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Frequently Asked Questions about the NOAA CORS Network (NCN)

  • Decommissioning NGS Public FTP Service

    Be advised that beginning on/after August 16, 2021, any FTP attempts made to NGS CORS will fail.

    NGS provides access to all NCN data collected since 9 February (040) 1994. The data include observation, meteorological, navigation/ephemeris, station logs and NGS coordinate files for the stations.

    General Access Methods:

    Example using cURL for downloading RINEX data

    • From NOAA-NCN AWS:
       curl -O https://noaa-cors-pds.s3.amazonaws.com/rinex/YYYY/DDD/ssss/ssssDDD0.YYd.gz

      e.g.: curl -O https://noaa-cors-pds.s3.amazonaws.com/rinex/2021/027/ab02/ab020270.21o.gz

    • From NGS servers: 
       curl -O https://geodesy.noaa.gov/corsdata/rinex/YYYY/DDD/ssss/ssssDDD0.YYd.gz

      e.g.: curl -O https://geodesy.noaa.gov/corsdata/rinex/2021/027/ab02/ab020270.21o.gz

    • where:
      • -O : save file to local system using remote filename(s)

    More details about NCN data and products at NCN Data and Products page.

  • A .gz file is an archive file compressed by the standard GNU zip (gzip) compression algorithm. You can open .gz files with the GNU zip (gzip) compression utility. You can also decompress .gz files and extract their content with other decompression utilities such as 7-zip (Windows) and Apple Archive Utility (MacOS).

  • Station Log! Also known as site log or logfile.

    We have that information in the station's logfile. Please visit NCN GPS/GNSS Data and Products page, section Station Logs for more information.

  • Please visit International GNSS Service (IGS) Formats and Standards for more information.

  • We now keep all of the data that we have online. It is possible that we were not able to get some data due to many reasons. Since many of our stations are owned and operated by other agencies, it may be possible to retrieve data from those agencies. You can find the responsible agency from the station log file.

  • Try our User Friendly CORS (UFCORS) to get customized dataset.

  • The National Geodetic Survey has designed UFCORS to facilitate the retrieval of GPS data from the NOAA CORS network (NCN) . In particular, UFCORS enables users to:

    • Obtain CORS data for an exact time interval specified in international or local time,
    • Choose a sampling rate for the requested data,
    • Specify how the data should be compressed to speed data transfer to the user,
    • Receive the requested data within a few minutes,
    • Obtain pertinent descriptive text (log files and datasheets) for CORS stations automatically,
    • Retrieve adopted NAD83 and ITRF positions and velocities for CORS stations automatically,
    • Obtain meteorological data for those CORS stations that have "met" sensors automatically, and
    • Retrieve both the highly accurate GPS satellite orbits calculated by the International GNSS Service for Geodynamics (IGS) and the "broadcast" GPS orbits automatically.

    A utility connected with UFCORS automatically provides the CORS data exactly as requested, saving users from tedious data manipulation and file management tasks.

    The National Geodetic Survey welcomes suggestions for improving UFCORS.

  • If the four-character identifier for a particular station does not appear in the list, then either the National Geodetic Survey (NGS) does not distribute GPS data for that station or NGS does not have GPS data for the requested time period for that station. Using the starting date, starting time, and duration that you specify, the UFCORS software looks at the records for each station to determine whether or not GPS data are available for the corresponding time period. If GPS data are available, then the station's name is added to the list.

  • We supply one "global" navigation file each day which can be used for all GPS processing. Visit the NCN Data & Products page, Ephemeris and Orbit section for more information.

  • The "Broadcast" orbit (navigation file) information is sent by the satellites. Since the satellite's clock may be off by as much as 7 nanosecs, its position could be off as much as 210 cm. The precise orbit is calculated after the fact by several analysis centers around the world ( the National Geodetic Survey has its own analysis center located in Silver Spring, MD). A combination of the precise orbits yield a global precise called the IGS orbit and is good to less than 2 cm !

    Visit the NCN Data & Products page, Ephemeris and Orbit section for more information about our GPS/GNSS Orbit files.

  • Once a station has passed the test to become a NOAA/NGS (National) CORS, a daily solution will be run for a minimum of 12 days. After that time the coordinates will entered into the NGSIDB and a coordinate file will be produced.

  • A "coordinate" file for a CORS contains the officially adopted position and velocity for the station's antenna reference point (ARP) as well as for its L1 phase center. Hence, this file is also called a "position-velocity" file. This file may also contain officially adopted positions and velocities for selected geodetic monuments located near the station. Note that the ARP usually corresponds to the center point on the bottom of the pre-amp on the CORS antenna.

