VLBI on the GBT

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

Proposals

VLBI observing with the GBT is supported with a VLBA backend using the Mark6 recording system. Recording bandwidth up to 1024 MHz is supported. The maximum recording data rate is 4096 Mbits/sec.

Proposals requesting the GBT as part of the VLBA and/or HSA should be submitted to the VLBA; no separate proposal to the GBT is required. In contrast, proposals requesting the GBT as part of the EVN or GMVA or other non-NRAO arrays should be submitted to the GBT and other arrays or telescopes simultaneously.

Refer to: VLBA Proposals
Proposing for the HSA


For more VLBA information, refer to: VLBA guide and proposer information

VLBA-compatible recording

The data acquisition system is similar to those at the VLBA stations: the new RDBE units and Mark6 recorder are in use, allowing wide-band recording up to 4 Gbits/sec. Two modes are available, "PFB" mode provides 16 32-MHz channels and a maximum recording rate of 2 Gbits/sec. The "DDC" mode allows up to 4 channels of bandwidth 1 to 128 MHz. With two RDBEs available, up to 8 DDC channels may be used.

The SCHED default frequency setups should be correct for writing schedules for the new system.

The old data acquisition system with the DAR rack and Mark5A or Mark5C recorder has been retired. No proposals should request it.

Schedule Preparation

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

Differences between the GBT and VLBA stations

Differences with the standard VLBA stations should be noted:

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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:
  • The GBT Proposer's Guide, Chapter 4, for antenna and receiver performance.
  • Gain Curves.

    • GBT Receivers

    VLBA
    Band     		VLBA
    Frequency
    Range (GHz)     		GBT
    Frequency
    Range(GHz)     		GBT
    Receiver
    AstrID Name     		GBT
    Receiver
    Common Name     		Available Net
    Sideband     		RDBE
    channels     		Polarization Primary Beam
    FWHM    		 Est.
    SEFD
    (Jy)     		Typical
    Tsys
    (K)
    90 cm 0.312 - 0.342 0.290 - 0.395 Rcvr_342 (PF1) P-band yes lower any Lin/Circ 36' 25 20-70
    ---- --- 0.385 - 0.520 Rcvr_450 (PF1) 400-MHz yes lower any Lin/Circ 27' 22 20-50
    50 cm 0.596 - 0.626 0.510 - 0.690 Rcvr_600 (PF1) 600-MHz yes lower any Lin/Circ 21' 12 20-35
    ----- --- 0.680 - 0.920 Rcvr_800 (PF1) 800-MHz yes lower any Lin/Circ 15' 15 18-25
    ----- --- 0.910 - 1.230 Rcvr_1070 (PF2) PF2 yes lower any Lin/Circ 12' 10 18-22
    21/18 cm 1.35 - 1.75 1.1 - 1.8 Rcvr1_2 L-band yes lower any Lin/Circ 9' 10 15-18
    13 cm 2.15 - 2.35 1.68 - 2.60 Rcvr2_3 S-band yes lower any Lin/Circ 5.8' 12 18
    6 cm 3.9 - 7.9 3.9 - 7.9 Rcvr4_6 C-band yes lower any Lin/Circ 2.5' 10 23
    4 cm 8.0 - 8.8 7.9 - 12.0 Rcvr8_10 X-band yes lower any Circ 1.4' 15 27
    2 cm 12.0 - 15.4 11.8 - 18.0 Rcvr12_18 Ku yes upper A/C Circ 54" 20 27
    1 cm 21.7 - 24.1 18.0 - 27.5 RcvrArray18_26 KFPA yes lower B/D Circ 32" 25 40
    ---- ---- 26.0 - 40.0 Rcvr26_40 Ka
    (Dual beam)     		yes
    (winter only)     		upper ? Lin
    (single-pol/beam)     		22" 20-40 40
    7 mm 41.0 - 45.0 40.0 - 50.0 Rcvr40_52 Q-band yes
    (winter only)     		upper A/C Circ 16" 60 80
    3 mm 80.0 - 90.0 68 - 92 Rcvr68_92 W-band yes
    (winter only)    		 upper A/C Circ 10" 100 110
    Notes:

  • Receivers marked "PF1" or "PF2" are at the prime focus; the others are at the Gregorian focus.
  • Rcvr26_40 has linear polarization only, 2 beams but one polarization state per beam; all other receivers can receive dual circular polarizations.
  • Pulse cal (or phase cal) is injected in receivers of 2cm wavelength and longer; pulse cal is injected in the 7mm receiver after the first mix; other receivers have no pulse cal injection.
  • For the Rcvr68_92 (3mm/W-band) receiver there is no pulse cal or noise cal injection; there is a system for measuring Tsys relative to built-in hot and cold loads.

    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
    4-6, 8-10 GHz 4-5 hours
    12-16 GHz 3-4 hours
    18-26 GHz 1.5-2 hours
    40-90 GHz 30-60 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, or ≥1.0 Jy for ν>20 GHz) 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 6 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 = 06:00  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.

    W-band (68-92 GHz) calibration

    System Temperature (Tsys) calibration with this receiver uses a calibration wheel that can place hot and cold loads in front of the feed. There is no noise injection as happens with the other receivers. A "cal sequence" procedure is done just after each peak/focus to provide a Tsys measurement. This happens automatically; the user does not have to specify it. The cal sequence takes about one minute, and will happen just after the peak/focus. The user should use a dwell time of 8 minutes for the pointing scan, and that will include the cal sequence.

    Pointing Sources for high frequency observing should be strong, i.e., stronger than 3 Jy if possible.

    High Frequency (40-90 GHz) active surface considerations.

    When using the 40-50 or 68-92 GHz receivers, one should tune up the active surface by doing an "AutoOOF" procedure. This is so-called "Out of focus holography" in which a strong point source is observed both in and out of focus, and large-scale deviations of the surface can be derived. The surface corrections are applied to the active surface model. This improves the aperture efficiency by a factor of 2 at 86 GHz. One should do an AutoOOF, which takes about 30 minutes, at the beginning of any high-frequency observing. The user does not have to specify this in the observing file; the operator or telescope friend will do an AutoOOF calibration prior to starting the observing.

    When observing with the 68-92 GHz receiver, one should repeat the AutoOOF about every 3-4 hours. This means that the user should allow a 30 minute gap in the schedule about every 3-4 hours. The user does not have to specify anything about an autoOOF in the schedule; just allow the 30 minute gap. The operator or telescope friend will do the calibration.

    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.

    To write a schedule using the slower azimuth slew speed, specify the GBT in your key file as: stations = gbt_cold ...
    instead of the usual: stations = gbt_vlba ...

    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

  [For more information on VLBI station positions contact the IVS
	(International VLBI Service) at https://ivscc.gsfc.nasa.gov/ ]

    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 Apr 2009 -- F. Ghigo Updated Aug -Sep 2013 -- F. Ghigo Updated April 2020 -- F. Ghigo