| subslr_c |
|
Table of contents
Procedure
subslr_c ( Sub-solar point )
void subslr_c ( ConstSpiceChar * method,
ConstSpiceChar * target,
SpiceDouble et,
ConstSpiceChar * fixref,
ConstSpiceChar * abcorr,
ConstSpiceChar * obsrvr,
SpiceDouble spoint [3],
SpiceDouble * trgepc,
SpiceDouble srfvec [3] )
AbstractCompute the rectangular coordinates of the sub-solar point on a target body at a specified epoch, optionally corrected for light time and stellar aberration. The surface of the target body may be represented by a triaxial ellipsoid or by topographic data provided by DSK files. This routine supersedes subsol_c. Required_ReadingDSK FRAMES NAIF_IDS PCK SPK TIME KeywordsGEOMETRY Brief_I/OVARIABLE 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 Aberration correction. obsrvr I Name of observing body. spoint O Sub-solar point on the target body. trgepc O Sub-solar point epoch. srfvec O Vector from observer to sub-solar 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:
"NEAR POINT/ELLIPSOID"
The sub-solar point computation uses a triaxial
ellipsoid to model the surface of the target body.
The sub-solar point is defined as the nearest
point on the target relative to the sun.
The word "NADIR" may be substituted for the phrase
"NEAR POINT" in the string above.
For backwards compatibility, the older syntax
"Near point: ellipsoid"
is accepted as well.
"INTERCEPT/ELLIPSOID"
The sub-solar point computation uses a triaxial
ellipsoid to model the surface of the target body.
The sub-solar point is defined as the target
surface intercept of the line containing the sun
and the target's center.
For backwards compatibility, the older syntax
"Intercept: ellipsoid"
is accepted as well.
"NADIR/DSK/UNPRIORITIZED[/SURFACES = <surface list>]"
The sub-solar point computation uses DSK data to
model the surface of the target body. The
sub-solar point is defined as the intercept, on
the surface represented by the DSK data, of the
line containing the sun and the nearest point on
the target's reference ellipsoid. If multiple such
intercepts exist, the one closest to the sun is
selected.
Note that this definition of the sub-solar point
is not equivalent to the "nearest point on the
surface to the sun." The phrase "NEAR POINT" may
NOT be substituted for "NADIR" in the string
above.
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 escaped 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.
"INTERCEPT/DSK/UNPRIORITIZED[/SURFACES = <surface list>]"
The sub-solar point computation uses DSK data to
model the surface of the target body. The
sub-solar point is defined as the target surface
intercept of the line containing the sun and the
target's center.
If multiple such intercepts exist, the one closest
to the sun is selected.
The surface list specification is optional. The
syntax of the list is identical to that for the
NADIR option described above.
Neither case nor white space are significant in
`method', except within double-quoted strings. For
example, the string " eLLipsoid/nearpoint " is valid.
Within double-quoted strings, 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. The target body is
an ephemeris object (its trajectory is given by
SPK data), and is an extended object.
The string `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.
When the target body's surface is represented by a
tri-axial ellipsoid, this routine assumes that a
kernel variable representing the ellipsoid's radii is
present in the kernel pool. Normally the kernel
variable would be defined by loading a PCK file.
et is the epoch of participation of the observer,
expressed as ephemeris seconds past J2000 TDB: `et' is
the epoch at which the observer's state is computed.
When aberration corrections are not used, `et' is also
the epoch at which the position and orientation of
the target body and the position of the Sun are
computed.
When aberration corrections are used, `et' is the epoch
at which the observer's state relative to the solar
system barycenter is computed; in this case the
position and orientation of the target body are
computed at et-lt, where `lt' is the one-way light time
between the sub-solar point and the observer. See the
description of `abcorr' below for details.
fixref is the name of a body-fixed reference frame centered
on the target body. `fixref' may be any such frame
supported by the SPICE system, including built-in
frames (documented in the Frames Required Reading)
and frames defined by a loaded frame kernel (FK). The
string `fixref' is case-insensitive, and leading and
trailing blanks in `fixref' are not significant.
