This sub-section describes the instrumentation that collects and amplifies radio-frequency (RF) radiation from a source, converts, and transmits it to the station control building.  Napier et al. (1994) includes further information on most of the following aspects.

The antenna brings the RF signals to a focus at one of ten feeds.  The main reflector is a 25-m diameter shaped figure of revolution with a focal-length-to-diameter ratio of 0.354.   A 3.5-m diameter Cassegrain subreflector with a shaped asymmetric figure is used at observing wavelengths shorter than 30 cm, while the prime focus is used at longer wavelengths.  The antenna features a wheel-and-track mount, with an advanced-design reflector support structure.  Antenna motions, designed to facilitate rapid source changes, are at 30° per minute in elevation and 90° per minute in azimuth.

The feed couples free-space electromagnetic waves into waveguides for transmission to the receiver system. Feeds at observing wavelengths shorter than 30 cm are located on a ring at the offset Cassegrain focus, and are selected by rotation of the subreflector with a maximum transition time of about 20 seconds. A frequency-selective dichroic system enables simultaneous 13/4-cm observations. The 90- and 50-cm feeds are crossed dipoles mounted on the subreflector near prime focus; simultaneous 90/50-cm observations are possible.

The polarizer extracts orthogonal circularly-polarized signals, which are routed separately to dual receiver channels. For receivers below 30 cm, the polarizer is cooled to cryogenic temperatures.

The receiver amplifies the signal. Most VLBA receivers use HFET (Heterostructure Field Effect Transistor) amplifiers at a physical temperature of 15 K, but the 90- and 50-cm receivers use GaAsFETs (Gallium Arsenide FETs) at room temperature. All receivers produce dual-polarization outputs, in opposite hands of circular polarization.

The IF converter mixes the receiver output signals with the first LO generated by a front end synthesizer. Two signals between 512 and 1024 MHz are output by each IF converter, one for each sense of circular polarization. The same LO signal is used for mixing with both polarizations in most cases. However, the 4 cm IF converter has a special mode that allows both output signals to be connected to the RCP output of the receiver and to use separate LO signals, thereby allowing the use of spanned bandwidths exceeding 512 MHz. Also, the 90 cm and 50 cm signals are combined and transmitted on the same IFs. The 50 cm signals are not frequency converted, while the 90 cm signals are upconverted to 827 MHz before output.

Four IFs, designated A, B, C, D, carry the IF signals from the antenna vertex room via cables to the station control building. Normally only two IFs are in use at a time, with the signals from each IF converter transmitted via A and C, or B and D; by convention, RCP is normally carried by IFs A and B, and LCP by C and D. However, switching is available to support several 4-IF configurations needed for special cases. These include dual-polarization observations at two arbitrary frequencies anywhere within the 4-8 GHz range of the new 6-cm receiver, and combinations of dual IF outputs from both the 13- and 4-cm receivers (using the dichroic system described in the paragraph on feeds above), or the 90- and 50-cm receivers.

Essential auxiliary instrumentation, required to make simultaneous observations feasible at VLBA stations separated by thousands of kilometers, is described in this sub-section.

A hydrogen maser provides an ultra-stable frequency reference at each VLBA station.  Its standard signals, at 100 MHz and 5 MHz, and multiplied versions thereof, are used throughout the station electronics, both in the antenna and in the station building.

The front end synthesizer generates the reference signals used to convert the receiver output from RF to IF. The lock points are at (n× 500) ± 100 MHz, where n is an integer. The synthesizer output frequency is between 2.1 and 15.9 GHz. There are 3 such synthesizers, each of which is locked to the maser. One synthesizer is used for most frequency bands, but two are used at 1 cm, at 7 mm, 3 mm, and for the 4 cm wideband mode.

VLBA stations support several different types of calibration measurements.

Two calibration signals are injected near the beginning of the primary data path, detected elsewhere in the VLBA system, and applied subsequently:

The switched-noise system injects well calibrated, broadband noise, switched at 80 Hz in a 50% duty-cycle square wave.  This noise signal is synchronously detected in the RDBE, to provide a time-tagged system-temperature table that is delivered with the primary fringe visibility data.  Application of these measurements for amplitude calibration is discussed separately.

