Table of contents
CSPICE_GFOCLT determines time intervals when an observer sees one target
body occulted by, or in transit across, another.
The surfaces of the target bodies may be represented by triaxial
ellipsoids or by topographic data provided by DSK files.
Given:
occtyp the scalar string naming the type of occultation to find.
help, occtyp
STRING = Scalar
Note that transits are considered to be a type of occultation.
Supported values and corresponding definitions are:
'FULL' denotes the full occultation of the
body designated by `back' by the body
designated by `front', as seen from the
location of the observer. In other
words, the occulted body is completely
invisible as seen from the observer's
location.
'ANNULAR' denotes an annular occultation: the
body designated by `front' blocks part
of, but not the limb of, the body
designated by `back', as seen from the
location of the observer.
'PARTIAL' denotes a partial, non-annular
occultation: the body designated by
`front' blocks part, but not all, of the
limb of the body designated by `back', as
seen from the location of the observer.
'ANY' denotes any of the above three types of
occultations: 'PARTIAL', 'ANNULAR', or
'FULL'.
'ANY' should be used to search for
times when the body designated by `front'
blocks any part of the body designated
by `back'.
The option 'ANY' must be used if either
the front or back target body is
modeled as a point.
Case and leading or trailing blanks are not
significant in the string `occtyp'.
Case and leading or trailing blanks are not significant in
the string `occtyp'.
front the scalar string naming the target body that occults---that is,
passes in front of---the other.
help, front
STRING = Scalar
Optionally, you may supply the integer NAIF ID code for the
body as a string. For example both 'MOON' and '301' are
legitimate strings that designate the Moon.
The `front' string lacks sensitivity to case, leading
and trailing blanks.
fshape a string indicating the geometric model used to represent the
shape of the front target body.
help, fshape
STRING = Scalar
The supported options are:
'ELLIPSOID'
Use a triaxial ellipsoid model with radius
values provided via the kernel pool. A kernel
variable having a name of the form
'BODYnnn_RADII'
where nnn represents the NAIF integer code
associated with the body, must be present in
the kernel pool. This variable must be
associated with three numeric values giving the
lengths of the ellipsoid's X, Y, and Z
semi-axes.
'POINT'
Treat the body as a single point. When a point
target is specified, the occultation type must
be set to 'ANY'.
'DSK/UNPRIORITIZED[/SURFACES = <surface list>]'
Use topographic data provided by DSK files to
model the body's shape. These data must be
provided by loaded DSK files.
The surface list specification is optional. The
syntax of the list is
<surface 1> [, <surface 2>...]
If present, it indicates that data only for the
listed surfaces are to be used; however, data
need not be available for all surfaces in the
list. If absent, loaded DSK data for any surface
associated with the target body are used.
The surface list may contain surface names or
surface ID codes. Names containing blanks must
be delimited by double quotes, for example
SURFACES = "Mars MEGDR 128 PIXEL/DEG"
If multiple surfaces are specified, their names
or IDs must be separated by commas.
See the -Particulars section below for details
concerning use of DSK data.
The combinations of the shapes of the target bodies
`front' and `back' must be one of:
One ELLIPSOID, one POINT
Two ELLIPSOIDs
One DSK, one POINT
Case and leading or trailing blanks are not
significant in the string `fshape'.
fframe the scalar string naming the body-fixed, body-centered reference
frame associated with the front target body.
help, fframe
STRING = Scalar
Examples of such names are 'IAU_SATURN' (for Saturn) and
'ITRF93' (for the Earth).
If the front target body is modeled as a point, `fframe'
should be left empty or blank.
The `fframe' string lacks sensitivity to case, leading
and trailing blanks.
back the scalar string naming the target body that is occulted
by---that is, passes in back of---the other.
help, back
STRING = Scalar
Optionally, you may supply the integer NAIF ID code for the
body as a string. For example both 'MOON' and '301' are
legitimate strings that designate the Moon.
