| subpnt_c |
|
Table of contents
Procedure
subpnt_c ( Sub-observer point )
void subpnt_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-observer 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 subpt_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 TDB seconds past J2000 TDB. fixref I Body-fixed, body-centered target body frame. abcorr I Aberration correction flag. obsrvr I Name of observing body. spoint O Sub-observer point on the target body. trgepc O Sub-observer point epoch. srfvec O Vector from observer to sub-observer 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-observer point computation uses a
triaxial ellipsoid to model the surface of the
target body. The sub-observer point is defined
as the nearest point on the target relative to
the observer.
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-observer point computation uses a
triaxial ellipsoid to model the surface of the
target body. The sub-observer point is defined
as the target surface intercept of the line
containing the observer 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-observer point computation uses DSK data
to model the surface of the target body. The
sub-observer point is defined as the intercept, on
the surface represented by the DSK data, of the
line containing the observer and the nearest point
on the target's reference ellipsoid. If multiple
such intercepts exist, the one closest to the
observer is selected.
Note that this definition of the sub-observer
point is not equivalent to the "nearest point on
the surface to the observer." 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-observer point computation uses DSK data
to model the surface of the target body. The
sub-observer point is defined as the target
surface intercept of the line containing the
observer and the target's center.
If multiple such intercepts exist, the one closest
to the observer 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 TDB 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 are computed.
When aberration corrections are used, the position
and orientation of the target body are computed at
et-lt or et+lt, where `lt' is the one-way light time
between the sub-observer point and the observer, and
the sign applied to `lt' depends on the selected
correction. 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-observer point `spoint' and the
observer-to-sub-observer point vector `srfvec' will be
expressed relative to this reference frame.
abcorr indicates the aberration corrections to be applied
when computing the target's position and orientation.
For remote sensing applications, where the apparent
sub-observer 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-observer point on the
target body.
Let `lt' represent the one-way light time between the
observer and the sub-observer 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-observer 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-observer
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.
Both the target position as seen by the
observer, and rotation of the target
body, are corrected for light time.
"LT+S" Correct for one-way light time and
stellar aberration using a Newtonian
formulation. This option modifies the
sub-observer 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-observer 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 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.
The following values of `abcorr' apply to the
"transmission" case in which photons *depart* from
the observer's location at `et' and arrive at the
sub-observer point at the light-time corrected epoch
et+lt:
"XLT" "Transmission" case: correct for
one-way light time using a Newtonian
formulation. This correction yields the
sub-observer location at the moment it
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
"LT" 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
sub-observer point obtained with the
"XLT" option to account for the
observer's velocity relative to the
solar system barycenter.
"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.
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.
Detailed_Output
spoint is the sub-observer point on the target body.
For target shapes modeled by ellipsoids, the
sub-observer point is defined either as the point on
the target body that is closest to the observer, or
the target surface intercept of the line from the
observer to the target's center.
For target shapes modeled by topographic data
provided by DSK files, the sub-observer point is
defined as the target surface intercept of the line
from the observer 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 observer is selected.
The input argument `method' selects the target shape
model and sub-observer 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-observer epoch `trgepc' (see description below).
When light time correction is used, the duration of
light travel between `spoint' to the observer is
considered to be the one way light time.
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-observer point is computed.
The components of `spoint' have units of km.
trgepc is the "sub-observer point epoch." `trgepc' is defined
as follows: letting `lt' be the one-way light time
between the observer and the sub-observer point,
`trgepc' is 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 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 );
The second example in the -Examples header section
below presents a complete program that demonstrates
this procedure.
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 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.
3) 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.
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-observer 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 the target and observer have distinct identities but are at
the same location (for example, the target is Mars and the
observer is the Mars barycenter), the error
SPICE(NOSEPARATION) is signaled by a routine in the call tree
of this routine.
9) If insufficient ephemeris data have been loaded prior to
calling subpnt_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 both observer and
target to the solar system barycenter.
10) 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 subpnt_c, an error
is signaled by a routine in the call tree of this routine.
11) The target must be an extended body, and must have a shape
for which a sub-observer point can be defined.
