| trgsep |
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Table of contents
Procedure
TRGSEP ( Separation quantity from observer )
DOUBLE PRECISION FUNCTION TRGSEP ( ET,
. TARG1, SHAPE1, FRAME1,
. TARG2, SHAPE2, FRAME2,
. OBSRVR, ABCORR )
Abstract
Compute the angular separation in radians between two spherical
or point objects.
Required_Reading
ABCORR
Keywords
ANGLE
GEOMETRY
Declarations
IMPLICIT NONE
INCLUDE 'zzabcorr.inc'
INCLUDE 'zzdyn.inc'
DOUBLE PRECISION ET
CHARACTER*(*) TARG1
CHARACTER*(*) SHAPE1
CHARACTER*(*) FRAME1
CHARACTER*(*) TARG2
CHARACTER*(*) SHAPE2
CHARACTER*(*) FRAME2
CHARACTER*(*) OBSRVR
CHARACTER*(*) ABCORR
Brief_I/O
VARIABLE 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 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 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. See the header of
SPKEZR 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_Output
The 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.
Parameters
None.
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.
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.
Particulars
This 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
IMPLICIT NONE
C
C SPICELIB functions
C
DOUBLE PRECISION TRGSEP
DOUBLE PRECISION DPR
C
C Local variables.
C
CHARACTER*(32) TARG (2)
CHARACTER*(32) SHAPE (2)
CHARACTER*(32) FRAME (2)
CHARACTER*(64) TDBSTR
CHARACTER*(32) OBSRVR
CHARACTER*(32) ABCORR
DOUBLE PRECISION ET
DOUBLE PRECISION VALUE
DATA FRAME / 'IAU_MOON', 'IAU_EARTH' /
DATA TARG / 'MOON', 'EARTH' /
DATA SHAPE / 'POINT', 'SPHERE' /
C
C Load the kernels.
C
CALL FURNSH( 'trgsep_ex1.tm')
TDBSTR = '2007-JAN-11 11:21:20.213872 (TDB)'
OBSRVR = 'SUN'
ABCORR = 'LT+S'
CALL STR2ET ( TDBSTR, ET )
VALUE = TRGSEP( ET,
. TARG(1), SHAPE(1), FRAME(1),
. TARG(2), SHAPE(1), FRAME(2),
. OBSRVR, ABCORR )
WRITE(*, FMT='(A,A6,A6)') 'Bodies: ',
. TARG(1), TARG(2)
WRITE(*, FMT='(A,A6)') 'as seen from: ', OBSRVR
WRITE(*, FMT='(A,A36)') 'at TDB time: ', TDBSTR
WRITE(*, FMT='(A,A)') 'with correction: ', ABCORR
WRITE(*,*)
WRITE(*, FMT='(A)') 'Apparent angular separation:'
WRITE(*, FMT='(A,F12.8)')
. ' point body models (deg.): ',
. VALUE * DPR()
VALUE = TRGSEP( ET,
. TARG(1), SHAPE(2), FRAME(1),
. TARG(2), SHAPE(2), FRAME(2),
. OBSRVR, ABCORR )
WRITE(*, FMT='(A,F12.8)')
. ' sphere body models (deg.): ',
. VALUE * DPR()
END
When this program was executed on a Mac/Intel/gfortran/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
Restrictions
None.
Literature_References
None.
Author_and_Institution
M. Costa Sitja (JPL)
J. Diaz del Rio (ODC Space)
E.D. Wright (JPL)
Version
SPICELIB Version 1.0.0, 07-AUG-2021 (EDW) (JDR) (MCS)
Based on code originally found in zzgfspu.f.
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Fri Dec 31 18:37:03 2021