Corn Belt coverage
Sep 1, 2008 12:00 PM, By David Hest
Up to the task?
State DOTs across the U.S. began constructing regional and statewide RTK networks in 2000. The primary goal was to improve surveying accuracy and efficiency as they managed road and other transportation construction projects.
The technologies and processes that the networks use to generate corrections vary, depending on whose equipment they use. Corrections typically provide accuracy to a radius of an inch or less, comparable to accuracies of dedicated agricultural RTK networks.
In general, they rely upon fixed RTK base stations, which are placed at relatively wide intervals of about every 30 miles. That compares to the six-mile grid typical of dedicated agricultural networks. The DOT base stations feed positional data into central computers, which use software to compute refined corrections before data are streamed to the Internet. This differs from dedicated ag RTK networks, where RTK base stations broadcast correction data directly to users and base stations typically are not linked through a computer network.
Some experts question whether the technologies used by some DOT networks are capable of providing correction signals reliably 24 hours a day, seven days a week.
“When you are controlling a machine, you are affected by latency [delays] in the Internet, as well as questions about cellular coverage,” says Bressler of Trimble, whose equipment is used in the Minnesota and Ohio DOT networks. “It is very effective for surveying but not as effective for machine control.”
But Leica's Hill says technologies used in newer DOT networks have eliminated latency as a practical concern. “This has been proven in 24/7 machine autosteer by our testing, as well as others,” he says.
Another concern is that DOT networks generally do not provide 24/7 network support, raising questions about downtime in off hours. The Minnesota system, for example, has technical support available only during normal working hours. However, the administrators say their system has a strong track record of being up and running more than 99% of the time.
In general, the systems have redundant backup server computers to reduce the chance of shutting down.
Proven in the field
After testing the Minnesota DOT RTK network last year, Dave Lagerstedt of GPS Services in Adams, MN, now works with 15 ag customers. “Our customers are very pleased with it,” he says. “If we have any issues at all, it is with the cell phone providers.”
Even cell phone problems have been relatively minor. In most instances, customers have required only external antennas to get a reliable cell signal, even in bluff terrain.
“We use an external antenna most of the time,” Lagerstedt says. “In some areas, we also use a booster [amplifier]” to improve the signal. Data transmission often continues even where voice calls tend to drop. (For more information about improving cellular communications signals with antennas and amplifiers, see “Wireless farm,” August 2008, page 31.)
Customers access corrections with a “smart” cell phone (the HTC 6800 is the company's preferred model). The cell phone relays signals to GPS controllers via Bluetooth wireless and software from InTime (www.gointime.com) for a package price of $4,400, excluding the phone.
Accessing DOT networks
Not all RTK guidance systems can use DOT RTK corrections, but most can, Lagerstedt says. Deere systems currently cannot use DOT RTK corrections because the company uses a proprietary communications protocol, says Beuligmann of Deere. But the company is looking into adding that capability.
Leica plans to add, to its mojoRTK product line, the ability to access DOT RTK corrections without an external modem or cell phone. Current survey-grade RTK receivers from Leica, Trimble and others typically have this capability.
For information about RTK networks in Michigan, visit www.mdotcors.org and click on RTK User Agreement. In Minnesota, go to www.olmweb.dot.state.mn.us/CORS.GPS/cors.html. In Ohio, send an e-mail to aerial@dot.state.oh.us or call the survey operations manager at 
614/275-1359
. Information about the Iowa DOT network is available at www.dot.state.ia.us/rtn.
Additional regional public and private surveying-based RTK networks also have been built in the Midwest. Individual locations and operators of these RTK base stations are listed on a Web site of the National Geodetic Survey (NGS), an arm of the National Oceanic and Atmospheric Administration (www.ngs.noaa.gov/CORS/sort_sites).
NGS coordinates a cooperative network of these stations, which it calls Continuously Operating Reference Stations (CORS). Additional information can be found by searching for “CORS RTK” with a Web search engine.
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