Green Bank 20-meter telescope.

The 20-meter diameter telescope at the National Radio Astronomy Observatory (NRAO) in Green Bank, WV, was constructed by RSI and completed in late 1994.

Run by NRAO and funded by the US Naval Observatory (USNO), it is part of the National Earth Orientation Service (NEOS) telescope network, and part of the USNO Navy Network (NAVNET) participating in a global program of Earth Orientation VLBI measurements in cooperation with the International Earth Rotation Service (IERS), and with the NASA Space Geodesy program.

The 20-meter program was shut down in June 2000, due to budget cutbacks at USNO.

The results of these measurements are published weekly in the IERS Bulletin-A, which lists the most recent polar motion and UT1 corrections and predictions for three months in the future. Also, a summary of VLBI results data is also available.

The experiments are designed to measure small wobbling motions of the Earth's polar axis and irregularities in the Earth's rate of rotation with reference to positions of quasars (distant bright explosions in nuclei of galaxies). Quasars are the most distant point-like radio sources known, and therefore form a good set of stable reference points. The resulting data are needed for high accuracy of world-wide navigation systems. The data are also used for studies of continental drift and of atmospheric and oceanic currents, in collaboration with the NASA Geodetic VLBI program.

Prior to the completion of the 20-meter, these geodetic VLBI experiments have been using telescope 85-3, part of the Green Bank Interferometer.

Other telescopes participating in the NEOS network are: Kokee Park Geophysical Observatory (Kauai, Hawaii); Gilmore Creek Geophysical Observatory (Fairbanks, Alaska); Wettzell, Germany; Fortaleza, Brazil; and Algonquin Park (Ontario, Canada).

Telescope status:

The antenna was completed in November of 1994. The Green-Bank-built prime focus receiver was installed on January 11, 1995. VLBI observing was done in Jan-Feb to test and shake down the system. In early March, the antenna encoders were borrowed to fix a problem at the Kokee antenna (a twin of ours), so the Green Bank 20-meter was inactive until replacement encoders were installed in October of 1995. After further tests, regular operation was begun in November 1995.

Telescope specifications:

 Type               : Elevation over azimuth.
 Reflector Diameter : 20 meters  
 F/D ratio          : 0.43 
 Receiver           : dual frequency/dual polarizations
                       Cooled (15K) HEMT at prime focus.
 Frequency bands    : 2.2 to 2.6 GHz; 8.2 to 9.0 GHz 
 Slew Speed         : 2 degrees per second, each axis. 
 Surface Accuracy   : 0.8 mm rms. 
 Geodetic Position  : 38 26'12.661" N.Lat (ref: NAD1983 datum)
                      79 49'31.865" W.Long.
                       Approx. height of elev.axis: 842 m

Performance Data:

Calibrations in single-dish mode done in February find rms pointing residuals of 34", and other performance data as follows:

                      X-band        S-band
 System Temps          46 K          31 K
 SEFD                 720 Jy        440 Jy
 Aperture efficiency  55-59%        58-61%
    
Geocentric CIO position from VLBI data in Reference Frame Navy 1996-9.
    X =   883772.7974 m  +- 4.3 mm
    Y = -4924385.5975 m  +- 5.9 mm
    z =  3944042.4991 m  +- 5.2 mm

Field System documentation.

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fghigo last changed this page 06 Oct 2000