## 2 Positioning in the Forest
### GNSS in under Forest Canopies
The researchers concluded that a higher stand density causes a larger positional error when the basal area is about the same. [PERSON] _et al._[31] studied the dynamic accuracy of recreational-grade GPS receivers in an oak-hickory forest by measuring the area of the test plot (0.9 ha) defined by eight ground-based control points. They concluded that the average area agreement was 93% during the winter season (leaf-off), and 84% during the summer season (leaf-on). In the study by [PERSON] _et al._[32], the accuracy of estimation of the location of the tree stems when using a mobile laser scanning system setup in a backpack was 0.38 m. The main error source was in the positioning of the system. The study area was a homogenous mature pine forest and an IMU was used and post-processing was implemented. Past studies have concentrated on the positioning accuracy of GNSS in forest environments and on techniques for improving GNSS accuracy. In 2001, [PERSON] _et al._[33] used skyward-looking photography in conjunction with GPS data in an attempt to develop a quantitative method of classifying forest canopies and of relating this classification to the degradation of carrier phase differential GPS (DGPS) performance. This technique led to a definite correlation between canopy obstruction and DGPS performance with the \(R^{2}\) value increasing up to 0.74 when fitting a trend line to their findings. In 1999, [PERSON] and [PERSON] [34] found that with 300 GPS fixes, the Position Dilution of Precision (PDOP) is not as good an indicator of positional accuracy under a forest canopy as has been generally acclaimed. The study focused on the quantitative analysis of GPS accuracy under different forest canopy types and how PDOP affects positional accuracy as well as how foliage relates to signal blockage. The most significant conclusions presented in this paper were as follows: as canopy density increased, the signal attenuation increased; PDOP is not a good indicator of position precision; and the usability of skyward photography techniques as a measure of canopy closure. [PERSON] and [PERSON] [35] also found moderate correlation between average positional error and some forest structure attributes in their study on the influence of the juxtaposition of trees on consumer-grade GPS positioning quality. In 1998, [PERSON] and [PERSON] [36] demonstrated that NMEA (National Marine Electronics Association) messages in GPS receivers included individual SV (space vehicle, satellite) signal to noise ratios and are capable of showing significant signal attenuation based on the angle of the SV from the zenith. This particular research was measuring this signal attenuation from space-based platforms and so the attenuation was mainly attributed to the atmosphere. It is, therefore, rational to assume that the addition of forest canopy between the SV and antenna attenuates the signal further. The technique of signal-to-noise ratios from each SV can provide us with the data from each SV and the SV location relative to the antenna regarding the effect of forest canopy density and the effect of the signal propagation of as caused by the forest.