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cspice_gfrfov

Table of contents
Abstract
I/O
Parameters
Examples
Particulars
Exceptions
Files
Restrictions
Required_Reading
Literature_References
Author_and_Institution
Version
Index_Entries

Abstract


   CSPICE_GFRFOV determines time intervals when a specified ray intersects
   the space bounded by the field-of-view (FOV) of a specified instrument.

I/O


   Given:

      inst     the string naming the instrument, such as a
               spacecraft-mounted framing camera, the field of view
               (FOV) of which is to be used for an target intersection
               search: the direction from the observer to a target
               is represented by a ray, and times when the specified
               ray intersects the region of space bounded by the FOV
               are sought.

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

               The position of the instrument designated by `inst' is
               considered to coincide with that of the ephemeris
               object designated by the input argument `obsrvr' (see
               description below).

               `inst' must have a corresponding NAIF ID and a frame
               defined, as is normally done in a frame kernel. It
               must also have an associated reference frame and a FOV
               shape, boresight and boundary vertices (or reference
               vector and reference angles) defined, as is usually
               done in an instrument kernel.

               See the header of the Mice routine cspice_getfov for a
               description of the required parameters associated with
               an instrument.

      raydir   the ray pointing toward a target.

               [3,1] = size(raydir); double = class(raydir)

               The ray emanates from the location of the ephemeris object
               designated by the input argument `obsrvr' and is expressed
               relative to the reference frame designated by `rframe'
               (see descriptions below).

      rframe   the string naming the reference frame associated with
               the input ray's direction vector `raydir'.

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

               Since light time corrections are not supported for
               rays, the orientation of the frame is always evaluated
               at the epoch associated with the observer, as opposed
               to the epoch associated with the light-time corrected
               position of the frame center.

               Case and leading or trailing blanks bracketing a non-blank
               frame name are not significant in the string 1rframe'.

      abcorr   the string indicating the aberration corrections
               to apply when computing the `raydir' direction.

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

               The supported aberration correction options are

                  'NONE'          No correction.
                  'S'             Stellar aberration correction,
                                  reception case.
                  'XS'            Stellar aberration correction,
                                  transmission case.

               For detailed information, see the geometry finder required
               reading, gf.req.

               Case, leading and trailing blanks are not significant
               in the string `abcorr'.

      obsrvr   the string naming the body from which the target
               represented by `raydir' is observed.

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

               The instrument designated by `inst' is treated as if it were
               co-located with the observer.

               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.

      step     the step size to use in the search.

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

               `step' must be shorter than any interval, within the
               confinement window, over which the specified occultation
               condition is met. In other words, `step' must be shorter than
               the shortest occultation event the user wishes to detect;
               `step' must also be shorter than the shortest time interval
               between two occultation events that occur within the
               confinement window (see below). However, `step' must not be
               *too* short, or the search will take an unreasonable amount
               of time.

               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.

      cnfine   the SPICE window that confines the time
               period over which the specified search is conducted.

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

               `cnfine' may consist of a single interval or a collection
               of intervals.

               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.

      nintvls  the maximum number of intervals to return in `result'.

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

               Note: this value should equal at least the number of expected
               intervals. Recall two double precision values define
               an interval.

   the call:

      result = cspice_gfrfov( inst,   raydir, rframe, abcorr,             ...
                              obsrvr, step,   cnfine, nintvls )

   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.

Parameters


   All parameters described here are declared in the Mice include file
   MiceGF.m. 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.

   SPICE_GF_MAXVRT

               is the maximum number of vertices that may be used
               to define the boundary of the specified instrument's
               field of view.

   SPICE_GF_MARGIN

               is a small positive number used to constrain the
               orientation of the boundary vectors of polygonal
               FOVs. Such FOVs must satisfy the following constraints:

                  1)  The boundary vectors must be contained within
                      a right circular cone of angular radius less
                      than (pi/2) - SPICE_GF_MARGIN radians; in other
                      words, there must be a vector A such that all
                      boundary vectors have angular separation from
                      A of less than (pi/2)-SPICE_GF_MARGIN radians.