The `back' string lacks sensitivity to case, leading
and trailing blanks.
bshape the string scalar naming the shape specification for the body
designated by `back'.
help, bshape
STRING = Scalar
The supported options are those for `fshape'. See the
description of `fshape' above for details.
bframe the scalar string naming the body-fixed, body-centered reference
frame associated with the '`back'' target body.
help, bframe
STRING = Scalar
Examples of such names are 'IAU_SATURN' (for Saturn) and
'ITRF93' (for the Earth).
If the back target body is modeled as a point, `bframe'
should be left empty or blank.
The `bframe' string lacks sensitivity to case, leading
and trailing blanks.
abcorr indicates the aberration corrections to be applied to the state
of the target body to account for one-way light time.
help, abcorr
STRING = Scalar
Stellar aberration corrections are ignored if specified, since
these corrections don't improve the accuracy of the occultation
determination.
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 scalar string 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 Earth.
Case and leading or trailing blanks are not significant in
the string `obsrvr'.
step a double precision value defining the step size to use in the
search.
help, step
DOUBLE = Scalar
`step' must be shorter than any interval, within the
confinement window, over which the specified occultation
condition is met. In other words, `step' must be shorter than
the shortest occultation event that the user wishes to detect;
`step' must also be shorter than the shortest time interval
between two occultation events that occur within the confinement
window (see below). 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.
cnfine a double precision SPICE 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.
the call:
cspice_gfoclt, occtyp, front, fshape, fframe, back, bshape, $
bframe, abcorr, obsrvr, step, cnfine, result
returns:
result the double precision SPICE 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_gfoclt 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 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 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) Find occultations of the Sun by the Moon (that is, solar
eclipses)as seen from the center of the Earth over the month
December, 2001.
Use light time corrections to model apparent positions of Sun
and Moon. Stellar aberration corrections are not specified
because they don't affect occultation computations.
We select a step size of 3 minutes, which means we
ignore occultation events lasting less than 3 minutes,
if any exist.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: gfoclt_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
pck00008.tpc Planet orientation and
radii
naif0009.tls Leapseconds
\begindata
KERNELS_TO_LOAD = ( 'de421.bsp',
'pck00008.tpc',
'naif0009.tls' )
\begintext
End of meta-kernel
Example code begins here.
PRO gfoclt_ex1
MAXWIN = 1000
TIMFMT = 'YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND'
TIMLEN = 41
;;
;; Load kernels.
;;
cspice_furnsh, 'gfoclt_ex1.tm'
;;
;; Store the time bounds of our search interval in
;; the cnfine confinement window.
;;
cspice_str2et, [ '2001 DEC 01 00:00:00 TDB', $
'2002 JAN 01 00:00:00 TDB'], et
cnfine = cspice_celld( 2 )
cspice_wninsd, et[0], et[1], cnfine
;;
;; Select a 3-minute step. We'll ignore any occultations
;; lasting less than 3 minutes.
;;
step = 180.D
occtyp = 'any'
front = 'moon'
fshape = 'ellipsoid'
fframe = 'iau_moon'
back = 'sun'
bshape = 'ellipsoid'
bframe = 'iau_sun'
obsrvr = 'earth'
abcorr = 'lt'
;;
;; List the beginning and ending points in each interval
;; if result contains data.
;;
result = cspice_celld( MAXWIN*2)
cspice_gfoclt, occtyp, front, fshape, fframe, back, bshape, $
bframe, abcorr, obsrvr, step, cnfine, result
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, the output was:
From : 2001-DEC-14 20:10:14.195952 (TDB)
To : 2001-DEC-14 21:35:50.317994 (TDB)
2) Find occultations of Titan by Saturn or of Saturn by
Titan as seen from the center of the Earth over the
last three months of 2008. Search for every type
of occultation.
Use light time corrections to model apparent positions of
Saturn and Titan. Stellar aberration corrections are not
specified because they don't affect occultation computations.
We select a step size of 15 minutes, which means we
ignore occultation events lasting less than 15 minutes,
if any exist.
Use the SPK kernel below for providing the Titan ephemerides
and the meta-kernel from example 1 above.
sat427.bsp
Example code begins here.
PRO gfoclt_ex2
MAXWIN = 1000
TIMFMT = 'YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND'
TIMLEN = 41
OCCTYP = ['FULL', 'ANNULAR', 'PARTIAL', 'ANY' ]
;;
;; Load kernels.