The output sub-solar point `spoint' and the
observer-to-sub-solar point vector `srfvec' will be
expressed relative to this reference frame.
abcorr indicates the aberration correction to be applied
when computing the target position and orientation
and the position of the Sun.
For remote sensing applications, where the apparent
sub-solar point seen by the observer is 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" Apply no correction. Return the
geometric sub-solar point on the target
body.
Let `lt' represent the one-way light time between the
observer and the sub-solar point (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 the sub-solar
point's location at the light-time corrected epoch
et-lt and *arrive* at the observer's location at `et':
"LT" Correct for one-way light time (also
called "planetary aberration") using a
Newtonian formulation. This correction
yields the location of sub-solar
point at the moment it emitted photons
arriving at the observer at `et'.
The light time correction uses an
iterative solution of the light time
equation. The solution invoked by the
"LT" option uses one iteration.
The target position and orientation as
seen by the observer are corrected for
light time. The position of the Sun
relative to the target is corrected for
one-way light time between the Sun and
target.
"LT+S" Correct for one-way light time and
stellar aberration using a Newtonian
formulation. This option modifies the
sub-solar point obtained with the "LT"
option to account for the observer's
velocity relative to the solar system
barycenter. These corrections yield
the apparent sub-solar point.
"CN" Converged Newtonian light time
correction. In solving the light time
equation, the "CN" correction iterates
until the solution converges. Both the
position and rotation of the target
body, and the position of the Sun, are
corrected for light time.
"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. The observing body
is an ephemeris object: it typically is 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.
The observer may coincide with the target.
Detailed_Output
spoint is the sub-solar point on the target body.
For target shapes modeled by ellipsoids, the
sub-solar point is defined either as the point on the
target body that is closest to the sun, or the target
surface intercept of the line from the sun to the
target's center.
For target shapes modeled by topographic data
provided by DSK files, the sub-solar point is defined
as the target surface intercept of the line from the
sun to either the nearest point on the reference
ellipsoid, or to the target's center. If multiple
such intercepts exist, the one closest to the sun is
selected.
The input argument `method' selects the target shape
model and sub-solar point definition to be used.
`spoint' is expressed in Cartesian coordinates,
relative to the body-fixed target frame designated by
`fixref'. The body-fixed target frame is evaluated at
the sub-solar point epoch `trgepc' (see description
below).
When aberration corrections are used, `spoint' is
computed using target body position and orientation
that have been adjusted for the corrections
applicable to `spoint' itself rather than to the target
body's center. In particular, if the stellar
aberration correction applicable to `spoint' is
represented by a shift vector S, then the light-time
corrected position of the target is shifted by S
before the sub-solar point is computed.
The components of `spoint' have units of km.
trgepc is the "sub-solar point epoch." `trgepc' is defined as
follows: letting `lt' be the one-way light time between
the observer and the sub-solar point, `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' from a reference frame
`fixref' at time `trgepc' to a time-dependent reference
frame `ref' at time `et', the routine pxfrm2_c should be
called. Let `xform' be the 3x3 matrix representing the
rotation from the reference frame `fixref' at time
`trgepc' to the reference frame `ref' at time `et'. Then
`srfvec' can be transformed to the result `refvec' as
follows:
pxfrm2_c ( fixref, ref, trgepc, et, xform );
mxv_c ( xform, srfvec, refvec );
ParametersNone. 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 transmission aberration corrections are specified, the
error SPICE(NOTSUPPORTED) is signaled by a routine in the call
tree of this routine.
3) If either the target or observer 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 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.
5) 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.
6) If the input argument `method' is not recognized, the error
SPICE(INVALIDMETHOD) is signaled by this routine, or, the
error is signaled by a routine in the call tree of this
routine.
7) If the sub-solar point type is not specified or is not
recognized, the error SPICE(INVALIDSUBTYPE) is signaled by a
routine in the call tree of this routine.
8) If insufficient ephemeris data have been loaded prior to
calling subslr_c, 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.
