| subsol_c |
|
Table of contents
Procedure
subsol_c ( Sub-solar point )
void subsol_c ( ConstSpiceChar * method,
ConstSpiceChar * target,
SpiceDouble et,
ConstSpiceChar * abcorr,
ConstSpiceChar * obsrvr,
SpiceDouble spoint[3] )
AbstractDeprecated: This routine has been superseded by the CSPICE routine subslr_c. This routine is supported for purposes of backward compatibility only. Determine the coordinates of the sub-solar point on a target body as seen by a specified observer at a specified epoch, optionally corrected for planetary (light time) and stellar aberration. Required_ReadingFRAMES PCK SPK TIME KeywordsGEOMETRY Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- 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-solar point on the target body. Detailed_Input
method is a short string specifying the computation method
to be used. The choices are:
"Near point" The sub-solar point is defined
as the nearest point on the
target to the sun.
"Intercept" The sub-observer point is defined
as the target surface intercept of
the line containing the target's
center and the sun's center.
In both cases, the intercept computation treats the
surface of the target body as a triaxial ellipsoid.
The ellipsoid's radii must be available in the kernel
pool.
Neither case nor white space are significant in
method. For example, the string " NEARPOINT" 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 is modeled by
a tri-axial ellipsoid, and that a PCK file containing
its radii has been loaded into the kernel pool via
furnsh_c.
et is the epoch in ephemeris seconds past J2000 at which
the sub-solar point on the target body is to be
computed.
abcorr indicates the aberration corrections to be applied
when computing the observer-target state. abcorr
may be any of the following.
"NONE" Apply no correction. Return the
geometric sub-solar point on the target
body.
"LT" Correct for planetary (light time)
aberration. Both the state and rotation
of the target body are corrected for one
way light time from target to observer.
The state of the sun relative to the
target is corrected for one way light
from the sun to the target; this state
is evaluated at the epoch obtained by
retarding et by the one way light time
from target to observer.
"LT+S" Correct for planetary (light time) and
stellar aberrations. Light time
corrections are the same as in the "LT"
case above. The target state is
additionally corrected for stellar
aberration as seen by the observer, and
the sun state is corrected for stellar
aberration as seen from the target.
"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).
Whether the "CN+S" solution is
substantially more accurate than the
"LT" solution 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. See the -Particulars section
below for a discussion of precision of
light time corrections.
Light time corrections are applied as in
the "LT" case.
"CN+S" Converged Newtonian light time correction
and stellar aberration correction.
Light time and stellar aberration
corrections are applied as in the "LT+S"
case.
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-solar point on the target body at et,
expressed relative to the body-fixed frame of the
target body.
The sub-solar point is defined either as the point on
the target body that is closest to the sun, or the
target surface intercept of the line containing the sun's
center and the target's center; the input argument
method selects the definition to be used.
The 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 state of the target body is corrected for
aberration as specified by abcorr; the corrected
state is used in the geometric computation. As
indicated above, the rotation of the target is
retarded by one way light time if abcorr specifies
that light time correction is to be done.
The state of the sun as seen from the target body
body is also corrected for aberration as specified
by abcorr. The corrections, when selected, are
applied at the epoch et-lt, where lt is the one way
light time from target to observer.
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 by a routine in the call tree
of this routine.
2) If either of the input body names `target' or `obsrvr' cannot be
mapped to NAIF integer codes, the error SPICE(IDCODENOTFOUND)
is signaled by a routine in the call tree of this routine.
3) If `obsrvr' and `target' map to the same NAIF integer ID codes,
the error SPICE(BODIESNOTDISTINCT) is signaled by a routine in
the call tree of this routine.
4) If frame definition data enabling the evaluation of the state
of the target relative to the observer in target body-fixed
coordinates have not been loaded prior to calling subsol_c, 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 subsol_c, an error is signaled by a
routine in the call tree of this routine.
7) If the triaxial radii of the target body have not been loaded
into the kernel pool prior to calling subsol_c, an error is
signaled by a routine in the call tree of this routine.
8) If the size of the `target' body radii kernel variable is not
three, an error is signaled by a routine in the call tree of
this routine.
9) If any of the three `target' body radii is less-than or equal to
zero, an error is signaled by a routine in the call tree of
this routine.
10) If PCK data supplying a rotation model for the target body
have not been loaded prior to calling subsol_c, an error is
signaled by a routine in the call tree of this routine.
11) If any of the `method', `target', `abcorr' or `obsrvr' input
string pointers is null, the error SPICE(NULLPOINTER) is
signaled.
12) If any of the `method', `target', `abcorr' or `obsrvr' input
strings has zero length, the error SPICE(EMPTYSTRING) is
signaled.
Files
Appropriate 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:
- SPK data: ephemeris data for sun, target, and observer must be
loaded. If aberration corrections are used, the states of sun,
target, and observer relative to the solar system barycenter
must be calculable from the available ephemeris data. 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: a rotation model for the target body must be
loaded. This may be provided in a text or binary PCK file
which is loaded via furnsh_c.
