# Positioning Methods and the Use of Location and Activity Data in Forests
## 2 Types of Positioning Technologies  
### GNSS - Single, Dedicated Receiver  
The Navigation System with Timing and Ranging (NAVSTAR) GPS was developed by the United States (U.S.) Department of Defense during the early 1970s for military applications as well as for civilian use [24]. The term GNSS refers broadly to the American GPS system and other satellite-based navigation constellations developed subsequently in other regions to enhance terrestrial and maritime PNT for various fields [2]. The other three primary systems currently in use and continued development are the Russian Global Navigation Satellite System (GLONASS), the European Union's Galileo system, and the BeiDou-2 system developed by the People's Republic of China. While GPS and GLONASS have been in use for some time, Galileo first became operational in December 2016, and BeiDou-2 is expected to have operational global coverage in 2020 [1; 25]. This is of interest because improved positioning accuracy is possible with integrated multi-GNSS frameworks that incorporate all four GNSS systems as the total number of available satellites across all constellations increases from 70 to approximately 120 over a period of a few years [1]. Figure 1 depicts the GNSS receiver chip in a smartphone being used for vehicular navigation. The mobile device receives timing and range data from several satellites and subsequently uses these to determine its location on the Earth's surface. The single device user can use positioning information such as latitude, longitude, elevation, and vector heading to monitor his or her location on topographic maps, custom maps, and aerial imagery. Prior to 2000, selective availability (SA), an intentional degradation of GPS signal quality by the U.S. government for national security reduced the accuracy  
of satellite positioning in the U.S. [26]. [PERSON] et al. demonstrated the significant improvement to accuracy with the removal of SA in May 2000, with mean error for a survey-grade receiver decreasing from 30 meters (m) to less than 2 m in ideal conditions [27]. Of the three general categories of GNSS receivers (survey-, mapping-, and consumer-grade), survey-grade receivers are the most accurate, able to achieve positioning accuracies of 1 m or less under forest canopies and approximately 5 centimeters (cm) in open areas (see Table 1) [28; 29]. For survey-grade devices, accuracy increases with longer durations of continuous position logging [30]. Typical accuracy of consumer-grade GNSS in U.S. forests ranges from 1 to 5 m in open conditions [28; 29; 31; 32; 33] and from 3.8 to 12 m under mature canopies [28; 29; 31; 32; 33; 34]. The effect of canopy on GNSS signal transmission has been well-documented, with accuracy inversely proportional to stand density in most situations [31; 23; 33; 34; 35; 29; 31; 36]. GNSS accuracy does not vary by the time of year (season) or weather [34], but can be affected by device orientation when held [37]. The number and geometry of satellites visible to a GNSS receiver at any point in time also affect the accuracy of coordinate readings recorded by the device. Various dilution of precision (DOP) indices are used to characterize the influence of satellite geometry on GNSS measurements [38]. Positional DOP (PDOP) is a particularly common index of the uncertainty in overall position, with lower PDOP values indicating a satellite arrangement providing higher measurement accuracy and precision [38]. Individual GNSS receivers (Figure 2, panel A) have been commonplace in forestry and throughout natural resource science and management for decades. In forestry and wildland fire, handheld GNSS receivers have been used for a wide variety of geospatial mapping and navigation applications. These include navigation to forest inventory plots and monumentation [39], as well as delineating stand boundaries and other features on stand maps [40; 41].