The TRACKn parameters are mostly obsolete since tape is no longer used. For Mark5A disk systems, the track concept is retained, but as if there were only one pass, so TRACK1 might still be used. But SCHED generally takes care of track assignments without input from the user.
The TRACKn parameters are arrays of track specifications for use during the nth pass at a head position. One track is given for each baseband channel. For Mark III, there is a one-to-one correspondence between baseband channels and tracks so this makes sense. For Mark III observations, the VLBA track for a channel is just the Mark III track plus 3. For many VLBA modes, several tracks are used for each channel. In these cases, the first track used for the baseband channel is specified and the on-line system determines the rest. A fanned out channel uses adjacent tracks within the odd or even groups and the lowest VLBA track used for data is 2. For example, a 1:4 fan out channel uses tracks 2, 4, 6, and 8 for the low order bit. If 2-bit samples are used, tracks 10, 12, 14, and 16 would be used for those bits. In such a case, the only track mentioned in a TRACKn parameter is 2. If a fan-in mode is used, more than one channel should be given the same track specification. Fan-in modes are not yet tested or in use.
For modes VLBA1:1, VLBA1:2, VLBA1:4, Mark III mode B with ascending frequency order (alternating sidebands), and Mark III mode E, TRACK1 can be left out. All of the TRACKn commands will default to reasonable values. This is the recommended action for most users since track assignment is an easy area in which to make mistakes. SCHED will complain and stop if it does not know how to set the tracks for the requested mode. For the Mark III modes, extra baseband channels (2 for Mode B and 1 for Mode E) can be specified and will be put on the edge tracks that are not normally used for Mark III. For Mode B, they should follow the same alternating sideband pattern as the rest of the channels.