Table of contents
CSPICE_GFSEP determines the time intervals when the angular separation
between the position vectors of two target bodies relative to an observer
satisfies a numerical relationship.
Given:
targ1 the scalar string naming the first body of interest.
help, targ1
STRING = Scalar
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 the scalar string naming the geometric model used to represent
the shape of the `targ1' body.
help, shape1
STRING = Scalar
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 the scalar string naming the body-fixed reference frame
corresponding to `targ1'.
help, frame1
STRING = Scalar
cspice_gfsep 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 the scalar string naming the second body of interest.
help, targ2
STRING = Scalar
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 the scalar string naming the geometric model used to represent
the shape of the `targ2.'
help, shape2
STRING = Scalar
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 the scalar string naming the body-fixed reference frame
corresponding to `targ2'.
help, frame2
STRING = Scalar
cspice_gfsep 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.
abcorr the string scalar indicating the aberration corrections to apply
to the state evaluations to account for one-way light time and
stellar aberration.
help, abcorr
STRING = Scalar
This routine accepts the same aberration corrections as does
the Icy routine cspice_spkezr. See the header of cspice_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, and to embedded,
leading and trailing blanks.
obsrvr the string scalar naming the observing body.
help, obsrvr
STRING = Scalar
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 the
Earth.
relate the string identifying the relational operator used to define a
constraint on the angular separation.
help, relate
STRING = Scalar
The result window found by this routine indicates the time
intervals where the constraint is satisfied. Supported values of
relate and corresponding meanings are shown below:
'>' Separation is greater than the reference
value 'refval'.
'=' Separation is equal to the reference
value 'refval'.
'<' Separation is less than the reference
value 'refval'.
'ABSMAX' Separation is at an absolute maximum.
'ABSMIN' Separation is at an absolute minimum.
'LOCMAX' Separation is at a local maximum.
'LOCMIN' Separation is at a local minimum.
The caller may indicate that the region of interest
is the set of time intervals where the quantity is
within a specified angular separation of an absolute extremum.
The argument adjust (described below) is used to
specify this angular separation.
Local extrema are considered to exist only in the
interiors of the intervals comprising the confinement
window: a local extremum cannot exist at a boundary
point of the confinement window.
Negative Angular Separation
For those searches using a SPHERE shape identifier for
either target body, the angular separation function
returns a negative value when the bodies overlap (occult).
The `relate' string lacks sensitivity to case, leading
and trailing blanks.
refval the double precision reference value used together with `relate'
argument to define an equality or inequality to be satisfied by
the angular separation between the specified target and
observer.
help, refval
DOUBLE = Scalar
See the discussion of `relate' above for further information.
The units of `refval' are radians.
adjust a double precision value used to modify searches for absolute
extrema: when `relate' is set to 'ABSMAX' or 'ABSMIN' and adjust
is set to a positive value, cspice_gfsep finds times when the
angular separation between the bodies is within adjust radians
of the specified extreme value.
help, adjust
DOUBLE = Scalar
For `relate' set to 'ABSMAX', the result window contains
time intervals when the angular separation has
values between ABSMAX - adjust and ABSMAX.
For `relate' set to 'ABSMIN', the result window contains
time intervals when the angular separation has
values between ABSMIN and ABSMIN + adjust.
`adjust' is not used for searches for local extrema,
equality or inequality conditions.
step the scalar double precision time step size to use in the search.
help, step
DOUBLE = Scalar
`step' must be short enough to for a search using this step
size to locate the time intervals where the
specified angular separation function is monotone
increasing or decreasing. However, `step' must not be
*too* short, or the search will take an unreasonable
amount of time.
The choice of `step' affects the completeness but not
the precision of solutions found by this routine; the
precision is controlled by the convergence tolerance.
See the discussion of the parameter SPICE_GF_CNVTOL for
details.
