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Green Bank Observatory Call for Proposals: 2023A Semester

The Green Bank Observatory (GBO) invites scientists to participate in the 2023A Semester Call for Proposals for the Green Bank Telescope (GBT)The submission deadline for Semester 2023A proposals is Monday, 1 August 2021, at 17:00 EDT (21:00 UTC).

Special Proposal Call for GBT Filler Time

The Green Bank Observatory would like to invite Director’s Discretionary Time proposals for filler time observations on the Green Bank Telescope (GBT).    Proposals must be submitted by January 20, 2023 at 22:00 UT.

This request is for observing time at frequencies below 8 GHz.  Submissions for observations will be considered for scheduling in the 23-12 hour LST range.  Requests will be restricted to using

the DCR, VEGAS and VEGAS pulsar modes unless special permission is received in advance of submission.  Requests will also be restricted to using the Prime Focus 800 MHz, L-band, S-band and C-band receivers unless special permission is received in advance of submission. 

We will not consider any Large Proposal requests (> 400 hours) as a part of this call.

Please state that your DDT submission is in response to this Special  Proposal Call (SPC) by beginning your proposal’s title using “SPC: ”.  All proposals should be submitted through the Proposal Submission Tool at https:/my.nrao.edu/

Proposers who need assistance with proposal preparation or have questions regarding the Call or GBT capabilities should contact Observatory staff via the Helpdesk at https://help.nrao.edu/

The GBO would like to encourage low frequency (below 8 GHz) projects, especially those that may require significant amounts of observing time per source or field.

We would like to remind proposers that their submissions will be peer reviewed by a panel with a wide ranging background in astronomy.

A virtual Proposal Planning Workshop will be held July 12 and 15.  Learn more and register.

If commissioning is successful, the Ultrawideband Receiver (UWBR) may become available for general use observing sometime in the 23A semester.

Proposal preparation and submission remain via the NRAO Proposal Submission Tool (PST) available at NRAO Interactive Services. Proposers who need assistance with proposal preparation or have questions regarding the Call or GBT capabilities should contact Observatory staff via the Helpdesk.  Note that use of the PST and Helpdesk requires registration. 

The entire proposal call can be found on this page.

We strongly encourage proposers to carefully read through the “News and Opportunities” section below.

News and Opportunities

Proposal Deadline

The 1 August 2022 deadline (21:00 UTC) is for the 2023A Semester observing period on the GBT: 1 February 2023 – 31 July 2023.

Details of all GBT observing modes are inThe Proposer’s Guide for the Green Bank Telescope. Proposers should also consult the more general document The Performance of the GBT: A Guide for Planning Observations. Proposers should make sure that they are familiar with the latest versions of these documents before writing their proposal.

All proposals should state why the GBT is necessary for the requested observations in both the abstract and science justification.

Proposal preparation and submission remain via the NRAO Proposal Submission Tool (PST) available at NRAO Interactive Services. Proposers who need assistance with proposal preparation or have questions regarding the Call or GBT capabilities should contact Observatory staff via the Helpdesk.  Note that use of the PST and Helpdesk requires registration. 

A virtual Proposal Planning Workshop will be held July 12 and 15.  Learn more and register.

Proposers are encouraged to look at past proposal call results, especially the LST pressure plots, which

can be found in the TAC proposal result reports.  This information can be found here.

Low Frequency Observations

The GBO encourages the submission of low frequency (below 8 GHz) projects, especially those that may require significant amounts of observing time per source or field.

Ultra-wideband Receiver Expected to become available in 23A semester

We anticipate that the new Ultra-wideband Receiver (UWBR) will become available for general use some time during the 23A semester.

Proposers wishing to use the UWBR for monitoring observations are also encouraged to include another receiver that can be used for the monitoring should the UWBR not be available at the beginning of the semester. These projects must ensure that their scientific goals can be achieved without using the UWB receiver.

The UWB receiver is optimized for high-precision pulsar timing and wide-band observations of fast transients.  VEGAS will support coherent and incoherent dedispersion, and pulsar searching and timing modes over the full bandwidth of the receiver. 

For more information on the UWBR please visit here.

