Index Page limb_pl02

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 limb_pl02 ( SpiceInt              handle,
                        ConstSpiceDLADescr  * dladsc,
                        ConstSpiceChar      * target,
                        SpiceDouble           et,
                        ConstSpiceChar      * fixref,
                        ConstSpiceChar      * abcorr,
                        ConstSpiceChar      * obsrvr,
                        SpiceInt              npoints,
                        SpiceDouble         * trgepc,
                        SpiceDouble           obspos   [3],
                        SpiceDouble           limbpts  [][3],
                        SpiceInt              plateIDs []     )

#### Abstract

       Deprecated: This routine has been superseded by the CSPICE routine
       limbpt_c. This routine is supported for purposes of backward
       compatibility only.

       Compute a set of points on the limb of a specified target body,
       where the target body's surface is represented by a triangular plate
       model contained in a type 2 DSK segment.

#### Required_Reading

       NAIF_IDS
       PCK
       SPK
       TIME

#### Keywords

       BODY
       GEOMETRY
       MATH

#### Brief_I/O

       Variable  I/O  Description
       --------  ---  --------------------------------------------------
       handle     I   DSK handle.
       dlasdc     I   DLA descriptor of target body segment.
       target     I   Target body.
       et         I   Observation epoch.
       fixref     I   Body-fixed frame associated with target.
       abcorr     I   Aberration correction.
       obsrvr     I   Observer.
       npoints    I   Number of points in limb set.
       trgepc     O   Epoch associated with target center.
       obspos     O   Position of observer in body-fixed frame.
       limbpts    O   Limb point set.
       plateIDs   O   DSK plate IDs of surface points.

#### Detailed_Input

       handle      is the DAS file handle of a DSK file open for read
                   access.  This kernel must contain a type 2 segment that
                   provides a plate model representing the entire surface
                   of the target body.

       dladsc      is the DLA descriptor of a DSK segment representing the
                   surface of a target body.

       target      is the name of the target body. `target' is
                   case-insensitive, and leading and trailing blanks in
                   `target' are not significant. Optionally, you may supply
                   a string containing the integer ID code for the object.
                   For example both "MOON" and "301" are legitimate strings
                   that indicate the moon is the target body.

                   This routine assumes that a kernel variable representing
                   the target's radii is present in the kernel pool.
                   Normally the kernel variable would be defined by loading
                   a PCK file.

       et          is the epoch of participation of the observer,
                   expressed as ephemeris seconds past J2000 TDB: `et' is
                   the epoch at which the observer's position is
                   computed.

                   When aberration corrections are not used, `et' is also
                   the epoch at which the position and orientation of the
                   target body are computed.

                   When aberration corrections are used, `et' is the epoch
                   at which the observer's position relative to the solar
                   system barycenter is computed; in this case the position
                   and orientation of the target body are computed at
                   et-lt, where `lt' is the one-way light time between the
                   target body's center and the observer. See the
                   description of `abcorr' below for details.

       fixref      is the name of the reference frame relative to which the
                   output limb points are expressed. This must a
                   body-centered, body-fixed frame associated with the
                   target.  The frame's axes must be compatible with the
                   triaxial ellipsoidal shape model associated with the
                   target body (normally provide via a PCK): this routine
                   assumes that the first, second, and third ellipsoid radii
                   correspond, respectively, to the x, y, and z-axes of the
                   frame designated by `fixref'.

                   `fixref' may refer to a built-in frame (documented in
                   the Frames Required Reading) or a frame defined by a
                   loaded frame kernel (FK).

                   The orientation of the frame designated by `fixref' is
                   evaluated at epoch of participation of the target
                   body.  See the descriptions of `et' and `abcorr' for
                   details.

       abcorr      indicates the aberration correction to be applied
                   when computing the observer-target position, the
                   orientation of the target body, and the target-
                   source position vector.  `abcorr' may be any of
                   the following.

                      "NONE"     Apply no correction.  Compute the limb
                                 points using the position of the observer
                                 and target, and the orientation of the
                                 target, at `et'.

                   Let `lt' represent the one-way light time between the
                   observer and the target body's center. The following
                   values of `abcorr' apply to the "reception" case in
                   which photons depart from the target body's center at
                   the light-time corrected epoch et-lt and *arrive* at
                   the observer's location at `et':

                      "LT"       Correct for one-way light time (also
                                 called "planetary aberration") using a
                                 Newtonian formulation. This correction
                                 yields the location of the limb points at
                                 the approximate time they emitted photons
                                 arriving at the observer at `et' (the
                                 difference between light time to the
                                 target center and light time to the limb
                                 points is ignored).

