Note that much of what is below applies to the old VLA system. VLBI on the EVLA with the WIDAR correlator has not yet been implemented and SCHED does not have the ability to write EVLA control files. Most likely, control of VLBI at the VLA will be based on the VEX files and a translator that makes the VLA specific files. This section will be updated when this appears.
SCHED can be used to schedule essentially all types of VLBI observations on the VLA. It can also be used instead of OBSERVE to schedule pure VLA observations involving a limited subset of the special cards that can be in VLA schedules. This includes most continuum observations. This situation is likely to evolve as the EVLA upgrade, which includes a new control system, comes on line, so stay tuned. For more information on the VLA, check out the VLA from the NRAO home page at http://www.nrao.edu. For much detailed information on VLBI at the VLA, follow the links to VLBI at the VLA. The latter document is available in postscript form from the VLA home page.
All observations at the VLA need an observe file that is used by the VLA on-line computers to control the antennas and correlator. In addition, all VLBI observations at the VLA also require a VLBA-style control file for the VME that controls the VLBA data aquisition rack and recorder. SCHED produces both kinds of files.
The VLBA control file, called bq001crd.y by SCHED for an example project with EXPCODE=bq001, is similar to those provided for VLBA stations, except that some VLBA specific commands for receivers, etc. are omitted. This file controls BBC settings, when the recorder starts and stops, and other recording details.
The observe file, called bq001obs.y by SCHED in this example, is the file that is normally created by the NRAO program OBSERVE (can be found from the NRAO home page) and given to the VLA on-line system. While SCHED is intended for VLBI scheduling and this output is provided mainly for projects that use the VLA for VLBI, there is no reason that it cannot be used for scheduling pure VLA projects. In fact, a few parameters of interest only for non-VLBI projects are provided for VLA observers, including the ability to schedule in LST. However, SCHED has only limited ability to provide the special setup cards, namely those that start with ``//''.
SCHED can provide ``//LO'' and ``//FI'' cards with parameters to set synthesizers to fixed values. The setup file can be used to provide values for the synthesizer settings. If FREQ or DOPPLER is used, the BBC settings will be adjusted to try to reach the right frequency for the VLBI data. Note that this has no effect on the VLA observing frequency. ``//LO'' and ``//FI'' cards are provided if, and only if, the required information is present in the setup file or could be obtained for the setup file by SCHED from the frequency catalog. SCHED also provides the ``//PM'' cards if proper (or planetary) motion is present for the source in the source catalog. Since the epoch of zero offset is given in IAT, IAT-UTC must be provided with parameter IATUTC in the SCHED keyin file. If other ``//'' cards are needed, or other options are needed on the ``//LO'' and ``//FI' cards, the regular VLA OBSERVE must be used or they must be edited in by hand. Beware of the fixed formats: if values are put in the wrong columns, the VLA on-line system will not behave as expected!
SCHED checks the ranges of the frequency settings given for the ``//LO'' cards. But the EVLA has made it possible to reach frequencies far outside the range of the original VLA hardware. Such frequencies can be requested using frequency settings in the style of the VLA, but outside the allowed VLA ranges. In addition, the parrameter EVLA should be given in the setup file to turn off some of the checks of frequency ranges. This is all at a primitive state as of Fall 2007 and will be updated as the EVLA matures.
Support for the Pie Town link was being added although, in practice, Pie Town link observations were not generally scheduled with SCHED. Parameter PTLINK tells SCHED that the link is to be used. This causes //LO cards to be written to the VLA OBS file and causes SCHED to put the Pie Town first LO value on those cards. A setup parameter, NOISEFRQ has been added to control the noise cal switching frequency. This is a new project and features will be added as necessary. The example file ptlink.key shows how to construct a file for an observation using the link. The Pie Town link is not currently active as of mid 2007, and when it returns it will be based on the new VLBA digital backend and on the EVLA correlator. So control is likely to be different.