Please note that the actual system temperatures for the UWBR will not be know before the proposal call is released.  We have thus used system temperatures 20% higher than the theoretical values in the sensitivity calculator to ensure that proposed observations will be able achieve their science goals.

Potential Expanded Summer Maintenance in 2023

If funding is available, the GBT will undergo an expanded summer maintenance period during 2023.  This would allow all the GBT to be repainted, including removal of existing paint. The extended summer maintenance would be from May through September. This would only allow for one week of observing each month. If the UWBR is available then it will be the Prime Focus receiver during the summer.  Otherwise the 800 MHz feed will be available.

GBT Capabilities

The GBO encourages proposals that take advantage of the GBT’s unique capabilities across 0.29 to 116.2 GHz frequency range.  (Coverage is not available for 15.8-18.0 GHz, and 50.5-67.0 GHz).

Key science areas include, but are not limited to:

  • Low column density HI (NHI ≈1017 cm-2 galactic and extragalactic)
  • astrochemistry
  • cosmology
  • fast radio bursts
  • galaxy and cluster evolution
  • pulsars (searches and timing)
  • radio recombination lines
  • solar system science

Large Proposals

The GBT only accepts large proposals once per year at the February proposal deadlines. Large GBT proposals are not accepted at the August proposal deadlines. This policy ensures equality for the reviews of all large proposals that can be scheduled across a full year.

Recall that all large proposals are restricted to using no more than 50% of the open skies time available under any weather category (poor, good, excellent) at any LST during any semester.

Regular, and Large proposal size definitions for the GBO are as follows:

  • 0-8 GHz (Any weather)
    • Regular:  < 400 hours and lasting  ≤ 1 year
    • Large: ≥ 400 hours or lasting >1 year
  • 8-18 and 27.5-50 GHz (Good weather)
    • Regular:  < 200 hours and lasting ≤ 1 year
    • Large: ≥ 200 hours or lasting >1 year
  • 18-27.5 and > 50 GHz (Excellent weather)
    • Regular:  < 100 hours and lasting ≤ 1 year
    • Large:  ≥ 100 hours or lasting >1 year
  • Fixed and Monitoring proposals
    • Regular:  < 200 hours and lasting ≤ 1 year
    • Large:  ≥ 200 hours or lasting >1 year

Proposers submitting Large Proposals should read the Large Proposal Policy to ensure that they address all of the mandatory requirements.

High Frequency Observations

There are approximately 1000 hours of excellent weather (18-27.5 and > 50 GHz) available each semester for both open skies and sponsored time. Prior commitments typically account for slightly more than half of this time which leaves only 450 hours available to be scheduled each semester. 


The GBT receivers, backends, and observing modes that are available for Semester 2023A proposals are listed in Tables 1 and 2 below.

ReceiverFrequency RangeExpected Availability
Prime Focus 1  290-395 MHz and 680-920 MHzShort campaigns with irregular intervals between
Prime Focus 1385-520 MHz and 510-690 MHzHigh rank proposal demand only
Prime Focus 2910-1230 MHzHigh rank proposal demand only
Ultrawideband Receiver700-4200 MHzCampaigns.  May not be available for general use until partway through the 23A semester.
L-band1.15-1.73 GHzEntire semester
S-band1.73-2.60 GHzShort campaigns every 2 months
C-band (linear polarization only – see below)3.8-7.8 GHzMost of semester (except when S-band is available)
X-band7.8-11.6 GHzEntire semester
Ku-band12.0-15.4 GHzCampaigns
K-band Focal Plane Array (7 pixels)18.0-27.5 GHzEntire semester
Ka-band26.0-39.5 GHzCampaigns
Q-band39.2-50.5 GHzCampaigns
W-band67-93.3 GHzShort Campaigns
ARGUS75-116 GHzEntire semester
MUSTANG2 (shared-risk – see below)90 GHz, Shared RiskEntire semester
Table 1

BackendObserving Modes
Versatile Green Bank Astronomical Spectrometer (VEGAS)Continuum (see below), spectral line, pulsar
Digital Continuum Receiver (DCR)Continuum
Caltech Continuum Backend (CCB)Continuum (Ka receiver only)
Mark6 Very Long Baseline Array Disk RecorderVery Long Baseline Interferometry
JPL Radar backendPrivate PI Instrument – Open for Public Use
Breakthrough ListenPrivate PI Instrument, Shared Risk
Table 2

Instrument Availability

The availability of GBT Gregorian receivers and prime focus feeds will be based on demand from the highly ranked (Group A) proposals. Some receivers and feeds may be available only for a few short, two or three week periods during the semester.