                                 The light time correction uses an
                                 iterative solution of the light time
                                 equation. The solution invoked by the
                                 "LT" option uses one iteration.

                                 The target position as seen by the
                                 observer and the rotation of the target
                                 body are corrected for light time.

                      'LT+S'     Correct for one-way light time and stellar
                                 aberration using a Newtonian formulation.
                                 This option modifies the position obtained
                                 with the "LT" option to account for the
                                 observer's velocity relative to the solar
                                 system barycenter. The result is the
                                 apparent limb as seen by the observer.

                      "CN"       Converged Newtonian light time correction.
                                 In solving the light time equation, the
                                 "CN" correction iterates until the
                                 solution converges.  The position and
                                 rotation of the target body are corrected
                                 for light time.

                      'CN+S'     Converged Newtonian light time
                                 and stellar aberration corrections.

       obsrvr      is the name of the observing body.  This is typically
                   a spacecraft, the Earth, or a surface point on the
                   Earth.  `obsrvr' is case-insensitive, and leading and
                   trailing blanks in `obsrvr' are not significant.
                   Optionally, you may supply a string containing the
                   integer ID code for the object.  For example both
                   "EARTH" and "399" are legitimate strings that indicate
                   the Earth is the observer.

       npoints     is the number of limb points to compute.

#### Detailed_Output

       trgepc      is the "target epoch."  `trgepc' is defined as follows:
                   letting `lt' be the one-way light time between the
                   target center and observer, `trgepc' is either the
                   epoch et-lt or `et' depending on whether the requested
                   aberration correction is, respectively, for received
                   radiation or omitted. `lt' is computed using the
                   method indicated by `abcorr'.

                   `trgepc' is expressed as seconds past J2000 TDB.

       obspos      is the vector from the center of the target body at
                   epoch `trgepc' to the observer at epoch `et'.  `obspos' is
                   expressed in the target body-fixed reference frame
                   `fixref', which is evaluated at `trgepc'.

                   `obspos' is returned to simplify various related
                   computations that would otherwise be cumbersome.  For
                   example, the vector `xvec' from the observer to the
                   Ith limb point can be calculated via the call

                      vminus_c ( limbpts[i], obspos, xvec );

                   The components of `obspos' are given in units of km.

       limbpts     is an array of points on the limb of the target.
                   The ith point is contained in the array elements

                       limbpts[i][j],  j = 0, 1, 2

                   As described above, each limb point lies on a ray
                   emanating from the center of the target and passing
                   through a limb point on the target's reference
                   ellipsoid. Each limb point *on the reference ellipsoid*
                   is the point of tangency of a ray that emanates from the
                   observer.  Measured in a cylindrical coordinate system
                   whose Z-axis is parallel to the observer-target vector,
                   the magnitude of the separation in longitude between the
                   limb points is

                      2*Pi / npoints

                   The limb points are expressed in the body-fixed
                   reference frame designated by `fixref'; the
                   orientation of the frame is evaluated at `trgepc'.
                   Units are km.

       plateIDs    is an array of integer ID codes of the plates on which
                   the limb points are located.  The ith plate ID
                   corresponds to the ith limb point. These ID codes can
                   be use to look up data associated with the plates, such
                   as the plates' vertices or outward normal vectors.

                   `plateIDs' should be declared by the caller

                      SpiceInt plateIDs [npoints];

#### Parameters

       None.

#### Exceptions

       1)  If the target name `target' cannot be mapped
           to a body ID code, the error SPICE(IDCODENOTFOUND)
           signaled.

       2)  If the observer name `obsrvr' cannot be mapped to a body ID
           code, the error SPICE(IDCODENOTFOUND) is signaled.

       3)  If `obsrvr' and `target' map to the same NAIF integer ID codes,
           the error SPICE(BODIESNOTDISTINCT) is signaled.

       4)  If the input frame name `fixref' cannot be mapped
           to a frame ID code, the error SPICE(UNKNOWNFRAME) is
           signaled.

       5)  If the frame designated by `fixref' is not centered
           on the target, the error SPICE(INVALIDFRAME) is
           signaled.

       6)  If the set size `npoints' is not at least 1, the error
           SPICE(INVALIDCOUNT) is signaled.

