gftfov_c

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

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

                  If `result' is non-empty on input, its contents
                  will be discarded before gftfov_c conducts its
                  search.
#### 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, the error
           SPICE(INVALIDSTEPSIZE) will be signaled.

       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 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, the error will be diagnosed by
           a routine in the call tree of this routine.

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

       6)  If the target body coincides with the observer body `obsrvr',
           the error will be diagnosed by a routine in the call tree of
           this routine.

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

       10) If the instrument name `inst' does not have corresponding NAIF
           ID code, the error will be diagnosed 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, the error will be be diagnosed by routines
           in the call tree of this routine.

       12) If the FOV boundary has more than SPICE_GF_MAXVRT vertices, the
           error will be be diagnosed by routines 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, the error will be diagnosed
           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, the error will be diagnosed 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 will be diagnosed 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, the error will be diagnosed
           by a routine in the call tree of this routine. If the result
           window has size less than 2, the error SPICE(WINDOWTOOSMALL)
           will be signaled by this routine.

       17) If any input string argument pointer is null, the error
           SPICE(NULLPOINTER) will be signaled.

       18) If any input string argument other than `tframe' is empty, the
           error SPICE(EMPTYSTRING) will be 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 these examples 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
                ---------                     --------
                naif0009.tls                  Leapseconds
                cpck05Mar2004.tpc             Satellite orientation and
                                              radii
                981005_PLTEPH-DE405S.bsp      Planetary ephemeris
                020514_SE_SAT105.bsp          Satellite ephemeris
                030201AP_SK_SM546_T45.bsp     Spacecraft ephemeris
                cas_v37.tf                    Cassini FK
                04135_04171pc_psiv2.bc        Cassini bus CK
                cas00084.tsc                  Cassini SCLK kernel
                cas_iss_v09.ti                Cassini IK

             \begindata

                KERNELS_TO_LOAD = ( 'naif0009.tls',
                                    'cpck05Mar2004.tpc',
                                    '981005_PLTEPH-DE405S.bsp',
                                    '020514_SE_SAT105.bsp',
                                    '030201AP_SK_SM546_T45.bsp',
                                    'cas_v37.tf',
                                    '04135_04171pc_psiv2.bc',
                                    'cas00084.tsc',
                                    'cas_iss_v09.ti'            )
             \begintext

          Example code begins here.

             #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 PC/Linux/gcc platform, the
          output was:

             Instrument: CASSINI_ISS_NAC
             Target:     PHOEBE

              Visibility start time              Stop time
              2004-JUN-11 07:35:49.958589 (TDB)  2004-JUN-11 08:48:27.485965 (TDB)
              2004-JUN-11 09:03:19.767799 (TDB)  2004-JUN-11 09:35:27.634790 (TDB)
              2004-JUN-11 09:50:19.585474 (TDB)  2004-JUN-11 10:22:27.854253 (TDB)
              2004-JUN-11 10:37:19.332696 (TDB)  2004-JUN-11 11:09:28.116016 (TDB)
              2004-JUN-11 11:24:19.049485 (TDB)  2004-JUN-11 11:56:28.380304 (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)
       L.S. Elson    (JPL)
       E.D. Wright   (JPL)
#### Version
       -CSPICE Version 1.0.0, 15-APR-2009 (NJB) (LSE) (EDW)
#### Index_Entries
       GF target in instrument FOV search
#### Link to routine gftfov_c source file gftfov_c.c

    Thu Apr 16 08:58:17 2009