Table of contents
CSPICE_ILLUMG finds the illumination angles (phase, incidence, and
emission) at a specified surface point of a target body.
The surface of the target body may be represented by a triaxial
ellipsoid or by topographic data provided by DSK files.
The illumination source is a specified ephemeris object.
Given:
method a short string providing parameters defining the computation
method to be used.
help, method
STRING = Scalar
In the syntax descriptions below, items delimited by brackets
are optional.
`method' may be assigned the following values:
'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.
'DSK/UNPRIORITIZED[/SURFACES = <surface list>]'
The illumination angle computation uses
topographic data to model the surface of the
target body. These data must be provided by
loaded DSK files.
The surface list specification is optional. The
syntax of the list is
<surface 1> [, <surface 2>...]
If present, it indicates that data only for the
listed surfaces are to be used; however, data
need not be available for all surfaces in the
list. If absent, loaded DSK data for any surface
associated with the target body are used.
The surface list may contain surface names or
surface ID codes. Names containing blanks must
be delimited by double quotes, for example
'SURFACES = "Mars MEGDR 128 PIXEL/DEG"'
If multiple surfaces are specified, their names
or IDs must be separated by commas.
See the -Particulars section below for details
concerning use of DSK data.
Neither case nor white space are significant in `method',
except within double-quoted strings representing surface
names. For example, the string ' eLLipsoid ' is valid.
Within double-quoted strings representing surface names,
blank characters are significant, but multiple
consecutive blanks are considered equivalent to a single
blank. Case is not significant. So
"Mars MEGDR 128 PIXEL/DEG"
is equivalent to
" mars megdr 128 pixel/deg "
but not to
"MARS MEGDR128PIXEL/DEG"
target the name of the target body.
help, target
STRING = Scalar
`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.
ilusrc the name of the illumination source.
help, ilusrc
STRING = Scalar
This source may be any ephemeris object. Case, blanks, and
numeric values are treated in the same way as for the input
`target'.
et the epoch, expressed as seconds past J2000 TDB, for which the
apparent illumination angles at the specified surface point
on the target body, as seen from the observing body, are to
be computed.
help, et
DOUBLE = Scalar
fixref the name of the body-fixed, body-centered reference frame
associated with the target body.
help, fixref
STRING = Scalar
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 the aberration correction to be used in computing the
position and orientation of the target body and the location
of the illumination source.
help, abcorr
STRING = Scalar
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 the input surface point `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 illumination source as seen by
the target, for light time. Correct the
orientation of the target for light
time.
'LT+S' Correct both the position of `spoint' as
seen by the observer, and the position
of the illumination source as seen by
the target, for light time and stellar
aberration. Correct the orientation of
the target for light time.
'CN' Converged Newtonian light time
correction. In solving the light time
equations for `spoint' and the
illumination source, 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.
The following values of `abcorr' apply to the
"transmission" case in which photons *arrive* at
`spoint' at the light-time corrected epoch et+lt and
*depart* from the observer's location at `et':
'XLT' "Transmission" case: correct for
one-way light time using a Newtonian
formulation. This correction yields the
illumination angles at the moment that
`spoint' receives photons emitted from the
observer's location at `et'.
The light time correction uses an
iterative solution of the light time
equation. The solution invoked by the
'XLT' option uses one iteration.
Both the target position as seen by the
observer, and rotation of the target
body, are corrected for light time.
'XLT+S' "Transmission" case: correct for
one-way light time and stellar
aberration using a Newtonian
formulation This option modifies the
angles obtained with the 'XLT' option
to account for the observer's and
target's velocities relative to the
solar system barycenter (the latter
velocity is used in computing the
direction to the apparent illumination
source).
'XCN' Converged Newtonian light time
correction. This is the same as XLT
correction but with further iterations
to a converged Newtonian light time
solution.
'XCN+S' "Transmission" case: converged
Newtonian light time and stellar
aberration corrections. This option
produces a solution that is at least as
accurate at that obtainable with the
'XLT+S' option. Whether the 'XCN+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 the name of the observing body.
help, obsrvr
STRING = Scalar
The observing body is an ephemeris object: it typically
is a spacecraft, an extended body, or a surface point
for which ephemeris data are available. `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 a surface point on the target body, expressed in Cartesian
coordinates, relative to the body-fixed target frame
designated by `fixref'.
help, spoint
DOUBLE = Array[3]
`spoint' need not be visible from the observer's
location at the epoch `et'.