Availability of Receivers

Once the UWBR is released for general use observing, we do not expect other prime focus receivers to be available monthly. We will endeavor to accommodate proposals for prime focus receivers other than the UWBR but encourage observers to use Gregorian receivers whenever scientifically feasible and to consider flexible observing strategies when requesting a prime focus receiver. This is especially true for proposals that request regular monitoring observations, phase connection observations, and any fixed date or temporally constrained observations.

We anticipate that the L-band, X-band, Mustang2, ARGUS, and KFPA receivers will be available most of the semester. We expect the following receivers to be available for short campaigns on an irregular basis:  PF/342 MHz, PF/800 MHz, S-band, C-band, Ku-band, Ka-band, Q-band, and W-band. Other PF feeds would be made available for high-ranking proposals. It is anticipated that UWBR will be available monthly once it is released for general observing. Note that this may not occur until we are partway through the semester.

Permission required for instruments not listed as being available

Anyone requesting a receiver or instrument not listed as being available in the proposal call must have permission from the site director before the proposal is submitted.


Proposals wishing to use the GBT C-band receiver should only use linear polarization outputs.  The circular polarization of the receiver is currently not performing correctly and we will not accept any proposals to use the circular polarization output of this receiver.

C-band VLBI on the GBT

The GBO will only accept proposals using the GBT C-band receiver for VLBI Stokes I continuum observations (the observations will need to be done using full Stokes just to calibrate Stoke I).  Please see the HSA section of the Long Baseline Observatory call (available here) for proposals for more details.


  • Continuum: All modes of VEGAS may now be used for continuum observations.  The Proposal Submission Tool has not been updated to reflect this situation.  Proposers should use the spectral line modes of VEGAS to choose the desired bandwidth and then note in the technical justification that the observations will be for continuum measurements.
  • Pulsar: VEGAS pulsar modes are available for use.


Observers interested in using the ARGUS instrument should see The ARGUS Observer’s Web Page for further information. 


Proposers should justify the need for the GBT in the proposal’s text.  All Very Long Baseline Interferometry (VLBI) proposals requesting the GBT should include any needed setup and overhead time in the time request of their proposals.  C-band VLBI observers should see the C-band VLBI note above.

Proposals requesting the GBT as part of High Sensitivity Array (HSA) and Global 3mm VLBI Array (GMVA) should be submitted through the Very Long Baseline Array’s call (available here).


The GBO will accept proposals for shared risk observations using the MUSTANG2 instrument at the proposal deadline.  The GBO cannot guarantee that MUSTANG2 will be cold at the start of scheduled observing due to low observing elevations or rotations of the turret from prior observations.  Cool down time for MUSTANG2 observations should be included in the overhead for MUSTANG2 observations.  All MUSTANG2 proposals must have permission from the instrument development team. 

Breakthrough Listen backend

The Breakthrough Listen project makes its backend available for shared-risk observations during the 2023A semester.  The instrument consists of a cluster of 64 Titan X and 1080 GPU-based servers capturing 8-bit baseband voltages over up to 12 GHz of instantaneous bandwidth.  Data rates are typically tens of TB/hr, but a pipeline is available to generate spectra with adjustable frequency (> 3 Hz) and time (> 350 μs) resolutions, with possible science applications including fast radio transients pulsar observations, stellar flares, SETI, etc.  Before submitting a proposal, proposers must obtain permission from the Breakthrough Listen team at Berkeley SETI Research Center.  The team will consult on proposal preparation and data analysis.  Any data acquired using the backend will be proprietary to the proposer per the standard GBO policies.

More information can be found here, including a technical description of the backend and team contact details.

Continuum Observations

Proposers wishing to perform continuum observation should consult with a GBO scientist.  Some information on continuum observations can be found here.

