NAME:
  eventprob
 PURPOSE:   (one line only)
  Compute the success probability for an occultation from discrete sites
 DESCRIPTION:
 CATEGORY:
  Occultations
 CALLING SEQUENCE:
  eventprob,fn,in_diam,vel,sigma
 INPUTS:
  fn - Name of input file that has all of the cross track positions
         of the mobile assets.  This input variable can also be a structure
         containing the mobile site information.  Two tags are required:
            nsites: Number of sites, if 0 then the other tags are not needed
            name: string vector with the name of the mobile site
            xtrack: float vector with the cross track position in km
  diam - Diameter of the object in km
  vel  - Plane-of-the sky velocity of the object in km/sec
  sigma - Cross track uncertainty of centerline in km
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
  FIXED - File name containing cross-track locations of the fixed sites.
             Default is no fixed sites.  These are not affected by use
             of TRACKERR.  This can also be a structure similar to the
             fn input variable.
  NTRIALS - Number of random trials, default=10000
  TRACKERR - Mobile site placement error in km.  This is modeled as a uniform
               random error from -0.5 to 0.5 of this value relative to
               the planned location.
  SILENT - Flag, if set will suppress all non-error printed output
  NODISPLAY - Flag, if set will suppress all plotted output
  SYSTEMATIC - Total amount of systematic error.  This is modeled as a
                 uniform random offset relative to nominal.  The number you
                 provide is end to end of the range so the maximum it will
                 be from nominal is half of this number.  Default=0
 OUTPUTS:
  results - Structure containing all the computed statistics
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  Written by Marc W. Buie, Southwest Research Institute, 2018/10/27
  2020/05/18, MWB, added prob1 and prob2 fields to results.
  2022/02/09, MWB, fixed minor issue with reported cross-track sigma range
  2023/12/15, MWB, added PDF keyword