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Frequently asked questions (FAQ)

Anonymous, unencrypted FTP access to the archive was discontinued on October 31, 2020. The CDDIS has provided examples of downloading data using other methods.

This Frequently Asked Questions (FAQ) list provides answers to many popular questions put to the CDDIS staff. These questions are divided into the following categories:

Users are encouraged to check this list for answers to questions routinely posed to the staff. Of course, questions not found here, as well as requests for clarifications to any of these answers, should be directed to the CDDIS manager.

Users are also encouraged to visit the Earthdata User Forum to see if their question has been answered there.

General:

  1. How do I access the CDDIS data archive?
    There are several ways to access the CDDIS archive: Detalied information and examples for using these methods can be found on our Achive Access page.
  2. How can I learn more about the formats used for CDDIS data sets?
    All documents describing the formats of data available through the CDDIS can be found at URL ftp://cddis.gsfc.nasa.gov/formats.
  3. What is the meaning of the .Z extension on filenames?
    The .Z indicates the file was compressed using UNIX compression, e.g., the UNIX compress command. Users on UNIX systems can use the uncompress command to expand the file. Users on non-UNIX systems can consult the gzip web site. Note that ftps transfers of compressed files must be performed using binary mode.
  4. Is the timestamp on archived files local or UTC?
    The timestamp of all files in the archive refers to local time at the CDDIS (EST or EDT).
  5. Where can I find the list of satellite identifiers for SP3 orbit files?
    The list of satellite identifiers for SP3 orbit files is available at URL http://cddis.gsfc.nasa.gov/Techniques/sp3c_satlist.html.
  6. What do all the acronyms mean?
    The CDDIS acronym list is available at URL http://cddis.gsfc.nasa.gov/About/Acronyms.html.
  7. How can I find an address for a colleague?
    Space geodesy contacts can be found in the directory maintained at NASA GSFC.
  8. What is the index.html file found in the archive directories?
    An index.html file is required in order for web browsers to be able to display the contents of a directory in a proper format. This file is used when accessing the CDDIS archive through https, not ftp. There is no harm in downloading this file; however, if you prefer to not retrieve the index.html file, you will need to modify your scripts to exclude any file named 'index.html' when downloading all files from a particular directory.

GNSS:

  1. What type of GNSS data are available from the CDDIS?
    The CDDIS provides an archive of GNSS data in support of the International GNSS Service (IGS, www.igs.org). We have data in daily files (30 second sampling) going back to 1991, hourly files (30 second sampling), and high-rate files (1 second sampling). The daily data are typically delivered to the CDDIS within 1-6 hours after the end of the UTC day; the hourly data within 5 to 20 minutes after the start of the hour. The operational archive of GNSS data consists of the U.S. GPS (Global Positioning System) and Russian GLONASS (GLObal NAvigation Satellite System) systems. In support of the IGS Multi-GNSS Experiment (MGEX), the CDDIS provides access to other GNSS such as Galileo, Japan’s QZSS (Quasi-Zenith Satellite System), China’s Beidou, and Indian Regional Navigation Satellite System (IRNSS) systems.
  2. How can I access the GNSS data?
    The GNSS data are freely available for download through https and other methods. GPS data since January 1998 are available online; older data are offline and can be requested by contacting the CDDIS manager.

    The data are stored in subdirectories by type (daily, hourly, high-rate), by year, and by day of year.

    Access the GNSS data by using one of the following methods: Detalied information and examples for using these methods can be found on our Achive Access page.
  3. What is the RINEX format for GNSS data?
    The "Receiver Independent Exchange Format" RINEX format was first developed by the Astronomical Institute of the University of Berne for the easy exchange of the GPS data to be collected during GPS campaigns. The format consists of six ASCII file types, observation, navigation, meteorological, GLONASS navigation, GEO navigation, and satellite and receiver clock files. Each file type consists of a header section and a data section. The maximum record length for all types is 80 bytes per record. The current version utilized by most stations is Version 2.10 for 30-second data and Version 2.20 for high-rate data. The Version 2.10 documentation contains a complete revision history as well as explanatory text.
  4. How are the daily GNSS data files stored?
    GNSS data are available in daily subdirectories of the form /gnss/data/daily/yyyy/ddd, where yyyy is the four-digit year and ddd is the three-digit day of year. All data are stored in RINEX format.
  5. How can I access hourly GNSS data files?
    Hourly GNSS data files are available through anonymous ftp and other methods. Hourly GNSS data are available for the last three days. These data are removed after that time since files containing a full day's worth of data are available.

