| subpt_pl02 |
|
Table of contents
Procedure
subpt_pl02 ( Sub-observer point using DSK type 2 plate model )
void subpt_pl02 ( SpiceInt handle,
ConstSpiceDLADescr * dladsc,
ConstSpiceChar * method,
ConstSpiceChar * target,
SpiceDouble et,
ConstSpiceChar * abcorr,
ConstSpiceChar * obsrvr,
SpiceDouble spoint [3],
SpiceDouble * alt,
SpiceInt * plateID )
AbstractDeprecated: This routine has been superseded by the CSPICE routine subpnt_c. This routine is supported for purposes of backward compatibility only. Compute the rectangular coordinates of the sub-observer point on a target body at a particular epoch, optionally corrected for light time and stellar aberration. The target body's surface is represented by a triangular plate model contained in a type 2 DSK segment. Return the sub-observer point's coordinates expressed in the body-fixed frame associated with the target body. Also, return the observer's altitude above the target body. Required_ReadingFRAMES PCK SPK TIME KeywordsGEOMETRY Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- handle I DSK handle. dladsc I DLA descriptor of target body segment. method I Computation method. target I Name of target body. et I Epoch in ephemeris seconds past J2000 TDB. abcorr I Aberration correction. obsrvr I Name of observing body. spoint O Sub-observer point on the target body. alt O Altitude of the observer above the target body. plateID O DSK plate ID of sub-point. 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 the target body.
method is a short string specifying the computation method
to be used. The choices are:
"Intercept" The sub-observer point is defined
as the plate model surface
intercept of the ray starting at
the observer and passing through
the target's center.
"Ellipsoid
near point" The sub-observer point is defined
as the plate model surface
intercept of the ray starting at
the observer and passing through
the nearest point to the observer
on a reference ellipsoid
associated with the target body.
This option requires that the
reference ellipsoid's radii be
available in the kernel pool.
For both computation methods, this routine finds a
sub-point on the same side of the target body as the
observer. If the observer is inside the target body,
the "sub-point" will actually be above the observer.
In the case of multiple intercepts, the outermost one
(that is, the one farthest from the target center) is
selected.
Neither case nor white space are significant in the
string "method". For example, the string
" ellipsoidNEARPOINT"
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.
This routine assumes that the target body's surface is
represented by a plate model, and that a DSK file
containing the plate model has been loaded via dasopr_c.
et is the epoch, represented as seconds past J2000 TDB, at
which the sub-observer point on the target body is to be
computed. When aberration corrections are used, `et'
refers to the epoch at which radiation is received at
the observer.
abcorr indicates the aberration corrections to be applied to
the position and orientation of the target body to
account for one-way light time and stellar aberration.
See the discussion in the Particulars section for
recommendations on how to choose aberration corrections.
`abcorr' may be any of the following:
"NONE" Apply no correction. Use the geometric
position of the target body relative to
the observer; evaluate the target body's
orientation at `et'.
The following values of `abcorr' apply to the
"reception" case in which photons depart from the
target'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
uses the position and orientation of the
target 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 (see Particulars for details).
The solution invoked by the "LT" option
uses one iteration.
"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
sub-observer point computed using the
apparent position and orientation of the
target 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 (three
iterations on all supported platforms).
The "CN" correction typically does not
substantially improve accuracy because
the errors made by ignoring
relativistic effects may be larger than
the improvement afforded by obtaining
convergence of the light time solution.
The "CN" correction computation also
requires a significantly greater number
of CPU cycles than does the
one-iteration light time correction.
"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.
Detailed_Output
spoint is the sub-observer point on the target body expressed
relative to the body-fixed reference frame of the target
body.
The definition of sub-observer point depends on the
selected computation method. See the description of the
input argument `method' for details.
The target body-fixed frame, which is time-dependent, is
evaluated at `et' if `abcorr' is "NONE"; otherwise the
frame is evaluated at et-lt, where `lt' is the one-way
light time from target to observer.
