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.