Observing and Scheduling Constraints

The GBT is scheduled by the Dynamic Scheduling System (DSS). The DSS system is fully described in the GBT Proposer’s Guide and the GBT Observer’s Guide.

Session lengths

Proposers should be aware that long scheduling blocks (more than ~6 hours) become increasingly difficult to schedule due to the transition to reduced open skies time by the NSF.  Proposers must clearly state in the technical justification section how their project could be scheduled in smaller observing blocks that would more easily fit within the GBT scheduling constraints. This is especially critical for proposals that require fixed or coordinated observing dates, e.g., VLBI observations, pulsar monitoring, radar, etc.


Suppose you are considering mapping with the GBT such that significant major turns or moves (end of rows in raster map, petals in daisy maps, changes in position for pointed maps, etc.) occur with a cadence faster than every 30 seconds. In that case, you will need to consult with a GBT support scientist to ensure that the GBT can safely withstand the stresses induced by the mapping motions.

Fixed, Windowed and Temporally Constrained Observations

Due to varied pressures on the GBT’s scheduling resulting from the transition to reduced open skies by the NSF, fixed time observations (e.g., VLBI, pulsar transit observations, etc.), windowed observations (e.g., monitoring observations), and temporally constrained observations (pulsar phase connection, orbit phase constraints, etc.) will likely have to be ranked at least in or near the top 10-15% of all AUI telescope proposals to be accepted.

Observing Team Members

We would like to remind all project teams of the Green Bank Observatory policy that all observers must be listed as a member of the project team in the GBT Dynamic Scheduling System. 

Also, we would like to remind all observers that they should not log into any GBO computing system using another person’s account.  Co-Is and students must have their own GBO login and account if they are to participate in observing and data reduction. 

Scheduling Increments

Please note that the GBT is scheduled in 15-minute (0.25 hour) increments and that all proposals should request time appropriately.   Time requests will be rounded down to the nearest 0.25-hour increment. 

GBT Proposal Preparation

Proposers should consult The Performance of the GBT: A Guide for Planning Observations and the GBT Observer’s Guide. All proposers, including pulsar proposers, should use the GBT Sensitivity Calculator. Please see the Calculator’s User’s Guide for instructions. The Sensitivity Calculator results can be cut and pasted into the Technical Justification section of the proposal. This will streamline the creation of your Technical Justification and lessen the chances of error.  If you plan to make maps with the GBT, you should use the GBT Mapping Calculator tool.

The GBT observing policies describe the telescope’s remote observing restrictions.

Proposers requesting GBT participation in High Sensitivity Array (HSA), Very Long Baseline Array (VLBA), or Global Millimeter Very Long Baseline Interferometry (GMVA) observations should consult the VLBA, HSA, and GMVA Proposal Call.


RFI Monitoring Scans

The most recent RFI monitoring scans for the GBT can be found here.  These scans provide information on the frequencies that may encounter RFI.


A GUI interface to the GBT RFI scans is available on the GBO computing network.  Information and instructions can be found here.

Schools and Workshops

Observer Training Workshops

The Green Bank Telescope (GBT) Remote Observer Training Workshop will provide the essential skills and knowledge needed to use the GBT and maximize its scientific output. It is intended for experienced astronomers who need to learn the specifics of observing with the GBT. After completing the workshop, an attendee will be certified to use the GBT as a remote observer. The workshop will focus on hands-on training in the observing techniques most relevant to participants (e.g., high-frequency mapping, continuum, pulsar, etc.).

These workshops will be held several times a year and complement traditional on-site training.  The next workshops will be held in February 2023 and fall 2023.

More information can be found here.

Single Dish Training Workshop

 The Single Dish Training School will provide graduate students, post-docs, and experts in other fields of astronomy with both knowledge and practical experience of the techniques and applications of single-dish radio astronomy using the GBT as the primary example. The school will be based around an intensive series of lectures from experts, as well as hands-on radio-astronomy projects and tutorials. Topics to be covered include radio telescope fundamentals, key single-dish science areas, observing and calibration techniques, the impact of weather, the GBT observing procedures and software, and data reduction.

The school will be held once per year. An intensive GBT remote observer training workshop will be held immediately following the school for those who wish to obtain remote observing certification.  The next school is tentatively set to occur in late summer 2023.

