VLBI Data Products and Holdings

Data Holdings

VLBI is a geometric technique, measuring the time difference between the arrival at two Earth-based antennas of a radio wavefront emitted by a distant quasar. The simultaneous measurements are compared at correlators; the difference in the arrival times between antennas yields a determination of the distance between the antennas and thus the precise positions of the antennas themselves. The VLBI technique can also determine the inertial reference frame defined by these quasars as well as the orientation of the Earth in this frame.

The globally distributed networks of VLBI antennas participate in highly coordinated experiments in order to perform simultaneous measurements, which are compared and correlated, generating VLBI database files. As additional analysis is performed on the experiments (e.g., the application of meteorological information, theoretical quantities and models), new versions of the database files are generated and archived. All versions are retained at the CDDIS and other IVS data centers for documentation purposes. The VLBI experiment data consist of the observing site parameters, source information, correlated delay and delay rate between the observing antennas as a function of time, along with meteorological data and system parameters that must be applied to the correlated measurements.

The CDDIS is one of the primary data centers supporting the International VLBI Service for Geodesy and Astrometry (IVS). Network stations forward raw observations to correlators who then submit the experiment databases to the IVS data centers.

Data Summary

The Very Long Baseline Interferometry (VLBI) data are archived in experiment files, organized by experimental session and frequency band. These experiment files are available in two formats, designed for use by different processing packages: binary databases (Goddard database format) and an ASCII format (NGS card). Multiple versions of the binary databases are retained in the IVS archives; the NGS card version of the experiment data is created from the last updates made to the databases.

Data Type

• Level1A Correlator output
• Level2 vgosDB format data
• VLBI DB data
• VLBI NGS data
• VLBI Auxilliary Files

Spatial Coverage

• List of IVS VLBI sites
• Map of IVS VLBI sites

Temporal Coverage

• Experiment files from 1979 through present; master schedules can be found at: https://cddis.nasa.gov/archive/vlbi/ivscontrol

Data Volume

• Experiment files (binary Goddard database format): ~10 Kbytes-6Mbytes (compressed)
• Experiment files (ASCII NGS card format): ~10–150 Kbytes (compressed)

Data Formats

• Goddard database format
• NGS card format
• SINEX format (future)

Auxiliary Information

• Schedules
• Notes
• Session logs

These are available under the directory https://cddis.nasa.gov/archive/vlbi/ivscontrol/.

Auxiliary data useful to the analysis of the VLBI data can can be found in https://cddis.nasa.gov/archive/vlbi/ivsdata/aux/.

More detailed information about the current and historic VLBI observing programs, including experiment schedules, observing stations, and temporal coverage, is available through the IVS website.

VLBI experiment data are available from the CDDIS in subdirectories by format type (Goddard database, NGS, SINEX) and year.

The starting directory for these files is: https://cddis.nasa.gov/archive/vlbi/ivsdata/

Append the following directory and file names to the starting directory:

db/YYYY/YYMONDDBB_V###.gz

ngs/YYYY/YYMONDDBB_N###.gz

sinex (future)

as described in the table below.

Product Holdings

The CDDIS archive of VLBI products consists of solutions derived by IVS analysis centers from the analysis of VLBI experiment files. These products include the correlated delay and delay rate of simultaneous observations as a function of time, station positions and velocities, positions of quasars (input to the International Celestial Reference Frame, ICRF), EOPs (polar motion and rates, length of- day, UT1-UTC and long-term stability of nutation), and troposphere parameters. Derived product files are typically less than 1 Mbyte in size and are archived in subdirectories by product type.

The format and time span of the products varies by type. Product-specific formats are used: SINEX (Software Independent Exchange) for station positions and troposphere parameters, an IVS standard format for EOP, and an IERS standard format for CRF.

Product Summary

• VLBI TRF products
• VLBI Daily SINEX Independent Solution product (DSNX)
• VLBI Daily Intensive Solution product (DSNI)
• Global VLBI Celestial Reference Frame solution product
• Global VLBI Terrestrial Reference Frame solution product
• Intensive Earth Orientation Parameter Series (EOP-I) product
• Session Earth Orientation Parameter Series (EOP-S) product
• VLBI Tropospheric Parameters (TROP) product

Product Formats

• SP3 and SP3c formats
• Software INdependent EXchange (SINEX) format

The archive of IVS products is organized in subdirectories by product type (e.g., time series of station positions, EOP values, etc.).

The starting directory for IVS derived product files is: https://cddis.nasa.gov/archive/vlbi/ivsproducts/

The subdirectories under the main product directory are:

More information about IVS products can be found on the IVS website.