| ilumin |
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Table of contents
Procedure
ILUMIN ( Illumination angles )
SUBROUTINE ILUMIN ( METHOD, TARGET, ET, FIXREF,
. ABCORR, OBSRVR, SPOINT, TRGEPC,
. SRFVEC, PHASE, INCDNC, EMISSN )
Abstract
Find the illumination angles (phase, solar incidence, and
emission) at a specified surface point of a target body.
This routine supersedes ILLUM.
Required_Reading
DSK
FRAMES
NAIF_IDS
PCK
SPK
TIME
Keywords
ANGLES
GEOMETRY
ILLUMINATION
Declarations
IMPLICIT NONE
CHARACTER*(*) METHOD
CHARACTER*(*) TARGET
DOUBLE PRECISION ET
CHARACTER*(*) FIXREF
CHARACTER*(*) ABCORR
CHARACTER*(*) OBSRVR
DOUBLE PRECISION SPOINT ( 3 )
DOUBLE PRECISION TRGEPC
DOUBLE PRECISION SRFVEC ( 3 )
DOUBLE PRECISION PHASE
DOUBLE PRECISION INCDNC
DOUBLE PRECISION EMISSN
Brief_I/O
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.
INCDNC O Solar incidence angle at the surface point.
EMISSN O Emission angle at the surface point.
Detailed_Input
METHOD is a short string providing parameters defining
the computation method to be used. 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 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, 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.
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 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 Sun 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 Sun 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 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.
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 is the name of the observing body. 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 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.
Detailed_Output
TRGEPC is the "target surface 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, 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 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 SPICELIB function VNORM to obtain the
distance between the observer and SPOINT:
DIST = VNORM ( 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:
CALL VSUB ( SPOINT, SRFVEC, OBSPOS )
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 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:
CALL PXFRM2 ( FIXREF, REF, TRGEPC, ET, XFORM )
CALL MXV ( XFORM, SRFVEC, REFVEC )
The following outputs depend on the existence of a well-defined
outward normal vector to the surface at SPOINT. See restriction 1.
PHASE is the phase angle at SPOINT, as seen from OBSRVR at time
ET. This is the angle between the negative of the vector
SRFVEC and the SPOINT-Sun vector at TRGEPC. Units are
radians. The range of PHASE is [0, pi]. See $Particulars
below for a detailed discussion of the definition.
INCDNC 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 at
TRGEPC. Units are radians. The range of INCDNC 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 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.
Parameters
None.
Exceptions
1) If the specified aberration correction is unrecognized, an
error is 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, an error is signaled
by a routine in the call tree of this routine.
3) If OBSRVR and TARGET map to the same NAIF integer ID code, an
error is signaled by a routine in the call tree of this
routine.
4) If the input target body-fixed frame FIXREF is not
recognized, an error 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.
5) If the input frame FIXREF is not centered at the target body,
an error is signaled by a routine in the call tree of this
routine.
6) If the input argument METHOD is not recognized, an error
is signaled by a routine in the call tree of this
routine.
7) If insufficient ephemeris data have been loaded prior to
calling ILUMIN, 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 Sun to the solar system barycenter.
8) 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, an error
is signaled by a routine in the call tree of this routine.
9) If any of the radii of the target body are non-positive, an
error is signaled by a routine in the call tree of this
routine. The target must be an extended body.
10) If PCK data specifying the target body-fixed frame orientation
have not been loaded prior to calling ILUMIN, 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.
Files
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 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 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 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.
Particulars
SPICELIB contains four routines that compute illumination angles:
ILLUMF (same as ILLUMG, except that illumination
and visibility flags are returned.)
ILLUMG (same as ILUMIN, except that the caller
specifies the illumination source.)
ILUMIN (this routine)
ILLUM (deprecated)
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 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 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 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-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.
Using DSK data
==============
DSK loading and unloading
-------------------------
DSK files providing data used by this routine are loaded by
calling FURNSH and can be unloaded by calling UNLOAD or
KCLEAR. See the documentation of 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 sets 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.
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 specified UTC time. Use
light time and stellar aberration corrections.
Use both an ellipsoidal Mars shape model and topographic data
provided by a DSK file.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File: ilumin_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_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
Example code begins here.