The components of `spoint' have units of km.
the call:
cspice_illumg, method, target, ilusrc, et, fixref, abcorr, obsrvr, $
spoint, trgepc, srfvec, phase, incdnc, emissn
returns:
trgepc the "target surface point epoch."
help, trgepc
DOUBLE = Scalar
`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, et+lt or `et'
depending on whether the requested aberration correction is,
respectively, for received radiation, transmitted radiation
or omitted. `lt' is computed using the method indicated by
`abcorr'.
`trgepc' is expressed as seconds past J2000 TDB.
srfvec 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'.
help, srfvec
DOUBLE = Array[3]
`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 function norm to obtain the
distance between the observer and `spoint':
dist = norm( 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 with:
obspos = spoint - srfvec
To transform the vector `srfvec' from a reference frame
`fixref' at time `trgepc' to a time-dependent reference
frame `ref' at time `et', the routine cspice_pxfrm2 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
`srfvec' can be transformed to the result `refvec' as
follows:
cspice_pxfrm2, fixref, ref, trgepc, et, xform
refvec = xform ## srfvec
The following outputs depend on the existence of a well-defined
outward normal vector to the surface at `spoint'. See restriction 1.
phase the phase angle at `spoint', as seen from `obsrvr' at time
`et'.
help, phase
DOUBLE = Scalar
This is the angle between the negative of the vector `srfvec'
and the spoint-illumination source vector at `trgepc'. Units
are radians. The range of `phase' is [0, pi]. See
-Particulars below for a detailed discussion of the
definition.
incdnc the illumination source incidence angle at `spoint', as seen
from `obsrvr' at time `et'.
help, incdnc
DOUBLE = Scalar
This is the angle between the surface normal vector at
`spoint' and the spoint-illumination source vector at
`trgepc'. Units are radians. The range of `incdnc' is [0, pi].
See -Particulars below for a detailed discussion of the
definition.
emissn the emission angle at `spoint', as seen from `obsrvr' at time
`et'.
help, emissn
DOUBLE = Scalar
This is the angle between the surface normal vector at
`spoint' and the negative of the vector `srfvec'. Units are
radians. The range of `emissn' is [0, pi]. See -Particulars
below for a detailed discussion of the definition.
None.
Any numerical results shown for these examples may differ between
platforms as the results depend on the SPICE kernels used as input
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: illumg_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
--------- --------
de430.bsp Planetary ephemeris
mar097.bsp Mars satellite ephemeris
pck00010.tpc Planet orientation and
radii
naif0011.tls Leapseconds
mgs_ext13_ipng_mgs95j.bsp MGS ephemeris
\begindata
KERNELS_TO_LOAD = ( 'de430.bsp',
'mar097.bsp',
'pck00010.tpc',
'naif0011.tls',
'mgs_ext13_ipng_mgs95j.bsp' )
\begintext
End of meta-kernel
Example code begins here.
PRO illumg_ex1
;;
;; Load kernel files.
;;
cspice_furnsh, 'illumg_ex1.tm'
;;
;; Convert the UTC request time to ET (seconds past J2000 TDB).
;;
utc = '2004 JAN 1 12:00:00'
cspice_str2et, 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'
;; style of sub-point definition.
;;
cspice_subslr, 'near point: ellipsoid', $
target, et, 'iau_mars', $
abcorr, obsrvr, ssolpt, trgepc, srfvec
;;
;; Now find the sub-spacecraft point.
;;
cspice_subpnt, '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.
;;
cspice_illumg, 'Ellipsoid', target, 'SUN', et, 'IAU_MARS', $
abcorr, obsrvr, ssolpt, trgepc, srfvec, $
sslphs, sslsol, sslemi
;;
;; Do the same for the sub-spacecraft point.
;;
cspice_illumg, 'Ellipsoid', target, 'SUN', et, 'IAU_MARS', $
abcorr, obsrvr, sscpt, trgepc, srfvec, $
sscphs, sscsol, sscemi
;;
;; Convert the angles to degrees and write them out.