`step' has units of TDB seconds.
nintvls a scalar integer value specifying the number of intervals in the
the internal workspace array used by this routine.
help, nintvls
LONG = Scalar
`nintvls' should be at least as large as the number of
intervals within the search region on which the specified
observer-target vector coordinate function is monotone
increasing or decreasing. It does no harm to pick a value of
`nintvls' larger than the minimum required to execute the
specified search, but if chosen too small, the search will fail.
cnfine a scalar double precision window that confines the time period
over which the specified search is conducted.
help, cnfine
STRUCT = cspice_celld(2*N)
`cnfine' may consist of a single interval or a collection of
intervals.
In some cases the confinement window can be used to greatly
reduce the time period that must be searched for the desired
solution. See the -Particulars section below for further
discussion.
See the -Examples section below for a code example
that shows how to create a confinement window.
In some cases the observer's state may be computed at
times outside of `cnfine' by as much as 2 seconds. See
-Particulars for details.
the call:
cspice_gfsep, targ1, shape2, frame1, targ2, shape2, $
frame2, abcorr, obsrvr, relate, refval, $
adjust, step, nintvls, cnfine, result
returns:
result the scalar double precision window of intervals, contained
within the confinement window `cnfine', on which the specified
constraint is satisfied.
help, result
STRUCT = cspice_celld(2*R)
If `result' is non-empty on input, its contents
will be discarded before cspice_gfsep conducts its
search.
`result' must be declared and initialized with sufficient
size to capture the full set of time intervals
within the search region on which the specified constraint
is satisfied.
If the search is for local extrema, or for absolute
extrema with `adjust' set to zero, then normally each
interval of `result' will be a singleton: the left and
right endpoints of each interval will be identical.
If no times within the confinement window satisfy the
constraint, `result' will be returned with a
cardinality of zero.
SPICE_GF_CNVTOL
is the convergence tolerance used for finding endpoints
of the intervals comprising the result window.
SPICE_GF_CNVTOL is also used for finding intermediate
results; in particular, SPICE_GF_CNVTOL is used for
finding the windows on which the specified coordinate
is increasing or decreasing. SPICE_GF_CNVTOL is used to
determine when binary searches for roots should
terminate: when a root is bracketed within an interval
of length SPICE_GF_CNVTOL; the root is considered to
have been found.
The accuracy, as opposed to precision, of roots found
by this routine depends on the accuracy of the input
data. In most cases, the accuracy of solutions will be
inferior to their precision.
See Icy include file IcyGF.pro for declarations and descriptions of
parameters used throughout the GF system.
Any numerical results shown for these examples may differ between
platforms as the results depend on the SPICE kernels used as input
and the machine specific arithmetic implementation.
1) Determine the times of local maxima of the angular separation
between the moon and sun as observed from earth from
Jan 1, 2007 to Jan 1 2008.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: gfsep_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.
PRO gfsep_ex1
MAXWIN = 1000
TIMFMT = 'YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND'
TIMLEN = 41
;;
;; Local variables
;;
targ1 = ''
targ2 = ''
obsrvr = ''
;;
;; Load kernels.
;;
cspice_furnsh, 'gfsep_ex1.tm'
;;
;; Store the time bounds of our search interval in
;; the cnfine confinement window.
;;
cspice_str2et, [ '2007 JAN 01', '2008 JAN 01'], et
cnfine = cspice_celld( 2 )
cspice_wninsd, et[0], et[1], cnfine
;;
;; Prompt for the inputs.
;;
read, targ1, PROMPT = 'First body > '
read, targ2, PROMPT = 'Second body > '
read, obsrvr, PROMPT = 'Observing body > '
;;
;; Search using a step size of 6 days (in units of seconds).