    Access the hourly GNSS data by using one of the following methods: Detalied information and examples for using these methods can be found on our Achive Access page.
  6. How are the hourly GNSS data files stored?
    Hourly GNSS data files are available in hourly subdirectories within the file system https://cddis.nasa.gov/archive/gnss/data/hourly/ YYYY/DDD/HH, where YYYY is the four-digit year, DDD is the three-digit day of year, and HH is the numeric hour of the day (00, 01, ... 23). All data are stored in RINEX format. All data are stored in RINEX format.
  7. How can I access high-rate GNSS data files?
    High-rate GNSS data files are available through anonymous ftp and other methods. These high-rate GNSS data are available since May 2001.

    Access the high-rate GNSS data by using one of the following methods: Detalied information and examples for using these methods can be found on our Achive Access page.
  8. How are the high-rate GNSS data files stored?
    High-rate GNSS data files are available in hourly subdirectories within the file system /archive/gnss/data/highrate//YYYY/DDD/YYt/HH, where YYYY is the four-digit year, DDDD is the three-digit day of year, YY is the two-digit year, t is the file type, and HH is the numeric hour of the day (00, 01, ... 23). High-rate data are stored in files containing fifteen minutes of data. All data are stored in the 2.20 enhancement to the RINEX format.
  9. What type of GNSS products are available from the CDDIS?
    Products derived from GNSS data and available from the CDDIS are provided by analysis centers supporting the International GNSS Service (IGS). IGS operational products include precise GNSS satellite ephemerides, positions and velocities of stations in the global GNSS network, EOPs (polar motion and rates, length-of-day), and station and satellite clock solutions. Other products derived from the CDDIS GNSS data holdings include troposphere Zenith Path Delay (ZPD) estimates (both dry and wet components) and global ionosphere products.
  10. How can I access products derived from GNSS data?
    GNSS products are available through anonymous ftp and other methods. GNSS products (preceise ephemerides) since June 1992 are available online.

    Access the GNSS products by using one of the following methods: Detalied information and examples for using these methods can be found on our Achive Access page.
  11. How accurate are the GNSS products produced by the IGS?
    The IGS Central Bureau maintains a table of accuracy information for the official IGS products. Additional information is available from a web page maintained by the Analysis Center Coordinator.
  12. How can I find out what data are archived?
    Users can view CDDIS daily status files, found in the daily GLONASS data directories, which detail data availability for a particular day.
  13. How can I obtain data not currently online?
    Access to GNSS data not accessible through anonymous ftp can be requested by contacting the CDDIS manager.
  14. How can I find out more about IGS stations?
    The IGS tracking network web page provides a list of current IGS sites and links to their site logs; a map is also available.
  15. What is the BRDC (and IFAG) navigation file?
    The file ftp://cddis.gsfc.nasa.gov/gnss/data/daily/yyyy/ddd/yyn/brdcddd0.yyn.Z (yy is the two-digit year, ddd is the three-digit day of year) is the daily broadcast ephemeris file. This file is a merge of the individual site navigation files into one, non-redundant file that can be utilized by users instead of the many individual navigation files. These files are also available in yearly subdirectories of ftp://cddis.gsfc.nasa.gov/gnss/data/daily/yyyy/brdc.
  16. What is the HOUR navigation file?
    The file ftp://cddis.gsfc.nasa.gov/gnss/data/hourly/hourddd0.yyn.Z (yy is the two-digit year, ddd is the three-digit day of year) is a combined broadcast ephemeris file. This file is generated on an hourly basis from all hourly navigation files archived at the CDDIS. The hourly navigation file contains all broadcast messages with the TOE of the day ddd that are available when the file is created at the top of the hour. The file is updated each hour with new navigation messages. At the end of the UTC day, when the final version of the file is generated, the file is copied to the ftps://gdc.cddis.gsfc.nasa.gov/gnss/data/daily/yyyy/ddd/yyn and ftps://gdc.cddis.gsfc.nasa.gov/gnss/data/daily/yyyy/brdc directories and becomes the "daily" broadcast ephemeris file (denoted as brdcddd0.yyn.Z) and described above.
  17. What are the data types in the GNSS daily subdirectories?
    Each daily subdirectory under ftps://gdc.cddis.gsfc.nasa.gov/gnss/data/daily/yyyy/ddd/ contains five subdirectories (yy denotes the two-digit year): yyd (compressed, Hatanaka-compacted RINEX observation data), yym (RINEX meteorological data for a subset of sites), yyn (compressed RINEX broadcast ephemeris files), yyo (compressed RINEX observation data), and yys (summary files of the observation data generated by UNAVCO's TEQC software).
  18. How can I use the yyd GNSS data type?
    The RINEX observation data have been "compacted" using software developed by Yuki Hatanaka/GSI and then compressed using standard UNIX compression. By using the Hatanka software with standard UNIX compression, the RINEX observation files have been reduced in size by a factor of 2.5. Users must first use the UNIX uncompress command (or equivalent software), then run the crx2rnx software to un-compact the uncompressed file; the resulting file is a GNSS observation file in RINEX format.
  19. Where can I find out more about GNSS/GPS data?
    An overview about GPS can be found at The Global Positioning System Overview, Geographer's Craft Project, by Peter Dana, Department of Geography, The University of Colorado at Boulder. Other links about GPS data can be found in the Links section of the CDDIS web site.

