void sincpt_c ( ConstSpiceChar * method,
ConstSpiceChar * target,
SpiceDouble et,
ConstSpiceChar * fixref,
ConstSpiceChar * abcorr,
ConstSpiceChar * obsrvr,
ConstSpiceChar * dref,
ConstSpiceDouble dvec [3],
SpiceDouble spoint [3],
SpiceDouble * trgepc,
SpiceDouble srfvec [3],
SpiceBoolean * found )
Given an observer and a direction vector defining a ray, compute
the surface intercept of the ray on a target body at a specified
epoch, optionally corrected for light time and stellar
aberration.
This routine supersedes srfxpt_c.
FRAMES
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.
dref I Reference frame of ray's direction vector.
dvec I Ray's direction vector.
spoint O Surface intercept point on the target body.
trgepc O Intercept epoch.
srfvec O Vector from observer to intercept point.
found O Flag indicating whether intercept was found.
method is a short string providing parameters defining
the computation method to be used.
The only choice currently supported is
"Ellipsoid" The intercept computation uses
a triaxial ellipsoid to model
the surface of the target body.
The ellipsoid's radii must be
available in the kernel pool.
Neither case nor white space are significant in
`method'. For example, the string ' eLLipsoid ' is
valid.
target is the name of the target body. `target' is
case-insensitive, and leading and trailing blanks in
`target' are not significant. Optionally, you may
supply a string containing the integer ID code
for the object. For example both "MOON" and "301"
are legitimate strings that indicate the Moon is the
target body.
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
intercept 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 intercept point `spoint' and the observer to
intercept 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
target surface intercept point seen by the observer is
desired, normally the correction
"CN+S"
should be used. This and the other supported options
are described below. `abcorr' may be any of the
following:
"NONE" Apply no correction. Return the
geometric surface intercept point on the
target body.
Let `lt' represent the one-way light time between the
observer and the surface intercept 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
intercept 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 the surface
intercept 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 surface intercept
obtained with the "LT" option to account
for the observer's velocity relative to
the solar system barycenter. These
computations yield the apparent surface
intercept 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.
For reception-case applications involving
intercepts near the target body limb, this
option should be used
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
intercept 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
intercept 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
"XLT" option uses one iteration.
Both the target position as seen by the
observer, and rotation of the target
body, are corrected for light time.
"XLT+S" "Transmission" case: correct for
one-way light time and stellar
aberration using a Newtonian
formulation This option modifies the
intercept 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. This option produces a
solution that is at least as accurate at
that obtainable with the "XLT+S" option.
Whether the "XCN+S" solution is
substantially more accurate depends on the
geometry of the participating objects and
on the accuracy of the input data. In all
cases this routine will execute more
slowly when a converged solution is
computed.
For transmission-case applications
involving intercepts near the target body
limb, this option should be used.
Case and embedded blanks are not significant in `abcorr'.
For example, the string
"Cn + s"
is valid.
obsrvr is the name of the observing body. This is typically
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.
dref is the name of the reference frame relative to which the
ray's direction vector is expressed. This may be any
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
`dref' is case-insensitive, and leading and trailing
blanks in `dref' are not significant.
When `dref' designates a non-inertial frame, the
orientation of the frame is evaluated at an epoch
dependent on the frame's center and, if the center is
not the observer, on the selected aberration
correction. See the description of the direction
vector `dvec' for details.
dvec Ray direction vector emanating from the observer. The
intercept with the target body's surface of the ray
defined by the observer and `dvec' is sought.
`dvec' is specified relative to the reference frame
designated by `dref'.
Non-inertial reference frames are treated as follows:
if the center of the frame is at the observer's
location, the frame is evaluated at `et'. If the
frame's center is located elsewhere, then letting
`ltcent' be the one-way light time between the observer
and the central body associated with the frame, the
orientation of the frame is evaluated at et-ltcent,
et+ltcent, or `et' depending on whether the requested
aberration correction is, respectively, for received
radiation, transmitted radiation, or is omitted.
`ltcent' is computed using the method indicated by
`abcorr'.
spoint is the surface intercept point on the target body of
the ray defined by the observer and the direction
vector. If the ray intersects the target body in
multiple points, the selected intersection point is
the one closest to the observer. The output argument
`found' (see below) indicates whether an intercept was
found.
`spoint' is expressed in Cartesian coordinates,
relative to the target body-fixed frame designated by
`fixref'. The body-fixed target frame is evaluated at
the intercept 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 both light
time and stellar aberration corrections are used,
`spoint' is selected such that, when `spoint' is
corrected for light time an stellar aberration, `spoint'
lies on the ray defined by the observer's location and
`dvec'.
The components of `spoint' are given in units of km.
trgepc is the "intercept epoch." `trgepc' is defined as
follows: letting `lt' be the one-way light time between
the observer and the intercept 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.
found A logical flag indicating whether or not the ray
intersects the target. If an intersection exists
`found' will be returned as SPICETRUE If the ray misses
the target, `found' will be returned as SPICEFALSE.
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 sincpt_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 sincpt_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 sincpt_c, the error will
be diagnosed and signaled by a routine in the call tree of
this routine.
