All of the VLA parameters are, at temporarily, out of use. The old VLA system has been turned off and the EVLA is not yet doing VLBI. The control scheme may change with the EVLA.
The VLA on-line system normally applies a Tsys-based amplitude correction to data from the VLA correlator. In order to calibrate 3-antenna and phased VLA data taken with the VLA as a VLBI station, it is necessary to have raw correlation coefficients. If the Tsys correction is made, the raw correlation coefficients are lost. The old DEC10 scheme for obtaining the VLBI calibration data was based on the standard VLA correlator output as archived, so it was necessary to request that the Tsys correction be turned off. The new scheme for obtaining the VLBI calibration information involves extraction of the correlation coefficients by the on-line system before the Tsys corrections are made. Therefore it is no longer necessary to turn the corrections off and leaving them on makes it easier to use the VLA correlator data to make VLA images. The default action of SCHED is now to leave the corrections on and that should be correct for essentially all observations. However, VLATSYS and VLANTSYS can be used to turn the corrections on and off, respectively, on a scan basis just in case someone still wants to do so.