NATIONAL GEODETIC SURVEY
CONTINUOUSLY OPERATING REFERENCE STATION PROJECT
STATUS AND PLANS
Paul R. Spofford
Neil D. Weston
National Geodetic Survey
National Ocean Service, NOAA
ABSTRACT
The National Geodetic Survey, National Ocean Service, NOAA, is assembling a multi-agency, cooperative,
Global Positioning System (GPS) base station network. The Continuously Operating Reference Station (CORS)
network supports post-processing activities in Federal, state, and local arenas across the nation with GPS carrier and
code range data from more than 100 sites. All data are available from one Internet distribution location. The
current network status and plans for the future will be discussed.
BRIEF HISTORY
NOAA's "National Geodetic Survey (NGS) has as its primary responsibility, the development and maintenance
of the National Spatial Reference System (NSRS). NSRS is a consistent national coordinate system that provides
America with a . . . foundation for all positioning requirements on, below, or above the Earth's surface. Most
commonly recognized as the latitude, longitude, and elevation contours on maps and charts, NSRS has become
increasingly important for delivering real time positional information to support transportation, emergency 911,
electric power distribution, and even for increasing bandwidth on the Internet."
"Like other basic Earth properties, the coordinate system is dynamic, affected on a daily basis by crustal
movement, glacial rebound, and polar motion. It has also been profoundly affected by changing technology. The
Global Positioning System (GPS) has had an almost revolutionary effect on our ability to position things more
accurately and with less time or effort. The National Spatial Reference System is in a transitional period, it must
become more accurate in all three dimensions and become more accessible for the utilization of GPS." (Lapine,
1997a)
In the early 1990s, NGS began to rethink its traditional method of making the NSRS available to the Nation.
NGS had converted its positioning field parties to GPS operations and was involved in the upgrade of NSRS to
support those activities. An analysis of the North American Datum of 1983 (NAD 83) had shown that the first order
framework was better than expected with a relative accuracy of about 1:250,000, but the accuracy fell short of the
capabilities of GPS-based surveying operations which regularly exceeded relative accuracies of 1:1,000,000. In
addition, many traditional "high accuracy" triangulation survey marks were in isolated, GPS unfriendly locations,
such as mountain peaks. Triangulation marks (or stations) had been established using traditional surveying
techniques which required extended lines of sight for the interconnection of nearby survey stations. Easily
accessible national survey control points were generally of low accuracy and/or were intersected radio towers,
church steeples, grain silos, or similar objects. None of these low accuracy control points were useful for GPS
operations. The rationale of the time had been to raise the primary survey points as high as possible. Natural
features such as mountain tops were used to extend the lines of sight and by placing the marks in remote locations
we protected the survey marks from vandalism. NGS had estimated that the cost of establishing a first order control
point was as much as $10,000. Reestablishing a lost or damaged monument to its former accuracy without a
significant investment of personnel time and money was difficult.
The upgrading of NAD 83 to support GPS operations was done cooperatively on a statewide or regional basis
with state and local agencies. The upgrade surveys, known as High Accuracy Reference Networks (HARNs),
showed very clearly that we needed easily accessible, highly accurate control stations to support the effort. Early
GPS receivers were very expensive (in excess of $100,000 each) and very heavy by today's standards and were
usually brought to control stations in specially outfitted trucks. Traditional control points could not be used.
However, GPS base stations, or fiducial points, had been used since the 1980s to support the computation of
globally precise GPS ephemerides (orbits) and in the early 1990s to support local surveyors in code range
operations. After much discussion in the early 1990s, NGS decided to establish a system of national GPS fiducial
points to support the HARN projects and to support local surveys, where possible, for both code range and carrier
phase surveys. Other Federal agencies like NASA's Jet Propulsion Laboratory (JPL) were already heavily invested
in continuously operating GPS stations which supported GPS orbit computations and the study of crustal motion
areas such as Southern California. We studied the number of required sites to cover the coterminous 48 states and
we estimated, at that time, that about 100-150 sites in a 200-km grid would be sufficient. In addition, we anticipated
there would be a need for closer spacing in areas where episodic crustal motion was occurring, in areas of surface
ground subsidence was occurring due to sub-surface fluid withdrawal, and where population density was greater.
