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Abstract
I/O
Examples
Particulars
Required Reading
Version
Index_Entries

Abstract


   CSPICE_LIMBPT finds limb points on a target body. The limb is the set
   of points of tangency on the target of rays emanating from the observer.
   The caller specifies half-planes bounded by the observer-target
   center vector in which to search for limb points.

I/O


   Given:

      method   is a short string providing parameters defining
               the computation method to be used. In the syntax
               descriptions below, items delimited angle brackets
               "<>" are to be replaced by actual values. Items
               delimited by brackets "[]" are optional.

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

                  or

               [1,1] = size(method); cell = class(method)

               `method' may be assigned the following values:

                 'TANGENT/DSK/UNPRIORITIZED[/SURFACES = <surface list>]'

                     The limb point computation uses topographic data
                     provided by DSK files (abbreviated as "DSK data"
                     below) to model the surface of the target body. A
                     limb point is defined as the point of tangency, on
                     the surface represented by the DSK data, of a ray
                     emanating from the observer.

                     Limb points are generated within a specified set
                     of "cutting" half-planes that have as an edge the
                     line containing the observer-target vector.
                     Multiple limb points may be found within a given
                     half-plane, if the target body shape allows for
                     this.

                     The surface list specification is optional. The
                     syntax of the list is

                        <surface 1> [, <surface 2>...]

                     If present, it indicates that data only for the
                     listed surfaces are to be used; however, data need
                     not be available for all surfaces in the list. If
                     the list is absent, loaded DSK data for any
                     surface associated with the target body are used.

                     The surface list may contain surface names or
                     surface ID codes. Names containing blanks must
                     be delimited by double quotes, for example

                        SURFACES = "Mars MEGDR 128 PIXEL/DEG"

                     If multiple surfaces are specified, their names
                     or IDs must be separated by commas.

                     See the Particulars section below for details
                     concerning use of DSK data.

                     This is the highest-accuracy method supported by
                     this subroutine. It generally executes much more
                     slowly than the "GUIDED" method described below.


                 'GUIDED/DSK/UNPRIORITIZED[/SURFACES = <surface list>]'

                     This method uses DSK data as described above, but
                     limb points generated by this method are "guided"
                     so as to lie in the limb plane of the target
                     body's reference ellipsoid, on the target body's
                     surface. This method produces a unique limb point
                     for each cutting half-plane. If multiple limb
                     point candidates lie in a given cutting
                     half-plane, the outermost one is chosen.

                     This method may be used only with the "CENTER"
                     aberration correction locus (see the description
                     of `refloc' below).

                     Limb points generated by this method are
                     approximations; they are generally not true
                     ray-surface tangent points. However, these
                     approximations can be generated much more quickly
                     than tangent points.

                 'TANGENT/ELLIPSOID'
                 'GUIDED/ELLIPSOID'

                     Both of these methods generate limb points on the
                     target body's reference ellipsoid. The 'TANGENT'
                     option may be used with any aberration correction
                     locus, while the 'GUIDED' option may be used only
                     with the 'CENTER' locus (see the description of
                     `refloc' below).

                     When the locus is set to 'CENTER', these methods
                     produce the same results.


                  Neither case nor white space are significant in
                  `method', except within double-quoted strings. For
                  example, the string ' eLLipsoid/tAnGenT ' is valid.

                  Within double-quoted strings, blank characters are
                  significant, but multiple consecutive blanks are
                  considered equivalent to a single blank. Case is
                  not significant. So

                     'Mars MEGDR 128 PIXEL/DEG'

                  is equivalent to

                     ' mars megdr  128  pixel/deg '

                  but not to

                     'MARS MEGDR128PIXEL/DEG'


      target      is the name of the target body. The target body is
                  an extended ephemeris object.

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

                     or

                  [1,1] = size(target); cell = class(target)

                  The string `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.

                  When the target body's surface is represented by a
                  tri-axial ellipsoid, this routine assumes that a
                  kernel variable representing the ellipsoid'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 TDB seconds past J2000 TDB: `et' is
                  the epoch at which the observer's state 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, the position
                  and orientation of the target body are computed at
                  et-lt, where `lt' is the one-way light time between the
                  aberration correction locus and the observer. The
                  locus is specified by the input argument `corloc'.
                  See the descriptions of `abcorr' and `corloc' below for
                  details.


      fixref      is the name of a body-fixed reference frame centered
                  on the target body. `fixref' may be any such frame
                  supported by the SPICE system, including built-in
                  frames (documented in the Frames Required Reading)
                  and frames defined by a loaded frame kernel (FK). The
                  string `fixref' is case-insensitive, and leading and
                  trailing blanks in `fixref' are not significant.

