Proposal submission information for the following three combinations of telescopes are detailed in individual sections below:

• Very Long Baseline Array (VLBA) and High Sensitivity Array (HSA) proposals requesting time in Semester 2024B (2024 August 01 – 2025 January 31) or multi-semester proposals.
• Global mm VLBI Array (GMVA) proposals for 2024 Session II (October 10 – 15), or later sessions, including proposals requesting the GMVA with ALMA for 2025 Session I (spring; date T.B.D.).

VLBA Proposals

The VLBA is a truly unique facility that provides unprecedented, ultra-high angular resolution for studying a wide range of astrophysical phenomenon. Sources that may be studied by the VLBA include, but are not limited to:

• Non-thermal continuum emission, including polarimetry, from active galactic nuclei (AGN), Galactic micro-quasars, pulsars, and other sources.
• Maser emission lines of OH (1.7 and 6.0 GHz), CH₃OH (6.7 and 12.2 GHz), H₂O (22 GHz), SiO (43 and 86 GHz) and other molecules, and numerous thermal absorption lines, in a variety of Galactic and extragalactic circumstances.
• Multiple-phase-center surveys across the primary beam.
• Parallax and proper motion via differential astrometry of a variety of stars, star-forming regions, and nearby extragalactic objects, at accuracies as good as 10 microarcsec.
• Absolute astrometry at accuracies of ~200 microarcsec to expand the International Celestial Reference Frame.

Overall information about the VLBA is available in the VLBA Observational Status Summary (OSS); specific sections relevant to various proposal types are linked below.

VLBA proposals must be prepared and submitted using the NRAO Proposal Submission Tool (PST), accessible via NRAO Interactive Services. Use of the PST requires registration by all proposers, including co-investigators, in the NRAO User Database.

Approximately 800 hours of VLBA observing time are expected to be available for new Open Skies proposals in observing semester 2024B. In recent semesters, there has been less proposal pressure in the GST range 06:00 – 18:00 than in the GST range 18:00 – 06:00, and we expect this trend to continue.

Most approved VLBA observations are performed dynamically; for those dynamic observations, users must either submit their observing (.key) files (to vlbiobs@nrao.edu) before the beginning of the semester (February 01 or August 01 for A and B semesters, respectively), or contact the VLBA Scheduler (schedsoc@nrao.edu) before those dates to avoid a reduction in scheduling priority. Early submission of schedules maximizes the opportunity of dynamic observing and assists in the efficient scheduling of the VLBA.

VLBA Large and Filler Proposals

Large proposals (those which request 200 hours or more) are particularly encouraged and welcome on the VLBA.  Such proposals should follow the instructions for Large proposals in the NRAO & GBO Users' Policy.  Large proposals which require multi-semester observations are often supported.

Filler proposals are also encouraged – these are scientifically useful programs, preferably utilizing the lower frequency bands of the VLBA, that can be scheduled over a large range of GST, require fewer than eight VLBA stations, and use short (2 – 6 hours) scheduling blocks.  Such projects can help to fill gaps in the dynamic observing schedule.  Filler proposals can be submitted as Regular proposals or even Large proposals (the Users' Policy instructions for Large proposals still apply); either Large or Regular filler proposals can be multi-semester proposals if scientifically justified. Filler proposals should be explicitly identified as such and will normally only be considered for filler time (i.e., priority C) regardless of their ranking. See the instructions for Filler Programs in the NRAO & GBO Users' Policy for further information.

Observing Capabilities for 2024B

For the 2024B semester the General Observing (GO) capabilities are given in the Offered VLBA Capabilities during the Next Semester section of the Observational Status Summary (OSS) and are summarized in the following table. Several additional capabilities are available to proposers through the Shared Risk Observing (SRO) and Resident Shared Risk Observing (RSRO) programs, as described below.

New and Updated:

• The VLBA 3mm sensitivities have been updated for 24B. The new values have been included in the EVN calculators and can be found at Frequency Bands and Performance in the OSS. 

The GO capabilities being offered are:

The VLBA operates two data systems, a Polyphase Filterbank (PFB), and a Digital Downconverter (DDC). These are described in detail in the Roach Digital Backend (RDBE) section of the VLBA OSS, which also includes suggestions for selecting the optimal observing system for various scientific goals. For the best continuum sensitivity (i.e. 4096 Mbps) at most receiver bands, or for the most flexible observing setups, the DDC is the better choice. For continuum observations using the 20cm or 13cm receiver bands, the PFB provides a setup using 2048 Mbps that can reduce the impact of prevalent radio-frequency interference (RFI). It is worth noting that while the DDC mode provides wider bandwidth (4096 Mbps recording) and tuning flexibility, the PFB mode (2048 Mbps recording) provides more accurate amplitude calibration and should be used if <10% flux density accuracy is required.