The position and orientation of the target body are
corrected for aberration as specified by `abcorr'; the
corrected position and orientation are used in the
computation of `spoint'.
alt is the signed distance between the observer and the
sub-point. When the observer is outside the body
`alt' is positive; when the observer is inside, `alt'
is negative.
Note that `alt' is not truly an `altitude' unless the
observer-to-sub-point vector happens to be perpendicular
to the target body's surface at the sub-point. In
general this condition should not be expected to hold,
unless the plate model representation of the target
body's surface very nearly matches the target body's
reference ellipsoid and the "ellipsoid near point"
computation method is selected.
plateID is the integer ID code of the plate on which the
sub-observer point is located. This ID code can be
use to look up data associated with the plate, such
as the plate's vertices or outward normal vector.
ParametersNone. Exceptions
If any of the listed errors occur, the output arguments are
left unchanged.
1) If the input argument `method' is not recognized, the error
SPICE(DUBIOUSMETHOD) is signaled.
2) If either of the input body names `target' or `obsrvr' cannot be
mapped to NAIF integer codes, 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 frame definition data enabling the evaluation of the state
of the target relative to the observer in the target
body-fixed frame have not been loaded prior to calling
subpt_pl02, an error is signaled by a routine in the call tree
of this routine.
5) If the specified aberration correction is not recognized, an
error is signaled by a routine in the call tree of this
routine.
6) If insufficient ephemeris data have been loaded prior to
calling subpt_pl02, an error is signaled by a
routine in the call tree of this routine.
7) If a DSK providing a DSK type 2 plate model has not been
loaded prior to calling subpt_pl02, an error is signaled by a
routine in the call tree of this routine.
8) If the computation method is "ellipsoid near point" and radii
of the target body have not been loaded into the kernel pool,
an error is signaled by a routine in the call tree of this
routine.
9) If PCK data supplying a rotation model for the target body
have not been loaded prior to calling subpt_pl02, an error is
signaled by a routine in the call tree of this routine.
10) If the segment associated with the input DLA descriptor does not
contain data for the designated target, the error
SPICE(TARGETMISMATCH) is signaled.
11) If the segment associated with the input DLA descriptor is not
of data type 2, the error SPICE(WRONGDATATYPE) is signaled.
12) If the sub-point cannot be computed because the ray from the
observer to the aim point designated by `method' fails to
intersect the target surface as defined by the plate model,
the error SPICE(NOINTERCEPT) is signaled.
13) Use of transmission-style aberration corrections is not
permitted. If abcorr specified such a correction, the
error SPICE(NOTSUPPORTED) is signaled.
14) If any input string pointer is null, the error SPICE(NULLPOINTER)
is signaled.
15) If any input string has length zero, the error SPICE(EMPTYSTRING)
is signaled.
Files
Appropriate DSK, SPK, PCK, and frame data must be available to
the calling program before this routine is called. Typically
the data are made available by loading kernels; however the
data may be supplied via subroutine interfaces if applicable.
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
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: triaxial radii for the target body must be loaded
into the kernel pool if the "Ellipsoid Near Point" method is
selected. Typically these data are made available 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.
Either type of file may be loaded via furnsh_c.
- 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.
Particulars
subpt_pl02 computes the sub-observer point (abbreviated as
"sub-point") on a target body. subpt_pl02 also determines the
distance from the observer to the sub-observer point.
Sub-point Definitions
=====================
This routine offers two ways of defining the sub-point:
- The "intercept" method. In general, this definition
calls for defining a ray emanating from the observer and
passing through the center of the target body. The intercept
on the first plate (the one closest to the observer) hit by this
ray is the sub-point.
The details of this definition are a bit more complex, because
this routine handles the case where the observer is inside the
target. In such cases, the sub-point is actually the point
that would be obtained by scaling up the target center-
observer vector so as to place the observer outside the target,
then computing the sub-point in the usual way. This handling
of the interior observer case prevents an observer location
that is slightly below the surface from accidentally being
associated with a sub-point on the opposite side of the target.
