The time string can either be in a year/month/day format:

      yyyy/mm/dd hh:mm:ss.sss

   or in a year/day-of-year format:

      yyyy//ddd hh:mm:ss.sss


      Choosing Saturn barycentric or Saturn geocentric for the origin
   will not change the longitude value.

   Note that the longitude value is considered west longitude (WL).
      
   The SLS4 longitude system (North) is only valid for this time period:
   
      5 April 2006 to 16 September 2009.
      
   The SLS4 longitude system (South) is only valid for this time period:
   
      12 September 2004 to 16 October 2009.
      
      If the user chooses a time outside these time periods, a fill
   value of -1 will be used in the output.

   Revision History
   ======== =======

   v1.0  Initial version.  30 Aug 2010

   v1.1  Phase constant changed so that the corrected phase
         will be approximately 100 degrees for both north
         and south.  10 Sep 2010

   v1.2  Two types of electric field measurements are now being
         used.  The first is from the x-axis dipole electric field
         antenna, and the second is from the two- or three-axis
         electric antennas.  The first type is used to measure
         the southern SKR source and the second is used to measure
         the northern SKR source.  Also, the time ranges for north
         and south have been extended.  13 Jan 2011

   v1.3  Two ASCII files have been added that contain the times when
         the sub-solar longitude is zero using the SLS4 longitude
         system (north and south).  A PDF file has been added that
         describes how to determine the longitude for a given time
         using interpolation.  The same file also describes how to
         compute the spacecraft longitude if the local time of the
         spacecraft is known.  An option has been added to let the user
         decide if the longitude of the SKR peak will be zero or one
         hundred degrees.  The time range for SLS4 south has been
         extended by thirty days.  24 Feb 2011

   v1.3a The time range for SLS4 south has been extended by thirty
         days.   5 Apr 2011

   v1.3b The time range for SLS4 south has been extended by thirty
         days.  26 Apr 2011

   v1.3c The time range for SLS4 south has been extended by thirty
         days.  25 May 2011

   v1.3d The time range for SLS4 south has been extended by sixty
         days.  03 Aug 2011

   v1.4  Due to uncertainties in the SKR starting in early 2010,
         the time range for SLS4 south has been changed.  It is
         now 12 September 2004 to 16 October 2009.  28 Dec 2012