Global Navigation Satellite System in Precision Agriculture
Global Positioning System (United States)
Satellite navigation is based on a global network of satellites that revolve around the Earth in a precise orbit. The satellite constellation most people are familiar with is the United States Global Positioning System (GPS). At the time of this publication, the GPS constellation consists of 31 operational satellites that orbit the earth twice a day. These satellites are positioned so that a user can view at least four satellites from any location on Earth. Each satellite transmits a unique signal and orbital information that enable GPS devices to calculate your location by measuring the time it takes for the signal to travel from the satellite to the GPS receiver.
Components of a GPS Guidance System
The main components of GNSS guidance systems include a differential GPS receiver and antenna, a computer or microprocessor, and an LCD graphics display that provides easy-to-use guidance patterns and real-time “as-applied” coverage mapping (Figure 5.2). The GPS antenna is used to receive signals from satellites orbiting the Earth. The location of the antenna is critical to the performance of the guidance system; GPS antennas should be mounted on top of a tractor, sprayer, or spreader truck along the centerline to ensure the antenna has a direct line of sight to satellites.
Global Positioning System Accuracy
The absolute positioning using GPS provides a moderate level of positioning geometric accuracy, typically within a few meters. Errors in range measurements and uncertainties in satellite locations introduce errors into GPS-determined positions.
Sources of GPS Errors
The primary causes of GNP position errors include the clock, the satellite’s orbital location, poor satellite configuration, atmospheric interference, and multipath errors. Clock errors result from the limited precision of the physical clocks contained within the receiver and the exact synchronization between the receiver and satellite clocks. More precise GPS receivers will use higher quality, more expensive clock and timing measurement components. Because the satellites are orbiting in gravitational fields, their positions and movements can be known and predicted quite accurately. Nevertheless, there still may be some errors in the satellite’s orbital location that is broadcast due to either miscalculation of the data or variations in the orbital path.
Correction Techniques for GPS Errors
The GPS of satellites allows persons with standard GPS receivers to know where they are with an accuracy of 5 meters or so. When more precise locations are needed, errors in GPS data must be corrected. A number of ways of correcting GPS data have been developed. Some can correct the data in real-time (differential GPS and the wide area augmentation system).
Differential Global Positioning Service
The U.S. Coast Guard has developed a system called the Differential Global Positioning Service (DGPS). In general, access to this correction information makes differential GPS and GNSS receivers much more accurate than other receivers; with these errors removed, a GNSS receiver has the potential to achieve accuracies of up to 10 centimeters.
Wide-Area Augmentation System
The Wide Area Augmentation System (WAAS) provides extremely accurate navigation capability by augmenting the Global Positioning System (GPS). It was developed for civil aviation by the Federal Aviation Administration (FAA) and covers most of the U.S. National Airspace System (NAS) as well as parts of Canada and Mexico.
L-band Satellite-Based Correction
The L-band frequencies can be used to download information from satellites. Before correction data became widely available through beacon stations and WAAS, some areas of the country could only receive correction information from commercial satellite service providers over L-band frequencies.
Real-Time Kinematic
Real-time kinematic (RTK) positioning is widely adopted for higher accuracy requirements, frequently as a Continuously Operating Reference Station (CORS) network, consisting of permanent reference stations, i.e., base stations, that monitor satellite signals and provide correction data through the internet or wireless communication. These corrections are transmitted to the mobile receiver, often referred to as the “rover,” in real time, allowing the rover to correct its position with centimeter-level accuracy.
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