;;
cspice_furnsh, 'gfoclt_ex1.tm'
cspice_furnsh, 'sat427.bsp'
;;
;; Store the time bounds of our search interval in
;; the cnfine confinement window.
;;
cspice_str2et, [ '2008 SEP 01 00:00:00 TDB', $
'2009 JAN 01 00:00:00 TDB'], et
cnfine = cspice_celld( 2 )
cspice_wninsd, et[0], et[1], cnfine
;;
;; Select a 15-minute step. We'll ignore any occultations
;; lasting less than 15 minutes.
;;
step = 900.D
;;
;; The observation location is the Earth.
;;
obsrvr = 'earth'
shape = 'ellipsoid'
abcorr = 'lt'
result = cspice_celld( MAXWIN*2)
;;
;; Loop over the occultation types.
;;
for i=0, n_elements(OCCTYP)-1 do begin
;;
;; For each type, do a search for both transits of
;; Titan across Saturn and occultations of Titan by
;; Saturn.
;;
for j=0, 1 do begin
if ( j EQ 0 ) then begin
front = 'TITAN'
fframe = 'IAU_TITAN'
back = 'SATURN'
bframe = 'IAU_SATURN'
endif else begin
front = 'SATURN'
fframe = 'IAU_SATURN'
back = 'TITAN'
bframe = 'IAU_TITAN'
endelse
;;
;; Perform the search.
;;
cspice_gfoclt, OCCTYP[i], front, shape, fframe, $
back, shape, bframe, $
abcorr, obsrvr, step, cnfine, result
;;
;; List the beginning and ending points in each interval
;; if result contains data.
;;
print, 'Condition : ', OCCTYP[i]
print, 'Occultation of : ', back
print, 'by : ', front
print
count = cspice_wncard( result )
if ( count eq 0 ) then begin
print, 'Result window is empty.'
print
endif else begin
for k= 0L, (count - 1L ) do begin
cspice_wnfetd, result, k, left, right
cspice_timout, [left, right], TIMFMT, TIMLEN, timstr
if ( left eq right ) then begin
print, 'Event time: ', timstr[0]
print
endif else begin
print, 'From : ', timstr[0]
print, 'To : ', timstr[1]
print
endelse
endfor
endelse
;;
;; We've finished displaying the results of the
;; current search.
;;
endfor
;;
;; We've finished displaying the results of the
;; searches using the current occultation type.
;;
endfor
;;
;; 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, the output was:
Condition : FULL
Occultation of : SATURN
by : TITAN
Result window is empty.
Condition : FULL
Occultation of : TITAN
by : SATURN
From : 2008-OCT-27 22:08:01.672540 (TDB)
To : 2008-OCT-28 01:05:03.332576 (TDB)
From : 2008-NOV-12 21:21:59.270692 (TDB)
To : 2008-NOV-13 02:06:05.034713 (TDB)
From : 2008-NOV-28 20:49:02.415745 (TDB)
To : 2008-NOV-29 02:13:58.978005 (TDB)
From : 2008-DEC-14 20:05:09.258916 (TDB)
To : 2008-DEC-15 01:44:53.517960 (TDB)
From : 2008-DEC-30 19:00:56.586894 (TDB)
To : 2008-DEC-31 00:42:43.219312 (TDB)
Condition : ANNULAR
Occultation of : SATURN
by : TITAN
From : 2008-OCT-19 21:29:20.694709 (TDB)
To : 2008-OCT-19 22:53:34.442728 (TDB)
From : 2008-NOV-04 20:15:38.652651 (TDB)
To : 2008-NOV-05 00:18:59.130645 (TDB)
From : 2008-NOV-20 19:38:59.674044 (TDB)
To : 2008-NOV-21 00:35:26.726756 (TDB)
From : 2008-DEC-06 18:58:34.093679 (TDB)
To : 2008-DEC-07 00:16:17.653067 (TDB)
From : 2008-DEC-22 18:02:46.308376 (TDB)
To : 2008-DEC-22 23:26:52.721881 (TDB)
Condition : ANNULAR
Occultation of : TITAN
by : SATURN
Result window is empty.
