Feature: From Surveys to Snowplows: Virtual Reference Station Use by Minnesota DOT
Professional Surveyor Magazine - February 2003
Dave Gorg, PE, Don Seitz, and Craig Shankwitz
Since 1985, the Minnesota Department of Transportation (Mn/DOT) has used GPS technology for all our surveying needs. From our original use of GPS for geodetic control, we added handheld GPS receivers for mapping, then RTK GPS receivers with portable GPS base stations. Then came the snowplow.
In 1998, the University of Minnesota Intelligent Vehicles Lab (UMN IV Lab) and Mn/DOT partnered to continue the development of technology that had originated in the safetruck, a tractor-trailer with a GPS-based driver-assistive system. Researchers realized the benefits of GPS for guiding trucks on low-volume test roads; additional research would help develop the possibilities of GPS use with snowplows during adverse weather conditions. International Truck, Inc. joined the partnership by donating the custom-built safeplow snowplow truck.
We knew that GPS used for machine control required RTK. With our district surveyors asking for more RTK systems, we started looking for solutions to permanently add base stations.
We found that Trimble's Virtual Reference Station (VRS) network enabled us to gain centimeter-accuracy RTK GPS for our surveying needs within a 3,000 square mile Twin City metro area, without the limitations of portable reference stations. VRS has also allowed the Twin Cities' Metro Transit Authority to use RTK GPS for non-survey applications: to help guide express buses in narrow bus shoulders. And then there's the snowplows.
It's a system that is benefitting surveyors, researchers, transit agencies and commuters. We're still looking to the sky to navigate; only this time, it's with satellites rather than stars.
GPS in Minnesota
The 12th largest state in the U.S., Minnesota boasts rugged ridges, deep lakes, and gently rolling plains with some of the nation's richest farmland. (83,574 sq. miles of Minnesota are land areas, with 7,326 sq. miles covered by water.) With annual temperatures ranging from -50º to 110º F, Mn/DOT surveyors have to deal with blizzards, humidity, and everything in between. As geodetic engineer for Mn/DOT since the early 1980's, Dave Gorg saw early on how accurate and powerful a tool GPS was for doing geodetic control and quickly brought one in-house. Mn/DOT was the one of only a few government agencies using GPS back then, but other state and county agencies quickly became interested and started budgeting for equipment. By the early 90s, the state had so many GPS users that they started a GPS Users Group. Along with survey-grade equipment, agencies were using non-survey grade units like the Trimble GPS Pathfinder receiver to collect information on water supply, highway signs and other uses. They also became the state advisory group, delving into issues such as developing a statewide base station network. The seeds for the current VRS network were sown early: today, those seeds are blooming.
Once RTK had been implemented, district surveyors didn't want to use anything else; the value of collecting accurate positioning in real-time rather than needing to download the data into a computer for postprocessing back in the office was clear. With RTK they could confirm data accuracy in the field; though RTK wasn't required for all uses, no one wanted anything less.
As RTK use proliferated throughout the state, several problems occurred. Each user required a base station and wireless communication link; so there'd be a county surveyor setting up a portable base station a mile away from a DOT surveyor and base station with both users trying to transmit RTK corrections to their respective rovers on the same frequency. It created a lot of radio interference, especially in the Twin City metro area, not to mention the ongoing threat of losing our unsecured base stations to theft, as has happened in some areas.
A solution was found for one-meter accuracy requirements when the private sector started selling access to radio broadcast corrections. But for survey-required centimeter accuracy, no options were then available. In addition, UMN started researching the use of GPS to navigate vehicles, which required RTK. So about five years ago the process of installing permanent base stations was investigated.
After researching the options for a pilot project in the Twin City metro area, Mn/DOT purchased Trimble 4000 SSi Continuously Operating Reference Station (CORS) receivers. They initially positioned five stations 60 miles apart, which was the maximum spacing for accurate post processing in a CORS network. Stations were mounted on government buildings with the post processing data stored on one central computer and available free to users via the Internet. Users need only proper software and one or more survey grade roving receivers. The CORS sites formed a 60-mile (90 km) grid around the Minneapolis/St. Paul metro area, with St. Paul as a center point; the network covered 3,000 sq. miles (7,770 sq. km) and was used by metro area surveyors.