                  2)  There must be a pair of boundary vectors U, V
                      such that all other boundary vectors lie in
                      the same half space bounded by the plane
                      containing U and V. Furthermore, all other
                      boundary vectors must have orthogonal
                      projections onto a plane normal to this plane
                      such that the projections have angular
                      separation of at least 2*SPICE_GF_MARGIN radians
                      from the plane spanned by U and V.

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.

   1) This example is an extension of the example in the
      header of

         cspice_gftfov

      The problem statement for that example is


         Search for times when Saturn's satellite Phoebe is within the
         FOV of the Cassini narrow angle camera (CASSINI_ISS_NAC). To
         simplify the problem, restrict the search to a short time
         period where continuous Cassini bus attitude data are
         available.

         Use a step size of 10 seconds to reduce chances of missing
         short visibility events.


      Here we search the same confinement window for times when a
      selected background star is visible. We use the FOV of the
      Cassini ISS wide angle camera (CASSINI_ISS_WAC) to enhance the
      probability of viewing the star.

      The star we'll use has catalog number 6000 in the Hipparcos
      Catalog. The star's J2000 right ascension and declination,
      proper motion, and parallax are taken from that catalog.

      Use the meta-kernel shown below to load the required SPICE
      kernels.


         KPL/MK

         File name: gfrfov_ex1.tm

         This meta-kernel is intended to support operation of SPICE
         example programs. The kernels shown here should not be
         assumed to contain adequate or correct versions of data
         required by SPICE-based user applications.

         In order for an application to use this meta-kernel, the
         kernels referenced here must be present in the user's
         current working directory.

         The names and contents of the kernels referenced
         by this meta-kernel are as follows:

            File name                       Contents
            -----------------------------   ----------------------
            naif0012.tls                    Leapseconds
            041014R_SCPSE_01066_04199.bsp   CASSINI, planetary and
                                            Saturn satellite
                                            ephemeris
            cas_v40.tf                      Cassini FK
            04161_04164ra.bc                Cassini bus CK
            cas00071.tsc                    Cassini SCLK kernel
            cas_iss_v10.ti                  Cassini IK


         \begindata

            KERNELS_TO_LOAD = ( 'naif0012.tls',
                                '041014R_SCPSE_01066_04199.bsp',
                                'cas_v40.tf',
                                '04161_04164ra.bc',
                                'cas00071.tsc',
                                'cas_iss_v10.ti'            )
         \begintext

         End of meta-kernel


      Example code begins here.


      function gfrfov_ex1()

         MAXWIN  =  1000;
         TIMFMT  = 'YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND';
         AU      =  149597870.693;

         %
         % Load kernels.
         %
         cspice_furnsh( 'gfrfov_ex1.tm' )

         %
         % Store the time bounds of our search interval in
         % the `cnfine' confinement window.
         %
         et = cspice_str2et( { '2004 JUN 11 06:30:00 TDB',                ...
                               '2004 JUN 11 12:00:00 TDB' } );

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

         %
         %Initialize inputs for the search.
         %
         inst = 'CASSINI_ISS_WAC';

         %
         % Create a unit direction vector pointing from observer to star.
         % We'll assume the direction is constant during the confinement
         % window, and we'll use 'et[0]' as the epoch at which to compute
         % the direction from the spacecraft to the star.
         %
         % The data below are for the star with catalog number 6000
         % in the Hipparcos catalog. Angular units are degrees; epochs
         % have units of Julian years and have a reference epoch of J1950.
         % The reference frame is J2000.
         %

         parallax_deg = 0.000001056;

         ra_deg_0     = 19.290789927;
         ra_pm        = -0.000000720;
         ra_epoch     = 41.2000;

         dec_deg_0    =  2.015271007;
         dec_pm       =  0.000001814;
         dec_epoch    = 41.1300;

         rframe       = 'J2000';
         nintvls      = MAXWIN;


         %
         % Correct the star's direction for proper motion.
         %
         % The argument `t' represents `et[0]' as Julian years past J1950.
         %

         t       = et(1)/cspice_jyear + ( cspice_j2000-cspice_j1950 )/365.25;

         dtra    = t - ra_epoch;
         dtdec   = t - dec_epoch;

         ra_deg  = ra_deg_0  + dtra * ra_pm;
         dec_deg = dec_deg_0 + dtra * dec_pm;

         ra      = ra_deg  * cspice_rpd;
         dec     = dec_deg * cspice_rpd;

         starpos = cspice_radrec( 1, ra, dec );


