| limb_pl02 |
|
Table of contents
Procedure
limb_pl02 ( Limb using DSK type 2 plate model )
void limb_pl02 ( SpiceInt handle,
ConstSpiceDLADescr * dladsc,
ConstSpiceChar * target,
SpiceDouble et,
ConstSpiceChar * fixref,
ConstSpiceChar * abcorr,
ConstSpiceChar * obsrvr,
SpiceInt npts,
SpiceDouble * trgepc,
SpiceDouble obspos [3],
SpiceDouble lmbpts [][3],
SpiceInt pltids [] )
AbstractDeprecated: This routine has been superseded by the CSPICE routine limbpt_c. This routine is supported for purposes of backward compatibility only. Compute a set of points on the limb of a specified target body, where the target body's surface is represented by a triangular plate model contained in a type 2 DSK segment. Required_ReadingNAIF_IDS PCK SPK TIME KeywordsBODY GEOMETRY MATH Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- handle I DSK handle. dladsc I DLA descriptor of target body segment. target I Target body. et I Observation epoch. fixref I Body-fixed frame associated with target. abcorr I Aberration correction. obsrvr I Observer. npts I Number of points in limb set. trgepc O Epoch associated with target center. obspos O Position of observer in body-fixed frame. lmbpts O Limb point set. pltids O DSK plate IDs of surface points. Detailed_Input
handle is the DAS file handle of a DSK file open for read
access. This kernel must contain a type 2 segment that
provides a plate model representing the entire surface
of the target body.
dladsc is the DLA descriptor of a DSK segment representing the
surface of a target body.
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.
This routine assumes that a kernel variable representing
the target'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 position 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, `et' is the epoch
at which the observer's position relative to the solar
system barycenter is computed; in this case the position
and orientation of the target body are computed at
et-lt, where `lt' is the one-way light time between the
target body's center and the observer. See the
description of `abcorr' below for details.
fixref is the name of the reference frame relative to which the
output limb points are expressed. This must a
body-centered, body-fixed frame associated with the
target. The frame's axes must be compatible with the
triaxial ellipsoidal shape model associated with the
target body (normally provide via a PCK): this routine
assumes that the first, second, and third ellipsoid radii
correspond, respectively, to the x, y, and z-axes of the
frame designated by `fixref'.
`fixref' may refer to a built-in frame (documented in
the Frames Required Reading) or a frame defined by a
loaded frame kernel (FK).
The orientation of the frame designated by `fixref' is
evaluated at epoch of participation of the target
body. See the descriptions of `et' and `abcorr' for
details.
abcorr indicates the aberration correction to be applied
when computing the observer-target position, the
orientation of the target body, and the target-
source position vector. `abcorr' may be any of
the following.
"NONE" Apply no correction. Compute the limb
points using the position of the observer
and target, and the orientation of the
target, at `et'.
Let `lt' represent the one-way light time 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 target body's center 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 limb points at
the approximate time they emitted photons
arriving at the observer at `et' (the
difference between light time to the
target center and light time to the limb
points is ignored).
The light time correction uses an
iterative solution of the light time
equation. The solution invoked by the
"LT" option uses one iteration.
The target position as seen by the
observer and the 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 position obtained
with the "LT" option to account for the
observer's velocity relative to the solar
system barycenter. The result is the
apparent limb as seen by the observer.
"CN" Converged Newtonian light time correction.
In solving the light time equation, the
"CN" correction iterates until the
solution converges. The position and
rotation of the target body are corrected
for light time.
'CN+S' Converged Newtonian light time
and stellar aberration corrections.
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
"EARTH" and "399" are legitimate strings that indicate
the Earth is the observer.
npts is the number of limb points to compute.
Detailed_Output
trgepc is the "target epoch." `trgepc' is defined as follows:
letting `lt' be the one-way light time between the
target center and observer, `trgepc' is either the
epoch et-lt or `et' depending on whether the requested
aberration correction is, respectively, for received
radiation or omitted. `lt' is computed using the
method indicated by `abcorr'.
`trgepc' is expressed as seconds past J2000 TDB.
obspos is the vector from the center of the target body at
epoch `trgepc' to the observer at epoch `et'. `obspos' is
expressed in the target body-fixed reference frame
`fixref', which is evaluated at `trgepc'.
`obspos' is returned to simplify various related
computations that would otherwise be cumbersome. For
example, the vector `xvec' from the observer to the
Ith limb point can be calculated via the call
vminus_c ( lmbpts[i], obspos, xvec );
The components of `obspos' are given in units of km.
lmbpts is an array of points on the limb of the target.