Moving to VRS
Even while the CORS network was developing, Don Seitz had his eye on Trimble's VRS technology. After seeing its potential at a conference in Munich, he became a strong proponent of bringing the technology to Mn/DOT. While CORS provided postprocessing, VRS would provide real-time positioning, a better, more cost-effective way to assist their customers.
When Mn/DOT made the decision to fund VRS, they needed only to increase the already active CORS network with five additional reference stations—four 4000 SSi and a 5700 CORS system—and decrease station spacing to 30 miles apart to create the first VRS network in the U.S. Antennas were initially installed just on permanent state buildings; as the system expanded, county buildings were included as well. They now use Cellular Digital Packet Data (CDPD) wireless data services and a modem hooked into the TSCe controller for the wireless communication link in the metro area. With eight reference stations and one integrity monitoring station, the Mn/DOT VRS network was declared operational June 7, 2002.
VRS includes a network of fixed reference stations, software, and a control center to "virtually create" reference stations for users in the region, providing centimeter accuracy with enhanced performance and reliability. Users no longer are required to invest in and set up local reference stations; with VRS surveyors can achieve long-range RTK precision with a much reduced ppm error compared to single base RTK—and over greater distances in the field. The VRS software uses the data from the entire network of reference stations to calculate GPS error corrections that are applicable over a wide area. Users receive these error corrections in the field via wireless communication devices. They can also retrieve stored GPS correction data from the control center via the Internet for post-processing. Because the system significantly reduces systematic errors—caused by ephemeris, tropospheric and ionospheric effects—in the reference station data, users can increase the distance at which the rover receiver is located from the physical reference stations while improving on-the-fly (OTF) initialization times.
Currently, VRS is predominantly being used by Mn/DOT, however, it's available free-of-cost to anyone. Most of the counties already have the equipment and need only a license, cellular modem, and the VRS software to gain the 1-2 cm horizontal accuracy that has been consistently gained with the new system. Three-centimeter vertical accuracy is common, though in the Twin City area it can jump to 5-6 cm; gravity anomalies due to underground rock structures in the area make it difficult to get accurate GPS elevations and require accurate gravity modeling. In addition, they make sure the satellite geometry allows for optimal VDOP in surveys where vertical measurements are critical. In addition, DOT users have used the system to gain good solutions at up to 18 miles (29 km) from the nearest reference station.
New users need no special training; in fact, experience has shown that it's quicker and easier to get a new user up and running on VRS than with RTK and a base station. The Trimble 4700 and 5700 GPS Total Station systems are used as rovers, as well as the cableless 5800 RTK Rover; basically, if you know how to work Survey Controller software on a TSCe, you know how to use VRS. And because you don't need a base station, using the VRS reduces the cost up to 50 percent for individual users to take advantage of RTK GPS.
The goal is to now seed the state with clusters or pods of four to six VRS stations. Locations would be determined by workload and funding availability. As VRS use increases, each pod will be expanded to ultimately create a 125-station unified statewide network. Currently, they're relying on the user to monitor the system. As it expands they may require dedicated personnel and additional servers, but they're initially using one computer and a bunch of part-time people to make it work. In fact, the expansion of VRS is already underway; a new server has been installed at UMN and testing is in progress.
Benefits to Minnesota
Benefits to the state include gaining a reliable, stable system for both postprocessing data and real-time positioning, cost-savings to individual surveyors and counties who now don't need base stations, and greater data integrity, efficiency and accuracy.
For example, Mn/DOT consultants do a lot of aerial photography. To collect digital elevation data using LiDAR, they need postprocessing, requiring base stations in the proximity. Permanent base stations provide better integrity, without the possibility of base station misplacement or out-of-adjustment equipment negatively impacting the data.
They also plan to use the real-time system for daily work. Users could show landowners the exact property line locations, highway rights-of-way or construction limits for roads. Utility companies could walk highway rights-of-way and point out gas or electrical line locations and easements. All this would help keep projects on schedule. Unforeseen delays can and do occur: negotiating with land owners, finding something unexpected underground, or certifying that everyone knows the exact construction limits. These take time; the VRS can streamline program delivery and provide completion of transportation projects faster to the public. And there are big payoffs in speeding up the process.