    A coordinate file presents positions and velocities in both the International Terrestrial Reference Frame (ITRF) and the North American Datum of 1983 (NAD 83). Also, it presents positions in both geocentric cartesian coordinates (X, Y, Z) and in geodetic coordinates (latitude, longitude, and ellipsoidal height) for the GRS80 ellipsoid. NAD 83 positions given in these files are identical to those contained in the National Geodetic Survey's Integrated Database

    See NCN Data and Products page, section Published Coordinates and Velocities, for more information.

  • Please visit our Multi-Year CORS Solution 2 (MYCS2) Coordinates page for more information about our most current reprocessing campaign.

  • We supply 2 positions for our CORS users: the L1 Phase Center (L1PC), and the center of the bottom of the antenna which we call the antenna reference point (ARP). We supply these positions in 2 reference frames, NAD83 (2011,MA11,PA11)epoch 2010.00 and ITRF2014 (epoch 2010.0) . We do not use the WGS84 to position points. You can read more about WGS here: Mordern Terrestrial Reference Systems: WGS84 and ITRS

    The published coordinates and velocities of each CORS are stored in the coordinate file. See NCN Data and Products page, section Published Coordinates and Velocities, for more information of how to access the coordinate file.

    We recommend that you use the published coordinates and velocities in the coordinate file, and consider the coordinate in the RINEX file header and station logs as APPROXIMATE coordinates. It is the policy of NGS to overwrite the appoximate position in the header with the published NAD83 position, but some older files may not have these corrections.

  • The L1 phase center is the theoretical point in space where the L1 carrier phase is received "on average." The actual location where this signal is received, however, varies as a function of the direction of the incoming GPS signal, and hence an averaging process is required.

    The antenna reference point (ARP) is a specified physical point on the antenna.

    The relative spatial relationship between these two points is determined via a calibration process in a laboratory-type environment. This process involves collecting and processing several hours of GPS data, and it involves several assumptions about antenna characteristics.

    The National Geodetic Survey (NGS) has calibrated many antennas and has determined the average spatial relationship between these two points for each of several classes of antennas. See NGS's Antenna Calibrations (ANTCAL) page for more information.

    For stations in the NOAA CORS Network, NGS provides spatial coordinates for both the L1 phase center and the ARP, as different GPS-processing software packages may use one or the other of these points or possibly even some other reference point.

    To find out whether your GPS-processing software uses the L1 phase center or the ARP, you need to contact the company that produced this software.

    NGS encourages GPS companies to have their software use the ARP, but our agency can not require this practice.

  • The Antenna Reference Point (ARP) is the physical point on the antenna to which antenna calibration values are referenced. ARP is preferably an easily accessible point on the lowest non-removable horizontal surface of the antenna. Typically, the ARP coincides with the axis of attachment of the antenna to a monument or surveying instrument.

    The ARP can be a point at Bottom of Antenna Mount (BAM), or Bottom of Chokering (BCR), or Bottom of PreAmplifier (BPA), etc..

    For detailed information, please visit IGS's Antenna Working Group web page for more documentation about antenna calibration, ANTEX - the Antenna Exchange Format, IGS naming convention for GNSS equipment, and ARP/BPA/BCR.. definitions.

  • We agree that the Brand X antennas are fairly symmetric but we still require the level and orient device. Basically the environment around the antenna will bias this calibration a bit and we want to ensure that over time other antennas which may or may not be symmetric, have the correct phase center applied, otherwise we can get a 1+ cm of distortion. In addition the orient and level device allows the antenna to always be returned to the same position in 3-D space on the antenna which is critical.

  • Here is the value for your coordinates as computed at Magnetic Field Calculators
    Declination = 12º 32' W changing by 0º 1' E/year

    So if you put the antenna aligned to magnetic north then the log file should read -12.53 deg . The best solution is when first putting up the antenna, to always align it with true north, that is, align it with magnetic north then turn it (at this location) 12.53 degrees clockwise. However, it is best not to touch the antenna once it is set it place and collecting data.

  • You should use the coordinate and the method that minimizes the confusion and the amount of work required of you. Most people find that the ARP coordinate fits that description.

  • NGS does not provide streaming services at this time.

    The NCN is solely for static post-processing endeavors. However, many of our contributors such as state DOTs, research organizations, and commercial partners might provide RTK/RTN streaming services with or without subscription fees. Below are some of our providers with public RTK/Network RTK service: (last visited: 13MAY2023)

    Public Network RTK and/or Single-base RTK:

    Public Single-base RTK:

    • UNAVCO GPS/GNSS Real-time Network by UNAVCO. UNAVCO publicly broadcasts real-time streaming GPS/GNSS) data from some stations in the Network of the Americas (NOTA), locating in many states such as AK, AR, AZ, CA, CO, HI, ID, KS, MT, ND, SD, UT, WY and more. See UNAVCO Real-time GPS/GNSS Data page for more information.

