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 )
Find the illumination angles (phase, solar incidence, and
emission) at a specified surface point of a target body.
This routine supersedes illum_c.
FRAMES
NAIF_IDS
PCK
SPK
TIME
GEOMETRY
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.
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.
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.
None.
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.
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.
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.
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.
None.
None.
N.J. Bachman (JPL)
-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)
illumination angles
lighting angles
phase angle
emission angle
solar incidence angle
Link to routine ilumin_c source file ilumin_c.c
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