Index Page illumg_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 illumg_c ( ConstSpiceChar        * method,
                       ConstSpiceChar        * target,
                       ConstSpiceChar        * ilusrc,
                       SpiceDouble             et,
                       ConstSpiceChar        * fixref,
                       ConstSpiceChar        * abcorr,
                       ConstSpiceChar        * obsrvr,
                       ConstSpiceDouble        spoint [3],
                       SpiceDouble           * trgepc,
                       SpiceDouble             srfvec [3],
                       SpiceDouble           * phase,
                       SpiceDouble           * incdnc,
                       SpiceDouble           * emissn     )

#### Abstract

       Find the illumination angles (phase, incidence, and
       emission) at a specified surface point of a target body.

       The surface of the target body may be represented by a triaxial
       ellipsoid or by topographic data provided by DSK files.

       The illumination source is a specified ephemeris object.

#### Required_Reading

       DSK
       FRAMES
       NAIF_IDS
       PCK
       SPK
       TIME

#### Keywords

       ANGLES
       GEOMETRY
       ILLUMINATION

#### Brief_I/O

       Variable  I/O  Description
       --------  ---  --------------------------------------------------
       method     I   Computation method.
       target     I   Name of target body.
       ilusrc     I   Name of illumination source.
       et         I   Epoch in TDB seconds past J2000 TDB.
       fixref     I   Body-fixed, body-centered target body frame.
       abcorr     I   Aberration correction flag.
       obsrvr     I   Name of observing body.
       spoint     I   Body-fixed coordinates of a target surface point.
       trgepc     O   Target surface point epoch.
       srfvec     O   Vector from observer to target surface point.
       phase      O   Phase angle at the surface point.
       incdnc     O   Source incidence angle at the surface point.
       emissn     O   Emission angle at the surface point.

#### Detailed_Input

       method      is a short string providing parameters defining
                   the computation method to be used. In the syntax
                   descriptions below, items delimited by brackets
                   are optional.

                   `method' may be assigned the following values:

                      "ELLIPSOID"

                         The illumination angle computation uses a
                         triaxial ellipsoid to model the surface of the
                         target body. The ellipsoid's radii must be
                         available in the kernel pool.

                      "DSK/UNPRIORITIZED[/SURFACES = <surface list>]"

                         The illumination angle computation uses
                         topographic data to model the surface of the
                         target body. These data must be provided by
                         loaded DSK files.

                         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 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 escaped 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.

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

                   Within double-quoted strings representing surface names,
                   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. `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.

       ilusrc      is the name of the illumination source. This source
                   may be any ephemeris object. Case, blanks, and
                   numeric values are treated in the same way as for the
                   input `target'.

       et          is the epoch, expressed as seconds past J2000 TDB,
                   for which the apparent illumination angles at the
                   specified surface point on the target body, as seen
                   from the observing body, are to be computed.

       fixref      is the name of the body-fixed, body-centered
                   reference frame associated with the target body. The
                   input surface point `spoint' and the output vector
                   `srfvec' are expressed relative to this reference
                   frame. The string `fixref' is case-insensitive, and
                   leading and trailing blanks in `fixref' are not
                   significant.

       abcorr      is the aberration correction to be used in computing
                   the position and orientation of the target body and
                   the location of the illumination source.

                   For remote sensing applications, where the apparent
                   illumination angles 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. `abcorr' may be any of the
                   following:

                      "NONE"     No aberration correction.

                   Let `lt' represent the one-way light time between the
                   observer and the input surface point `spoint' (note: NOT
                   between the observer and the target body's center). The
                   following values of `abcorr' apply to the "reception"
                   case in which photons depart from `spoint' at the
                   light-time corrected epoch et-lt and *arrive* at the
                   observer's location at `et':

                      "LT"       Correct both the position of `spoint' as
                                 seen by the observer, and the position
                                 of the illumination source as seen by
                                 the target, for light time. Correct the
                                 orientation of the target for light
                                 time.

                      "LT+S"     Correct both the position of `spoint' as
                                 seen by the observer, and the position
                                 of the illumination source as seen by
                                 the target, for light time and stellar
                                 aberration. Correct the orientation of
                                 the target for light time.

                      "CN"       Converged Newtonian light time
                                 correction. In solving the light time
                                 equations for `spoint' and the
                                 illumination source, the "CN"
                                 correction iterates until the solution
                                 converges.