The pulse-cal system injects a series of pulses at intervals of 1.0 or 0.2 microseconds, to generate monochromatic, phase-stable tones, spaced at multiples of 1 MHz or 5 MHz.  The tones are detected currently in the VLBA DiFX correlator, and eventually will be available from the RDBE. Application of these measurements for phase calibration is discussed separately.

There is also a round trip cable calibration scheme that monitors the length of the signal cables, to enable corrections for temperature and pointing induced variations.

The RDBE replaces much of the VLBA's original analog signal processing in the station control building.  The baseband converters, in particular, are eliminated by sampling directly from the IF outputs of each station's receivers, with 8-bit precision.  All subsequent processing is performed digitally.  To preclude confusion in the following descriptions, please refer to these definitions of essential VLBA terminology:

An "IF" refers to one of a maximum of four 512-MHz wide intermediate-frequency analog signals transmitted from the receiver(s) to the RDBE.  As described in the section on Receivers, most receivers provide two IFs, in opposite circular polarizations.  However, four IFs are available in certain specialized observing modes: two dual-polarization pairs, at arbitrary frequencies within the full range of the new 6-cm receiver; or from different receivers in 13/4-cm or 90/50-cm dual-receiver operation.

A "channel" refers to a single contiguous frequency range (of any bandwidth), observed in a single polarization, that is sampled, filtered, and recorded as a separate entity.  This approach is essential for the VLBA, where capabilities are fundamentally limited by the overall data-transmission bandwidth.

'RDBE' is an acronym for "ROACH Digital Backend''.  ROACH, in turn, refers to the FPGA-based central signal processing board ("Reconfigurable Open Architecture Computing Hardware'') that was developed in a collaboration among NRAO, the South African KAT project, and the Collaboration for Astronomy Signal Processing and Electronics Research (CASPER) at UC Berkeley.  In addition to the ROACH, the RDBE includes an input analog level control module, a sampler developed by CASPER, and a synthesizer board that generates the 1024-MHz sample clock. RDBEs accept two 512-1024 MHz IF inputs, and deliver packetized output via a 10G Ethernet interface.  For observing semester 2013B, each VLBA station will be equipped with two RDBE units.  When operating in dual-RDBE mode, their outputs will be sequenced by a software-based Ethernet switch before transmission to the Mark 5C recording system

Two separate "observing systems" are currently available within the VLBA's RDBE.  Some suggestions for choosing between these two options, where both are possible, follow the functional outlines below.

PFB:  The RDBE's initial observing system, in regular use for scientific observations since 2012 February 19, implements a polyphase filterbank (PFB) digital signal-processing algorithm.  It produces sixteen fixed-bandwidth 32-MHz channels within a single RDBE unit, which can be selected flexibly between two input IFs, and can be placed at 32-MHz steps along the entire IF frequency range. Some typical selection modes include (a) a compact dual-polarization configuration of eight contiguous 32-MHz channels at matching frequencies in each polarization; (b) a spanned-band dual-polarization configuration, with eight 32-MHz channels spaced every 64 MHz in each polarization; and (c) a single-polarization configuration of 16 channels, contiguous across the entire width of one IF.  The selected channels are requantized at two bits per Nyquist sample and transmitted to the recording system, at a total data rate of 2.048 Gbps (referred to subsequently as "2 Gbps'').  An important auxiliary function, detection of the switched broadband noise calibration signal, is also supported by the PFB.

DDC: A newer observing system, available at a wide range of bandwidths in observing semester 2013B, implements a digital downconverter (DDC) algorithm in the RDBE's FPGA.  A total of 1, 2, or 4 channels are supported within a single RDBE unit; 8 channels are available using both RDBEs.  Available bandwidths range downward from 128 MHz to 1 MHz by factors of two; however, recording rate limitations restrict the 128-MHz bandwidth to a maximum of 4 channels.  All channels must use the same bandwidth within an observing scan.  Channels can be selected flexibly among up to four input IFs, and in either sideband.  Tuning of individual channels can be set in steps of 15.625 kHz, although 250-kHz steps are recommended when compatibility with legacy systems is required.  Channels may not cross IF zone boundaries at 640 and 896 MHz.  Each channel is requantized at two bits per Nyquist sample and transmitted to the recording system, at a total data rates ranging from 4 Mbps to 2 Gbps.  The DDC also incorporates an advanced switched-noise detection methodology.

