Guidance and Steering Systems in Precision Agriculture
GNSS/GPS Guidance Systems
As with any application of GNSS, the ability to accurately determine geographic coordinates is essential to ensure quality performance. Despite the type of system used, the radio signals from the satellites processed by receivers can be affected by several factors (atmospheric interference, configuration of satellites in the sky, time estimation uncertainties, etc.) that can degrade the quality of the position estimates. Various differential correction services may be used to improve the accuracy of estimated geographic coordinates in real-time. Each differential correction system relies on a base station receiver or a network of receivers at a known location(s). GNSS errors are calculated at the base station(s) and transmitted in real-time to the roving receivers either directly through land-based radio transmission or via a communications satellite. In addition to the differential correction, most receivers apply signal filtering techniques to assure the best possible predictability of antenna location. Based on the quality of differential correction and internal signal processing, positioning receivers used for auto-guidance have been advertised according to the level of anticipated accuracy: sub-meter, decimeter, and centimeter.
GNSS/GPS Accuracy
Positioning accuracy indicates the proximity of measurement results to the actual value. Precision refers to the repeatability or reproducibility of the measurement. What is essential from the standpoint of GPS use is to make positioning measurements with the least amount of unexplained variation (i.e., accuracy) and to ensure that these measurements remain unchanged as time progresses (i.e., precision). GPS receivers are built to achieve different accuracy levels depending on their internal components, communication protocols, and available firmware capabilities. Many manufacturers list the rated accuracy of their system in the equipment specifications, but what does that mean, exactly? Most guidance systems are concerned with position accuracy, the difference between the unit’s measured and actual positions.
Year-to-Year Accuracy
Year-to-year or static accuracy is measured as the GPS receiver’s ability to return to a point measured more than 15 minutes ago. Typically, static accuracy is measured by collecting position data over a minimum of 24 hours. Repeatability is tied to the absolute accuracy of the system, changes in satellite configurations, atmospheric conditions, local interferences, and other satellite data errors.
Pass-to-Pass Accuracy
When working their fields, farmers move their machinery across the field in swaths or rows, eventually covering the entire area. As they drive towards one end of the field, they turn to drive back down the other direction. Pass-to-pass accuracy is the performance level, or precision (of error), between two subsequent swaths. Accuracy is critical for farmers to optimize the distribution of seeds, water, and other crop inputs across their fields. For some crops, it’s integral for their health to be consistently planted. Pass-to-pass accuracy can be achieved over a 15-minute window, with 15 minutes the approximate time to make a pass in a typical field. If the pass-to-pass accuracy of a system is 6 inches, then the GPS should be able to return to within 6 inches of that point if no more than 15 minutes have passed since measuring the point initially. Pass-to-pass accuracy is the most commonly presented accuracy by equipment manufacturers because it is the most important for swath guidance.
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