;;
sslphs = sslphs * cspice_dpr()
sslsol = sslsol * cspice_dpr()
sslemi = sslemi * cspice_dpr()
sscphs = sscphs * cspice_dpr()
sscsol = sscsol * cspice_dpr()
sscemi = sscemi * cspice_dpr()
print
print, 'UTC epoch is ', utc
print
print, 'Illumination angles at the sub-solar point:'
print
print, 'Phase angle (deg): ', sslphs
print, 'Solar incidence angle (deg): ', sslsol
print, 'Emission angle (deg): ', sslemi
print
print, 'The solar incidence angle should be 0.'
print, 'The emission and phase angles should be '
print, 'equal.'
print
print
print, 'Illumination angles at the sub-s/c point:'
print
print, 'Phase angle (deg): ', sscphs
print, 'Solar incidence angle (deg): ', sscsol
print, 'Emission angle (deg): ', sscemi
print
print, 'The emission angle should be 0.'
print, 'The solar incidence and phase angles '
print, 'should be equal.'
print
;;
;; It's always good form to unload kernels after use,
;; particularly in IDL due to data persistence.
;;
cspice_kclear
END
When this program was executed on a Mac/Intel/IDL8.x/64-bit
platform, the output was:
UTC epoch is 2004 JAN 1 12:00:00
Illumination angles at the sub-solar point:
Phase angle (deg): 115.54200
Solar incidence angle (deg): 1.2827388e-14
Emission angle (deg): 115.54200
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): 62.083999
Solar incidence angle (deg): 62.083999
Emission angle (deg): 1.2715891e-09
The emission angle should be 0.
The solar incidence and phase angles
should be equal.
2) 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 specified UTC time.
Use both an ellipsoidal Mars shape model and topographic data
provided by a DSK file. For both surface points, use the 'near
point' and 'nadir' definitions for ellipsoidal and DSK shape
models, respectively.
Use converged Newtonian light time and stellar aberration
corrections.
The topographic model is based on data from the MGS MOLA DEM
megr90n000cb, which has a resolution of 4 pixels/degree. A
triangular plate model was produced by computing a 720 x 1440
grid of interpolated heights from this DEM, then tessellating
the height grid. The plate model is stored in a type 2 segment
in the referenced DSK file.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File: illumg_ex2.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
--------- --------
de430.bsp Planetary ephemeris
mar097.bsp Mars satellite ephemeris
pck00010.tpc Planet orientation and
radii
naif0011.tls Leapseconds
mgs_ext12_ipng_mgs95j.bsp MGS ephemeris
megr90n000cb_plate.bds Plate model based on
MEGDR DEM, resolution
4 pixels/degree.
\begindata
KERNELS_TO_LOAD = ( 'de430.bsp',
'mar097.bsp',
'pck00010.tpc',
'naif0011.tls',
'mgs_ext12_ipng_mgs95j.bsp',
'megr90n000cb_plate.bds' )
\begintext
End of meta-kernel
Example code begins here.
PRO illumg_ex2
;;
;; Load kernel files.
;;
cspice_furnsh, 'illumg_ex2.tm'
;;
;; Convert the UTC request time string to seconds past
;; J2000 TDB.
;;
utc = '2003 OCT 13 06:00:00 UTC'
cspice_str2et, utc, et
;;
;; Assign observer, target, and illumination source
;; names. The acronym MGS indicates Mars Global
;; Surveyor. See NAIF_IDS for a list of names
;; recognized by SPICE.
;;
;; Also set the target body-fixed frame and
;; the aberration correction flag.
;;
target = 'Mars'
obsrvr = 'MGS'
ilusrc = 'Sun'
fixref = 'IAU_MARS'
abcorr = 'CN+S'
ilumth = ['Ellipsoid', 'DSK/Unprioritized' ]
submth = ['Near Point/Ellipsoid', 'DSK/Nadir/Unprioritized' ]
for i = 0, n_elements(ilumth)- 1L do begin
;;
;; 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.
;;
cspice_subslr, submth[i], target, et, fixref, $
abcorr, obsrvr, ssolpt, trgepc, srfvec
;;
;; Now find the sub-spacecraft point.
;;
cspice_subpnt, submth[i], target, et, fixref, $
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.
;;
cspice_illumg, ilumth[i], target, 'SUN', et, fixref, $
abcorr, obsrvr, ssolpt, trgepc, srfvec, $
sslphs, sslsol, sslemi
;;
;; Do the same for the sub-spacecraft point.
;;
cspice_illumg, ilumth[i], target, 'SUN', et, fixref, $
abcorr, obsrvr, sscpt, trgepc, srfvec, $
sscphs, sscsol, sscemi
;;
;; Convert the angles to degrees and write them out.
