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 2025A (2025 February 01 – 2025 July 31) or multi-semester proposals.
• Global mm VLBI Array (GMVA) proposals for 2025 Session I (2025 April 24 – 29), or later sessions.

VLBA Proposals

It is anticipated that there will be around 570 hours available for new Open Skies proposals on the VLBA in observing semester 2025A. In addition to Regular proposals, Large proposals (those which request 200 hours or more) are welcome on the VLBA. Such proposals should follow the instructions for Large proposals in the NRAO & GBO Users' Policy. It is anticipated that there will be around 150 hours available for new Large proposals on the VLBA in observing semester 2025A. 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.

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.

Science Time Available

The availability of science time varies with GST, with less time available during daytime periods due to maintenance and at other GSTs (in particular in the 18:00 – 06:00 range) due to multi-semester proposals approved at earlier calls that are continuing in 25A. Plots below detail the anticipated time that is available to be allocated for different observing priorities in 25A.

The upper plot shows the total estimated availability across all frequencies, while the lower plot shows the estimated availability for high frequency (> 12 GHz) observing. The upper plot also shows the estimated time available by GST hour for large proposals. This is determined by the limitation of time allocated to large proposals to 50% of the open skies time at any given GST and the time already allocated to large proposals from previous calls. 

Proposal pressure is a function of both availability and demand. In recent semesters, there has been lower demand in the GST range 06:00 – 18:00 than in the GST range 18:00 – 06:00 (around the Galactic center), leading to lower proposal pressure in the 06:00 – 18:00 range. We expect this trend to continue. 

Heightened pressure for VLBA time has increased the importance of setting the GST ranges of sessions accurately in the PST and, if possible, avoiding the regions of highest pressure (normally 18:00 - 06:00 GST, as mentioned above) that can be seen on the pressure plot in the TAC Report.

As detailed in the PST Manual, the sessions defined in the PST are used by the TAC in their assignment of scheduling priorities, thus sessions that include the times of high pressure are likely to receive a lower scheduling priority than sessions with the same linear-rank science score in times of lower pressure. In particular, sessions with a 24-hour GST range (00:00 - 24:00) are guaranteed to pass through high pressure times and should be avoided if not essential (e.g., for actual 24 hour observations).

The "Guidelines for Creating VLA Sessions" in the PST Manual contain advice that is also useful in creating VLBA sessions; in particular, proposals with a small number of sources should put each target source into a separate session and sources with a large number of sources should be broken into smaller sessions, grouping nearby sources by GST range.

Users are also encouraged to use Sched or Planobs to manually define their GST range as detailed in the Guide to Proposing rather than using the Calculate Min/Max GST button in the PST.

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.

Observing Capabilities for 2025A

For the 2025A 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 Recently Updated:

• The VLBA 3mm sensitivities were updated prior to semester 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 capabilities are offered under the SRO program during the 2025A 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.

and 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 to future semesters if they cannot be scheduled for reasons associated with the shared risk component(s) of the observations, even if awarded scheduling 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 scheduling priority A, owing to the level of risk associated with these observations. Also, RSRO proposals will not be carried over to future semesters 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 2025A in the D (to ~ Feb-May 2025) and C (from ~ May-Aug 2025) configurations, and during reconfigurations. If requesting future semesters, please note that BnA (~ Jan – Feb 2026) 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. 

The installed receivers change regularly on the GBT and not all VLBA bands are available every semester. 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 2025A deadline will be considered for scheduling in 2025 Session I (2025 April 24–29), 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 was updated for the 24B semester, so sensitivity calculations from proposals submitted in earlier semesters will need to be re-made.

The GBT is expected to be available for 2025 Session I.

The Apex telescope in Chile is expected to be available at 3mm in 2025 Session I.

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 in a GMVA proposal should be quantitatively justified. Apex, the KVN, or the Greenland Telescope (GLT) must be specified by entering "Apex", "KVN" or "GLT" as "Other" entries in the PST. If the Hancock (HN) or Saint Croix (SC) VLBA antennas (which are not normally included in the GMVA) are requested at 7mm, these should be specified by entering "HN" or "SC" 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.

New GMVA proposals requesting phased ALMA should not be submitted at the semester 2025A deadline.  Only proposals which were re-proposed at the Cycle 11 ALMA deadline – those that were approved for GMVA Session 2024/I but failed at ALMA – will be considered.

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.