The cost of such a system was daunting for an agency the size of NGS. Geodetic quality GPS receiver systems
by then were about $35,000-50,000 each, and we had the challenge of installing, maintaining, and operating a large
group of receivers. It meant a capital outlay of as much as $7,500,000, which was about 33% of NGS' annual
budget. On top of this, significant personnel expenses would have to be added. Serendipitously, two other Federal
agencies were going through the same thought process - but for different reasons. The U.S. Coast Guard (USCG)
wished to upgrade its radionavigation system using a Differential GPS (DGPS) system to promote and support safe
marine navigation in or near U.S. ports and coastal waters (USCG, 1996). The U.S. Army Corps of Engineers
(USACE) wanted a cost efficient navigation system to support their inland waterway operations (dredging,
hydrographic surveys, etc.). Because of project similarities, the U.S. General Accounting Office directed the three
agencies (NGS, USCG, and USACE) to work together and to coordinate activities and equipment procurements to
reduce the expense to the Federal government and the U.S. taxpayer.
NGS found itself then in an advisory role helping USCG and USACE define the receiver specifications needed
to support the missions of all three agencies. The CORS program had the most stringent receiver requirements and
both USCG and USACE agreed to support those requirements. Thus, NGS obtained access to more than 50
geodetic quality GPS receivers when they were installed without the need to install, maintain, or operate any of the
systems. NGS began the CORS network with a single site in February 1994, when Trimble Navigation Corporation
installed one of its geodetic receivers at the National Institutes of Standards and Technology, in Gaithersburg,
Maryland, on a long term loan. The receiver, still on loan, was upgraded in 1997. NGS installed a receiver 6
months later near Boulder, Colorado. We had also established a number of continuously operating GPS receivers as
part of the Cooperative International GPS Network (CIGNET). Data from both CIGNET and JPL sites were used to
support global GPS orbit computations. All data were available on the Internet and selected sites within the United
States were added to the CORS network.
USCG and USACE began installing receivers at their sites in 1995. The USCG network was largely completed
by January 1996, although both USCG and USACE have added sites since then. Other Federal, state, and locally
sponsored continually operating receivers were identified and added to the network gradually from 1995 through
1997. The Texas Department of Transportation provided significant coverage in Texas with their 10-station
Regional Reference Point network, established well before the CORS network. By making contact with these
agencies and arranging to exchange data, NGS expanded the network to 108 sites by December 1997. (see Cors Coverage Map)
For those agencies whose sites are included in the network, NGS has computed highly accurate three-dimensional positions
in the NSRS of the L-1 phase centers of the site antennas, provided a national data distribution mechanism,
monitored the positions of the antennas on a daily basis, and notified the agencies when we
detected movement of the antennas. In exchange, the agencies notified NGS when they made equipment or
software changes so that NGS could keep CORS users abreast of the status of the CORS sites. Scientific users monitoring
very small movements of the Earth's crust are especially interested in any equipment changes so that
they can account for those effects when they undertake long-term analyses of site locations.
THE PRESENT
We can see a demonstration of the growing popularity and impact of the CORS program in the increase in
CORS data usage recently. "Below is a summary showing the increase in CORS data usage over the last 2 years.
The summary shows the amount of data distributed during the month of September in 1995, 1996, and 1997. [The
units are Gigabytes (1,000 megabytes).]
Users Sept. 1995 Sept. 1996 Sept. 1997
Academic 0.56 3.33 11.40
Other 0.44 0.72 5.68
Total 1.00 4.05 17.08
Notice that non-academic users have increased the rate of their data downloads by almost ninefold in the last year.
It appears to be an important indication of how CORS usage is beginning to take hold in the commercial and
government communities." (Strange, 1997) During September 1997 data were accessed by 1,600 identifiable
E-mail addresses and many users whose E-mail addresses or methods of access we could not track.
"NGS collects and distributes GPS observational data from a nationwide network of permanently operating
GPS receivers. Because these stations meet rigid standards for instrumentation and operation and are monitored
continuously, and sufficient data has been amassed to enable us to establish their positional accuracy at the
centimeter level - coast to coast. CORS has been declared by NGS as absolute, i.e., without error, relative to all
other positions. That means that any new positions determined relative to CORS will have only the error associated
with the relative measurement between CORS and the new point. For surveyors and mapmakers requiring the
highest accuracy, this means that their work can concentrate on the local survey requirements without the need to
build in additional checks and balances to ensure the accuracy of their final product. It also means that surveyors do
not need to allocate GPS receivers and personnel to occupy existing control stations." (Lapine, 1997b) The
leveraging effect of the CORS sites can have a significant economic savings impact on many survey projects. We
foresee that, for some applications, a surveyor with one GPS receiver will be able to observe an entire project by
him or herself. The density of CORS sites is not sufficient yet in many areas to allow such a survey, but the
capability is nearly here.