More information can be found here.

Joint Observatory Observation Opportunities

Joint Observations with XMM-Newton

The XMM-Newton Project may award up to 3% of GBT open skies observing time by agreement with the Green Bank Observatory, detailed in a Memorandum of Understanding. Similarly, the GBT Time Allocation Committee may award up to 150 ks of XMM-Newton time per year. See the Joint Observations with XMM-Newton page for details.

Joint Observations with Chandra X-ray Observatory

In previous semesters, the community has had the opportunity to propose observing time on NRAO facilities through a joint program with the Chandra X-ray Observatory. The Green Bank Observatory will continue with this program and allocate up to 3% of the open skies time to highly ranked proposals that request time on both Chandra and the GBT.  Proposers to the GBO will have the opportunity to request time on Chandra, to be awarded on the GBO Telescope Allocation Committee (TAC) recommendation and approved by the GBO Director. Up to 120 ksec will be available to GBO/NRAO proposers annually. 

Due to Chandra’s increasingly challenging thermal constraints, the amount of Chandra exposure time available for High Ecliptic Latitude (HEL) targets with |bGal| > 55deg is extremely limited.  If you request joint time on Chandra, please avoid long exposures on such targets if possible.  You must explicitly note the requested amount of Chandra HEL time in the body of your science justification.

N.B. Chandra ToO proposals are not supported under the Chandra-NRAO joint program.  See the Joint Observations with Chandra page for details.

Joint Observations with the Hubble Space Telescope (HST)

By agreement between the NRAO (and continued honoring by GBO) and the Space Telescope Science Institute, STScI will be able to award up to 3% of the available open skies time to highly ranked proposals that request time on both HST and the GBT. In return, STScI has offered 30 orbits per year of HST time for allocation by the GBO/NRAO TAC. See the Joint Observations with HST page for details.

N.B., HST “Snapshot” observations are not supported under the HST-NRAO Joint program since there is no guarantee that Snapshot targets will be completed.

Joint Observations with Swift Gamma-Ray Burst Mission

A joint Swift/NRAO observing program was established to foster correlative observations, detailed in a Memorandum of Understanding. The GBO will continue to honor this agreement.  By this agreement, the Swift Program permits GBO/NRAO to award up to 300 kiloseconds of Swift observing time per year.   Similarly, GBO/NRAO allows the Swift Guest Investigator (GI) Program to award GBO observing time.  See the Joint Observations with Swift page for details.

Joint Observations with Fermi Gamma-ray Space Telescope

We remind the community that it is possible to propose observing time on the GBT through the Fermi Gamma-ray Space Telescope Joint Proposal Opportunity or the Cooperative Proposal Opportunity.  See the Joint Observations with Fermi page for details.

Joint Proposals Between the GBT and NRAO

Observing programs that require combinations of the GBT, VLBA, and/or the VLA should submit a proposal for each of the requested telescopes, with a clear justification for each, as has been the case to date. The submissions will be reviewed as before and considered jointly by the Time Allocation Committee. VLBI proposals that request the GBT or VLA (or the HSA, for example) as elements of the VLBI array do not need separate proposals. Those telescopes can be selected as separate VLBI stations from a VLBA/HSA proposal.

Other Proposal Opportunities

The GBO would like to make users aware that there are additional proposal opportunities as follows:

High Risk Proposals

To maximize its scientific impact through cutting-edge observations, the observatory encourages the submission of high-risk/high-reward proposals.  The high risk/high reward should be designated in the proposal’s abstract and in the science justification.

Filler Programs

Some programs that are not time-critical or require highly subscribed LST ranges can request designation as a “filler program.” Such programs may take advantage of gaps in the GBT schedule, but there is no guarantee that any GBT time will be allocated. Proposals requesting a designation as filler should do so in the proposal abstract and science justification. All filler programs will be given a ranking of C.

Triggered Proposals

Observations for unknown sources that would be triggered by a celestial event (e.g., near-Earth asteroid, comet, fast radio burst, etc.) can be submitted as a triggered proposal. Any accepted triggered proposal will have proprietary rights to observations over any Director’s Discretionary Time (DDT) proposal.