                      "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.

                   The following values of `abcorr' apply to the
                   "transmission" case in which photons *arrive* at
                   `spoint' at the light-time corrected epoch et+lt and
                   *depart* from the observer's location at `et':

                      "XLT"      "Transmission" case: correct for
                                 one-way light time using a Newtonian
                                 formulation. This correction yields the
                                 illumination angles at the moment that
                                 `spoint' receives photons emitted from the
                                 observer's location at `et'.

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

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

                      "XLT+S"    "Transmission" case: correct for
                                 one-way light time and stellar
                                 aberration using a Newtonian
                                 formulation  This option modifies the
                                 angles obtained with the "XLT" option
                                 to account for the observer's and
                                 target's velocities relative to the
                                 solar system barycenter (the latter
                                 velocity is used in computing the
                                 direction to the apparent illumination
                                 source).

                      "XCN"      Converged Newtonian light time
                                 correction. This is the same as XLT
                                 correction but with further iterations
                                 to a converged Newtonian light time
                                 solution.

                      "XCN+S"    "Transmission" case: converged
                                 Newtonian light time and stellar
                                 aberration corrections. This option
                                 produces a solution that is at least as
                                 accurate at that obtainable with the
                                 "XLT+S" option. Whether the "XCN+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.

                   Neither case nor white space are significant in
                   `abcorr'. For example, the string

                     "Lt + s"

                   is valid.

       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.

                   `obsrvr' may be not be identical to `target'.

       spoint      is a surface point on the target body, expressed in
                   Cartesian coordinates, relative to the body-fixed
                   target frame designated by `fixref'.

                   `spoint' need not be visible from the observer's
                   location at the epoch `et'.

                   The components of `spoint' have units of km.

#### Detailed_Output

       trgepc      is the "target surface point epoch." `trgepc' is defined
                   as follows: letting `lt' be the one-way light time
                   between the observer and the input surface point
                   `spoint', `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 TDB seconds past J2000 TDB.

       srfvec      is the vector from the observer's position at `et' to
                   the aberration-corrected (or optionally, geometric)
                   position of `spoint', where the aberration corrections
                   are specified by `abcorr'. `srfvec' is expressed in the
                   target body-fixed reference frame designated by
                   `fixref', evaluated at `trgepc'.

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

                   One can use the CSPICE function vnorm_c to obtain the
                   distance between the observer and `spoint':

                      dist = vnorm_c ( srfvec );

                   The observer's position `obspos', relative to the
                   target body's center, where the center's position is
                   corrected for aberration effects as indicated by
                   `abcorr', can be computed via the call:

                      vsub_c ( spoint, srfvec, obspos );

                   To transform the vector `srfvec' from a reference frame
                   `fixref' at time `trgepc' to a time-dependent reference
                   frame `ref' at time `et', the routine pxfrm2_c should be
                   called. Let `xform' be the 3x3 matrix representing the
                   rotation from the reference frame `fixref' at time
                   `trgepc' to the reference frame `ref' at time `et'. Then
                   `srfvec' can be transformed to the result `refvec' as
                   follows:

                      pxfrm2_c ( fixref, ref,    trgepc, et, xform );
                      mxv_c    ( xform,  srfvec, refvec            );

       phase       is the phase angle at `spoint', as seen from `obsrvr' at
                   time `et'. This is the angle between the spoint-obsrvr
                   vector and the spoint-illumination source vector. Units
                   are radians. The range of `phase' is [0, pi]. See
                   Particulars below for a detailed discussion of the
                   definition.

       incdnc      is the illumination source incidence angle at `spoint',
                   as seen from `obsrvr' at time `et'. This is the angle
                   between the surface normal vector at `spoint' and the
                   spoint-illumination source vector. Units are radians.
                   The range of `incdnce' is [0, pi]. See Particulars below
                   for a detailed discussion of the definition.

       emissn      is the emission angle at `spoint', as seen from `obsrvr'
                   at time `et'. This is the angle between the surface
                   normal vector at `spoint' and the spoint-observer
                   vector. Units are radians. The range of `emissn' is [0,
                   pi]. See Particulars below for a detailed discussion of
                   the definition.

#### Parameters

       None.

#### Exceptions

       1)  If the specified aberration correction is relativistic or
           calls for stellar aberration but not light time correction,
           the error SPICE(NOTSUPPORTED) is signaled. If the specified
           aberration correction is any other unrecognized value, the
           error will be diagnosed and signaled by a routine in the call
           tree of this routine.

