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NASA Goddard Geophysical and Astronomical Observatory

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The Planetary Geodynamics Laboratory (GSFC Code 698) is the home to VLBI scientists supporting NASA's Space Geodesy Program. They use GGAO for a variety of VLBI and related GNSS programs. The GGAO VLBI radio telescope is used as: (1) a fiducial location in the global terrestrial reference frame, (2) as one of two unique test sites for the next generation VLBI system, and (3) as one of several sites participating in a series of VLBI observing sessions to gather data used to model tropospheric refraction. The mobile VLBI system MV-3 was permanently located at GGAO starting in 1989 and was eventually transformed into the fixed antenna configuration in the 1992-1993 timeframe.

MV-3 at GGAO

This picture shows the GGAO VLBI antenna. Electronics for the VLBI system are located in the van to the left of the antenna. The GODE GPS antenna is shown in the left foreground..

VLBI is a geometric technique: it measures the time difference between the arrival at two Earth-based antennas of a radio wavefront emitted by a distant quasar. Using large numbers of time difference measurements from many quasars observed with a global network of antennas, VLBI determines the inertial reference frame defined by the quasars and simultaneously the precise positions of the antennas. Because the time difference measurements are precise to a few picoseconds, VLBI determines the relative positions of the antennas to a few millimeters and the quasar positions to fractions of a milliarcsecond. Since the antennas are fixed to the Earth, their locations track the instantaneous orientation of the Earth in the inertial reference frame.

The GGAO operational VLBI 5-meter antenna is part of the network supporting the International VLBI Service for Geodesy and Astrometry (IVS). The IVS is an international collaboration of organizations which operate or support Very Long Baseline Interferometry (VLB I) components. The objectives of IVS are (1) to provide a service to support geodetic, geophysical, and astrometric research and operational activities; (2) to promote research and development activities in all aspects of the geodetic and astrometric VLBI technique; and (3) to interact with the community of users of VLBI products and to integrate VLBI into a global Earth observing system. The next generation system, VLBI2010, is a 12-meter VLBI antenna currently under installation and development at GGAO.

For more information about VLBI or the IVS contact:

  • Chopo Ma, NASA GSFC Code 698, 301-614-6101

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