       7)  If any of the reference ellipsoid's semi-axis lengths is
           non-positive, the error will be diagnosed by a routine in the
           call tree of this routine.

       8)  If radii for the target body are not available in the kernel
           pool, the error will be diagnosed by a routine in the call tree
           of this routine.  If radii are available but the target body
           does not have three radii, the error SPICE(INVALIDCOUNT) will be
           signaled.

       9)  If any SPK look-up fails, the error will be diagnosed by
           a routine in the call tree of this routine.

      10)  If a DSK providing a DSK type 2 plate model has not been
           loaded prior to calling llgrid_pl02, the error will be
           diagnosed and signaled by a routine in the call tree of this
           routine.

      11)  If the segment associated with the input DLA descriptor is not
           of data type 2, the error SPICE(WRONGDATATYPE) is signaled.

      12)  If a surface point cannot be computed because the ray corresponding
           to a longitude/latitude pair fails to intersect the target
           surface as defined by the plate model, the error will be
           diagnosed by a routine in the call tree of this routine.

      13)  If the DSK segment identified by `dladsc' is not for the
           body identified by `target', the error SPICE(DSKTARGETMISMATCH)
           is signaled.

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

      15)  If any input string has length zero, the error SPICE(EMPTYSTRING)
           will be signaled.

#### Files

       Appropriate DSK, SPK, PCK, and frame kernels must be loaded by the
       calling program before this routine is called.

       The following data are required:

          - DSK data:  a DSK file containing a plate model representing the
            target body's surface must be loaded. This kernel must contain
            a type 2 segment that contains data for the entire surface of
            the target body.

          - SPK data: ephemeris data for target and observer must be
            loaded. If aberration corrections are used, the states of both
            objects 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.

          - PCK data: triaxial radii for the target body must be loaded
            into the kernel pool. Typically this is done by loading a text
            PCK file via furnsh_c.

          - Further PCK data:  rotation data for the target body must
            be loaded.  These may be provided in a text or binary PCK
            file.

          - Frame data:  if a frame definition is required to convert
            the observer and target states to the target body-fixed
            frame designated by `fixref', 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.

       In all cases, kernel data are normally loaded once per program
       run, NOT every time this routine is called.

#### Particulars

       Boundaries of visible regions on an arbitrary surface are often
       complicated point sets:  boundaries of mountains and craters, if
       present, may contribute to the overall set.  To make the limb
       computation tractable, we simplify the problem by using a reference
       ellipsoid for guidance.  We compute a set of limb points on the
       reference ellipsoid for the target body, then use those points to
       define the latitudes and longitudes of limb points on the surface
       defined by the specified triangular shape model.  As such, the set
       of limb points found by this routine is just an approximation.

#### 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)  Compute sets of limb points on Phobos as seen from Mars. Perform
           a consistency check using the emission angle at each point,
           where the emission angle is computed using both a reference
           ellipsoid and the actual plate model surface and surface normal.
           We expect to see an emission angle of approximately 90 degrees.

           This program loads SPICE kernels via a meta-kernel.  The
           contents of the meta-kernel used to produce the results shown
           below are:

              \begindata

                 KERNELS_TO_LOAD = ( 'naif0009.tls'
                                     'pck00009.tpc'
                                     'mar085.bsp'
                                     'de421.bsp'   )
              \begintext

           The DSK is loaded separately (this is a temporary inconvenience
           that will be remedied when the KEEPER system is updated to handle
           DSKs).  The DSK used for this computation is

              phobos.3.3.bds

           Source code follows.

              #include <stdio.h>
              #include <math.h>
              #include "SpiceUsr.h"

              int main()
              {
                 /.
                 Prototypes
                 ./
                 void limb_pl02 ( SpiceInt              handle,
                                  ConstSpiceDLADescr  * dladsc,
                                  ConstSpiceChar      * target,
                                  SpiceDouble           et,
                                  ConstSpiceChar      * fixref,
                                  ConstSpiceChar      * abcorr,
                                  ConstSpiceChar      * obsrvr,
                                  SpiceInt              npoints,
                                  SpiceDouble         * trgepc,
                                  SpiceDouble           obspos   [3],
                                  SpiceDouble           limbpts   [][3],
                                  SpiceInt              plateIDs []     );

                 /.
                 Local parameters
                 ./
                 #define  FILSIZ         256
                 #define  NPOINTS        3
                 #define  NTYPES         2
                 #define  TOL            ( 1.e-12 )
                 #define  CORLEN         15
                 #define  TYPLEN         81
                 #define  TIMLEN         41