         %
         % Correct star position for parallax applicable at
         % the Cassini orbiter's position. (The parallax effect
         % is negligible in this case; we're simply demonstrating
         % the computation.)
         %
         parallax = parallax_deg * cspice_rpd;

         stardist = AU/tan(parallax);

         %
         % Scale the star's direction vector by its distance from
         % the solar system barycenter. Subtract off the position
         % of the spacecraft relative to the solar system barycenter;
         % the result is the ray's direction vector.
         %
         starpos = stardist * starpos;

         [pos, lt] = cspice_spkpos( 'cassini', et(1), 'J2000',            ...
                                      'NONE', 'solar system barycenter' );

         raydir = starpos - pos;

         %
         % Correct the star direction for stellar aberration when
         % we conduct the search.
         %
         abcorr = 'S';
         obsrvr = 'CASSINI';
         stepsz = 100;

         result = cspice_gfrfov( inst,   raydir, rframe, abcorr,          ...
                                 obsrvr, stepsz, cnfine, nintvls );

         %
         % 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,:)] )
               disp( ' ' )

            end

         end

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


      When this program was executed on a Mac/Intel/Octave6.x/64-bit
      platform, the output was:


      From : 2004-JUN-11 06:30:00.000000 (TDB)
      To   : 2004-JUN-11 12:00:00.000000 (TDB)


      Note that the star is visible throughout the confinement window.

Particulars


   This routine determines a set of one or more time intervals when
   the specified ray in contained within the field of view of a
   specified instrument. We'll use the term "visibility event" to
   designate such an appearance. The set of time intervals resulting
   from the search is returned as a SPICE window.

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

   The search for visibility events is treated as a search for state
   transitions: times are sought when the state of the ray
   changes from "not visible" to "visible" or vice versa.

   Step Size
   =========

   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 visibility state will be sampled.
   Starting at the left endpoint of an interval, samples will be
   taken at each step. If a state change is detected, a root has
   been bracketed; at that point, the "root"--the time at which the
   state change occurs---is found by a refinement process, for
   example, by a binary search.

   Note that the optimal choice of step size depends on the lengths
   of the intervals over which the visibility state is constant:
   the step size should be shorter than the shortest visibility event
   duration and the shortest period between visibility events, within
   the confinement window.

   Having some knowledge of the relative geometry of the ray 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
   =====================

   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.

Exceptions


   1)  In order for this routine to produce correct results,
       the step size must be appropriate for the problem at hand.
       Step sizes that are too large may cause this routine to miss
       roots; step sizes that are too small may cause this routine
       to run unacceptably slowly and in some cases, find spurious
       roots.

       This routine does not diagnose invalid step sizes, except that
       if the step size is non-positive, an error is signaled by a
       routine in the call tree of this routine.

   2)  Due to numerical errors, in particular,

          - Truncation error in time values
          - Finite tolerance value
          - Errors in computed geometric quantities

       it is *normal* for the condition of interest to not always be
       satisfied near the endpoints of the intervals comprising the
       result window.

       The result window may need to be contracted slightly by the
       caller to achieve desired results. The SPICE window routine
       cspice_wncond can be used to contract the result window.

   3)  If the observer's name cannot be mapped to an ID code, an
       error is signaled by a routine in the call tree of this
       routine.

   4)  If the aberration correction flag calls for light time
       correction, an error is signaled by a routine in the call tree
       of this routine.

   5)  If the ray's direction vector is zero, an error is signaled by
       a routine in the call tree of this routine.

   6)  If the instrument name `inst' does not have corresponding NAIF
       ID code, an error is signaled by a routine in the call
       tree of this routine.

   7)  If the FOV parameters of the instrument are not present in
       the kernel pool, an error is signaled by a routine
       in the call tree of this routine.

   8)  If the FOV boundary has more than SPICE_GF_MAXVRT vertices, an error
       is signaled by a routine in the call tree of this
       routine.