Facilities' management people can use it to position light standards, underground utilities and sign locations. Today, Mn/DOT provides permits for utility companies to place utilities on our rights-of-way. If they're building a new highway, they need to know the utilities' exactlocation. Now, utility companies usually mark their utilities' locations on a paper map; Mn/DOT survey crews then position these marked areas with coordinates. With VRS we can know the precise location of all utilities and store them in a database for future projects.
VRS can become a tool used by maintenance people for drainage, highway maintenance, and underground drainage. And when VRS expands outside the metro area, surveyors in our natural resources department are interested in using it.
Besides state use, private companies are beginning to sign on. Bolten & Menk, Inc.'s (Mankato, MN) Pete Blethen is one early VRS user. Today, utilizing VRS mainly for topography work, Blethen uses half the equipment and greatly increases setup speed. In the future, VRS use by the entire company—including 25 survey crews—could cut hardware costs in half, double the crews that use GPS daily and increase productivity. It would also benefit the company to have all surveys on a known coordinate system. As Blethen recently said, "It's as close to a black box for surveying as I've seen yet—you just set up your rover and go."
Beyond these practical applications are safety of life issues. People don't readily think of these, but keeping emergency vehicles going or roads open in blizzards can save lives, which is where the snowplow comes in.
Semis, Snowplows and Technobuses
Mn/DOT operates MnROAD, a pavement research facility, where experimental pavements are tested by driving a heavily loaded safetruck continuously around a two-mile long test track. Worried that the driver may get bored, fall asleep, and veer into the adjacent highway, a DGPS-based system was developed that was capable of steering the vehicle if the driver dozed. It worked—so well, in fact, that the technology was quickly transferred to snowplows to improve driver visibility problems in a state notorious for whiteouts and low visibility.
The system uses a head-up display, GPS, and a high accuracy (5-10 cm) onboard digital map database. A Trimble MS750 DGPS determines a plow's location to within centimeters 10 times per second. Snowplow position information is used to query the map database for road information near the snowplow's location. That road information is projected onto the head-up display, similar to displays in fighter aircraft. The display gives drivers a computer representation of the road that is aligned with the actual road, so they know where they are even when snow or fog blinds the actual view. Three radar units on the roof of the snowplow and one on the front grill provide information to drivers regarding obstacles so collisions can be prevented. DGPS applied to snowplow operations is presently being expanded at MnROAD and funded through Mn/DOT as part of the safeplow Hwy. 101 project. Testing will continue during the 2002-2003 winter on Hwy. 101; the tests will focus on ‘gang plowing,' where snowplows ‘fly' in formation on wide highways, passing the snow to the second and/or third plow to finally get it off the road. The problem: impatient commuters at times try to jump between the plows, get hit by the transferred snow - and by the next plow. Using GPS and VRS, the plows can tighten their formation with centimeter precision and ultimately decrease the potential fatalities caused by these accidents.
The IV Lab
When Mn/DOT deployed the VRS in the metro area, the IV Lab quickly jumped on board. VRS is being used with a cellular wireless link to support lane assistance for buses operating on narrow bus-only shoulders. For this application, the IV Lab and Metro Transit migrated the snowplow technology to an experimental transit bus. Funded by the Federal Transit Administration and Metro Transit, the IV Lab has equipped a retired Metro Transit express bus with Trimble GPS, laser scanners, virtual mirrors and a rumbling driver's seat. All three partners hope to increase the safety of buses as they operate in narrow lanes. Currently, the 9-foot-wide "Technobus" uses the VRS-corrected GPS to assist a driver along the 10-foot-wide narrow shoulder lanes to bypass freeway congestion. Radar and laser scanners are used to provide a driver collision avoidance information for the periphery of the bus using a "virtual mirror." The steering wheel uses torque feedback to help the driver to stay on course; the seat also vibrates to let drivers know if the bus veers off course. Metro Transit is hoping to deploy more buses to more safely make use of the Twin Cities' extensive 200-mile bus-only shoulder network. In addition, the federal government is keeping an eye on the study.
VRS for these applications works extremely well, offering exceptional positioning performance with just a quarter of the infrastructure needed for conventional RTK applications.
Dave Gorg recently retired as a Mn/DOT Surveying and Mapping Engineer.
Don Seitz is an Engineering Specialist in Survey Research and Support for Mn/DOT.
Craig Shankwitz is Program Director for UMN ITS.
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