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About the CDDIS GNSS archive:The CDDIS has been actively archiving GNSS data since 1990. This GPS and GLONASS archive includes several different categories of GPS experiments: local, intercomparison surveys between existing SLR and VLBI monuments, regional surveys, footprint surveys, and global GNSS network tracking data. In early 1992, the CDDIS was selected to serve as a global archive center for the International GPS Service (IGS), by supporting the daily GNSS data retrieved from identified core observatories. The purpose of this international service is to provide GNSS data products and highly accurate ephemerides to the global science community to further understanding in geophysical research. The IGS has demonstrated the near real-time capability of the global GNSS community to retrieve data and produce products (e.g., satellite ephemerides and Earth rotation parameters) that are of use to a broader community. A description of the structure of the CDDIS GPS data and product ftp archive is available.
Daily 30-second GNSS data files. Several regional data collection centers forward their data holdings of daily, 30-second GPS (and some GPS+GLONASS) data in compressed RINEX format to the CDDIS on a daily basis. At present time, an average of over 200 sites are archived in this fashion each day. Data analysis centers located worldwide retrieved these data daily to produce IGS data products. These products, such as the daily and weekly satellite ephemerides and the Earth rotation parameters, are in turn, submitted to the CDDIS for availability to the global science community. In general, a majority of the data delivered to and archived on the CDDIS are available to the user community within a few hours after the observation day. These data are checked for data quality, summarized, and archived on-line in daily subdirectories; the summary/inventory information is also loaded into the on-line data base. The daily files are archived in subdirectories by day within the filesystem ftp://cddis.gsfc.nasa.gov/pub/gps/data/daily/YYYY/DDD/YYt, where YY is the two-digit year, DDD is the three-digit day of year, and t is the type of file (d is Hatanaka-compressed observation data, m is compressed meteorological data, n is broadcast ephemeris data, o is observation data, and s is observation summary files). All observation, navigation, and met data are stored in RINEX format.
Hourly 30-second GPS data files. The CDDIS also maintains an archive of GPS data at a 30-second sampling rate stored in files containing an hour's worth of data. These data are utilized by the IGS analysis centers to produce rapid and ultra-rapid orbit and clock products. The IGS Analysis Center Coordinator generates a daily, combined, rapid product with a 17-hour latency. The ultra-rapid (predicted) orbit and clock products are generated twice daily, at 07:00 and 19:00 UTC. The hourly GPS data are archived online for three days; after that time, the data are deleted since files containing a full day's worth of data are available in the daily GPS data directories. These hourly files are stored in hourly subdirectories within the filesystem ftp://cddis.gsfc.nasa.gov/pub/gps/data/hourly/YYYY/DDD/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.
High-rate GPS data files. In 1999, the CDDIS proposed to serve as a data center for the IGS Low Earth Orbiters (LEO) Pilot Project. In that capacity, the CDDIS has begun archiving high-rate (typically, one-second) GPS data from a network of 30-40 global IGS sites. The files contain fifteen minutes of GPS data and are available ftp://cddis.gsfc.nasa.gov/pub/gps/data/highrate/YYYY/DDD/YYt/HH, where YY is the two-digit year, DDD is the three-digit day of year, t is the file type (d, n, and m) and HH is the numeric hour of the day (00, 01, ... 23). All data are stored in RINEX format.
On-board LEO receiver data files. Also in support of the IGS LEO-PP the CDDIS archives GPS data from flight receivers on board LEO satellites. These data are now available in subdirectories by satellite from ftp://cddis.gsfc.nasa.gov/pub/gps/data/satellite. All LEO receiver data are stored in RINEX format.
Broadcast ephemeris files. The file ftp://cddis.gsfc.nasa.gov/pub/gps/data/daily/YYn/brdcDDD0.YYn.Z (YY is the two-digit year, DDD is the three-digit day of year) is the daily GPS 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/pub/gps/data/daily/YYYY/brdc. Similarly, the file ftp://cddis.gsfc.nasa.gov/pub/gps/data/daily/YYYY/DDD/YYn/ifagDDD0.YYn.Z is created at BKG each day and contains unique navigation messages from sites in Europe. The file ftp://cddis.gsfc.nasa.gov/pub/gps/data/hourly/YYYY/DDD/hourDDD0.YYn.Z (YY is the two-digit year, DDD is the three-digit day of year) is a combined broadcast ephemeris file 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/pub/gps/data/daily/YYYY/DDD/YYn and ftp://cddis.gsfc.nasa.gov/pub/gps/data/daily/YYYY/brdc directories and becomes the "daily" broadcast ephemeris file (denoted as brdcDDD0.YYn.Z) and described above. Similarly, the daily GLONASS-only broadcast ephemeris file can be found in the GLONASS navigation file subdirectory ftp://cddis.gsfc.nasa.gov/pub/gps/data/daily/YYg/brdcDDD0.YYg.Z (YY is the two-digit year, DDD is the three-digit day of year). These files are also available in yearly subdirectories of ftp://cddis.gsfc.nasa.gov/pub/gps/data/daily/YYYY/brdc.
GPS experiment data. The CDDIS also archives regional experiment data, sponsored by the NASA DOSE investigation as well as other international cooperative programs. Starting in 1990, studies in the geophysical integrity of the NASA SLR and VLBI sites have been undertaken utilizing GPS. Small networks of GPS sites have been selected in an approximate 10-20 km radius around the main observing monuments at several of key sites. The measurements taken at these "footprint" sites will allow analysts to estimate the local tectonic stability of the area they encompass, assuring the integrity of the space geodetic measurements taken at these critical sites. In addition to the surrounding footprint sites, measurements are taken at the reference monuments placed at a distance of 50 to 100 meters to the main marker. GPS measurements at the reference monuments will allow analysts to continue to monitor the physical stability of the main observing monuments and provide additional data for the estimation of the regional tectonic stability of the area.
GNSS product files. Products derived from GNSS data by the IGS analysis and associate analysis centers are also available from the CDDIS. These products include precise satellite ephemerides, station positions, and Earth rotation parameters accessible in subdirectories by GPS week at ftp://cddis.gsfc.nasa.gov/pub/gps/products. Information about the accuracy of these products can be found at the IGS Central Bureau and from the IGS Analysis Center Coordinator's (ACC's) web site. Ionosphere maps of total electron content (TEC) from IGS associate analysis centers in support of the IGS Ionosphere Working Group are available in yearly and daily subdirectories at ftp://cddis.gsfc.nasa.gov/pub/gps/products/ionex. Tropospheric parameters, in the form of zenith path delay (ZPD) corrections, are estimated by IGS associate analysis centers in support of the IGS Troposphere Working Group and can be found in subdirectories within the weekly GPS product directories (ftp://cddis.gsfc.nasa.gov/pub/gps/products/WWWW/trop, where WWWW equals the GPS week number).
Obtaining access to the CDDIS GNSS data archive:The archive of GNSS data and products are accessible through anonymous ftp and the WWW at URL ftp://cddis.gsfc.nasa.gov/pub/gps; contact the CDDIS manager regarding questions about obtaining GNSS data and products.
Summary of CDDIS GNSS tracking data holdings:
Reports of CDDIS GNSS tracking data holdings:
Summary of CDDIS GPS experiment data holdings:
Related GNSS data and information links:
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Author: Carey E. Noll |