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 2021B (2021 August 01 - 2022 January 31) or multi-semester proposals.
• Global mm VLBI Array (GMVA) proposals for 2021 Session I (Apr 22 - 27), 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 to 1200 hours of VLBA observing time are expected to be available for Open Skies in observing semester 2021B.  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.

NRAO Staff Assistance with VLBA Observation Setup and Data Reduction Continues

The NRAO offers extra staff assistance with VLBA observational setups and data reduction. This offer is currently limited in scope to new or novice VLBA users, and pertains to proposals which request only the VLBA (i.e. no HSA, GMVA, or cm Global proposals). There are also limitations in some capabilities which will be supported under this program, for example, receiver bands or correlator setups.  In order to request this assistance, please mark the checkbox on the General page of VLBA proposals in the PST, and provide input on your proficiency, and/or specific requests in the associated text box.  Note that requesting this service does not automatically mean it will be provided for your proposal. An internal NRAO committee will review these requests, and the proposal disposition letter will inform the proposers whether or not the service can be provided.

VLBA Filler Proposals

Filler proposals, which are scientifically useful programs that can be scheduled at a large range of GST times, with fewer than 8 stations, low-frequency weather requirements, and short (2 - 6 hours) scheduling blocks, are especially encouraged on the VLBA.  Such  projects can help to fill gaps in the dynamic observing schedule.

Observing Capabilities for 2021B

For the 2021B 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).

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 2021B semester:

• Baseband Data Copy: The 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

We are saddened by the loss of the 305-m Arecibo telescope; obviously the HSA will no longer be able to use Arecibo as an HSA station. The HSA thus now 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.  Arecibo is no longer selectable as an HSA station in the PST.

HSA Station Notes

• The phased Very Large Array (Y27) will be available for HSA observing in Semester 2021B, in the C (~Aug - Sep 2021) and B (~Sep 2021 - Jan 2022) configurations.  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 3mm VLBI Array (GMVA) Proposals

GMVA proposals submitted by the 2021 February 1 deadline will be considered for scheduling in 2021 Session II  (Sep 30 - Oct 05), 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.

The GMVA will record at the highest bitrate which instrumentation and resources permit. Currently all telescopes will record at 4096 Mbps.  (A note about KVN recording was removed from this paragraph on Jan 07, 2021; the KVN can record at 4096 MBps.)

Note that the availability of NOEMA (Plateau de Bure) for GMVA Session II in 2021 and later sessions is not confirmed.

The Mexican Large Millimeter Telescope Alfonso Serrano (LMT) may be available to participate in some GMVA observations:

To use the LMT as part of the GMVA, a separate proposal (in addition to the GMVA proposal) must be submitted to the LMT, with a PI from the LMT community, as described on the LMT web page.

The PI of this LMT proposal need not be the leader (i.e., PI) of the GMVA proposal, but must agree to take responsibility for all necessary arrangements for observing at the LMT, and must be identified in the GMVA proposal.

The proposal(s) should emphasize the value and importance of adding the LMT to the GMVA observations.

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 8 (~Oct 2021 - Sept 2022; it is anticipated that the ALMA Cycle 8 Call for Proposals will open again in 2021 March).   GMVA session dates for 2022 are not yet fixed but Session I in 2022, which is traditionally in the period March - May, would provide an opportunity for  GMVA + ALMA observing.

GMVA+ALMA proposals which were submitted to the GMVA in February 2020 for ALMA Cycle 8 have been reviewed by the GMVA (but not by ALMA) and will be held over for possible scheduling in GMVA Session I 2022.  A separate proposal to ALMA for these projects needs to be submitted following the (new) ALMA Call for Cycle 8 proposals in March 2021.   However, proposers may instead submit a revised version of their GMVA+ALMA proposal to the GMVA by the February 2021 deadline, which will then be re-evaluated by the GMVA observatories.  Proposers should give the code number of the earlier proposal which is superseded.  Original and re-submitted proposals will compete with any new GMVA+ALMA proposals submitted via the NRAO PST at the February 01, 2021 deadline.

Proposers should:

• specify "ALMA" in the Other Stations text field in the PST
• select the default GMVA 3mm 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 8 proposals.  For this, all proposers (PI and Co-I's) must be registered ALMA users (see:  http://www.almascience.org).

Restrictions on GMVA+ALMA proposals in Cycle 8:

• GMVA observations with ALMA will be limited to a fixed recording mode, which currently provides 4096 Mbps on all baselines.  The KVN will not be available in this mode.  Only VLBA telescopes will be available at 7mm in this mode.
• Direct phasing of the ALMA array is limited to targets with a correlated flux density > 0.5 Jy 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 will be 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 5 degrees from the science target.  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 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 8 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.