S2

The S2 system has been developed in Canada to support the Orbiting VLBI missions VSOP and Radioastron, in addition to ground based astronomy. It is based on VCR technology and is relatively inexpensive. It is being used in Australia on the AT and at a number of other sites around the world.

SCHED  supports observations on telescopes that have S2 recorders and are controlled through the Field System (FS9.3.7 or higher). This is also done by creating a *.skd file in the VEX format (see section on MkIV).

The telescopes are characterized by RECORDER = S2 and CONTROL = VEX. Currently DAR = VLBA is completely supported (it could be possible to also implement DAR = MKIV). For any other DAR SCHED  will attempt to schedule the observation with a ``standard'' S2 mode, not using any knowledge on the content of the ``user signal''.

The modes supported for DAR = VLBA can be derived from a document ``Compatibility of S2 and VSOPT recordings at S2 and VSOPT Correlators'' by R. Wietfeldt, available at ftp::s2.sgl.ists.ca:pub/s2/svlbi/s2vsop_compat_memo_v*.ps.Z.

Because the S2 recorders work in a way conceptually different from VLBA (or MkIV) recorders, the user should bear the following in mind when scheduling for S2 recorders.

Please note that no operational tests with SCHED  and S2 recorders have been completed yet, except for VSOP recording. Recently (Dec 2000) it was discovered that observations that use more than 1 group of transports (thus not VSOP modes) would not switch groups. This should have been fixed.

• The S2 recorder internally works with a number of recorders which each have one head and track. The S2 box uses 1, 2, 4 or 8 recorders depending of the mode. In SCHED  this is characterized by setting TPMODE to the number of groups will be used by the S2 recorder.

• The concept of fan-in or fan-out does not apply to S2 recorders in SCHED.

• Tape lengths for S2 tapes are specified as the length for standard US VCR recording in seconds. Thus, when the VCR cassette has 120 (minutes) on the label the tape lengths is 7200 (ft) in SCHED. European buyers should be careful; a ST-120 tape presumably is labeled SE-180, but should still be scheduled with TPLENGTH = 7200. The S2 LP is obtained by using low density for which the TPSPEEDL = 6.3 (ips) and SLP by using TPSPEEDH = 4.2 (ips). These speeds are chosen in order to simulate that a 120 minute cassette will last for 7200*12/(6.3*60)=229 minutes, consistent with the S2 documentation. For high density the same tape lasts for 343 minutes per group (multiply by TPMODE to get the total time).

• S2 tape motion is supposed to be ``adaptive''. The S2 recorders should start 2.5 minutes before a scan, stop only 0.1 minutes after the scan and generally continue to spin if any gaps are shorter than 2.6 minutes. This is the default and only mode of control for S2 recorders through SCHED.