GPS and Reference Station networks
| March 25, 2009
At the beginning of this decade if you wanted to use global positioning satellite signals to direct your survey crews or earthmoving equipment you first had to buy a base station to receive the signals. You’d mount the base station on a surveyed point (what surveyors call “control”) and then rig your machines with roving GPS receivers that triangulated between the base station and the satellites to deliver centimeter-level positioning accuracy.
This system worked well, but a base station can cost upwards of $20,000 and crews have to secure it at the end of every day and set it up every morning. The range for individual base stations was limited and to go beyond those limits you had to set up repeaters. If you set your base station up on a construction trailer and the trailer settled or sank a little bit, that could throw off all your measurements unless you caught it and corrected for it.
But over the past few years many state DOTs and private companies have started setting up a new type of GPS network. The individual receivers in these networks are called Continually Operating Reference Stations (CORS) – essentially base stations that are always on with an Ethernet connection to the Internet. If you spread out four or more of these CORS over a dispersed geographic area and create a wide area network of corrected GPS transmissions that anybody can tap into from any location, it makes the individual, contractor-owned base station unnecessary.
Iowa gets wired
The Iowa Department of Transportation is in the final stages of setting up a state-wide CORS network that will be open and free to the public this month. Wendy Watson, Leica Geosystems project manager for the Iowa DOT project, says the network will provide GPS for machine control, agricultural applications, engineering and surveying professionals; and it will be used to monitor the movement of bridges and other DOT assets.
The Iowa network consists of 80 Leica CORS spaced about 43.5 miles (70 km) apart. It will be able to pick up transmissions from more than 40 currently operational GPS satellites and 40 more satellites as those come on line in the near future. To use the system, all you have to do is register with the DOT and the state sends you back a user name and password.
Accuracy with the network is better than using a single base station, Watson says. “The corrections received have error sources reduced even further because they have been modeled over the network rather than from a single station,” she says. And you get the same quality signal no matter where you are in the state. “The corrections are distributed via the Internet so the user cell modem connects via an IP address. As the user moves from one area of coverage to another, the transition from one base station to another is seamless. With radios, each base transition would require a new setup,” she says.
More coverage, better accuracy
All GPS systems require signal corrections, because of the atmospheric and environmental conditions that distort GPS signals from the satellites. When you’re operating with a single base station, the further you get from the base station, the less accurate it becomes. The upper limit is about 10 kilometers.
CORS wide area networks solve this problem by having multiple base stations each sending correction data to a common server. The server then compiles the correction from all its base stations, analyzes the imperfections and discrepancies, corrects for the errors and then sends out the corrected data to users. Such applications weren’t possible until a few years ago when manufacturers developed high-powered software to model all the correction streams. But now these modeled network systems can use base stations up to 30 to 40 miles apart and give users anywhere in the network highly accurate positioning data regardless of how far they are from the base station.
To get this data to users who are many miles from the nearest base station CORS systems broadcast information via cell phone transmissions (rather than radio signals, which have a limited range). So cell phone coverage is required, but in many cases contractors can use their existing cell phones if they’re Bluetooth enabled, or buy a cell phone that has the proper cable port to connect to their receivers.
Mixing public and private
Florida was one of the early adaptors of GPS networks, says Charles Rihner, vice president of planning for emerging business units at Topcon. Many states allow private access to their network but in some cases, network users choose a fee-for-service private network provider due to the higher level of service, coverage or support they give. So Florida has a mixture of free state networks and fee-for-service private network service providers.
Georgia offers only private networks, Rihner says. Typically in states like Georgia, the networks are clustered around major metropolitan areas, since there isn’t enough construction or agricultural demand for GPS service in rural areas. GPS equipment dealers have become big players in these private networks.
“Our dealers in different states install networks and run them,” Rihner says. “The dealers are leveraging their networks to increase hardware sales to contractors and surveyors. Our dealer in Florida has coverage over pretty much the entire state, and we have dealers with networks in Maryland, Pennsylvania, New Jersey, Northern California and Dallas-Fort Worth.”
Despite the manufacturer-specific dealer setups, the networks are designed so you can use most any brand of GPS equipment in the network, Rihner says. Fees to tap into the network vary from as low as $125 a month to as much as $500 a month, he says.
In Iowa, where agriculture is as important as construction and survey work, a state funded network made sense. “Typically if the government has funded the network it is against policy to charge the public because in essence the public has paid for it,” Watson says. “In some states the government understands the need for the system and maintains it like roads and other infrastructure, while in others getting the funding into the budget each year can be difficult. What some are starting to do is charge a nominal fee to cover maintenance.”