If the target body's shape is modeled by DSK data, the shape
must be such that the specified sub-observer 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
observer and target.
12) If PCK data specifying the target body-fixed frame orientation
have not been loaded prior to calling subpnt_c, 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 no DSK files are loaded for the specified
target, an error is signaled by a routine in the call tree
of this routine.
14) 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.
15) 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.
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 and observer must be
loaded. If aberration corrections are used, the states of
target and observer 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 assignments 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
For ellipsoidal target bodies, there are two different popular
ways to define the sub-observer point: "nearest point on the
target to the observer" or "target surface intercept of the line
containing observer and target." These coincide when the target
is spherical and generally are distinct otherwise.
For target body shapes modeled using topographic data provided by
DSK files, the "surface intercept" notion is valid, but the
"nearest point on the surface" computation is both inefficient to
execute and may fail to yield a result that is "under" the
observer in an intuitively clear way. The NADIR option for DSK
shapes instead finds the surface intercept of a ray that passes
through the nearest point on the target reference ellipsoid. For
shapes modeled using topography, there may be multiple
ray-surface intercepts; the closest one to the observer is
selected.
The NADIR definition makes sense only if the target shape is
reasonably close to the target's reference ellipsoid. If the
target is very different---the nucleus of comet
Churyumov-Gerasimenko is an example---the intercept definition
should be used.
This routine computes light time corrections using light time
between the observer and the sub-observer 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-observer 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-observer point, in particular when the
observer to sub-observer point distance is much less than the
observer to target center distance.
When comparing sub-observer 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 these examples 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-Earth point on Mars for a specified time.
Compute the sub-Earth points 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 Earth and the sub-Earth
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: subpnt_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 subpnt_ex1
./
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
/.
Local parameters
./
#define META "subpnt_ex1.tm"
#define MTHLEN 81
#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 obspos [3];
SpiceDouble odist;
SpiceDouble opclat;
SpiceDouble opclon;
SpiceDouble opcrad;
SpiceDouble opgalt;
SpiceDouble opglat;
SpiceDouble opglon;
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 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++ )
{
subpnt_c ( method[i],
"mars", et, "iau_mars", "cn+s",
"earth", spoint, &trgepc, srfvec );
/.
Compute the observer's distance from `spoint'.
./
odist = vnorm_c ( 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 sub-observer 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 observer's position relative to the center
of the target, where the center's location has been
adjusted using the aberration corrections applicable
to the sub-point. Express the observer's location in
planetographic coordinates.
./
vsub_c ( spoint, srfvec, obspos );
recpgr_c ( "mars", obspos, re, f,
&opglon, &opglat, &opgalt );
opglon *= dpr_c ();
opglat *= dpr_c ();
/.
Convert the observer's rectangular coordinates to
planetocentric radius, longitude, and latitude.
Convert radians to degrees.
./
reclat_c ( obspos, &opcrad, &opclon, &opclat );
opclon *= dpr_c();
opclat *= dpr_c();
/.
Write the results.
