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:

• General
• GNSS
  • GPS
  • GLONASS
• SLR
• VLBI
• DORIS

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 extention 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 use software provided by IGS Central Bureau or consult the gzip web site. Note that ftp transfers of compressed files must be performed using binary mode.
  4. What do all the acronyms mean?
     The CDDIS acronym list is available at URL http://cddis.gsfc.nasa.gov/acronyms.html.
  5. How can I find an address for a colleague?
     Space geodesy contacts can be found in the directory maintained at NASA GSFC; alternatively, users can query the on-line personnel data base.

     

• GPS:
  1. What kind of GPS data are available from the CDDIS?
     The CDDIS archives GPS data at a 30 second sampling rate in files containing a single day's worth of data as well as files (for a subset of the network) containing one hour's worth of data. 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.
  2. How can I access daily GPS data files?
     GPS data are available through anonymous ftp at URL ftp://cddis.gsfc.nasa.gov/gps/data/daily. 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 GPS 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 GPS data files stored?
     GPS data are available in daily subdirectories of the form /gps/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 GPS data files?
     Hourly GPS data files are available through anonymous ftp at URL ftp://cddis.gsfc.nasa.gov/gps/data/hourly. Hourly GPS 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 GPS data files stored?
     Hourly GPS data files are available in hourly subdirectories within the filesystem /gps/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 GPS data files?
     High-rate GPS data files are available through anonymous ftp at URL ftp://cddis.gsfc.nasa.gov/gps/data/highrate. These high-rate GPS data are available since May 2001.
  8. How are the high-rate GPS data files stored?
     High-rate GPS data files are available in hourly subdirectories within the filesystem /gps/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 GPS data?
     GPS products are also available through anonymous ftp at URL ftp://cddis.gsfc.nasa.gov/gps/products. GPS products (preceise ephemerides) since June 1992 are available on-line.
  10. How accurate are the GPS 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 query the CDDIS data base to view GPS data holdings by monument name or geographic region. Alternatively, users can view CDDIS data holdings reports. Daily status files, found in the daily GPS data directories detail data availability for a particular day.
  12. How can I obtain data not currently online?
      Access to GPS 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/gps/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/gps/data/daily/yyyy/brdc. Similarly, the file ftp://cddis.gsfc.nasa.gov/gps/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/gps/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/gps/data/daily/yyyy/ddd/yyn and ftp://cddis.gsfc.nasa.gov/gps/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 GPS daily subdirectories?
      Each daily subdirectory under ftp://cddis.gsfc.nasa.gov/gps/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 GPS 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 GPS observation file in RINEX format.
  18. Where can I find out more about 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?
     As of June 11, 2002 (day 02162) 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/gps/data/daily (for GPS+GLONASS receiver data). GLONASS data since the start of 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 /igex/data/yyddd, where yy is the two-digit year and ddd is the three-digit day of year. All data are stored in RINEX format.
  3. How can I access products derived from GLONASS data?
     GLONASS products are also available through anonymous ftp at URL ftp://cddis.gsfc.nasa.gov/glonass/products. GLONASS products (preceise ephemerides) since October 1998 are available on-line.
  4. How can I find out what data are archived?
     Users can query the CDDIS data base to view GLONASS data holdings by monument name or geographic region. Alternatively, users can view CDDIS data holdings reports. Daily status files, found in the daily GLONASS data directories detail data availability for a particular day.
  5. What is the IGEX file?
     The file ftp://cddis.gsfc.nasa.gov/glonass/data/daily/yyyy/ddd/yyn/igexddd0.yyn.Z (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.
  6. 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).
  7. 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.
  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.
  3. How can I find out what data are archived?
     Users can query the CDDIS data base to view GLONASS data holdings by satellite, satellite and date, station, or station and date. Alternatively, users can view CDDIS data holdings reports.
  4. 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 at 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.
  5. 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.
  6. 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 data bases 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) "All about DORIS" page.