;;
step = 6.D*cspice_spd()
adjust = 0.D
refval = 0.D
shape1 = 'SPHERE'
frame1 = 'NULL'
shape2 = 'SPHERE'
frame2 = 'NULL'
abcorr = 'NONE'
relate = 'LOCMAX'
nintvls= MAXWIN
result = cspice_celld( MAXWIN*2)
cspice_gfsep, targ1, shape2, frame1, targ2, shape2, $
frame2, abcorr, obsrvr, relate, refval, $
adjust, step, nintvls, cnfine, result
;;
;; List the beginning and ending times in each interval
;; if result contains data.
;;
count = cspice_wncard( result )
if ( count eq 0 ) then begin
print, 'Result window is empty.'
endif else begin
for i= 0L, (count - 1L ) do begin
cspice_wnfetd, result, i, left, right
cspice_timout, [left, right], TIMFMT, TIMLEN, timstr
if ( left eq right ) then begin
print, 'Event time: ', timstr[0]
endif else begin
print, 'From : ', timstr[0]
print, 'To : ', timstr[1]
print
endelse
endfor
endelse
;;
;; It's always good form to unload kernels after use,
;; particularly in IDL due to data persistence.
;;
cspice_kclear
END
When this program was executed on a Mac/Intel/IDL8.x/64-bit
platform, using 'MOON' as first body, 'EARTH' as second body
and 'SUN' as observing body, the output was:
First body > MOON
Second body > EARTH
Observing body > SUN
Event time: 2007-JAN-11 11:21:20.214305 (TDB)
Event time: 2007-JAN-26 01:43:41.027309 (TDB)
Event time: 2007-FEB-10 04:49:53.431964 (TDB)
Event time: 2007-FEB-24 13:18:18.953256 (TDB)
Event time: 2007-MAR-11 20:41:59.571964 (TDB)
Event time: 2007-MAR-26 01:20:26.860201 (TDB)
Event time: 2007-APR-10 10:24:39.017514 (TDB)
Event time: 2007-APR-24 14:00:49.422728 (TDB)
Event time: 2007-MAY-09 21:53:25.643532 (TDB)
Event time: 2007-MAY-24 03:14:05.873982 (TDB)
Event time: 2007-JUN-08 07:24:13.686616 (TDB)
Event time: 2007-JUN-22 16:45:56.506850 (TDB)
Event time: 2007-JUL-07 15:30:03.706532 (TDB)
Event time: 2007-JUL-22 06:26:17.397353 (TDB)
Event time: 2007-AUG-05 23:03:21.625229 (TDB)
Event time: 2007-AUG-20 20:14:56.801678 (TDB)
Event time: 2007-SEP-04 07:13:25.162360 (TDB)
Event time: 2007-SEP-19 10:16:42.721117 (TDB)
Event time: 2007-OCT-03 17:11:17.188939 (TDB)
Event time: 2007-OCT-19 00:30:31.300060 (TDB)
Event time: 2007-NOV-02 05:43:48.902220 (TDB)
Event time: 2007-NOV-17 14:38:21.314771 (TDB)
Event time: 2007-DEC-01 20:50:27.562519 (TDB)
Event time: 2007-DEC-17 04:04:46.933247 (TDB)
Event time: 2007-DEC-31 13:43:52.558812 (TDB)
2) Determine the time of local maxima elongation of the
Moon as seen from Earth for the same time interval
as the previous example, i.e. find the local maxima of
the angular separation between the Moon and the Sun as
seen from the Earth, by running the code in example #1.
When Example #1 was executed on a Mac/Intel/IDL8.x/64-bit
platform, using 'MOON' as first body, 'SUN' as second body
and 'EARTH' as observing body, the output was:
First body > MOON
Second body > SUN
Observing body > EARTH
Event time: 2007-JAN-03 14:20:24.617627 (TDB)
Event time: 2007-FEB-02 06:16:24.101517 (TDB)
Event time: 2007-MAR-03 23:22:41.994972 (TDB)
Event time: 2007-APR-02 16:49:16.135505 (TDB)
Event time: 2007-MAY-02 09:41:43.830081 (TDB)
Event time: 2007-JUN-01 01:03:44.527470 (TDB)
Event time: 2007-JUN-30 14:15:26.576292 (TDB)
Event time: 2007-JUL-30 01:14:49.000963 (TDB)
Event time: 2007-AUG-28 10:39:01.388249 (TDB)
Event time: 2007-SEP-26 19:25:51.509426 (TDB)
Event time: 2007-OCT-26 04:30:56.625105 (TDB)
Event time: 2007-NOV-24 14:31:04.331185 (TDB)
Event time: 2007-DEC-24 01:40:12.235392 (TDB)
This routine determines a set of one or more time intervals
within the confinement window for which the angular separation
between the two bodies satisfies some defined relationship.
