Reaching for the Stars
Julio Rivera, GPS coordinator for Southeastern Surveying & Mapping Corp. based in Orlando, Fla., remembers using single-base RTK to collect GPS data. “When I started working for the firm in 1997, we were sending out multiple receivers for each project. Each receiver was limited to a range of about 4 miles and needed a base, so we had to send a lot of equipment and personnel into the field.”
The firm was skilled at managing large commercial and infrastructure projects, but Rivera kept looking for a better way to handle RTK corrections. Then he began reading about reference networks in trade magazines. “The idea was appealing,” he says. “Instead of sending two or more receivers out for one project, I could send just one rover and connect to the network, or I could still send two receivers and use both of them to collect data rather than having to set one of them up as a base station. I knew that type of service would allow us to operate more efficiently. I started talking to people from Lengemann, our equipment supplier, and essentially told them that, as soon as they were ready to do a network, we would get on board.”
Lengemann was ready; in fact, the company had already begun exploring the idea. “We believed our customers needed more than the very-limited, single-constellation network offered by the state’s DOT,” says Ralph Lengemann, the company’s owner. “Our customers include everyone from those involved in agriculture, construction, mining, outright surveying and more. These professionals are craving greater levels of accuracy than ever before, so we wanted to cater to that need.
In 2004, with the support of Southeastern Surveying & Mapping as well as other firms, Lengemann launched L-Net, a network powered by Topcon’s NET G-3 receivers. It was a move that would have a far-reaching impact.
Something for EveryoneA major challenge facing anyone creating a reference network is making the resultant product universal enough to serve the greatest number of customers, flexible enough to accommodate a full range of available software, and of course, able to provide the greatest possible level of accuracy for its users. Lengemann went to great lengths to address each of those issues, says Justin Farthing, Lengemann’s network administrator.
“We wanted to offer L-Net subscribers a multi-base network solution,” he says. “It uses the entire network and models the atmosphere and ionosphere then creates a simulated station for the rover based on both the closest reference stations and what it is modeling. Essentially, the network software creates a virtual base station. Then, the rover connects, the network software models the ionosphere, resolves the ambiguities between all the stations, creates that simulated station, and the rover gets its position from that virtual station. Doing so not only decreases initialization times, it maintains accuracy over longer distances. That’s a real plus for our customers.”
One of the benefits Lengemann sought to bring to L-Net subscribers was a distinct advantage over public alternatives. Farthing says they believe they accomplished this goal largely through the hardware they chose. “The most dramatic difference between our network and a public one like FDOT’s is the fact that ours is a dual-constellation reference network, so we generate both GPS and Glonass with L2C coming online shortly,” he says. “Those advantages were made possible because we selected Topcon’s NET G-3 as our network-wide receiver. In fact, all of our hardware--and by extension, the firmware--is from a single source, in this case, Topcon. We feel this uniformity definitely enhances network performance over one that is of the mix-and-match variety. And the performance to date bears that out.”
Certifiably AccurateUpgrades have also played a big part in L-Net’s current success. The network was initially based on the WGS84 broadcast, but Lengemann realized that a move to the NAD83 reference frame was needed to accommodate more software and more versions of software. To make this move, Lengemann contracted with a company that is also one of its best customers, the Wantman Group Inc. (WGI) of West Palm Beach, Fla.
According to Robin Petzold, WGI’s senior vice president, the goal was simple: Place the network on the State Plane Coordinate System in the North American Datum of 1983, 2007 adjustment (SPCS’83-2007). “Lengemann provided us with the GNSS data collected over a period of one year (2007-2008) from an area that covered Key West to Savannah, Georgia, and west to Pensacola,” he says. “The total network consisted of 101 stations, of which 41 were existing Continuous Operating Reference Stations (CORS) ‘blue booked’ control stations, and seven were control stations within FDOT’s network. The remaining 53 stations were Lengemann’s L-NET stations requiring adjustment. The project was broken down into seven regions for ease of processing and was ‘constrained’ using the existing CORS and FDOT stations in each area. Over a five-month period, a total of five sessions, each lasting four hours, were processed for each point.”
Upon completion, Petzold says L-Net showed horizontal accuracies of less than 0.05 foot and vertical accuracies to less than 0.10 foot. The final “constrained” network holding a total of 41 CORS stations and seven FDOT stations in the final adjustment currently meets or exceeds the Federal Geodetic Control Committee (FGCC) guidelines for “Geometric Geodetic Accuracy Standards and Specifications for Using GPS Relative Positioning Techniques” as well as the Minimum Technical Standards set forth by the Florida Board of Land Surveyors in Chapter 61G17-6.0051 Florida Administrative Code, pursuant to Chapter 472.027 Florida Statutes.
The effort put forth by WGI was outstanding, though not at all surprising, says Ralph Lengemann. “When we got into this, we were ‘green’ as far as network creation goes,” he says. “Wantman did a lot to help us improve on things as we went along. This is just one example of that--though a big one.”
A "Living, Breathing' NetworkToday, L-Net is nearly 70 stations strong and provides dual-constellation RTK correction to a growing list of customers and subscribers throughout the southeast. According to Ralph Lengemann, a commitment to customer service--more a part of company policy and philosophy than specifically L-Net related--has served the network well as it has grown. “The reference station network is like a living and breathing entity; it is constantly evolving and changing,” he says. “And the demand is almost constant: we have seen people accessing it at midday, at midnight, at 5 a.m.; it is endless. As a result, it needs constant attention, and we work hard through our Lengemann Support Group (LSG) to ensure there are no glitches, no downtime.”
The steps Lengemann has taken to minimize service interruptions include establishing a pair of parallel, mirrored services set up at different locations, each working though a different internet provider, on different power grids and each with battery backup. In addition, L-Net uses parallel servers, each with a mirrored backup. If a hard drive fails on a server, Lengemann has a second drive that is ready to be turned on. The same level of redundancy extends to the server itself.
“Hearing our customers say they are pleased with our service tell us we are doing our jobs and gives us a vote to confidence to continue developing the network to help them further,” Lengemann says.
In general, the ability to work anywhere in the county and even across county lines with less equipment and fewer personnel has given the company a significant advantage. “With the network, we can now be 20 to 25 miles from a reference station and still get reliable data,” Rivera says. “It’s provided an added level of efficiency and accuracy for the company as a whole.”
A Secure SolutionHaving access to a network means not having to set up a base station, tie into it and be responsible for it on a long-term basis. By way of illustration, Ralph Lengemann cites two subscribers, one involved in placing boulders for a rip-rap project, and the other using sounding equipment and software to map the sea floor of an area harbor.
“The company doing the rip-rap was looking to get accuracies for their placements to within 2/10 or better, which of course they easily got from the network,” he says. “When I asked how this was done before, they said the spec was never obtainable, and no one really cared. Now that they had the capability, however, they wanted to capitalize on it. As to the company doing the soundings, they too benefitted from the accuracy, but they also liked the fact that they no longer had to set up a base and worry about it while they were out on the water taking soundings.”