Index Page
cspice_gfsep
A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X 

Abstract
I/O
Examples
Particulars
Required Reading
Version
Index_Entries

Abstract


   CSPICE_GFSEP determines the time intervals when the angular separation
   between the position vectors of two target bodies relative to an
   observer satisfies a numerical relationship.

I/O


   Given:

      Parameters-

      All parameters described here are declared in the header file
      SpiceGF.h. See that file for parameter values.

      SPICE_GF_CNVTOL

               is the convergence tolerance used for finding endpoints of
               the intervals comprising the result window.
               SPICE_GF_CNVTOL is used to determine when binary searches
               for roots should terminate: when a root is bracketed
               within an interval of length SPICE_GF_CNVTOL, the root is
               considered to have been found.

               The accuracy, as opposed to precision, of roots found
               by this routine depends on the accuracy of the input
               data. In most cases, the accuracy of solutions will be
               inferior to their precision.

      Arguments-

      targ1    the name of the first body of interest. You can
               also supply the integer ID code for the object as an
               integer string.  For example both 'MOON' and '301'
               are legitimate strings that indicate the moon is the
               target body.

               [1,c1] = size(targ1); char = class(targ1)

                  or

               [1,1] = size(targ1); cell = class(targ1)

      shape1   the name of the geometric model used to represent the shape
               of the 'targ1' body.

               [1,c2] = size(shape1); char = class(shape1)

                  or

               [1,1] = size(shape1); cell = class(shape1)

               Models supported by this routine:

                  'SPHERE'        Treat the body as a sphere with radius
                                  equal to the maximum value of
                                  BODYnnn_RADII

                  'POINT'         Treat the body as a point;
                                  radius has value zero.

               The 'shape1' string lacks sensitivity to case, leading
               and trailing blanks.

      frame1   the name of the body-fixed reference frame
               corresponding to 'targ1'. cspice_gfsep does not currently use
               this argument's value, its use is reserved for future
               shape models. The value 'NULL' will suffice for
               "POINT" and "SPHERE" shaped bodies.

               [1,c3] = size(frame1); char = class(frame1)

                  or

               [1,1] = size(frame1); cell = class(frame1)

      targ2    the name of the second body of interest. You can
               also supply the integer ID code for the object as an
               integer string.  For example both 'MOON' and '301'
               are legitimate strings that indicate the moon is the
               target body.

               [1,c4] = size(targ2); char = class(targ2)

                  or

               [1,1] = size(targ2); cell = class(targ2)

      shape2   the name of the geometric model used to represent
               the shape of the 'targ2.'

               [1,c5] = size(shape2); char = class(shape2)

                  or

               [1,1] = size(shape2); cell = class(shape2)

               Models supported by this routine:

                 'SPHERE'        Treat the body as a sphere with radius
                                 equal to the maximum value of
                                 BODYnnn_RADII

                 'POINT'         Treat the body as a single point;
                                 radius has value zero.

               The 'shape2' string lacks sensitivity to case, leading
               and trailing blanks.

      frame2   the name of the body-fixed reference frame
               corresponding to 'targ2'. cspice_gfsep does not currently use
               this argument's value, its use is reserved for future
               shape models. The value 'NULL' will suffice for
               "POINT" and "SPHERE" shaped bodies.

               [1,c6] = size(frame2); char = class(frame2)

                  or

               [1,1] = size(frame2); cell = class(frame2)

      abcorr   describes the aberration corrections to apply to the state
               evaluations to account for one-way light time and stellar
               aberration.

               [1,c7] = size(abcorr); char = class(abcorr)

                  or

               [1,1] = size(abcorr); cell = class(abcorr)

               This routine accepts the same aberration corrections as does
               the routine spkezr_c. See the header of spkezr_c for a
               detailed description of the aberration correction options.
               For convenience, the options are listed below:

                  'NONE'     Apply no correction.

                  'LT'       "Reception" case:  correct for
                             one-way light time using a Newtonian
                             formulation.

                  'LT+S'     "Reception" case:  correct for
                             one-way light time and stellar
                             aberration using a Newtonian
                             formulation.

                  'CN'       "Reception" case:  converged
                             Newtonian light time correction.

                  'CN+S'     "Reception" case:  converged
                             Newtonian light time and stellar
                             aberration corrections.

                  'XLT'      "Transmission" case:  correct for
                             one-way light time using a Newtonian
                             formulation.

                  'XLT+S'    "Transmission" case:  correct for
                             one-way light time and stellar
                             aberration using a Newtonian
                             formulation.