PROGRAM ILUMIN_EX1
IMPLICIT NONE
C
C SPICELIB functions
C
DOUBLE PRECISION DPR
C
C Local parameters
C
CHARACTER*(*) F1
PARAMETER ( F1 = '(A,F15.9)' )
CHARACTER*(*) F2
PARAMETER ( F2 = '(A)' )
CHARACTER*(*) F3
PARAMETER ( F3 = '(A,2(2X,L))' )
CHARACTER*(*) META
PARAMETER ( META = 'ilumin_ex1.tm' )
INTEGER NAMLEN
PARAMETER ( NAMLEN = 32 )
INTEGER TIMLEN
PARAMETER ( TIMLEN = 25 )
INTEGER CORLEN
PARAMETER ( CORLEN = 5 )
INTEGER MTHLEN
PARAMETER ( MTHLEN = 50 )
INTEGER NMETH
PARAMETER ( NMETH = 2 )
C
C Local variables
C
CHARACTER*(CORLEN) ABCORR
CHARACTER*(NAMLEN) FIXREF
CHARACTER*(MTHLEN) ILUMTH ( NMETH )
CHARACTER*(NAMLEN) OBSRVR
CHARACTER*(MTHLEN) SUBMTH ( NMETH )
CHARACTER*(NAMLEN) TARGET
CHARACTER*(TIMLEN) UTC
DOUBLE PRECISION ET
DOUBLE PRECISION SRFVEC ( 3 )
DOUBLE PRECISION SSCEMI
DOUBLE PRECISION SSCPHS
DOUBLE PRECISION SSCPT ( 3 )
DOUBLE PRECISION SSCSOL
DOUBLE PRECISION SSLEMI
DOUBLE PRECISION SSLPHS
DOUBLE PRECISION SSLSOL
DOUBLE PRECISION SSOLPT ( 3 )
DOUBLE PRECISION TRGEPC
INTEGER I
C
C Initial values
C
DATA ILUMTH / 'Ellipsoid',
. 'DSK/Unprioritized' /
DATA SUBMTH / 'Near Point/Ellipsoid',
. 'DSK/Nadir/Unprioritized' /
C
C Load kernel files.
C
CALL FURNSH ( META )
C
C Convert the UTC request time string to seconds past
C J2000 TDB.
C
UTC = '2003 OCT 13 06:00:00 UTC'
CALL UTC2ET ( UTC, ET )
WRITE (*,F2) ' '
WRITE (*,F2) 'UTC epoch is '//UTC
C
C Assign observer and target names. The acronym MGS
C indicates Mars Global Surveyor. See NAIF_IDS for a
C list of names recognized by SPICE. Also set the
C aberration correction flag.
C
TARGET = 'Mars'
OBSRVR = 'MGS'
FIXREF = 'IAU_MARS'
ABCORR = 'CN+S'
DO I = 1, NMETH
C
C Find the sub-solar point on Mars as
C seen from the MGS spacecraft at ET. Use the
C "near point" style of sub-point definition
C when the shape model is an ellipsoid, and use
C the "nadir" style when the shape model is
C provided by DSK data. This makes it easy to
C verify the solar incidence angle when
C the target is modeled as an ellipsoid.
C
CALL SUBSLR ( SUBMTH(I), TARGET, ET,
. FIXREF, ABCORR, OBSRVR,
. SSOLPT, TRGEPC, SRFVEC )
C
C Now find the sub-spacecraft point.
C
CALL SUBPNT ( SUBMTH(I), TARGET, ET,
. FIXREF, ABCORR, OBSRVR,
. SSCPT, TRGEPC, SRFVEC )
C
C Find the phase, solar incidence, and emission
C angles at the sub-solar point on Mars as
C seen from MGS at time ET.
C
CALL ILUMIN ( ILUMTH(I), TARGET,
. ET, FIXREF, ABCORR,
. OBSRVR, SSOLPT, TRGEPC,
. SRFVEC, SSLPHS, SSLSOL,
. SSLEMI )
C
C Do the same for the sub-spacecraft point.
C
CALL ILUMIN ( ILUMTH(I), TARGET,
. ET, FIXREF, ABCORR,
. OBSRVR, SSCPT, TRGEPC,
. SRFVEC, SSCPHS, SSCSOL,
. SSCEMI )
C
C Convert the angles to degrees and write them out.
