| trgsep_c |
|
Table of contents
Procedure
trgsep_c ( Separation quantity from observer )
SpiceDouble trgsep_c ( SpiceDouble et,
ConstSpiceChar * targ1,
ConstSpiceChar * shape1,
ConstSpiceChar * frame1,
ConstSpiceChar * targ2,
ConstSpiceChar * shape2,
ConstSpiceChar * frame2,
ConstSpiceChar * obsrvr,
ConstSpiceChar * abcorr )
AbstractCompute the angular separation in radians between two spherical or point objects. Required_ReadingABCORR KeywordsANGLE GEOMETRY Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- et I Ephemeris seconds past J2000 TDB. targ1 I First target body name. shape1 I First target body shape. frame1 I Reference frame of first target. targ2 I Second target body name. shape2 I First target body shape. frame2 I Reference frame of second target. obsrvr I Observing body name. abcorr I Aberration corrections flag. The function returns the angular separation between two targets, `targ1' and `targ2', as seen from an observer `obsrvr', possibly corrected for aberration corrections. Detailed_Input
et is the time in ephemeris seconds past J2000 TDB at
which the separation is to be measured.
targ1 is the string naming the first body of interest. You can
also supply the integer ID code for the object as an
integer string. For example both "MOON" and "301"
are legitimate strings that indicate the moon is the
target body.
shape1 is the string naming the geometric model used to
represent the shape of the `targ1' body. Models
supported by this routine:
"SPHERE" Treat the body as a sphere with
radius equal to the maximum value of
BODYnnn_RADII.
"POINT" Treat the body as a point;
radius has value zero.
The `shape1' string lacks sensitivity to case, leading
and trailing blanks.
frame1 is the string naming the body-fixed reference frame
corresponding to `targ1'. trgsep_c does not currently use
this argument's value, its use is reserved for future
shape models. The value "NULL" will suffice for
"POINT" and "SPHERE" shaped bodies.
targ2 is the string naming the second body of interest. You can
also supply the integer ID code for the object as an
integer string. For example both "MOON" and "301"
are legitimate strings that indicate the moon is the
target body.
shape2 is the string naming the geometric model used to
represent the shape of the `targ2'. Models supported by
this routine:
"SPHERE" Treat the body as a sphere with
radius equal to the maximum value of
BODYnnn_RADII.
"POINT" Treat the body as a single point;
radius has value zero.
The `shape2' string lacks sensitivity to case, leading
and trailing blanks.
frame2 is the string naming the body-fixed reference frame
corresponding to `targ2'. trgsep_c does not currently use
this argument's value, its use is reserved for future
shape models. The value "NULL" will suffice for
"POINT" and "SPHERE" shaped bodies.
obsrvr is the string naming the observing body. Optionally, you
may supply the ID code of the object as an integer
string. For example, both "EARTH" and "399" are
legitimate strings to supply to indicate the
observer is Earth.
abcorr is the string description of the aberration corrections
to apply to the state evaluations to account for
one-way light time and stellar aberration.
This routine accepts the same aberration corrections
as does the SPICE routine spkezr_c. See the header of
spkezr_c for a detailed description of the aberration
correction options. For convenience, the options are
listed below:
"NONE" Apply no correction.
"LT" "Reception" case: correct for
one-way light time using a Newtonian
formulation.
"LT+S" "Reception" case: correct for
one-way light time and stellar
aberration using a Newtonian
formulation.
"CN" "Reception" case: converged
Newtonian light time correction.
"CN+S" "Reception" case: converged
Newtonian light time and stellar
aberration corrections.
"XLT" "Transmission" case: correct for
one-way light time using a Newtonian
formulation.
"XLT+S" "Transmission" case: correct for
one-way light time and stellar
aberration using a Newtonian
formulation.
"XCN" "Transmission" case: converged
Newtonian light time correction.
"XCN+S" "Transmission" case: converged
Newtonian light time and stellar
aberration corrections.
The `abcorr' string lacks sensitivity to case, leading
and trailing blanks.
Detailed_OutputThe function returns the angular separation between two targets, `targ1' and `targ2', as seen from an observer `obsrvr' expressed in radians. The observer is the angle's vertex. The angular separation between the targets may be measured between the centers or figures (limbs) of the targets, depending on whether the target shapes are modeled as spheres or points. If the target shape is either a spheroid or an ellipsoid, the radius used to compute the limb will be the largest of the radii of the target's tri-axial ellipsoid model. If the targets are modeled as points the result ranges from 0 to Pi radians or 180 degrees. If the target shapes are modeled as spheres or ellipsoids, the function returns a negative value when the bodies overlap (occult). Note that in this situation the function returns 0 when the limbs of the bodies start or finish the overlap. The positions of the targets may optionally be corrected for light time and stellar aberration. ParametersNone. Exceptions
1) If the three objects `targ1', `targ2' and `obsrvr' are not
distinct, an error is signaled by a routine in the call tree
of this routine.
2) If the object names for `targ1', `targ2' or `obsrvr' cannot resolve
to a NAIF body ID, an error is signaled by a routine in the
call tree of this routine.