GLONASS:

  1. How can I access GLONASS data?
    Data from GLONASS-only GNSS receivers are available from June 11, 2002 (day 2002162) through July 09, 2005 (day 2005190). The GLONASS data are available through https and other methods at two URLs. GLONASS data obtained through the IGEX-98 campaign (August 1998) through June 11, 2002 are available online.

    Access the GLONASS data by using one of the following methods: Detalied information and examples for using these methods can be found on our Achive Access page.
  2. How are GLONASS data stored?
    GLONASS data are available in daily subdirectories of the form /glonass/data/daily/yyyy/ddd, where yyyy is the four-digit year and ddd is the three-digit day of year. All data are stored in RINEX format.
  3. How can I find out what data are archived?
    Users can view CDDIS daily status files, found in the daily GLONASS data directories, which detail data availability for a particular day.
  4. What is the IGEX file?
    The file ftps://gdc.cddis.gsfc.nasa.gov/glonass/data/daily/yyyy/ddd/yyn/igexddd0.yyn.Z (yyyy is the four-digit year, yy is the two-digit year, ddd is the three-digit day of year) is the daily broadcast ephemeris file for the GLONASS satellites. This file is a merge of the individual site navigation files into one, non-redundant file that can be utilized by users instead of the many individual navigation files.
  5. What are the data types in the GLONASS daily subdirectories?
    Each daily subdirectory under ftps://gdc.cddis.gsfc.nasa.gov/glonass/data/daily/ contains five subdirectories (yy denotes the two-digit year): yyd (compressed, Hatanaka-compacted RINEX observation data), yym (RINEX meteorological data for a subset of sites), yyn (compressed RINEX broadcast ephemeris files), yyo (compressed RINEX observation data), and yys (summary files of the observation data generated by UNAVCO's TEQC software).
  6. How can I use the yyd GLONASS data type?
    The RINEX observation data have been "compacted" using software developed by Yuki Hatanaka/GSI and then compressed using standard UNIX compression. By using the Hatanka software with standard UNIX compression, the RINEX observation files have been reduced in size by a factor of 2.5. Users must first use the UNIX uncompress command (or equivalent software), then run the cmp2rnx software to un-compact the uncompressed file; the resulting file is a GLONASS observation file in RINEX format.
  7. How can I access products derived from GLONASS data?
    GLONASS-only derived products are available through anonymous ftp and other methods. GLONASS products (precise ephemerides) since October 1998 are available online.