./
printf( "\n"
" Computation method = %s\n\n"
" Observer altitude relative to spheroid (km) = %21.9f\n"
" Length of SRFVEC (km) = %21.9f\n"
" Sub-observer point altitude (km) = %21.9f\n",
method[i],
opgalt,
odist,
spgalt );
printf( " Sub-observer planetographic longitude (deg) = %21.9f\n"
" Observer planetographic longitude (deg) = %21.9f\n"
" Sub-observer planetographic latitude (deg) = %21.9f\n"
" Observer planetographic latitude (deg) = %21.9f\n",
spglon,
opglon,
spglat,
opglat );
printf( " Sub-observer planetocentric longitude (deg) = %21.9f\n"
" Observer planetocentric longitude (deg) = %21.9f\n"
" Sub-observer planetocentric latitude (deg) = %21.9f\n"
" Observer planetocentric latitude (deg) = %21.9f\n"
"\n",
spclon,
opclon,
spclat,
opclat );
}
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Computation method = Intercept/ellipsoid
Observer altitude relative to spheroid (km) = 349199089.540947139
Length of SRFVEC (km) = 349199089.577642739
Sub-observer point altitude (km) = 0.000000000
Sub-observer planetographic longitude (deg) = 199.302305029
Observer planetographic longitude (deg) = 199.302305029
Sub-observer planetographic latitude (deg) = 26.262401237
Observer planetographic latitude (deg) = 25.994936751
Sub-observer planetocentric longitude (deg) = 160.697694971
Observer planetocentric longitude (deg) = 160.697694971
Sub-observer planetocentric latitude (deg) = 25.994934171
Observer planetocentric latitude (deg) = 25.994934171
Computation method = Near point/ellipsoid
Observer altitude relative to spheroid (km) = 349199089.540938556
Length of SRFVEC (km) = 349199089.540938556
Sub-observer point altitude (km) = -0.000000000
Sub-observer planetographic longitude (deg) = 199.302305029
Observer planetographic longitude (deg) = 199.302305029
Sub-observer planetographic latitude (deg) = 25.994936751
Observer planetographic latitude (deg) = 25.994936751
Sub-observer planetocentric longitude (deg) = 160.697694971
Observer planetocentric longitude (deg) = 160.697694971
Sub-observer planetocentric latitude (deg) = 25.729407227
Observer planetocentric latitude (deg) = 25.994934171
Computation method = Intercept/DSK/Unprioritized
Observer altitude relative to spheroid (km) = 349199089.541017234
Length of SRFVEC (km) = 349199091.785406649
Sub-observer point altitude (km) = -2.207669751
Sub-observer planetographic longitude (deg) = 199.302304999
Observer planetographic longitude (deg) = 199.302304999
Sub-observer planetographic latitude (deg) = 26.262576677
Observer planetographic latitude (deg) = 25.994936751
Sub-observer planetocentric longitude (deg) = 160.697695001
Observer planetocentric longitude (deg) = 160.697695001
Sub-observer planetocentric latitude (deg) = 25.994934171
Observer planetocentric latitude (deg) = 25.994934171
Computation method = Nadir/DSK/Unprioritized
Observer altitude relative to spheroid (km) = 349199089.541007638
Length of SRFVEC (km) = 349199091.707172275
Sub-observer point altitude (km) = -2.166164622
Sub-observer planetographic longitude (deg) = 199.302305000
Observer planetographic longitude (deg) = 199.302305000
Sub-observer planetographic latitude (deg) = 25.994936752
Observer planetographic latitude (deg) = 25.994936751
Sub-observer planetocentric longitude (deg) = 160.697695000
Observer planetocentric longitude (deg) = 160.697695000
Sub-observer planetocentric latitude (deg) = 25.729237570
Observer planetocentric latitude (deg) = 25.994934171
2) Use subpnt_c to find the sub-spacecraft point on Mars for the
Mars Reconnaissance Orbiter spacecraft (MRO) at a specified time,
using both the "Ellipsoid/Near point" computation method and an
ellipsoidal target shape, and the "DSK/Unprioritized/Nadir"
method and a DSK-based shape model.
Use both LT+S and CN+S aberration corrections to illustrate
the differences.
Convert the spacecraft to sub-observer point vector obtained from
subpnt_c into the MRO_HIRISE_LOOK_DIRECTION reference frame at
the observation time. Perform a consistency check with this
vector: compare the Mars surface intercept of the ray emanating
from the spacecraft and pointed along this vector with the
sub-observer point.
Perform the sub-observer point and surface intercept computations
using both triaxial ellipsoid and topographic surface models.
For this example, the topographic model is based on the MGS MOLA
DEM megr90n000eb, which has a resolution of 16 pixels/degree.
Eight DSKs, each covering longitude and latitude ranges of 90
degrees, were made from this data set. For the region covered by
a given DSK, a grid of approximately 1500 x 1500 interpolated
heights was produced, and this grid was tessellated using
approximately 4.5 million triangular plates, giving a total plate
count of about 36 million for the entire DSK set.
All DSKs in the set use the surface ID code 499001, so there is
no need to specify the surface ID in the `method' strings passed
to sincpt_c and subpnt_c.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: subpnt_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
mro_psp4_ssd_mro95a.bsp MRO ephemeris
mro_v11.tf MRO frame specifications
mro_sclkscet_00022_65536.tsc MRO SCLK coefficients
parameters
mro_sc_psp_070925_071001.bc MRO attitude
megr90n000eb_*_plate.bds Plate model DSKs based
on MEGDR DEM, resolution
16 pixels/degree.