The ith point is contained in the array elements
lmbpts[i][j], j = 0, 1, 2
As described above, each limb point lies on a ray
emanating from the center of the target and passing
through a limb point on the target's reference
ellipsoid. Each limb point *on the reference ellipsoid*
is the point of tangency of a ray that emanates from the
observer. Measured in a cylindrical coordinate system
whose Z-axis is parallel to the observer-target vector,
the magnitude of the separation in longitude between the
limb points is
2*Pi / npts
The limb points are expressed in the body-fixed
reference frame designated by `fixref'; the
orientation of the frame is evaluated at `trgepc'.
Units are km.
pltids is an array of integer ID codes of the plates on which
the limb points are located. The ith plate ID
corresponds to the ith limb point. These ID codes can
be use to look up data associated with the plates, such
as the plates' vertices or outward normal vectors.
`pltids' should be declared by the caller
SpiceInt pltids [npts];
ParametersNone. Exceptions
1) If the target name `target' cannot be mapped
to a body ID code, the error SPICE(IDCODENOTFOUND)
is signaled.
2) If the observer name `obsrvr' cannot be mapped to a body ID
code, the error SPICE(IDCODENOTFOUND) is signaled.
3) If `obsrvr' and `target' map to the same NAIF integer ID codes,
the error SPICE(BODIESNOTDISTINCT) is signaled.
4) If the input frame name `fixref' cannot be mapped
to a frame ID code, the error SPICE(UNKNOWNFRAME) is
signaled.
5) If the frame designated by `fixref' is not centered
on the target, the error SPICE(INVALIDFRAME) is
signaled.
6) If the set size `npts' is not at least 1, the error
SPICE(INVALIDCOUNT) is signaled.
7) If any of the reference ellipsoid's semi-axis lengths is
non-positive, an error is signaled by a routine in the
call tree of this routine.
8) If radii for the target body are not available in the kernel
pool, an error is signaled by a routine in the call tree
of this routine.
9) If radii are available but the target body does not have three
radii, the error SPICE(INVALIDCOUNT) is signaled.
10) If any SPK look-up fails, an error is signaled by
a routine in the call tree of this routine.
11) If a DSK providing a DSK type 2 plate model has not been
loaded prior to calling llgrid_pl02, an error is signaled by a
routine in the call tree of this routine.
12) If the segment associated with the input DLA descriptor is not
of data type 2, the error SPICE(WRONGDATATYPE) is signaled.
13) If a surface point cannot be computed because the ray
corresponding to a longitude/latitude pair fails to intersect
the target surface as defined by the plate model, an error is
signaled by a routine in the call tree of this routine.
14) If the DSK segment identified by `dladsc' is not for the
body identified by `target', the error SPICE(DSKTARGETMISMATCH)
is signaled.
15) If any input string pointer is null, the error SPICE(NULLPOINTER)
is signaled.
16) If any input string has length zero, the error SPICE(EMPTYSTRING)
is signaled.
Files
Appropriate DSK, SPK, PCK, and frame kernels must be loaded by the
calling program before this routine is called.
The following data are required:
- DSK data: a DSK file containing a plate model representing the
target body's surface must be loaded. This kernel must contain
a type 2 segment that contains data for the entire surface of
the target body.
- SPK data: ephemeris data for target and observer must be
loaded. If aberration corrections are used, the states of both
objects 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: 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 target body-fixed
frame designated by `fixref', that definition must be
available in the kernel pool. Typically the definitions of
frames not already built-in to SPICE are supplied by loading
a frame kernel.
In all cases, kernel data are normally loaded once per program
run, NOT every time this routine is called.
ParticularsBoundaries of visible regions on an arbitrary surface are often complicated point sets: boundaries of mountains and craters, if present, may contribute to the overall set. To make the limb computation tractable, we simplify the problem by using a reference ellipsoid for guidance. We compute a set of limb points on the reference ellipsoid for the target body, then use those points to define the latitudes and longitudes of limb points on the surface defined by the specified triangular shape model. As such, the set of limb points found by this routine is just an approximation. Examples
The numerical results shown for this example may differ across
platforms. The results depend on the SPICE kernels used as input,
the compiler and supporting libraries, and the machine specific
arithmetic implementation.
1) Compute a set of limb points on Phobos as seen from Mars. Perform
a consistency check using the emission angle at each point,
where the emission angle is computed using both a reference
ellipsoid and the actual plate model surface and surface normal.
