# GNSS Signal Quality in Forest Stands for Off-Road Vehicle Navigation
## 1 Introduction  
Rescue and military operations usually include both movement over existing roads and also through natural terrain. In cases when drivers are not able to use some segments of roads (damaged or destroyed objects, traffic jam, etc. ), they have to be provided with complete cross-country mobility (CCM) analyses considering all main geographical factors to solve transportation problems [1, 2]. The cross-country movement of off-road vehicles is affected by various terrain features such as slope, terrain roughness, soil type, vegetation, seasonal changes to the ground surface, rivers, etc. [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]. One of the most difficult environments in which to maneuver is forested terrain. Two-thirds of European countries are at least 30% forested. In Sweden, Finland, Estonia, Latvia, Bosnia and Herzegovina, and Slovenia, vegetation covers over 50% of the country--see Figure 1 and [14, 15]--severely restricting the off-road movement of terrain vehicles. Types of vegetation in a specific area can also give an indication of climatic conditions, hydrologic transformation, drainage systems, water supply, and soil type. The ability of a vehicle to cross a forested area depends on whether or not it can either maneuver between tree stems or override individual trees. The planning and success of moving off-road vehicles through a forest stand depends notonly on vegetation factors such as stem spacing, tree diameter, and root structure (by tree overpassing), and vehicle parameters (width, length, turning radius, weight, and traction force), but also on the quality of navigation. There are several possible ways to navigate off-road vehicles. One option is to use global satellite navigation systems (GNSS). A satellite navigation system is a system that uses satellites to provide autonomous geospatial positioning. It allows small electronic receivers to determine their location (longitude, latitude, and altitude/elevation) with high precision, using time signals transmitted along a line of sight by radio from satellites. Current fully-operational GNSS applications include the United States' Global Positioning System (GPS), Russia's Global Navigation Satellite System (GLONASS), China's BeiDou Navigation Satellite System (BDS), and the European Union's Galileo. Japan's Quasi-Zenith Satellite System (QZSS) is a (U.S.) GPS satellite-based augmentation system to enhance the accuracy of GPS, with satellite navigation independent of GPS, scheduled for 2023. The Indian Regional Navigation Satellite System (IRNSS) plans to expand to a global version in the long term. Outside of GNSS, multisensory navigation systems are increasingly being used which routinely include the sensing of non-GNSS (acceleration, radio frequency signals, gravity, magnetic fields, imaging, barometric pressure, sound waves, etc.). However, each sensor type has its own set of limitations. The aim of this presented study was to verify the quality of the GNSS signal under forest vegetation, with a focus on coniferous and deciduous vegetation of varying densities in the temperate vegetation zone of Central Europe. The prerequisite for GNSS navigation is that data of the structure of the forest will be available, including the coordinates of the position of individual trees. We are currently seeing a growing number of studies dealing with the use of LIDAR for mapping the structure of forest vegetation, e.g., [PERSON] and [PERSON] (16), [PERSON] et al. (17), and [PERSON] et al. (18). Navigation of an off-road vehicle using GNSS methods in a forest depends both on the accuracy of mapping the positions of individual trees and on the quality of the navigation itself. Both the determination of tree positions and the quality of the GNSS signal depend on many factors, such as the density of the trees, their branching, and meteorological conditions, etc. There are several studies that focus on positioning under tree crowns using various GNSS systems. [PERSON] et al.