The HSA comprises the VLBA, phased Very Large Array (VLA), Green Bank Telescope (GBT), Effelsberg, and Arecibo telescopes, and subsets thereof.  All of these are equipped with instrumentation compatible with the VLBA observing capabilites described in the Station Signal Processing section.  VLBI observations combining the VLBA with any one or more of the other four HSA stations can be requested in a single HSA proposal.  Proposal deadlines for the HSA coincide with those for the VLBA alone, as described in Section 15.  Further information on "Observing with the High Sensitivity Array" is available in a separate document.

• The VLA is available as a single phased array ("Y27"; no subarrays) with two independently-tunable subband pairs, one polarization pair (RCP+LCP) in the A0/C0 basebands and the other (RCP+LCP) in the B0/D0 basebands.  Setups also matching the VLBA PFB and DDC (4- and 8-channel) observing systems are available on the VLA.  The VLA must be set up to match the VLBA; mixed modes are not allowed.

Bandwidths must be uniform 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.  Subband bandwidths of 16 MHz and wider are available as a general capability.  Bandwidths narrower than 16 MHz may work if the source is strong enough, but are expected to be of limited use, have not been tested, and are available only on a shared-risk basis.

Two adjunct documents: VLBI at the VLA and Guide to Observing with the VLBA: HSA/GMVA/Global VLBI, discuss the available phased-VLA capabilities, and provide instructions for their use.

• The GBT, like the VLBA, will be transitioning to a new partnership arrangement for semester 2017A.  Time available for VLBI on the GBT will be dramatically reduced compared to past observing semesters due to this change.  Proposers should only include the GBT in the proposal if it is essential for the science and if it is clearly justified in the text.

The GBT is frequency agile (with some limitations) at all its bands.

The GBT's 6 cm receiver is similar to the VLBA's new system, but does differ in converting to circular polarization at ambient temperature.  Tests have seen substantial polarization leakage between the RCP and LCP channels. Proposals to use this receiver will be considered only for total-intensity observations.  Such proposals should request full dual-polarization modes for both observation and correlation, and careful calibration of the leakage terms should be included in the data analysis.

Proposals including the GBT in an HSA observation must include time to set up the telescope (pointing, focus, etc.) prior to the start of the observation. This can take 0.5-1 hour depending on the frequency.

Further information on use of the GBT may be found in Section 5.7 and Chapter 7 of the GBT Proposer's Guide, in the GBT Observer's Guide, and in the VLBI at the GBT page.

• The Effelsberg telescope supports both of the VLBA observing systems, and is frequency agile at 5 GHz and above.

Further information on use of Effelsberg is available at the Effelsberg HSA page.

• The Arecibo 305-m telescope is currently available only with the PFB observing system.  An RSRO project is encouraged to help qualify the 4-channel DDC observing system at Arecibo.

The telescope operates at frequencies up to 10 GHz, and  can observe between declinations of -1 and +38 degrees, with transits ranging up to 2.75 hours at 18.5 degrees.

Further information on use of Arecibo is available in The VLBI Guide at Arecibo and The New Users Guide.

The GMVA observes at 3mm wavelength, using 8 VLBA stations, GBT, Effelsberg, Pico Veleta, Onsala, Metsaehovi, Yebes, and Korean VLBI Network (KVN) telescopes. (The VLBA stations in high-humidity locations at HN and SC are not equipped with 3mm receivers.)

The European part of the GMVA is coordinated by the Max-Planck-Institut für Radioastronomie (MPIfR), which hosts the main GMVA website.  Proposal deadlines for the GMVA coincide with those for the VLBA alone, as described in Section 15.  Observations are block-scheduled in two sessions per year, typically in April-May and September-October, and correlated at the MPIfR in Bonn.

In GMVA observations, all telescopes operate at their highest possible bandwidth.  Currently, this is a 2 Gbps data rate in all cases except at the KVN telescopes, which operate in a compatible 1 Gbps mode.

The EVN is a VLBI network of stations operated by an international consortium of institutes (Schilizzi 1995).  The EVN home page provides access to the EVN User's 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.  Unlike the VLBA, HSA, and GMVA, the EVN operates on a trimester cycle, with proposal deadlines on February 1, June 1, and October 1.

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.  Approved observations will be carried out during EVN sessions.