Linked Without Baselines
October 1, 2007
When you read a road map, you see a network of interconnecting highways with the interstate system providing the backbone. When you think of the Internet, you imagine a huge cloud of magically linked data that moves through space from server to server, PC to PC. Like the super highways and the Internet, reference station networks create a similarly linked infrastructure.
Active control reference station networks, which are interconnected GPS base stations, work together to provide a single, real-time spatial reference frame with reliability and productivity benefits that exceed survey control expectations; many users report more than 50 percent increase in productivity and accuracies. With precision requirements to the centimeter, these networks eliminate the need for field crews to set up costly real-time kinematic (RTK) base stations. They also remove a fair number of common problems typically associated with classic “point-to-point” RTK, such as baseline-dependent errors.
Simple, reliable and cost efficient, these networks are currently in operation across entire states as well as on structures and facilities around the world including dams, buildings, bridges and highways. They are increasingly used for machine control on road construction projects, on snow removal equipment positioning in winter time, and in water and wastewater management, public safety and much more.
Bottom line, reference station networks are an integral part of the surveying and mapping infrastructure currently under construction around the world. Just type “reference station networks” into your Internet search engine and watch this infrastructure unfold.
Continuous OperationNetworked reference station technology has actually been around for more than a decade. Over time, with the widespread use of GPS and the Internet, the technology has become increasingly available and affordable--most notably at the national level with the introduction of active GPS networks that incorporate continuously operating reference stations (CORS). Hosted by the National Geodetic Survey (NGS), these stations are rapidly becoming the backbone of our communities, the “iron core” of a stable and permanent infrastructure.
Many see CORS as the future, if not for their technology merit and simple pragmatics, then for economics; they have a cost benefit ratio of 16:1 or better over classical RTK base and rover methods used for mapping and survey techniques, according to recent audit reports posted by state DOTs in Michigan and Florida.
In September 2001, the Michigan Department of Transpor-tation (MDOT) was the first institution in the nation to apply an advanced application of networked reference station technology by implementing an active GPS control network of 14 reference stations across the state. This network forms the backbone of the Michigan Spatial Reference Network, a statewide network of more than 60 stations providing network RTK solutions.
Today, MDOT hosts 67 such stations. The overall network provides hourly data files--both locally and to the NGS--as well as real-time streaming data to a central control facility in Lansing, Mich. Any surveyor with a suitable connection and GPS receiver can gain access to the online network and obtain RTK corrections without setting up a local base station. This network supports RTK activi-ties throughout the state such as surveying, photogrammetry, construction, public works, emergency services and more.
Quickly following suit was the Florida DOT with more than 50 reference stations providing network RTK over the entire state.
With these networks, an unlimited number of surveyors using a GPS rover never have to re-initialize as long as the communications link stays connnected and the user does not leave the area covered by the network. Users can maintain consistent accuracy over the entire network, stay connected to a “real” reference station at all times, and use all reference stations simultaneously while instantaneously resolving ambiguities. This is due in large part to the fact that the user is always connected to a real reference station. Because it’s a true network solution derived from all the reference stations, the baseline-dependent errors are almost completely removed, essentially providing a homogeneous platform whether the user is one meter or 35 kilometers away; he will have almost the same error residuals. A GPS control network makes a smart rover (a rover that uses the available network correction to solve ambiguities and determine its position) even smarter.
Statewide reference stations provide an active, always-online network with plenty of redundancy and reliability that is linked in a homogenous coordinate reference frame, reducing the effort surveyors need to make to establish static control (on the ground) during projects. Some states require user fees, which vary; Alabama, Florida, Michigan, New Jersey, New York and Oklahoma have no fees while North Carolina and Ohio each charge user fees.
The blossoming of this technology is leading hundreds of state agencies and private entities across the country to actively sign up.
Powerful PossibilitiesThe New York State Department of Transportation (NYSDOT) and the Indiana Department of Transportation (INDOT) became two of several states to opt for the benefits of a CORS network, primarily to take advantage of the speed, accuracy and 24/7 accessibility afforded by this online, fully automated network. For NYSDOT, this network provides improved accessibility to the New York State Plane Coordinate System and the National Spatial Reference System.
The New York statewide network includes 46 permanent CORS, each connected to a system of servers and processing software located in the NYSDOT facility in Albany. These dual-frequency stations are spaced about 50 to 100 kilometers apart, particularly along dense major transportation corridors. This is only the beginning; the state ultimately needs about 65 total with 70-kilometer spacing. Static and real-time data from the network is available through the Internet.