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 subslr_c, an error
is signaled by a routine in the call tree of this routine.
10) The target must be an extended body, and must have a shape
for which a sub-solar point can be defined.
If the target body's shape is modeled by DSK data, the shape
must be such that the specified sub-solar point definition is
applicable. For example, if the target shape is a torus, both
the NADIR and INTERCEPT definitions might be inapplicable,
depending on the relative locations of the sun and target.
11) If PCK data specifying the target body-fixed frame orientation
have not been loaded prior to calling subslr_c, 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 no DSK files are loaded for the specified
target, 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, and the ray from the observer to the sub-observer
point doesn't intersect the target body's surface, the error
SPICE(SUBPOINTNOTFOUND) is signaled by a routine in the call
tree of this routine.
14) If the surface intercept on the target body's reference
ellipsoid of the observer to target center vector cannot not
be computed, the error SPICE(DEGENERATECASE) is signaled by a
routine in the call tree of this routine. Note that this is a
very rare case.
15) If the target body is the sun, the error SPICE(INVALIDTARGET)
is signaled by a routine in the call tree of this routine.
16) 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.
17) 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.
18) 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.
19) If any of the `method', `target', `fixref', `abcorr' or
`obsrvr' input string pointers is null, the error
SPICE(NULLPOINTER) is signaled.
20) If any of the `method', `target', `fixref', `abcorr' or
`obsrvr' input strings has zero length, the error
SPICE(EMPTYSTRING) is signaled.
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 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.
- Target body orientation data: these may be provided in a text or
binary PCK file. In some cases, target body orientation may
be provided by one more more CK files. In either case, data
are made available by loading the files via furnsh_c.
- 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_c.
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_c.
- 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
- SCLK data: if the target body's orientation is provided by
CK files, an associated SCLK kernel must be loaded.
In all cases, kernel data are normally loaded once per program
run, NOT every time this routine is called.
Particulars
There are two different popular ways to define the sub-solar
point: "nearest point on target to the Sun" or "target surface
intercept of the line containing the Sun and target." These
coincide when the target is spherical and generally are distinct
otherwise.
This routine computes light time corrections using light time
between the observer and the sub-solar point, as opposed to the
center of the target. Similarly, stellar aberration corrections
done by this routine are based on the direction of the vector
from the observer to the light-time corrected sub-solar point,
not to the target center. This technique avoids errors due to the
differential between aberration corrections across the target
body. Therefore it's valid to use aberration corrections with
this routine even when the observer is very close to the
sub-solar point, in particular when the observer to sub-solar
point distance is much less than the observer to target center
distance.
When comparing sub-solar point computations with results from
sources other than SPICE, it's essential to make sure the same
geometric definitions are used.
Using DSK data
==============
DSK loading and unloading
-------------------------
DSK files providing data used by this routine are loaded by
calling furnsh_c and can be unloaded by calling unload_c or
kclear_c. See the documentation of furnsh_c 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
"NADIR/DSK/UNPRIORITIZED/<surface list>"
"DSK/NADIR/<surface list>/UNPRIORITIZED"
"UNPRIORITIZED/<surface list>/DSK/NADIR"
The simplest form of the `method' argument specifying use of
DSK data is one that lacks a surface list, for example:
"NADIR/DSK/UNPRIORITIZED"
"INTERCEPT/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 strings suffice. 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\""
Escaped 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
"NADIR/DSK/UNPRIORITIZED/SURFACES= \"Mars MEGDR 64 PIXEL/DEG\",3"
Aberration corrections
----------------------
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 sub-solar point on Mars as seen from the Earth for a
specified time.
Compute the sub-solar point using both triaxial ellipsoid
and topographic surface models. Topography data are provided by
a DSK file. For the ellipsoid model, use both the "intercept"
and "near point" sub-observer point definitions; for the DSK
case, use both the "intercept" and "nadir" definitions.
Display the locations of both the sun and the sub-solar
point relative to the center of Mars, in the IAU_MARS
body-fixed reference frame, using both planetocentric and
planetographic coordinates.