POLICIES
We have developed several operating policies along with the CORS network. Several have been in effect from
the earliest days. The rest have evolved in response to the growth of the network and requests received from the
user public. These policies include:
NGS will provide GPS receiver data and site coordinates on line to the public free of charge for the
foreseeable future.
NGS will convert all receiver-specific data formats to the Receiver Independent Exchange (RINEX)
format, version 2.
NGS will provide site-specific meteorological data, where available, in RINEX format.
NGS will store data from all sites on line and make the data available to the user public for 31 days. At the
end of that period, to regulate computer data storage requirements, the data will be moved off line and
archived on CD ROMs. However, CORS users may request temporary restoration of limited amounts of
data back on line for up to 7 days. Large data restoration requests will be evaluated. If NGS considers the
request beyond the ability of CORS personnel to fulfill easily, NGS will only provide copies of CD ROMs
for a published fee. (NGS, 1996)
NGS will collect GPS data at most sites at a 30-second epoch rate. For a few sites, the data will be
collected at a 5-second epoch rate. For the latter sites, NGS stores data on line in hourly and daily files.
When we archive the data, we decimate all files to the 30-second epoch rate and save only the daily files.
If we have not generated daily files by that time, we will create them. NGS assumes that users who require
5-second data will have downloaded that data during the initial 31 days. The archive CD ROMs will then
have standardized data files and less duplication of data will occur. This process maximizes the data stored
on each archive CD ROM.
New sites will be added to the network according to a set of published set of guidelines which specify
minimum communications and equipment requirements. (NGS, 1996 (updated 1997))
PARTICIPATING AGENCIES
Many organizations representing Federal, state, and local governments, academic institutions, and private
industry are participating in the CORS project. The number of organizations is expected to increase in the future.
This representation provides a wider breadth of experience, and ensures that the project is dependent on no one
agency, communications system, or GPS equipment type. The drawback of such a system is the need for closer
communications with the participating agencies.
Participating Federal agencies include NOAA (National Geodetic Survey and Forecast Systems Laboratory),
NASA (Jet Propulsion Laboratory and Goddard Space Flight Center), U.S. Coast Guard, U.S. Army Corps of
Engineers, and the Federal Railway Administration (FRA). State agencies and academic institutions include North
Carolina Geodetic Survey, Pennsylvania Department of Transportation/Pennsylvania State University, Southern
Polytechnic University (GA), Texas Department of Transportation, University of California at Berkeley, University
of Maine at Orono/SATLOC, Inc., University of Utah/University NAVSTAR Consortium, Vermont Agency of
Transportation, and Virginia Department of Transportation/SATLOC, Inc. Local and private agencies include
Harris-Galveston Coastal Subsidence District (TX), NCAD Corporation (KY), Riverside County Flood Control
District (CA), and SATLOC, Inc.
DATA ACCESS
All GPS data collected as part of the CORS network are available to the public via the Internet. Access to the
NGS FTP and World Wide Web sites may be achieved using the following electronic addresses:
FTP: cors.ngs.noaa.gov WWW: http://www.ngs.noaa.gov
Within the web site click on "Products and Services" and then "GPS Continuously Operating Reference Station
(CORS)." We recommend that the user download and review the Readme file and the Frequently Asked Questions
file on the first visit to the web site. Software to work with the GPS data files is available through both methods of
access. Users may transmit E-mail messages to the CORS staff while logged onto the web site.
Each Sunday, NGS generates a weekly newsletter reporting new and recent events which affect the CORS
network. In addition, the newsletter contains a table showing how many data files were collected at each site per
day for the preceding week. The table does not indicate data quality or completeness, but is a guideline to show
whether or not we have received any data. For some sites, the lack of an entry means only that the controlling
organization has not yet processed or released the data. The user may retrieve the newsletter from the Web site as
needed or the user can request that the newsletter be sent to him or her by E-mail when we generate it.