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

                     or

                  [1,1] = size(fixref); cell = class(fixref)

                  The output limb points in the array `points' and the
                  output observer-target tangent vectors in the array
                  `tangts' are expressed relative to this reference frame.


      abcorr      indicates the aberration corrections to be applied
                  when computing the target's position and orientation.
                  Corrections are applied at the location specified by
                  the aberration correction locus argument `corloc',
                  which is described below.

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

                     or

                  [1,1] = size(abcorr); cell = class(abcorr)
                  For remote sensing applications, where apparent limb
                  points seen by the observer are desired, normally
                  either of the corrections

                     'LT+S'
                     'CN+S'

                  should be used. The correction 'NONE' may be suitable
                  for cases in which the target is very small and the
                  observer is close to, and has small velocity relative
                  to, the target (e.g. comet Churyumov-Gerasimenko and
                  the Rosetta Orbiter).

                  These and the other supported options are described
                  below. `abcorr' may be any of the following:

                     'NONE'     Apply no correction. Return the
                                geometric limb points on the target
                                body.

                  Let `lt' represent the one-way light time between the
                  observer and the aberration correction locus. The
                  following values of `abcorr' apply to the "reception"
                  case in which photons depart from the locus 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 locus at the moment it
                                emitted photons arriving at the
                                observer at `et'.

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

                                Both the target position as seen by the
                                observer, and 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
                                locus obtained with the 'LT' option to
                                account for the observer's velocity
                                relative to the solar system
                                barycenter. These corrections yield
                                points on the apparent limb.

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

                     'CN+S'     Converged Newtonian light time and
                                stellar aberration corrections. This
                                option produces a solution that is at
                                least as accurate at that obtainable
                                with the 'LT+S' option. Whether the
                                'CN+S' solution is substantially more
                                accurate depends on the geometry of the
                                participating objects and on the
                                accuracy of the input data. In all
                                cases this routine will execute more
                                slowly when a converged solution is
                                computed.


      corloc      is a string specifying the aberration correction
                  locus: the point or set of points for which
                  aberration corrections are performed.

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

                     or

                  [1,1] = size(corloc); cell = class(corloc)

                  `corloc' may be assigned the values:

                     'CENTER'

                         Light time and stellar aberration corrections
                         are applied to the vector from the observer to
                         the center of the target body. The one way
                         light time from the target center to the
                         observer is used to determine the epoch at
                         which the target body orientation is computed.

                         This choice is appropriate for small target
                         objects for which the light time from the
                         surface to the observer varies little across
                         the entire target. It may also be appropriate
                         for large, nearly ellipsoidal targets when the
                         observer is very far from the target.

                         Computation speed for this option is faster
                         than for the 'ELLIPSOID LIMB' option.

                     'ELLIPSOID LIMB'

                         Light time and stellar aberration corrections
                         are applied to individual limb points on the
                         reference ellipsoid. For a limb point on the
                         surface described by topographic data, lying
                         in a specified cutting half-plane, the unique
                         reference ellipsoid limb point in the same
                         half-plane is used as the locus of the
                         aberration corrections.

                         This choice is appropriate for large target
                         objects for which the light time from the limb
                         to the observer is significantly different
                         from the light time from the target center to
                         the observer.

                         Because aberration corrections are repeated for
                         individual limb points, computational speed for
                         this option is relatively slow.


      obsrvr      is the name of the observing body. The observing body
                  is an ephemeris object: it typically is 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 'MOON' and '301' are
                  legitimate strings that indicate the Moon is the
                  observer.

                  [1,c7] = size(obsrvr); char = class(obsrvr)

                     or

                  [1,1] = size(obsrvr); cell = class(obsrvr)

      refvec,
      rolstp,
      ncuts       are, respectively, a reference vector, a roll step
                  angle, and a count of cutting half-planes.

                  [3,1] = size(refvec); double = class(refvec)
                  [1,1] = size(rolstp); double = class(rolstp)
                  [1,1] = size(ncuts); int32 = class(ncuts)

                  `refvec' defines the first of a sequence of cutting
                  half-planes in which limb points are to be found.
                  Each cutting half-plane has as its edge the line
                  containing the observer-target vector; the first
                  half-plane contains `refvec'.