\begindata
KERNELS_TO_LOAD = (
'de430.bsp',
'mar097.bsp',
'pck00010.tpc',
'naif0011.tls',
'mro_psp4_ssd_mro95a.bsp',
'mro_v11.tf',
'mro_sclkscet_00022_65536.tsc',
'mro_sc_psp_070925_071001.bc',
'megr90n000eb_LL000E00N_UR090E90N_plate.bds'
'megr90n000eb_LL000E90S_UR090E00S_plate.bds'
'megr90n000eb_LL090E00N_UR180E90N_plate.bds'
'megr90n000eb_LL090E90S_UR180E00S_plate.bds'
'megr90n000eb_LL180E00N_UR270E90N_plate.bds'
'megr90n000eb_LL180E90S_UR270E00S_plate.bds'
'megr90n000eb_LL270E00N_UR360E90N_plate.bds'
'megr90n000eb_LL270E90S_UR360E00S_plate.bds' )
\begintext
End of meta-kernel
Example code begins here.
/.
Program subpnt_ex2
./
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
/.
Local constants
./
#define META "subpnt_ex2.tm"
#define NCORR 2
#define NMETH 2
/.
Local variables
./
SpiceBoolean found;
static SpiceChar * abcorr[NCORR] =
{
"LT+S", "CN+S"
};
static SpiceChar * fixref = "IAU_MARS";
static SpiceChar * sinmth[NMETH] =
{
"Ellipsoid",
"DSK/Unprioritized"
};
static SpiceChar * submth[NMETH] =
{
"Ellipsoid/Near point",
"DSK/Unprioritized/Nadir"
};
static SpiceChar * hiref;
SpiceDouble alt;
SpiceDouble et;
SpiceDouble lat;
SpiceDouble lon;
SpiceDouble mrovec [3];
SpiceDouble radius;
SpiceDouble spoint [3];
SpiceDouble srfvec [3];
SpiceDouble trgepc;
SpiceDouble xepoch;
SpiceDouble xform [3][3];
SpiceDouble xpoint [3];
SpiceDouble xvec [3];
SpiceInt i;
SpiceInt j;
/.
Load kernel files via the meta-kernel.
./
furnsh_c ( META );
/.
Convert the TDB request time string to seconds past
J2000, TDB.
./
str2et_c ( "2007 SEP 30 00:00:00 TDB", &et );
/.
Compute the sub-spacecraft point using each method.
Compute the results using both LT+S and CN+S aberration
corrections.
./
for ( i = 0; i < NMETH; i++ )
{
printf ( "\nSub-observer point computation "
"method = %s\n", submth[i] );
for ( j = 0; j < NCORR; j++ )
{
subpnt_c ( submth[i],
"mars", et, fixref, abcorr[j],
"mro", spoint, &trgepc, srfvec );
/.
Compute the observer's altitude above `spoint'.
./
alt = vnorm_c ( srfvec );
/.
Express `srfvec' in the MRO_HIRISE_LOOK_DIRECTION
reference frame at epoch `et'. Since `srfvec' is expressed
relative to the IAU_MARS frame at `trgepc', we must
call pxfrm2_c to compute the position transformation matrix
from IAU_MARS at `trgepc' to the MRO_HIRISE_LOOK_DIRECTION
frame at time `et'.
To make code formatting a little easier, we'll store
the long MRO reference frame name in a variable:
./
hiref = "MRO_HIRISE_LOOK_DIRECTION";
pxfrm2_c ( "iau_mars", hiref, trgepc, et, xform );
mxv_c ( xform, srfvec, mrovec );
/.
Convert sub-observer point rectangular coordinates to
planetocentric latitude and longitude. Convert radians to
degrees.
./
reclat_c ( spoint, &radius, &lon, &lat );
lon *= dpr_c();
lat *= dpr_c();
/.
Write the results.
