| illum_c |
|
Table of contents
Procedure
illum_c ( Illumination angles )
void illum_c ( ConstSpiceChar * target,
SpiceDouble et,
ConstSpiceChar * abcorr,
ConstSpiceChar * obsrvr,
ConstSpiceDouble spoint [3],
SpiceDouble * phase,
SpiceDouble * solar,
SpiceDouble * emissn )
AbstractDeprecated: This routine has been superseded by the CSPICE routine ilumin_c.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_ReadingKERNEL NAIF_IDS SPK TIME KeywordsGEOMETRY Brief_I/OVARIABLE 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.
ParametersNone. Exceptions
1) If `target' and `obsrvr' are not distinct, the error
SPICE(BODIESNOTDISTINCT) is signaled by a routine in the call
tree of this routine.
2) If no SPK (ephemeris) data are available for the observer,
target, and Sun at the time specified by `et', an error is
signaled by a routine in the call tree of 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, an error is signaled by a routine in the
call tree of this routine.
4) If no body-fixed frame is associated with the target body, the
error SPICE(NOFRAME) is signaled by a routine in the call tree
of this routine.
5) If name of target or observer cannot be translated to its NAIF
ID code, the error SPICE(IDCODENOTFOUND) is signaled by a
routine in the call tree of this routine.
6) If radii for `target' are not found in the kernel pool, an error
is signaled by a routine in the call tree of this routine.
7) If the size of the `target' body radii kernel variable is not
three, an error is signaled by a routine in the call tree of
this routine.
8) If any of the three `target' body radii is less-than or equal to
zero, an error is signaled by a routine in the call tree of
this routine.
9) If any of the `target', `abcorr' or `obsrvr' input string
pointers is null, the error SPICE(NULLPOINTER) is signaled.
10) If any of the `target', `abcorr' or `obsrvr' input strings has
zero length, the error SPICE(EMPTYSTRING) is signaled.
FilesNo 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.
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: illum_ex1.tm
This meta-kernel is intended to support operation of SPICE
example programs. The kernels shown here should not be
assumed to contain adequate or correct versions of data
required by SPICE-based user applications.
In order for an application to use this meta-kernel, the
kernels referenced here must be present in the user's
current working directory.
The names and contents of the kernels referenced
by this meta-kernel are as follows:
File name Contents
--------- --------
mar097.bsp Mars satellite ephemeris
pck00010.tpc Planet orientation and
radii
naif0011.tls Leapseconds
mgs_ext12_ipng_mgs95j.bsp MGS ephemeris
\begindata
KERNELS_TO_LOAD = ( 'mar097.bsp',
'pck00010.tpc',
'naif0011.tls',
'mgs_ext12_ipng_mgs95j.bsp' )
\begintext
End of meta-kernel
Example code begins here.
/.
Program illum_ex1
./
#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 kernels.
./
furnsh_c ( "illum_ex1.tm" );
/.
Convert the UTC request time to ET (seconds past J2000 TDB).
./
utc = "2003 AUG 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 was executed on a Mac/Intel/cc/64-bit
platform, the output was:
UTC epoch is 2003 AUG 1 12:00:00
Illumination angles at the sub-solar point:
Phase angle (deg): 149.314573
Solar incidence angle (deg): 0.000000
Emission angle (deg): 149.314573
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): 121.932334
Solar incidence angle (deg): 121.932334
Emission angle (deg): 0.000000
The emission angle should be 0.
The solar incidence and phase angles should be equal.
RestrictionsNone. Literature_ReferencesNone. Author_and_InstitutionC.H. Acton (JPL) N.J. Bachman (JPL) J. Diaz del Rio (ODC Space) B.V. Semenov (JPL) Version
-CSPICE Version 1.0.6, 01-NOV-2021 (JDR)
Edited the header to comply with NAIF standard. Added
example's problem statement and meta-kernel.
Added entries #6 and #7 to -Exceptions section.
-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_EntriesDEPRECATED illumination angles DEPRECATED lighting angles DEPRECATED phase angle DEPRECATED emission angle DEPRECATED solar incidence angle |
Fri Dec 31 18:41:08 2021