We expect to see an emission angle of approximately 90 degrees.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File: limb_pl02_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
--------- --------
mar097.bsp Mars satellite ephemeris
pck00010.tpc Planet orientation and
radii
naif0010.tls Leapseconds
\begindata
KERNELS_TO_LOAD = ( 'mar097.bsp',
'pck00010.tpc',
'naif0010.tls' )
\begintext
End of meta-kernel
Use the DSK kernel below to provide the plate model representation
of the surface of Phobos.
phobos_3_3.bds
Example code begins here.
/.
Program limb_pl02_ex1
./
#include <stdio.h>
#include <math.h>
#include "SpiceUsr.h"
int main()
{
/.
Prototypes
./
void limb_pl02 ( SpiceInt handle,
ConstSpiceDLADescr * dladsc,
ConstSpiceChar * target,
SpiceDouble et,
ConstSpiceChar * fixref,
ConstSpiceChar * abcorr,
ConstSpiceChar * obsrvr,
SpiceInt npts,
SpiceDouble * trgepc,
SpiceDouble obspos [3],
SpiceDouble lmbpts [][3],
SpiceInt pltids [] );
/.
Local parameters
./
#define FILSIZ 256
#define NPOINTS 3
#define NTYPES 2
#define TOL ( 1.e-12 )
#define CORLEN 15
#define TYPLEN 81
#define TIMLEN 41
/.
Local variables
./
SpiceBoolean found;
SpiceChar * abcorr = "LT+S";
SpiceChar dsk [ FILSIZ ];
SpiceChar * fixref = "IAU_PHOBOS";
SpiceChar meta [ FILSIZ ];
SpiceChar * obsrvr = "Mars";
SpiceChar * target = "Phobos";
SpiceChar * utcstr = "2007 FEB 9 00:00:00 UTC";
SpiceChar timstr [ TIMLEN];
SpiceDLADescr dladsc;
SpiceDouble emissn;
SpiceDouble obspos [3];
SpiceDouble phase;
SpiceDouble radius;
SpiceDouble solar;
SpiceDouble trgepc;
SpiceDouble et;
SpiceDouble lat;
SpiceDouble lon;
SpiceDouble lmbpts [NPOINTS][3];
SpiceInt handle;
SpiceInt i;
SpiceInt pltids [NPOINTS];
/.
Prompt for the name of a meta-kernel specifying
all of the other kernels we need. Load the
metakernel.
./
prompt_c ( "Enter meta-kernel name > ", FILSIZ, meta );
furnsh_c ( meta );
/.
Prompt for the name of the DSK to read.
./
prompt_c ( "Enter DSK name > ", FILSIZ, dsk );
/.
Open the DSK file for read access.
We use the DAS-level interface for
this function.
./
dasopr_c ( dsk, &handle );
/.
Begin a forward search through the
kernel, treating the file as a DLA.
In this example, it's a very short
search.
./
dlabfs_c ( handle, &dladsc, &found );
if ( !found )
{
/.
We arrive here only if the kernel
contains no segments. This is
unexpected, but we're prepared for it.
./
setmsg_c ( "No segments found in DSK file #.");
errch_c ( "#", dsk );
sigerr_c ( "SPICE(NODATA)" );
}
/.
If we made it this far, `dladsc' is the
DLA descriptor of the first segment.
Convert the observation time to seconds past J2000 TDB.
./
str2et_c ( utcstr, &et );
timout_c ( et,
"YYYY-MON-DD "
"HR:MN:SC.### ::TDB(TDB)",
TIMLEN,
timstr );
printf ( "\n\n"
" Observer: %s\n"
" Target: %s\n"
" Observation epoch: %s\n"
" Aberration correction: %s\n"
" Body-fixed frame: %s\n",
obsrvr,
target,
timstr,
abcorr,
fixref );
/.
Now compute grid of limb points.
./
limb_pl02 ( handle, &dladsc,
target, et, fixref, abcorr,
obsrvr, NPOINTS, &trgepc, obspos,
lmbpts, pltids );
/.
Display the limb points.
./
for ( i = 0; i < NPOINTS; i++ )
{
printf ( "\n" );
reclat_c ( lmbpts[i], &radius, &lon, &lat );
printf (
" Limb point %d:\n"
" Radius (km): %f\n"
" Planetocentric longitude (deg): %f\n"
" Planetocentric latitude (deg): %f\n"
" Plate ID: %d\n",
(int)i,
radius,
lon * dpr_c(),
lat * dpr_c(),
(int)pltids[i] );
/.