./
printf ( "\n"
" Aberration correction = %s\n\n"
" MRO-to-sub-observer vector in\n"
" MRO HIRISE look direction frame\n"
" X-component (km) = %21.9f\n"
" Y-component (km) = %21.9f\n"
" Z-component (km) = %21.9f\n"
" Sub-observer point radius (km) = %21.9f\n"
" Planetocentric latitude (deg) = %21.9f\n"
" Planetocentric longitude (deg) = %21.9f\n"
" Observer altitude (km) = %21.9f\n",
abcorr[j],
mrovec[0],
mrovec[1],
mrovec[2],
radius,
lat,
lon,
alt );
/.
Consistency check: find the surface intercept on
Mars of the ray emanating from the spacecraft and having
direction vector `mrovec' in the MRO HIRISE look direction
reference frame at `et'. Call the intercept point
`xpoint'. `xpoint' should coincide with `spoint', up to a
small round-off error.
./
sincpt_c ( sinmth[i], "mars", et, "iau_mars",
abcorr[j], "mro", hiref, mrovec,
xpoint, &xepoch, xvec, &found );
if ( !found )
{
printf ( "Bug: no intercept\n" );
}
else
{
/.
Report the distance between `xpoint' and `spoint'.
./
printf ( " Intercept comparison error (km) = "
"%21.9f\n\n",
vdist_c( xpoint, spoint ) );
}
}
}
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Sub-observer point computation method = Ellipsoid/Near point
Aberration correction = LT+S
MRO-to-sub-observer vector in
MRO HIRISE look direction frame
X-component (km) = 0.286933229
Y-component (km) = -0.260425939
Z-component (km) = 253.816326385
Sub-observer point radius (km) = 3388.299078378
Planetocentric latitude (deg) = -38.799836378
Planetocentric longitude (deg) = -114.995297227
Observer altitude (km) = 253.816622175
Intercept comparison error (km) = 0.000002144
Aberration correction = CN+S
MRO-to-sub-observer vector in
MRO HIRISE look direction frame
X-component (km) = 0.286933107
Y-component (km) = -0.260426683
Z-component (km) = 253.816315915
Sub-observer point radius (km) = 3388.299078376
Planetocentric latitude (deg) = -38.799836382
Planetocentric longitude (deg) = -114.995297449
Observer altitude (km) = 253.816611705
Intercept comparison error (km) = 0.000000001
Sub-observer point computation method = DSK/Unprioritized/Nadir
Aberration correction = LT+S
MRO-to-sub-observer vector in
MRO HIRISE look direction frame
X-component (km) = 0.282372596
Y-component (km) = -0.256289313
Z-component (km) = 249.784871247
Sub-observer point radius (km) = 3392.330239436
Planetocentric latitude (deg) = -38.800230156
Planetocentric longitude (deg) = -114.995297338
Observer altitude (km) = 249.785162334
Intercept comparison error (km) = 0.000002412
Aberration correction = CN+S
MRO-to-sub-observer vector in
MRO HIRISE look direction frame
X-component (km) = 0.282372464
Y-component (km) = -0.256290075
Z-component (km) = 249.784860121
Sub-observer point radius (km) = 3392.330239564
Planetocentric latitude (deg) = -38.800230162
Planetocentric longitude (deg) = -114.995297569
Observer altitude (km) = 249.785151209
Intercept comparison error (km) = 0.000000001
RestrictionsNone. Literature_ReferencesNone. Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) S.C. Krening (JPL) Version
-CSPICE Version 2.0.1, 01-NOV-2021 (JDR)
Edited the header to comply with NAIF standard.
Updated first example code to split printf statement in three
in order to comply with ANSI-C maximum string literal of length.
Added entries #16 and #17 in -Exceptions section.
-CSPICE Version 2.0.0, 05-APR-2017 (NJB)
Fixed a few header comment typos.
Updated to support surfaces represented by DSK data.
-CSPICE Version 1.0.2, 02-APR-2011 (NJB) (SCK)
References to the new pxfrm2_c routine were added, which
changed the Detailed Output section and the second example.
Miscellaneous, minor header comment corrections were made.
-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-observer point on target body find sub-spacecraft point on target body find nearest point to observer on target body |
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