ilumin_c

Index Page ilumin_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 ilumin_c ( ConstSpiceChar          * method,
                       ConstSpiceChar          * target,
                       SpiceDouble               et,
                       ConstSpiceChar          * fixref,
                       ConstSpiceChar          * abcorr,
                       ConstSpiceChar          * obsrvr,
                       ConstSpiceDouble          spoint [3],
                       SpiceDouble             * trgepc,
                       SpiceDouble               srfvec [3],
                       SpiceDouble             * phase,
                       SpiceDouble             * solar,
                       SpiceDouble             * emissn     )
#### Abstract
       Find the illumination angles (phase, solar incidence, and
       emission) at a specified surface point of a target body.

       This routine supersedes illum_c.
#### Required_Reading
       FRAMES
       NAIF_IDS
       PCK
       SPK
       TIME
#### Keywords
       GEOMETRY
#### Brief_I/O
       Variable  I/O  Description
       --------  ---  --------------------------------------------------
       method     I   Computation method.
       target     I   Name of target body.
       et         I   Epoch in ephemeris seconds past J2000 TDB.
       fixref     I   Body-fixed, body-centered target body frame.
       abcorr     I   Desired aberration correction.
       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.
       solar      O   Solar 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. Parameters
                   include, but are not limited to, the shape model
                   used to represent the surface of the target body.

                   The only choice currently supported is

                      "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.

                   Neither case nor white space are significant in
                   `method'. For example, the string ' eLLipsoid ' is
                   valid.

       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.

       et          is the epoch, specified as ephemeris seconds past J2000
                   TDB, at 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 Sun.

                   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 `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 Sun as seen by the target, for light
                                 time.

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

                      "CN"       Converged Newtonian light time correction.
                                 In solving the light time equations for
                                 target and the Sun, 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.

                   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.  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 "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 "surface point 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 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' to a time-dependent
                   reference frame `ref' at `et', a sequence of two frame
                   transformations is required. For example, let `mfix'
                   and `mref' be 3x3 matrices respectively describing the
                   target body-fixed to J2000 frame transformation at
                   `trgepc' and the J2000 to (time-dependent frame) `ref'
                   transformation at `et', and let `xform' be the 3x3 matrix
                   representing the composition of `mref' with `mfix'. Then
                   `srfvec' can be transformed to the result `refvec' as
                   follows:

                       pxform_c ( fixref,  "j2000", trgepc, mfix   );
                       pxform_c ( "j2000", ref,     et,     mref   );
                       mxm_c    ( mref,    mfix,            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-sun vector. Units are radians. The
                   range of `phase' is [0, pi]. See Particulars below for a
                   detailed discussion of the definition.

       solar       is the solar 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-sun
                   vector. Units are radians. The range of `solar' 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 either the target or observer 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 the target and observer have distinct identities but are
           at the same location (for example, the target is Mars and
           the observer is the Mars barycenter), the error
           SPICE(NOSEPARATION) is signaled.

       8)  If insufficient ephemeris data have been loaded prior to
           calling ilumin_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 Sun to the solar system barycenter.

       9)  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 ilumin_c, the error will
           be diagnosed and signaled by a routine in the call tree of
           this routine.

       10) 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.

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

       12) The error SPICE(EMPTYSTRING) is signaled if any input string
           argument does not contain at least one character, since the
           input string cannot be converted to a Fortran-style string in
           this case.

       13) The error SPICE(NULLPOINTER) is signaled if any input
           string argument pointer is null.
#### 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
            Sun must be loaded. If aberration corrections are used, the
            states of target, observer, and the Sun 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: 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.

          - 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 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.

       In all cases, kernel data are normally loaded once per program
       run, NOT every time this routine is called.
#### Particulars
       The term "illumination angles" refers to following set of
       angles:

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

          solar incidence angle    Angle between the surface normal at
                                   the specified surface point and the
                                   vector from the surface point to the
                                   Sun.

          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 solar incidence,
       emission, and phase angles are "s.i.", "e.", and "phase".

                                                        *
                                                       Sun

                      surface normal vector
                                ._                 _.
                                |\                 /|  Sun vector
                                  \    phase      /
                                   \   .    .    /
                                   .            .
                                     \   ___   /
                                .     \/     \/
                                      _\ s.i./
                               .    /   \   /
                               .   |  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-sun vector are coplanar,
       then phase is the sum of incidence and emission. This is rarely
       true; usually

          phase angle  <  solar 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 body 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 -- Sun 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 Sun as seen from the target body at
             et-lt. So, a second light time correction is used to compute
             the position of the Sun 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 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-Sun vector
             ----------------------

             The target body-Sun vector is the apparent position of the
             Sun, corrected for light time and stellar aberration, as
             seen from the target body at time et-lt.
#### 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 user-specified UTC time. Use
          light time and stellar aberration corrections.