       2)  If any of the target, observer, or illumination source
           input strings cannot be converted to an integer ID code, the
           error SPICE(IDCODENOTFOUND) is signaled.

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

       4)  If the input target body-fixed frame `fixref' is not
           recognized, the error SPICE(NOFRAME) is signaled. A frame
           name may fail to be recognized because a required frame
           specification kernel has not been loaded; another cause is a
           misspelling of the frame name.

       5)  If the input frame `fixref' is not centered at the target body,
           the error SPICE(INVALIDFRAME) is signaled.

       6)  If the input argument `METHOD' is not recognized, the error
           SPICE(INVALIDMETHOD) is signaled.

       7)  If insufficient ephemeris data have been loaded prior to
           calling illumg_c, the error will be diagnosed and signaled by a
           routine in the call tree of this routine. Note that when
           light time correction is used, sufficient ephemeris data must
           be available to propagate the states of observer, target, and
           the illumination source to the solar system barycenter.

       8)  If the computation method specifies an ellipsoidal target
           shape and triaxial radii of the target body have not been
           loaded into the kernel pool prior to calling illumg_c, the
           error will be diagnosed and signaled by a routine in the call
           tree of this routine.

       9)  The target must be an extended body: if any of the radii of
           the target body are non-positive, the error will be
           diagnosed and signaled by routines in the call tree of this
           routine.

       10) If PCK or CK data specifying the target body-fixed frame
           orientation have not been loaded prior to calling illumg_c,
           the error will be diagnosed and signaled by a routine in the
           call tree of this routine.

       11) If ``method'' specifies that the target surface is represented by
           DSK data, and no DSK files are loaded for the specified
           target, the error is signaled by a routine in the call tree
           of this routine.

       12) If `method' specifies that the target surface is represented
           by DSK data, and data representing the portion of the surface
           on which `spoint' is located are not available, an error will
           be signaled by a routine in the call tree of this routine.

       13) If `method' specifies that the target surface is represented
           by DSK data, `spoint' must lie on the target surface, not above
           or below it. A small tolerance is used to allow for round-off
           error in the calculation determining whether `spoint' is on the
           surface. If, in the DSK case, `spoint' is too far from the
           surface, an error will be signaled by a routine in the call
           tree of this routine.

           If the surface is represented by a triaxial ellipsoid, `spoint'
           is not required to be close to the ellipsoid; however, the
           results computed by this routine will be unreliable if `spoint'
           is too far from the ellipsoid.

#### Files

       Appropriate kernels must be loaded by the calling program before
       this routine is called.

       The following data are required:

          - SPK data: ephemeris data for target, observer, and the
            illumination source must be loaded. If aberration corrections
            are used, the states of target, observer, and the illumination
            source relative to the solar system barycenter must be
            calculable from the available ephemeris data. Ephemeris data
            are made available by loading one or more SPK files via
            furnsh_c.

          - Target body orientation data: these may be provided in a text or
            binary PCK file. In some cases, target body orientation may
            be provided by one more more CK files. In either case, data
            are made available by loading the files via furnsh_c.

          - Shape data for the target body:

              PCK data:

                 If the target body shape is modeled as an ellipsoid,
                 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.

                 Triaxial radii are also needed if the target shape is
                 modeled by DSK data, and the DSK NADIR method is
                 selected.

              DSK data:

                 If the target shape is modeled by DSK data, DSK files
                 containing topographic data for the target body must be
                 loaded. If a surface list is specified, data for at
                 least one of the listed surfaces must be loaded. DSK
                 files are loaded via furnsh_c.

       The following data may be required:

          - 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 definition is supplied by loading a
            frame kernel via furnsh_c.

          - Surface name-ID associations: if surface names are specified
            in `method', the association of these names with their
            corresponding surface ID codes must be established by
            assignments of the kernel variables

               NAIF_SURFACE_NAME
               NAIF_SURFACE_CODE
               NAIF_SURFACE_BODY

            Normally these associations are made by loading a text
            kernel containing the necessary assignments. An example
            of such assignments is

               NAIF_SURFACE_NAME += 'Mars MEGDR 128 PIXEL/DEG'
               NAIF_SURFACE_CODE += 1
               NAIF_SURFACE_BODY += 499

          - SCLK data: if the target body's orientation is provided by
            CK files, an associated SCLK kernel must be loaded.