The resulting set of intervals is returned as a SPICE window.
Below we discuss in greater detail aspects of this routine's
solution process that are relevant to correct and efficient
use of this routine in user applications.
The Search Process
==================
Regardless of the type of constraint selected by the caller, this
routine starts the search for solutions by determining the time
periods, within the confinement window, over which the specified
angular separation function is monotone increasing and monotone
decreasing. Each of these time periods is represented by a SPICE
window. Having found these windows, all of the angular separation
function's local extrema within the confinement window are known.
Absolute extrema then can be found very easily.
Within any interval of these "monotone" windows, there will be at
most one solution of any equality constraint. Since the boundary
of the solution set for any inequality constraint is contained in
the union of
- the set of points where an equality constraint is met
- the boundary points of the confinement window
the solutions of both equality and inequality constraints can be
found easily once the monotone windows have been found.
Step Size
=========
The monotone windows (described above) are found using a two-step
search process. Each interval of the confinement window is
searched as follows: first, the input step size is used to
determine the time separation at which the sign of the rate of
change of angular separation (angular separation rate) will be
sampled. Starting at the left endpoint of an interval, samples
will be taken at each step. If a change of sign is found, a
root has been bracketed; at that point, the time at which the
angular separation rate is zero can be found by a refinement
process, for example, using a binary search.
Note that the optimal choice of step size depends on the lengths
of the intervals over which the distance function is monotone:
the step size should be shorter than the shortest of these
intervals (within the confinement window).
The optimal step size is *not* necessarily related to the lengths
of the intervals comprising the result window. For example, if
the shortest monotone interval has length 10 days, and if the
shortest result window interval has length 5 minutes, a step size
of 9.9 days is still adequate to find all of the intervals in the
result window. In situations like this, the technique of using
monotone windows yields a dramatic efficiency improvement over a
state-based search that simply tests at each step whether the
specified constraint is satisfied. The latter type of search can
miss solution intervals if the step size is longer than the
shortest solution interval.
Having some knowledge of the relative geometry of the target and
observer can be a valuable aid in picking a reasonable step size.
In general, the user can compensate for lack of such knowledge by
picking a very short step size; the cost is increased computation
time.
Note that the step size is not related to the precision with which
the endpoints of the intervals of the result window are computed.
That precision level is controlled by the convergence tolerance.
Convergence Tolerance
=====================
As described above, the root-finding process used by this routine
involves first bracketing roots and then using a search process
to locate them. "Roots" are both times when local extrema are
attained and times when the distance function is equal to a
reference value. All endpoints of the intervals comprising the
result window are either endpoints of intervals of the
confinement window or roots.
Once a root has been bracketed, a refinement process is used to
narrow down the time interval within which the root must lie.
This refinement process terminates when the location of the root
has been determined to within an error margin called the
"convergence tolerance." The default convergence tolerance
used by this routine is set by the parameter SPICE_GF_CNVTOL (defined
in IcyGF.pro).
The value of SPICE_GF_CNVTOL is set to a "tight" value so that the
tolerance doesn't become the limiting factor in the accuracy of
solutions found by this routine. In general the accuracy of input
data will be the limiting factor.
