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XMM-Newton Science Analysis System Page: 1
gtibuild
August 15, 2005
Abstract
Constructs a GTI table from a plain text description le.
1 Instruments/Modes
Instrument Mode
2 Use
pipeline processing no
interactive analysis yes
3 Description
gtibuild constructs an OGIP-compliant table of Good Time-Intervals (GTI) from a plain text description
le. 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 specied, 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 le is always
irrelevant. (See Section 6 for a description of the syntax of the description le.)
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 innite space to represent in the output GTI table. Therefore the number of periods
must be nite. 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 nal 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 denition. Similarly a
periodic bad interval contains none of the good segments that are implicit in its denition. The periodic
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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 rst 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 species 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 le. Having simple bad intervals together with a periodic bad interval
in the same description le 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.
4 Parameters
This section documents the parameters recognized by this task (if any).
Parameter Mand Type Default Constraints
le yes le gti.text
The name of the input description le.
table no table gti.ds
The name of the output GTI table.
5 Errors
This section documents warnings and errors generated by this task (if any). Note that warnings and
errors can also be generated in the SAS infrastructure libraries, in which case they would not be docu-
mented here. Refer to the index of all errors and warnings available in the HTML version of the SAS
documentation.
badFileName (error)
The input description le could not be opened.
negativeTime (error)
Negative starting times are not permitted.
tooManyIntervals (error)
A periodic interval with an unreasonably large number of segments. The interval must have
a nite upper bound.
badSegment (error)
Both phase-segments of a periodic interval must have positive duration.
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badNumericValue (error)
A non-numeric token was encountered where a number is required.
badOperator (error)
A bad symbol in the third column: check description syntax.
unexpectedEOL (error)
The line ended unexpectedly: check description syntax.
6 Input Files
The input le is formatted in plain text as an unordered sequence of time-interval descriptions. Each
non-empty line describes an interval, which may be simple or periodic, and good or bad. All intervals
are bounded by a non-negative start and stop time. Zero as the stop time indicates eternity as the upper
bound. The rst two columns of the le are start and stop. In the third column, which is optional, is
one of six operators: '+' or '-', for simple intervals, or '+g', '+b', '-g', '-b' for periodic intervals. Plus
indicates a good interval and minus indicates bad. If not specied, the operator defaults to '+'.
Periodic intervals break each period into two phase-segments, one good and the other bad, both having
positive duration. The second character of the operator species whether the rst segment is the good
one or the bad one ('g' or 'b', respectively). The lengths of the rst and second segments are the fourth
and fth columns, which are required for periodic intervals. The sum of the two segment lengths is the
period, and the stop time is automatically adjusted to the end of whatever period it falls in.
Consistently with the SAS conventions, the time values used to build a good time interval are inter-
preted as osets from the XMM-Newton Mission Reference Time (MRT, 1998-01-01T00:00:00.00 TT or
1997-12-31T23:58:56.816 UTC). For convenience it is possible to specify a dierent reference time with
the keyword timeref followed by a time descriptor of the form yyyy-mm-ddThh:mm:ss[.s], as shown in
the examples below. Time values are then interpreted as timeref + value - MRT. The time timeref is
taken to be expressed in UTC. Not using timeref is equivalent to timeref 1997-12-31T23:58:56.816.
Comments may be placed on empty lines and at the end of non-empty lines, preceded by '#'. Times are
given as oating-point numbers. The following examples illustrates each point of syntax.
100 0 # Good time extends from 100 seconds onward,
50 1000 -g 40 20 # punctuated by 20 second bad intervals every
# 60 seconds beginning at 90 seconds.
200 250 - # The instrument had a brief tantrum here.
When using a dierent timeref:
timeref 2001-12-20T01:02:03
timesystem TT
100 0 # Good time extends from timeref + 100s onward
7 Output Files
 An OGIP-compliant Good Time-Interval (GTI) table.
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8 Algorithm
9 Comments
 gtiedit is expected to supersede this task eventually.
References
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