    Access the GLONASS products using one of the following methods: Detalied information and examples for using these methods can be found on our Achive Access page.
  8. Where can I find out more about GLONASS?
    The Russian home page on GLONASS is the official source of information about the system.

SLR:

  1. How can I access SLR data?
    Satellite laser ranging data are available through several methods. SLR data since 1976 are available online.

    Access the SLR data using one of the following methods: Detalied information and examples for using these methods can be found on our Achive Access page.
  2. How are SLR data stored?
    SLR data are available in subdirectories by data type, satellite, and year. The data type supported by the ILRS is on-site normal points, found in directories at URL https://cddis.nasa.gov/archive/slr/data/npt_crd.
  3. What are the data types in the SLR subdirectories?
    The primary data product for SLR is the on-site normal point data format. These data can be found a URL ftp://cddis.gsfc.nasa.gov/slr/data/npt in subdirectories by satellite and year. SLR full-rate data are available from 1976 through the present; however, since 1994, only a small subset of the global network are providing full-rate data. The full-rate data can be accessed at URL ftps://gdc.cddis.gsfc.nasa.gov/slr/data/fr_crd.
  4. How can I access hourly SLR data files?
    Hourly SLR data files are available through anonymous ftp at URL ftps://gdc.cddis.gsfc.nasa.gov/slr/data/npt/allsat/ in subdirectories by four-digit year. The filenames are of the form new_qlyymmdd#.allsat where yymmdd is the date and # is the hour of the day (a-x). Each file contains all the data received at the ILRS operations center within the last sixty minute period. Therefore, the hourly file could contain several hours worth of data. Hourly SLR data are available for the last five days. These data are removed after that time since the next day's daily files also contain these data.
  5. Where can I find out more about SLR and LLR?
    An overview of the SLR technique has been written by NASA. The home page of the ILRS (International Laser Ranging Service) is also an excellent source of information about SLR and LLR.

VLBI:

  1. How can I access VLBI data?
    VLBI data are available through anonymous ftp and other methods. VLBI databases since 1979 are available online. Access the VLBI data using one of the following methods: Detalied information and examples for using these methods can be found on our Achive Access page.
  2. Where can I find out more about VLBI?
    The International VLBI Service for Geodesy and Astrometry (IVS) has extensive information about VLBI, particularly the links found on their "What Is VLBI" page.

DORIS:

  1. How can I access DORIS data?
    DORIS data are available in satellite subdirectories through anonymous ftp and other methods. DORIS data since 1992 are available online.

    Access the DORIS data using one of the following methods: Detalied information and examples for using these methods can be found on our Achive Access page.
  2. How are DORIS data stored?
    DORIS data are stored in files by satellite and cycle number where a cycle typically spans ten days. DORIS data are available in subdirectories by satellite; files follow the naming convention satdatacyc.dat.Z, where sat is the three-character satellite code, and cyc is the three-digit cycle number. All data are stored in the standard format for DORIS.
  3. How can I find out what data are archived?
    Files are available within each satellite subdirectory and the general DORIS https directory that list DORIS filename and timespan of data within each file.
  4. What DORIS products are available?
    Products derived DORIS data include satellite orbits (SP1 format), time series of station coordinates (SINEX format), time series of coordinates of the TRF origin (geocenter), Earth orientation parameters (EOP), and ionosphere parameters. DORIS products archived at the CDDIS are available in subdirectories by product type at https://cddis.nasa.gov/archive/doris/products. Additional information about DORIS analysis can be found on the IDS Analysis Coordinators website.
  5. How can I find out more about DORIS stations?
    The DORIS network web page provides a list of current sites and links site photos to their site logs.
  6. Where can I find out more about DORIS?
    DORIS information can be accessed through the IDS (International DORIS Service) page.

Data Caster:

  1. What do I do if I forgot my username or password?
    The Earthdata login is required to access the NTRIP caster website. If you have forgotten your username or password, please visit https://urs.earthdata.nasa.gov to confirm and then choose one of their options: "I don't remember my username" or "I don't remember my password" and follow their instructions to reset your credentials.