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: http://xmm.vilspa.esa.es/sas/6.1.0/doc/gtibuild/node3.html
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gtibuild constructs an OGIP-compliant table of Good Time-Intervals (GTI) from a plain text description file. A complex fragmentation of the time-line may be described in terms of good and bad intervals, which may be simple or periodic. The description elements are combined as the union of all the good intervals intersected with the complement of the union of all the bad intervals. In other words, a given point in time is good if it is covered by any good intervals and not also covered by any bad intervals. If no good intervals are specified, a single good interval covering all time is assumed; hence all times not covered by any bad intervals are considered good. The relative order of elements in the description file is always irrelevant. (See Section 6 for a description of the syntax of the description file.)
A periodic interval is a good or bad simple interval repeated through a whole number of periods. Because a periodic interval must be translated into a sequence of explicit simple intervals, an open-ended periodic interval would take infinite space to represent in the output GTI table. Therefore the number of periods must be finite. The syntax for describing a periodic interval is intended to be convenient for the purpose of analyzing two-phase phenomena in the data. The starting and ending times are given, as with simple intervals, but the ending time serves only to specify the final period, which may extend beyond that point. Each period is broken into two segments, one good and the other bad. This may be confusing because it sounds like a periodic interval describes both good and bad intervals at the same time; it does not. A periodic good interval contains none of the bad segments that are implicit in its definition. Similarly a periodic bad interval contains none of the good segments that are implicit in its definition. The periodic interval description syntax is simply a convenient shorthand for a long list of simple intervals, regularly spaced and all having the same length and type.
The periodic interval description designates whether the first segment of each period is good or bad, and the second segment is then implicitly the opposite. Any periodic interval becomes its own complement simply by switching this designation. Regardless of which segment is the good segment, the description also specifies whether it is the set of good segments or the set of bad segments that comprise the periodic interval; that is, whether it is a periodic good interval or a periodic bad interval. This choice must be made carefully based on an understanding of how all the description elements are combined, as explained above. To avoid unexpected results, it is best not to have simple good intervals along with a periodic good interval in the same description file. Having simple bad intervals together with a periodic bad interval in the same description file does not create the possibility of unexpected good time. This is a foible of human cognition, not an asymmetry in the description semantics. If these last two paragraphs have not made much sense yet, try again after reading Section 6, below.