                  `refvec' is expressed in the body-fixed reference frame
                  designated by `fixref'.

                  `rolstp' is an angular step by which to roll the
                  cutting half-planes about the observer-target vector.
                  The first half-plane is aligned with `refvec'; the ith
                  half-plane is rotated from `refvec' about the
                  observer-target vector in the counter-clockwise
                  direction by (i-1)*rolstp. Units are radians.
                  `rolstp' should be set to

                     2*pi/ncuts

                  to generate an approximately uniform distribution of
                  limb points along the limb.

                  `ncuts' is the number of cutting half-planes used to
                  find limb points; the angular positions of
                  consecutive half-planes increase in the positive
                  sense (counterclockwise) about the target-observer
                  vector and are distributed roughly equally about that
                  vector: each half-plane has angular separation of
                  approximately

                     `rolstp' radians

                  from each of its neighbors. When the aberration
                  correction locus is set to 'CENTER', the angular
                  separation is the value above, up to round-off. When
                  the locus is 'ELLIPSOID LIMB', the separations are
                  less uniform due to differences in the aberration
                  corrections used for the respective limb points.


      schstp,
      soltol      are used only for DSK-based surfaces. These inputs
                  are, respectively, the search angular step size and
                  solution convergence tolerance used to find tangent
                  rays and associated limb points within each cutting
                  half plane. These values are used when the `method'
                  argument includes the 'TANGENT' option.

                  [1,1] = size(schstp); double = class(schstp)
                  [1,1] = size(soltol); double = class(soltol)

                  In this case, limb points are found by a two-step
                  search process:

                     1) Bracketing: starting with the direction
                        opposite the observer-target vector, rays
                        emanating from the observer are generated
                        within the half-plane at successively greater
                        angular separations from the initial direction,
                        where the increment of angular separation is
                        `schstp'. The rays are tested for intersection
                        with the target surface. When a transition
                        between non-intersection to intersection is
                        found, the angular separation of a tangent ray
                        has been bracketed.

                     2) Root finding: each time a tangent ray is
                        bracketed, a search is done to find the angular
                        separation from the starting direction at which
                        a tangent ray exists. The search terminates
                        when successive rays are separated by no more
                        than `soltol'. When the search converges, the
                        last ray-surface intersection point found in
                        the convergence process is considered to be a
                        limb point.


                   `schstp' and `soltol' have units of radians.

                   Target bodies with simple surfaces---for example,
                   convex shapes---will have a single limb point within
                   each cutting half-plane. For such surfaces, `schstp'
                   can be set large enough so that only one bracketing
                   step is taken. A value greater than pi, for example
                   4.0, is recommended.

                   Target bodies with complex surfaces can have
                   multiple limb points within a given cutting
                   half-plane. To find all limb points, `schstp' must be
                   set to a value smaller than the angular separation
                   of any two limb points in any cutting half-plane,
                   where the vertex of the angle is the observer.
                   `schstp' must not be too small, or the search will be
                   excessively slow.

                   For both kinds of surfaces, `soltol' must be chosen so
                   that the results will have the desired precision.
                   Note that the choice of `soltol' required to meet a
                   specified bound on limb point height errors depends
                   on the observer-target distance.


      maxn         is the maximum number of limb points that can be
                   stored in the output array `points'.

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

   the call:

      [npts, points, epochs, tangts] = cspice_limbpt( method,        ...
                                       target, et,   fixref, abcorr, ...
                                       corloc, obsrvr, refvec,       ...
                                       rolstp, ncuts,  schstp,       ...
                                       soltol, maxn )

   returns:

      npts         is an array of counts of limb points within the
                   specified set of cutting half-planes. The Ith
                   element of `npts' is the limb point count in the Ith
                   half-plane.

                   [1,ncuts] = size(npts); int32 = class(npts)

                   For most target bodies, there will be one limb point
                   per half-plane. For complex target shapes, the limb
                   point count in a given half-plane can be greater
                   than one (see example 3 below), and it can be zero.


      points       is an array containing the limb points found by this
                   routine. Sets of limb points associated with
                   half-planes are ordered by the indices of the
                   half-planes in which they're found. The limb points
                   in a given half-plane are ordered by decreasing
                   angular separation from the observer-target
                   direction; the outermost limb point in a given
                   half-plane is the first of that set.