Compute the illumination angles using an ellipsoidal
representation of the target's surface. The role of
this representation is to provide an outward surface
normal.
./
illum_c ( target, et, abcorr,
obsrvr, lmbpts[i], &phase,
&solar, &emissn );
printf ( " emission angle derived using:\n"
" - an ellipsoidal\n"
" reference surface (deg): %f\n",
emissn * dpr_c() );
/.
Compute the illumination angles at the limb point
using the actual plate model surface normal.
./
illum_pl02 ( handle, &dladsc, target, et,
abcorr, obsrvr, lmbpts[i],
&phase, &solar, &emissn );
printf ( " - plate model's surface\n"
" and normal vector (deg): %f\n",
emissn * dpr_c() );
}
printf ( "\n" );
/.
Close the kernel. This isn't necessary in a stand-
alone program, but it's good practice in subroutines
because it frees program and system resources.
./
dascls_c ( handle );
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, using the meta-kernel file named limb_pl02_ex1.tm
and the DSK file named phobos_3_3.bds, the output was:
Enter meta-kernel name > limb_pl02_ex1.tm
Enter DSK name > phobos_3_3.bds
Observer: Mars
Target: Phobos
Observation epoch: 2007-FEB-09 00:01:05.184 (TDB)
Aberration correction: LT+S
Body-fixed frame: IAU_PHOBOS
Limb point 0:
Radius (km): 11.563501
Planetocentric longitude (deg): 91.739066
Planetocentric latitude (deg): -0.000811
Plate ID: 229468
emission angle derived using:
- an ellipsoidal
reference surface (deg): 90.001006
- plate model's surface
and normal vector (deg): 110.821665
Limb point 1:
Radius (km): 9.537023
Planetocentric longitude (deg): -87.847223
Planetocentric latitude (deg): 59.998792
Plate ID: 235885
emission angle derived using:
- an ellipsoidal
reference surface (deg): 89.999961
- plate model's surface
and normal vector (deg): 97.681554
Limb point 2:
Radius (km): 9.046773
Planetocentric longitude (deg): -88.051727
Planetocentric latitude (deg): -59.997991
Plate ID: 17961
emission angle derived using:
- an ellipsoidal
reference surface (deg): 89.996966
- plate model's surface
and normal vector (deg): 64.808794
Restrictions
1) The quality of the results produced by this routine depend on how
well the target body's surface is approximated by the target's
reference ellipsoid. This routine will *not* produce meaningful
results for body shapes such as "dumbbells."
Literature_ReferencesNone. Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) E.D. Wright (JPL) Version
-CSPICE Version 2.1.0, 26-OCT-2021 (JDR)
Changed the argument names "npoints", "limbpts" and "plateIDs" to
"npts", "lmbpts" and "pltids" for consistency with other routines.
Edited the header to comply with NAIF standard. Updated
example code to reformat its output.
Index lines now state that this routine is deprecated.
-CSPICE Version 2.0.0, 23-JUL-2016 (NJB) (EDW)
Added and moved failed_c calls.
Include file references have been updated. Integer output
format in the example program has been updated. Updated
example program output to reflect bug fix of 29-APR-2014.
Beta Version 3.0.0, 24-JUN-2014 (NJB) (EDW)
Added failed_c checks to ensure return before
performance of arithmetic computations using
invalid operands. Added check for target equal
to observer.
Last update was 29-APR-2014 (NJB)
Bug fix: corrected limb point selection so that
points have uniform spacing in longitude, measured
in a frame centered on the observer-target center
line.
Bug fix: added sigerr_c call in error handling
branch for invalid value of `npts'.
Bug fix: added check for body-fixed frame not
centered on target.
Bug fix: added error handling for null input string
pointers and empty strings.
Added check for mismatch between target and central
body of DSK segment.
Added test of failed_c after spkpos_c call.
Changed argument name `fixfrm' to `fixref'.
The target name to ID conversion is now performed
in-line. The short error message for a failed
conversion is not SPICE(IDCODENOTFOUND).
Header correction: the error case of an unmapped
observer name is now listed.
Beta Version 2.1.0, 14-MAY-2010 (NJB)
Updated header example. Added include statement
for header pl02.h.
Beta Version 2.0.0, 12-FEB-2010 (NJB)
Updated to include
SpiceDSK.h
Beta Version 1.0.0, 27-FEB-2007 (NJB)
Index_EntriesDEPRECATED find limb on plate model DEPRECATED find limb on triangular shape model |
Fri Dec 31 18:41:09 2021