- Frame data: if a frame definition is required to convert
the sun, 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.
Particularssubsol_c computes the sub-solar point on a target body, as seen by a specified observer. There are two different popular ways to define the sub-solar point: "nearest point on target to the sun" or "target surface intercept of line containing target and sun." These coincide when the target is spherical and generally are distinct otherwise. When comparing sub-point computations with results from sources other than SPICE, it's essential to make sure the same geometric definitions are used. 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-solar point on Mars as seen from the Mars Global
Surveyor (MGS) spacecraft for a specified time. Perform the
computation twice, using both the "intercept" and "near point"
options.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File: subsol_ex1.tm
This meta-kernel is intended to support operation of SPICE
example programs. The kernels shown here should not be
assumed to contain adequate or correct versions of data
required by SPICE-based user applications.
In order for an application to use this meta-kernel, the
kernels referenced here must be present in the user's
current working directory.
The names and contents of the kernels referenced
by this meta-kernel are as follows:
File name Contents
--------- --------
de430.bsp Planetary ephemeris
mar097.bsp Mars satellite ephemeris
pck00010.tpc Planet orientation and
radii
naif0011.tls Leapseconds
mgs_ext12_ipng_mgs95j.bsp MGS ephemeris
\begindata
KERNELS_TO_LOAD = ( 'de430.bsp',
'mar097.bsp',
'pck00010.tpc',
'naif0011.tls',
'mgs_ext12_ipng_mgs95j.bsp')
\begintext
End of meta-kernel
Example code begins here.
/.
Program subsol_ex1
./
#include <stdio.h>
#include "SpiceUsr.h"
int main( void )
{
#define METHLN 26
SpiceChar method [2][ METHLN ] =
{
"Intercept",
"Near point"
};
SpiceDouble et;
SpiceDouble lat;
SpiceDouble lon;
SpiceDouble radius;
SpiceDouble spoint[3];
SpiceInt i;
/.
Load kernel files.
./
furnsh_c ( "subsol_ex1.tm" );
/.
Convert the UTC request time to ET (seconds past
J2000, TDB).
./
str2et_c ( "2003 OCT 13 06:00:00 UTC", &et );
/.
Compute sub-spacecraft point using light time and stellar
aberration corrections. Use the "target surface intercept"
definition of sub-spacecraft point on the first loop
iteration, and use the "near point" definition on the
second.
./
for ( i = 0; i < 2; i++ )
{
subsol_c ( method[i], "mars", et, "lt+s", "mgs", spoint );
/.
Convert rectangular coordinates to planetocentric
latitude and longitude. Convert radians to degrees.
./
reclat_c ( spoint, &radius, &lon, &lat );
lon = lon * dpr_c ();
lat = lat * dpr_c ();
/.
Write the results.
./
printf ( "\n"
"Computation method: %s\n"
"\n"
" Radius (km) = %f\n"
" Planetocentric Latitude (deg) = %f\n"
" Planetocentric Longitude (deg) = %f\n"
"\n",
method[i], radius, lat, lon );
}
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Computation method: Intercept
Radius (km) = 3392.623653
Planetocentric Latitude (deg) = -24.888050
Planetocentric Longitude (deg) = 31.764895
Computation method: Near point
Radius (km) = 3392.692317
Planetocentric Latitude (deg) = -24.630785
Planetocentric Longitude (deg) = 31.764895
Restrictions
1) The appropriate kernel data must have been loaded before this
routine is called. See the -Files section above.
Literature_ReferencesNone. Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) J.E. McLean (JPL) B.V. Semenov (JPL) Version
-CSPICE Version 1.0.6, 01-NOV-2021 (JDR)
Edited the header to comply with NAIF standard.
Updated example to use a meta-kernel to load the required
kernels. Added example's problem statement and solution.
-CSPICE Version 1.0.5, 10-JUL-2014 (NJB)
Discussion of light time corrections was updated. Assertions
that converged light time corrections are unlikely to be
useful were removed.
-CSPICE Version 1.0.4, 19-MAY-2010 (BVS)
Index line now states that this routine is deprecated.
-CSPICE Version 1.0.3, 07-FEB-2008 (NJB)
-Abstract now states that this routine is deprecated.
-CSPICE Version 1.0.2, 22-JUL-2004 (NJB)
Updated header to indicate that the `target' and `observer'
input arguments can now contain string representations of
integers. Deleted references to kernel-specific loaders.
Made miscellaneous minor corrections to header comments.
-CSPICE Version 1.0.1, 12-DEC-2002 (NJB)
Corrected and updated code example in header.
-CSPICE Version 1.0.0, 03-SEP-1999 (NJB) (JEM)
Index_EntriesDEPRECATED sub-solar point |
Fri Dec 31 18:41:13 2021