                  [3,maxn] = size(points); double = class(points)

                   The limb points for the half-plane containing `refvec'
                   occupy array elements

                         points(1,1)                       through
                         points(3,npts(1))

                   Limb points for the second half plane occupy
                   elements

                         points(1,npts(1)+1)               through
                         points(3,npts(1)+npts(2))

                   and so on.

                   Limb points are expressed in the reference frame
                   designated by `fixref'. For each limb point, the
                   orientation of the frame is evaluated at the epoch
                   corresponding to the limb point; the epoch is
                   provided in the output array `epochs' (described
                   below).

                   Units of the limb points are km.


      epochs       is an array of epochs associated with the limb
                   points, accounting for light time if aberration
                   corrections are used. `epochs' contains one element
                   for each limb point.

                  [1,maxn] = size(epochs); double = class(epochs)

                   The element

                         epochs(i)

                   is associated with the limb point

                         points(j,i), j = 1 to 3

                   If `corloc' is set to 'CENTER', all values of `epochs'
                   will be the epoch associated with the target body
                   center. That is, if aberration corrections are used,
                   and if `lt' is the one-way light time from the target
                   center to the observer, the elements of `epochs' will
                   all be set to

                      et - lt

                   If `corloc' is set to 'ELLIPSOID LIMB', all values of
                   `epochs' for the limb points in a given half plane
                   will be those for the reference ellipsoid limb point
                   in that half plane. That is, if aberration
                   corrections are used, and if lt[i] is the one-way
                   light time to the observer from the reference
                   ellipsoid limb point in the ith half plane, the
                   elements of `epochs' for that half plane will all be
                   set to

                         et - lt(i)

      tangts       is an array of tangent vectors connecting the
                   observer to the limb points. The tangent vectors are
                   expressed in the frame designated by `fixref'. For the
                   Ith vector, the orientation of the frame is
                   evaluated at the Ith epoch provided in the output
                   array `epochs' (described above).

                   [3,maxn] = size(tangts); double = class(tangts)

                   The elements

                         tangts(j,i), j = 1 to 3

                   are associated with the limb point

                         points(j,i), j = 1 to 3

                   Units of the tangent vectors are km.

Examples


   Any numerical results shown for this example may differ between
   platforms as the results depend on the SPICE kernels used as input
   and the machine specific arithmetic implementation.

   Find apparent limb points on Phobos as seen from Mars.

   Due to Phobos' irregular shape, the TANGENT limb point
   definition will used. It suffices to compute light time and
   stellar aberration corrections for the center of Phobos, so
   the "CENTER" aberration correction locus will be used. Use
   converged Newtonian light time and stellar aberration
   corrections in order to model the apparent position and
   orientation of Phobos.

   For comparison, compute limb points using both ellipsoid
   and topographic shape models.

   Use the target body-fixed +Z axis as the reference direction
   for generating cutting half-planes. This choice enables the
   user to see whether the first limb point is near the target's
   north pole.

   For each option, use just three cutting half-planes, in order
   to keep the volume of output manageable. In most applications,
   the number of cuts and the number of resulting limb points
   would be much greater.

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

         KPL/MK

         File: limbpt_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
            ---------                        --------
            de430.bsp                        Planetary ephemeris
            mar097.bsp                       Mars satellite ephemeris
            pck00010.tpc                     Planet orientation and
                                             radii
            naif0011.tls                     Leapseconds
            phobos512.bds                    DSK based on
                                             Gaskell ICQ Q=512
                                             Phobos plate model
         \begindata

            PATH_SYMBOLS    = "GEN"
            PATH_VALUES     = '/ftp/pub/naif/generic_kernels'

            KERNELS_TO_LOAD = ( 'de430.bsp',
                                'mar097.bsp',
                                'pck00010.tpc',
                                'naif0011.tls',
                                '$GEN/dsk/phobos/phobos512.bds' )
         \begintext


     function limbpt_t1( meta )

         %
         % Local constants
         %
         NMETH  =          2;
         MAXN   =      10000;

         %
         % Local variables
         %

         method = { 'TANGENT/ELLIPSOID', ...
                    'TANGENT/DSK/UNPRIORITIZED' };

         z = [ 0.0, 0.0, 1.0 ]';

         %
         % Load kernel files via the meta-kernel.
         %
         cspice_furnsh( meta )