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

#### Particulars

       CSPICE contains four routines that compute illumination angles:

          illumf_c (same as this routine, except that illumination
                    and visibility flags are returned)

          illumg_c (this routine)

          ilumin_c (same as illumg_c, except that the sun is fixed
                    as the illumination source)

          illum_c  (deprecated)

       illumf_c is the most capable of the set.

       Illumination angles
       ===================

       The term "illumination angles" refers to the following set of
       angles:

          phase angle              Angle between the vectors from the
                                   surface point to the observer and
                                   from the surface point to the
                                   illumination source.

          incidence angle          Angle between the surface normal at
                                   the specified surface point and the
                                   vector from the surface point to the
                                   illumination source.

          emission angle           Angle between the surface normal at
                                   the specified surface point and the
                                   vector from the surface point to the
                                   observer.

       The diagram below illustrates the geometric relationships
       defining these angles. The labels for the incidence, emission,
       and phase angles are "inc.", "e.", and "phase".

                                                        *
                                                illumination source

                      surface normal vector
                                ._                 _.
                                |\                 /|  illumination
                                  \    phase      /    source vector
                                   \   .    .    /
                                   .            .
                                     \   ___   /
                                .     \/     \/
                                      _\ inc./
                               .    /   \   /
                               .   |  e. \ /
           *             <--------------- *  surface point on
        viewing            vector            target body
        location           to viewing
        (observer)         location

       Note that if the target-observer vector, the target normal vector
       at the surface point, and the target-illumination source vector
       are coplanar, then phase is the sum of the incidence and emission
       angles. This rarely occurs; usually

          phase angle  <  incidence angle + emission angle

       All of the above angles can be computed using light time
       corrections, light time and stellar aberration corrections, or no
       aberration corrections. In order to describe apparent geometry as
       observed by a remote sensing instrument, both light time and
       stellar aberration corrections should be used.

       The way aberration corrections are applied by this routine
       is described below.

          Light time corrections
          ======================

             Observer-target surface point vector
             ------------------------------------

             Let `et' be the epoch at which an observation or remote
             sensing measurement is made, and let et-lt ("lt" stands
             for "light time") be the epoch at which the photons
             received at `et' were emitted from the surface point `spoint'.
             Note that the light time between the surface point and
             observer will generally differ from the light time between
             the target body's center and the observer.

             Target body's orientation
             -------------------------

             Using the definitions of `et' and `lt' above, the target body's
             orientation at et-lt is used. The surface normal is
             dependent on the target body's orientation, so the body's
             orientation model must be evaluated for the correct epoch.

             Target body -- illumination source vector
             -----------------------------------------

             The surface features on the target body near `spoint' will
             appear in a measurement made at `et' as they were at et-lt.
             In particular, lighting on the target body is dependent on
             the apparent location of the illumination source as seen
             from the target body at et-lt. So, a second light time
             correction is used to compute the position of the
             illumination source relative to the surface point.

          Stellar aberration corrections
          ==============================

          Stellar aberration corrections are applied only if
          light time corrections are applied as well.

             Observer-target surface point body vector
             -----------------------------------------

             When stellar aberration correction is performed, the
             direction vector `srfvec' is adjusted so as to point to the
             apparent position of `spoint': considering `spoint' to be an
             ephemeris object, `srfvec' points from the observer's
             position at `et' to the light time and stellar aberration
             corrected position of `spoint'.

             Target body-illumination source vector
             --------------------------------------

             The target body-illumination source vector is the apparent
             position of the illumination source, corrected for light
             time and stellar aberration, as seen from the target body
             at time et-lt.

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

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

          DSK files providing data used by this routine are loaded by
          calling furnsh_c and can be unloaded by calling unload_c or
          kclear_c. See the documentation of furnsh_c 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 a set 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

             "DSK/<surface list>/UNPRIORITIZED"
             "DSK/UNPRIORITIZED/<surface list>"
             "UNPRIORITIZED/<surface list>/DSK"

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

             "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 string suffices. 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\""

          Escaped 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

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

          Aberration corrections using DSK data
          -------------------------------------

          For irregularly shaped target bodies, the distance between the
          observer and the nearest surface intercept need not be a
          continuous function of time; hence the one-way light time
          between the intercept and the observer may be discontinuous as
          well. In such cases, the computed light time, which is found
          using an iterative algorithm, may converge slowly or not at all.
          In all cases, the light time computation will terminate, but
          the result may be less accurate than expected.

