A Carter & Burgess RTK survey for CDOT at Highway 160 over Wolf Creek Pass. An RTRN would eliminate the need to set a traditional base station at a high point many miles from the project location.

Evolving technology in this profession requires surveyors to develop an understanding and to gain knowledge of the offerings available, and to subsequently meld those offerings with fundamental surveying techniques into their respective practices. One of the latest technological advancements gaining the attention of many surveyors is scalable GPS infrastructure and RTK networks; these solutions omit the traditional RTK base station setup by allowing users to feed off a network of GPS reference stations. The system allows for the network to be built-out and provides corrections in real-time.

In Colorado, Pam Fromhertz, the NGS state geodetic advisor, began an initiative to implement this technology in the Denver metro area. Several users had approached her about the concept, but Fromhertz explains, "There seemed to be a lack of knowledge of what a real-time network [is] and how bases should be installed to be part of the CORS [Continuously Operating Reference Station] system, or for that matter a real-time network system." As a result, a users group was established. Fromhertz explains the purpose of the group: "The GPS Users Group educates everyone in the community, surveying profession and the GIS industry; determines who [has] base stations and what their capabilities are; and determines if we want a real-time network and how to implement it." The creation of the GPS Users Group was just the beginning; following a period of intense research and work, the group created a four-phase proposal to establish an RTK network in Colorado.

Getting Started

Great numbers from the public and private surveying community in Colorado attended the first meeting of the GPS Users Group on Jan. 26, 2005. I attended on behalf of the Denver office of Carter & Burgess Inc., where I work as a project manager. There was a wide spectrum of GPS users present, from the advanced user to the novice. Because of this knowledge gap, the group needed education on the fundamentals of RTK network technology and its probable uses. Over the next several meetings the group members discussed their experiences and understanding of the technology. This led to the invitation of three manufacturers-Trimble, Leica Geosystems and Topcon-to present their respective RTK network technology solutions, followed by a question and answer session with the group.

This forum, held on March 31, allowed the manufacturers to present on the differences in their hardware and software approaches to establishing GPS infrastructure and RTK networks. Trimble (Sunnyvale, Calif.) presented its Virtual Reference Station (VRS) technology, which is suited for full scalability from the reference stations to the network configurations. This gives users the ability to have from one to a multitude of reference stations and to scale the network and architecture to suit the number of reference stations. The software processes the entire network of reference stations simultaneously, yielding higher data accuracy at distances greater than that of traditional RTK. The user at the rover connects to the system via a wireless connection; the system then accepts the user's position and fully models a solution right next to the rover to create a virtual reference station. This allows users to work at greater distances and significantly reduces PPM error (ionospheric and tropospheric factors).

Leica Geosystems (Atlanta, Ga.) presented its GNSS Reference Network technology. This approach has a master reference station and from one to a multitude of auxiliary reference stations as its foundation. This concept is also fully scalable from the reference stations to the network configurations. The process transmits the raw observation data from the reference stations to the network server, running SpiderNet. The network estimation process, including ambiguity resolution to reduce the stations to the common ambiguity level, is performed. Then the formation and transmission of RTCM 3.0 network messages using corrections for the master station and correction differences for the auxiliary reference stations in the network are sent to the user at the rover from the reference station nearest the rover's location.

Topcon (Livermore, Calif.) discussed its current and upcoming technology. According to Topcon sources, Topcon's concept is to provide a suite of TopNET Reference Station Software-TopNET CORS, TopNET RTK and TopNET VRS-that offer a wide range of functionality that is also scalable from a single site to networks covering a large geographical area. TopNET can be configured to offer complete control over a single CORS site or a network of CORS sites with additional options to provide GPS+ (GPS + GLONASS satellite) RTK correction data from individual reference stations as well as RTK correction data generated using information from all reference stations to model and reduce errors. The TopNET VRS solution currently under development will support both a point-to-multipoint solution as well as a point-to-point Virtual Reference Station type solution.

