# GNSS Signal Quality in Forest Stands for Off-Road Vehicle Navigation
## 4 Discussion  
The results of vehicle location analyses within deciduous forest (oak stand) and mostly coniferous forest (spruce and pine stand) confirmed that the accuracy of the measurement depends mainly on stand density, season (deciduous forest), vegetation age and meteorological conditions, which can significantly reduce the quality of the received GNSS signal, especially during precipitation. For this reason, it is necessary to take into account the results obtained, which were not affected by the same humidity in both tested localities (60% and 90%).  
During the experiment, the number of satellites (GPS and GLONASS) varied between 15 and 20 and was never less than 11 (see Figures 10 and 11). The horizontal DOP (dilution of precision) was in the interval of 0.8-4. Only two position records out of more than 400 records had DOP = 17. The dependence of the accuracy of determining the horizontal position (\(\sigma_{HZ}\) or \(d_{HZ}\)) on HDOP parameters or total number of satellites was not statistically proven (significant).  
The average values of the actual deviations of vehicle positions determined using GNSS from two terrestrially measured routes reached values of 2.0 m and 1.9 m in locality 1 (deciduous forest), and 1.2 m in locality 2 (mainly coniferous forest). These values are sufficient for the navigation of an off-road vehicle, provided that we evaluate the possibilities of movement using measured distances and directions (azimuths) from the start to the destination. If we are to consider the possibility of moving a vehicle in a dense forest, we have to count on greater errors in determining the position and the fact that the possibility of moving the vehicle will decrease with its increasing dimensions and with turning radius--see [41].  
The achieved positioning was sufficient for locating a moving vehicle, however, ultimately, the type of vegetation and terrain topography (such as deep valleys) will have a significant effect on the ability of the vehicle to maneuver between trees, but this factor was not the subject of research.  
Another factor that can significantly affect the movement of the vehicle is the time of day. At night, orientation in the forest is much more difficult.  
The canopy differences may be best described by the hemispheric photographs taken with the camera pointed vertically at the forest stand--see Figures 17 and 18.  
Figure 17: _Cont._The achieved results of determining the accuracy of vehicle navigation corresponded to the character of both locations where the tests were carried out. These results can be found in Table 1.  
Figure 17: Hemispheric photographs—deciduous forest.  
Figure 18: Hemispheric photographs—coniferous forest.  
used to assess the possibilities of navigation in similar stands that are typical for the Central European region, that is, for forests in a temperate climate zone. It can be assumed that in more northerly latitudes, drier arid regions, and inland, where vegetation is sparser, the accuracy of vehicle navigation determination will also be better. On the other hand, in denser vegetation in a monsoon forest region and in the equatorial climate zone, where the vegetation is denser and air humidity is greater, the accuracy of vehicle navigation will be worse.  
In the future, the authors of this article would like to focus on research to determine the accuracy of vehicle navigation in different types of vegetation with different parameters of tree density, the nature of their crowns (density, height), and in different seasons.