However, the possibility that the "sub-point" may be "above"
the observer may seem counterintuitive.
- The "ellipsoid near point" method. When a target's surface is
modeled by a set of triangular plates, the notion of "dropping
a perpendicular segment to the surface," which makes sense
for convex surfaces, becomes problematic: there need not be
any plate whose normal vector is parallel to a segment from
the observer to some point on that plate, or there could be
more than one such plate. If such a plate exists, it might
be located anywhere on the visible surface---not necessarily
"below" the observer.
To work around these problems, the ellipsoid near point method
uses a reference ellipsoid to define a preliminary sub-point:
for an exterior observer, this is the unique point on the
ellipsoid's surface at which the outward surface normal points
toward the observer. Then the plate model sub-point is defined
as the plate intercept closest to the observer of a ray
emanating from the observer and passing through the preliminary
sub-point on the ellipsoid.
For a large target such as Mars, or for any target whose reference
ellipsoid deviates significantly from spherical, the results
obtained using the two sub-point definitions can be quite different.
The example program provided below demonstrates this fact; Phobos is
the target body in this case. Some analysis on the user's part will
be needed to select the "best" definition for a given application.
When comparing sub-point computations with results from sources
other than SPICE, it's essential to make sure the same geometric
definitions are used.
Aberration Corrections
======================
Below, we indicate the aberration corrections to use for some
common applications:
1) Compute the sub-observer point using the apparent direction
and orientation of a target. This is the most common case for
a remote-sensing observation. When the observer's altitude
is more than one target radius above the surface:
Use "LT+S": apply both light time and stellar
aberration corrections.
Note that when the observer is close to the target surface,
this choice may yield inaccurate results, since light time is
measured between the observer and the target center. When the
observer has altitude of less than one target radius above the
surface, aberration corrections should be omitted, so in this
case abcorr should be set to:
"NONE"
2) Use a geometric position vector and uncorrected target
orientation as low-accuracy estimates for an application where
execution speed is critical.
Use "NONE".
See the header of the CSPICE routine spkezr_c for a detailed
discussion of aberration corrections.
Examples
The numerical results shown for this example may differ across
platforms. The results depend on the SPICE kernels used as input,
the compiler and supporting libraries, and the machine specific
arithmetic implementation.
1) Find the sub-observer point on Phobos as seen from Earth for a
specified sequence of times, using a DSK file to provide
topographic data. Perform the computation twice,
using both the "intercept" and "ellipsoid near point"
options. Compute the corresponding sub-observer point values
using an ellipsoidal surface for comparison.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File: subpt_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 subpt_pl02_ex1
./
#include <stdio.h>
#include <math.h>
#include "SpiceUsr.h"
int main()
{
/.
Local parameters
./
#define FILSIZ 256
#define NCORR 2
#define NSAMP 3
#define NMETHOD 2
#define TOL ( 1.e-12 )
#define CORLEN 15
#define METHLEN 81
#define TIMLEN 41
/.
Local variables
./
SpiceBoolean found;
SpiceChar * abcorr;
SpiceChar * abcorrs[ NCORR ] =
{
"NONE",
"LT+S"
};
SpiceChar dsk [ FILSIZ ];
SpiceChar * emethod;
SpiceChar * emethods[ NMETHOD ] =
{
"Intercept",
"Near point"
};
SpiceChar meta [ FILSIZ ];
SpiceChar * method;
SpiceChar * methods [ NMETHOD ] =
{
"Intercept",
"Ellipsoid near point"
};
SpiceChar * obsrvr = "Earth";
SpiceChar * target = "Phobos";
SpiceChar timstr [ TIMLEN ];
SpiceDLADescr dladsc;
SpiceDouble alt;
SpiceDouble elat;
SpiceDouble elon;
SpiceDouble erad;
SpiceDouble et0;
SpiceDouble et;
SpiceDouble f;
SpiceDouble radii [3];
SpiceDouble re;
SpiceDouble rp;
SpiceDouble stepsize ;
SpiceDouble xlat;
SpiceDouble xlon;
SpiceDouble xpt [3];
SpiceDouble xr;
SpiceInt coridx;
SpiceInt handle;
SpiceInt i;
SpiceInt midx;
SpiceInt n;
SpiceInt plid;
/.
