Scalable GPS infrastructure: The building blocks of tomorrow

As the designers and builders of infrastructure; surveyors, engineers and construction contractors know well the benefits gained from a strong infrastructure. Whether transportation, utilities or communication systems, infrastructure constitutes a society’s basic structure, the foundation upon which the growth of a community, state or entire nation depends.

But infrastructure doesn’t happen overnight. Consider the nation’s the nation’s massive transportation system. Built over several decades, the Interstate Highway system in the USA is a good example of infrastructure’s scalability—the ability to grow over time as needs increase—and the need for sound urban planning. A powerful network based on smaller interconnecting state roadway systems, the massive Interstate Highway system was discussed and designed as early as the mid-1930s, but not fully approved and started until the mid-1950s. Funded by the Federal-Aid Highway Act of 1956, President Dwight Eisenhower pushed for a national system after being impressed with the strong autobahn network in Germany.

Today, similar planning foresight is taking place in states across the nation as private firms, municipalities, and state and federal organizations are providing real-time kinematic (RTK) Global Positioning System (GPS) infrastructure capabilities to their areas. Similar to the national highway system’s importance to the nation as a whole, GPS infrastructure can be said to be essential to the growth and development of the surveying community.

GPS Infrastructure

For two decades, the tools of GPS have mainly been individual receivers designed for various accuracies and capabilities. Initially, surveyors who used GPS in the early 1980s endured long observation periods in the field and time-intensive postprocessing back in the office. As a result, GPS was really only feasible for establishing control. To gain centimeter-level accuracy positioning in the field, surveyors in 1993 began using RTK GPS technology, which also minimized data postprocessing. For RTK positioning, a reference receiver (station) transmits its raw measurements or observation corrections to a rover receiver via a data communication link, whether radio modem or cell phone. With the introduction of RTK, GPS became a valuable tool for applications other than control work, including topographic mapping, high-accuracy GIS (Geographic Information Systems) and construction
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The most recent advancement in GPS technology, however, is scalable GPS reference station infrastructure. GPS infrastructure consists of permanent or semi-permanent GPS receivers operating continuously (24/7). Users no longer need to set up a separate base station to achieve RTK positioning; they simply use a GPS rover to connect to the established infrastructure. GPS infrastructure can range from a single reference station to a widearea Virtual Reference Staion network; for each option, GPS infrastructure offers several benefits:
• Ubiquitous positioing over a large area
• Common coordinate reference frame
• Reference station security
• Decreased learning curve to achieve precise GPS surveying
• Cost savings for capital improvement projects (government) or larger profit margin on the same type of jobs (private sector)
• Reduced cost for field crews for field setup and equipment costs The choice of each option depends on requirements and coverage area. Let’s look at each.

Single reference station

The first step in scalable Global Positioning Systems (GPS) infrastructure is an independently operated community reference station providing data for multiple applications. Private firms, municipalities and larger agencies all find single reference stations a good starting point to gain network RTK benefits. Generally, a single reference station is connected to one computer for a variety of application including:
• Postprocessed file logging for static surveying
• Single-base RTK positioningfor precision applications within a 20 km radius
• DGPS corrections for submeter accuracy within a 200 km radius

The prime example of single reference station infrastructures is the National Geodetic Survey (NGS) Cooperative Continuously Operating Reference Station (CORS) network. The Cooperative CORS network consists of single reference stations independently operated by governmental, academic, commercial and private organizations. Through a link on the NGS Web site, users can access the data by contacting the individual station for threedimensional (3D) positioning activities throughout the U.S. and its territories. CORS sites have to meet established criteria for inclusion in the national database.

Multiple reference stations controlled centrally

The next step in GPS infrastructure is multi-station networks that are controlled at one central site. Analogous to having multiple offices linked together through a wide area network (WAN), these networks cover a larger area. Each station offers single-base RTK positioning but all stations are managed centrally. This level enables an organization with multiple offices to be on a common coordinate reference frame; similar to their IT network, the GPS infrastructure can be controlled using the same architecture as the IT network.

This second infrastructure level expands the geographic territory covered by single reference stations and enables a single administrator to operate an unlimited number of receivers in a network. Cities, counties, states, nations and private firms can establish and control a network of fixed reference stations to provide RTK corrections or postprocessed data for their area of operation.

Quality control is also enhanced at this level. Administrators can monitor the coordinates relative to the other reference stations, holding one fixed and monitoring the base lines. This enables administrators to ensure the network stations aren’t moving over time and that coordinates—and thus data quality—are correct.

Full atmospheric and systematic error modeling

Offering the largest coverage area while minimizing the number of reference stations, the third step in GPS infrastructure is the VRS™ network (Virtual Reference Station). Including three up to a multitude of stations, VRS network software processes the entire network simultaneously, offering greater quality control and higher data accuracy at greater distances. Additionally, along with offering scalability in the number of reference stations, network configuration and architecture are also scalable. VRS networks can run on just one server, or have 10 or more servers running GPS solutions, depending on the redundancy, reliability and processing power required.

In the field, the farther users get from a reference station using conventional RTK, the more susceptible they become to reduced accuracy and performance due to ionospheric and tropospheric factors, also called PPM errors. With a VRS infrastructure, network software provides a fully modeled solution that factors in potential PPM errors. Users connect into the system using a wireless connection; the software acknowledges the users’ field positions and allows them to operate as though there is a reference station—a virtual reference station—right next to their rover. As a result, the PPM error is significantly reduced, enabling surveyors to work at long distances from the physical reference stations.

In Asia-Pacific, several countries have installed GPS infrastructure networks , amongst them China (for e.g, in the cities of Sichuan, Beijing, Shanghai and Wuhan) , Malaysia (myRTKNET was launched recently in May 2005), Australia (in the states of Queensland and Victoria), Taiwan and Japan.