#### 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) Find the phase, solar incidence, and emission angles at the
          sub-solar and sub-spacecraft points on Mars as seen from the Mars
          Global Surveyor spacecraft at a specified UTC time.

          Use both an ellipsoidal Mars shape model and topographic data
          provided by a DSK file.  For both surface points, use the "near
          point" and "nadir" definitions for ellipsoidal and DSK shape
          models, respectively.

          Use converged Newtonian light time and stellar aberration
          corrections.

          The topographic model is based on data from the MGS MOLA DEM
          megr90n000cb, which has a resolution of 4 pixels/degree. A
          triangular plate model was produced by computing a 720 x 1440
          grid of interpolated heights from this DEM, then tessellating
          the height grid. The plate model is stored in a type 2 segment
          in the referenced DSK file.

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

             KPL/MK

             File: illumg_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
                mgs_ext12_ipng_mgs95j.bsp        MGS ephemeris
                megr90n000cb_plate.bds           Plate model based on
                                                 MEGDR DEM, resolution
                                                 4 pixels/degree.

             \begindata

                KERNELS_TO_LOAD = ( 'de430.bsp',
                                    'mar097.bsp',
                                    'pck00010.tpc',
                                    'naif0011.tls',
                                    'mgs_ext12_ipng_mgs95j.bsp',
                                    'megr90n000cb_plate.bds'      )
             \begintext

          Example code begins here.

             /.
                Program illumg_ex1
             ./

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

             int main()
             {

                /.
                Local constants
                ./
                #define                 META   "illumg_ex1.tm"
                #define                 MTHLEN 81
                #define                 NMETH  2

                /.
                Local variables
                ./
                SpiceChar             * abcorr;
                SpiceChar             * fixref;
                SpiceChar               ilumth [NMETH][MTHLEN] =
                   {"Ellipsoid", "DSK/Unprioritized" };

                SpiceChar             * ilusrc;
                SpiceChar             * target;
                SpiceChar             * obsrvr;
                SpiceChar               submth [NMETH][MTHLEN] =
                   {"Near Point/Ellipsoid", "DSK/Nadir/Unprioritized" };

                SpiceChar             * utc;

                SpiceDouble             et;
                SpiceDouble             srfvec [3];
                SpiceDouble             sscemi;
                SpiceDouble             sscphs;
                SpiceDouble             sscpt  [3];
                SpiceDouble             sscsol;
                SpiceDouble             sslemi;
                SpiceDouble             sslphs;
                SpiceDouble             sslsol;
                SpiceDouble             ssolpt [3];
                SpiceDouble             trgepc;

                SpiceInt                i;

                /.
                Load kernel files.
                ./
                furnsh_c ( META );

                /.
                Convert the UTC request time string to seconds past
                J2000 TDB.
                ./
                utc = "2003 OCT 13 06:00:00 UTC";

                str2et_c ( utc, &et );

                printf ( "\n"
                         "UTC epoch is %s\n", utc );

                /.
                Assign observer, target, and illumination source
                names. The acronym MGS indicates Mars Global
                Surveyor. See NAIF_IDS for a list of names
                recognized by SPICE.

                Also set the target body-fixed frame and
                the aberration correction flag.
                ./

                target = "Mars";
                obsrvr = "MGS";
                fixref = "IAU_MARS";
                abcorr = "CN+S";
                ilusrc = "Sun";

                for ( i = 0;  i < NMETH;  i++ )
                {
                   /.
                   Find the sub-solar point on Mars as
                   seen from the MGS spacecraft at `et'. Use the
                   "near point" style of sub-point definition
                   when the shape model is an ellipsoid, and use
                   the "nadir" style when the shape model is
                   provided by DSK data. This makes it easy to
                   verify the solar incidence angle when
                   the target is modeled as an  ellipsoid.
                   ./
                   subslr_c ( submth[i], target,  et,
                              fixref,    abcorr,  obsrvr,
                              ssolpt,    &trgepc, srfvec  );

                   /.
                   Now find the sub-spacecraft point.
                   ./
                   subpnt_c ( submth[i], target,  et,
                              fixref,    abcorr,  obsrvr,
                              sscpt,     &trgepc, srfvec  );

