MARK IV (PCFS)

SCHED supports Mark IV observations on Mark IV (and VLBA4) antennas equipped with the Field System (PCFS). This is done by creating a *.vex file in the VEX format. The VEX file is a global and self-contained description of the experiment that is scheduled. The *.vex file can be read by the control system (PCFS) and translated (by a program called DRUDG) into a list of commands for the telescope, data acquisition system and recorder.

SCHED produces VEX files with ``adaptive'' recorder control that enables continuous recording on PCFS controled telescopes, but can only be correctly interpreted by FS9.3.11 (and higher). This aided the VLBA hardware correlator in synchronizing playback, but is no longer of much interest as the sync time with Mark5 disk systems and current correlators, including the DiFX software correlator on the VLBA, do not take more than a second to sync.

SCHED now writes VEX files by default. It can be prevented using the input parameter DOVEX.

Mark IV (MkIV) telescopes are characterized by DAR = MKIV in the station catalog and can only be requested to record in any of the MkIV FORMATs. Most MkIV telescopes only support specific BBC's (called Video Converters) to be connected to certain IF distribution boxes. SCHED supports ``astronomical patching'' or a version of geodetic patching -- see M4PATCH. The astronomical patching requires odd BBC's to be connected to either IFCHAN = 1N (normal input, usually LCP) or IFCHAN = 1A (alternate, usually RCP). Even BBC's have to be connected to IFCHAN = 2N (usually RCP) or IFCHAN = 2A for LCP. Also, you must use at least one of IFCHAN = 1N or IFCHAN = 2N (i.e. a schedule that uses both IFCHAN = 1A and IFCHAN = 2A is not permitted). This can be controlled by explicitly setting BBC. See the details about M4PATCH for the setup implied by the geodetic patching.

In December 2000 the capability to record 512 Mb/s was introduced. This is achieved by recording 64 tracks simultaneously on the tapes or Mark5A. It requires NHEADS = 2 in the station catalogue. All MkIV stations now record using the Mark5 disk-based system. MkIV Stations equipped with Mk5 systems can record data at up to 1 Gbps. This requires DISK = MARK5A in the station catalogue.

Several features are not yet available or supported for MkIV (and MarkIV based Mark5 systems). Some as of late 2004 (some updates in early 2008) are:

• Continuous recording has been used widely, however the ability to acquire the information needed to flag data which is recorded during slewing is not universal (though is standard for EVN stations). The SCHED output .flg file can be used by the AIPS task UVFLG if telescope flagging data are not available. In addition users need to insert gaps during continuous recording whenever they want calibration data.

• Phase Cal detection at the telescopes is not implemented. This will likely become available with the new digital backends. Also, DiFX is acquiring the ability to detect phase cal tones.

• For MkIV (VLBA4) telescopes there is flexibility for controling when Tsys measurements will be made. Tsys measurements will typically be made at the beginning of a scan if there is a gap in the recording. When scheduling long scans or continuous recording, one may need to insert gaps in order to obtain Tsys data.

In addition to requiring use of tested modes, SCHED will not allow speedup factor changes during mixed VLBA and Mark IV mode observations. This is because of track bit rate dependent delays that are different in the two formats. The correlator would require different clocks for different modes, which is not yet available. Fairly soon, this restriction will be relaxed. Again, MODETEST will turn off the test.