;;
sslphs = sslphs * cspice_dpr()
sslsol = sslsol * cspice_dpr()
sslemi = sslemi * cspice_dpr()
sscphs = sscphs * cspice_dpr()
sscsol = sscsol * cspice_dpr()
sscemi = sscemi * cspice_dpr()
print
print, ' illumg_c method: ', ilumth[i]
print, ' subpnt_c method: ', submth[i]
print, ' subslr_c method: ', submth[i]
print
print, ' Illumination angles at the ' +$
'sub-solar point:'
print
print, ' Phase angle (deg): ', sslphs
print, ' Solar incidence angle (deg): ', sslsol
print, ' Emission angle (deg): ', sslemi
print
if i eq 0 then begin
print, ' The solar incidence angle should be 0.'
print, ' The emission and phase angles should ' +$
'be equal.'
endif
print
print, ' Illumination angles at the ' +$
'sub-s/c point:'
print
print, ' Phase angle (deg): ', sscphs
print, ' Solar incidence angle (deg): ', sscsol
print, ' Emission angle (deg): ', sscemi
if i eq 0 then begin
print, ' The emission angle should be 0.'
print, ' The solar incidence and phase ' +$
'angles should be equal.'
endif
print
endfor
;;
;; It's always good form to unload kernels after use,
;; particularly in IDL due to data persistence.
;;
cspice_kclear
END
When this program was executed on a Mac/Intel/IDL8.x/64-bit
platform, the output was:
illumg_c method: Ellipsoid
subpnt_c method: Near Point/Ellipsoid
subslr_c method: Near Point/Ellipsoid
Illumination angles at the sub-solar point:
Phase angle (deg): 138.37027
Solar incidence angle (deg): 1.7127811e-14
Emission angle (deg): 138.37027
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): 101.43933
Solar incidence angle (deg): 101.43933
Emission angle (deg): 1.5147226e-09
The emission angle should be 0.
The solar incidence and phase angles should be equal.
illumg_c method: DSK/Unprioritized
subpnt_c method: DSK/Nadir/Unprioritized
subslr_c method: DSK/Nadir/Unprioritized
Illumination angles at the sub-solar point:
Phase angle (deg): 138.38707
Solar incidence angle (deg): 0.96712274
Emission angle (deg): 137.62148
Illumination angles at the sub-s/c point:
Phase angle (deg): 101.43933
Solar incidence angle (deg): 101.55599
Emission angle (deg): 0.11786116
Icy contains four routines that compute illumination angles:
cspice_illumf (same as this routine, except that illumination
and visibility flags are returned)
cspice_illumg (this routine)
cspice_ilumin (same as cspice_illumg, except that the sun is fixed
as the illumination source)
cspice_illum (deprecated)
cspice_illumf is the most capable of the set.
Illumination angles
===================
The term "illumination angles" refers to the following set of
angles:
phase angle Angle between the vectors from the
surface point to the observer and
from the surface point to the
illumination source.
incidence angle Angle between the surface normal at
the specified surface point and the
vector from the surface point to the
illumination source.
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 incidence, emission,
and phase angles are "inc.", "e.", and "phase".
*
illumination source
surface normal vector
._ _.
|\ /| illumination
\ phase / source vector
\ . . /
. .
\ ___ /
. \/ \/
_\ inc./
. / \ /
. | 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-illumination source vector
are coplanar, then phase is the sum of the incidence and emission
angles. This rarely occurs; usually
phase angle < 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 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 -- illumination source 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 illumination source as seen
from the target body at et-lt. So, a second light time
correction is used to compute the position of the
illumination source 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 body 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-illumination source vector
--------------------------------------
The target body-illumination source vector is the apparent
position of the illumination source, corrected for light
time and stellar aberration, as seen from the target body
at time et-lt.
Using DSK data
==============
DSK loading and unloading
-------------------------
DSK files providing data used by this routine are loaded by
calling cspice_furnsh and can be unloaded by calling cspice_unload or
cspice_kclear. See the documentation of cspice_furnsh for limits on
numbers of loaded DSK files.
For run-time efficiency, it's desirable to avoid frequent
loading and unloading of DSK files. When there is a reason to
use multiple versions of data for a given target body---for
example, if topographic data at varying resolutions are to be
used---the surface list can be used to select DSK data to be
used for a given computation. It is not necessary to unload
the data that are not to be used. This recommendation presumes
that DSKs containing different versions of surface data for a
given body have different surface ID codes.
