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Frequently asked questions (FAQ)

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.

General:

  1. How do I access the CDDIS data archive?
    The majority of CDDIS data are available through anonymous ftp at URL ftp://cddis.gsfc.nasa.gov.
  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 command uncompress to expand the file. Users on non-UNIX systems can consult the gzip web site. Note that ftp 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. Why is my FTP connection not working?
    1. Is your network connection working?

    Check that your network connection works by hitting a few websites you've never been to before. If they come up then try an anonymous login to a few public FTP servers (ftp.openbsd.org, ftp.microsoft.com). After you login make sure you can do an directory listing with the "ls" command. If this works move on to step 2.

    2. Can you connect to the CDDIS servers at all?

    Try to pull up the main CDDIS website. Located at http://cddis.gsfc.nasa.gov. If you can access that site then the main network is up and running correctly. If this works go to step 3.

    3. Can you login to the FTP server, cddis.gsfc.nasa.gov?

    For the first part of FTP (control connection) to work correctly you need to be able to connect to the FTP server. After connecting you should be presented with a banner message and login prompt.

    From a command line client it is easy to tell if the FTP login is working. You will be presented with a CDDIS login banner. Then a username and password prompt. You login anonymously with the username "anonymous" or "ftp". The password is your e-mail address. After that you should be at an FTP prompt. If you're using a GUI client set it to login anonymously. Then to see if you logged in correctly there is usually a window that scrolls all of the commands issued by the client. You will have to scroll back through this window to see if the login completed correctly.

    If you can login successfully and get to a FTP prompt move on to step 4.

    4. Can you get a file listing or retrieve a file?

    After logging in to the FTP server and getting to a prompt you should be able to issue an "ls" command from a command line client. GUI clients issue this command for you automatically when you login successfully. If this command is issued and you get nothing back you might not be using the correct mode of FTP transfer. To check this, please refer to the question "Why is it when I login to your FTP servers and try to get a file it fails?" in our FAQ section.

    After you have read this and verified you're using the correct client and FTP settings please move to step 5.

    5. All other steps have checked out and are working. Now what?

    You should be to the point where you've made sure your FTP client supports passive FTP, that it is turned on, successfully logged in, and tried to retrieve a file or get a directory listing. At this poin,t if retrieving a file or getting a directory listing fails then there is not much you can do from here.

    Our final suggestion is that you get in contact with your system and network administrators. They deal with these types of problems on a daily basis. They usually have better access to information about the network setup at your location and can provide you with more insight into why FTP may not be working correctly.

    If your system administrators say everything should work correctly and can reproduce the same problem you're having then your last resort is sending CDDIS an e-mail.

    If you are going to send us an e-mail about an FTP problem we ask that you take the time to read the whole FAQ first. In the body of the email you will need to answer the following questions. If you do not answer each and every question/request we will not be able to help you with your problem.

    • What is the public IP address you're coming from? If you don't know please visit
      http://www.ipchicken.com from the machine you are trying to connect to CDDIS with. Copy and
      paste into the e-mail the big number under the words "Current IP Address".
    • What is the data you are trying to retrieve?
    • Where is the data you are trying to retrieve? The hostname of the server you are trying to contact
      and (if you have it) the path to the data. Example: ftp://cddis.gsfc.nasa.gov/pub/gps/data/daily/2014/
    • How many times have you tried to retrieve the data?
    • What are the exact times you tried to retrieve the data?
    • What is the name and version of FTP client (program) you are using?
    • What is the name and version of operating system you're using?
    • Copy and paste the full text of your failing FTP session to your e-mail.
    • What is the e-mail address of your system administrator?
    Send the answers to these questions and a description of the problem to: CDDIS help.
  9. Why is it when I login to your FTP servers and try to get a file it fails?
    There are a few reasons why this could be happening. First and foremost, you must make sure your FTP client (program) is using passive mode to connect to our servers. FTP has 2 primary modes it can use. Active and passive. Active is used by many FTP clients as the default transfer method. The problem with this is active FTP does not work well with firewalls. When you login successfully but your connection hangs on a "get" or "ls" command you're probably using FTP in active mode.

