# Positioning Methods and the Use of Location and Activity Data in Forests
## 2 Types of Positioning Technologies
### Augmented GNSS and GNSS with RTK Correction  
With either dedicated, single unit GNSS receivers or mobile device-based GNSS solutions, several methods exist to improve upon the accuracy of GNSS positioning. One such method is a satellite-based augmentation system (SBAS), in which reference stations located at known points are used to create correction messages that are sent to various satellites and then broadcast to end users (Figure 3, left panel) [63]. The Wide Area Augmentation System (WAAS) in the United States, which is managed by the Federal Aviation Authority (FAA), is an example of an SBAS [64]. The WAAS encompasses a network of terrestrial reference stations that estimate errors in GPS signals and send correction messages to a geostationary communications satellite (GEO) which then re-transmits the corrections to any WAAS-enabled GNSS device [65] as shown in Figure 3. WAAS differential correction is frequently built into GPS devices and even some smartwatches, but has demonstrated little positioning accuracy improvement for consumer-grade receivers [64]. [PERSON] and [PERSON] compared WAAS-enabled modes to autonomous (uncorrected) modes for Garmin, Trimble, and DeLorme receivers under ideal conditions; WAAS improved horizontal positioning accuracy only for the Delorme units [64]. Additionally, WAAS signaling is limited under forest canopy, typically available less than one quarter of the time for stationary, recreational-grade devices, and even less for mobile receivers [65].  
In a ground-based augmentation system (GBAS), correction messages are transmitted to the end user via radio or mobile data network [63]. Real-time kinematic (RTK) positioning is a GBAS differential correction method in which one or more base (reference) stations at fixed, precisely-surveyed locations are used to improve the accuracy of mobile rover units [63; 66; 67]. Specifically, both the reference and rover units receive pseudorange and carrier phase measurements from similar satellites, which allows common errors between the units to be estimated and used to correct the rover's accuracy in real time [66; 67]. Real-time kinematic is typically built into survey-grade devices, enabling them to achieve centimeter-level accuracy in unobstructed conditions (see Table 1) [29] and is integrated into heavy equipment for road construction (Figure 3, right panel) [68; 69]. Real-time kinematic methods improve accuracy under the canopy as well, but are not immune to multipath error inherent in forests; therefore, positioning accuracies are lower (about 1 m) in these highly reflective environments [29; 63; 66].  
Precise point positioning (PPP) is an absolute positioning technique that uses a single GNSS receiver and corrections for satellite positions and clocks based on processing of reference station data available through the International GNSS Service (IGS) [70]. Receivers simultaneously use dual frequency pseudorange and carrier phase measurements in order to improve accuracy [67; 70]. Data downloaded from the IGS can be used in real-time or for post-processing correction [63; 71]. With PPP, GNSS devices can correctly position within 5 cm in open areas and less than 0.5 m under forest canopies (Table 1) [67; 70].