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States offer RTK satellite signal alternative

Jeff Caldwell 08/30/2010 @ 2:20pm Multimedia Editor for Agriculture.com and Successful Farming magazine.

The best satellite receivers and RTK equipment in your machinery is useless without a solid signal to keep all your precision tools on the straight and narrow.

If your farm's spread out over a lot of area, placing satellite base stations in the right places to get coverage in all of your farms can be difficult, if not impossible in some cases.

But, if you've got decent cell phone reception in your area, most states now offer statewide satellite data transmission capabilities through Continuously Operating Reference Station (CORS) networks. Using a state's CORS network eliminates the need for individual base stations on the farm and relies on a signal that's typically monitored by that state's Department of Transportation (DOT).

In Iowa, for example, around 600 registered users tap into the CORS network. About half of those are in the surveying industry, with just under 150 in construction and just over 150 in agriculture. That includes farmers, cooperatives and others dealing in precision ag, says Iowa DOT statewide Real Time Network (RTN) coordinator Steve Milligan.

"The network was designed so basically any GPS receiver that can use satellite communications to get their directions can use it," he says.

Brent Johnson uses Iowa's CORS network on his farm near Manson in the northwestern part of the state. For his farm, it's a "key component" to all the technology tools he uses.

"It gives us RTK accuracy on our receivers anywhere our operation goes," Johnson says. "It's not tied to a particular base station or impeded by obstacles or terrain."


Brent Johnson of Manson, Iowa, talks about the RTK satellite signal he uses in all his equipment from planting to harvest. He uses Iowa's CORS network for his connections, eliminating the need for base stations (video by Doug Hetherington).


Johnson uses his CORS-directed RTK in every step of his corn and soybean production, starting at planting and going on through harvest. "Autosteer correction is by the DOT network," says Johnson, who uses a Leica Mojo RTK system in his 30-series Deere tractor. "It's modem technology, not remote technology."

While this latter characteristic of the CORS network is a big pro for Johnson (it eliminates the need for base stations around his farm), that can also be a drawback, Milligan says. With base stations, the satellite signal's the only one needed to make a connection. But, when using a statewide CORS system, good cellular data coverage is required to keep a solid connection. The CORS signal uses cellular transmission to reach RTK units that are themselves communicating via satellite transmission.

"The GPS unit would actually connect with the Internet to access the RTK correction messages that our network's putting out," Milligan says. "That would be through a private cellular provider."

So basically, Milligan says 2 connections have to be bulletproof for the CORS system to work on the farm.

"The big thing that determines whether you can use it is whether you have cellular data coverage. You have to connect to the Internet, then it sends out a radio signal to your area," he says. "So, that's the biggest limitation. You've got to have good, constant cell data service."

Johnson adds his CORS connection -- which comes free of charge in the state of Iowa -- ultimately proves its worth at harvest. "We use variable rate with a strip-till system and on-board steering," he says. "We variable-rate fertilizer. So, when the planter comes in, we have proper fertility for our corn to grow in."

One final thing to think about when weighing base stations versus the CORS network is simplicity. Milligan admits that, though using a CORS network can carry lower costs than owning and maintaining base stations for RTK signal, the statewide network can complicate things in some cases.

"Until this started, owning base stations was how it was done. That's a lot less complicated," he says. "A lot more goes into a network like this. There's more chance of having downtime, but in terms of the cost, you can get by with a lot less with a network like this. Especially if you'd otherwise have to move base stations or have more than one."

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