Precision agriculture — the practice of optimizing inputs of seed, water, and fertilizers while maximizing yields by mapping variations in soil characteristics and guiding machinery accordingly — began in the United States in the early 1980s and has been growing steadily. Key components include soil mapping based on sampling and remote sensing, proximal sensing of soils and crops, variable rate irrigation and variable rate spraying of fertilizers and herbicides, and automatic tractor navigation.
“GNSS-based guidance is probably the most highly adopted precision ag technology, followed by variable rate and section control,” said John Fulton, associate professor at The Ohio State University. “I suspect that somewhere around 40% of those GNSS receivers use RTK-level corrections — which provide sub-inch accuracy — and that number is increasing.”
Need for sub-inch accuracy
Water runs downhill, of course, which makes vertical accuracy critical for hydrology. “AgLeader builds a plow to put tile in soil to drain water,” said Bill Cran, AgLeader Technology’s GNSS product specialist. “It might only be 4 inches round; so, if we are off by 2 or 3 inches vertically, that affects where water can run.” To get the best vertical accuracy possible, he recommends farmers install a base station in the field where they are operating.
Sub-inch accuracy also enables farmers to determine where to plant each seed, rather than monitoring planters at the row level. “That may not be a requirement today, but it is certainly coming,” Cran said.
Market demand for RTK in agriculture is increasing, an important factor for drone guidance or machine control, said Gustavo Lopez, market access manager at Septentrio. “The robots are very close to the crops. When small robots are working in a corn field, the corn plants are causing multipath or shadowing GNSS signals,” Lopez said. “You need either a good RTK or GNSS-INS, because if you lose satellite lock you can still coast for a while with an IMU.”
Services and options
AgLeader’s displays have a built-in networked transport of RTCM via internet protocol (NTRIP) client that enables it to connect to NTRIP networks and CORS networks, as well as other free and subscription-based networks. “That allows us to get RTK from the internet for customers that want to go that route,” Cran said. Alternatively, the company offers NovAtel GPS receivers, including Satel- or Freewave-based RTK options with 400 MHz and 900 MHz radio options that can communicate with a similar base station. This spring, it will begin to offer NovAtel’s TerraStar-X service. “We are calling that ‘RTK from the sky,’” Cran said. “The expectation is sub-inch accuracy, with a convergence time of less than one minute. Many of our customers and dealers are very excited about that option.”
Septentrio’s GNSS modules for agriculture are used mostly in mapping drones, Lopez said. The modules mitigate interference and spoofing. “We have also been quite successful in robotics for agriculture,” Lopez said. Septentrio is working closely with the French agriculture robotics company Naïo Technologies, which integrates its robots with Septentrio’s smart antenna GNSS products, providing a full RTK solution as well as autonomy.
For areas without RTK networks, some farmers buy and install Septentrio base stations that provide corrections to their robots or UAVs. Septentrio provides agricultural mapping software for post-processing data gathered without RTK. Also on offer are L-band receivers — while not as accurate as a local RTK network and possibly with higher convergence time, the relative accuracy of L-band corrections is more than good enough for many ag robots, Lopez explained.
On the baseline
Most RTK users are on a short baseline — under 5 miles from the base station to the rover, according to Al Savage, manager of John Deere’s StarFire network. Medium baseline is about 5–8 miles, and long baseline is up to 12 miles. In 2015 John Deere released its Base Station Manager, which enables dealers to remotely upload firmware, upkeep the rover access list, and monitor their base stations.
As dealerships and their RTK networks merged and farms expanded, it became difficult for farmers to keep track of which base station to use. So, in 2019 John Deere released an Automatic Base Station Switching feature that links the RTK radio configuration to the field boundary in its Generation 4 display.
Also new: A John Deere StarFire receiver can continue to operate if it loses connection to a base station using the RTK Extend feature. The StarFire SF6000 rover receiver can continue operating with RTK-like accuracy for up to 14 days without connecting to an RTK base station, compared to only 14 minutes for a previous receiver, Savage explained. The increase “was very well received by customers, especially those operating in areas challenged by line of sight or trees and foliage on field boundaries.”
In South America, John Deere’s RTK Flex feature, “will automatically switch between RTK and SF3 during a time in the day when scintillation causes the greatest interference,” Savage said, enabling farmers “to continue working with similar accuracy when RTK is unavailable due to scintillation.”
Remaining obstacles to adoption
Despite’s RTK’s growing popularity, there are a few remaining obstacles to its adoption.
Cost. “Though the cost has been greatly reduced over the years, RTK is still more expensive than other correction signals out there,” Fulton said. Part of the cost is due to the hardware, Cran pointed out. “There are rover and base station radios, there are towers to put up. On the NTRIP side, there are cell modems to put in vehicles, and they require keeping a data plan active.” The agriculture market traditionally has been very cost-sensitive and conservative, Lopez said. “Farmers expect to implement very low-cost solutions. They want to know whether they will have an ROI (return on investment) on these solutions.”
Satellite services offer a cheaper alternative to RTK. TerraStar-X, for example, gets rid of the cell modem hardware and the requirement for base station hardware, Cran said. “At a lower accuracy level, there are other satellite-based TerraStar signals: TerraStar-C and TerraStar-C Pro, which get an accuracy maybe under 5 to 20 cm. Those are less-expensive alternatives that growers are using without making the leap to RTK.”
Lack of Internet Connectivity. While most RTK services, including NTRIP, require an internet connection, many rural areas have limited broadband and even cellular connections. Some areas lack support for RTK, Fulton said.
Lack of cross-platform compatibility. Farmers with a mixed fleet want to run a mix of receivers. “For example, John Deere and Trimble RTK do not work together,” Cran said. “It is still very manufacturer-specific. I cannot take a NovAtel receiver that is AgLeader branded and use it with a John Deere RTK network.” NTRIP partially enables cross-platform mixing and matching. “We’re excited about TerraStar-X, too, because, while it is specific to NovAtel receivers, it is not tied to any base station hardware,” Cran said. “So, a John Deere guy can put an AgLeader receiver on their vehicle and use TerraStar-X and still get that accuracy without being tied to the Deere RTK network.”
Liability. While safety is not nearly as big an issue as it is with autonomous vehicles on the roads, liability questions will soon loom. “If, for example, a robot destroys a whole plantation, someone must be liable,” Lopez pointed out. “Was it the robot? The GPS receiver? Other sensors? The farmer? What if there is a spoofing attack and the robot goes to a neighbor’s field?” Such challenges are slowing adoption. “That is where the reliability of the GNSS is becoming important,” he said.
The future
Soon, satellite-based internet connections could make RTK correction more widely available and give more growers the option of using NTRIP, though at a cost. WAAS, a free service of the U.S. government, is broadcast by satellites but does not achieve the accuracy level of RTK. “RTK is still a localized correction,” Fulton said. “We may see that shift to satellite, but it will more likely be an online or some other type of communication.
“Once farmers start using RTK, it is very unlikely that they will ever revert to another type of correction,” he added.“RTK is a very addictive correction service for folks.”
Savage concurs. “RTK is addictive because of its accuracy, efficiency and repeatability.” Ultimately, however, to achieve universal adoption, RTK solutions will need to work everywhere, with little intervention by the farmer.