Condition : PARTIAL
Occultation of : SATURN
by : TITAN
From : 2008-OCT-19 20:44:30.377190 (TDB)
To : 2008-OCT-19 21:29:20.694709 (TDB)
From : 2008-OCT-19 22:53:34.442728 (TDB)
To : 2008-OCT-19 23:38:26.219866 (TDB)
From : 2008-NOV-04 19:54:40.368045 (TDB)
To : 2008-NOV-04 20:15:38.652651 (TDB)
From : 2008-NOV-05 00:18:59.130645 (TDB)
To : 2008-NOV-05 00:39:58.607160 (TDB)
From : 2008-NOV-20 19:21:46.714397 (TDB)
To : 2008-NOV-20 19:38:59.674044 (TDB)
From : 2008-NOV-21 00:35:26.726756 (TDB)
To : 2008-NOV-21 00:52:40.606954 (TDB)
From : 2008-DEC-06 18:42:36.120123 (TDB)
To : 2008-DEC-06 18:58:34.093679 (TDB)
From : 2008-DEC-07 00:16:17.653067 (TDB)
To : 2008-DEC-07 00:32:16.331200 (TDB)
From : 2008-DEC-22 17:47:10.796148 (TDB)
To : 2008-DEC-22 18:02:46.308376 (TDB)
From : 2008-DEC-22 23:26:52.721881 (TDB)
To : 2008-DEC-22 23:42:28.860689 (TDB)
Condition : PARTIAL
Occultation of : TITAN
by : SATURN
From : 2008-OCT-27 21:37:17.003994 (TDB)
To : 2008-OCT-27 22:08:01.672540 (TDB)
From : 2008-OCT-28 01:05:03.332576 (TDB)
To : 2008-OCT-28 01:35:49.235671 (TDB)
From : 2008-NOV-12 21:01:47.121213 (TDB)
To : 2008-NOV-12 21:21:59.270692 (TDB)
From : 2008-NOV-13 02:06:05.034713 (TDB)
To : 2008-NOV-13 02:26:18.211754 (TDB)
[...]
Warning: incomplete output. Only 100 out of 156 lines have been
provided.
This routine provides a simple interface for conducting searches for
occultation events.
This routine determines a set of one or more time intervals
within the confinement window when a specified type of
occultation occurs. 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
==================
The search for occultations is treated as a search for state
transitions: times are sought when the state of the `back' body
changes from "not occulted" to "occulted" or vice versa.
Step Size
=========
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 occultation state will be sampled.
Starting at the left endpoint of an interval, samples will be
taken at each step. If a state change is detected, a root has
been bracketed; at that point, the "root"--the time at which the
state change occurs---is found by a refinement process, for
example, by a binary search.
Note that the optimal choice of step size depends on the lengths
of the intervals over which the occultation state is constant:
the step size should be shorter than the shortest occultation
duration and the shortest period between occultations, within
the confinement window.
Having some knowledge of the relative geometry of the targets 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 convergence tolerance used by this
routine is set by the parameter SPICE_GF_CNVTOL.
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 in seconds
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 affect 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.
Using DSK data
==============
DSK loading and unloading
-------------------------
DSK files providing data used by this routine are loaded by
calling cspice_furnsh and can be unloaded by calling cspice_unload or
cspice_kclear. See the documentation of cspice_furnsh for limits on
numbers of loaded DSK files.
For run-time efficiency, it's desirable to avoid frequent
loading and unloading of DSK files. When there is a reason to
use multiple versions of data for a given target body---for
example, if topographic data at varying resolutions are to be
used---the surface list can be used to select DSK data to be
used for a given computation. It is not necessary to unload
the data that are not to be used. This recommendation presumes
that DSKs containing different versions of surface data for a
given body have different surface ID codes.
DSK data priority
-----------------
A DSK coverage overlap occurs when two segments in loaded DSK
files cover part or all of the same domain---for example, a
given longitude-latitude rectangle---and when the time
intervals of the segments overlap as well.
When DSK data selection is prioritized, in case of a coverage
overlap, if the two competing segments are in different DSK
files, the segment in the DSK file loaded last takes
precedence. If the two segments are in the same file, the
segment located closer to the end of the file takes
precedence.
