This parameter was developed originally for the old VLA system. It has not yet been converted for use with the VLA. But that is on the to-do list.
VLAPSRC can be used to specify the source to use for phasing the VLA. If the scan is mode VX and VLAPSRC differs from SOURCE, then a scan will be inserted in the VLA observe file in VLAPSRC with mode VA. The VLBI control file will not be affected. The stop time of the phasing scan will be 1 minute before the start of the VLBI scan or 3 minutes after the stop of the previous VLBI scan, whichever is later. If less than 2 minutes of the VLBI scan remain as a result, a complaint will be generated. It is wise to take the VLA observe file into the NRAO VLA OBSERVE program to check the slew and dwell times, since the default timing used for VLAPSRC will not always be reasonable and the phasing scan stop time may need to be adjusted. Note that, since SCHED does not generate a phasing scan for a mode VA scan or when VLAPSRC equals SOURCE, VLAPSRC need not be specified or changed for such scans; this may minimize the number of times VLAPSRC needs to be specified.