Index Page edterm_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 edterm_c ( ConstSpiceChar     * trmtyp,
                       ConstSpiceChar     * source,
                       ConstSpiceChar     * target,
                       SpiceDouble          et,
                       ConstSpiceChar     * fixref,
                       ConstSpiceChar     * abcorr,
                       ConstSpiceChar     * obsrvr,
                       SpiceInt             npts,
                       SpiceDouble        * trgepc,
                       SpiceDouble          obspos  [3],
                       SpiceDouble          trmpts  [ ][3] )

#### Abstract

       Compute a set of points on the umbral or penumbral terminator of
       a specified target body, where the target shape is modeled as an
       ellipsoid.

#### Required_Reading

       FRAMES
       PCK
       SPK
       TIME

#### Keywords

       BODY
       GEOMETRY
       MATH

#### Brief_I/O

       Variable  I/O  Description
       --------  ---  --------------------------------------------------
       trmtyp     I   Terminator type.
       source     I   Light source.
       target     I   Target body.
       et         I   Observation epoch.
       fixref     I   Body-fixed frame associated with target.
       abcorr     I   Aberration correction.
       obsrvr     I   Observer.
       npts       I   Number of points in terminator set.
       trgepc     O   Epoch associated with target center.
       obspos     O   Position of observer in body-fixed frame.
       trmpts     O   Terminator point set.

#### Detailed_Input

       trmtyp      is a string indicating the type of terminator to
                   compute: umbral or penumbral. The umbral terminator is
                   the boundary of the portion of the ellipsoid surface in
                   total shadow. The penumbral terminator is the boundary
                   of the portion of the surface that is completely
                   illuminated. Note that in astronomy references, the
                   unqualified word "terminator" refers to the umbral
                   terminator. Here, the unqualified word refers to either
                   type of terminator.

                   Possible values of `trmtyp' are

                      "UMBRAL"
                      "PENUMBRAL"

                   Case and leading or trailing blanks in `trmtyp' are
                   not significant.

       source      is the name of the body acting as a light source.
                   `source' 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 "SUN" and "10" are
                   legitimate strings that indicate the Sun is the light
                   source.

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

                   The shape of the light source is always modeled as a
                   sphere, regardless of whether radii defining a triaxial
                   ellipsoidal shape model are available in the kernel
                   pool. The maximum radius of the body is used as the
                   radius of the sphere.

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

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

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

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

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

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

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

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

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

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

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

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

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

                                 The target position as seen by the
                                 observer, the position of the light
                                 source as seen from the target at
                                 et-lt, and the rotation of the target
                                 body, are corrected for light time.

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

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

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

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

       npts        is the number of terminator points to compute.

#### Detailed_Output

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

                   `trgepc' is expressed as seconds past J2000 TDB.

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

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

                      vsub_c ( trmpts[i], obspos, xvec );

                   To transform the vector `obspos' 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
                   `obspos' can be transformed to the result `refvec' as
                   follows:

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

       trmpts      is an array of points on the umbral or penumbral
                   terminator of the ellipsoid, as specified by the
                   input argument `trmtyp'. The ith point is contained in
                   the array elements

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

                   Each terminator point is the point of tangency of a
                   plane that is also tangent to the light source. These
                   associated points of tangency on the light source have
                   uniform distribution in longitude when expressed in a
                   cylindrical coordinate system whose Z-axis is the target
                   center to source center vector. The magnitude of the
                   separation in longitude between the tangency points on
                   the light source is

                      2*pi / npts

                   If the target is spherical, the terminator points
                   also are uniformly distributed in longitude in the
                   cylindrical system described above. If the target is
                   non-spherical, the longitude distribution of the
                   points generally is not uniform.

                   The terminator points are expressed in the body-fixed
                   reference frame designated by `fixref'. Units are km.

#### Parameters

       None.

#### Exceptions

       1)  If the input frame name `fixref' cannot be mapped to a frame ID
           code, the error will be diagnosed by a routine in the call tree
           of this routine.

       2)  If the target name `target' cannot be mapped to a body ID code,
           the error will be diagnosed by a routine in the call tree of
           this routine.

       3)  If the frame designated by `fixref' is not centered on the
           target, the error will be diagnosed by a routine in the call
           tree of this routine.

       4)  If the terminator type is not recognized, the error will be
           diagnosed by a routine in the call tree of this routine.

       5)  If the terminator point count`npts' is not at least 1, the error
           will be diagnosed by a routine in the call tree of this routine.

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

       7)  If the light source has non-positive radius, the error will be
           diagnosed by a routine in the call tree of this routine.

       8)  If the light source intersects the smallest sphere centered at
           the origin and containing the ellipsoid, the error will be
           diagnosed by a routine in the call tree of this routine.

       9)  If radii for the target body or light source are not available
           in the kernel pool, or if radii are available but either body
           does not have three radii, the error will be diagnosed by a
           routine in the call tree of this routine.

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

       11) If any array pointer argument is null, the error SPICE(NULLPOINTER)
           will be signaled.

