This is an obsolete parameter because it only affected the Green Bank control files and the SNAP file, neither of which is now in use. Note that the GBT now does continuous system temperatures in the same manner as the VLBA.
By default, SCHED requests that system temperatures be measured every scan in the NRAO (Green Bank) and snap control file types. The VLA and VLBA measure Tsys continuously so no such request is needed. For Green Bank, the system temperatures are measured at the start of every scan (as part of the ``MARKIII'' proceedure). They are no longer measured at the end of a scan. For stations using snap files, there is no concept of ``go to source, then do something'', so Tsys requests are only made at the end of each scan. NOTSYS turns off the Tsys requests (For Green Bank, it also turns off all other actions included in the ``MARKIII'' proceedure which might include antenna temperature measurements and peaking up poining). The Tsys measurements take a bit of time so it may be desirable to turn them off when switching rapidly between sources; however, by doing so, calibration may be compromised. The measurement of system temperatures can be turned back on with TSYS.
This has no effect on VEX files; system temperatures are measured by PCFS controled at the beginning of every scan. Note that with thin tapes and fan-out, the time between measurements could be as long as 44 minutes. SCHED will warn if there is more than 30 minutes between Tsys measurements for PCFS systems, except on a last reverse scan which may be forced to be long in order to avoid post-passing.