Since 2011, time on the VLBA and other NRAO instruments is scheduled on a semester basis, with each semester lasting six months.  Proposal deadlines are February 1 and August 1, with the February 1 proposal deadline nominally covering time to be scheduled during the following August through January, and the August 1 deadline covering time to be scheduled from February through July.   Time can be requested over multiple semesters if scientifically justified.

Observing proposals may specify the VLBA, or the VLBA in combination with various other VLBI arrays.  It should be noted, however, that proposals to use the European VLBI Network (EVN) and Global cm VLBI are handled by the EVN on a trimester system, with proposal submission deadlines of February 1, June 1, and October 1.  Further instructions are available on proposal preparation and submission for the various types of VLBI arrays.

1. The VLBA alone. A Call for Proposals is published in the NRAO eNews approximately four weeks in advance of each semester submission deadline.  Currently, these deadlines are 5pm (1700) Eastern Time on February 1, and August 1.   (If the deadline falls on a holiday or weekend, it is extended to the next working day.)  VLBA proposals must be prepared and submitted using the NRAO Proposal Submission Tool (PST), available via NRAO Interactive Services.

   All proposals will be reviewed by a Science Review Panel (SRP) in relevant subdisciplines (e.g., solar system, stellar, Galactic, extragalactic, etc.).  The SRP's comments and rating are strongly advisory to the NRAO Time Allocation Committee (TAC), and the comments of both groups are passed on to the proposers soon after each meeting of the TAC (twice yearly) and prior to the next proposal submission deadline.  A detailed description of the time allocation process is available.

   Approved programs are scheduled by the VLBA scheduling officers, who may be contacted at schedsoc@nrao.edu.  A "Guide to Using the VLBA" is available, aimed specifically at inexperienced users but also useful to fill in knowledge gaps for more experienced observers.

2. The High Sensitivity Array (HSA). The HSA comprises the VLBA in combination with the phased VLA, the GBT, Effelsberg, and/or Arecibo; observing time of up to 100 hours per trimester has been reserved for these observations.  Subsets of the HSA may also be requested.  All deadlines and procedures are the same as for the VLBA above.  Further information on "Observing with the High Sensitivity Array" is available in a separate document.
   
   The phased VLA rejoined the HSA as of observing semester 2013A, after a three-year gap during construction of the EVLA.  The functionality of the VLBA's RDBE unit is supported in the VLA by various elements dispersed throughout the system; phased-array output is written directly from the WIDAR correlator to a Mark 5C recorder.  An expanded (but still partial) set of modes will be available for semester 2014B.
   
   Arecibo only operates at frequencies up to 10 GHz, and can view sources only within 19.7° of its zenith; see http://www.naic.edu for further information about Arecibo's properties.
3. The European VLBI Network (EVN) and Global cm VLBI. The EVN consists of a VLBI network of stations operated by an international consortium of institutes (Schilizzi 1995).  The EVN home page provides access to the EVN User Guide.  Included in the guide is an EVN Status Table, giving details of current observing capabilities of all EVN stations; and the EVN Call for Proposals, which specifies EVN session dates and the wavelengths to be observed.  The EVN provides proposal, review, and scheduling mechanisms for such programs, and conducts regular sessions of 2-3 weeks, 3 times per year, to carry out these observations.  EVN proposal deadlines are February 1, June 1, and October 1, with no allowance made for weekends.  Proposals requesting the EVN in combination with the VLBA or other affiliates are classified as "Global cm VLBI".  EVN and Global cm VLBI proposals must be prepared and submitted to the EVN using the EVN's NorthStar Tool.  Such observations will be carried out during EVN sessions.
4. The Global Millimeter VLBI Array (GMVA).  This array consists of the eight VLBA stations outfitted with 3 mm receivers, together with Effelsberg, Pico Veleta, Plateau de Bure, Onsala, Metsähovi, and Yebes.  The European part of the GMVA is coordinated by the Max-Planck-Institut für Radioastronomie.  Observations are block-scheduled in two sessions per year.  More  details about the GMVA are available here.   GMVA proposals are submitted twice per year at the 1 August and 1 February NRAO deadlines, and beginning for the 2014A semester (deadline 1 August 2013), GMVA proposals are only submitted through the NRAO PST, via NRAO Interactive Services, as for the VLBA and HSA.
   

The NRAO SCHED program (Walker 2011) can be used to determine the Greenwich Sidereal Time range during which the VLBI target sources are visible at various stations.  This program can also be used to evaluate the u-v plane coverage and synthesized beams provided by the selected array.