       12) If any input string pointer is empty, the error SPICE(EMPTYSTRING)
           will be signaled.

#### Files

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

       The following data are required:

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

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

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

          -  Frame data: if a frame definition is required to convert the
             observer and target states to the target body-fixed frame
             designated by `fixref', that definition must be available in
             the kernel pool. Typically the definitions of frames not
             already built-in to SPICE are supplied by loading a frame
             kernel.

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

#### Particulars

       This routine models the boundaries of shadow regions on an
       ellipsoidal target body "illuminated" by a spherical light
       source. Light rays are assumed to travel along straight lines;
       refraction is not modeled.

       Points on the target body's surface are classified according to
       their illumination as follows:

          -  A target surface point X for which no vector from X to any
             point in the light source intersects the target, except at
             X, is considered to be "completely illuminated."

          -  A target surface point X for which each vector from X to a
             point in the light source intersects the target at points
             other than X is considered to be "in total shadow."

          -  All other target points are considered to be in partial
             shadow.

       In this routine, we use the term "umbral terminator" to denote
       the curve usually called the "terminator": this curve is the
       boundary of the portion of the target body's surface that lies in
       total shadow. We use the term "penumbral terminator" to denote
       the boundary of the completely illuminated portion of the
       surface.

       In general, the terminator on an ellipsoid is a more complicated
       curve than the limb (which is always an ellipse). Aside from
       various special cases, the terminator does not lie in a plane.

       However, the condition for a point X on the ellipsoid to lie on
       the terminator is simple: a plane tangent to the ellipsoid at X
       must also be tangent to the light source. If this tangent plane
       does not intersect the vector from the center of the ellipsoid to
       the center of the light source, then X lies on the umbral
       terminator; otherwise X lies on the penumbral terminator.

#### Examples

       The numerical results shown for these examples may differ across
       platforms. The results depend on the SPICE kernels used as
       input, the compiler and supporting libraries, and the machine
       specific arithmetic implementation.

       1)  Compute sets of umbral and penumbral terminator points on the
           Moon. Perform a consistency check using the solar incidence
           angle at each point. We expect to see a solar incidence angle of
           approximately 90 degrees. Since the solar incidence angle is
           measured between the local outward normal and the direction to
           the center of the Sun, the solar incidence angle at an umbral
           terminator point should exceed 90 degrees by approximately the
           angular radius of the Sun, while the angle at a penumbral
           terminator points should be less than 90 degrees by that amount.

           This program loads SPICE kernels via a meta-kernel. The
           meta-kernel used to produce the results shown below is

              KPL/MK

              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.

              \begindata

                 KERNELS_TO_LOAD = ( 'de421.bsp',
                                     'pck00010.tpc',
                                     'naif0010.tls'  )

              \begintext

           Program source code:

              /.
                 Program EX1
              ./

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

              int main()
              {
                 /.
                 Local constants
                 ./

                 #define META            "edterm.tm"
                 #define NPTS            3

                 /.
                 Local variables
                 ./
                 SpiceBoolean            first = SPICETRUE;

                 SpiceChar             * abcorr;
                 SpiceChar             * fixref;
                 SpiceChar             * obsrvr;
                 SpiceChar             * source;
                 SpiceChar             * target;
                 SpiceChar             * trmtyps [2] = { "UMBRAL",
                                                         "PENUMBRAL" };
                 SpiceChar             * utc;

                 SpiceDouble             angrad;
                 SpiceDouble             emissn;
                 SpiceDouble             et;
                 SpiceDouble             lat;
                 SpiceDouble             lon;
                 SpiceDouble             lt;
                 SpiceDouble             obspos  [3];
                 SpiceDouble             phase;
                 SpiceDouble             radius;
                 SpiceDouble             s       [2] = { -1.0,  1.0};

                 SpiceDouble             solar;
                 SpiceDouble             srcpos  [3];
                 SpiceDouble             srcrad  [3];
                 SpiceDouble             srfvec  [3];
                 SpiceDouble             trgepc;
                 SpiceDouble             trmpts  [NPTS][3];

                 SpiceInt                i;
                 SpiceInt                n;
                 SpiceInt                trmidx;

                 /.
                 Load meta-kernel.
                 ./
                 furnsh_c ( META );

                 /.
                 Set observation time.
                 ./
                 utc    = "2007 FEB 3 00:00:00.000";

                 str2et_c ( utc, &et );

                 /.
                 Set participating objects, frame, and aberration
                 corrections.
                 ./
                 obsrvr = "EARTH";
                 target = "MOON";
                 source = "SUN";
                 fixref = "IAU_MOON";
                 abcorr = "LT+S";

                 /.
                 Look up the radii of the sun.
                 ./
                 bodvrd_c ( source, "RADII", 3, &n, srcrad );

                 /.
                 Compute terminator points.
                 ./
                 for ( trmidx = 0;  trmidx < 2;  trmidx++  )
                 {
                    edterm_c ( trmtyps[trmidx],  source,  target,
                               et,               fixref,  abcorr,
                               obsrvr,           NPTS,    &trgepc,
                               obspos,           trmpts            );
                    /.
                    Validate terminator points.