Learning about the technology directly from these three manufacturers aided the GPS Users Group's understanding of RTK networks.

Moving Forward

Over the next couple of months the group continued to develop a better understanding of RTK network technology and its possible uses in Colorado. The group wanted to take the idea of a statewide RTK network from a vision to a reality, but recognized there were many issues that needed to be resolved. One issue was that, within the collective group, individual public and private surveyors had their own ideology of how the network would benefit their own surveying/mapping goals. Overall, the group had no clear identity or defined set of goals. Other issues involved funding, and maintenance and control of such a network. Would this be a public or privately held network? We quickly realized that we were creating more questions than developing answers.

Enter Dan Smith, PLS, Colorado Department of Transportation (CDOT) statewide survey coordinator. Smith suggested the formation of a committee to create a charter for the group. Smith authored the charter, which defined the objectives and organization of the initiative, and the group then became the more official Colorado GPS Users Group.

Focusing on Research and Funding

An initiative of this magnitude needed support and funding from an entity other than the charter itself. This is when the focus shifted to the Colorado Department of Transportation. Smith and Fromhertz conducted a preliminary meeting with CDOT to gauge the public organization's interest in funding the initiative.

Smith says, "DOTs today are constantly looking for new ways to accelerate project delivery while maintaining accuracy and reliability. Identifying innovative technologies and developing realistic implementation plans are key goals in moving our program forward and being a leader in the surveying, engineering and construction industry."

There was enough interest for the charter to continue; however, obtaining the support of CDOT would require the group to define a cost benefit analysis that would balance the needs of an entire state, with population and growth considered, along with demographics and future projects. Such a proposal would need to be well-thought out and carefully orchestrated. To accomplish this, we needed to create working committees to research and plan each element of a statewide RTK network. Four working committees were formed with chairs assigned:

Infrastructure - Kevin Hoffman, PLS, Woolpert
Objective: To identify current equipment used as a base station for both public and private practitioners; and to identify state-owned property (maintenance facilities, tower sites, etc.) in strategic areas for the purpose of placing base stations for the network. These locations were to have electrical components in place, and be safe and secure since state employees would have to occasionally enter the property to check on reference stations in place for data transmission. Utilizing state-owned property would eliminate the need to lease privately held properties.

Equipment - Mathew J. Barr, LSI, Carter & Burgess
Objective: To research and perform cost analysis on GPS hardware and software, IT needs (servers, routers, etc.) and the installation of each individual base station (both rooftop- and ground-mounted).

Design - Dan Smith, PLS, CDOT
Objective: To oversee the design of the overall network, including the densification of CORS in strategic areas and the identification of a pilot project area for the RTK network.

Communication - Pam Fromhertz, Colorado State Geodetic Advisor
Objective: To facilitate the communication between the working groups and with outside groups; and to keep track of the members of the charter.

From August through October, these committees worked diligently to research all the possible information in the specified areas. The committees worked independently but shared research and information collaboratively. Once this was completed, the charter had all the research and information needed to completely define the type of RTK network suitable for the state and the plan to implement it.

Defining the Planning Phases

The naming convention for the statewide network took form in the planning stage and became known as the CDOT Real-Time Reference Network (RTRN). To accommodate the hopeful funding and support from CDOT, the final plan for the RTRN took on four phases. These phases were developed to illustrate to CDOT the true benefit of having one GPS infrastructure to accommodate all survey, engineering, construction, environmental and GIS applications.

Phase I - Statewide Infrastructure
This phase involves the creation of a core scalable GPS infrastructure throughout the state. This process requires the densification of the existing CORS network currently in place. The addition of 20 reference stations will be placed throughout the state at strategic geographic locations. In Phase I, there would not be an RTK network; however, the initial benefits of the additional reference stations would still be useful to CDOT and the surveying community, and provide the ability to expand the network to an RTRN in the future.

