The topic of controlling scan timing is discussed in detail in the Scan Times section.
GAP is the minimum gap in time between the previous stop time and the current nominal scan start time when scheduling with DURation or DWELL. For scheduling with DURation, it will be that time interval. For scheduling with DWELL, the interval might be longer if required for slews, but it will not be shorter. Note that PRESTART, PRESCAN (old, mostly obsolete parameter) and MINPAUSE are applied after the nominal scan times are established and could make the period during which the recording is stopped shorter than GAP. MINPAUSE is used to keep the recordings going through short gaps. Note that, if START is specified, GAP will be ignored.
If LST scheduling, GAP is a sidereal time, so it will be slightly shorter in UT (by about 1.0027).
For all types of scheduling, if the gap between scans is less than the value of MINPAUSE, the recordings are left running. The default MINPAUSE is 10 seconds for legacy systems and the DBBC. It is zero for the RDBE and WIDAR.
For Field System stations (most non-VLBA stations), gaps should be inserted to allow bank changes. The VLBA can switch between mounted disk banks (modules) on the fly, but the field systems need a pause in the data recording. Such gaps should be inserted every 22 minutes for recordings at 1 Gbps and proportionally less often at lower bit rates. These gaps need to be more than 10s long.
The PCFS software requires an interval of 36 seconds at tape reversals (not needed for disk) to produce a valid VEX file. A 40 second gap is required to change setup. Apart from these restrictions continuous recording is implied for gaps shorter than 10 seconds. In addition, users should realize that during continuous recording no calibration data is taken. Starting and stopping the recording can be forced using GAP or PRESCAN, or by using START times. GAP is probably the prefered mechanism. As of Feb. 2014, we are trying to determine what is the current minimum time for bank changes. It is actually likely to be less than 40 seconds.
There is an interaction between GAP and AUTOPEAK. Any specified GAP will be enforced for the interval between the input scans, but the inserted pointing scan can occur during that interval. Note, prior to Feb 2014, the gap was set to zero when pointing was inserted, so under some circumstances, the input scans could end up closer together than GAP.