    Today, most modern FTP clients can be used in either active or passive FTP modes. You just have to tell the client which mode to use. Most Windows-based clients have a configuration settings section where the mode can be set. Try searching through the help section of the program with the word "passive" or "firewall" if you have trouble finding the section with the setting. The command line FTP clients use command line switches to set passive mode. If you're using a Unix type operating system, use the system's man pages (Ex. man ftp) to check the switches and options of the FTP client you're trying to use. If you don't see anything in these sections with the words passive or firewall then it's a good possibility that FTP client does not support passive.

    Please refer to this website for a good explanation of active vs passive FTP.

    http://slacksite.com/other/ftp.html

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 anonymous ftp; the data are stored in subdirectories by type (daily, hourly, high-rate), by year, and by day of year: ftp://cddis.gsfc.nasa.gov/gnss/data. GPS data since January 1998 are available on-line; older data are off-line and can be requested by contacting the CDDIS manager
  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 at URL ftp://cddis.gsfc.nasa.gov/gnss/data/hourly. 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.
  6. How are the hourly GNSS data files stored?
    Hourly GNSS data files are available in hourly subdirectories within the filesystem /gnss/nrtdata/yyddd/hh, where yy is the two-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.
  7. How can I access high-rate GNSS data files?
    High-rate GNSS data files are available through anonymous ftp at URL ftp://cddis.gsfc.nasa.gov/gnss/data/highrate. These high-rate GNSS data are available since May 2001.
  8. How are the high-rate GNSS data files stored?
    High-rate GNSS data files are available in hourly subdirectories within the filesystem /gnss/hrdata/yyddd/yyt/hh, where yy is the two-digit year, ddd is the three-digit day of year, t is the file type (d or n), 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. How can I access products derived from GNSS data?
    GNSS products are also available through anonymous ftp at URL ftp://cddis.gsfc.nasa.gov/gnss/products. GNSS products (preceise ephemerides) since June 1992 are available on-line.
  10. 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.
  11. 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.
  12. 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.
  13. 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.
  14. 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. Similarly, the file ftp://cddis.gsfc.nasa.gov/gnss/data/daily/yyyy/ddd/yyn/ifagddd0.yyn.Z is created at BKG each day and contains unique navigation messages from sites in Europe.
  15. 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 ftp://cddis.gsfc.nasa.gov/gnss/data/daily/yyyy/ddd/yyn and ftp://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.
  16. What are the data types in the GNSS daily subdirectories?
    Each daily subdirectory under ftp://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).
  17. 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.
  18. 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 maps.
  19. How can I access the GNSS products?
    The GNSS derived data products are freely available for download through anonymous ftp; the data are stored in subdirectories by type or GPS week: ftp://cddis.gsfc.nasa.gov/gnss/products.
  20. 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 anonymous ftp at two URLs: ftp://cddis.gsfc.nasa.gov/glonass/data/daily/ (for GLONASS-only receiver data) and ftp://cddis.gsfc.nasa.gov/gnss/data/daily/ (for GPS+GLONASS receiver data). GLONASS data obtained through the IGEX-98 campaign (August 1998) through June 11, 2002 are available on-line in the ftp://cddis.gsfc.nasa.gov/glonass/data/daily/ filesystem.
  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 ftp://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 ftp://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 at URL ftp://cddis.gsfc.nasa.gov/glonass/products/. GLONASS products (precise ephemerides) since October 1998 are available on-line.
  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?
    Laser ranging data are available through anonymous ftp at URL ftp://cddis.gsfc.nasa.gov/slr. SLR data since 1976 are available on-line.
  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 ftp://cddis.gsfc.nasa.gov/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 ftp://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 ftp://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 at URL ftp://cddis.gsfc.nasa.gov/vlbi/ivsdata. VLBI databases since 1979 are available on-line.
  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 at URL ftp://cddis.gsfc.nasa.gov/doris. DORIS data since 1992 are available on-line.
  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 ftp 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 ftp://cddis.gsfc.nasa.gov/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.