                 /.
                 Local variables
                 ./
                 SpiceBoolean            found;

                 SpiceChar             * abcorr = "LT+S";
                 SpiceChar               dsk     [ FILSIZ  ];
                 SpiceChar             * fixref = "IAU_PHOBOS";
                 SpiceChar               meta    [ FILSIZ  ];

                 SpiceChar             * obsrvr = "Mars";
                 SpiceChar             * target = "Phobos";
                 SpiceChar             * utcstr = "2007 FEB 9 00:00:00 UTC";
                 SpiceChar               timstr  [ TIMLEN];

                 SpiceDLADescr           dladsc;

                 SpiceDouble             emissn;
                 SpiceDouble             obspos    [3];
                 SpiceDouble             phase;
                 SpiceDouble             radius;
                 SpiceDouble             solar;
                 SpiceDouble             trgepc;
                 SpiceDouble             et;
                 SpiceDouble             lat;
                 SpiceDouble             lon;
                 SpiceDouble             limbpts    [NPOINTS][3];

                 SpiceInt                handle;
                 SpiceInt                i;
                 SpiceInt                plateIDs  [NPOINTS];

                 /.
                 Prompt for the name of a meta-kernel specifying
                 all of the other kernels we need.  Load the
                 metakernel.
                 ./
                 prompt_c ( "Enter meta-kernel name > ", FILSIZ, meta );
                 furnsh_c ( meta );

                 /.
                 Prompt for the name of the DSK to read.
                 ./
                 prompt_c ( "Enter DSK name         > ", FILSIZ, dsk );

                 /.
                 Open the DSK file for read access.
                 We use the DAS-level interface for
                 this function.
                 ./
                 dasopr_c ( dsk, &handle );

                 /.
                 Begin a forward search through the
                 kernel, treating the file as a DLA.
                 In this example, it's a very short
                 search.
                 ./
                 dlabfs_c ( handle, &dladsc, &found );

                 if ( !found  )
                 {
                    /.
                    We arrive here only if the kernel
                    contains no segments.  This is
                    unexpected, but we're prepared for it.
                    ./
                    setmsg_c ( "No segments found in DSK file #.");
                    errch_c  ( "#",  dsk                         );
                    sigerr_c ( "SPICE(NODATA)"                   );
                 }

                 /.
                 If we made it this far, `dladsc' is the
                 DLA descriptor of the first segment.

                 Convert the observation time to seconds past J2000 TDB.
                 ./
                 str2et_c ( utcstr, &et );

                 timout_c ( et,
                            "YYYY-MON-DD "
                            "HR:MN:SC.### ::TDB(TDB)",
                            TIMLEN,
                            timstr                    );

                 printf ( "\n\n"
                          "   Observer:               %s\n"
                          "   Target:                 %s\n"
                          "   Observation epoch:      %s\n"
                          "   Aberration correction:  %s\n"
                          "   Body-fixed frame:       %s\n",
                          obsrvr,
                          target,
                          timstr,
                          abcorr,
                          fixref                            );

                 /.
                 Now compute grid of limb points.
                 ./
                 limb_pl02 ( handle,   &dladsc,
                             target,   et,       fixref,   abcorr,
                             obsrvr,   NPOINTS,  &trgepc,  obspos,
                             limbpts,   plateIDs                    );

                 /.
                 Display the limb points.
                 ./
                 for ( i = 0;  i < NPOINTS;  i++  )
                 {
                    printf ( "\n" );

                    reclat_c ( limbpts[i], &radius, &lon, &lat );

                    printf (
                    "      Limb point %d:\n"
                    "         Radius                     (km):  %f\n"
                    "         Planetocentric longitude   (deg): %f\n"
                    "         Planetocentric latitude    (deg): %f\n"
                    "         Plate ID:                         %d\n",
                    (int)i,
                    radius,
                    lon * dpr_c(),
                    lat * dpr_c(),
                    (int)plateIDs[i]                                );

                    /.
                    Compute the illumination angles using an ellipsoidal
                    representation of the target's surface. The role of
                    this representation is to provide an outward surface
                    normal.
                    ./
                    illum_c ( target,  et,         abcorr,
                              obsrvr,  limbpts[i],  &phase,
                              &solar,  &emissn             );

                    printf (
                    "            "
                    "emission angle derived using\n"
                    "            "
                    "   - an ellipsoidal reference surface        (deg):"
                    " %f\n",
                    emissn  * dpr_c()                                      );

