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March 1, 2004
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Michigan earns status for some of nation's best CORS.

CORS at the Marine City Airport (CLRE), St. Clair County, Mich.
A new technology has emerged in surveying, and Michigan is leading the way. It is a satellite-based Global Positioning System (GPS) called the National Continuously Operated Reference Station (CORS) network.

Within the national network, the Michigan Spatial Reference System consists of twenty-four permanent CORS across the state, with four residing in St. Clair County. At twelve feet tall, the stations resemble futuristic light poles, mounted on cement bases that run twelve feet into the ground. In other parts of the United States, CORS are located on the roofs of buildings. Testing has found those sites to be less accurate than the Michigan stations that are mounted on a deep cement base, probably due to the instability of a frame building and the ever-present reflective surfaces and obstructions that cause noise in the GPS solution. The CORS system technology is catching on across Michigan and the nation because it saves a tremendous amount of money. This is due to the fact that time spent surveying a site is almost cut in half by using the CORS system.

CORS in St. Clair County, Michigan

Chuck Koob, an engineer at BMJ Engineers and Surveyors Inc., Port Huron, Mich., and Bob Kreger, PLS, an engineer-surveyor for the St. Clair County Road Commission heard about the CORS technology from Rick Sauve, technical sales representative for Leica Geosystems.

At the 2001 Michigan Surveyors Conference, Sauve was informing his customers of a project by the Michigan Department of Transportation (MDOT) to set up the Michigan Spatial Reference System. Rick informed the surveyors that MDOT had agreed to handle all the positioning, integrity monitoring and data archiving of the CORS if counties invested in constructing one. That inspired Koob and Kreger to call a meeting to discuss the idea.

Funding for the construction of the CORS in St. Clair County came through Michigan's Remonumentation Fund. Every time a document is recorded at the Register of Deeds, a fee is collected by the state and given back in the form of a grant through the Remonumentation Fund. Larry O'Keefe, environmental services director and remonumentation grant administrator, spearheaded the CORS project. O'Keefe and Troy Feltman, St. Clair County administrator, worked persistently on the paperwork required for the CORS system.

The project was a joint effort between several county departments: landfill, metropolitan planning, road commission, lands and graphics, drain office and MDOT's transportation design survey division. MDOT was pleased by the CORS project because it will greatly enhance the network across the state. There is no fee to use the CORS; anyone who uses advanced GPS techniques will benefit.

CORS at Smiths Creek Landfill (OKEE), St. Clair County, Mich.

Overcoming Obstacles

The need for a survey crew to set up a temporary base station is eliminated with the CORS system. Traditionally, when a surveyor (the "rover") collects GPS points in the field, a temporary base station is set up on a known (the base) point. By simultaneously observing a minimum of five Navstar GPS satellites at both locations, a very precise 3-D vector measurement is computed between base and rover. When this computed vector (dx,dy,dz) is applied to the base station coordinate, a new rover position is calculated. This entire process of computing a new rover position is performed 10 times a second. The geographic data collected includes latitude, longitude, and elevation, earth-centered Cartesian (X, Y and Z) or local coordinates (N, E, elevation) and is centimeter accurate. Simply, CORS are permanent reference stations with a fixed point on the earth, an unchanging X, Y and Z value.

Under normal circumstances, a standard UHF 450-470 MHz radio signal is emitted ten times per second from each station, reaching in excess of five miles from the base via radio link. However, utilizing these radio frequencies presented a problem in Michigan because St. Clair County shares a 37-mile border with Canada. The Canadian itinerate frequencies are in the 430 - 450 MHz bandwidth, and cross-pollination of U.S.-based itinerated frequencies with Canada's is regulated by international law. As a result of this radio issue, St. Clair County turned to the newest technologies linked to the existing cell tower network and wireless Cellular Digital Packet Data (CDPD) to deliver the real-time kinematic (RTK) corrections to the surveyors working in the county.

The radio frequency problem was a major hurdle in making the St. Clair County project work. The whole idea behind the CORS technology is for a surveyor, base station and a satellite to communicate. The solution, inspired by Brian Breise, information technology (IT) director for St. Clair County, was to use unregulated cellular frequencies for the radio link. Normally, CORS are built with a radio transmitter onsite. Instead, Breise has each of four CORS in the county linked via the IT network to a phone modem bank housed at the St. Clair County administration office building in Port Huron, as well as the state web server in Lansing. A surveyor can use a cell phone in the field to call the phone number for the CORS housed at the administration building. That call accesses the CORS data stream, providing the surveyor with real-time data.

The St. Clair county system is also equipped to transmit wireless CDPD information. In an area with good wireless coverage, a surveyor in the field could access the data from a CORS via the wireless modem built into a GPS receiver. Accessing a station in this way would eliminate the need for a radio or cellular phone, and it would be much faster and easier to operate, eliminating some hardware, cables and batteries. However, densification of CDPD within St. Clair County is an expensive endeavor and CDPD will soon be replaced with the newer Code Division Multiple Access (CDMA) system.