Another type of network is a private network maintained by large construction companies. “Contractors with big jobs that cover a large geographic area can’t always get one base station to cover it,” Rihner says. So they’ll create a CORS network and a computer/server bank to process the information and let their subcontractors tap into the system. “I know one contractor who set up 12 reference stations to create his own network,” he says.
Do your homework
Integrating your GPS earthmoving or survey work with a wide area network, however, is not a simple, plug-and-play operation, like setting up a laser-guided system, cautions Tom Mackie, sales and applications engineering manager for the Americas at Trimble Infrastructure. “This isn’t a boxed product anymore,” he says. “They can’t take it out of a box, set it on a tripod and go to work. It’s a system and you have to have integration with third party items – the Internet and the cell phone provider.”
Mackie also urges contractors to look into the quality of the network provider. Some of the early adaptor states such as Minnesota, have a robust, full-service system. Depending on user’s needs, it is sometimes advisable to supplement what a state run system offers you with services from a private network. In some cases, the free network corrections are only single base station data being supplied by the state. “Having the ability to use multiple sources for corrections can be advantageous to users for accuracy, quality analysis and control as well as redundancy,” he says.
In states like Florida users can take a combination of the free data from the state and supplement it with data from the paid, private networks to insure the best possible accuracy and uptime. And getting on a paid network can be as easy as giving the service provider a credit card number over the Internet, Mackie says.
But before you subscribe, Mackie says you should research your network provider. “When somebody goes to choose a network, they want to look at the installation standards. What is the experience of the owner and operator of the network? What is the backup plan should the network go down, or communications go down? What are the backup plans for power?”
Aside from not having to buy, secure and maintain a base station, there are lots of benefits for contractors who use CORS networks to guide their GPS earthmoving machines and survey rovers.
“First, you’re operational 24/7, with instant-on, real-time kinematic GPS,” Rihner says. “You don’t have to worry about setting up, turning on and hooking up your base station. Second, you get clear, reliable communications via cell modem. Contractors and surveyors don’t like to deal with radio problems. They can be difficult to manage – the radio doesn’t work; it locks up or gets interference. If you have it, cell phone communication is head and shoulders above radio communications.”
Rihner adds that a lot of earthmoving contractors don’t understand everything they need to know about the technical side of GPS and surveying and control. CORS networks take care of a lot of that. “When you operate off a network then you don’t have to worry about setting up control. The control infrastructure is already there. And all you have to do is make sure you’re getting the signal and you have good design data and you’re off and running your machines. You can eliminate errors from many sources.”
Survey and staking crews also benefit. In many cases a single surveyor with a rover can do the job that previously required two people. And scratching around in the woods looking for control points or monuments is eliminated.
States gain too
Many state governments and in particular their departments of transportation are big advocates of wide area networks – and for good reason. Mackie cites New Orleans as an example of the benefits of modeled networks versus the old method of survey and control using brass monuments set in concrete piers.
After Katrina it was discovered that many of the area’s brass survey markers had sunk below their original installation level – what’s called subsidence. “The benchmarks were sinking and never redefined for the height component, so you had inconsistent heights on the levies when it came to flooding and rebuilding,” Mackie says.
“Now Louisiana State University has put in a network of CORS stations covering the state and they’re very focused on the height component, Mackie says. “They have this continuous spatial reference across the state that contractors can use to build and do pre-planning and as-builds. Everybody is working on the same spatial reference, the same heights and the same X and Y coordinates without question. You don’t have one contractor bringing in control from one benchmark and another bringing in control from another benchmark that didn’t match because of subsidence of the benchmark.”
Mackie estimates that up to 25 states are now operating some sort of wide area GPS network.
Networks are the future
Clearly there is a growing interest and activity around GPS networks. “The networks are getting more popular as more surveyors and contractors work with them on a regular basis,” Rihner says. But contractors who already own base stations will continue to use them for the foreseeable future, if for no other reason just to amortize their expense. And base stations are the only GPS solution in areas where cellular coverage is spotty or non-existent.
But it’s likely over time that the states will drive more and more contractors to use their CORS networks, Mackie says. “The Texas DOT operates a Trimble network of over 100 stations,” he says. “It’s not open to the public, only contractors and subcontractors. If you’re going to do a job for the Texas DOT you have to use their coordinates from their network. It also provides them with feedback as to when and where those contractors were on the network. TXDOT keeps a log and does an analysis of efficiencies. We’re going to see more of that. A lot of states are beginning to require the correction data come from them, because it assures them it all comes from a common frame.”