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gftfov_c

Table of contents
Procedure
Abstract
Required_Reading
Keywords
Brief_I/O
Detailed_Input
Detailed_Output
Parameters
Exceptions
Files
Particulars
Examples
Restrictions
Literature_References
Author_and_Institution
Version
Index_Entries

Procedure

   gftfov_c ( GF, is target in FOV? ) 

   void gftfov_c ( ConstSpiceChar     * inst,
                   ConstSpiceChar     * target,
                   ConstSpiceChar     * tshape,
                   ConstSpiceChar     * tframe,
                   ConstSpiceChar     * abcorr,
                   ConstSpiceChar     * obsrvr,
                   SpiceDouble          step,
                   SpiceCell          * cnfine,
                   SpiceCell          * result  )

Abstract

   Determine time intervals when a specified ephemeris object
   intersects the space bounded by the field-of-view (FOV) of a
   specified instrument.

Required_Reading

   CK
   FRAMES
   GF
   KERNEL
   NAIF_IDS
   PCK
   SPK
   TIME
   WINDOWS

Keywords

   EVENT
   FOV
   GEOMETRY
   INSTRUMENT
   SEARCH
   WINDOW


Brief_I/O

   VARIABLE  I/O  DESCRIPTION
   --------  ---  --------------------------------------------------
   SPICE_GF_MARGIN
              P   Minimum complement of FOV cone angle.
   SPICE_GF_CNVTOL
              P   Convergence tolerance.
   SPICE_GF_MAXVRT
              P   Maximum number of FOV boundary vertices.
   inst       I   Name of the instrument.
   target     I   Name of the target body.
   tshape     I   Type of shape model used for target body.
   tframe     I   Body-fixed, body-centered frame for target body.
   abcorr     I   Aberration correction flag.
   obsrvr     I   Name of the observing body.
   step       I   Step size in seconds for finding FOV events.
   cnfine    I-O  SPICE window to which the search is restricted.
   result     O   SPICE window containing results.

Detailed_Input

   inst        indicates the name of an 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.

               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 CSPICE routine getfov_c for a
               description of the required parameters associated with
               an instrument.

   target      is the name of the target body, the appearances of
               which in the specified instrument's field of view are
               sought. 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.

               Case and leading or trailing blanks are not
               significant in the string `target'.

   tshape      is a string indicating the geometric model used to
               represent the shape of the target body. 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.

               Case and leading or trailing blanks are not
               significant in the string `tshape'.

   tframe      is the name of the body-fixed, body-centered reference
               frame associated with the target body. 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.

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

   abcorr      indicates the aberration corrections to be applied
               when computing the target's position and orientation.

               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.

               Supported aberration correction options for
               observation (the case where radiation is received by
               observer at ET) are:

                  "NONE"         No correction.

                  "LT"           Light time only

                  "LT+S"         Light time and stellar aberration.

                  "CN"           Converged Newtonian (CN) light time.

                  "CN+S"         CN light time and stellar aberration.

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

                  "XLT"          Light time only.

                  "XLT+S"        Light time and stellar aberration.

                  "XCN"          Converged Newtonian (CN) light time.

                  "XCN+S"        CN light time and stellar aberration.

               For detailed information, see the GF Required Reading,
               gf.req.

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

   obsrvr      is the name of the body from which the target is
               observed. The instrument designated by `inst' is treated
               as if it were co-located with the observer.

               Optionally, you may supply the integer NAIF ID code
               for the body as a string.

               Case and leading or trailing blanks are not
               significant in the string `obsrvr'.

   step        is the step size to be used in the search. `step' must
               be shorter than any interval, within the confinement
               window, over which the specified condition is met. In
               other words, `step' must be shorter than the shortest
               visibility event that the user wishes to detect. `step'
               also must be shorter than the minimum duration
               separating any two visibility events. 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 seconds.

   cnfine      is 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.