Proposals requiring significant additional correlator resources, such as multiple phase centers per field or multiple pulsar phase bins, should consider mechanisms to support the correlation without adversely affecting the throughput of other projects. These should be entered in the technical justification section of the proposal.

VLBA Shared Risk Observing

The VLBA Shared Risk Observing (SRO) program allows observers access to capabilities that are essentially commissioned, but are not well tested. The following capability is offered under the SRO program during the 2024B semester:

• Baseband Data Copy: Limited amounts of raw data recorded at each station can be copied to user-supplied media for correlation at a different location. See the VLBA OSS SRO section for more details.

For HSA observations only: 

• Wideband VLA for VLBI: Enables recording of VLA WIDAR continuum-mode correlations at full 2-GHz BW during VLA phased array (Y27) VLBI observations. See the HSA Station Notes for more details.

Proposers should be aware that SRO projects will not be carried over if they cannot be scheduled for reasons associated with the shared risk component(s) of the observations, even if awarded priority A.

VLBA Resident Shared Risk Observing

The VLBA Resident Shared Risk Observing (RSRO) program provides users with early access to new capabilities in exchange for a period of residency in Socorro to help commission those capabilities. For example, the phased-VLA system was developed through RSRO programs. Users are encouraged to conceive and propose innovative ideas for new VLBA capabilities. Some staff suggestions, such as VLBA real time correlation, can be found at the VLBA RSRO program section of the VLBA OSS. For details about participating in the RSRO program, see the RSRO Considerations section of the Submission Guidelines page in the Guide to Proposing for the VLBA.

Users wishing to propose a RSRO program should select the VLBA RSRO observing mode in the resources section of the PST in order to submit their proposal as a RSRO program. 

Proposers should be aware that RSRO projects are generally not approved at priority A, owing to the level of risk associated with these observations. Also, RSRO proposals will not be carried over if they cannot be scheduled, similar to SRO proposals.

High Sensitivity Array (HSA) Proposals

The HSA comprises the VLBA, phased VLA, GBT, and Effelsberg telescopes. Similar to the VLBA, all of the HSA stations can observe at 4096 Mbps (General Observing). The EVN Observation Planner or old EVN Calculator can select 4096 Mbps for HSA sensitivity estimates. Details on the HSA telescopes are documented in the HSA section of the VLBA OSS, and special considerations on proposing and observing are listed in the HSA page of the Guide to Proposing for the VLBA.

VLBI observations combining the VLBA with any one or more of the other three HSA stations can be requested in a single HSA proposal. HSA proposals requesting only VLBI use of HSA telescopes should not be identified as Joint proposals. However, separate proposals must be submitted for any non-VLBI use of any requested telescopes.

HSA proposals must be prepared and submitted using the NRAO Proposal Submission Tool (PST), accessible via NRAO Interactive Services. Use of the PST requires registration by all proposers, including co-investigators, in the NRAO User Database. The inclusion of HSA stations should be quantitatively justified in the proposal. 

HSA Station Notes

• The phased Very Large Array (Y27) will be available for HSA observing in Semester 2024B in the B (to ~ Sep 2024) and A (from ~ Oct 2024) configurations, and during reconfigurations. Note that BnA (~ Sep – Oct 2024) configuration is reserved for VLASS therefore scheduling HSA observations during BnA may be difficult. Please note that high frequencies (at receiver bands 22 GHz and above) have better phasing in the more compact configurations (C and D). High frequency phasing in the extended configurations in the summer can be quite difficult. For more details, see VLBI at the VLA.

Wide-band correlation of VLA-only data in parallel with the VLBI recording is offered as Shared Risk Observing (SRO). This supports standard VLA 8-bit continuum modes with a 2-GHz bandwidth. If your science requires these wide-band correlations, you should explain this and justify your need for wide-band correlations in the top box of the Technical Justification of your proposal in the PST. Proposals for recording wide-band correlation of VLA-only data using modes other than standard VLA 8-bit continuum with 2-GHz bandwidth can be submitted as Resident Shared Risk Observing (RSRO) proposals; see the VLBA RSRO section of the OSS for details of the RSRO program.

• The Green Bank Telescope (Gb): Proposers should clearly justify the need for the GBT in the text of the proposal. All proposers requesting the GBT should include any needed setup and overhead time in the total time request for their proposals.

Observations using the GBT 6-cm (C band) receiver as part of the HSA must be taken, correlated, and calibrated in full Stokes mode. Due to the large cross-talk between polarizations, only total intensity (Stokes I) data will be usable. 

Please see the GBT proposal call for receiver availability, time availability, and other relevant factors regarding the GBT's participation in HSA observations.

• The Effelsberg (Eb) 100-m telescope supports both the PFB and DDC observing systems available on the VLBA. Consult this web page for more detailed information about the Eb HSA station.

Global mm VLBI Array (GMVA) Proposals

GMVA proposals submitted for the semester 2024B deadline will be considered for scheduling in 2024 Session II (October 10–15), or later sessions.

