PN3DB tells SCHED to write a VLBA tracking test sequence in the VLBA control file for this scan. NOPN3DB tells SCHED to stop writing such sequences. This parameter should only be of interest to VLBA operations staff. The sequence, which is not a loop, starts with a scan of length PTSLEW to set levels at the half power point, then does a 15 second scan at the same place to fix those levels, then does one scan each of length PTDUR at off source and on peak positions, then trackes the source at the azimuth half power point. That is followed by an off source, on source, and half power in elevation tracking sequence. PTINCR in the setup file is used to set the distance to the half power point. The periods of tracking are the full scan duration minus all the level fixing, off, and on scan durations, divided by two. Scan durations of about an hour are appropriate.