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 );
/.
Look up radii of the target; compute flattening
coefficient.
./
bodvrd_c ( target, "RADII", 3, &n, radii );
re = radii[0];
rp = radii[2];
f = ( re - rp ) / re;
/.
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.
Now compute sub-points using both computation
methods. We'll vary the aberration corrections
and the epochs.
./
et0 = 0.0;
stepsize = 1.e6;
for ( i = 0; i < NSAMP; i++ )
{
/.
Set the computation time for the ith
sample.
./
et = et0 + i*stepsize;
timout_c ( et,
"YYYY-MON-DD "
"HR:MN:SC.### ::TDB(TDB)",
TIMLEN,
timstr );
printf ( "\n\nObservation epoch: %s\n",
timstr );
for ( coridx = 0; coridx < NCORR; coridx++ )
{
/.
Select the aberration correction.
./
abcorr = abcorrs[coridx];
printf ( "\n"
" abcorr = %s\n", abcorr );
for ( midx = 0; midx < NMETHOD; midx++ )
{
/.
Select the computation method.
./
method = methods [midx];
emethod = emethods[midx];
printf ( "\n"
" Method = %s\n", method );
/.
Compute the sub-observer point using a plate model
representation of the target's surface.
./
subpt_pl02 ( handle, &dladsc, method,
target, et, abcorr,
obsrvr, xpt, &alt, &plid );
/.
Represent the intercept in latitudinal
coordinates.
./
reclat_c ( xpt, &xr, &xlon, &xlat );
printf (
" Sub-point on plate model surface:\n"
" Planetocentric Longitude (deg): %f\n"
" Planetocentric Latitude (deg): %f\n"
" Radius (km): %f\n"
" Observer altitude (km): %f\n"
" ID of intercept plate : %d\n",
xlon * dpr_c(),
xlat * dpr_c(),
xr,
alt,
(int)plid );
/.
Compute the sub-observer point using an ellipsoidal
representation of the target's surface.
./
subpt_c ( emethod, target, et, abcorr,
obsrvr, xpt, &alt );
/.
Represent the intercept in latitudinal
coordinates.
./
reclat_c ( xpt, &erad, &elon, &elat );
printf (
" Sub-point on ellipsoid surface:\n"
" Planetocentric Longitude (deg): %f\n"
" Planetocentric Latitude (deg): %f\n"
" Radius (km): %f\n"
" Observer altitude (km): %f\n",
elon * dpr_c(),
elat * dpr_c(),
erad,
alt );
}
}
}
/.
