CORS Stations in St. Clair County
By Lori J. Eschenburg, GIS Specialist

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 CORS (Continuously Operated Reference Station) Network.

Within the national network, the Michigan Spatial Reference System consists of twenty-four permanent CORS Stations across the state, with four residing in St. Clair County. At twelve foot tall, the Stations resemble futuristic light poles, mounted on a cement base that runs twelve feet into the ground. In other parts of the United States, CORS Stations 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 which 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. With the CORS System, the need for a survey crew to set up a temporary base station is eliminated. Traditionally, when a surveyor (the ‘rover’) collected GPS points in the field, a temporary ‘base station’ was 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 is latitude, longitude, and elevation, Earth Centered Cartesian (X, Y, and Z), or local coordinates (N, E, Elevation) and is centimeter accurate. Simply, a CORS Station is a permanent ‘reference station’, 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 CORS Station, reaching in excess of 12 miles from the base via radio link. However, St. Clair County shares a 37-mile border with Canada and utilizing these radio frequencies presents a problem. The Canadian itinerate frequencies are in the 430 – 450 MHz bandwidth and cross-pollination of U.S. based itinerated frequencies with the Canadian’s is highly regulated and problematic. As a result of this radio issue, St. Clair County turned to the newest technologies linked to the existing cell tower network and wireless CDPD (Cellular Digital Packet Data) towers to deliver the RTK (real-time kinematic) 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 Station 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, a CORS Station is built with a radio transmitter on site. Instead, Brian has each of four CORS Stations 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, Michigan, 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 Station 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 (Cellular Digital Packet Data) information. In an area with good wireless coverage, a surveyor in the field could access the data from a CORS Station 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 CDMA (Code Division Multiple Access) system.

In addition to “real time” surveying applications, surveyors can verify and adjust field data by “post processing” field data in coincidence with the data collected simultaneously at the CORS Station via the Michigan Department of Transportation, Design Survey Division CORS website. The other alternative is to collect data in the field and upload it to the National Geodetic Survey’s OPUS (Online Processing Users Service) website. With two hours of collected dual frequency data, the website will email 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 stake out a position in real-time. It only facilitates the positioning of fixed locations established in the field.

The method of post processing is very straightforward and is a very convenient way to bring 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 ‘real time’, with no post processing. For more technical information on the St. Clair County CORS Stations, see http://www.stclaircounty.org/offices/cors/.

Individuals from private engineering companies like Michael J. Rossow P.S. of Tetra Tech MPS, Senior Program/Project manager, in Richmond, Michigan, are embracing the CORS System. Mike indicated that the cost of an expensive GPS unit is offset by the fact that one survey staff person spends just 25% 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 very accurate.

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 a 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.

Funding for the construction of the CORS Stations in St. Clair County came through the Remonumentation Fund. Every time a document is recorded at the Register of Deeds a fee is collected by the State of Michigan, 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 Station project. Mr. 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.

Chuck Koob, an Engineer at BMJ Engineering, and Bob Kreger, a St. Clair County Road Commission Professional Land Surveyor, heard about the CORS Station technology from Rick Suave of the Leica Corporation, a manufacturer of GPS equipment.

At the 2001 Michigan Surveyors Conference, Rick 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 a CORS Station if counties invested in constructing one. That inspired Chuck and Bob to call a meeting to discuss the idea.

The project was a joint effort between several county departments: Landfill, Metropolitan Planning, Road Commission, Lands and Graphics, Drain Office, and the Michigan Department of Transportation Design Survey Division of MDOT. In fact, MDOT was very pleased by the CORS System project, because it will greatly enhance the network across the state. There is no fee to use a CORS Station; anyone who uses advanced GPS techniques will benefit.

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 world wide, according to Richard (Rick) Sauve, Michigan Technical Sales Representative for Leica, a leading manufacturer of GPS equipment.

Rick had this to say about the St. Clair County CORS System:

“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 seam-less exchange of centimeter level geo-spatial data between private and government entities. In place and operational, the St. Clair County Reference Stations facilitates the use of “one receiver” phase data collection on a contiguous coordinate system. As illustrated every working day in St. Clair County, 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. By definition, when a GPS field measurement is made in St. Clair County, that position is fixed in both time and space and directly tied to the entire State of Michigan Coordinate System.

As is the case with any government sponsored project, it requires a very high level of dedication and perseverance of a few forward looking individuals to insure the success and completion of the project. In the case of St. Clair County, the project could not have been initiated without Mr. Troy Feltman (County Administrator) and Mr. Larry O’Keefe (Remonumentation Administrator). Both of these fine individuals were persistent in the task of moving the required paperwork through the system which resulted in the most advanced reference station network in the United States.

St. Clair County is considered an averaged size county in the state of Michigan. Despite that fact, they are a definite leader in Reference Station operations in the United States and extremely advanced in the downstream applications resulting from the implementation of the Reference station network, particularly their advanced Geographical Information System.

In addition to the internal utilization of the Reference Station Network, the private sector Professional Land Surveyors utilize the network to improve both the accuracy and precision of their tasks in St. Clair County resulting in a much better product for the citizens of St. Clair County that require their services.

As the St. Clair County Reference Station Network gains popularity within both the various county agencies and the private sector organizations, we can expect many more advanced utilizations of the system including:

Certainly, the St. Clair County Reference Station Network will provide the valued 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 Atmoshpheric Administration (NOAA) can analyze the data from a CORS Station and determine the amount of water vapor in the atmosphere, which helps meteorologists determine weather patterns. To learn more about specific case studies on GPS Monitoring, visit http://www.leica-geosystems.com/solutions/engineering-monitoring/monitoring/index.htm

The National Geodetic Survey, NOS/NOAA, a branch of the National Oceanic and Atmoshpheric Administration, studied the CORS Station 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 Station 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, using the Leica RS500 receiver.

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 mentioned above routinely. We know this because on the rare occasion when that station is down, Brian Breise, Information Technology Director for St. Clair County, gets calls from the state of California to fix it! Brian had this to say about the CORS technology: “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 our imaginations and how creatively we can apply the technology.