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From the Ground Up

September 1, 2004
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CORS: Still a national asset.

When I last wrote about the Continuously Operating Reference Station (CORS) network in this magazine in August 1998 (see "CORS-A National Asset"), I felt that the leadership of the National Geodetic Survey (NGS), and the professionalism and cooperation of the individual CORS operators had led to the CORS network becoming a true national asset. The old saying is that you don't get anything for free, but in the case of CORS, nothing is further from the truth. The CORS network continues to be extremely beneficial to the surveying, mapping and engineering community. But the benefits of the network extend well beyond the traditional positioning community.

Then and Now

The implementation of the CORS network began in 1994 with the installation of a GPS receiver at the National Institute of Standards and Technology (NIST) facility in Gaithersburg, Md. At the time I wrote the article on CORS in 1998, there were 121 sites in the network. That number has grown significantly today to 479 National CORS and 72 Cooperative CORS sites. The usefulness of the data has grown proportionately with the number of stations.

Richard Snay, manager of the National and Cooperative CORS Program for NGS, noted that, "NGS leadership did anticipate the rapid growth of the CORS network because they have always recognized the importance of GPS to the positioning community. The CORS network grew at a rate of about 30 new stations per year from 1994 to 2001. During the last three years, it has been growing at a rate of more than 100 new stations per year. I estimate that more than 250 new GPS base stations will be established in the United States during each of the next three years."

Moreover, the collection frequencies of the individual CORS sites have undergone significant changes from the early years. In 1998, most of the individual sites collected data at a 30-second sampling rate. There were a few sites that collected at an increased rate of either 5 or 15 seconds. Today, many of the new stations coming online have a 1-second collection rate. This is a significant improvement for many applications including airborne mapping as well as many land-based GPS projects. Snay added that "GPS data is being collected at a 1-second sampling rate at many of the newer CORS because the sponsors of these stations intend to use them to support kinematic positioning as well as static positioning. Indeed, many state and local governments are establishing regional real-time kinematic (RTK) networks in their jurisdictions."

Several states have been extremely progressive in implementing a dense CORS network with higher frequencies of either 5- or 1-second samplings. Take a look at the CORS map on the NGS website. Each site is color-coded based on its collection frequency. The brown sites are 1-second sites; the red collect at 5 seconds. You will notice that Florida, Ohio, Michigan and North Carolina have been very aggressive in establishing a dense network of CORS that have either a 1- or 5-second collection frequency.

Maps available at the NGS website clearly show the general location of the CORS sites and are color-coded to reflect the collection frequency.

CORS and OPUS

In addition to managing CORS, the NGS also provides another significant benefit to the professional community. The Online Positioning User Service (OPUS), located at www.ngs.noaa.gov/OPUS, is a great tool, and again, it is free. OPUS allows GPS users to submit data files electronically to NGS where they are quickly processed against CORS data and the positioning results are E-mailed back to the user.

RINEX and certain receiver specific formats may be used with OPUS. A minimum of two hours of data is required; more data will improve the results. Moreover, only dual-frequency data is supported. Each data file submitted is processed with respect to three CORS sites. The CORS sites are selected by OPUS based on the distance from the observation, site stability and number of observations.

Generally, users can expect positional accuracies of a few centimeters, although there are a number of things users can do to maximize accuracy. The length of the observation is very important. Also, waiting two days to submit the data can prove significant. This is because a more precise post-fit orbit file will be used in the processing when available. A predicted orbit will likely be used if users submit the data soon after the observation. Typically, a more-accurate IGS (International GPS Service) "rapid" precise orbit is available within one to two days of the observation. Finally, an IGS "final" precise orbit is typically available within 10 to 14 days after observation. According to Snay, it is seldom worth waiting more than two days because final precise orbits are only marginally more accurate than the corresponding rapid precise orbit.

Significant Cost Savings

The CORS network can result in significant cost savings for the right project. Our work at Photo Science flying the entire state of West Virginia last year is a great example. We were collecting airborne GPS (ABGPS) data with the photography and needed a dense network of ground-based GPS to accurately post-process that data. We were able to work with individual CORS operators to have them change their collection frequency to one second during our flight window. The spirit of cooperation from each of the individual CORS operators was extremely gratifying.

In all, nine CORS sites in West Virginia, Ohio, Pennsylvania and Maryland were used for our project. We installed two temporary CORS receivers in parts of the state that had little coverage from the existing network. We were able to save the state somewhere between $50,000 and $75,000 through this arrangement with the CORS operators. Logistics were also greatly simplified, as we weren't trying to coordinate the placement of three or four ground GPS personnel for every morning that we had clear weather.

We used a similar strategy for a combination of ABGPS and inertial collection for two seagrass mapping projects in the state of Florida. CORS were used to control these flights along the coast of St Petersburg and in the Florida Bay running along the Florida Keys. Again, the cost savings and simplification of the project were critical.

Innovative Uses for CORS

When asked if they had realized additional benefits from the CORS network that were not originally anticipated, Scott Harris of the Florida Department of Transportation responded, "We started looking at a single functional area, the survey [and] mapping community. Other applications began to emerge. In my opinion, and as far as our network is concerned, the coolest application we have been developing is structural monitoring. So far we have managed to install sensors on two cable-stayed bridges here in Florida. One is at the Dames Point Bridge in Jacksonville and the other is the Sunshine Skyway in Tampa. Both structures are being continuously monitored and being controlled from nearby reference stations from our network."

Moreover, both atmospheric scientists and meteorologists use CORS data in very innovative ways in their profession. "NGS has been pleasantly surprised by the ability of the CORS data to accurately monitor the distribution of free electrons in the ionosphere and the distribution of moisture (water vapor) in the atmosphere," Snay said. "While these applications primarily benefit the weather community, they will also benefit our positioning community. Namely, the ionospheric models being derived from the CORS data may soon enable GPS users to better resolve the integer ambiguities associated with their carrier phase data, even for short observing sessions [of less than 1 hour] over long base lines [greater than 100 km]. Also, the moisture models may soon enable users to obtain more accurate (ellipsoidal) heights from their GPS data."

Tomorrow's CORS

We will continue to see changes and growth with CORS. According to Snay, users currently download about 400,000 data files per month from the CORS archives via the Internet. The total volume of the files downloaded during a month amounts to approximately 400 Gb. Also, users are currently submitting more than 8,500 GPS data sets per month to OPUS for positioning. Snay estimates that the CORS data as of June 2004 provides the positioning community with about $12.5 million worth of benefits per month.

If anything, I think that that estimate of benefit may be conservative. In many ways I think the impact of CORS on our profession is only surpassed by the development of GPS itself. I hope you are putting it to use in your practice.

A great deal of information regarding CORS is available from the NGS website at www.ngs.noaa.gov. Maps are available that show the location and collection frequency of all stations in the network. Easy download tools are available to help users acquire the data needed.

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