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Continuously Operating Reference Station (CORS) |
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In May 2003, USSRC GTAC met to discuss installation of a CORS (Continuously Operating Referece Station) to be monitored at the main campus of the U.S. Space & Rocket Center in Huntsville, AL. The operating range of the CORS would supply coverage that extends 50 miles, capable of serving the Tennessee Valley Experiment Station, which is used, as the field site for this grant. The reference station will do the following:
In January of 2004, the U.S. Space & Rocket Center held and Open House for the public to learn more about the Continuously Operating Reference Station. The Open House was attended by over 100 participants from 22 counties and 3 states. Exhibits included ACES, Tennessee Valley Experiment Station, Auburn University, Leica Geosystems, and the National Geodetic Survey. GPS Reference Stations or CORS are key pieces of the GPS infrastructure. As GPS grows, so does the need for Reference Stations. The key issue in GPS is accuracy. A single GPS receiver can give you a position with accuracy of about 10-15 meters (33-50 feet). While this is sufficient for many people, numerous applications require positioning with significantly higher accuracy. In fact, GPS can readily produce positions that are accurate to 1 meter, 0.5 meter, or even 1-2 centimeters (less than one inch). These higher accuracies are achieved by using a technique called “Differential GPS”. The differential technique requires one GPS receiver to be located at a known position (often called a Control or Reference Point) and a second “Rover” receiver at the location to be measured. The information from the two GPS receivers (Rover and Control) is combined to determine the position of the Rover. The GPS Reference Station functions as the Control Point, and serves potentially unlimited users and applications The GTAC site has been certified as part of a national network of CORS systems coordinated by the National Geodetic Survey (NGS) www.ngs.noaa.gov/CORS , and the data is available at no charge to users through the GTAC website at www.gstac.org. In addition, the USSRC also convert the observational data available to the receiver independently exchange (RINEX) format so that these data are available for use with data from various types of GPS receivers and with various types of GPS processing software, thus expanding the user base. Moreover, the USSRC distributes the data over the Internet accessible through the AgraVision and GSTAC websites, archive the data, and publish site description and coordinate information. A few users for GPS are as follows (All of these applications usually require accuracies better than one meter, and many require accuracy to the nearest inch):
Users usually download the data within a few days after the fieldwork is completed. However, the data sets are available on the website for 90 days. After that period, the data is archived and can be accessed by special request. Real time data is immediately transmitted to users by radios or cellular phones.
A GPS Reference Station is like the anchor point for the GPS activities in the community. It is the foundation for all of the Differential GPS measurements. The GPS users can link data in their Rover units with the Reference Station to establish accurate positions quickly, easily, and economically. In 2005, a statewide initiative to create a network of CORS sites across the state in coordination with the NGS nationwide Cooperative CORS program. The goal of the statewide initiative is to provide a consistent network of GPS correction signal to advance the availability, timely distribution, and widespread use of GPS correction data and technology for practical applications in the state of Alabama. The initiative was officially launched in November 2003 with the U.S. Space & Rocket Center’s GTAC serving as the initial site. The initiative is in the operational stages with the immediate goals of installing new sites and creating cooperative agreements with various agencies that are interested in network participation. Further, the initiative is creating a web-based infrastructure for cost-effective and timely access to correction data to be used for post-process by users across the state of Alabama and the nation. Current Partners: U.S. Space & Rocket Center (GTAC), National Geodetic Survey Cooperative CORS program, Alabama Department of Transportation, Alabama Society of Professional Land Surveyors (ASPLS) Alabama Continuously Operating Reference Station Network -Who will benefit? Many who require accuracy to the nearest inch
A CORS GPS reference station benefits users via: Accurate positions are available 24/7. Think of a crew searching for a water cutoff valve covered by 24 inches of water, at night. GPS users can work at any time, and use the Reference Station to establish accurate results. The crew can find the valve in minutes. It’s as accurate as you need it. Users seeking accuracy of one meter can use the same Reference Station as a person working to 2 centimeters. One size really does fit all. Operating efficiency. In order to provide their own local Control Points, high accuracy GPS users typically need to buy at least two GPS receivers for each field crew. A GPS Reference Station eliminates the need for a second receiver. For surveying and engineering, GPS Reference Stations reduce the need for an extensive network of physical control point monuments. Field crews can start work immediately. There is no need to set up a reference station or rely on survey crews to set up local control points. GPS Reference Stations can serve entire communities. See “Who will us it?” above.
The GTAC Cooperative CORS site happens to be a Leica GPS Reference Stations and is compatible with all major manufacturers. GPS data for post processing is stored in the industry standard RINEX (Receiver Independent Exchange) format. Data for real time GPS can be provided in several formats including RTCM, CMR, and Leica Standard. This ensures support for Leica, Ashtech, Trimble, Topcon, Javad, and other brands of GPS equipment for virtually all applications. The GTAC CORS data is broadcast at 464.50 mHz in CMR format. Brand compatibility provides complete flexibility in selecting GPS equipment for the various applications and is not tied to any single brand or provider.
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