When DSK data selection is unprioritized, data from competing
segments are combined. For example, if two competing segments
both represent a surface as a set of triangular plates, the
union of those sets of plates is considered to represent the
surface.
Currently only unprioritized data selection is supported.
Because prioritized data selection may be the default behavior
in a later version of the routine, the UNPRIORITIZED keyword is
required in the `fshape' and `bshape' arguments.
Syntax of the shape input arguments for the DSK case
----------------------------------------------------
The keywords and surface list in the target shape arguments
`bshape' and `fshape' are called "clauses." The clauses may
appear in any order, for example
"DSK/<surface list>/UNPRIORITIZED"
"DSK/UNPRIORITIZED/<surface list>"
"UNPRIORITIZED/<surface list>/DSK"
The simplest form of the `method' argument specifying use of
DSK data is one that lacks a surface list, for example:
"DSK/UNPRIORITIZED"
For applications in which all loaded DSK data for the target
body are for a single surface, and there are no competing
segments, the above string suffices. This is expected to be
the usual case.
When, for the specified target body, there are loaded DSK
files providing data for multiple surfaces for that body, the
surfaces to be used by this routine for a given call must be
specified in a surface list, unless data from all of the
surfaces are to be used together.
The surface list consists of the string
"SURFACES = "
followed by a comma-separated list of one or more surface
identifiers. The identifiers may be names or integer codes in
string format. For example, suppose we have the surface
names and corresponding ID codes shown below:
Surface Name ID code
------------ -------
"Mars MEGDR 128 PIXEL/DEG" 1
"Mars MEGDR 64 PIXEL/DEG" 2
"Mars_MRO_HIRISE" 3
If data for all of the above surfaces are loaded, then
data for surface 1 can be specified by either
'SURFACES = 1'
or
'SURFACES = "Mars MEGDR 128 PIXEL/DEG"'
Double quotes are used to delimit the surface name because
it contains blank characters.
To use data for surfaces 2 and 3 together, any
of the following surface lists could be used:
'SURFACES = 2, 3'
'SURFACES = "Mars MEGDR 64 PIXEL/DEG", 3'
'SURFACES = 2, Mars_MRO_HIRISE'
'SURFACES = "Mars MEGDR 64 PIXEL/DEG", Mars_MRO_HIRISE'
An example of a shape argument that could be constructed
using one of the surface lists above is
'DSK/UNPRIORITIZED/SURFACES = "Mars MEGDR 64 PIXEL/DEG", 3'
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.
The result window may need to be contracted slightly by the
caller to achieve desired results. The SPICE window routine
cspice_wncond can be used to contract the result window.
3) If name of either target or the observer cannot be translated
to a NAIF ID code, an error is signaled by a routine
in the call tree of this routine.
4) If the radii of a target body modeled as an ellipsoid cannot
be determined by searching the kernel pool for a kernel
variable having a name of the form
'BODYnnn_RADII'
where nnn represents the NAIF integer code associated with
the body, an error is signaled by a routine in the
call tree of this routine.
5) If either of the target bodies `front' or `back' coincides with
the observer body `obsrvr', an error is signaled by a
routine in the call tree of this routine.
6) If the body designated by `front' coincides with that
designated by `back', an error is signaled by a routine
in the call tree of this routine.
7) If either of the body model specifiers `fshape' or `bshape'
is not recognized, an error is signaled by a routine
in the call tree of this routine.
8) If both of the body model specifiers `fshape' and `bshape'
specify point targets, an error is signaled by a
routine in the call tree of this routine.
9) If one of the body model specifiers `fshape' and `bshape'
specifies a DSK model, and the other argument does not
specify a point target, an error is signaled by a routine in
the call tree of this routine.
10) If a target body-fixed reference frame associated with a
non-point target is not recognized, an error is signaled by a
routine in the call tree of this routine.
11) If a target body-fixed reference frame is not centered at the
corresponding target body, an error is signaled by a routine
in the call tree of this routine.
12) If the loaded kernels provide insufficient data to compute any
required state vector, an error is signaled by a routine in
the call tree of this routine.