C
SSLPHS = DPR() * SSLPHS
SSLSOL = DPR() * SSLSOL
SSLEMI = DPR() * SSLEMI
SSCPHS = DPR() * SSCPHS
SSCSOL = DPR() * SSCSOL
SSCEMI = DPR() * SSCEMI
WRITE (*,F2) ' '
WRITE (*,F2) ' ILUMIN method: '//ILUMTH(I)
WRITE (*,F2) ' SUBPNT method: '//SUBMTH(I)
WRITE (*,F2) ' SUBSLR method: '//SUBMTH(I)
WRITE (*,F2) ' '
WRITE (*,F2) ' Illumination angles at the '
. // 'sub-solar point:'
WRITE (*,F2) ' '
WRITE (*,F1) ' Phase angle (deg.): ',
. SSLPHS
WRITE (*,F1) ' Solar incidence angle (deg.): ',
. SSLSOL
WRITE (*,F1) ' Emission angle (deg.): ',
. SSLEMI
WRITE (*,F2) ' '
IF ( I .EQ. 1 ) THEN
WRITE (*,F2) ' The solar incidence angle '
. // 'should be 0.'
WRITE (*,F2) ' The emission and phase '
. // 'angles should be equal.'
WRITE (*,F2) ' '
END IF
WRITE (*,F2) ' Illumination angles at the '
. // 'sub-s/c point:'
WRITE (*,F2) ' '
WRITE (*,F1) ' Phase angle (deg.): ',
. SSCPHS
WRITE (*,F1) ' Solar incidence angle (deg.): ',
. SSCSOL
WRITE (*,F1) ' Emission angle (deg.): ',
. SSCEMI
WRITE (*,F2) ' '
IF ( I .EQ. 1 ) THEN
WRITE (*,F2) ' The emission angle '
. // 'should be 0.'
WRITE (*,F2) ' The solar incidence '
. // 'and phase angles should be equal.'
END IF
END DO
END
When this program was executed on a Mac/Intel/gfortran/64-bit
platform, the output was:
UTC epoch is 2003 OCT 13 06:00:00 UTC
ILUMIN method: Ellipsoid
SUBPNT method: Near Point/Ellipsoid
SUBSLR method: Near Point/Ellipsoid
Illumination angles at the sub-solar point:
Phase angle (deg.): 138.370270685
Solar incidence angle (deg.): 0.000000000
Emission angle (deg.): 138.370270685
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.): 101.439331040
Solar incidence angle (deg.): 101.439331041
Emission angle (deg.): 0.000000002
The emission angle should be 0.
The solar incidence and phase angles should be equal.
ILUMIN method: DSK/Unprioritized
SUBPNT method: DSK/Nadir/Unprioritized
SUBSLR method: DSK/Nadir/Unprioritized
Illumination angles at the sub-solar point:
Phase angle (deg.): 138.387071677
Solar incidence angle (deg.): 0.967122745
Emission angle (deg.): 137.621480599
Illumination angles at the sub-s/c point:
Phase angle (deg.): 101.439331359
Solar incidence angle (deg.): 101.555993667
Emission angle (deg.): 0.117861156
Restrictions
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.
Literature_References
None.
Author_and_Institution
N.J. Bachman (JPL)
J. Diaz del Rio (ODC Space)
S.C. Krening (JPL)
Version
SPICELIB Version 2.1.0, 20-NOV-2021 (NJB) (JDR)
Changed the name of the argument SOLAR to INCDNC
for consistency with other illumination angle
routines.
Edited the header to comply with NAIF standard.
SPICELIB Version 2.0.0, 04-APR-2017 (NJB)
Fixed some header comment typos.
Now supports DSK usage. No longer includes zzabcorr.inc.
String 'SUN' passed to ILLUMG has been changed to '10'.
Now supports transmission aberration corrections.
SPICELIB Version 1.2.0, 04-APR-2011 (NJB) (SCK)
The routine has been completely re-implemented:
it now calls ILLUMG.
The meta-kernel used for the header example program
has been updated. The example program outputs have
been updated as well.
References to the new PXFRM2 routine were added
to the Detailed Output section.
SPICELIB Version 1.1.0, 17-MAY-2010 (NJB)
Bug fix: ILUMIN now returns immediately if a target
radius lookup fails.
SPICELIB Version 1.0.1, 06-FEB-2009 (NJB)
Typo correction: changed FIXFRM to FIXREF in header
documentation. Meta-kernel name suffix was changed to
".tm" in header code example.
SPICELIB Version 1.0.0, 02-MAR-2008 (NJB)
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Fri Dec 31 18:36:27 2021