Index Page illum_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 illum_c  ( ConstSpiceChar          * target,
                       SpiceDouble               et,
                       ConstSpiceChar          * abcorr,
                       ConstSpiceChar          * obsrvr,
                       ConstSpiceDouble          spoint [3],
                       SpiceDouble             * phase,
                       SpiceDouble             * solar,
                       SpiceDouble             * emissn     )

#### Abstract

       Deprecated: This routine has been superseded by the CSPICE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

       Find the illumination angles at a specified surface point of a
       target body.

#### Required_Reading

       KERNEL
       NAIF_IDS
       SPK
       TIME

#### Keywords

       GEOMETRY

#### Brief_I/O

       Variable  I/O  Description
       --------  ---  --------------------------------------------------
       target     I   Name of target body.
       et         I   Epoch in ephemeris seconds past J2000.
       abcorr     I   Desired aberration correction.
       obsrvr     I   Name of observing body.
       spoint     I   Body-fixed coordinates of a 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

       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 in ephemeris seconds past
                      J2000, 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.

       abcorr         is the aberration correction to be used in
                      computing the location and orientation of the
                      target body and the location of the Sun.  Possible
                      values are:

                         "NONE"        No aberration correction.

                         "LT"          Correct the position and
                                       orientation of target body for
                                       light time, and correct the
                                       position of the Sun for light
                                       time.

                         "LT+S"        Correct the observer-target vector
                                       for light time and stellar
                                       aberration, correct the
                                       orientation of the target body
                                       for light time, and correct the
                                       target-Sun vector for light time
                                       and stellar aberration.

                         "CN"          Converged Newtonian light time
                                       correction. In solving the light
                                       time equation, the "CN" correction
                                       iterates until the solution
                                       converges (three iterations on all
                                       supported platforms). Whether the
                                       "CN+S" solution is substantially
                                       more accurate than the "LT" solution
                                       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. See the Particulars
                                       section of spkezr_c for a discussion
                                       of precision of light time
                                       corrections.

                                       Both the state and rotation of the
                                       target body are corrected for light
                                       time.

                         "CN+S"        Converged Newtonian light time
                                       correction and stellar aberration
                                       correction.

                                       Both the state and rotation of the
                                       target body are corrected for light
                                       time.

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

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

       spoint         is a surface point on the target body, expressed
                      in rectangular body-fixed (body equator and prime
                      meridian) coordinates.  `spoint' need not be visible
                      from the observer's location at time `et'.

#### Detailed_Output

       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 `target' and `obsrvr' are not distinct, the error
           SPICE(BODIESNOTDISTINCT) will be signaled.

       2)  If no SPK (ephemeris) data are available for the observer,
           target, and Sun at the time specified by `et', the error will
           be diagnosed by routines called by this routine.  If light
           time corrections are used, SPK data for the target body must
           be available at the time et - lt, where `lt' is the one-way
           light time from the target to the observer at `et'.
           Additionally, SPK data must be available for the Sun at the
           time et - lt - lt2, where lt2 is the light time from the Sun
           to the target body at time et - lt.

       3)  If PCK data defining the orientation or shape of the target
           body are unavailable, the error will be diagnosed by routines
           called by this routine.

       4)  If no body-fixed frame is associated with the target body,
           the error SPICE(NOFRAME) is signaled.

       5)  If name of `target' or `obsrvr' cannot be translated to its
           NAIF ID code, the error SPICE(IDCODENOTFOUND) is signaled.

#### Files

       No files are input to this routine.  However, illum_c expects
       that the appropriate SPK and PCK files have been loaded via
       furnsh_c.

#### Particulars

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

          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.

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

       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.  The way aberration corrections
       are used is described below.

       Care must be used in computing light time corrections.  The
       guiding principle used here is "describe what appears in
       an image."  We ignore differential light time; the light times
       from all points on the target to the observer are presumed to be
       equal.

          Observer-target 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 body (we use the term "emitted"
          loosely here).

          The correct observer-target vector points from the observer's
          location at `et' to the target body's location at et - lt.
          The target-observer vector points in the opposite direction.

          Since light time corrections are not symmetric, the correct
          target-observer vector CANNOT be found by computing the light
          time corrected position of the observer as seen from the
          target body.

          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
          -------------------------

          All surface features on the target body 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
          in finding the apparent location of the Sun.

       Stellar aberration corrections, when used, are applied as follows:

          Observer-target body vector
          ---------------------------

          In addition to light time correction, stellar aberration is
          used in computing the apparent target body position as seen
          from the observer's location at time `et'.  This apparent
          position defines the observer-target body vector.