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

             KPL/MK

             File: mgs_example.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
                ---------                     --------
                de418.bsp                     Planetary ephemeris
                pck00008.tpc                  Planet orientation and
                                              radii
                naif0008.tls                  Leapseconds
                mgs_ext13_ipng_mgs95j.bsp     MGS ephemeris

             \begindata

                KERNELS_TO_LOAD = ( 'de418.bsp',
                                    'pck00008.tpc',
                                    'naif0008.tls',
                                    'mgs_ext13_ipng_mgs95j.bsp'  )
             \begintext

          Example code begins here.

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

          int main()
          {
             /.
             Local constants
             ./
             #define  META           "mgs_example.tm"

             /.
             Local variables
             ./
             SpiceChar             * abcorr;
             SpiceChar             * obsrvr;
             SpiceChar             * target;
             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;

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

             /.
             Convert the UTC request time string to seconds past J2000 TDB.
             ./
             utc = "2004 JAN 1 12:00:00";

             str2et_c ( utc, &et );

             /.
             Assign observer and target names. The acronym MGS
             indicates Mars Global Surveyor. See NAIF_IDS for a
             list of names recognized by SPICE. Also set the
             aberration correction flag.
             ./
             target = "Mars";
             obsrvr = "MGS";
             abcorr = "CN+S";

             /.
             Find the sub-solar point on the Earth as seen from
             the MGS spacecraft at et. Use the "near point: ellipsoid"
             style of sub-point definition.
             ./
             subslr_c ( "near point: ellipsoid",
                        target, et,     "iau_mars", abcorr,
                        obsrvr, ssolpt, &trgepc,    srfvec  );

             /.
             Now find the sub-spacecraft point.
             ./
             subpnt_c ( "near point: ellipsoid",
                        target, et,    "iau_mars", abcorr,
                        obsrvr, sscpt, &trgepc,    srfvec  );

             /.
             Find the phase, solar incidence, and emission
             angles at the sub-solar point on the Earth as seen
             from MGS at time et.
             ./
             ilumin_c ( "Ellipsoid",
                        target,  et,      "IAU_MARS", abcorr,
                        obsrvr,  ssolpt,  &trgepc,    srfvec,
                        &sslphs, &sslsol, &sslemi            );

             /.
             Do the same for the sub-spacecraft point.
             ./
             ilumin_c ( "Ellipsoid",
                        target,  et,      "IAU_MARS", 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"
                      "UTC epoch is %s\n"
                      "\n"
                      "Illumination angles at the sub-solar point:\n"
                      "\n"
                      "Phase angle             (deg):  %f\n"
                      "Solar incidence angle   (deg):  %f\n"
                      "Emission angle          (deg):  %f\n"
                      "\n"
                      "The solar incidence angle should be 0.\n"
                      "The emission and phase angles should be "
                      "equal.\n"
                      "\n"
                      "\n"
                      "Illumination angles at the sub-s/c point:\n"
                      "\n"
                      "Phase angle             (deg):  %f\n"
                      "Solar incidence angle   (deg):  %f\n"
                      "Emission angle          (deg):  %f\n"
                      "\n"
                      "The emission angle should be 0.\n"
                      "The solar incidence and phase angles "
                      "should be equal.\n"
                      "\n",
                      utc,
                      sslphs,
                      sslsol,
                      sslemi,
                      sscphs,
                      sscsol,
                      sscemi                                    );

             return ( 0 );
          }

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

          UTC epoch is 2004 JAN 1 12:00:00

          Illumination angles at the sub-solar point:

          Phase angle             (deg):  115.542001
          Solar incidence angle   (deg):  0.000000
          Emission angle          (deg):  115.542001

          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):  62.084003
          Solar incidence angle   (deg):  62.084003
          Emission angle          (deg):  0.000000

          The emission angle should be 0.
          The solar incidence and phase angles should be equal.
#### Restrictions
       None.
#### Literature_References
       None.
#### Author_and_Institution
       N.J. Bachman   (JPL)
#### Version
       -CSPICE Version 1.0.1, 06-FEB-2009 (NJB)

           Incorrect frame name fixfrm was changed to fixref in
           documentation.

           In the header examples, meta-kernel names were updated to use
           the suffix

              ".tm"

       -CSPICE Version 1.0.0, 02-MAR-2008 (NJB)
#### Index_Entries
       illumination angles
       lighting angles
       phase angle
       emission angle
       solar incidence angle
#### Link to routine ilumin_c source file ilumin_c.c

    Thu Apr 16 08:58:19 2009