NGS visits each non-USCG/USACE site electronically hourly or daily to download new data files. We also
check to see if any data files are missing at each site for the previous 6 days and to try to retrieve the missing file(s).
After 6 days we assume that the data could not be saved at the site, have been lost, are corrupted, or are unavailable
for some other reason.
Within the NGS FTP site, the README.txt and FAQ (Frequently Asked Questions) files are found at the login
level. To download data change to the "rinex" directory, and then select the directory for the day of year needed.
For example, one could select 97330 or 98001, i.e., year/day of the year. Find the directory of the site needed and
change to that directory, e.g., gait. Download the file(s) needed. We have compressed all files using the utility
GZIP, so one must pay attention to the naming convention and download in binary mode. Helpful software,
including GZIP, is available in the UTILITY directory for DOS, UNIX, and Windows NT platforms. We support
several versions of UNIX.
ORGANIZATION
Some NGS personnel support the CORS project by interacting with the various components of the hardware
and software systems. Computer systems experts keep the hardware running as part of their support mission in
NGS. Procurement personnel help with equipment upgrades and replacements as needed - and as resources allow.
Senior NGS management provides administrative support and program funds to keep the CORS effort operating.
However, central to all the operations are a small group of personnel who keep the CORS program running. Two
senior geodesists interact with NGS management, outside organizations, and other CORS personnel to try to keep
the program operating smoothly. A computer/technical specialist writes unique software, establishes and maintains
communications with the various sites, and monitors daily coordinate computations and computer status. Two
geodetic specialists inrchive data, monitor data files, temporarily restore data for users after
it has been moved off line, and monitor, upgrade, and maintain the CORS portion of the NGS web site. Two
geodesists monitor daily site coordinates, long-term site coordinate variations, certify new site coordinates, research
and resolve local site interconnection problems, and compute new site coordinates. Two computer programmers
write operating systems and monitoring programs, monitor the specialized telephone connections, interact with the
telephone company, monitor computer loads, and research and initiate computer procurements and upgrades. Nine
people are directly involved with the project in part or in whole and, as often happens, all personnel are involved
with other NGS activities as well.
A list of the CORS personnel can be found on the CORS web page along with their telephone numbers and
E-mail addresses. They may be contacted for questions about sites, data restoration, problems, etc.
THE FUTURE
Although the CORS network has grown to more than 100 sites located in 35 states, Guam, Puerto Rico, and the
U.S. Virgin Islands, there are still areas of the country, as seen in figure 1, which lack nearby coverage. As with
many projects, expensive equipment is usually placed where the population density is greatest or where there is
some special scientific interest. Potential users who live in areas of low population density or where there is no
potentially dangerous (i.e., life or property threatening) movements of the Earth's crust are unfortunately the least
served by the CORS network to date. We expect to eliminate most of this sparseness in the next 2-3 years. The
USCG, FRA, and USACE are beginning a joint project to extend the coastal and inland waterways navigation
networks to cover the "dry" portion of America. Negotiations are underway with the Department of Defense to turn
over to the Department of Transportation (home of USCG and FRA) 60-70 Ground Wave Emergency Network
(GWEN) sites for conversion into DGPS/CORS sites. Coverage will be expanded or established in some 33 states
by retaining broadcast equipment on most GWEN sites and moving some equipment to new locations, largely
eliminating current "bare spots" in the CORS coverage.
In addition, NOAA's Environmental Research Laboratory will be installing about 10 additional sites in areas
where weather causes significant problems each year. USACE will be adding 5-10 sites on the inland waterways.
The Federal Aviation Administration will be establishing about 30 sites for its Wide Area Augmentation System
supporting civil aviation. State agencies will continue to establish continuously operating systems supporting the
operations of their Departments of Transportation or Natural Resources. Other sites will become available through
academia and most sites will be CORS compatible. The problem eventually will not be the lack of sufficient
coverage, but what sites to select to give the best coverage with sufficient redundancy. We anticipate that over the
next 2-5 years, the CORS network will expand to between 200 and 250 sites. At some point redundancy in some
areas may become excessive and we may eliminate some sites from the network.