Suggestions for Observing System Selection: Wideband science will be possible using either the PFB observing system, at its fixed 2048 Mbps data rate, or the DDC system at 2048 Mbps or lower rates.  The primary instrumental differences are in the numbers and bandwidths of channels.  The DDC's wide-bandwidth channels will be more subject to bandwidth smearing in wide-field observations, while the smaller number of channels it requires may be preferable for simplicity of data analysis.

Spectroscopic and other narrow-band observations will generally be best supported by the DDC system, which incorporates scientifically compatible counterparts for all modes of the VLBA legacy system.  16-channel legacy modes require using channels in both sidebands of the analog BBCs, and thus can be replaced with 8-channel equivalents at double the channel bandwidth.  And while the DDC cannot currently match the narrowest legacy channel bandwidths due to formatting restrictions, nevertheless much narrower effective bandwidths can be achieved by exploiting the DiFX correlator's high spectral resolution and spectral zoom capabilities.

Observations using all four of the IFs available from any of the 4-IF capabilities require the dual-RDBE capability of the DDC.

The VLBA's data transmission system comprises the recorder units at the stations, playback units at the correlator, and the magnetic disk modules that are shipped between those units.  The new Mark 5C system was developed jointly by NRAO, Haystack Observatory, and Conduant Corporation.  It closely resembles the Mark 5A version used previously by the VLBA, and the Mark 5B used at some other observatories.  In particular, identical disk modules are used.  However, Mark 5C is functionally more straightforward than its predecessors.   It simply records the payload of each 10G Ethernet packet received from the RDBE without imposing any special recording format.  All formatting of the observed data — most essentially, the precision time tags — is internal to the packet payloads, which are transmitted directly from recorder to playback by the Mark 5C system.  Initially, Mark 5B format is being used internally, for compatibility with some existing correlators.  An eventual transition is planned to the VLBI Data Interchange Format (VDIF).

Each Mark 5C unit accommodates two removable modules, each in turn comprising eight commercial disk drives.  As used on the VLBA, these modules are recorded sequentially at a maximum rate of 2 Gbps, matching the current maximum RDBE output rate.  Modules of 16-TB capacity were procured with funding awarded through NSF's MRI-R² program.  We anticipate that our pool of recording media will support wideband modes for approximately 40% to 50% of observing hours.

Further information on the Mark 5C system is available in the Sensitivity Upgrade memo series, and in the Haystack Mark 5 series.

The correlator is situated in the SOC, at the end of the data path.  Its role is to reproduce the signals recorded at the VLBA stations and any others involved in the observation, and to combine them in two-station baseline pairs, to yield the visibility function which is the fundamental measurement produced by the VLBA.  VLBA observations are processed using the DiFX software correlator. DiFX was developed at Swinburne University in Melbourne, Australia (Deller et al. 2007), and adapted to the VLBA operational environment by NRAO staff (Brisken 2008).

We encourage users to include the following text in the Acknowledgments section of any publication arising from VLBA observations made since December 2009:

This work made use of the Swinburne University of Technology software correlator, developed as part of the Australian Major National Research Facilities Programme and operated under licence.

... and to cite the following recent paper by the developers: Deller, et al. 2011, PASP, 123, 275.

Software correlation has become feasible in recent years, and is especially well suited to applications like VLBI with bandwidth-limited data-transmission systems and non-realtime processing. Among its several advantageous aspects are: (1) flexible allocation of processing resources to support correlation of varying numbers of stations, frequency and time resolution, and various special processing modes, with no fundamental fixed limits other than the finite performance of the processing cluster; (2) optimization of resource usage to minimize processing time; (3) integration of control and processing functions; (4) continuously scalable, incremental upgrade paths; and (5) relatively straightforward implementation of special modes and tests. These and other virtues of software correlation are discussed in more detail by Deller et al. (2007).