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 subpt_pl02_ex1.tm
and the DSK file named phobos_3_3.bds, the output was:
Enter meta-kernel name > subpt_pl02_ex1.tm
Enter DSK name > phobos_3_3.bds
Observation epoch: 2000-JAN-01 12:00:00.000 (TDB)
abcorr = NONE
Method = Intercept
Sub-point on plate model surface:
Planetocentric Longitude (deg): 68.184433
Planetocentric Latitude (deg): -22.013938
Radius (km): 10.904558
Observer altitude (km): 276700025.580291
ID of intercept plate : 154969
Sub-point on ellipsoid surface:
Planetocentric Longitude (deg): 68.184433
Planetocentric Latitude (deg): -22.013938
Radius (km): 11.111635
Observer altitude (km): 276700025.373214
Method = Ellipsoid near point
Sub-point on plate model surface:
Planetocentric Longitude (deg): 62.358996
Planetocentric Latitude (deg): -13.611694
Radius (km): 11.189797
Observer altitude (km): 276700025.467226
ID of intercept plate : 182987
Sub-point on ellipsoid surface:
Planetocentric Longitude (deg): 62.501544
Planetocentric Latitude (deg): -13.828255
Radius (km): 11.480112
Observer altitude (km): 276700025.172491
abcorr = LT+S
Method = Intercept
Sub-point on plate model surface:
Planetocentric Longitude (deg): 80.397650
Planetocentric Latitude (deg): -22.012145
Radius (km): 11.102641
Observer altitude (km): 276710248.044155
ID of intercept plate : 161027
Sub-point on ellipsoid surface:
Planetocentric Longitude (deg): 80.397650
Planetocentric Latitude (deg): -22.012145
Radius (km): 10.997901
Observer altitude (km): 276710248.148895
Method = Ellipsoid near point
Sub-point on plate model surface:
Planetocentric Longitude (deg): 77.596717
Planetocentric Latitude (deg): -14.326159
Radius (km): 11.278719
Observer altitude (km): 276710247.981509
ID of intercept plate : 184540
Sub-point on ellipsoid surface:
Planetocentric Longitude (deg): 77.592522
Planetocentric Latitude (deg): -14.314136
Radius (km): 11.261264
Observer altitude (km): 276710247.999143
Observation epoch: 2000-JAN-13 01:46:40.000 (TDB)
abcorr = NONE
Method = Intercept
Sub-point on plate model surface:
Planetocentric Longitude (deg): -28.168808
Planetocentric Latitude (deg): -23.838824
Radius (km): 12.939782
Observer altitude (km): 286106844.028053
ID of intercept plate : 105656
Sub-point on ellipsoid surface:
Planetocentric Longitude (deg): -28.168808
Planetocentric Latitude (deg): -23.838824
Radius (km): 11.740687
Observer altitude (km): 286106845.227148
Method = Ellipsoid near point
Sub-point on plate model surface:
Planetocentric Longitude (deg): -22.941997
Planetocentric Latitude (deg): -13.930327
Radius (km): 13.832136
Observer altitude (km): 286106843.393083
ID of intercept plate : 135034
Sub-point on ellipsoid surface:
Planetocentric Longitude (deg): -22.381480
Planetocentric Latitude (deg): -12.794856
Radius (km): 12.437687
Observer altitude (km): 286106844.817034
abcorr = LT+S
Method = Intercept
Sub-point on plate model surface:
Planetocentric Longitude (deg): -15.758498
Planetocentric Latitude (deg): -23.837297
Radius (km): 13.159728
[...]
Warning: incomplete output. Only 100 out of 179 lines have been
provided.
Restrictions
1) This routine assumes that the origin of the body-fixed reference
frame associated with the target body is located in the interior
of that body.
Literature_ReferencesNone. Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) Version
-CSPICE Version 2.1.0, 26-OCT-2021 (JDR) (NJB)
Bug fix: call to ljust_ is now followed by call to F2C_ConvertStr.
Edited the Examples section to comply with NAIF standard.
Index lines now state that this routine is deprecated.
-CSPICE Version 2.0.0, 23-JUL-2016 (NJB)
Bug fix: the DSK segment's surface ID code is no longer
required to match that of the target. The segment's
center ID must match.
Added failed_c calls.
Include file references have been updated. Now calls zzdsksgr_
instead of dsksgr_. Updated integer output format in example
program.
-Beta Version 1.3.0, 30-APR-2014 (NJB) (BVS)
Adding missing "return" statement after chkout_c call
in branch for bad input method string.
Now includes dsk_proto.h.
Last update was 07-APR-2014 (BVS)
Changed FRAME to FRAMES in the Required_Reading section.
-Beta Version 1.2.0, 14-MAY-2010 (NJB)
Updated for compatibility with new DSK design.
-Beta Version 1.1.0, 09-FEB-2007 (NJB)
Bug fix: type of local variable fDLADescr was changed to SpiceInt.
-Beta Version 1.0.0, 06-NOV-2006 (NJB)
Index_EntriesDEPRECATED sub-observer point |
Fri Dec 31 18:41:13 2021