In addition to real-time surveying applications, surveyors can verify and adjust field data by post processing it in coincidence with the data collected simultaneously at the CORS via the MDOT's design survey division CORS website. The other alternative is to collect data in the field and upload it to the National Geodetic Survey's (NGS) Online Processing Users Service (OPUS) website. With at least two hours of collected dual frequency data, the website will E-mail back to the surveyor a precise position of where the data was collected.

This post processing method does not enable a surveyor to extrapolate or stakeout a position in real time. It only facilitates the positioning of fixed locations established in the field. The method of post processing is straightforward and conveniently brings state plane coordinate data to a local survey location, but now surveyors in St. Clair County with a cell phone can immediately get the data in real time, with no post processing.

Reaping the Rewards of CORS: Time and Money Savings

Individuals from private engineering companies like Michael J. Rossow, PS, senior program/project manager of Tetra Tech MPS, Richmond, Mich., are embracing the CORS system. Rossow indicated that the cost of an expensive GPS unit is offset by the fact that one survey staff member spends just 25 percent of the time formerly utilized by a two-person crew in the field, and radios do not have to be purchased because of the cellular link. They are also finding the retrieval of data to be fast, user-friendly and accurate. Leica's representative Sauve said, "GPS crews save countless hours of time by not having to set up their own base station for the day's work and performing sufficient field checks to insure the authenticity of their base station location."

Savings are also achieved when points don't have to be re-surveyed. The values obtained by GPS are "˜fixed in time and space' and can be located by any competent surveyor equipped with a high precision dual frequency GPS receiver. If a point has already been located with GPS, that point may never have to be surveyed again. Any surveyor with a survey grade GPS unit can enter the coordinate values prior to going into the field, eliminating the repetitious need to locate that point again.

The county has also noted the savings created by the CORS system. O'Keefe said the expensive GPS equipment that was purchased to access the frequencies emitted from the towers paid for itself within the first year and a half. These savings are primarily due to a decrease in the time required to perform the survey work necessary in maintaining a landfill facility.

Future Benefits of CORS

The St. Clair County project is creating quite a buzz in the realm of surveying and construction engineering. The county is on the cutting edge of satellite-based GPS, and is considered a model project worldwide, according to Sauve. He said, "The St. Clair County Michigan Spatial Reference Network project is looked upon both nationally and internationally as a prime example of how GPS technology can enable the seamless exchange of centimeter level geo-spatial data between private and government entities." Sauve explained that the St. Clair County reference stations facilitate the use of "one receiver" phase data collection on a contiguous coordinate system.

Sauve also noted that, as the St. Clair County CORS become more recognized and utilized by the government and private sector organizations, more advances will follow. These technological developments could include improved machine control operations, crime scene investigations, maritime construction and precise navigation, county-wide digital terrain modeling and emergency vehicle tracking. "Certainly," he said, "the St. Clair County reference station network will provide the citizens of St. Clair County with applications that have not yet been invented, which will enable cost savings and tax money resources for years to come."

The Michigan Spatial Reference System has already detected minute changes in Michigan's elevation, known to geologists as glacial rebound. The reference stations are also very sensitive to water vapor. The National Oceanic and Atmospheric Administration (NOAA) can analyze data from the CORS and determine the amount of water vapor in the atmosphere, which helps meteorologists determine weather patterns.

NGS Declares Michigan CORS the Best

The NGS, which is a branch of NOAA, studied the CORS network in the United States and its territories. The findings of the study were accumulated in a poster paper titled "Evaluating Pseudorange Multipath Effects at Stations in the National CORS Network" in October 2002. The main objectives of the study were to identify the most affected and least affected sites in the network, to closely investigate problematic sites and to compare various receiver/antenna combinations. Over 390 sites were compared. The study found that the location and mounting base affected the performance of the stations, as well as equipment combinations. Many negatively affected sites had signal interference from other sources.

The least affected sites were the state networks installed in Ohio and Michigan; these sites used excellent antenna mounts, choke ring antennas, and new receiver technology. The CORS located at the Smiths Creek Landfill (OKEE) in St. Clair County, rated the best overall performer in the Americas! In fact, the top sixteen performing stations in the study were all in the Michigan network.

The Michigan network is not only a benefit to those surveyors who use it within the state; anyone using advanced survey techniques needing a reference point can use the system. Surveyors from all over the nation access the OKEE station routinely. On the rare occasion when that station is down, Breise's St. Clair County IT department gets calls from the state of California to fix it! Breise said, "The CORS system brings centimeter accuracy to an industry that not so long ago accepted plus or minus one foot as state-of-the-art; but then again didn't we once believe the world was flat?"

The CORS system project in St. Clair County will save hundreds of thousands of dollars in surveying costs over the next few years while improving accuracy. Potential applications involving these reference stations are only limited to the imaginations of those who creatively apply the technology.

For more technical information on the St. Clair County CORS, see www.stclaircounty.org/offices/cors/.

To learn more about specific case studies on GPS monitoring, visit www.leica-geosystems.com/solutions/engineering-monitoring/monitoring/index.htm.

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