Despite the absence of fixed limits cited in item (1) above, NRAO has established guidelines for the extremes of spectral resolution, integration period, and output rate, for routine DiFX processing, as specified in the appropriate sections below. Exceptions will be considered for proposals including a sufficiently compelling scientific justification.

DiFX processes 2-bit samples with substantially greater efficiency than 1-bit samples over double the bandwidth, basically because only half as many samples must be correlated.  Since these two cases have nearly equivalent sensitivity, 1-bit sampling is no longer supported by the VLBA's data systems.

Operation of DiFX is governed primarily by an observation description in VEX format.  This format is used for both station and correlator control functions in a number of VLBI arrays, and NRAO program SCHED (Walker 2011) has been producing it for many years.

DiFX processes input data recorded in Mark 5A, Mark 5B, and the new Mark 5C formats.  Correlator output is written according to the FITS Interferometry Data Interchange Convention (Greisen 2009).  In addition to the fundamental visibility function measurements and associated meta-data, the FITS files include amplitude and phase calibration measurements, weather data, and editing flags, derived from data logged at the VLBA stations (Ulvestad 1999).  AIPS release 31DEC08 or later is required to handle DiFX data properly.

Conversion of DiFX correlator output to the Mark 4 format that is used primarily in analysis of geodetic observations is also available.  To enable this additional output, a SCHED parameter CORDFMT=MARK4 should be specified.

A recent change to DiFX allows a more flexible selection of the desired number of "spectral points" spanning each individual data channel.  Any number that can be factored as 2ⁿ · 5^m can now be specified, subject to a maximum of 4096 for routine DiFX processing.  The total number of spectral points, summed over all data channels and polarization products, is now 132,096 (for compatibility with AIPS).   The number of spectral points must be the same for all data channels at any given time, although multiple passes are possible with different sets of channels. The actual spectral resolution obtained, and statistical independence of the spectral points, depends on subsequent smoothing and other processing.

DiFX also supports "spectral zooming'', selection of a subset of correlated spectral points from any or all data channels.  Only the selected spectral points are included in the output dataset.  This capability will be of value mainly in maser studies, where a recorded data channel may be much wider than the maser emission in two main categories of observations:  (1) Maser astrometry with in-beam continuum calibrators.  Wideband observing is required for maximum sensitivity on the calibrators, while zooming allows high spectral resolution at the frequencies where maser emission appears.  (2) Multiple maser transitions.  When wideband data channels are used to cover a large number of widely separated maser transitions, spectral zooming allows the empty portions of high-resolution spectrum to be discarded.

In proposing observations that will use spectral zooming, the required number of spectral points before zooming should be specified in the Proposal Submission Tool.  Currently, the location and width of the "zoom" bands must be communicated directly to VLBA operations before correlation.

DiFX accommodates a nearly continuous range of correlator integration periods over the range of practical interest. Individual integrations are quantized in multiples of the indivisible internal FFT interval, which is equal to the number of spectral points requested, divided by the data channel bandwidth.

For most cases, with low to moderate spectral resolution, and/or wide data channels, the FFT intervals are fairly short, and it is straightforward to find an integration period in any desired range that is an optimal integral multiple of the FFT interval. ("Optimal'' refers here to the performance of DiFX.) Extreme cases of very high spectral resolution (many spectral points across a narrow data channel - resolution of less than about 100 Hz) imply FFT intervals long enough that only limited choices of integral multiples are available.

For flexibility in these situations (although the option exists in all cases), integration periods other than an integral multiple of the FFT interval can be approximated, in a long-term mean, by an appropriate sequence of nearby optimal integral multiples. In this case, output records are time-tagged as if correlated with exactly the requested period.

SCHED now accepts an additional parameter so that users can indicate that the requested integration period is to be implemented exactly, as described above. Otherwise, the nearest optimal integral multiple of the FFT interval is passed to the correlator.