          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.  Note that the target body's
          position is not affected by the stellar aberration correction
          applied in finding its apparent position as seen by the
          observer.

       Once all of the vectors, as well as the target body's
       orientation, have been computed with the proper aberration
       corrections, the element of time is eliminated from the
       computation.  The problem becomes a purely geometric one,
       and is described by the diagram above.

#### 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.

       In the following example program, the file

          spk_m_031103-040201_030502.bsp

       is a binary SPK file containing data for Mars Global Surveyor,
       Mars, and the Sun for a time interval bracketing the date

          2004 JAN 1 12:00:00 UTC.

       pck00007.tpc is a planetary constants kernel file containing
       radii and rotation model constants.  naif0007.tls is a
       leapseconds kernel.

       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.

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

          int main()
          {
             /.
             Local variables
             ./
             SpiceChar             * obsrvr;
             SpiceChar             * target;
             SpiceChar             * utc;

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

             /.
             Load kernel files.
             ./
             furnsh_c ( "naif0007.tls"                   );
             furnsh_c ( "pck00007.tpc"                   );
             furnsh_c ( "spk_m_031103-040201_030502.bsp" );

             /.
             Convert the UTC request time to ET (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.
             ./
             target = "Mars";
             obsrvr = "MGS";

             /.
             Find the sub-solar point on the Earth as seen from
             the MGS spacecraft at et.  Use the "near point"
             style of sub-point definition.  This makes it easy
             to verify the solar incidence angle.
             ./
             subsol_c ( "near point",  target,  et,
                        "LT+S",        obsrvr,  ssolpt  );

             /.
             Now find the sub-spacecraft point.  Use the
             "nearest point" definition of the sub-point
             here---this makes it easy to verify the emission
             angle.
             ./
             subpt_c  ( "near point",  target,  et,
                        "LT+S",        obsrvr,  sscpt,  &alt );

             /.
             Find the phase, solar incidence, and emission
             angles at the sub-solar point on the Earth as seen
             from Mars Observer at time et.
             ./
             illum_c ( target,  et,       "LT+S",    obsrvr,
                       ssolpt,  &sslphs,  &sslsol,   &sslemi );

             /.
             Do the same for the sub-spacecraft point.
             ./
             illum_c ( target,  et,       "LT+S",    obsrvr,
                       sscpt,   &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"
                      "\n",
                      utc,
                      sslphs,
                      sslsol,
                      sslemi,
                      sscphs,
                      sscsol,
                      sscemi                                    );

             printf ( "\n" );

             return ( 0 );
          }

       When this program is executed, the output will be:

          UTC epoch is 2004 JAN 1 12:00:00

          Illumination angles at the sub-solar point:

          Phase angle             (deg):  150.210714
          Solar incidence angle   (deg):  0.000000
          Emission angle          (deg):  150.210714

          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):  123.398202
          Solar incidence angle   (deg):  123.398202
          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

       C.H. Acton     (JPL)
       N.J. Bachman   (JPL)

#### Version

       -CSPICE Version 1.0.5, 10-JUL-2014 (NJB)

           Discussion of light time corrections was updated. Assertions
           that converged light time corrections are unlikely to be
           useful were removed.

       -CSPICE Version 1.0.4, 19-MAY-2010 (BVS)

            Index lines now state that this routine is deprecated.

       -CSPICE Version 1.0.3, 07-FEB-2008 (NJB)

            Abstract now states that this routine is deprecated.

       -CSPICE Version 1.0.2, 22-JUL-2004 (NJB)

           Updated header to indicate that the `target' and `observer'
           input arguments can now contain string representations of
           integers.

       -CSPICE Version 1.1.2, 27-JUL-2003   (NJB) (CHA)

           Various header corrections were made.  The example program
           was upgraded to use real kernels, and the program's output is
           shown.

       -CSPICE Version 1.1.1, 04-SEP-2002   (NJB)

           Updated Index_Entries header section.  Corrected error in
           erract_c call in header example.

       -CSPICE Version 1.1.0, 24-JUL-2001   (NJB)

           Changed prototype:  input spoint is now type
           (ConstSpiceDouble [3]).  Implemented interface macro for
           casting spoint array to const.

       -CSPICE Version 1.0.0, 25-MAY-1999 (NJB)

#### Index_Entries

       DEPRECATED illumination angles
       DEPRECATED lighting angles
       DEPRECATED phase angle
       DEPRECATED emission angle
       DEPRECATED solar incidence angle

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

    Wed Apr  5 17:54:36 2017