Phase II - Pilot RTRN
Phase II sets up the initial RTRN in the pilot project area chosen and tests the results of the network. The geographic region is a densely populated front-range area from Fort Collins to Pueblo. The design for this phase includes the additional densification of the reference stations applied in Phase I. Fifteen reference stations are set at 50 km spacing in the pilot project area. The IT and communication components of the RTRN are then implemented to bring the Phase I reference stations and the pilot project area online to one central server. The overall core GPS infrastructure could then be seeded (a process that involves collecting raw data for a specified length of time and then adjusting it to the National CORS to get solid positional solutions, which would be the basis for the real-time corrections) and real-time corrections could be transmitted within the pilot project RTRN. This will allow for significant testing on both RTRN results, and any and all communication and IT issues.

Phase III - Statewide Coverage
After all testing and analysis has been performed on the pilot project RTRN, we plan to cover the entire state and create one statewide RTRN. To accomplish this, the further densification of the core GPS infrastructure from Phase 1 would be done. The addition of 120 reference stations spaced at 50 km will be placed throughout the state; these stations will be added online to the pilot project RTRN, creating a seamless scalable RTRN throughout the state. This stage will also allow CDOT to pursue partnerships with municipalities, counties, and other public or private entities to share costs in specific areas.

Phase IV - Height Modernization
Phase IV involves utilizing the NGS height modernization program to further amplify our RTK initiative and solidify the vertical component of the RTRN. NGS currently has a draft guideline for establishing GPS-derived orthometric heights that Colorado would use as a guide in the development of the vertical component for the RTRN. The approach would be to run level circuits to all reference stations inside the RTRN. This process could be time-consuming, but would allow the vertical (orthometric heights) from GPS to be much more accurate than traditional RTK, ultimately saving CDOT and the surveying community money and time.

Waiting Out Our Proposal

"For the past decade CDOT has utilized GPS surveys referenced and tied to the HARN of passively monumented control stations for all of its engineering and construction projects," Smith says. "With the technological advances of CORS and RTRN, we are looking at the next generation of GPS surveys that will be referenced and tied to "˜live' control stations that are accessible 24 hours a day, every day of the year, without the need to send a survey crew with GPS equipment to the station."

Smith and Fromhertz took our four-phase initial plan and developed an in-depth proposal for CDOT that included proposed costs and potential benefits for CDOT and the surveying/engineering/construction community. The proposal was presented to CDOT on Nov. 1, 2005. The research and development of this proposal was done on a volunteer basis by an outstanding group of professionals both in the public and private sectors, each with vastly different backgrounds. The decision on funding such an exciting project is not yet known; however, we are all very hopeful that this project will come to fruition.

Sidebar: Benefits and Challenges of a Statewide RTRN

As the research and planning for this proposal has progressed, the Colorado GPS Users Group realizes the benefits of a project of this degree as well as the challenges. These challenges will not all be overcome, but proper planning can limit their overall impact.

Benefits of a statewide RTRN in Colorado

  • Allows surveyors to start from a common datum
  • Refines the accuracy of RTK GPS temporary base station surveys
  • Obtains accuracies of 1 - 1.5 cm horizontal and 1 - 3 cm vertical (after Phase IV - Height Modernization)
  • Omits the need for HARN monuments to be occupied
  • Reduces the number of GPS receivers, hardware and personnel needed to perform survey components of a project
  • Allows for communications to occur via non-radio means
  • Allows for 50 - 75% reduction in infrastructure for local networked RTK surveys
Challenges of a statewide RTRN in Colorado
  • Lack of statewide cellular phone coverage
  • Colorado's mountainous and remote terrain makes obtaining accurate GPS heights difficult in some areas
  • Network and information technology complexities
  • Ongoing hardware and software updates, operation and maintenance (set by the state and NGS)
  • Establishment of agreements with partners for cost-sharing initiatives, including federal agencies (FHWA, BLM, FAA, USGS), state agencies (CO State Division of Water Resources), local government (cities and counties), local utilities, educational facilities, military bases, and private surveying and engineering firms