The field of view in VLBI observations is very small, around 10^-4 of the primary antenna beam area. This restricted interferometer beam arises in the correlation process, from smearing due to averaging in time (with, typically, a 2-second period) and/or across bandwidth ("chromatic aberration'' over, typically, 0.5 MHz spectral resolution), at positions away from the correlation phase center. Thus, imaging of targets that are widely spaced in the primary beam requires multiple processing passes in typical correlator implementations. If the visibilities are maintained at high time and frequency resolution, it is possible to perform a u-v shift after correlation, essentially repointing the correlated dataset to a new phase center. However, this approach would require prohibitively large visibility datasets.

DiFX implements multiple u-v shifts inside the correlator, to generate as many phase centers as are necessary, in a single correlation pass. The output consists of one dataset of normal size for each phase center. This mode consumes around three times the correlator resources of a normal continuum correlation, due to the need for finer frequency resolution before the u-v shift, but the additional cost is only weakly dependent on the number of phase centers. For reasonable spectral and temporal resolution requirements (for example, adequate for smearing < 10% at the 50% contour of the VLBA primary beam), 200 phase centers require only 20% more correlator time than 2 phase centers. Extremely high spectral and/or temporal resolution (e.g. for shifts even closer to the edge of the primary beam) carry a higher overhead per additional phase center. This mode thus should be requested only for imaging of three or more sources within any single antenna pointing. The output data rate must be justified if it exceeds the current limit.

Multiple phase-center correlation is requested in the NRAO Proposal Submission Tool by setting the "Number of Fields'' item in the resource section to the maximum number of phase centers required for any antenna pointing specified in a given resource. The requested spectral resolution and integration time should correspond to the desired initial number of spectral points per data channel (required to minimize bandwidth smearing) and the desired integration between u-v shifts (to minimize time smearing).  SCHED includes facilities to support specification of the actual phase center locations.

For more details on wide-field imaging techniques, see Bridle & Schwab (1999), and Garrett et al. (1999).

Correlation parameters should result in an output rate less than 10 MBytes per second (of observing time) for routine DiFX processing; higher rates may be considered if required and adequately justified. Observers should ensure that their data-analysis facilities can handle the dataset volumes that will result from the correlation parameters they specify.

An approximate parametrization of the output rate is given by

[display]R = 4 \cdot \frac{N_{\rm stn} \cdot (N_{\rm stn}+1) \cdot N_{\rm sbb} \cdot N_{\rm spc}}{T_{\rm int}}\cdot N_{\rm phc} \cdot p[/display]

where the rate [inline]R[/inline] is in Byte/s;

[inline] N_{\rm stn} , \; N_{\rm sbb} , \; N_{\rm spc} [/inline] are the numbers of observing stations, data channels, and spectral points per data channel, respectively;

[inline]T_{\rm int}[/inline] is the correlator integration period;   and

[inline]N_{\rm phc}[/inline] is the number of phase centers.

The polarization factor [inline]p=1[/inline] for single-polar, or dual-polar parallel-hand output;  or [inline]p=2[/inline] for cross-polar, four-Stokes processing.

Output data rates are also estimated by SCHED.

Fringe phases should be coherent across the entire set of sub-bands produced by each RDBE.  Correction of phase offsets between the two RDBEs at each station, and/or between the oppositely polarized signal channels, can be determined using the "phase cal'' or "pulse cal'' system (Thompson 1995).   In conjunction with the LO cable length measuring system, this system can also be used to measure changes in the delays through the cables and electronics which must be removed for accurate geodetic and astrometric observations.

The pulse cal system consists of a pulse generator and a sine-wave detector.  The interval between the pulses can be either 0.2 or 1 microsecond.  They are injected into the signal path at the receivers and serve to define the delay reference point for astrometry.  The pulses appear in the spectrum as a "comb'' of very narrow, weak spectral lines at integral multiples of 1 or 5 MHz.  The phases of one or more of these lines is measured by the detector, logged as a function of time, and delivered in a PC table.