12) If the reference frame designated by `dref' is not recognized
by the SPICE frame subsystem, the error SPICE(NOFRAME)
will be signaled.
13) If the direction vector `dvec' is the zero vector, the error
SPICE(ZEROVECTOR) will be signaled.
14) 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.
15) 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 computation method is specified as
"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.
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. Similarly, the frame definition required to map
between the frame designated by `dref' and the target
body-fixed frame must be available. Typically the
definitions of frames not already built-in to SPICE are
supplied by loading a frame kernel.
- CK data: if the frame to which `dref' refers is fixed to a
spacecraft instrument or structure, at least one CK file will
be needed to permit transformation of vectors between that
frame and both the J2000 and the target body-fixed frames.
- SCLK data: if a CK file is needed, an associated SCLK
kernel is required to enable conversion between encoded SCLK
(used to time-tag CK data) and barycentric dynamical time
(TDB).
In all cases, kernel data are normally loaded once per program
run, NOT every time this routine is called.
Given a ray defined by a direction vector and the location of an
observer, sincpt_c computes the surface intercept point of the ray
on a specified target body. sincpt_c also determines the vector
from the observer to the surface intercept point.
When aberration corrections are used, this routine finds the
value of `spoint' such that, if `spoint' is regarded as an ephemeris
object, after the selected aberration corrections are applied to
the vector from the observer to `spoint', the resulting vector is
parallel to the direction vector `dvec'.
This routine computes light time corrections using light time
between the observer and the surface intercept 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
intercept 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 intercept point, in particular when the
observer-intercept point distance is much less than the
observer-target center distance. It's also valid to use stellar
aberration corrections even when the intercept point is near or
on the limb (as may occur in occultation computations using a
point target).
When comparing surface intercept 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 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) The following program computes surface intercept points on
Mars for the boresight and FOV boundary vectors of the MGS MOC
narrow angle camera. The intercepts are computed for a single
observation epoch. Light time and stellar aberration corrections
are used. For simplicity, camera distortion is ignored.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File: mgs_example2.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
mgs_moc_v20.ti MGS MOC instrument
parameters
mgs_sclkscet_00061.tsc MGS SCLK coefficients
mgs_sc_ext12.bc MGS s/c bus attitude
mgs_ext12_ipng_mgs95j.bsp MGS ephemeris
\begindata
KERNELS_TO_LOAD = ( 'de418.bsp',
'pck00008.tpc',
'naif0008.tls',
'mgs_moc_v20.ti',
'mgs_sclkscet_00061.tsc',
'mgs_sc_ext12.bc',
'mgs_ext12_ipng_mgs95j.bsp' )
\begintext
Example code begins here.
#include <stdio.h>
#include <string.h>
#include "SpiceUsr.h"
#include "SpiceZmc.h"
int main()
{
/.
Local parameters
./
#define META "mgs_example2.tm"
#define ABCLEN 20
#define LNSIZE 81
#define NAMLEN 33
#define TIMLEN 51
#define SHPLEN 81
#define NCORNR 4
/.
Local variables
./
SpiceBoolean found;
SpiceChar * abcorr = "CN+S";
SpiceChar * camera = "MGS_MOC_NA";
SpiceChar dref [NAMLEN];
SpiceChar * fixref = "IAU_MARS";
SpiceChar * method = "Ellipsoid";
SpiceChar * obsrvr = "MGS";
SpiceChar shape [ SHPLEN ];
SpiceChar * target = "Mars";
SpiceChar title [ LNSIZE ];
SpiceChar * utc = "2003 OCT 13 06:00:00 UTC";
SpiceDouble bounds [NCORNR][3];
SpiceDouble bsight [3];
SpiceDouble dist;
SpiceDouble dvec [3];
SpiceDouble et;
SpiceDouble lat;
SpiceDouble lon;
SpiceDouble radius;
SpiceDouble spoint [3];
SpiceDouble srfvec [3];
SpiceDouble trgepc;
SpiceInt camid;
SpiceInt i;
SpiceInt n;
/.
Load kernel files:
./
furnsh_c ( META );
/.
Convert the UTC request time to ET (seconds past
J2000, TDB).
./
str2et_c ( utc, &et );
/.
Get the MGS MOC Narrow angle camera (MGS_MOC_NA)
ID code. Then look up the field of view (FOV)
parameters.
./
bodn2c_c ( camera, &camid, &found );
if ( !found )
{
setmsg_c ( "Could not find ID code for "
"instrument #." );
errch_c ( "#", camera );
sigerr_c ( "SPICE(NOTRANSLATION)" );
}
/.
getfov_c will return the name of the camera-fixed frame
in the string `dref', the camera boresight vector in
the array `bsight', and the FOV corner vectors in the
array `bounds'.
./
getfov_c ( camid, NCORNR, SHPLEN, NAMLEN,
shape, dref, bsight, &n, bounds );
printf ( "\n"
"Surface Intercept Locations for Camera\n"
"FOV Boundary and Boresight Vectors\n"
"\n"
" Instrument: %s\n"
" Epoch: %s\n"
" Aberration correction: %s\n"
"\n",
camera, utc, abcorr );
/.