DSK data priority
-----------------
A DSK coverage overlap occurs when two segments in loaded DSK
files cover part or all of the same domain---for example, a
given longitude-latitude rectangle---and when the time
intervals of the segments overlap as well.
When DSK data selection is prioritized, in case of a coverage
overlap, if the two competing segments are in different DSK
files, the segment in the DSK file loaded last takes
precedence. If the two segments are in the same file, the
segment located closer to the end of the file takes
precedence.
When DSK data selection is unprioritized, data from competing
segments are combined. For example, if two competing segments
both represent a surface as a set of triangular plates, the
union of those sets of plates is considered to represent the
surface.
Currently only unprioritized data selection is supported.
Because prioritized data selection may be the default behavior
in a later version of the routine, the UNPRIORITIZED keyword is
required in the `method' argument.
Syntax of the `method' input argument
-------------------------------------
The keywords and surface list in the `method' argument
are called "clauses." The clauses may appear in any
order, for example
'DSK/<surface list>/UNPRIORITIZED'
'DSK/UNPRIORITIZED/<surface list>'
'UNPRIORITIZED/<surface list>/DSK'
The simplest form of the `method' argument specifying use of
DSK data is one that lacks a surface list, for example:
'DSK/UNPRIORITIZED'
For applications in which all loaded DSK data for the target
body are for a single surface, and there are no competing
segments, the above string suffices. This is expected to be
the usual case.
When, for the specified target body, there are loaded DSK
files providing data for multiple surfaces for that body, the
surfaces to be used by this routine for a given call must be
specified in a surface list, unless data from all of the
surfaces are to be used together.
The surface list consists of the string
'SURFACES ='
followed by a comma-separated list of one or more surface
identifiers. The identifiers may be names or integer codes in
string format. For example, suppose we have the surface
names and corresponding ID codes shown below:
Surface Name ID code
------------ -------
"Mars MEGDR 128 PIXEL/DEG" 1
"Mars MEGDR 64 PIXEL/DEG" 2
"Mars_MRO_HIRISE" 3
If data for all of the above surfaces are loaded, then
data for surface 1 can be specified by either
"SURFACES = 1"
or
'SURFACES = "Mars MEGDR 128 PIXEL/DEG"'
Double quotes are used to delimit the surface name
because it contains blank characters.
To use data for surfaces 2 and 3 together, any
of the following surface lists could be used:
'SURFACES = 2, 3'
'SURFACES = "Mars MEGDR 64 PIXEL/DEG", 3'
'SURFACES = 2, Mars_MRO_HIRISE'
'SURFACES = "Mars MEGDR 64 PIXEL/DEG", Mars_MRO_HIRISE'
An example of a `method' argument that could be constructed
using one of the surface lists above is
'DSK/UNPRIORITIZED/SURFACES = "Mars MEGDR 64 PIXEL/DEG", 3'
Aberration corrections using DSK data
-------------------------------------
For irregularly shaped target bodies, the distance between the
observer and the nearest surface intercept need not be a
continuous function of time; hence the one-way light time
between the intercept and the observer may be discontinuous as
well. In such cases, the computed light time, which is found
using an iterative algorithm, may converge slowly or not at all.
In all cases, the light time computation will terminate, but
the result may be less accurate than expected.
Please refer to the Aberration Corrections Required Reading (abcorr.req)
for detailed information describing the nature and calculation of the
applied corrections.
1) If the specified aberration correction is relativistic or
calls for stellar aberration but not light time correction,
the error SPICE(NOTSUPPORTED) is signaled by a routine in the
call tree of this routine.
2) If the specified aberration correction is any other
unrecognized value, an error is signaled by a routine in the
call tree of this routine.
3) If any of the target, observer, or illumination source input
strings cannot be converted to an integer ID code, the error
SPICE(IDCODENOTFOUND) is signaled by a routine in the call
tree of this routine.
4) If `obsrvr' and `target' map to the same NAIF integer ID code, the
error SPICE(BODIESNOTDISTINCT) is signaled by a routine in the
call tree of this routine.
5) If the input target body-fixed frame `fixref' is not recognized,
the error SPICE(NOFRAME) is signaled by a routine in the call
tree of this routine. 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.