AIPS tasks can load and apply the PC data.  However, some VLBA observers may still want to use a strong compact source to do a "manual'' phase cal if necessary (Diamond 1995).  Spectral line users will not want the pulse cal comb to appear in their observations, and should ensure that their observing schedules both disable the pulse cal generators and include observations suitable for a manual phase cal.   Manual phase calibration also is likely to be necessary for non-VLBA stations that have no tone generators or detectors, and in VLBA observations at 3 mm, where the VLBA receivers have no pulse calibration tones.

Even after global fringe fitting, averaging, and editing, the phases on a VLBA target source can still vary rapidly with time.  Most of these variations are due to inadequate removal of station-based atmospheric phases, but some variations also can be caused by an inadequate model of the source structure during fringe fitting.   If the VLBA target source is sufficiently strong and if absolute positional information is not needed, then it is possible to reduce these phase fluctuations by looping through cycles of Fourier transform imaging and deconvolution, combined with phase self-calibration in a time interval shorter than that used for the fringe fit (Cornwell 1995; Walker 1995b; Cornwell & Fomalont 1999).  Fourier transform imaging is straightforward (Briggs, Schwab, & Sramek 1999), and done with AIPS task IMAGR or the Caltech program Difmap (Shepherd 1997).  The resulting VLBI images are deconvolved to rid them of substantial sidelobes arising from relatively sparse sampling of the u-v plane (Cornwell, Braun, & Briggs 1999).  Such deconvolution is achieved with AIPS tasks based on the CLEAN or Maximum Entropy methods or with the Caltech program Difmap.

Phase self-calibration just involves minimizing the difference between observed phases and model phases based on a trial image, by solving for station-based instrumental phases (Cornwell 1995; Walker 1995b; Cornwell & Fomalont 1999).  After removal of these instrumental phases, the improved visibilities are used to generate an improved set of model phases, usually based on a new deconvolved trial image.  This process is iterated several times until the phase variations are substantially reduced.  The method is then generalized to allow estimation and removal of complex instrumental antenna gains, leading to further image improvement.  Both phase and complex self-calibration can be accomplished using AIPS task CALIB or with the Caltech program Difmap.  Self-calibration should only be done if the VLBA target source is detected with sufficient signal-to-noise in the self-calibration time interval (otherwise, fake sources can be generated!), and if absolute positional information is not needed.

The useful field of view in VLBI images can be limited by finite bandwidth, integration time, and non-coplanar baselines (Wrobel 1995; Cotton 1999b; Bridle & Schwab 1999; Perley 1999b). Measures of image correctness -- image fidelity and dynamic range -- are discussed by Walker (1995a) and Perley (1999a).

If the VLBA target source is not sufficiently strong for self-calibration or if absolute positional information is needed but geodetic techniques are not used, then VLBA phase referenced observations must be employed (Beasley & Conway 1995).  Currently, 63% of all VLBA observations employ phase referencing.  Wrobel et al. (2000) recommend strategies for phase referencing with the VLBA, covering the proposal, observation, and correlation stages.  A VLBA phase reference source should be observed frequently and be within a few degrees of the VLBA target region, otherwise differential atmospheric (tropospheric and ionospheric) propagation effects will prevent accurate phase transfer.  VLBA users can draw candidate phase calibrators from VLBA correlator's source catalog, which is distributed with SCHED.  Easy searching for the nearest calibrators is available online through the VLBA Calibrator Survey (Beasley et al. 2002).  Most of these candidate phase calibrators now have positional uncertainties below 1 mas.

Calibration of atmospheric effects for either imaging or astrometric observations can be improved by the use of multiple phase calibrators that enable multi-parameter solutions for phase effects in the atmosphere.  See AIPS Memos 110 (task DELZN, Mioduszewski 2004) and 111 (task ATMCA, Fomalont & Kogan 2005), available from the AIPS home page, for further information.

