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 2023B (2023 August 01 – 2024 January 31) or multi-semester proposals.
• Global mm VLBI Array (GMVA) proposals for 2023 Session II (October 11 – 16), or later sessions.

VLBA Proposals

The VLBA provides ultra-high angular resolution for astrophysical studies including:

• 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 1000 hours of VLBA observing time are expected to be available for Open Skies in observing semester 2023B. In recent semesters, there has been less pressure in the GST range 0600 - 1800 hours as compared to 1800 - 0600 GST, 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 Large Proposal Policy.  Large proposals which require multi-semester observations are often supported.

Filler proposals are also encouraged - these are scientifically useful programs that can be scheduled over a large range of GST, requiring fewer than eight VLBA stations, with low-frequency weather constraints, and 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 Large proposal policy still applies); either Large or Regular filler proposals can be multi-semester proposals if scientifically justified.

Observing Capabilities for 2023B

For the 2023B 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.

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 2023B 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.

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 can be found at the VLBA RSRO program. For details about participating in the RSRO program, see the RSRO Considerations section in the Guide to Proposing for the VLBA.

Proposers should be aware that RSRO capabilities are generally not approved at priority A, owing to the level of risk associated with these observations.

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 Sensitivity 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. 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 2023B, mostly in the D (~ Oct 2023 – mid Jan 2024) configuration, although a limited amount of time should be available in the A (~ Aug 2023 – Sep 2023) and C (~ late Jan 2024 – Feb 2024) configurations, and during reconfigurations. 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.

• The Green Bank Telescope's (Gb) has significant scheduling constraints because of reduced open-skies time. Therefore, HSA proposals requesting Gb will likely have to be ranked at least in or near the top 10-15% of all AUI telescope proposals in order for GBT time to be approved. 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. Additionally, proposers should be aware that long scheduling blocks (more than 6 hours) will be difficult to schedule owing to constraints coming from non-open skies time. Proposers are encouraged to make clear in the technical justification section any constraints about how observing time could be broken into smaller pieces without adversely affecting the proposed science; include information as relevant regarding maximum elapsed time of a split schedule and minimum scheduling block lengths.

Observations using the GBT 6-cm 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. 

• 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 by the 2023 February 01 deadline will be considered for scheduling in 2023 Session II (October 11 - 16), 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 HSA section above, GBT time available for VLBI has been reduced due to its new partnership arrangements.

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. 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.

Another opportunity to propose VLBI observations using the phased ALMA telescope will be available at this Call for GMVA proposals:

It is expected that phased ALMA will participate in some GMVA observations during ALMA Cycle 10 (~Oct 2023 - Sept 2024; it is anticipated that the ALMA Cycle 10 Call for Proposals will be open in 2023 March). GMVA session dates for 2024 are not yet fixed but Session I in 2024, which is traditionally in the period March - May, should provide an opportunity for GMVA + ALMA observing. In ALMA Cycle 10, 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 10 proposals. For this, all proposers (PI and Co-Is) must be registered ALMA users (see: http://www.almascience.org).

Restrictions on GMVA+ALMA proposals in Cycle 10:

• 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, the option of "passive" phasing was introduced in Cycle 8. In this mode, the ALMA array is periodically phased on a bright calibrator close in angular distance to the science target. (This mode has been in use for VLBI at the VLA for many years.) There are thus no restrictions on the flux density of science targets using passive phasing (aside from SNR considerations on VLBI baselines). The properties of the phasing calibrator must meet the same criteria as for actively phased observations, and it is recommended that the phasing calibrator lie within an angular separation of no more than 6 degrees from the science target at 3mm or 10 degrees 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 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.
• There is a cap for VLBI of 5% of ALMA Cycle 10 observing time. As time for GMVA observations will thus 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 may also be directed to the NRAO Helpdesk.