A source position service is available through NRAO to obtain accurate positions for use in correlation (Walker 1999a).  This should be requested simultaneously with the proposal, if not earlier.  Requirements for source position accuracy in correlation are discussed in the "Guide to Using the VLBA".

HSA proposals can request the phased VLA in conjunction with the VLBA and other HSA telescopes, subject to availability of matching observing systems.  Two adjuncts to the VLA Observational Status Summary, "VLBI at the VLA" and VLBI @ the VLA: Scheduling Hints, discuss the available phased-VLA capabilities, and provides instructions for their use.  This sub-section of the VLBA Observational Status Summary describes only the specific compatible modes between the two instruments.

Phased-array data will be limited to two VLA subband pairs.  Any matching bandwidths available on both the VLA and the 4-channel VLBA DDC data system can be used.  Bandwidths must be uniform within each station, across the entire VLBI array, and throughout the entire duration of the observation. In particular, VLA phasing and VLBI observing must be carried out at the same bandwidth.  Bandwidths narrower than 16 MHz are not expected to be useful in most cases, and have not been tested at this time.  Such observations are therefore available only on a shared-risk basis.

Table 7 shows the available bandwidth options, their recorded data rates, and shared-risk status, for maximum VLA-VLBA sensitivity with four DDC channels on the VLBA.

Table 7:  Phased-VLA / VLBA Compatibility

The NRAO has established two categories of proposals for Director's Discretionary Time (DDT). DDT is limited to a maximum of 5% of the total observing time on the VLBA. All DDT proposals should be submitted using the standard NRAO procedures, using the on-line proposal tool. Proposals submitted by any other means (e.g., phone calls, e-mails, faxes, word-of-mouth) will be not be considered.

1. Target of Opportunity. Target of Opportunity (ToO) proposals are for unexpected or unpredicted phenomena such as supernovae in nearby galaxies or extreme X-ray or radio flares. ToO Proposals are evaluated rapidly, with scheduling done as quickly as possible and as warranted by the nature of the transient phenomenon. ToO Proposals are evaluated on the basis of scientific merit by Observatory staff with the necessary scientific expertise. Those staff also assess the technical feasibility of the ToO proposals.  The proprietary period for data obtained by ToO Proposals will be assessed on a case-by-case basis but will be no more than six months.
2. Exploratory Time. Exploratory Time proposals are normally for requests of small amounts of time, typically a few hours or less, in response to a recent discovery, possibly to facilitate future submission of a larger proposal. In general, there will not be a need for immediate scheduling with these proposals, but they may need to be observed with the VLBA without waiting for an entire proposal cycle. The possibility that a proposer forgets about or misses an NRAO proposal deadline, or just discovered that he/she was granted time for a particular source on some other telescope, will not constitute sufficient justification for granting observing time by this process. Thus, Exploratory Time proposals must include a clear description of why the proposal could not have been submitted for normal review at a previous NRAO proposal deadline, and why it should not wait for the next proposal deadline. Exploratory Time proposals are evaluated on the basis of scientific merit by Observatory scientific staff.  Those staff also assess the technical feasibility of the Exploratory Time proposals. Notification of the disposition of an Exploratory Proposal normally will be within three weeks of receipt of the proposal; some of these proposals may be put in a queue such that they may or may not be observed. The proprietary period for data obtained by Exploratory Proposals normally will be six months.

Students planning to use one or more NRAO telescopes for their PhD dissertation must submit a "Plan of Dissertation Research" of no more than 1000 words with their first proposal.  This plan can be referred to in later proposals.  At a minimum it should contain a thesis time line and an estimate of the level of NRAO telescope resources needed.  The plan provides some assurance against a dissertation being impaired by an adverse review of one proposal when the full scope of the project is not seen.  The plan can be submitted via NRAO Interactive Services.  Proposers are reminded to prepare the plan comfortably in advance of the proposal deadline.  New thesis plans must be in pdf format to enable science reviewers to easily access the plans.  Students who have not yet graduated but have active plans on file should consider updating those plans to a pdf format if they are not already in that form.

NRAO maintains a program to support research by students, both graduate and undergraduate, at U.S. universities and colleges.  Regular and Large proposals submitted for the VLA, VLBA, and GBT, and any combination of these telescopes, are eligible.  New applications to the program may be submitted along with new observing proposals at any proposal deadline.  Details of this program can be found at https://science.nrao.edu/opportunities/student-programs/studentprograms