Walker & Chatterjee (1999) have investigated ionospheric corrections.  Such corrections can even be of significant benefit for frequencies as high as 5 GHz or 8 GHz (Ulvestad & Schmitt 2001).  These corrections may be made with the AIPS task TECOR, as described in AIPS Cookbook Appendix C (NRAO 2006), or the procedure VLBATECR.  In addition, it is strongly recommended that the most accurate Earth-Orientation values be applied to the calibration, since correlation may have taken place before final values were available; this may be done with AIPS task CLCOR or more easily with the AIPS procedure VLBAEOPS.

The rapid motion of VLBA antennas often can lead to very short time intervals for the slew between target source and phase reference source.  Some data may be associated with the wrong source, leading to visibility points of very low amplitude at the beginnings of scans.  Application of the AIPS program QUACK using the `TAIL' option will fix this problem.

In VLBA polarimetric observations, sub-bands are assigned in pairs to opposite hands of circular polarization at each frequency.  Typical "impurities" of the antenna feeds are about 3% for the center of most VLBA bands and degrade toward the band edges and away from the pointing center in the image plane.  Without any polarization calibration, an unpolarized source will appear to be polarized at the 2% level.  Furthermore, without calibration of the RCP-LCP phase difference, the polarization angle is undetermined.  With a modest investment of time spent on calibrators and some increased effort in the calibration process, the instrumental polarization can be reduced to less than 0.5%.

To permit calibration of the feed impurities (sometime also called "leakage" or "D-terms"), VLBA users should include observations of a strong (≈ 1 Jy) calibration source, preferably one with little structure.  This source should be observed during at least 5 scans covering a wide range (> 100 degrees) of parallactic angle, with each scan lasting for several minutes.  The electric vector polarization angle (EVPA) of the calibrator will appear to rotate in the sky with parallactic angle while the instrumental contribution stays constant.  Some popular calibrator choices are J0555+3948=DA193 and J1407+2827=OQ208, although either or both may be inappropriate for a given frequency or an assigned observing time.  Fortunately, many calibrators satisfying the above criteria are available.

A viable alternative approach to measuring polarization leakage is to use an unpolarized calibrator source.  This can be done with a single scan.

To set the absolute EVPA on the sky, it is necessary to determine the phase difference between RCP and LCP.  For VLBA users at frequencies of 5 GHz and above, the best method for EVPA calibration is to observe one or two of the compact sources that are being monitored with the VLA; see the VLA/VLBA Polarization Calibration Page (Taylor & Myers 2000).  At 1.6 GHz it may be preferable to observe a source with a stable, long-lived jet component with known polarization properties.  At frequencies of 5 GHz and below one can use J0521+1638=3C138 (Cotton et al. 1997a), J1331+3030=3C286 (Cotton et al. 1997b), J1829+4844=3C380 (Taylor 1998), or J1902+3159=3C395 (Taylor 2000). At 8 GHz and above one may use J1256-0547=3C279 (Taylor 1998) or J2136+0041=2134+004 (Taylor 2000), although beware that some of these jet components do change on timescales of months to years.  It will be necessary to image the EVPA calibrator in Stokes I, Q, and U, and  to determine the appropriate correction to apply.  Thus it is recommended to obtain 2 to 4 scans, each scan lasting at least 3 minutes, over as wide a range in hour angle as is practical.

To permit calibration of the RCP-LCP delays, VLBA users should include a 2-minute observation of a very strong (≈ 10 Jy) calibration source.  While 3C279 is a good choice for this delay calibration, any very strong fringe-finder will suffice.

Post-processing steps include amplitude calibration; fringe-fitting; solving for the RCP-LCP delay; self-calibration and Stokes I image formation; instrumental polarization calibration; setting the absolute position angle of electric vectors on the sky; and correction for ionospheric Faraday rotation, if necessary (Cotton 1995b, 1999a; Kemball 1999).  All these post-processing steps can currently be done in AIPS, as can the polarization self-calibration technique described by Leppänen, Zensus, & Diamond (1995).