                  'XCN'      "Transmission" case:  converged
                             Newtonian light time correction.

                  'XCN+S'    "Transmission" case:  converged
                             Newtonian light time and stellar
                             aberration corrections.

               The 'abcorr' string lacks sensitivity to case, and to embedded,
               leading and trailing blanks.

      obsrvr   the name of the observing body. Optionally, you
               may supply the ID code of the object as an integer
               string. For example, both 'EARTH' and '399' are
               legitimate strings to supply to indicate the
               observer is Earth.

               [1,c8] = size(obsrvr); char = class(obsrvr)

                  or

               [1,1] = size(obsrvr); cell = class(obsrvr)

      relate   the constraint relational operator on the angular separation.
               The result window found by this routine indicates the time
               intervals where the constraint is satisfied. 

               [1,c9] = size(relate); char = class(relate)

                  or

               [1,1] = size(relate); cell = class(relate)


               Supported values of relate and corresponding meanings are 
               shown below:

                  '>'      Separation is greater than the reference
                           value 'refval'.

                  '='      Separation is equal to the reference
                           value 'refval'.

                  '<'      Separation is less than the reference
                           value 'refval'.

                  'ABSMAX'  Separation is at an absolute maximum.

                  'ABSMIN'  Separation is at an absolute  minimum.

                  'LOCMAX'  Separation is at a local maximum.

                  'LOCMIN'  Separation is at a local minimum.

               The caller may indicate that the region of interest
               is the set of time intervals where the quantity is
               within a specified angular separation of an absolute extremum.
               The argument adjust (described below) is used to
               specify this angular separation.

               Local extrema are considered to exist only in the
               interiors of the intervals comprising the confinement
               window:  a local extremum cannot exist at a boundary
               point of the confinement window.

               Negative Angular Separation

                  For those searches using a SPHERE shape identifier for
                  either target body, the angular separation function
                  returns a negative value when the bodies overlap (occult).

               The 'relate' string lacks sensitivity to case, leading
               and trailing blanks.

      refval   reference value used together with 'relate' argument to define
               an equality or inequality to be satisfied by the angular
               separation between the specified target and observer. See
               the discussion of relate above for further information.

               [1,1] = size(refval); double = class(refval)

               The units of 'refval' are radians.

      adjust   value used to modify searches for absolute extrema: when
               'relate' is set to ABSMAX or ABSMIN and adjust is set to a
               positive value, cspice_gfsep finds times when the angular
               separation between the bodies is within adjust
               radians of the specified extreme value.

               [1,1] = size(adjust); double = class(adjust)

               For relate set to ABSMAX, the result window contains
               time intervals when the angular separation has
               values between ABSMAX - 'adjust' and ABSMAX.

               For relate set to ABSMIN, the result window contains
               time intervals when the angular separation has
               values between ABSMIN and ABSMIN + 'adjust'.

               'adjust' is not used for searches for local extrema,
               equality or inequality conditions.

      step     time step size to use in the search. 'step' must be short
               enough for a search using this step size to locate the time
               intervals where coordinate function of the observer-target
               vector is monotone increasing or decreasing. However, step must
               not be *too* short, or the search will take an unreasonable
               amount of time.

               [1,1] = size(step); double = class(step)

               The choice of step affects the completeness but not
               the precision of solutions found by this routine; the
               precision is controlled by the convergence tolerance.
               See the discussion of the parameter SPICE_GF_CNVTOL for
               details.

               'step' has units of TDB seconds.

      nintvls  value specifying the number of intervals in the internal
               workspace array used by this routine. 'nintvls' should be at
               least as large as the number of intervals within the search
               region on which the specified observer-target vector coordinate
               function is monotone increasing or decreasing. It does no harm
               to pick a value of 'nintvls' larger than the minimum required
               to execute the specified search, but if chosen too small, the
               search will fail.

               [1,1] = size(nintvls); int32 = class(nintvls)

      cnfine   a SPICE window that confines the time period over which the
               specified search is conducted. 'cnfine' may consist of a
               single interval or a collection of intervals.

               [2m,1] = size(cnfine); double = class(cnfine)

               In some cases the confinement window can be used to
               greatly reduce the time period that must be searched
               for the desired solution. See the Particulars section
               below for further discussion.

               See the Examples section below for a code example
               that shows how to create a confinement window.

   the call:

      result = cspice_gfsep( targ1,  shape1, frame1,          ...
                             targ2,  shape2, frame2,          ...
                             abcorr, obsrvr, relate,  refval, ...
                             adjust, step,   nintvls, cnfine )

   returns:

      result   the SPICE window of intervals, contained within the
               confinement window 'cnfine', on which the specified
               constraint is satisfied.