               The endpoints of the time intervals comprising `cnfine'
               are interpreted as seconds past J2000 TDB.

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

               `cnfine' must be declared as a double precision SpiceCell.

               CSPICE provides the following macro, which declares and
               initializes the cell

                  SPICEDOUBLE_CELL        ( cnfine, CNFINESZ );

               where CNFINESZ is the maximum capacity of `cnfine'.

Detailed_Output

   cnfine      is the input confinement window, updated if necessary so the
               control area of its data array indicates the window's size
               and cardinality. The window data are unchanged.

   result      is a SPICE window representing the set of time intervals,
               within the confinement period, when the target body is
               visible; that is, when the target body intersects the
               space bounded by the specified instrument's field of
               view.

               `result' must be declared and initialized with sufficient
               size to capture the full set of time intervals within the
               search region on which the specified condition is satisfied.

               If `result' is non-empty on input, its contents will be
               discarded before gftfov_c conducts its search.

               The endpoints of the time intervals comprising `result' are
               interpreted as seconds past J2000 TDB.

               If no times within the confinement window satisfy the
               search criteria, `result' will be returned with a
               cardinality of zero.

               `result' must be declared as a double precision SpiceCell.

               CSPICE provides the following macro, which declares and
               initializes the cell

                  SPICEDOUBLE_CELL        ( result, RESULTSZ );

               where RESULTSZ is the maximum capacity of `result'.

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.


   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 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 specific
                     plane normal to this plane (the normal plane
                     contains the angle bisector defined by U and V)
                     such that the projections have angular separation
                     of at least 2*SPICE_GF_MARGIN radians from the
                     plane spanned by U and V.

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
       wncond_c 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)-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.

   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 size less than 2, the
       error SPICE(WINDOWTOOSMALL) is signaled by a routine in the
       call tree of this routine.

   17) 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.

   18) If any of the `inst', `target', `tshape', `abcorr', `obsrvr'
       or `tframe' input string pointers is null, the error
       SPICE(NULLPOINTER) is signaled.

   19) If any of the `inst', `target', `tshape', `abcorr' or `obsrvr'
       input strings has zero length, the error SPICE(EMPTYSTRING) is
       signaled.

   20) If any the `cnfine' or `result' cell arguments has a type
       other than SpiceDouble, the error SPICE(TYPEMISMATCH) is
       signaled.

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 furnsh_c.

   -  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 furnsh_c.

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

   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 furnsh_c.

      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
      furnsh_c.

   -  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.

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.

   This routine provides a simpler, but less flexible, interface
   than does the CSPICE routine gffove_c for conducting searches for
   visibility events. Applications that require support for progress
   reporting, interrupt handling, non-default step or refinement
   functions, or non-default convergence tolerance should call
   gffove_c rather than this routine.

   To treat the target as a ray rather than as an ephemeris object,
   use either the higher-level CSPICE routine gfrfov_c or gffove_c.
   Those routines may be used to search for times when distant
   target objects such as stars are visible in an instrument FOV, as
   long the direction from the observer to the target can be modeled
   as a ray.

   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
   via 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.

   To use a different tolerance value, a lower-level GF routine such as
   gffove_c  must be called. Making 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 effect 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. For an example, see the program CASCADE in the GF
   Example Programs chapter of the GF Required Reading, gf.req.

Examples

   The numerical results shown for this example may differ across
   platforms. The results depend on the SPICE kernels used as
   input, the compiler and supporting libraries, 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.