Pulsar observing is an expert mode of the VLBA, requiring additional understanding and effort on the part of the user.  Those willing to learn to use them can take advantage of the following enhanced capabilities supporting pulsar observations, available in the DiFX correlator:

1. Binary Gating: A simple pulse-phase driven on-off accumulation window can be specified, with "on" and "off" phases.  Such gating increases the signal to noise ratio of pulsar observations by a factor of typically 3 to 6, and can also be used to search for off-pulse emission.
2. Matched-filter Gating: If the pulse profile at the observation frequency is well understood and the pulse phase is very well predicted by the provided pulse ephemeris, additional signal to noise over binary gating can be attained by appropriately scaling the correlation coefficients as a function of pulse phase.  Depending on the pulse shape, additional gains of up to 50% in sensitivity over binary gating can be realized.
3. Pulsar Binning: This mode entails generating a separate visibility spectrum for each requested range of pulse phase.  There are no explicit limits to the number of pulse phase bins that are supported, however, data rates can become increasingly large.  Currently AIPS does not support databases with multiple phase bins.  Until post-processing support is available, a separate FITS file will be produced for each pulsar phase bin.

In all cases, the user will be responsible for providing a pulsar spin ephemeris.  Except for certain applications of mode 3, the ephemeris must be capable of predicting the absolute rotation phase of the pulsar.  Pulsar modes incur a minimum correlation-time penalty of about 50%.  High output data rates may require greater correlator resource allocations.  Details of pulsar observing, including practical aspects of using the pulsar modes, and limitations imposed by operations, are documented by Brisken (2009).

Diamond (1995) and Reid (1995, 1999) describe the special problems encountered during data acquisition, correlation, and post-processing of a spectral line program.  The spectral line user must know the transition rest frequency, the approximate velocity and velocity width for the line target, and the corresponding observing frequency and bandwidth.  The schedule should include observations of a strong continuum source to be used for fringe-finding, "manual" phase calibration, and bandpass calibration; as well as scans of a continuum source reasonably close to the line target to be used as a fringe-rate and delay calibrator.  The pulse cal generators should be disabled.

Post-processing steps include performing Doppler corrections for the Earth's rotation and orbital motion (the correction for rotation is not necessary for observations when station-based fringe rotation is applied, as is the case for the VLBA); amplitude calibration using single-antenna spectra; fringe fitting the continuum calibrators and applying the results to the line target; referencing phases to a strong spectral feature in the line source itself; and deciding whether to do fringe rate mapping or normal synthesis imaging and then form a spectral line cube.  All these post-processing steps can currently be done in AIPS.

Data reduction techniques for VLBI spectral line polarimetry are discussed by Kemball, Diamond, & Cotton (1995) and Kemball (1999).

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.

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 two 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 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 2013B.
   
   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 3 mm Array. This array consists of the VLBA stations outfitted with 3 mm receivers, together with Effelsberg, Pico Veleta, Plateau de Bure, Onsala, and Metsähovi.  The European part of the 3 mm Array is coordinated by the Max-Planck-Institut für Radioastronomie.  Further details, including instructions on proposal submission, are available.

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 by Ulvestad (2004).

HSA proposals for observing semester 2013B can request the phased VLA in conjunction with the VLBA, and for processing only at the Socorro DiFX correlator.  A comprehensive adjunct to the VLA Observational Status Summary, "VLBI at the VLA", discusses 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, in any combination of polarizations, but with each pair in different IFs (AC and BD).  Any matching bandwidths available on both the VLA and the VLBA DDC data system described above 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 the Chair of the relevant Science Review Panel and Observatory staff with the necessary scientific expertise. The technical feasibility of the proposed observations will be assessed by Observatory staff. 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 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 a 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 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. Proposals for exploratory time will be evaluated on the basis of scientific merit by the relevant Science Review Panel. Observatory staff will assess their technical feasibility. 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.

Please see https://science.nrao.edu/observing/proposal-types/directorsdiscretionarytime for further information.

Students planning to use one or more NRAO telescopes for their PhD dissertation (particularly if more than one proposal will be required) may 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.

NRAO maintains a Student Observing Support program for 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.

Acknowledgment to NRAO and NSF: Any papers using observational material taken with NRAO instruments (VLBA or otherwise) or papers where a significant portion of the work was done at NRAO, should include the following acknowledgment to NRAO and NSF:

The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.