         %
         % Set target, observer, and target body-fixed,
         % body-centered reference frame.
         %
         obsrvr = 'MARS';
         target = 'PHOBOS';
         fixref = 'IAU_PHOBOS';

         %
         % Set aberration correction and correction locus.
         %
         abcorr = 'CN+S';
         corloc = 'CENTER';

         %
         % Convert the UTC request time string seconds past
         % J2000, TDB.
         %
         et = cspice_str2et( '2008 AUG 11 00:00:00');

         %
         % Compute a set of limb points using light time and
         % stellar aberration corrections. Use both ellipsoid
         % and DSK shape models. Use a step size of 100
         % microradians to ensure we don't miss the limb.
         % Set the convergence tolerance to 100 nanoradians,
         % which will limit the height error to about 1 meter.
         % Compute 3 limb points for each computation method.
         %
         schstp = 1.0d-4;
         soltol = 1.0d-7;
         ncuts  = 3;

         fprintf( ['\n'                     ...
                  'Observer:       %s\n'    ...
                  'Target:         %s\n'    ...
                  'Frame:          %s\n'    ...
                  '\n'                      ...
                  'Number of cuts: %d\n'],  ...
                  obsrvr,                   ...
                  target,                   ...
                  fixref,                   ...
                  ncuts            )

         delrol = cspice_twopi() / ncuts;

         for i = 1:NMETH

            [ npts, points, trgeps, tangts] = cspice_limbpt( method(i), ...
                       target, et,     fixref,            ...
                       abcorr,    corloc, obsrvr, z,      ...
                       delrol,    ncuts,  schstp, soltol, ...
                       MAXN );

            %
            % Write the results.
            %
            fprintf ( ['\n\n'                      ...
                     'Computation method = %s\n'   ...
                     'Locus              = %s\n'], ...
                     char(method(i)),...
                     corloc                     );

            start = 0;

            for j = 1:ncuts

               roll = (j-1) * delrol;

               fprintf ( ['\n'                           ...
                        '  Roll angle (deg) = %21.9f\n'  ...
                        '     Target epoch  = %21.9f\n'  ...
                        '     Number of limb points at this ' ...
                        'roll angle: %d\n'],                  ...
                        roll * cspice_dpr(),                  ...
                        trgeps(j),                            ...
                        npts(j)                            );

               fprintf ( '      Limb points\n' );

               for k = 1:npts(j)

                  fprintf( ' %20.9f %20.9f %20.9f\n', ...
                           points(1, k+start),        ...
                           points(2, k+start),        ...
                           points(3, k+start)        )
               end

               start = start + npts(j);
            end

         end

         fprintf ( '\n' )

   Matlab outputs:

      >> limbpt_t1( 'limbpt_t1.tm' )
      
      Observer:       MARS
      Target:         PHOBOS
      Frame:          IAU_PHOBOS
      
      Number of cuts: 3
      
      
      Computation method = TANGENT/ELLIPSOID
      Locus              = CENTER
      
        Roll angle (deg) =           0.000000000
           Target epoch  =   271684865.152078211
           Number of limb points at this roll angle: 1
            Limb points
                0.016445326         -0.000306114          9.099992715
      
        Roll angle (deg) =         120.000000000
           Target epoch  =   271684865.152078211
           Number of limb points at this roll angle: 1
            Limb points
               -0.204288375         -9.235230829         -5.333237706
      
        Roll angle (deg) =         240.000000000
           Target epoch  =   271684865.152078211
           Number of limb points at this roll angle: 1
            Limb points
                0.242785221          9.234520095         -5.333231253
      
      
      Computation method = TANGENT/DSK/UNPRIORITIZED
      Locus              = CENTER
      
        Roll angle (deg) =           0.000000000
           Target epoch  =   271684865.152078211
           Number of limb points at this roll angle: 1
            Limb points
               -0.398901673          0.007425178          9.973720555
      
        Roll angle (deg) =         120.000000000
           Target epoch  =   271684865.152078211
           Number of limb points at this roll angle: 1
            Limb points
               -0.959300281         -8.537573427         -4.938700447
      
        Roll angle (deg) =         240.000000000
           Target epoch  =   271684865.152078211
           Number of limb points at this roll angle: 1
            Limb points
               -1.380536729          9.714334047         -5.592916790

Particulars


   Using DSK data
   ==============

      DSK loading and unloading
      -------------------------

      DSK files providing data used by this routine are loaded by
      calling cspice_furnsh and can be unloaded by calling cspice_unload or
      cspice_kclear. See the documentation of cspice furnsh for limits on
      numbers of loaded DSK files.