      /.
         Program gftfov_ex1
      ./
      #include <stdio.h>
      #include "SpiceUsr.h"
      #include "SpiceZmc.h"

      int main()
      {
         /.
         PROGRAM EX1
         ./

         /.
         Local constants
         ./
         #define  META           "gftfov_ex1.tm"
         #define  TIMFMT         "YYYY-MON-DD HR:MN:SC.######::TDB (TDB)"
         #define  TIMLEN         41
         #define  MAXWIN         10000

         /.
         Local variables
         ./
         SPICEDOUBLE_CELL ( cnfine, MAXWIN );
         SPICEDOUBLE_CELL ( result, MAXWIN );

         SpiceChar             * abcorr;
         SpiceChar             * inst;
         SpiceChar             * obsrvr;
         SpiceChar             * target;
         SpiceChar             * tframe;
         SpiceChar               timstr  [2][ TIMLEN ];
         SpiceChar             * tshape;

         SpiceDouble             endpt   [2];
         SpiceDouble             et0;
         SpiceDouble             et1;
         SpiceDouble             stepsz;

         SpiceInt                i;
         SpiceInt                j;
         SpiceInt                n;

         /.
         Load kernels.
         ./
         furnsh_c ( META );

         /.
         Insert search time interval bounds into the
         confinement window.
         ./
         str2et_c ( "2004 JUN 11 06:30:00 TDB", &et0 );
         str2et_c ( "2004 JUN 11 12:00:00 TDB", &et1 );

         wninsd_c ( et0, et1, &cnfine );

         /.
         Initialize inputs for the search.
         ./
         inst   = "CASSINI_ISS_NAC";
         target = "PHOEBE";
         tshape = "ELLIPSOID";
         tframe = "IAU_PHOEBE";
         abcorr = "LT+S";
         obsrvr = "CASSINI";
         stepsz = 10.0;

         printf ( "\n"
                  " Instrument: %s\n"
                  " Target:     %s\n"
                  "\n",
                  inst,
                  target            );

         /.
         Perform the search.
         ./
         gftfov_c ( inst,   target, tshape, tframe,
                    abcorr, obsrvr, stepsz, &cnfine, &result );


         n = wncard_c ( &result );

         if ( n == 0 )
         {
            printf (  "No FOV intersection found.\n" );
         }
         else
         {
            printf ( "  Visibility start time              Stop time\n" );

            for ( i = 0;  i < n;  i++ )
            {
               wnfetd_c ( &result, i, endpt, endpt+1 );

               for ( j = 0;  j < 2;  j++ )
               {
                  timout_c ( endpt[j], TIMFMT, TIMLEN, timstr[j] );
               }

               printf ( "  %s  %s\n",
                        timstr[0],
                        timstr[1]                                   );
            }
         }

         printf ( "\n" );

         return ( 0 );
      }


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


       Instrument: CASSINI_ISS_NAC
       Target:     PHOEBE

        Visibility start time              Stop time
        2004-JUN-11 07:35:27.066980 (TDB)  2004-JUN-11 08:48:03.954696 (TDB)
        2004-JUN-11 09:02:56.580045 (TDB)  2004-JUN-11 09:35:04.038509 (TDB)
        2004-JUN-11 09:49:56.476397 (TDB)  2004-JUN-11 10:22:04.242879 (TDB)
        2004-JUN-11 10:36:56.283772 (TDB)  2004-JUN-11 11:09:04.397165 (TDB)
        2004-JUN-11 11:23:56.020645 (TDB)  2004-JUN-11 11:56:04.733535 (TDB)

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 gftfov_c must be loaded (normally
       via the CSPICE routine furnsh_c) before gftfov_c is called.

Literature_References

   None.

Author_and_Institution

   N.J. Bachman        (JPL)
   J. Diaz del Rio     (ODC Space)
   L.S. Elson          (JPL)
   E.D. Wright         (JPL)

Version

   -CSPICE Version 1.0.1, 06-AUG-2021 (JDR)

       Edited the header to comply with NAIF standard.

       Updated Example's kernels set to use PDS archived data.

       Updated the description of "cnfine" and "result" arguments.

       Added entry #20 in -Exceptions section.

   -CSPICE Version 1.0.0, 15-APR-2009 (NJB) (LSE) (EDW)

Index_Entries

   GF target in instrument FOV search
Fri Dec 31 18:41:07 2021