 |
|
|
|||||
|
|
December 30, 2004 Geospatial Applications: GPS Differential Correction Options for Farmers
Farmers and others who use the Global Positioning System (GPS) often need greater accuracy than the 10- to 30-foot GPS signal available. GPS users employ differential correction (DGPS) services to increase the positional accuracy of GPS. There are several options available for differential correction; some are available for a subscription fee and others at no cost. Differential correction reduces GPS errors and provides position accuracy to less than 10 feet. The Global Positioning System is a space-based radio-navigation system consisting of a constellation of satellites and a network of ground stations. Twenty-eight GPS satellites orbit the earth at an altitude of approximately 11,000 miles, providing users with accurate information on position, velocity and time anywhere in the world and in all weather conditions. Two U.S. government agencies provide GPS differential correction services at no cost to users. The Federal Aviation Administration provides the Wide Area Augmentation System (WAAS) differential correction service designed for use by commercial aircraft. The U.S. Coast Guard provides the Coast Guard DGPS Service intended for use by the Coast Guard on navigable waters in and around the continental United States. The WAAS DGPS service is a system of satellites and ground stations that provide GPS signal corrections to increase position accuracy. A WAAS-capable receiver can provide position accuracy of less than 10 feet 95 percent of the time anywhere in North Dakota. WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. The corrected differential message then is broadcast through one of two geostationary satellites with a fixed position over the equator. Any WAAS-enabled GPS receiver can receive the signal and use it to increase position accuracy. Farmers use the WAAS service in equipment guidance systems, combine yield monitors and variable rate crop input application equipment, and to mark points and areas in fields. The Coast Guard Beacon DGPS service consists of two control centers and more than 60 remote land-based broadcast sites. A Beacon DGPS-capable receiver typically will provide position accuracy of less than 10 feet anywhere in North Dakota. The Coast Guard Beacon DGPS service broadcasts correction signals in amplitude modulated (AM) radio frequency. AM radio signals can be degraded by surface weather conditions that decrease accuracy. Producers use the Coast Guard Beacon DGPS service for the same purposes as the WAAS DGPS service. There are several commercial differential correction services available for annual subscription fees. Commercial submeter differential correction services are available from OmniSTAR, John Deere, Case IH and Agco for approximately $800 per year. The accuracy of the commercial differential correction services varies by company and the length of time accuracy is checked. Commercial suppliers often measure the accuracy of their DGPS in terms of “pass-to-pass,” which refers to the length of time it takes to travel from one end of the field and back again, usually about 15 minutes. Differential correction services that increase GPS position accuracy to less than 6 inches also are available commercially. Farmers need this higher accuracy differential correction service when they incorporate guidance technology directly into the equipment steering system to provide automatic steering. The higher accuracy DGPS often is referred to as decimeter-accuracy or centimeter-accuracy DGPS. More information is available on the NDSU Agriculture and Biosystems Engineering Department Web site at www.ageng.ndsu.nodak.edu/. ### Source:
John Nowatzki, (701) 231-8213, jnowatzk@ndsuext.nodak.edu |
Market Advisor: |
|
North Dakota State University |