               [2n,1] = size(result); double = class(result)

               If the search is for local extrema, or for absolute
               extrema with adjust set to zero, then normally each
               interval of result will be a singleton: the left and
               right endpoints of each interval will be identical.

               If no times within the confinement window satisfy the
               constraint, 'result' will return with cardinality zero.

Examples


   Any numerical results shown for this example may differ between
   platforms as the results depend on the SPICE kernels used as input
   and the machine specific arithmetic implementation.

      Determine the times of local maxima of the angular separation
      between the moon and sun as observed from earth from
      Jan 1, 2007 to Jan 1 2008.

      MAXWIN  =  1000;
      TIMFMT  = 'YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND';

      %
      % Load kernels.
      %
      cspice_furnsh( 'standard.tm' );

      %
      % Store the time bounds of our search interval in
      % the cnfine confinement window.
      %
      et = cspice_str2et( { '2007 JAN 01', '2008 JAN 01'} );

      cnfine = cspice_wninsd( et(1), et(2) );

      %
      % Search using a step size of 6 days (in units of seconds).
      %
      step   = 6.*cspice_spd;
      adjust = 0.;
      refval = 0;

      targ1  = 'MOON';
      shape1 = 'SPHERE';
      frame1 = 'NULL';
      targ2  = 'SUN';
      shape2 = 'SPHERE';
      frame2 = 'NULL';
      abcorr = 'NONE';
      relate = 'LOCMAX';
      obsrvr = 'EARTH';
      nintvls = MAXWIN;

      result = cspice_gfsep( targ1,  shape1, frame1, ...
                             targ2,  shape2, frame2, ...
                             abcorr, obsrvr, relate, ...
                             refval, adjust, step,   ...
                             nintvls, cnfine );

      %
      % List the beginning and ending times in each interval
      % if result contains data.
      %
      for i=1:numel(result)/2

         [left, right] = cspice_wnfetd( result, i );

         output = cspice_timout( [left,right], TIMFMT );

         if( isequal( left, right) )

            disp( ['Event time: ' output(1,:)] )

         else

            disp( ['From : ' output(1,:)] )
            disp( ['To   : ' output(2,:)] )

         end

      end

      %
      % It's always good form to unload kernels after use,
      % particularly in Matlab due to data persistence.
      %
      cspice_kclear

   MATLAB outputs:

      Event time: 2007-JAN-03 14:20:24.618884 (TDB)
      Event time: 2007-FEB-02 06:16:24.101655 (TDB)
      Event time: 2007-MAR-03 23:22:41.994289 (TDB)
      Event time: 2007-APR-02 16:49:16.134481 (TDB)
      Event time: 2007-MAY-02 09:41:43.829169 (TDB)
      Event time: 2007-JUN-01 01:03:44.527040 (TDB)
      Event time: 2007-JUN-30 14:15:26.576639 (TDB)
      Event time: 2007-JUL-30 01:14:49.002265 (TDB)
      Event time: 2007-AUG-28 10:39:01.390508 (TDB)
      Event time: 2007-SEP-26 19:25:51.512445 (TDB)
      Event time: 2007-OCT-26 04:30:56.628530 (TDB)
      Event time: 2007-NOV-24 14:31:04.334590 (TDB)
      Event time: 2007-DEC-24 01:40:12.238389 (TDB)

Particulars


   This routine provides a simple interface for conducting searches
   for angular separation events.

   This routine determines a set of one or more time intervals
   within the confinement window for which the angular separation
   between the two bodies satisfies some defined relationship.
   The resulting set of intervals is returned as a SPICE window.

   Below we discuss in greater detail aspects of this routine's
   solution process that are relevant to correct and efficient
   use of this routine in user applications.

   The Search Process
   ==================

   Regardless of the type of constraint selected by the caller, this
   routine starts the search for solutions by determining the time
   periods, within the confinement window, over which the specified
   angular separation function is monotone increasing and monotone
   decreasing. Each of these time periods is represented by a SPICE window.
   Having found these windows, all of the angular separation function's
   local extrema within the confinement window are known. Absolute extrema
   then can be found very easily.

   Within any interval of these "monotone" windows, there will be at
   most one solution of any equality constraint. Since the boundary
   of the solution set for any inequality constraint is contained in
   the union of

      - the set of points where an equality constraint is met
      - the boundary points of the confinement window

   the solutions of both equality and inequality constraints can be
   found easily once the monotone windows have been found.