3) If the reference frame associated with `targ1', `frame1', is not
centered on `targ1', or if the reference frame associated with
`targ2', `frame2', is not centered on `targ2', an error is signaled
by a routine in the call tree of this routine. This
restriction does not apply to shapes "SPHERE" and "POINT", for
which the frame input is ignored.
4) If the frame name for `frame1' or `frame2' cannot resolve to a
NAIF frame ID, an error is signaled by a routine in the call
tree of this routine.
5) If the body shape for `targ1', `shape1', or the body shape for
`targ2', `shape2', is not recognized, an error is signaled by a
routine in the call tree of this routine.
6) If the requested aberration correction `abcorr' is not
recognized, an error is signaled by a routine in the call tree
of this routine.
7) If either one or both targets' shape is modeled as sphere, and
the required PCK data has not been loaded, an error is
signaled by a routine in the call tree of this routine.
8) If the ephemeris data required to perform the needed state
look-ups are not loaded, an error is signaled by a routine in
the call tree of this routine.
9) If the observer `obsrvr' is located within either one of the
targets, an error is signaled by a routine in the call tree of
this routine.
10) If an error is signaled, the function returns a meaningless
result.
11) If any of the `targ1', `shape1', `frame1', `targ2', `shape2',
`frame2', `obsrvr' or `abcorr' input string pointers is null,
the error SPICE(NULLPOINTER) is signaled. The function returns
the value 0.0.
12) If any of the `targ1', `shape1', `frame1', `targ2', `shape2',
`frame2', `obsrvr' or `abcorr' input strings has zero length,
the error SPICE(EMPTYSTRING) is signaled. The function returns
the value 0.0.
Files
Appropriate SPICE kernels must be loaded by the calling program
before this routine is called.
The following data are required:
- An SPK file (or files) containing ephemeris data sufficient to
compute the position of each of the targets with respect to the
observer. 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.
- A PCK file containing the targets' tri-axial ellipsoid model,
if the targets are modeled as spheres.
- If non-inertial reference frames are used, then PCK files,
frame kernels, C-kernels, and SCLK kernels may be needed.
ParticularsThis routine determines the apparent separation between the two objects as observed from a third. The value reported is corrected for light time. Moreover, if at the time this routine is called, stellar aberration corrections are enabled, this correction will also be applied to the apparent positions of the centers of the two objects. Please refer to the Aberration Corrections Required Reading (abcorr.req) for detailed information describing the nature and calculation of the applied 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) Calculate the apparent angular separation of the Earth and
Moon as observed from the Sun at a TDB time known as a time
of maximum separation. Calculate and output the separation
modeling the Earth and Moon as point bodies and as spheres.
Provide the result in degrees.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: trgsep_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
--------- --------
de421.bsp Planetary ephemeris
pck00009.tpc Planet orientation and
radii
naif0009.tls Leapseconds
\begindata
KERNELS_TO_LOAD = ( 'de421.bsp',
'pck00009.tpc',
'naif0009.tls' )
\begintext
End of meta-kernel
Example code begins here.
/.
Program trgsep_ex1
./
#include <stdio.h>
#include "SpiceUsr.h"
int main( )
{
/.
Local variables.
./
SpiceChar * tdbstr;
SpiceChar * obsrvr;
SpiceChar * abcorr;
SpiceDouble et;
SpiceDouble value;
SpiceChar frame [2][33] = { "IAU_MOON","IAU_EARTH" };
SpiceChar targ [2][33] = { "MOON","EARTH" };
SpiceChar shape [2][33] = { "POINT","SPHERE" };
/.
Load the kernels.
./
furnsh_c ( "trgsep_ex1.tm" );
tdbstr = "2007-JAN-11 11:21:20.213872 (TDB)";
obsrvr = "SUN";
abcorr = "LT+S";
str2et_c ( tdbstr, &et );
value = trgsep_c ( et, targ[0], shape[0],
frame[0], targ[1], shape[0],
frame[1], obsrvr, abcorr );
printf( "Bodies: %-6s%-6s\n", targ[0], targ[1] );
printf( "as seen from: %-6s\n", obsrvr );
printf( "at TDB time: %-36s\n", tdbstr );
printf( "with correction: %s\n", abcorr );
printf( "\n" );
printf( "Apparent angular separation:\n" );
printf( " point body models (deg.): %11.8f\n", value * dpr_c() );
value = trgsep_c ( et, targ[0], shape[1],
frame[0], targ[1], shape[1],
frame[1], obsrvr, abcorr );
printf( " sphere body models (deg.): %11.8f\n", value * dpr_c() );
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Bodies: MOON EARTH
as seen from: SUN
at TDB time: 2007-JAN-11 11:21:20.213872 (TDB)
with correction: LT+S
Apparent angular separation:
point body models (deg.): 0.15729276
sphere body models (deg.): 0.15413221
RestrictionsNone. Literature_ReferencesNone. Author_and_InstitutionM. Costa Sitja (JPL) J. Diaz del Rio (ODC Space) Version-CSPICE Version 1.0.0, 07-AUG-2021 (MCS) (JDR) Index_Entriescompute the angular separation between two target bodies |
Fri Dec 31 18:41:14 2021