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: subslr_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
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',
'megr90n000cb_plate.bds' )
\begintext
End of meta-kernel
Example code begins here.
/.
Program subslr_ex1
./
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
/.
Local parameters
./
#define META "subslr_ex1.tm"
#define NMETH 4
/.
Local variables
./
static SpiceChar * method[NMETH] =
{
"Intercept/ellipsoid",
"Near point/ ellipsoid",
"Intercept/DSK/Unprioritized",
"Nadir/DSK/Unprioritized"
};
SpiceDouble et;
SpiceDouble f;
SpiceDouble radii [3];
SpiceDouble re;
SpiceDouble rp;
SpiceDouble spclat;
SpiceDouble spclon;
SpiceDouble spcrad;
SpiceDouble spgalt;
SpiceDouble spglat;
SpiceDouble spglon;
SpiceDouble spoint [3];
SpiceDouble srfvec [3];
SpiceDouble sunlt;
SpiceDouble sunpos [3];
SpiceDouble sunst [6];
SpiceDouble supcln;
SpiceDouble supclt;
SpiceDouble supcrd;
SpiceDouble supgal;
SpiceDouble supgln;
SpiceDouble supglt;
SpiceDouble trgepc;
SpiceInt i;
SpiceInt n;
/.
Load kernel files via the meta-kernel.
./
furnsh_c ( META );
/.
Convert the UTC request time string to seconds past
J2000, TDB.
./
str2et_c ( "2008 aug 11 00:00:00", &et );
/.
Look up the target body's radii. We'll use these to
convert Cartesian to planetographic coordinates. Use
the radii to compute the flattening coefficient of
the reference ellipsoid.
./
bodvrd_c ( "MARS", "RADII", 3, &n, radii );
/.
Let `re' and `rp' be, respectively, the equatorial and
polar radii of the target.
./
re = radii[0];
rp = radii[2];
f = ( re - rp ) / re;
/.
Compute sub-observer point using light time and stellar
aberration corrections. Use both ellipsoid and DSK
shape models, and use all of the "near point,"
"intercept," and "nadir" sub-observer point definitions.
./
for ( i = 0; i < NMETH; i++ )
{
subslr_c ( method[i],
"mars", et, "iau_mars", "cn+s",
"earth", spoint, &trgepc, srfvec );
/.
Convert the sub-observer point's rectangular coordinates
to planetographic longitude, latitude and altitude.
Convert radians to degrees.
./
recpgr_c ( "mars", spoint, re, f,
&spglon, &spglat, &spgalt );
spglon *= dpr_c();
spglat *= dpr_c();
/.
Convert the sub-solar point's rectangular coordinates to
planetocentric radius, longitude, and latitude. Convert
radians to degrees.
./
reclat_c ( spoint, &spcrad, &spclon, &spclat );
spclon *= dpr_c();
spclat *= dpr_c();
/.
Compute the Sun's apparent position relative to the
sub-solar point at `trgepc'. Add the position of the
sub-solar point relative to the target's center to
obtain the position of the sun relative to the target's
center. Express the latter position in planetographic
coordinates.
./
spkcpo_c ( "sun", trgepc, "iau_mars", "OBSERVER",
"lt+s", spoint, "mars", "iau_mars",
sunst, &sunlt );
vadd_c ( sunst, spoint, sunpos );
recpgr_c ( "mars", sunpos, re, f,
&supgln, &supglt, &supgal );
supgln *= dpr_c ();
supglt *= dpr_c ();
/.
Convert the Sun's rectangular coordinates to
planetocentric radius, longitude, and latitude.
Convert radians to degrees.
./
reclat_c ( sunpos, &supcrd, &supcln, &supclt );
supcln *= dpr_c();
supclt *= dpr_c();
/.
Write the results.
