <-- 66 -->        PDF

shading trees close to measured point. At point fields densifying it is important to pay an attention on the optimal point arrangement. Points should be placed in open areas (borders of the forest stands, forest rides), to have secured an optimal signal receiving, but also with the optimal possibility of the projected point field utilization for next terrestrial measurements (El-Mowafy 2000), (Kaartinen et all 2015), (Pirti 2013), (Pirti 2016), (Pirti 2005), (Pirti 2008), (Wright et all 2017).
Kinematic surveying can provide immediate results using the real-time kinematic (RTK) mode or in the office using the post-process kinematic (PPK) mode. In post-processed kinematic (PPK) surveys, the collected data are stored on the survey controller or receiver until the fieldwork is completed. The data are then processed in the office using the same software and processing techniques used in static surveys. Data latency is not a problem in PPK surveys since the data is post processed. Other advantages of PPK surveys are that (1) precise ephemeris can be combined with the observational data to remove errors in the broadcast ephemeris and (2) the base station coordinates can be resolved after the fieldwork is complete. Thus, the base station’s coordinates do not have to be known prior to the survey. The lack of data latency and use of a precise ephemeris results in PPK surveys having slightly higher accuracies than those obtainable from RTK surveys. After the data is collected, it is loaded into the processing software. An advantage PPK surveys have over RTK surveys is that precise ephemeris can be used in the processing (Kaartinen et all 2015), (El-Mowafy), (Wolf 2008).
Forest canopy affects the GNSS signals because of obstruction, attenuation, and reflection. So, line-of-sight GNSS signals are obstructed by solid objects. The signal is blocked by tree trunks, larger branches, and terrain features such as mountains. The signals are weakened and attenuated by leaves and small branches. This attenuation can make it very difficult for a GNSS receiver to track the signals. At some point, the receiver will not be able to track the signal at all and the effect will be the same as if the signal was obstructed. Even if the signal can be tracked, some receivers will have difficulty in measuring the pseudoranges accurately. The phenomenon of a satellite signal reaching an antenna by more than one path (direct and some reflected paths) is called multipath. This multipath can cause large variations in position estimates in a variety of environment, e.g., under forest canopy. Modern GNSS systems have been improved for the satellite tracking technology so that weaker signals can be observed under trees with foliage (Note that dense foliage will still cause cycle slips). In spite of this advanced tracking capability, the signals are noisier, weaker and more likely to be subject to multipath and diffraction. The surveyor should be aware that positions may not be accurate despite the quality indicators showing good solutions (Kaartinen 2015), (Wright 2017), (Pirti 2016), (Pirti 2005), (Pirti 2008), (Pirti 2010). The aim of this study is to assess the achievable accuracy of PPK surveys in the forest area.
The work was performed in Turgutlu, Manisa, Turkey (Figure 1). The site was selected in the Derbent-Çamlık Forest District, Turgutlu. The four points (P5, P6, P7 and P8) were marked in Derbent-Çamlık District, in the forest area. The four points were selected with the intention of achieving maximally different measurement conditions, See Figures 2 and 3. This means that points were selected in both normal and difficult survey conditions. The four points to be measured were fixed with either asphalt nails. All static GNSS surveys were performed using four Satlab SL600 receivers. A static GNSS survey was performed in order to determine the coordinates of these four points. The surveys in this primary network were carried out with at least 5 hours (10:30-15:30) of observation times. The minimum