                    Look up the target-sun vector at the light-time
                    corrected target epoch.
                    ./
                    if ( first )
                    {
                       spkpos_c ( source, trgepc, fixref,
                                  abcorr, target, srcpos, &lt );

                       first = SPICEFALSE;
                    }

                    printf ( "\n"
                             " Terminator type: %s\n", trmtyps[trmidx] );

                    for ( i = 0;  i < NPTS;  i++ )
                    {
                       /.
                       Convert the ith terminator point to latitudinal
                       coordinates. Display the point.
                       ./
                       reclat_c ( trmpts[i], &radius, &lon, &lat );

                       printf ( "\n"
                                "   Terminator point %d:\n"
                                "     Radius                     (km):  %18.9f\n"
                                "     Planetocentric longitude   (deg): %18.9f\n"
                                "     Planetocentric latitude    (deg): %18.9f\n",
                                (int)i,
                                radius,
                                lon * dpr_c(),
                                lat * dpr_c()                            );

                       /.
                       Find the illumination angles at the
                       ith terminator point.
                       ./
                       ilumin_c  ( "Ellipsoid", target,  et,         fixref,
                                   abcorr,      obsrvr,  trmpts[i],  &trgepc,
                                   srfvec,      &phase,  &solar,     &emissn  );

                       printf ( "     Solar incidence angle      (deg): %18.9f\n",
                                solar * dpr_c()                                   );

                       /.
                       Find the angular radius of the Sun as seen from
                       the terminator point.
                       ./
                       angrad = asin (   srcrad[0]
                                       / vdist_c ( srcpos, trmpts[i] )  );

                       /.
                       Display the solar incidence angle after
                       adjusting the angular radius of the Sun
                       as seen from the terminator point.The
                       result should be approximately 90 degrees.
                       ./
                       printf ( "     Solar incidence angle adjusted for\n"
                                "     sun's angular radius (deg):       %18.9f\n",
                                ( solar + ( s[trmidx]*angrad ) ) * dpr_c()        );
                    }
                 }

                 return ( 0 );
              }

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

             Terminator type: UMBRAL

               Terminator point 0:
                 Radius                     (km):      1737.400000000
                 Planetocentric longitude   (deg):      -95.084552819
                 Planetocentric latitude    (deg):        0.004052763
                 Solar incidence angle      (deg):       90.269765819
                 Solar incidence angle adjusted for
                 sun's angular radius (deg):             90.000000129

               Terminator point 1:
                 Radius                     (km):      1737.400000000
                 Planetocentric longitude   (deg):       84.228091534
                 Planetocentric latitude    (deg):       59.995755519
                 Solar incidence angle      (deg):       90.269765706
                 Solar incidence angle adjusted for
                 sun's angular radius (deg):             90.000000016

               Terminator point 2:
                 Radius                     (km):      1737.400000000
                 Planetocentric longitude   (deg):       87.216417974
                 Planetocentric latitude    (deg):      -59.979550515
                 Solar incidence angle      (deg):       90.269765730
                 Solar incidence angle adjusted for
                 sun's angular radius (deg):             90.000000040

             Terminator type: PENUMBRAL

               Terminator point 0:
                 Radius                     (km):      1737.400000000
                 Planetocentric longitude   (deg):       84.914100511
                 Planetocentric latitude    (deg):       -0.004073047
                 Solar incidence angle      (deg):       89.730234406
                 Solar incidence angle adjusted for
                 sun's angular radius (deg):             90.000000126

               Terminator point 1:
                 Radius                     (km):      1737.400000000
                 Planetocentric longitude   (deg):      -95.769215814
                 Planetocentric latitude    (deg):      -59.995785101
                 Solar incidence angle      (deg):       89.730234298
                 Solar incidence angle adjusted for
                 sun's angular radius (deg):             90.000000018

               Terminator point 2:
                 Radius                     (km):      1737.400000000
                 Planetocentric longitude   (deg):      -92.780892017
                 Planetocentric latitude    (deg):       59.979498997
                 Solar incidence angle      (deg):       89.730234322
                 Solar incidence angle adjusted for
                 sun's angular radius (deg):             90.000000042

#### Restrictions

       1) This routine models light paths as straight lines.

#### Literature_References

       None.

#### Author_and_Institution

       N.J. Bachman    (JPL)
       E.D. Wright     (JPL)

#### Version

       -CSPICE Version 1.0.1, 12-JUL-2016 (EDW)

          Edit to example program to use "%d" with explicit casts
          to int for printing SpiceInts with printf.

       -CSPICE Version 1.0.0, 13-JUN-2012 (NJB) (EDW)

#### Index_Entries

       find terminator on ellipsoid
       find umbral terminator on ellipsoid
       find penumbral terminator on ellipsoid

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

    Wed Apr  5 17:54:33 2017