While NGS encourages all agencies to maintain CORS sites for a long period, we are also realistic enough to
know budget shortfalls, technology advances, changes in priorities, and other equally valid reasons will terminate
some sites. It is rare for all current sites to operate simultaneously and without problems. Weather affects sites year
round (snow and ice buildup, lightning strikes, tornadoes, hurricanes and typhoons, etc.) with effects ranging from
equipment destruction to deliberate shutdowns or antenna movement. Communications problems add to the
complexity when Internet connections are interrupted, telephone service is disconnected, computer software fails,
computer hardware fails, and other hard to predict site-specific problems occur. Many sites have on site data and
power backups, and data interruptions and/or apparent losses are only temporary.
USCG and USACE sites do not fall in the latter category, however. Those stations have no on site data
backups. NGS receives the only copy of the data, "real time," from the sites by direct telephone line. Interruptions
of the data flow from these sites, for whatever reason, result in real and permanent data losses with no possible
recovery from backup sources. USCG and USACE minimize equipment problems, however, since two "hot"
receivers are operating at each site and two Master Control Stations monitor the network - one in Virginia and one
in California. When USCG or USACE receivers experience a problem, a USCG systems operator immediately
switches over to the backup receiver.
NGS has established two identical "hot" data collection computer systems at its Silver Spring headquarters.
These systems, on two different floors in the same building, work in parallel - each system monitoring the other,
looking for systems problems. Each system contains identical copies of the incoming data. We switch to the
backup system when computer problems occur or when the prime system requires maintenance.
No system is completely safe from problems, however, and the CORS program does have recognized
weaknesses. By having both computer systems in the same building we could lose power or telephone connections
for both computers simultaneously. We connect both systems to the same Internet node and a loss of the local
connection would affect service to CORS users. Data from the USCG/USACE sites come through the national
telephone system and interruptions locally at the GPS receiver sites or in Silver Spring could result in data loss.
NGS employees do not work on weekends or holidays, and hardware problems which could occur during those
times might go undetected for several days. We do not believe that we are taking an undue risk, however. Loss of
our Internet connection would only mean a temporary delay of data delivery from some sites and inconvenience to
CORS users. We mitigate problems due to a temporary loss of power by using uninterruptable power sources
(battery backups). A long-term loss of power would probably suggest a problem beyond the ability of NGS to deal
with. Loss of the telephone connection locally at a CORS site is beyond our control and, for some sites, occurs
regularly - much to our discomfort. Loss of the telephone connections at Silver Spring is a rare occurrence and,
again, is beyond our control.
We have considered moving one of the parallel computer systems, with telephone and Internet connections, to a
site other than in Silver Spring (different power grid, telephone lines, and Internet connections). However,
budgetary restrictions must be considered and NGS does not have a secure second site to which we could move a
computer system. In addition, the problem of having personnel at a second location solely to monitor a backup
computer system would be hard to justify given the relative lack of systems problems to date. Therefore, we do not
anticipate moving any of the main computer equipment.
SUMMARY
The NGS CORS project is a growing, multi agency endeavor designed to aid GPS users in their post-survey
processing activities. In about 3 years we have extended continuous GPS data coverage to most of the nation.
Within the next few years, we will cover the coterminous 48 states with a relatively dense network of continuously
operating GPS stations with data available from a single Internet location. Coverage exists in parts of Alaska,
Hawaii, and Puerto Rico and we expect increased site availablility there as well. Barring unforeseen budgetary
problems over the next 3-5 years, the project will grow from the current 110 sites to about 200-250 sites.
REFERENCES
Lapine, Lewis A., Capt., NOAA; briefing paper for NOAA Senior Management, pp. 1, September 22, 1997(a).
Lapine, Lewis A., Capt., NOAA; briefing paper for NOAA Senior Management, pp. 2, September 22, 1997(b).
Strange, William E., Chief Geodesist, National Geodetic Survey, NOAA; Intra-office E-mail, October 6, 1997.
NGS CORS Special Data Request Policy; National Geodetic Survey, NOAA; World Wide Web Site; Silver Spring,
Maryland; October 25, 1996.
National Geodetic Survey CORS System, Station Selection Criteria; National Geodetic Survey, NOAA, Silver
Spring, Maryland; World Wide Web Site; August 22, 1996 (updated July 14, 1997).
The U.S. Coast Guard's Radionavigation History; Differential GPS System Initial Operational Capability
Ceremony; U.S. Coast Guard, Electronics Engineering Center, Wildwood, New Jersey; January 1966.