6) If the input frame `fixref' is not centered at the target body,
the error SPICE(INVALIDFRAME) is signaled by a routine in the
call tree of this routine.
7) If the input argument `method' is not recognized, the error
SPICE(INVALIDMETHOD) is signaled by a routine in the call tree
of this routine.
8) If insufficient ephemeris data have been loaded prior to
calling cspice_illumg, an error is 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 illumination source 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 cspice_illumg, an error
is signaled by a routine in the call tree of this routine.
10) If PCK data specifying the target body-fixed frame orientation
have not been loaded prior to calling cspice_illumg, an error is
signaled by a routine in the call tree of this routine.
11) If `method' specifies that the target surface is represented by
DSK data, and no DSK files are loaded for the specified
target, an error is signaled by a routine in the call tree
of this routine.
12) If `method' specifies that the target surface is represented by
DSK data, and data representing the portion of the surface on
which `spoint' is located are not available, an error is
signaled by a routine in the call tree of this routine.
13) If `method' specifies that the target surface is represented
by DSK data, `spoint' must lie on the target surface, not above
or below it. A small tolerance is used to allow for round-off
error in the calculation determining whether `spoint' is on the
surface.
If, in the DSK case, `spoint' is too far from the surface, an
error is signaled by a routine in the call tree of this
routine.
If the surface is represented by a triaxial ellipsoid, `spoint'
is not required to be close to the ellipsoid; however, the
results computed by this routine will be unreliable if `spoint'
is too far from the ellipsoid.
14) 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.
15) 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.
16) 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.
17) If any of the input arguments, `method', `target', `ilusrc',
`et', `fixref', `abcorr', `obsrvr' or `spoint', is undefined,
an error is signaled by the IDL error handling system.
18) If any of the input arguments, `method', `target', `ilusrc',
`et', `fixref', `abcorr', `obsrvr' or `spoint', is not of the
expected type, or it does not have the expected dimensions and
size, an error is signaled by the Icy interface.
19) If any of the output arguments, `trgepc', `srfvec', `phase',
`incdnc' or `emissn', is not a named variable, an error is
signaled by the Icy interface.
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
illumination source must be loaded. If aberration
corrections are used, the states of target, observer, and
the illumination source 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 cspice_furnsh.
- PCK data: rotation data for the target body must be
loaded. These may be provided in a text or binary PCK file.
- Shape data for the target body:
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 cspice_furnsh.
Triaxial radii are also needed if the target shape is
modeled by DSK data, but the DSK NADIR method is
selected.
DSK data:
If the target shape is modeled by DSK data, DSK files
containing topographic data for the target body must be
loaded. If a surface list is specified, data for at
least one of the listed surfaces must be loaded.
The following data may be required:
- 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 cspice_furnsh.
- Surface name-ID associations: if surface names are specified
in `method', the association of these names with their
corresponding surface ID codes must be established by
assignments of the kernel variables
NAIF_SURFACE_NAME
NAIF_SURFACE_CODE
NAIF_SURFACE_BODY
Normally these associations are made by loading a text
kernel containing the necessary assignments. An example
of such an assignment is
NAIF_SURFACE_NAME += 'Mars MEGDR 128 PIXEL/DEG'
NAIF_SURFACE_CODE += 1
NAIF_SURFACE_BODY += 499
In all cases, kernel data are normally loaded once per program
run, NOT every time this routine is called.
1) Results from this routine are not meaningful if the input
point lies on a ridge or vertex of a surface represented by
DSK data, or if for any other reason the direction of the
outward normal vector at the point is undefined.
ABCORR.REQ
DSK.REQ
FRAMES.REQ
ICY.REQ
NAIF_IDS.REQ
PCK.REQ
SPK.REQ
TIME.REQ
None.
J. Diaz del Rio (ODC Space)
M. Liukis (JPL)
E.D. Wright (JPL)
-Icy Version 1.0.1, 20-NOV-2021 (JDR)
Added -Parameters, -Exceptions, -Files, -Restrictions,
-Literature_References and -Author_and_Institution sections.
Edited the header to comply with NAIF standard. Added meta-kernel
to example 1.
Removed reference to the routine's corresponding CSPICE header from
-Abstract section.
Added arguments' type and size information in the -I/O section.
-Icy Version 1.0.0, 04-APR-2017 (ML) (EDW)
illumination angles general source
lighting angles general source
phase angle general source
incidence angle general source
emission angle general source
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