The stations automatically measure errors in the GPS satellite signals and compute error correction factors. DGPS (mapping grade) and survey grade data are streamed in real time from the reference stations via Internet connections to the Albany facility. There, data integrity is checked prior to being streamed to the Internet for public access, and redirected to the NGS site in the industry-standard RINEX (Receiver Independent Exchange) format. Static raw data files are downloaded every half hour and are also available to the public for post processing.
Interoperability is one of the benefits of today’s leading reference stations. While post-processed in the industry standard RINEX format, real-time GPS reference station network data can be delivered and provided in several formats including RTCM, CMR and Leica Standard, thus ensuring support for Leica Geosystems, Ashtech, Trimble, Topcon, Javad and many other brands of GPS equipment, while staying connected to a real reference station so that every correction stands on its own. The network maintains GPS 3D accuracy at 2 centimeters horizontal and 3 centimeters vertical. The RTK network provides users with the benefit of the same consistent accuracy rates 24 hours a day, seven days a week, or any time a shot is taken. This is only achievable in a true network.
Set for implementation beginning January 2008, INDOT’s CORS network will include 17 primary CORS stations with choke ring antennae and approximately 25 secondary stations with geodetic quality dual-frequency antennae, for a total of 45 located strategically around the state. Each CORS will incorporate a Leica Geosystems (Norcross, Ga.) GRX1200 GG Pro GNSS receiver with Ethernet directly to the Internet and high-precision frequency import capabilities. INDOT plans to space primary stations about 100 kilometers apart and then densify the network as needed with the secondary stations. Once the network is implemented in 2008, DGPS and RTK data will be available to the agency via the Internet 24/7.
INDOT will implement Leica Geo-systems’ SpiderNET, an integrated suite of software that adjusts for the orbit (i.e., ephemeris) errors as well as ionospheric and tropospheric conditions, then analyzes and calculates solutions for the entire network, and finally sends a network correction (RTCM V3.1) to the rover or end user--all while staying connected to a real reference station.
The result is that rather than merely getting information from the closest reference station utilizing the classical point-to-point methodology, the surveyor is getting network corrections based on the entire network.
Bill Schmidt, manager of Aerial Engin-eering with INDOT, says, “Our survey community is very excited about the CORS network and its ability to minimize the need to set up base stations and create greater opportunity for RTK surveying. An added plus is that every state agency will operate on the same coordinate system.”
A Local EdgeWhile the value of a statewide CORS network is readily clear, active GPS reference station networks also support a host of local and regional survey and engineering activities on complex and long-term projects. These GPS reference station networks, linked via the Internet, provide engineering and surveying professionals a foundation to manage a variety of project aspects.
For instance, in 2006, the Minnesota Department of Transportation (Mn/DOT) implemented a fully automated, on-site geodetic monitoring system to manage train track shifts and ground settling during the Roosevelt Avenue Underpass reconstruction project in Detroit Lakes. During the two-year construction period, crews must carefully monitor the area for ground settlement that might be caused by the ongoing de-watering process or the vibration effects from nearby construction of the bridges and underpass infrastructure. Any settlement could cause the railways to sink or shift, resulting in potentially deadly derailments.
In addition to a system utilizing GPS reference stations and software to monitor movement at the stations, the monitoring system technology allows the use of the RTK network to create high accuracy control for total station monitoring of components that can’t be easily monitored directly with GPS.
A Look to the FutureSurveyors cannot execute high precision or even mapping-grade GPS without a reference station; if a project’s accuracy requirements are for less than 3 meters, a reference station is needed. For projects with accuracy requirements of less than 2 centimeters continuously (as opposed to discrete measurements), a GPS network is needed.
GPS networks are the enabler to creating the 21st century surveying and mapping infrastructure. “Got GPS Networks?”
Leica Geosystems has participated in the design, development and deployment of more than 67 CORS networks worldwide including those for the Arkansas, Florida, Ohio, Oklahoma and Texas DOTs, Lane County, Oregon, Ford Engineering in Texas, Eastside Reservoir project and Lands Department of the Government of the Hong Kong Special Administrative Region. The 4,500-acre Eastside Reservoir project in Riverside, Calif., the largest dam project in the United States, is completely managed in real time by the reference stations, as is the world’s longest cable-stayed concrete bridge, Florida’s Sunshine Skyway.