      For run-time efficiency, it's desirable to avoid frequent
      loading and unloading of DSK files. When there is a reason to
      use multiple versions of data for a given target body---for
      example, if topographic data at varying resolutions are to be
      used---the surface list can be used to select DSK data to be
      used for a given computation. It is not necessary to unload
      the data that are not to be used. This recommendation presumes
      that DSKs containing different versions of surface data for a
      given body have different surface ID codes.


      DSK data priority
      -----------------

      A DSK coverage overlap occurs when two segments in loaded DSK
      files cover part or all of the same domain---for example, a
      given longitude-latitude rectangle---and when the time
      intervals of the segments overlap as well.

      When DSK data selection is prioritized, in case of a coverage
      overlap, if the two competing segments are in different DSK
      files, the segment in the DSK file loaded last takes
      precedence. If the two segments are in the same file, the
      segment located closer to the end of the file takes
      precedence.

      When DSK data selection is unprioritized, data from competing
      segments are combined. For example, if two competing segments
      both represent a surface as sets of triangular plates, the
      union of those sets of plates is considered to represent the
      surface.

      Currently only unprioritized data selection is supported.
      Because prioritized data selection may be the default behavior
      in a later version of the routine, the UNPRIORITIZED keyword is
      required in the `method' argument.


      Syntax of the `method' input argument
      -------------------------------------

      The keywords and surface list in the `method' argument
      are called "clauses." The clauses may appear in any
      order, for example

         UMBRAL/TANGENT/DSK/UNPRIORITIZED/<surface list>
         DSK/UMBRAL/TANGENT/<surface list>/UNPRIORITIZED
         UNPRIORITIZED/<surface list>/DSK/TANGENT/UMBRAL

      The simplest form of the `method' argument specifying use of
      DSK data is one that lacks a surface list, for example:

         'PENUMBRAL/TANGENT/DSK/UNPRIORITIZED'
         'UMBRAL/GUIDED/DSK/UNPRIORITIZED'

      For applications in which all loaded DSK data for the target
      body are for a single surface, and there are no competing
      segments, the above strings suffice. This is expected to be
      the usual case.

      When, for the specified target body, there are loaded DSK
      files providing data for multiple surfaces for that body, the
      surfaces to be used by this routine for a given call must be
      specified in a surface list, unless data from all of the
      surfaces are to be used together.

      The surface list consists of the string

         SURFACES =

      followed by a comma-separated list of one or more surface
      identifiers. The identifiers may be names or integer codes in
      string format. For example, suppose we have the surface
      names and corresponding ID codes shown below:

         Surface Name                              ID code
         ------------                              -------
         "Mars MEGDR 128 PIXEL/DEG"                1
         "Mars MEGDR 64 PIXEL/DEG"                 2
         "Mars_MRO_HIRISE"                         3

      If data for all of the above surfaces are loaded, then
      data for surface 1 can be specified by either

         'SURFACES = 1'

      or

         'SURFACES = "Mars MEGDR 128 PIXEL/DEG"'

      Double quotes are used to delimit the surface name because
      it contains blank characters.

      To use data for surfaces 2 and 3 together, any
      of the following surface lists could be used:

         'SURFACES = 2, 3'

         'SURFACES = "Mars MEGDR  64 PIXEL/DEG", 3'

         'SURFACES = 2, Mars_MRO_HIRISE'

         'SURFACES = "Mars MEGDR 64 PIXEL/DEG", Mars_MRO_HIRISE'

      An example of a `method' argument that could be constructed
      using one of the surface lists above is

      'NADIR/DSK/UNPRIORITIZED/SURFACES= "Mars MEGDR 64 PIXEL/DEG",3'

Required Reading


   For important details concerning this module's function, please refer to
   the CSPICE routine limbpt_c.

   MICE.REQ
   ABCORR.REQ
   CK.REQ
   DSK.REQ
   FRAMES.REQ
   NAIF_IDS.REQ
   PCK.REQ
   SPK.REQ
   TIME.REQ

Version


   -Mice Version 1.0.0, 15-DEC-2016, EDW (JPL), NJB (JPL), ML (JPL)

Index_Entries


   find limb points on target body


Wed Apr  5 18:00:33 2017