    Commercial Network RTK and/or Single-base RTK:

  • We are working on revising the Guidelines for Establishing, Operating, and Maintaining Continuously Operating Reference Station (CORS)!

    Send us an email at ✉ ngs.proposed.cors at noaa.gov with Proposed CORS Site in the subject line before installing your CORS!. Please include a short description of the site, its location, and contact information in the email.

    Our soon-to-be-retired NCN Guidelines for Establishing and Operating CORS can be found at:
    https://geodesy.noaa.gov/CORS/Establish_Operate_CORS.shtml

  • NO!

    Your station is an existing station with a published coordinate for the ARP (antenna reference point or the bottom of the antenna). When the ARP is relocated in 3-D space, such as either by reconstructing the monument that supports the antenna or by adding an adapter between the monument and the antenna, then the prior location in 3-D space has been destroyed. The antenna will have moved too much to simply ignore the movement and continue to use the same published coordinate.

  • Yes. This potential station might be rejected.

    The antennas require a clear view of the sky with minimal obstructions and minimum likelihood of change in the environment surrounding the monument, e.g avoid location with future tree or shrub growing, building addition, rooftop addition, or new antenna masts, etc.. This is because the greater the volume through which uninterupted/unreflected signal can reach the CORS's antenna, the greater the likelihood of a robust position estimate. Therefore, no lightning rods, other RTK broadcast antennas, or any objects should extend above the CORS antenna or be anywhere within 3 meters of the CORS antenna, and all should be below the - degree of the horizontal surface containing the CORS's ARP.

  • Most of the handheld units designed for recreation purposes are good for general location and navigation. They are not designed for precise data collection. The location coordinates are determined from using one GPS receiver without the benefit of corrections from a secondary source, it is known as single-point positioning. Consumer grade handheld GPS units generally provides coordinates with around 3 meters horizontal accuracy and usually with poor vertical accuracy. Users can get more realiable solutions by tracking more satellites from a secondary satellite constellation such as Galileo, GLONASS, etc.. Users can also improve postion accuracy by enabling connection (if capable) with satellite-based augmentation systems (SBAS) such as Wide Area Augmentation System (WAAS, avaialble in North America) or European Geostationary Navigation Overlay System (EGNOS, avaialble in Europe).

    More information about the GPS augmentation systems can be found at GPS.gov.

  • The NCN data has been compressed to save space. After download and decompression, the file should be a readable ASCII file in RINEX format. If you cannot open the file and recognize numbers and words in the file then the transfer and decompression has not worked properly. If the file can be opened and read easily, then the reduction software may not be expecting a RINEX file. Ensure that you are importing the data properly. If all else fails contact the manufacturer of your software.

  • This problem may result from several causes but the chance the NCN position is seriously in error is very small. First, you must ensure that you are comparing NAD 83 positions with NAD 83 positions or the equivalent. Different datums can introduce over 100 meters in position differences, e.g. NAD 27 to NAD 83. If the datums are the same, are you trying to compare an uncorrected point position (yours) with an NGS published position? An uncorrected point position can be in error by as much as 100 meters (horizontally when SA was on) 95% of the time due to Department of Defense signal dithering. If that is not the problem, have you checked to ensure that the reference position you think you are using in your computations is actually being used by the reduction software? The positions in the RINEX files are ONLY APPROXIMATE and may be tens of meters or more in error. You must enter into your reduction software an accurate position from the appropriate coordinate file. There are other possible problems, but these are the most obvious and most common.

  • There are two types of CORS stations which fit this naming problem. Those that are in a suburb of some large city where the major city name was used and those stations which were funded by the US Army Corps of Engineers (now by the U.S. Coast Guard). These are named after the local district headquarters.

    Some stations include:

    • Louisville, KY (LOU1 & LOU2) - Taylorsville, KY
    • Memphis, TN (MEM1 & MEM2) - Frenchman's Bayou, AR
    • Kansas City, MO (KAN1 & KAN2) - Perry, KS
    • Omaha, NE (OMH1 & OMH2) - Pisgah, IA
    • Rock Island, IL (RIS1 & RIS2) - Teeds Grove, IA
    • St Louis, MO (STL3 & STL4) - Summerfield, IL
    • St Paul, MN (STP1 & STP2) - Alma, WI

    All Decommissioned