# Web Mapping for Farm Management Information Systems: A Review and Australian Orchard Case Study
## 2 Background  
### A Primer on the GNSS  
The Global Positioning System (GPS) was first operationalized by the U.S. Department of Defense in 1978, although its satellite constellation was not fully complete until 1995. The Russian GLONASS, European Galileo, and Chinese BeiDou systems achieved full operational capability in 1995, 2021, and 2020, respectively, albeit functioning at a range of position accuracies and levels of global coverage. The four systems are collectively referred to as Global Navigation Satellite Systems (GNSS). The GPS system was originally restricted to military use until the [PERSON] administration allowed degraded civilian use following the destruction of a Korean Airlines passenger plane that strayed into Russian airspace in 1983. The [PERSON] administration allowed civilian access to un-degraded GPS signals in 2000, and this commitment was made permanent in 2007 [14]. Civilian access to data from GLONASS and BeiDou geo-positioning systems has followed. Galileo access was always open, as it was the only non-military system. The data sampling rate (also known as the update rate) of a GNSS module is the rate at which the position is calculated and reported. This is determined by both the satellite constellation accessed and the receiver chipset quality, with the primary limitation being the computing power of the receiver [15]. The GPS constellation outputs data at 1000 Hz. However, many receivers support only a 1 Hz rate, although receivers with update rates of 5, 10, and 20 Hz are available. A 1 Hz rate is adequate for non-moving applications. This specification is important to agricultural applications involving moving vehicles. For example, the orchard imaging system described by [PERSON] et al. [6] captures geolocated images at 10 fps from a ground vehicle moving at 7 km/h, i.e., 1.9 m/s. The matching of frame capture time to location data collected at this rate or higher will require interpolation of GNSS data. A single point receiver (also referred to as a standalone receiver) of GNSS data has a positioning error of 5-10 m because of fluctuations in the GNSS signal due to variations in the layers of the atmosphere, multipath signals, and receiver electrical noise, in addition to errors in satellite clocks and imperfect satellite orbits. In differential GNSS (DGNSS), data are collected from a reference (base) station(s) at a known location, with the estimated position error on the reference station used to adjust the estimated rover position. Real-time kinematics (RTKs) also use base station data but employ more sophisticated algorithms, correcting ionospheric changes and satellite clock errors. Position accuracies of DGNSS and RTK are approx. 0.5 m and 0.02 m, respectively. In Australian surveying practice, the use of DGPS supported by local base stations has been largely replaced by the use of RTK based on a public/private network of Continuously Operating Reference Stations (CORS). However, a data link is required to access the CORS-RTK service in real time, and internet or 4G cellular service has been limited in agricultural areas. The satellite-based augmentation system (SBAS, also known as wide-area differential GNSS) is an alternative that is widely adopted in Australian broadacre agriculture. In SBAS, a satellite signal is used to provide correction for satellite orbits and clocks, and information on the signal delay incurred in passes through the ionosphere that is calculated from multiple base stations. Such systems achieve a horizontal position 1 sigma accuracy below 1 m. Major agricultural technology providers such as John Deere (Moline, Illinois, United States), Case (Turin, Italy), Ag Leader (Ames, Iowa, United States), and Trimble (Westminister, Colorado, United States) have relied on the Inmarsat-4 constellation of three high-orbit satellites to deliver the correction data.