## 2 Positioning in the Forest
### GNSS in under Forest Canopies  
There are many studies focusing on positioning under forest canopies using various GNSS devices and processing techniques, and the results show that a dense forest canopy can reduce the accuracy of the GNSS device. In these cases, the measurements were obtained during static sessions of varying length, and the positioning accuracy varied from a few centimeters to several meters, depending on the instrumentation, length of data observation period, and processing. [PERSON] and [PERSON] [25] studied the performance of a GPS PPP application under coniferous forest canopies. A 20-channel, dual-frequency GPS receiver collecting pseudorange and carrier phase observations was used as a stand-alone receiver to determine the positional accuracy of 19 points under coniferous forest canopies. The mean positional accuracy ranged from 0.27 to 0.88 m for an observation period of 120 min, and 0.95 to 3.48 m for 15 min. In the case of the 15 min observation period, the computed positions could not be found for 8 to 44 percent of the locations. The accuracy increased as the forest stand density decreased. The stand basal area and number of trees correlated significantly with the expected positioning accuracy. The accuracies of recreational-grade and survey-grade GPS receivers were evaluated by [PERSON] _et al._[26] across a range of forest conditions in the Tanana Valley of interior Alaska. High-accuracy check points were then used to evaluate the accuracy of positions acquired in stands representing different forest types using a recreational-grade GPS unit and a GLONASS-enabled survey-grade unit, over a range of acquisition and post-processing alternatives, including distance to base station, or baseline length (0-10, 10-50, 50-100, and \(>\)100 km), use of GLONASS satellites, and data observation periods (5, 10, and 20 min). The accuracy of the recreational-grade GPS was 3-7 m across all sites. As regards the survey-grade units, their accuracy was influenced by the forest type and baseline length, with smaller errors observed in more open stands and with shorter baseline lengths. The use of GLONASS satellites improved positioning accuracy by a small but appreciable amount, and longer observation periods (20 min) resulted in more reliable and accurate positions across all sites. In general, these results indicate that if forest inventory plots in interior Alaska and other high-latitude regions of the world were occupied for 20 min by survey-grade instruments, positioning accuracy below 1 m errors can be consistently obtained across a wide range of conditions. [PERSON] _et al._[27] confirmed that survey-grade receivers provide higher horizontal positioning accuracy than consumer-grade receivers, better horizontal positioning accuracy during leaf-off forest conditions (_i.e._, winter), and that differential correction can improve horizontal positioning accuracy, and WAAS, when available, can improve horizontal positioning accuracy. Even centimeter-level accuracy can be achieved using differential computation and static observations, but usually this requires open-sky conditions [28, 29]. However, positioning while moving is a more complex problem as satellite visibility changes arbitrarily and the collecting of positional reference data is challenging. There are some studies addressing the positioning accuracy of a moving device evaluated under a forest canopy. [PERSON] _et al._[30] studied GPS-related positional errors produced when monitoring a mobile device under the forest canopy on two test sites: a deciduous forest stand and a coniferous forest stand with mean stand tree heights 8.4 and 8.9 m, and mean basal areas 41.1 and 38.4 m\({}^{3}\)/ha, respectively. Two GPS receivers were used, one of navigation-grade and the other of survey-grade, both with and without real-time differential correction. The mean horizontal accuracy (STD) was about 4.1 m for the navigation-grade receiver and about 2.7 m for the survey-grade receiver. Differential correction did not improve the accuracy for either receiver.