13) If an error occurs while reading an SPK or other kernel file,
the error is signaled by a routine in the call tree
of this routine.
14) If a point target is specified and the occultation type is set
to a valid value other than 'ANY', an error is signaled by a
routine in the call tree of this routine.
15) If the output SPICE window `result' has size less than 2, the
error SPICE(WINDOWTOOSMALL) is signaled by a routine in the
call tree of this routine.
16) If the output SPICE window `result' has insufficient capacity
to contain the number of intervals on which the specified
occultation condition is met, an error is signaled
by a routine in the call tree of this routine.
17) If the occultation type `occtyp' is invalid, an error is
signaled by a routine in the call tree of this routine.
18) If the aberration correction specification `abcorr' is invalid,
an error is signaled by a routine in the call tree of this
routine.
19) If either `fshape' or `bshape' specifies that the target surface
is represented by DSK data, and no DSK files are loaded for
the specified target, an error is signaled by a routine in
the call tree of this routine.
20) If either `fshape' or `bshape' specifies that the target surface
is represented by DSK data, but the shape specification is
invalid, an error is signaled by a routine in the call tree
of this routine.
21) If any of the input arguments, `occtyp', `front', `fshape',
`fframe', `back', `bshape', `bframe', `abcorr', `obsrvr',
`step', `cnfine' or `result', is undefined, an error is
signaled by the IDL error handling system.
22) If any of the input arguments, `occtyp', `front', `fshape',
`fframe', `back', `bshape', `bframe', `abcorr', `obsrvr',
`step', `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 SPICE 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, source and observer that cover the time
period specified by the window `cnfine'. If aberration
corrections are used, the states of the target bodies and of
the 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 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 via cspice_furnsh.
- FK data: if either of the reference frames designated by
`bframe' or `fframe' are not built in to the SPICE system,
one or more FKs specifying these frames must be loaded.
The following data may be required:
- DSK data: if either `fshape' or `bshape' indicates that DSK
data are to be used, DSK files containing topographic data
for the target body must be loaded. If a surface list is
specified, data for at least one of the listed surfaces must
be loaded.
- Surface name-ID associations: if surface names are specified
in `fshape' or `bshape', the association of these names with
their corresponding surface ID codes must be established by
assignments of the kernel variables
NAIF_SURFACE_NAME
NAIF_SURFACE_CODE
NAIF_SURFACE_BODY
Normally these associations are made by loading a text
kernel containing the necessary assignments. An example
of such a set of assignments is
NAIF_SURFACE_NAME += 'Mars MEGDR 128 PIXEL/DEG'
NAIF_SURFACE_CODE += 1
NAIF_SURFACE_BODY += 499
- CK data: either of the body-fixed frames to which `fframe' or
`bframe' refer might be a CK frame. If so, at least one CK
file will be needed to permit transformation of vectors
between that frame and the J2000 frame.
- SCLK data: if a CK file is needed, an associated SCLK
kernel is required to enable conversion between encoded SCLK
(used to time-tag CK data) and barycentric dynamical time
(TDB).
Kernel data are normally loaded once per program run, NOT every
time this routine is called.
1) The kernel files to be used by cspice_gfoclt must be loaded (normally
via the Icy routine cspice_furnsh) before cspice_gfoclt is called.
CK.REQ
FRAMES.REQ
GF.REQ
ICY.REQ
KERNEL.REQ
NAIF_IDS.REQ
SPK.REQ
TIME.REQ
WINDOWS.REQ
None.
N.J. Bachman (JPL)
J. Diaz del Rio (ODC Space)
E.D. Wright (JPL)
-Icy Version 2.0.1, 25-NOV-2021 (JDR) (NJB)
Added -Parameters, -Exceptions, -Files, -Restrictions,
-Literature_References and -Author_and_Institution sections.
Edited the header to comply with NAIF standard. Renamed
example's meta-kernel.
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 2.0.0, 04-APR-2017 (EDW) (NJB)
Updated to support use of DSKs.
-Icy Version 1.0.1, 14-MAY-2012 (EDW)
Header updated to describe use of cspice_gfstol.
-Icy Version 1.0.0, 15-APR-2009 (EDW)
GF occultation search
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