Sowing your rows with accuracy
March 17, 2008 By Hugh McElhone
Global Positioning Satellite technology, or GPS, has grown by leaps and bounds since it was first introduced in 1978, says Murray Hall with Halltech AgGPS, including agricultural applications such as automatic steering systems and row-crop planting accurate within two centimetres of the row centre.
The GPS system requires a minimum of three satellites’ signals to determine the latitude and longitude of a set location within the field. A fourth satellite signal is needed for the ‘z’ component that provides exacting accuracy, adds Hall.
Since the U.S. military does not want civilians to possess the pinpoint accuracy of a Cruise missile, or to even know how accurate they can be, they purposely distort their satellite signals, explains Hall. For farm applications, a correction service was developed based on the latitude and longitude of known landmarks from which the signal deviation is calculated. This deviation correction is then fed to a geo-stationary satellite that in turn provides the x, y and z triangulation accurate to within six inches.
Hall calls this correction service the Wide Area Augmentation System (WAAS) and it is used in three different agricultural GPS applications. One such system is the OmniStar HP with two-inch pass-to-pass accuracy and four-inch repeatability. There is also the Beacon system, which provides similar accuracy based on data from navigation satellites and is the most common in use, says Hall.
The third, the RTK system, is the “Cadillac of the line,” says Hall. With this, a mobile reference station is placed in the corner of the field to set precise parameters. Data from the station is continuously streamed to the six gyro mechanisms that can process up to 120 steering corrections per second. Row crop accuracy is within one to two centimetres, says Hall, and “we’re almost at the point where we can turn the tractor around at the end of the field.” Until that happens, one touch on the steering wheel and the system disengages and gives the operator full control.
“There are thousands of these systems out there already,” says Hall. One of their largest customers is a watermelon operation in Arizona with 24 such systems. The farm uses the system to plant two rows of melons in one row of mulch, he explains, adding that after it is harvested, workers run a root killer through the drip line, flush it through, and then replant another melon crop in the same holes. “They go 24 hours a day.”
Closer to home, a farmer near Brantford, Ont., erected a 110-foot tower for about $6,000, and can use his GPS system in fields 20 kilometres away. Hall says there are approximately 30 million acres set up in such GPS zones running from the southern U.S. north to Ontario and east to Prince Edward Island.
To establish a zone, the reference stations should ideally be placed where they cannot move, such as on concrete silos 200 to 300 feet high, with a clear view of the sky, Hall says. Hydro corridors should be avoided because the power lines run on a similar frequency. The signal can also be encrypted so no one else with a receiver can steal it. For fields slightly out of range, a repeater station can be set up.
“Iron out your field problems first, such as trees and other obstructions when choosing your antenna site,” Hall advises. And use topographical maps and local knowledge when selecting which fields are best suited for GPS, he concludes.
Print this page