void subpnt_c ( ConstSpiceChar * method,
ConstSpiceChar * target,
SpiceDouble et,
ConstSpiceChar * fixref,
ConstSpiceChar * abcorr,
ConstSpiceChar * obsrvr,
SpiceDouble spoint [3],
SpiceDouble * trgepc,
SpiceDouble srfvec [3] )
Compute the rectangular coordinates of the sub-observer point on
a target body at a specified epoch, optionally corrected for
light time and stellar aberration.
This routine supersedes subpt_c.
frame_c
NAIF_IDS
PCK
SPK
TIME
GEOMETRY
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 Aberration correction.
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.
method is a short string providing parameters defining
the computation method to be used.
The supported values of `method' are listed below.
Please note that the colon is a required delimiter;
using a blank will not work.
"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.
"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.
Neither case nor white space are significant in
`method'. For example, the string
" nearpoint:ELLIPSOID "
is valid.
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 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 the body-fixed, body-centered reference
frame associated with the target body. The output
sub-observer point `spoint' will be expressed relative
to this reference frame. The string `fixref' is
case-insensitive, and leading and trailing blanks in
`fixref' are not significant.
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.
spoint is the sub-observer point on the target body.
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; the input argument
`method' selects the 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' to a time-dependent
reference frame `ref' at `et', a sequence of two frame
transformations is required. For example, let `mfix'
and `mref' be 3x3 matrices respectively describing the
target body-fixed to J2000 frame transformation at
`trgepc' and the J2000 to (time-dependent frame) `ref'
transformation at `et', and let `xform' be the 3x3 matrix
representing the composition of `mref' with `mfix'. Then
`srfvec' can be transformed to the result `refvec' as
follows:
pxform_c ( fixref, "j2000", trgepc, mfix );
pxform_c ( "j2000", ref, et, mref );
mxm_c ( mref, mfix, xform );
mxv_c ( xform, srfvec, refvec );
The second example in the Examples header section
below presents a complete program that demonstrates
this procedure.
None.
1) If the specified aberration correction is relativistic or
calls for stellar aberration but not light time correction,
the error SPICE(NOTSUPPORTED) is signaled. If the specified
aberration correction is any other unrecognized value, the
error will be diagnosed and 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.
3) If `obsrvr' and `target' map to the same NAIF integer ID code,
the error SPICE(BODIESNOTDISTINCT) is signaled.
4) If the input target body-fixed frame `fixref' is not recognized,
the error SPICE(NOFRAME) is signaled. 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.
6) If the input argument `method' is not recognized, the error
SPICE(INVALIDMETHOD) is signaled.
7) 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.
8) If insufficient ephemeris data have been loaded prior to
calling subpnt_c, the error will be diagnosed and 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.
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 subpnt_c, the error will
be diagnosed and signaled by a routine in the call tree of
this routine.
10) The target must be an extended body: if any of the radii of
the target body are non-positive, the error will be diagnosed
and signaled by routines in the call tree of this routine.
11) If PCK data specifying the target body-fixed frame orientation
have not been loaded prior to calling subpnt_c, the error will
be diagnosed and signaled by a routine in the call tree of
this routine.
12) The error SPICE(EMPTYSTRING) is signaled if any input string
argument does not contain at least one character, since the
input string cannot be converted to a Fortran-style string in
this case.
13) The error SPICE(NULLPOINTER) is signaled if any input
string argument pointer is null.
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.
- 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.
- Further PCK data: rotation data for the target body must be
loaded. These may be provided in a text or binary PCK file.
- 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.
In all cases, kernel data are normally loaded once per program
run, NOT every time this routine is called.
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.
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.
The definition of the aberration-corrected sub-observer point is
implicit: `spoint' is defined by an equation of the form
spoint = f ( spoint )
Because of the contraction properties of both light time and
stellar aberration corrections---that is, the difference in the
corrections for two vectors is much smaller than the difference
between the vectors themselves---it's easy to solve this equation
accurately and fairly quickly.
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.
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. Perform
the computation twice, using both the "intercept" and "near
point" options. Display the location of both the Earth and the
sub-Earth point using both planetocentric and planetographic
coordinates.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
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.
\begindata
KERNELS_TO_LOAD = ( 'de418.bsp',
'pck00008.tpc',
'naif0008.tls' )
\begintext
Example code begins here.
/.
Program EX1
./
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
/.
Local parameters
./
#define META "example.tm"
/.
Local variables
./
static SpiceChar * method[2] =
{
"Intercept: ellipsoid",
"Near point: ellipsoid"
};
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 the "target surface intercept"
definition of the sub-observer point on the first loop
iteration, and use the "near point" definition on the
second.