   9)  If the instrument FOV is polygonal, and this routine cannot
       find a ray R emanating from the FOV vertex such that maximum
       angular separation of R and any FOV boundary vector is within
       the limit (pi/2)-SPICE_GF_MARGIN radians, an error is signaled
       by a routine in the call tree of this routine. If the FOV
       is any other shape, the same error check will be applied with
       the instrument boresight vector serving the role of R.

   10) If the loaded kernels provide insufficient data to compute a
       requested state vector, an error is signaled by a
       routine in the call tree of this routine.

   11) If an error occurs while reading an SPK or other kernel file,
       the error is signaled by a routine in the call tree
       of this routine.

   12) If the output SPICE window `result' has insufficient capacity
       to contain the number of intervals on which the specified
       visibility condition is met, an error is signaled
       by a routine in the call tree of this routine.

   13) If any of the input arguments, `inst', `raydir', `rframe',
       `abcorr', `obsrvr', `step', `cnfine' or `nintvls', is
       undefined, an error is signaled by the Matlab error handling
       system.

   14) If any of the input arguments, `inst', `raydir', `rframe',
       `abcorr', `obsrvr', `step', `cnfine' or `nintvls', is not of
       the expected type, or it does not have the expected dimensions
       and size, an error is signaled by the Mice interface.

Files


   Appropriate SPICE kernels must be loaded by the calling program
   before this routine is called.

   The following data are required:

   -  SPK data: ephemeris data for the observer for the period
      defined by the confinement window `cnfine' must be loaded.
      If aberration corrections are used, the state of the
      observer relative to the solar system barycenter must be
      calculable from the available ephemeris data. Typically
      ephemeris data are made available by loading one or more SPK
      files via cspice_furnsh.

   -  Data defining the reference frame associated with the
      instrument designated by `inst' must be available in the kernel
      pool. Additionally the name `inst' must be associated with an
      ID code. Normally these data are  made available by loading
      a frame kernel via cspice_furnsh.

   -  IK data: the kernel pool must contain data such that
      the Mice routine cspice_getfov may be called to obtain
      parameters for `inst'. Normally such data are provided by
      an IK via cspice_furnsh.

   The following data may be required:

   -  CK data: if the instrument frame is fixed to a spacecraft,
      at least one CK file will be needed to permit transformation
      of vectors between that frame and the J2000 frame.

   -  SCLK data: if a CK file is needed, an associated SCLK
      kernel is required to enable conversion between encoded SCLK
      (used to time-tag CK data) and barycentric dynamical time
      (TDB).

   -  Since the input ray direction may be expressed in any
      frame, FKs, CKs, SCLK kernels, PCKs, and SPKs may be
      required to map the direction to the J2000 frame.

   Kernel data are normally loaded once per program run, NOT every
   time this routine is called.

Restrictions


   1)  The kernel files to be used by cspice_gfrfov must be loaded (normally
       via the Mice routine cspice_furnsh) before cspice_gfrfov is called.

Required_Reading


   MICE.REQ
   CK.REQ
   FRAMES.REQ
   GF.REQ
   KERNEL.REQ
   NAIF_IDS.REQ
   PCK.REQ
   SPK.REQ
   TIME.REQ
   WINDOWS.REQ

Literature_References


   None.

Author_and_Institution


   J. Diaz del Rio     (ODC Space)
   E.D. Wright         (JPL)

Version


   -Mice Version 1.1.0, 27-AUG-2021 (EDW) (JDR)

       Edited the header to comply with NAIF standard. Updated
       Example's kernels set to use PDS archived data.

       Added -Parameters, -Exceptions, -Files, -Restrictions,
       -Literature_References and -Author_and_Institution sections.

       Eliminated use of "lasterror" in rethrow.

       Removed reference to the function's corresponding CSPICE header from
       -Required_Reading section.

   -Mice Version 1.0.1, 13-NOV-2014 (EDW)

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

   -Mice Version 1.0.1, 05-NOV-2013 (EDW)

       Corrected minor typos in header.

       Renamed the argument 'size' to 'nintvls'. "size" is a Matlab function
       name and it's seriously dumb to use a function name word as an
       argument name.

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

       Header updated to describe use of cspice_gfstol.

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

Index_Entries


   GF ray in instrument FOV search


Fri Dec 31 18:44:25 2021