The user may change the convergence tolerance from the default
SPICE_GF_CNVTOL value by calling the routine cspice_gfstol, e.g.
cspice_gfstol, tolerance value
Call cspice_gfstol prior to calling this routine. All subsequent
searches will use the updated tolerance value.
Setting the tolerance tighter than SPICE_GF_CNVTOL is unlikely to be
useful, since the results are unlikely to be more accurate.
Making the tolerance looser will speed up searches somewhat,
since a few convergence steps will be omitted. However, in most
cases, the step size is likely to have a much greater effect
on processing time than would the convergence tolerance.
The Confinement Window
======================
The simplest use of the confinement window is to specify a time
interval within which a solution is sought. However, the
confinement window can, in some cases, be used to make searches
more efficient. Sometimes it's possible to do an efficient search
to reduce the size of the time period over which a relatively
slow search of interest must be performed.
Certain types of searches require the state of the observer,
relative to the solar system barycenter, to be computed at times
slightly outside the confinement window `cnfine'. The time window
that is actually used is the result of "expanding" `cnfine' by a
specified amount "T": each time interval of `cnfine' is expanded by
shifting the interval's left endpoint to the left and the right
endpoint to the right by T seconds. Any overlapping intervals are
merged. (The input argument `cnfine' is not modified.)
The window expansions listed below are additive: if both
conditions apply, the window expansion amount is the sum of the
individual amounts.
- If a search uses an equality constraint, the time window
over which the state of the observer is computed is expanded
by 1 second at both ends of all of the time intervals
comprising the window over which the search is conducted.
- If a search uses stellar aberration corrections, the time
window over which the state of the observer is computed is
expanded as described above.
When light time corrections are used, expansion of the search
window also affects the set of times at which the light time-
corrected state of the target is computed.
In addition to the possible 2 second expansion of the search
window that occurs when both an equality constraint and stellar
aberration corrections are used, round-off error should be taken
into account when the need for data availability is analyzed.
Negative Angular Separation
===========================
For those searches using a SPHERE shape identifier for both
target bodies, the angular separation function returns a
negative value when the bodies overlap (occult), e.g.
a search for an ABSMIN of angular separation in a
confinement window covering an occultation event will
return the time when the apparent center of the
occulting body passes closest to the apparent center of
the occulted body.
Elongation
===========================
The angular separation of two targets as seen from an observer
where one of those targets is the sun is known as elongation.
1) In order for this routine to produce correct results,
the step size must be appropriate for the problem at hand.
Step sizes that are too large may cause this routine to miss
roots; step sizes that are too small may cause this routine
to run unacceptably slowly and in some cases, find spurious
roots.
This routine does not diagnose invalid step sizes, except
that if the step size is non-positive, an error is signaled
by a routine in the call tree of this routine.
2) Due to numerical errors, in particular,
- truncation error in time values
- finite tolerance value
- errors in computed geometric quantities
it is *normal* for the condition of interest to not always be
satisfied near the endpoints of the intervals comprising the
`result' window. One technique to handle such a situation,
slightly contract `result' using the window routine cspice_wncond.
3) If the number of intervals `nintvls' is less than 1, the error
SPICE(VALUEOUTOFRANGE) is signaled by a routine in the call
tree of this routine.
4) If result window, `result', is not at least 2 and an even value,
the error SPICE(INVALIDDIMENSION) is signaled by a routine in
the call tree of this routine.
5) If `result' has insufficient capacity to contain the
number of intervals on which the specified distance condition
is met, an error is signaled by a routine in the call
tree of this routine.
6) If an error (typically cell overflow) occurs during
window arithmetic, the error is signaled by a routine
in the call tree of this routine.
7) If the relational operator `relate' is not recognized, an
error is signaled by a routine in the call tree of this
routine.
8) If the aberration correction specifier contains an
unrecognized value, an error is signaled by a routine in the
call tree of this routine.
9) If `adjust' is negative, an error is signaled by a routine in
the call tree of this routine.
