Name

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  MASK2GTI

Author

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  Craig B. Markwardt, NASA/GSFC Code 662, Greenbelt, MD 20770
  craigm@lheamail.gsfc.nasa.gov

Purpose

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  Convert a gridded set of times to a set of Good Time Intervals (GTIs)

Calling Sequence

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  GTI = MASK2GTI(TIME, MASK, COUNT, INDICES=INDICES, $
                  TIMEDEL=, GOOD=, BAD=, PRE=, POST=)

Description

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  The function MASK2GTI accepts an array of times and mask, and
  converts valid data into corresponding good time intervals (GTIs).
  Elements of the MASK array are clustered together according to
  whether they are "good" or not. Contiguous segments of good
  elements are converted to single good time intervals. Time
  elements are considered to be regularly spaced, so any breaks in
  the MASK values are considered to be discontinuities.
  The time array *must* be evenly spaced and sorted in ascending
  order. Each element of MASK must correspond to the same element
  of TIME. The primary difference between GTISEG and MASK2GTI is
  that GTI2SEG allows time values to be irregularly sampled and no
  mask is passed. Also, MASK2GTI allows intervals to be enlarged or
  shrunk.
  It should be noted that this function is not constrained to
  operation only on time arrays. It should work on any
  one-dimensional quantity with intervals.

Inputs

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  TIME - an array of evenly spaced, ascending order, times.
  MASK - an array of values matched to TIME.
  COUNT - upon return, the number of resulting intervals. A value
          of zero indicates no good time intervals.

Keywords

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  INDICES - upon return, a 2xCOUNT array of integers which give the
            indices of samples which lie within each interval. The
            times TIME(INDICES(0,i) : INDICES(1,i)) fall within the
            ith interval.
  TIMEDEL - a scalar value giving the time spacing of the array.
            Default: TIME(1)-TIME(0)
  PRE - the amount each interval should be enlarged from its leading
        edge. A negative value indicates the interval should
        shrink.
        Default: 0
  POST - the amount each interval should be enlarged from its
          trailing edge. A negative value indicates the interval
          should shrink.
          Default: 0
 
  GOOD - the value of "good" in the input mask array.
          Default: 1b
  BAD - the value of "bad" in the input mask array.
        Default: 0b

Returns

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  A new GTI array containing the enlarged or shrunken intervals.
  The array is 2xCOUNT where COUNT is the number of resulting
  intervals. GTI(*,i) represents the start and stop times of
  interval number i. The intervals are non-overlapping and
  time-ordered.
  If COUNT is zero then the returned array is a scalar value of
  zero, indicating no good intervals were found.

See Also

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  GTI2MASK, GTITRIM, GTIMERGE, GTIWHERE

Modification History

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  Written, CM, 1997-2001
  Documented, CM, Apr 2001