   Step Size
   =========

   The monotone windows (described above) are found using a two-step
   search process. Each interval of the confinement window is
   searched as follows: first, the input step size is used to
   determine the time separation at which the sign of the rate of
   change of angular separation (angular separation rate) will be
   sampled. Starting at the left endpoint of an interval, samples
   will be taken at each step. If a change of sign is found, a
   root has been bracketed; at that point, the time at which the
   angular separation rate is zero can be found by a refinement
   process, for example, using a binary search.

   Note that the optimal choice of step size depends on the lengths
   of the intervals over which the distance function is monotone:
   the step size should be shorter than the shortest of these
   intervals (within the confinement window).

   The optimal step size is *not* necessarily related to the lengths
   of the intervals comprising the result window. For example, if
   the shortest monotone interval has length 10 days, and if the
   shortest result window interval has length 5 minutes, a step size
   of 9.9 days is still adequate to find all of the intervals in the
   result window. In situations like this, the technique of using
   monotone windows yields a dramatic efficiency improvement over a
   state-based search that simply tests at each step whether the
   specified constraint is satisfied. The latter type of search can
   miss solution intervals if the step size is longer than the
   shortest solution interval.

   Having some knowledge of the relative geometry of the target and
   observer can be a valuable aid in picking a reasonable step size.
   In general, the user can compensate for lack of such knowledge by
   picking a very short step size; the cost is increased computation
   time.

   Note that the step size is not related to the precision with which
   the endpoints of the intervals of the result window are computed.
   That precision level is controlled by the convergence tolerance.

   Convergence Tolerance
   =====================

   As described above, the root-finding process used by this routine
   involves first bracketing roots and then using a search process
   to locate them. "Roots" are both times when local extrema are
   attained and times when the distance function is equal to a
   reference value. All endpoints of the intervals comprising the
   result window are either endpoints of intervals of the
   confinement window or roots.

   Once a root has been bracketed, a refinement process is used to
   narrow down the time interval within which the root must lie.
   This refinement process terminates when the location of the root
   has been determined to within an error margin called the
   "convergence tolerance." The convergence tolerance used by this
   routine is set by the parameter SPICE_GF_CNVTOL.

   The value of SPICE_GF_CNVTOL is set to a "tight" value so that the
   tolerance doesn't become the limiting factor in the accuracy of
   solutions found by this routine. In general the accuracy of input
   data will be the limiting factor.

   The user may change the convergence tolerance from the default
   SPICE_GF_CNVTOL value by calling the routine cspice_gfstol, e.g.

      cspice_gfstol( tolerance value in seconds )

   Call cspice_gfstol prior to calling this routine. All subsequent
   searches will use the updated tolerance value.

   Setting the tolerance tighter than SPICE_GF_CNVTOL is unlikely to be
   useful, since the results are unlikely to be more accurate.
   Making the tolerance looser will speed up searches somewhat,
   since a few convergence steps will be omitted. However, in most
   cases, the step size is likely to have a much greater affect on
   processing time than would the convergence tolerance.

   The Confinement Window
   ======================

   The simplest use of the confinement window is to specify a time
   interval within which a solution is sought. However, the
   confinement window can, in some cases, be used to make searches
   more efficient. Sometimes it's possible to do an efficient search
   to reduce the size of the time period over which a relatively
   slow search of interest must be performed.

   Elongation
   ===========================

   The angular separation of two targets as seen from an observer
   where one of those targets is the sun is known as elongation.

Required Reading


   For important details concerning this module's function, please refer to
   the CSPICE routine gfsep_c.

   MICE.REQ
   GF.REQ
   SPK.REQ
   CK.REQ
   TIME.REQ
   WINDOWS.REQ

Version


   -Mice Version 1.0.3, 17-MAR-2015, EDW (JPL)

       Edited I/O section to conform to NAIF standard for Mice documentation.

       Typo correction in version IDs in Version section.

   -Mice Version 1.0.2, 05-SEP-2012, EDW (JPL)

      Edit to comments to correct search description.

      Header updated to describe use of cspice_gfstol.

    -Mice Version 1.0.1, 29-DEC-2009, EDW (JPL)

      Edited argument descriptions. Removed mention of "ELLIPSOID"
      shape from 'shape1' and 'shape2' as that option is not yet
      implemented.

   -Mice Version 1.0.0, 15-APR-2009, NJB (JPL), EDW (JPL)

Index_Entries


   GF angular separation search


Wed Apr  5 18:00:32 2017