10) If either of the input body names, `targ1', `targ2' do not map
to NAIF ID codes, an error is signaled by a routine in the
call tree of this routine.
11) If either of the input body shape names, `shape1', `shape2',
are not recognized by the GF subsystem, an error is signaled
by a routine in the call tree of this routine.
12) If either of the input body frame names, `frame1', `frame2',
are not recognized by the frame subsystem, an error is
signaled by a routine in the call tree of this routine.
13) If either of the input body frames, `frame1', `frame2',
are not centered on the corresponding body (`frame1' on `targ1',
`frame2' on `targ2'), an error is signaled by a routine in the
call tree of this routine.
14) If required ephemerides or other kernel data are not
available, an error is signaled by a routine in the call tree
of this routine.
15) If any of the input arguments, `targ1', `shape1', `frame1',
`targ2', `shape2', `frame2', `abcorr', `obsrvr', `relate',
`refval', `adjust', `step', `nintvls', `cnfine' or `result',
is undefined, an error is signaled by the IDL error handling
system.
16) If any of the input arguments, `targ1', `shape1', `frame1',
`targ2', `shape2', `frame2', `abcorr', `obsrvr', `relate',
`refval', `adjust', `step', `nintvls', `cnfine' or `result',
is not of the expected type, or it does not have the expected
dimensions and size, an error is signaled by the Icy
interface.
Appropriate SPK and PCK kernels must be loaded by the
calling program before this routine is called.
The following data are required:
- SPK data: the calling application must load ephemeris data
for the targets, observer, and any intermediate objects in
a chain connecting the targets and observer that cover the
time period specified by the window `cnfine'. 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 using
cspice_furnsh.
- PCK data: bodies modeled as triaxial ellipsoids must have
semi-axis lengths provided by variables in the kernel pool.
Typically these data are made available by loading a text
PCK file using cspice_furnsh.
- If non-inertial reference frames are used, then PCK
files, frame kernels, C-kernels, and SCLK kernels may be
needed.
- In some cases the observer's state may be computed at times
outside of `cnfine' by as much as 2 seconds; data required to
compute this state must be provided by loaded kernels. See
-Particulars for details.
Such kernel data are normally loaded once per program
run, NOT every time this routine is called.
1) The kernel files to be used by this routine must be loaded
(normally using the Icy routine cspice_furnsh) before this
routine is called.
2) This routine has the side effect of re-initializing the
angular separation quantity utility package. Callers may
need to re-initialize the package after calling this routine.
3) Due to the current logic implemented in SPICE, a direct
search for zero angular separation of two point targets will
always fails, i.e.,
relate = '='
refval = 0.d0
Use `relate' values of 'ABSMIN' or 'LOCMIN' to detect such an
event(s).
ICY.REQ
GF.REQ
SPK.REQ
CK.REQ
TIME.REQ
WINDOWS.REQ
None.
N.J. Bachman (JPL)
J. Diaz del Rio (ODC Space)
E.D. Wright (JPL)
-Icy Version 1.0.3, 03-NOV-2021 (JDR)
Updated header to describe use of expanded confinement window.
Edited the header to comply with NAIF standard. Added
example's meta-kernel, modified example code to prompt for
the required inputs and added a second example.
Added -Parameters, -Exceptions, -Files, -Restrictions,
-Literature_References and -Author_and_Institution sections.
Removed reference to the routine's corresponding CSPICE header from
-Abstract section.
Added arguments' type and size information in the -I/O section.
-Icy Version 1.0.2, 05-SEP-2012 (EDW)
Edit to comments to correct search description.
Header updated to describe use of cspice_gfstol.
-Icy Version 1.0.1, 29-DEC-2009 (EDW)
Edited argument descriptions. Removed mention of "ELLIPSOID"
shape from 'shape1' and 'shape2' as that option is not yet
implemented.
-Icy Version 1.0.0, 15-APR-2009 (NJB) (EDW)
GF angular separation search
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