VLBI on the GBT

The GBT does VLBI recording with a:
Setup information:
Related Information:

Proposals

Proposals requesting GBT participation in VLBA or global VLBI observations should be submitted to the VLBA only, not to the GBT.

Refer to: VLBA Proposal Preparation

Proposals requesting GBT participation but including no other NRAO telescopes should be submitted as GBT proposals as well as to the other telescopes or arrays (for example, the EVN).

VLBA-compatible recording

The data acquisition system is similar to those at the VLBA stations: there are eight double-sideband baseband converters with bandwidths settable between 62.5 KHz and 16 MHz. Recording is accomplished with a Mark5A disk pack recorder. Tape recording is no longer available. The wide-band (512 MB) mode is supported.

Scheduling is done through the VLBA analysts in Socorro. Schedules are prepared with the SCHED program. The GBT uses the standard VLBA schedule file ("*.crd" file). The user needs to prepare a ".key" file for SCHED in the usual way and send it to the VLBA analysts.

Differences with the standard VLBA stations should be noted:

Click here for general VLBA information.

S2 recording

S2 recording is no longer supported.

Available Receivers and Bands.

The receivers and frequency bands are listed in the following table. Note that some bands are available on the GBT but not on the VLBA.

Note also the time it takes to change bands, described above.

For more information, consult:
  • Typical system performance.
  • Gain Curves.
  • Gregorian Receivers.
  • Prime Focus Receivers.
  • GBT LO/IF System.

    • GBT Receivers

    VLBA
    Band     		GBT
    Frequency
    Range(GHz)     		GBT
    Receiver     		available net
    sideband     		primary
    beam     		est.
    SEFD
    90 cm 0.290 - 0.395 Rcvr_342 (PF1) yes lower 36' 35 Jy
    50 cm 0.385 - 0.520
    0.510 - 0.690     		Rcvr_450 (PF1)
    Rcvr_600     		yes lower 27'
    21'     		24 Jy
    ----- 0.680 - 0.920 Rcvr_800 (PF1) yes lower 15' 13 Jy
    ----- 0.910 - 1.230 Rcvr_1070 (PF2) yes lower 12' 12 Jy
    21-18 cm 1.1 - 1.8 Rcvr1_2 yes lower 9' 10 Jy
    13 cm 1.68 - 2.65 Rcvr2_3 yes lower 5.8' 12 Jy
    6 cm 3.9 - 5.9 Rcvr4_6 yes lower 2.5' 13 Jy
    4 cm 7.9 - 10.1 Rcvr8_10 yes lower 1.4' 15 Jy
    2 cm 11.8 - 15.6 Rcvr12_18 yes upper 54" 20 Jy
    1.3 cm 18.0 - 22.5 Rcvr18_22 yes upper 37" 25 Jy
    1 cm 22.0 - 26.5 Rcvr22_26 yes upper 30" 30 Jy
    ---- 26.0 - 40.0 Rcvr26_40 yes (winter only) upper 22" ??
    7 mm 40.0 - 52.0 Rcvr40_52 yes (winter only) upper 16" 60 Jy

    Include Pointing & Focus Checks

    It is recommended to allow for pointing and focus touch-ups when observing at the higher frequencies. The table shows the recommendations.
    Frequency Band Interval between pointing scans
    8-10 GHz 4-5 hours
    12-16 GHz 3-4 hours
    18-26 GHz 1.5-2 hours
    40-50 GHz 30-45 minutes
    At the higher frequencies (18-26 GHz and 40-50 GHz) also do a pointing check when the source elevation has changed by 15 degrees or more.

    The observer should select a strong continuum source (flux density ≥0.5 Jy) within about 15 degrees and at similar elevation as the program source. Include the pointing calibration source in the VLBI observing schedule at intervals as indicated above. Allow about 8 minutes for the pointing and focus check.

    Note also that significant pointing errors at 7mm can happen when the wind speed is greater than 3 m/sec (7 miles per hour). For 1.3 cm significant pointing errors can occur for wind speeds greater than 6 m/sec (14 miles per hour). Refer to PointingFocusGeneralStrategy for details.

    To include a pointing and focus scan in your schedule, put commands into your ".key" file similar to the following: 
    comment='GBT pointing scan.'  peak=1
stations =  gbt_vlba
source = 'J0920+4441'  dwell = 08:00  vlamode='VA' norecord /
nopeak
    It is important to specify only the GBT ("stations=gbt_vlba") when putting in "PEAK=1". Otherwise it may do a reference pointing for the whole VLBA, and if the selected pointing source is under about 5 Jy for the VLBA, it could produce bad results.

    Refer to the SCHED MANUAL for details of schedule preparation.

    Telescope move times and limits.

    To figure the approximate move times, we can use the following information:

    This translates to about 24 seconds for acceleration and deceleration. Adding software overhead, about 45 seconds should be added to the travel time at the maximum slew rates.

    For short moves, the telescope speed never reaches the maximum slew rate, and the move times are shorter than the above estimate, as listed in the following table. Also see a Report

    Total move + settling time (sec)
    Distance T(AZ) T(EL)
    < 0.2° 12 10
    0.5° 20 16
    1.0° 25 20
    1.5° 27 23
    2.0° 30 27
    Despite the implication of the above that 10-15 seconds is sufficient for short moves, it turns out that due to software overhead it is best to be a little conservative and never allow less than 30 seconds.

    Move limits:

    Bad Weather Considerations


    To get some idea of how often the low temperature condition happens, refer to Low Temperature statistics.
    If your project will run in December, January, or February you should use the slower azimuth slew rate of 18 deg/min when making the schedule. In other months, the probability of running into the low temperature limits is slight.

    Useful links for weather forecasts:

    GBT Position

    The geodetic position for the GBT (as of Jan 2000), based on a local survey referred to a standard NGS survey marker on the Green Bank site. The surveyed height refers to the top of the azimuth track. The phase center (intersection of azimuth and elevation axes) is 48.22m above the top of the azimuth track. The average geoid height = -31.10 meters with respect to the ellipsoid.

    Our best estimate of the ITRF97 earth-centered coordinates for the phase center of the GBT are derived from analysis of a "TIES" VLBI run with the GBT and GB 20-meter (December 2002). 

      (Epoch 1997)    Value         Error
  X     (mm)     882589641.08     1.7 
  Y     (mm)   -4924872324.81     6.9
  Z     (mm)    3943729358.48     5.4
  X vel (mm/yr)        -15.34      .1
  Y vel (mm/yr)         -1.38      .2
  Z vel (mm/yr)          3.51      .2

  Solution as of Oct 2007:
  x=  882589.638 meters
  y= -4924872.319  
  z= 3943729.355

  [Reference:
     ftp://gemini.gsfc.nasa.gov/pub/solutions/2004bn_may04/geocentric_positions-velocities.2004bn.1 ]

    The corresponding NAD83 position (and the best geodetic position to use) is: 

       Latitude  = 38° 25' 59.266" N           ( 38.433129 )
   Longitude = 79° 50' 23.423" W           ( 79.839840 )
   Height    = 824.363 m (above the ellipsoid)
   Height    = 855.46  m (above the geoid)
    Updated Feb 2008 -- F. Ghigo