Another method of differential GNSS positioning that has attracted a following among farmers over the years is the use of a SBAS, such as WAAS established by the U.S. Federal Aviation Administration.
PPP delivers a much more reliable and accurate solution than WAAS for Guy LaRochelle, owner of LaRochelle Farm in Zenon Park, Saskatchewan, and operator of an Ag Leader dealership. The Ag Leader positioning system with the embedded NovAtel PPP solution he installed on his Case IH Patriot Series sprayer last spring required much lower setup costs than the RTK system he had begun using a few years earlier.
For about 25 years, LaRochelle has managed a 2,000-acre farm, which produces wheat, canola, peas and canary feed. Having worked in the computer industry before coming back to take over the family farm, he has a deep understanding of the benefits of technology for optimizing his business. Precision agriculture is not a new concept for LaRochelle; he previously used WAAS for machine guidance and then set up the infrastructure for more accurate RTK several years ago.
“RTK is very expensive when you start putting up towers and so forth,” he says, adding that his own customers would have had to do the same to use that technology. “Customers were having a hard time justifying spending $30,000 to start RTK.”
And, while WAAS is free, the sub–metre accuracy it provides is not sufficient for most precision agriculture applications. So, this past spring, he had his sprayer configured with the Ag Leader GPS 6500 system utilizing TerraStar system to see if he could enjoy better-than-WAAS accuracy. The GNSS configuration also includes Ag Leader’s Integra monitor and Steer Command automated steering system.
“I put it on my sprayer because I use the sprayer throughout the year; I can monitor it better, whereas if I put it on my drill, I can only use it for a few weeks in the spring,” LaRochelle explains.
After a few months, it became clear that the accuracy is better than with WAAS. “Most people don’t need inch accuracy in our area, unless they want to do some ditching where they need elevation accuracy,” he says. “But most of the time, accuracy to within three to four inches [about 7–10 centimetres] is enough.”
Soon after having the system installed, LaRochelle would check its accuracy by setting up two parallel lines of flags, the width of the sprayer’s tires, in a section of his land. During the spring, he would run the sprayer via the guidance system, and the sprayer stayed within the flag lines every time, he reports.
“That’s how I was able to tell how consistent it was,” he says. “I have never turned it on and run over the flags because the accuracy was off. It’s not quite as accurate as RTK but that’s the nature of the beast.”
LaRochelle still repeats the tests from time to time. “Sometimes I go back and spray in that same field and try to follow the same tracks so I’m not making new tracks and trampling crops,” he says. “I just create a new job with my old pattern, and it pretty much falls into that same track every time.”
He adds that he inadvertently crushed more plants using WAAS-based positioning due to its lower accuracy.
“I think [PPP] is a very, very good system for farmers who don’t need inch accuracy but still want something more accurate than sub-metre accuracy like you get with WAAS,” LaRochelle says. “For the price you pay for the signal, I think it’s a very, very good alternative versus something like RTK, which involves expensive equipment and may be something that you’d need for elevation.”
LaRochelle reports that NovAtel CORRECT with TerraStar PPP is actually more reliable than RTK for his purposes: heavily wooded eastern Saskatchewan presents challenges for RTK systems, such as reliable transmission and reception of correction data from local base stations. Convergence time is very short if the sprayer is started up in the same location as it finished the previous day, longer if it is moved prior to startup, LaRochelle says.