./
for ( i = 0; i < 2; i++ )
{
subpnt_c ( method[i],
"mars", et, "iau_mars", "lt+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 (km) = %21.9f\n"
" Length of SRFVEC (km) = %21.9f\n"
" Sub-observer point altitude (km) = %21.9f\n"
" 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"
" 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",
method[i],
opgalt,
odist,
spgalt,
spglon,
opglon,
spglat,
opglat,
spclon,
opclon,
spclat,
opclat );
}
return ( 0 );
}
When this program was executed on a PC/Linux/gcc platform, the
output was:
Computation method = Intercept: ellipsoid
Observer altitude (km) = 349199089.542324722
Length of SRFVEC (km) = 349199089.579020321
Sub-observer point altitude (km) = 0.000000000
Sub-observer planetographic longitude (deg) = 199.302305055
Observer planetographic longitude (deg) = 199.302305055
Sub-observer planetographic latitude (deg) = 26.262401212
Observer planetographic latitude (deg) = 25.994936725
Sub-observer planetocentric longitude (deg) = 160.697694945
Observer planetocentric longitude (deg) = 160.697694945
Sub-observer planetocentric latitude (deg) = 25.994934146
Observer planetocentric latitude (deg) = 25.994934146
Computation method = Near point: ellipsoid
Observer altitude (km) = 349199089.542316437
Length of SRFVEC (km) = 349199089.542316437
Sub-observer point altitude (km) = -0.000000000
Sub-observer planetographic longitude (deg) = 199.302305055
Observer planetographic longitude (deg) = 199.302305055
Sub-observer planetographic latitude (deg) = 25.994936725
Observer planetographic latitude (deg) = 25.994936725
Sub-observer planetocentric longitude (deg) = 160.697694945
Observer planetocentric longitude (deg) = 160.697694945
Sub-observer planetocentric latitude (deg) = 25.729407202
Observer planetocentric latitude (deg) = 25.994934146
2) Use subpnt_c to find the sub-spacecraft point on Mars for the
Mars Reconnaissance Orbiter spacecraft (MRO) at a specified
time, using the "near point: ellipsoid" computation method.
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.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File: mro_example.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
--------- --------
de418.bsp Planetary ephemeris
pck00008.tpc Planet orientation and
radii
naif0008.tls Leapseconds
mro_psp4_ssd_mro95a.bsp MRO ephemeris
mro_v11.tf MRO frame specifications
mro_sclkscet_00022_65536.tsc MRO SCLK coefficients and
parameters
mro_sc_psp_070925_071001.bc MRO attitude
\begindata
KERNELS_TO_LOAD = ( 'de418.bsp',
'pck00008.tpc',
'naif0008.tls',
'mro_psp4_ssd_mro95a.bsp',
'mro_v11.tf',
'mro_sclkscet_00022_65536.tsc',
'mro_sc_psp_070925_071001.bc' )
\begintext
Example code begins here.
/.
Program EX2
./
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
/.
Local constants
./
#define META "mro_example.tm"
#define NCORR 2
/.
Local variables
./
SpiceBoolean found;
static SpiceChar * abcorr[NCORR] =
{
"LT+S", "CN+S"
};
static SpiceChar * hiref;
static SpiceChar * method;
SpiceDouble alt;
SpiceDouble et;
SpiceDouble lat;
SpiceDouble lon;
SpiceDouble mrovec [3];
SpiceDouble r1 [3][3];
SpiceDouble r2 [3][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;
/.
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 the
"NEAR POINT: ELLIPSOID" definition.
Compute the results using both LT+S and CN+S
aberration corrections.
./
method = "Near point: ellipsoid";
printf ( "\nComputation method = %s\n", method );
for ( i = 0; i < 2; i++ )
{
subpnt_c ( method,
"mars", et, "iau_mars", abcorr[i],
"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
compose two transformations: that from IAU_MARS to
J2000 at `trgepc', followed by the transformation from
J2000 to MRO_HIRISE_LOOK_DIRECTION at `et'.
(We could use any other inertial frame in place
of J2000; the result would be the same.)
To make code formatting a little easier, we'll store
the long MRO reference frame name in a variable:
./
hiref = "MRO_HIRISE_LOOK_DIRECTION";
pxform_c ( "iau_mars", "j2000", trgepc, r1 );
pxform_c ( "j2000", hiref, et, r2 );
mxm_c ( r2, r1, xform );
mxv_c ( xform, srfvec, mrovec );
/.
Convert 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[i],
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 ( "ellipsoid", "mars", et, "iau_mars",
abcorr[i], "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 PC/Linux/gcc platform, the
output was:
Computation method = Near point: ellipsoid
Aberration correction = LT+S
MRO-to-sub-observer vector in
MRO HIRISE look direction frame
X-component (km) = 0.286931987
Y-component (km) = -0.260417167
Z-component (km) = 253.816284981
Sub-observer point radius (km) = 3388.299078207
Planetocentric latitude (deg) = -38.799836879
Planetocentric longitude (deg) = -114.995294746
Observer altitude (km) = 253.816580760
Intercept comparison error (km) = 0.000002144
Aberration correction = CN+S
MRO-to-sub-observer vector in
MRO HIRISE look direction frame
X-component (km) = 0.286931866
Y-component (km) = -0.260417914
Z-component (km) = 253.816274506
Sub-observer point radius (km) = 3388.299078205
Planetocentric latitude (deg) = -38.799836883
Planetocentric longitude (deg) = -114.995294968
Observer altitude (km) = 253.816570285
Intercept comparison error (km) = 0.000000001
None.
None.
N.J. Bachman (JPL)
-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)
find sub-observer point on target body
find sub-spacecraft point on target body
find nearest point to observer on target body
Link to routine subpnt_c source file subpnt_c.c
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