                    /.
                    Compute the illumination angles at the limb point
                    using the actual plate model surface normal.
                    ./
                    illum_pl02 ( handle, &dladsc, target,   et,
                                 abcorr, obsrvr,  limbpts[i],
                                 &phase, &solar,  &emissn      );

                    printf (
                    "            "
                    "   - plate model's surface and normal vector (deg):"
                    " %f\n",
                    emissn  * dpr_c()                                      );
                 }

                 printf ( "\n" );

                 /.
                 Close the kernel.  This isn't necessary in a stand-
                 alone program, but it's good practice in subroutines
                 because it frees program and system resources.
                 ./
                 dascls_c ( handle );

                 return ( 0 );
              }

       When this program was executed on a PC/Linux/gcc platform,
       the output was:

         Enter meta-kernel name > limb_meta
         Enter DSK name         > phobos.3.3.bds

            Observer:               Mars
             Target:                 Phobos
             Observation epoch:      2007-FEB-09 00:01:05.184 (TDB)
             Aberration correction:  LT+S
             Body-fixed frame:       IAU_PHOBOS

                Limb point 0:
                   Radius                     (km):  11.548564
                   Planetocentric longitude   (deg): 91.916308
                   Planetocentric latitude    (deg): -0.001421
                   Plate ID:                         229468
                      emission angle derived using
                         - an ellipsoidal reference surface        (deg): 90.002150
                         - plate model's surface and normal vector (deg): 110.821147

                Limb point 1:
                   Radius                     (km):  9.539431
                   Planetocentric longitude   (deg): -87.644392
                   Planetocentric latitude    (deg): 59.998173
                   Plate ID:                         235885
                      emission angle derived using
                         - an ellipsoidal reference surface        (deg): 89.999635
                         - plate model's surface and normal vector (deg): 97.682098

                Limb point 2:
                   Radius                     (km):  9.040443
                   Planetocentric longitude   (deg): -87.860468
                   Planetocentric latitude    (deg): -59.996395
                   Plate ID:                         17964
                      emission angle derived using
                         - an ellipsoidal reference surface        (deg): 89.996577
                         - plate model's surface and normal vector (deg): 69.169892

#### Restrictions

       1)  The quality of the results produced by this routine depend on how
           well the target body's surface is approximated by the target's
           reference ellipsoid.  This routine will *not* produce meaningful
           results for body shapes such as "dumbbells."

#### Literature_References

       None.

#### Author_and_Institution

       N.J. Bachman    (JPL)

#### Version

       -CSPICE Version 2.0.0, 23-JUL-2016 (NJB)

          Added and moved failed_c calls.

          Include file references have been updated. Integer output
          format in the example program has been updated. Updated
          example program output to reflect bug fix of 29-APR-2014.

       -Beta Version 3.0.0, 24-JUN-2014 (NJB) (EDW)

           Added failed_c checks to ensure return before
           performance of arithmetic computations using
           invalid operands. Added check for target equal
           to observer.

        Last update was 29-APR-2014 (NJB)

           Bug fix: corrected limb point selection so that
           points have uniform spacing in longitude, measured
           in a frame centered on the observer-target center
           line.

           Bug fix: added sigerr_c call in error handling
           branch for invalid value of `npoints'.

           Bug fix: added check for body-fixed frame not
           centered on target.

           Bug fix: added error handling for null input string
           pointers and empty strings.

           Added check for mismatch between target and central
           body of DSK segment.

           Added test of failed_c after spkpos_c call.

           Changed argument name `fixfrm' to `fixref'.

           The target name to ID conversion is now performed
           in-line. The short error message for a failed
           conversion is not SPICE(IDCODENOTFOUND).

           Header correction: the error case of an unmapped
           observer name is now listed.

       -Beta Version 2.1.0, 14-MAY-2010 (NJB)

           Updated header example. Added include statement
           for header pl02.h.

       -Beta Version 2.0.0, 12-FEB-2010 (NJB)

           Updated to include

              SpiceDSK.h

       -Beta Version 1.0.0, 27-FEB-2007 (NJB)

#### Index_Entries

       find limb on plate model
       find limb on triangular shape model

#### Link to routine limb_pl02 source file limb_pl02.c

    Wed Apr  5 17:54:38 2017