Now compute and display the surface intercepts for the
boresight and all of the FOV boundary vectors.
./
for ( i = 0; i <= NCORNR; i++ )
{
if ( i < NCORNR )
{
sprintf ( title, "Corner vector %ld", i );
vequ_c ( bounds[i], dvec );
}
else
{
strcpy ( title, "Boresight vector" );
vequ_c ( bsight, dvec );
}
/.
Compute the surface intercept point using
the specified aberration corrections.
./
sincpt_c ( method,
target, et, fixref, abcorr,
obsrvr, dref, dvec, spoint,
&trgepc, srfvec, &found );
if ( found )
{
/.
Compute range from observer to apparent intercept.
./
dist = vnorm_c( srfvec );
/.
Convert rectangular coordinates to planetocentric
latitude and longitude. Convert radians to degrees.
./
reclat_c ( spoint, &radius, &lon, &lat );
lon *= dpr_c ();
lat *= dpr_c ();
/.
Display the results.
./
printf ( "\n"
"%s\n", title );
sprintf ( title, " Vector in %s frame = ", dref );
printf ( "\n"
"%s\n", title );
if ( i < NCORNR )
{
printf ( " %18.10e %18.10e %18.10e\n",
bounds[i][0], bounds[i][1], bounds[i][2] );
}
else
{
printf ( " %18.10e %18.10e %18.10e\n",
bsight[0], bsight[1], bsight[2] );
}
printf ( "\n"
" Intercept:\n"
"\n"
" Radius (km) = %18.10e\n"
" Planetocentric Latitude (deg) = %18.10e\n"
" Planetocentric Longitude (deg) = %18.10e\n"
" Range (km) = %18.10e\n"
"\n",
radius, lat, lon, dist );
}
else
{
printf ( "\n"
"Intercept not found.\n"
"\n" );
}
}
return ( 0 );
}
When this program was executed on a PC/Linux/gcc platform, the
output was:
Surface Intercept Locations for Camera
FOV Boundary and Boresight Vectors
Instrument: MGS_MOC_NA
Epoch: 2003 OCT 13 06:00:00 UTC
Aberration correction: CN+S
Corner vector 0
Vector in MGS_MOC_NA frame =
1.8571383810e-06 -3.8015622659e-03 9.9999277403e-01
Intercept:
Radius (km) = 3.3849411359e+03
Planetocentric Latitude (deg) = -4.8477481924e+01
Planetocentric Longitude (deg) = -1.2347407905e+02
Range (km) = 3.8898310366e+02
Corner vector 1
Vector in MGS_MOC_NA frame =
1.8571383810e-06 3.8015622659e-03 9.9999277403e-01
Intercept:
Radius (km) = 3.3849396987e+03
Planetocentric Latitude (deg) = -4.8481636340e+01
Planetocentric Longitude (deg) = -1.2339882297e+02
Range (km) = 3.8897512130e+02
Corner vector 2
Vector in MGS_MOC_NA frame =
-1.8571383810e-06 3.8015622659e-03 9.9999277403e-01
Intercept:
Radius (km) = 3.3849396899e+03
Planetocentric Latitude (deg) = -4.8481661910e+01
Planetocentric Longitude (deg) = -1.2339882618e+02
Range (km) = 3.8897466238e+02
Corner vector 3
Vector in MGS_MOC_NA frame =
-1.8571383810e-06 -3.8015622659e-03 9.9999277403e-01
Intercept:
Radius (km) = 3.3849411271e+03
Planetocentric Latitude (deg) = -4.8477507498e+01
Planetocentric Longitude (deg) = -1.2347408220e+02
Range (km) = 3.8898264472e+02
Boresight vector
Vector in MGS_MOC_NA frame =
0.0000000000e+00 0.0000000000e+00 1.0000000000e+00
Intercept:
Radius (km) = 3.3849404102e+03
Planetocentric Latitude (deg) = -4.8479579822e+01
Planetocentric Longitude (deg) = -1.2343645396e+02
Range (km) = 3.8897573572e+02
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
A cautionary note: if aberration corrections are used, and
if `dref' is the target body-fixed frame, the epoch at which that
frame is evaluated is offset from `et' by the light time between
the observer and the *center* of the target body. This light time
normally will differ from the light time between the observer and
intercept point. Consequently the orientation of the target
body-fixed frame at `trgepc' will not match that of the target
body-fixed frame at the epoch associated with `dref'. As a result,
various derived quantities may not be as expected: for example,
`srfvec' would not be parallel to `dvec'.
In many applications the errors arising from this frame
discrepancy may be insignificant; however a safe approach is to
always use as `dref' a frame other than the target body-fixed
frame.
None.
N.J. Bachman (JPL)
-CSPICE Version 1.0.1, 06-FEB-2009 (NJB)
Typos in the Detailed Input section's description of `dref'
were corrected. 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 surface intercept point
find intersection of ray and target body surface
find intercept of ray on target body surface
Link to routine sincpt_c source file sincpt_c.c
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