# GNSS Signal Quality in Forest Stands for Off-Road Vehicle Navigation
## 2 Material and Methods
### Measurement Methodology  
When designing the methodology for determining the accuracy of navigation of a moving vehicle, the authors based the above-mentioned literature on article [36], in which the authors determined the accuracy of navigation from data measured by GNSS, IMU and laser scanner (territory of Finland) and from publications focused on the combination of different navigation methods--see [37; 38; 39; 40]. Since determining the navigation accuracy of a moving vehicle also depends on the regional characteristics of forest units, the authors chose for their own experiment two representative localities of the temperate climate zone (coniferous and deciduous forest) with typical tree species for this area. Another important factor is the quality of the GNSS signal, resulting from the quantity and distribution of satellites during the vehicle's route, therefore, the analysis of the quantity and quality of the GNSS signal was part of the proposed methodology. The methodology for determining the accuracy of off-road vehicle navigation using GNSS systems consisted of the following steps, see also Figure 5:  
* Demarcation of forest stand for testing with a focus on finding a relatively homogeneous part of the forest in which there were representative tree types of stand;
* Surveying the coordinates of the reference ground control points around the forest and in the middle of the forest (places not covered by vegetation) using the GNSS, Trimble Geo XR with Zephyr model 2 Geo XR antenna, the Trimble Geo 7X cm version with antenna, Tornado pro 7X devices with 2-3 cm estimated position error, and using the real time VRS (virtual reference station) method. Selected parameters of receiver Geo 7X were: firmware GNSS: 4.50.8.; software GNSS: Terra Sync version 5.86.; number of channels: 142 (max. 220); GNSS types: GPS L1/L2 (L1E, L2C), GLONASS G1/G2, Galileo E1, and Beilou B1; SBAS types: WAAS and EGNOS;
* For field measurements, the GNSS receiver was set as: measurement mode: 1; external GNSS: 2; SBAS: 3; autonomous mode; logging interval: 1 s; elevation mask: 5\({}^{\circ}\); PDOP mask: 99; signal to noise ratio (SNR): 12; and height of the external antenna location: 1 m;
* The number of reference ground control points depended on the density of forest stands;
* Measuring tree positions using tachymetry. The measurement was performed from reference ground control points, from which there was good visibility to the surrounding trees. First, the distance was measured on a pole with a surveying prism touching the tree from the side. Then the pole was moved in front of the center of the tree and the azimuth was measured. This avoided measuring eccentricities, i.e., diameter at breast high (DBH) measurement. During the measurement, the surveying prism was placed at a height of 1.3 m above the ground surface;
* Measurement of vehicle positions while driving between pre-marked trees. The GNSS Tornado pro 7X antenna of the Geo 7X receiver and the device itself were attached to the front of the vehicle body at a height of 1.0 m. The vehicle was moving between reference trees for which position coordinates had previously been measured on a route previously targeted by the tachymetric method. Before the actual testing, the route along which the vehicle was moving was also marked with a spray. This route was also measured using tachymetry in order to determine the deviations between the reference route and the routes located using the GNSS method. Deviations of vehicle movement from the reference marked path depended on the terrain configuration and vegetation density. The speed of the moving vehicle was 3-5 km\(\cdot\)h\({}^{-1}\), the distances between the trees were 3 m to 5 m, the height of the trees was approximately 15 m. For most of the vehicle's path, the sky was obscured by tree branches.