B.3 Mark III Observations

To schedule Mark III projects using non-VLBA control file stations or a Mark III correlator, it is best to use PC-SCHED, written by Alan Rogers and Dave Schultz at Haystack (unrelated to SCHED), or SKED, written by Nancy Vandenberg at Goddard Space Flight Center. PC-SCHED can read a very basic SCHED keyin file to get scan times and generate Mark III schedules. This is mainly useful to help get the files needed by a Mark III correlator to process a Mark III observation on the VLBA that was scheduled with SCHED. PC-SCHED and SKED produce a “Standard Schedule File Format” file, which is also known as the “sked file”, “.vex file”, “.drg file”, or “drudg file”. This single file contains a complete description of the experiment for all stations.

There is a program called DRUDG, associated with the Mark III systems, that is used to convert the “.drg file” into snap files to control the antennas and tape systems. DRUDG can produce VLBA control files and is used to do so regularily by the geodetic groups. It is rarely used to make schedules for non-geodetic projects. PC-SCHED also is able to produce VLBA control files, but this capability has not been kept up-to-date and should not be used.

An NRAO program called SKEDCONV exists to convert “.drg files” files into SCHED keyin files. It is recommended that most VLBA Mark III observers send their “.drg files” files to the AOC, where the staff will use SKEDCONV and SCHED, along with current setup files, to make the VLBA control files. Instructions on how to submit schedules are provided to observers when they are allocated observing time.

There are no definite plans to support Mark III observations fully with SCHED because the system will soon be obsolete. It will be replaced by the Mark IV system, which is basically an upgrade that uses the same tape recorders (upgraded) and heads, but a new formatter and wider bandwidths from the video converters. SCHED supports the Mark IV system through the VEX control files. However, as mentioned elsewhere, the Japanese systems related to VSOP have been designed around the “.drg file”. Hooks have been provided in SCHED where they can attach routines to write “.drg file” output. If this project is completed, SCHED may acquire at limited Mark III capability.

Mark III observations on stations that use VLBA control files (VLBA, VLA, and Green Bank) can be scheduled directly with SCHED. The only difference between Mark III and VLBA format projects on these antennas is the data format on the tape (see the setup file input FORMAT) and various details in the setup files. Mark III observations have a one-to-one correspondence between tracks and channels, use one bit samples, and do not barrel roll. They typically use 7 or 14 channels, although Mark III format can be used on any project with no fan out. Anyone scheduling Mark III observations should either use standard setup files or, if making their own, should start with a standard file as a template. Such observations can be correlated on the VLBA correlator, but require production of a “.drg file” if sent to a Mark III correlator.

The PCFS can control Mark III observations on Mark III, Mark IV, and VLBA hardware. However, it will only do this with a Mark III type “.drg file” for input. Since SCHED does not produce such files, it cannot currently schedule Mark III format observations on systems that do not use the VLBA control files (e.g. the EVN). The “.drg file” is also required to control Mark III correlators, and therefore SCHED cannot be used to control experiments that are destinated for such correlators without causing considerable extra effort to generate such a file later. A possible future upgrade to the PCFS and SCHED would allow the use of VEX files to control observations on Mark III hardware or configure Mark III modes on Mark IV hardware.

The VLBA correlator is capable of correlating VLBA, Mark IV, and Mark III formats against each other as long as the sample rates and recording speeds were the same. Therefore the VLBA and Mark IV stations of a global observation that also involves Mark III stations could be scheduled with SCHED if it were to be processed in Socorro. Of course, the Mark III stations would still have to be scheduled somehow, so one of the other programs would probably be used anyway.