## 2 Positioning in the Forest
### Positioning Accuracy and Augmentation Systems  
The accuracy achievable using GNSS ranges from a few millimeters to tens of meters depending on the operating environment and receiver technology used (e.g., single- or dual-frequency use, code- or carrier-phase measurements, and static or kinematic receivers). Better positioning accuracy can be achieved using more complicated and thus more expensive technology. Millimeter accuracy can only be achieved in good, unobstructed signal conditions, such as those in open, outdoor areas, utilizingdifferential GNSS (DGNSS). The basic concept of DGNSS is the use of a minimum of two receivers, one at a known location and one at an unknown position, which can see common GNSS satellites. By fixing the location of one of the receivers, the other location can be found either by computing corrections to the position of the unknown receiver or by computing corrections to the pseudoranges. Satellite signals are generally noisy, especially in environments where signals are attenuated and reflected, which is also called multi-pathing, due to obstacles in the proximity, such as buildings or trees.  
Augmentation of GNSS means improving the navigation system's performance, such as accuracy, reliability, and availability, through the integration of external data into the positioning computation. Some systems transmit additional data on sources of error (such as clock drift, ephemeris, or ionospheric delay), others provide direct measurements of how much the signal is off, while a third group provides additional vehicle data for integration into the computation process.  
A satellite-based augmentation system (SBAS) is a system that supports augmentation through the use of additional satellite-broadcast messages. Such systems are commonly composed of multiple ground stations, located at precisely-surveyed points. The ground stations take measurements of one or more of the GNSS satellites, the satellite signals, or other environmental factors, which may impact the signal received by the users. Using these observations, correction messages are created and sent to one or more satellites to be broadcast to the end users. SBAS is sometimes synonymous with WADGNSS, wide-area DGNSS. Examples of SBAS are the Wide Area Augmentation System (WAAS), operated by the United States Federal Aviation Administration (FAA) and the European Geostationary Navigation Overlay Service (EGNOS), operated by the European Satellite Services Provider (ESSP).  
Ground-based augmentation systems (GBAS) and ground-based regional augmentation systems (GRAS) support augmentation through the use of messages transmitted by radio or mobile data networks. As with the satellite based augmentation systems detailed above, ground-based augmentation systems are commonly composed of one or more precisely-surveyed ground stations (references), which take measurements concerning the GNSS, and the data are transmitted directly to the end user (rover). The distance between the reference and the rover, _i.e._, baseline, has a major influence on the achievable accuracy. Examples of GBAS utilizing DGNSS are real-time kinematic (RTK) measurements, where correction messages are typically received from one reference station, and the virtual reference station (VRS) service, where a network of reference stations is used to compute a virtual reference for a location close to the rover.  
The augmentation can also include additional information being blended into the position computation. Often the additional positioning instruments used operate using principles that complement GNSS positioning. These instruments provide independent and uncorrelated data that are not necessarily subject to the same sources of error or interference. The additional sensors may include sensors such as an IMU, an electronic compass and an odometer.