Complete information on the GMVA is available at the GMVA website. Ongoing special considerations are documented in the GMVA section of the VLBA OSS; new features and/or special cases are cited here.

As noted in the VLBA section above, the VLBA sensitivity at 3mm has been updated so sensitivity calculations from proposals submitted in previous semesters will need to be re-made.

NOTE: The GBT will not be available for observations with the GMVA for 2024 Session II but is expected to be available for 2025 Session I (e.g., for observations proposed at this deadline including ALMA with the GMVA).

GMVA proposals must be prepared and submitted using the NRAO Proposal Submission Tool (PST), accessible via NRAO Interactive Services. Use of the PST requires registration by all proposers, including co-investigators, in the NRAO User Database. The inclusion of the GBT and/or ALMA in a GMVA proposal should be quantitatively justified. ALMA, the KVN, or the Greenland Telescope (GLT) must be specified by entering "ALMA", "KVN" or "GLT" as "Other" entries in the PST. Proposers should use the EVN Observation Planner tool to determine their sensitivities and upload the PDF summary or summaries on the Technical Justification page of the PST. 

NOTE: The 3mm noise estimates provided by the Observation Planner (and the old EVN Sensitivity Calculator) use SEFD values determined during reasonably good weather and while the antennas were performing well. Because real-world conditions are often less than ideal, the actual noise levels obtained during observations may be significantly worse.  This is especially true for GMVA observations, which are observed on fixed dates and cannot be rescheduled due to poor weather.  When planning for GMVA observations, users are encouraged to assume the actual noise will be roughly 3 times higher than the tool’s estimate.

Observations at 7mm with the VLBA antennas can be scheduled as part of a GMVA program during what would otherwise be gaps in observing while other antennas are making pointing or calibration observations. Such observations can be included in GMVA proposals.

The GMVA will record at the highest bit rate which instrumentation and resources permit. Currently all telescopes will record at 4096 Mbps.

An opportunity to propose VLBI observations using the phased ALMA telescope in conjunction with the GMVA is available at this Call for Proposals:

It is expected that phased ALMA will participate in some GMVA observations during ALMA Cycle 11 (~Oct 2024 - Sept 2025; it is anticipated that the ALMA Cycle 11 Call for Proposals will be open in April 2024). GMVA session I in 2025 (tentatively scheduled for April 24–29) should provide an opportunity for GMVA + ALMA observing. In ALMA Cycle 11, ALMA expects to support 3mm and 7mm VLBI observations. Spectral line VLBI is supported at both wavelengths, so GMVA + ALMA spectral line observations can be proposed.

Proposers should:

• specify "ALMA" in the Other Stations text field in the PST
• select the default GMVA 3mm or 7mm observing mode of 4096 Mbps, dual polarization
• specify the amount of time and GST range(s) needed for ALMA separately, either in Session Constraints or Comments, or in the Technical Justification.

A separate proposal to ALMA must also be submitted at the deadline for ALMA Cycle 11 proposals. For this, all proposers (PI and Co-Is) must be registered ALMA users (see the ALMA science website).

Restrictions on GMVA+ALMA proposals in Cycle 11:

• GMVA observations with ALMA will be limited to a fixed recording mode, which currently provides 4096 Mbps on all baselines.
• Direct phasing of the ALMA array is limited to targets with a correlated flux density > 0.5 Jy at 3mm or > 0.35 Jy at 7mm, contained within an unresolved core on ALMA baselines up to 1 km. Direct phasing on the science target ("active" phasing) thus puts a lower limit on the brightness of the science target.
• For weaker sources, ALMA offers the option of "passive" phasing. In this mode, the ALMA is phased on a bright phasing calibrator close in angular distance to the science target. (This mode has been in use for VLBI at the VLA for many years). The phasing calibrator has to be brighter than 0.5 Jy at 3mm (>0.35 Jy at 7mm) and be located within 6 degrees from the science target (<10 deg at 7mm). Proposers must specify the phasing calibrator in their proposal; consult the ALMA calibrator catalog.
• In order to make a clean linear-to-circular polarization transformation of ALMA recordings, any target source must be observed at each frequency for a duration of at least 3 hours (breaks for calibrators permitted) to sample a range of parallactic angles.
• Large ALMA Programs (>50 hours of observing time) are not permitted because phased ALMA is a non-standard mode.
• No long-term programs may be proposed, and no proposals will be carried over into the next cycle.
• As time for ALMA observations with the GMVA will be scarce, proposals should include a quantitative justification as to why ALMA is essential for the goals of the project.

Documentation and Assistance

Detailed information about the VLBA instrument, its capabilities, observing strategies, proposal preparation and submission, and observation preparation, can be found in the VLBA Observational Status Summary, at the Guide to Proposing for the VLBA, and at the Observing with the VLBA web pages. Questions should be directed to the NRAO Helpdesk.