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cspice_gftfov

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

Abstract


   CSPICE_GFTFOV determines time intervals when a specified ephemeris
   object 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 a target intersection
               search: times when the specified target intersects the
               region of space corresponding to 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.

      target   the string naming the `target' body, the appearances
               of which in the specified instrument's field of view are
               sought.

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

               The body must be an ephemeris object.

               Optionally, you may supply the integer NAIF ID code
               for the body as a string. For example both 'MOON' and
               '301' are legitimate strings that designate the Moon.

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

      tshape   the string naming the geometric model used to
               represent the shape of the `target' body.

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

               The supported options are:

                  'ELLIPSOID'   Use a triaxial ellipsoid model,
                                with radius values provided via the
                                kernel pool. A kernel variable
                                having a name of the form

                                   'BODYnnn_RADII'

                                where nnn represents the NAIF
                                integer code associated with the
                                body, must be present in the kernel
                                pool. This variable must be
                                associated with three numeric
                                values giving the lengths of the
                                ellipsoid's X, Y, and Z semi-axes.

                  'POINT'       Treat the body as a single point.

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

      tframe   the string naming the body-fixed, body-centered
               reference frame associated with the target body.

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

               Examples of such names are 'IAU_SATURN' (for Saturn) and
               'ITRF93' (for the Earth).

               If the target body is modeled as a point, `tframe'
               is ignored and should be left blank.

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

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

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

               For remote sensing applications, where the apparent
               position and orientation of the target seen by the
               observer are desired, normally either of the
               corrections

                  'LT+S'
                  'CN+S'

               should be used. These and the other supported options
               are described below.

                 'NONE'      Apply no correction.

               Supported aberration correction options for reception case
               (radiation is received by observer at `et') are:

                  'LT'       Correct for one-way light time using a Newtonian
                             formulation.

                  'LT+S'     Correct for one-way light time and stellar
                             aberration using a Newtonian formulation.

                  'CN'       Correct for one-way light time using a converged
                             Newtonian light time correction.

                  'CN+S'     Correct for one-way light time and stellar
                             aberration using a converged Newtonian light
                             time and stellar aberration corrections.

               Supported aberration correction options for transmission case
               (radiation is emitted from observer at ET) are:

                  'XLT'      Correct for one-way light time using a Newtonian
                             formulation.

                  'XLT+S'    Correct for one-way light time and stellar
                             aberration using a Newtonian formulation.

                  'XCN'      Correct for one-way light time using a converged
                             Newtonian light time correction.

                  'XCN+S'    Correct for one-way light time and stellar
                             aberration using a converged Newtonian light
                             time and stellar aberration corrections.

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

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

      obsrvr   the string naming the body from which the target is
               observed.

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

               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 to use in the search.

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

               `step' must be short enough for a search using step
               to locate the time intervals where the specified
               angular separation function is monotone increasing or
               decreasing. 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_gftfov( inst,   target, tshape, tframe, 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) 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.

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


         KPL/MK

         File name: gftfov_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
            pck00010.tpc                    Satellite orientation
                                            and radii
            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',
                                'pck00010.tpc',
                                '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 gftfov_ex1()

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

         %
         % Load kernels.
         %
         cspice_furnsh( 'gftfov_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_NAC';
         target  = 'PHOEBE';
         tshape  = 'ELLIPSOID';
         tframe  = 'IAU_PHOEBE';
         abcorr  = 'LT+S';
         obsrvr  = 'CASSINI';
         step    = 10.;
         nintvls = MAXWIN;

         result = cspice_gftfov( inst,   target, tshape, tframe, abcorr,  ...
                                 obsrvr, step,   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 07:35:27.066980 (TDB)
      To   : 2004-JUN-11 08:48:03.954696 (TDB)

      From : 2004-JUN-11 09:02:56.580046 (TDB)
      To   : 2004-JUN-11 09:35:04.038509 (TDB)

      From : 2004-JUN-11 09:49:56.476397 (TDB)
      To   : 2004-JUN-11 10:22:04.242879 (TDB)

      From : 2004-JUN-11 10:36:56.283772 (TDB)
      To   : 2004-JUN-11 11:09:04.397165 (TDB)

      From : 2004-JUN-11 11:23:56.020645 (TDB)
      To   : 2004-JUN-11 11:56:04.733536 (TDB)


Particulars


   This routine determines a set of one or more time intervals
   within the confinement window when any portion of a specified
   target body appears 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.

   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
   ==================

   The search for visibility events is treated as a search for state
   transitions: times are sought when the state of the target body
   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, via 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 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
   =====================

   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 name of either the target or observer cannot be
       translated to a NAIF ID code, an error is signaled by
       a routine in the call tree of this routine.

   4)  If the specified aberration correction is an unrecognized
       value, an error is signaled by a routine
       in the call tree of this routine.

   5)  If the radii of a target body modeled as an ellipsoid cannot
       be determined by searching the kernel pool for a kernel
       variable having a name of the form

          'BODYnnn_RADII'

       where nnn represents the NAIF integer code associated with
       the body, an error is signaled by a routine in the
       call tree of this routine.

   6)  If the target body coincides with the observer body `obsrvr', an
       error is signaled by a routine in the call tree of this
       routine.

   7)  If the body model specifier `tshape' is invalid, an error is
       signaled by either this routine or a routine in the call tree
       of this routine.

   8)  If a target body-fixed reference frame associated with a
       non-point target is not recognized, an error is signaled by a
       routine in the call tree of this routine.

   9)  If a target body-fixed reference frame is not centered at the
       corresponding target body, an error is signaled by a routine
       in the call tree of this routine.

   10) 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.

   11) 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.

   12) 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.

   13) 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)-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.

   14) 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.

   15) 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.

   16) 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.

   17) If any of the input arguments, `inst', `target', `tshape',
       `tframe', `abcorr', `obsrvr', `step', `cnfine' or `nintvls', is
       undefined, an error is signaled by the Matlab error handling
       system.

   18) If any of the input arguments, `inst', `target', `tshape',
       `tframe', `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 target and observer that
      describes the ephemeris of these objects for the period
      defined by the confinement window, `cnfine', must be
      loaded. If aberration corrections are used, the states of
      target and 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.

   -  Frame data: if a frame definition is required to convert
      the observer and target states to the body-fixed frame of
      the target, that definition must be available in the kernel
      pool. Typically the definitions of frames not already
      built-in to SPICE are supplied by loading a frame kernel.

      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:

   -  PCK data: bodies modeled as triaxial ellipsoids must have
      orientation data provided by variables in the kernel pool.
      Typically these data are made available by loading a text
      PCK file via cspice_furnsh.

      Bodies modeled as triaxial ellipsoids must have semi-axis
      lengths provided by variables in the kernel pool. Typically
      these data are made available by loading a text PCK file via
      cspice_furnsh.

   -  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 both J2000 and the target
      body-fixed 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).

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

Restrictions


   1)  The reference frame associated with `inst' must be
       centered at the observer or must be inertial. No check is done
       to ensure this.

   2)  The kernel files to be used by cspice_gftfov must be loaded (normally
       via the Mice routine cspice_furnsh) before cspice_gftfov 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 1.2.0, 11-AUG-2021 (EDW) (JDR)

       Changed input argument name "room" to "nintvls" for consistency
       with other routines.

       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 1.1.0, 12-MAY-2012 (EDW)

       Corrected minor typo in header.

       Renamed the argument 'size' to 'room'. "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 target in instrument FOV search


Fri Dec 31 18:44:25 2021