./
printf ( "\n"
" Computation method = %s\n\n"
" Sub-solar point altitude (km) = %21.9f\n"
" Sub-solar planetographic longitude (deg) = %21.9f\n"
" Sun's planetographic longitude (deg) = %21.9f\n"
" Sub-solar planetographic latitude (deg) = %21.9f\n"
" Sun's planetographic latitude (deg) = %21.9f\n"
" Sub-solar planetocentric longitude (deg) = %21.9f\n"
" Sun's planetocentric longitude (deg) = %21.9f\n"
" Sub-solar planetocentric latitude (deg) = %21.9f\n"
" Sun's planetocentric latitude (deg) = %21.9f\n"
"\n",
method[i],
spgalt,
spglon,
supgln,
spglat,
supglt,
spclon,
supcln,
spclat,
supclt );
}
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Computation method = Intercept/ellipsoid
Sub-solar point altitude (km) = 0.000000000
Sub-solar planetographic longitude (deg) = 175.810675508
Sun's planetographic longitude (deg) = 175.810675508
Sub-solar planetographic latitude (deg) = 23.668550281
Sun's planetographic latitude (deg) = 23.420823362
Sub-solar planetocentric longitude (deg) = -175.810675508
Sun's planetocentric longitude (deg) = -175.810675508
Sub-solar planetocentric latitude (deg) = 23.420819936
Sun's planetocentric latitude (deg) = 23.420819936
Computation method = Near point/ ellipsoid
Sub-solar point altitude (km) = 0.000000000
Sub-solar planetographic longitude (deg) = 175.810675408
Sun's planetographic longitude (deg) = 175.810675408
Sub-solar planetographic latitude (deg) = 23.420823362
Sun's planetographic latitude (deg) = 23.420823362
Sub-solar planetocentric longitude (deg) = -175.810675408
Sun's planetocentric longitude (deg) = -175.810675408
Sub-solar planetocentric latitude (deg) = 23.175085578
Sun's planetocentric latitude (deg) = 23.420819936
Computation method = Intercept/DSK/Unprioritized
Sub-solar point altitude (km) = -4.052254284
Sub-solar planetographic longitude (deg) = 175.810675512
Sun's planetographic longitude (deg) = 175.810675512
Sub-solar planetographic latitude (deg) = 23.668848891
Sun's planetographic latitude (deg) = 23.420823362
Sub-solar planetocentric longitude (deg) = -175.810675512
Sun's planetocentric longitude (deg) = -175.810675512
Sub-solar planetocentric latitude (deg) = 23.420819936
Sun's planetocentric latitude (deg) = 23.420819936
Computation method = Nadir/DSK/Unprioritized
Sub-solar point altitude (km) = -4.022302438
Sub-solar planetographic longitude (deg) = 175.810675412
Sun's planetographic longitude (deg) = 175.810675412
Sub-solar planetographic latitude (deg) = 23.420823362
Sun's planetographic latitude (deg) = 23.420823362
Sub-solar planetocentric longitude (deg) = -175.810675412
Sun's planetocentric longitude (deg) = -175.810675412
Sub-solar planetocentric latitude (deg) = 23.174793924
Sun's planetocentric latitude (deg) = 23.420819936
RestrictionsNone. Literature_ReferencesNone. Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) S.C. Krening (JPL) B.V. Semenov (JPL) Version
-CSPICE Version 2.0.1, 01-NOV-2021 (JDR)
Edited the header to comply with NAIF standard. Fixed
comments on example.
-CSPICE Version 2.0.0, 05-APR-2017 (NJB) (SCK) (BVS)
Fixed some header comment typos.
Updated to support use of DSKs.
-CSPICE Version 1.1.0, 08-MAY-2012 (NJB) (SCK)
Some changes to computation of the surface point-to-sun vector
were made in the underlying SPICELIB routine SUBSLR. These
changes have a small effect on the outputs of this routine.
-Exceptions removed: the observer and target are now
permitted to coincide.
The header example program was updated to reflect the new
method of computing the apparent sun location, and the set
of kernels referenced by the example meta-kernel were updated.
The display of the program's output was updated accordingly.
References to the new pxfrm2_c routine were added to the
Detailed Output section.
-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)
Index_Entriesfind sub-solar point on target body find nearest point to sun on target body |
Fri Dec 31 18:41:13 2021