                   /.
                   Find the phase, solar incidence, and emission
                   angles at the sub-solar point on Mars as
                   seen from MGS at time `et'.
                   ./
                   illumg_c ( ilumth[i], target,  ilusrc,
                              et,        fixref,  abcorr,
                              obsrvr,    ssolpt,  &trgepc,
                              srfvec,    &sslphs, &sslsol,
                              &sslemi                      );

                   /.
                   Do the same for the sub-spacecraft point.
                   ./
                   illumg_c ( ilumth[i], target,  ilusrc,
                              et,        fixref,  abcorr,
                              obsrvr,    sscpt,   &trgepc,
                              srfvec,    &sscphs, &sscsol,
                              &sscemi                      );

                   /.
                   Convert the angles to degrees and write them out.
                   ./
                   sslphs *= dpr_c();
                   sslsol *= dpr_c();
                   sslemi *= dpr_c();

                   sscphs *= dpr_c();
                   sscsol *= dpr_c();
                   sscemi *= dpr_c();

                   printf ( "\n"
                            "   illumg_c method: %s\n"
                            "   subpnt_c method: %s\n"
                            "   subslr_c method: %s\n"
                            "\n"
                            "      Illumination angles at the "
                            "sub-solar point:\n"
                            "\n"
                            "      Phase angle            (deg): %15.9f\n"
                            "      Solar incidence angle  (deg): %15.9f\n"
                            "      Emission angle         (deg): %15.9f\n",
                            ilumth[i],
                            submth[i],
                            submth[i],
                            sslphs,
                            sslsol,
                            sslemi                                    );

                   if ( i == 0 )
                   {
                      printf ( "        The solar incidence angle "
                               "should be 0.\n"
                               "        The emission and phase "
                               "angles should be equal.\n"          );
                   }

                   printf ( "\n"
                            "      Illumination angles at the "
                            "sub-s/c point:\n"
                            "\n"
                            "      Phase angle            (deg): %15.9f\n"
                            "      Solar incidence angle  (deg): %15.9f\n"
                            "      Emission angle         (deg): %15.9f\n",
                            sscphs,
                            sscsol,
                            sscemi                                    );

                   if ( i == 0 )
                   {
                      printf ( "        The emission angle "
                               "should be 0.\n"
                               "        The solar incidence "
                               "and phase angles should be equal.\n"  );
                   }
                }
                printf ( "\n" );

                return ( 0 );
             }

       When this program was executed on a PC/Linux/gcc/64-bit platform,
       the output was:

          UTC epoch is 2003 OCT 13 06:00:00 UTC

             illumg_c method: Ellipsoid
             subpnt_c method: Near Point/Ellipsoid
             subslr_c method: Near Point/Ellipsoid

                Illumination angles at the sub-solar point:

                Phase angle            (deg):   138.370270685
                Solar incidence angle  (deg):     0.000000000
                Emission angle         (deg):   138.370270685
                  The solar incidence angle should be 0.
                  The emission and phase angles should be equal.

                Illumination angles at the sub-s/c point:

                Phase angle            (deg):   101.439331040
                Solar incidence angle  (deg):   101.439331041
                Emission angle         (deg):     0.000000002
                  The emission angle should be 0.
                  The solar incidence and phase angles should be equal.

             illumg_c method: DSK/Unprioritized
             subpnt_c method: DSK/Nadir/Unprioritized
             subslr_c method: DSK/Nadir/Unprioritized

                Illumination angles at the sub-solar point:

                Phase angle            (deg):   138.387071677
                Solar incidence angle  (deg):     0.967122745
                Emission angle         (deg):   137.621480599

                Illumination angles at the sub-s/c point:

                Phase angle            (deg):   101.439331359
                Solar incidence angle  (deg):   101.555993667
                Emission angle         (deg):     0.117861156

#### Restrictions

       None.

#### Literature_References

       None.

#### Author_and_Institution

       N.J. Bachman   (JPL)
       B.V. Semenov   (JPL)

#### Version

       -CSPICE Version 1.0.0, 04-APR-2017 (NJB) (BVS)

          16-AUG-2016

             Original version.

#### Index_Entries

       illumination angles general source
       lighting angles general source
       phase angle general source
       incidence angle general source
       emission angle general source

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

    Wed Apr  5 17:54:37 2017