| gfocce |
|
Table of contents
Procedure
GFOCCE ( GF, occultation event )
SUBROUTINE GFOCCE ( OCCTYP, FRONT, FSHAPE, FFRAME,
. BACK, BSHAPE, BFRAME, ABCORR,
. OBSRVR, TOL, UDSTEP, UDREFN,
. RPT, UDREPI, UDREPU, UDREPF,
. BAIL, UDBAIL, CNFINE, RESULT )
Abstract
Determine time intervals when an observer sees one target
occulted by another. Report progress and handle interrupts
if so commanded.
Required_Reading
FRAMES
GF
KERNEL
NAIF_IDS
SPK
TIME
WINDOWS
Keywords
EVENT
GEOMETRY
SEARCH
WINDOW
Declarations
IMPLICIT NONE
INCLUDE 'gf.inc'
INCLUDE 'zzdsk.inc'
INTEGER LBCELL
PARAMETER ( LBCELL = -5 )
CHARACTER*(*) OCCTYP
CHARACTER*(*) FRONT
CHARACTER*(*) FSHAPE
CHARACTER*(*) FFRAME
CHARACTER*(*) BACK
CHARACTER*(*) BSHAPE
CHARACTER*(*) BFRAME
CHARACTER*(*) ABCORR
CHARACTER*(*) OBSRVR
DOUBLE PRECISION TOL
EXTERNAL UDSTEP
EXTERNAL UDREFN
LOGICAL RPT
EXTERNAL UDREPI
EXTERNAL UDREPU
EXTERNAL UDREPF
LOGICAL BAIL
LOGICAL UDBAIL
EXTERNAL UDBAIL
DOUBLE PRECISION CNFINE ( LBCELL : * )
DOUBLE PRECISION RESULT ( LBCELL : * )
Brief_I/O
VARIABLE I/O DESCRIPTION
-------- --- --------------------------------------------------
LBCELL P SPICE Cell lower bound.
OCCTYP I Type of occultation.
FRONT I Name of body occulting the other.
FSHAPE I Type of shape model used for front body.
FFRAME I Body-fixed, body-centered frame for front body.
BACK I Name of body occulted by the other.
BSHAPE I Type of shape model used for back body.
BFRAME I Body-fixed, body-centered frame for back body.
ABCORR I Aberration correction flag.
OBSRVR I Name of the observing body.
TOL I Convergence tolerance in seconds.
UDSTEP I Name of the routine that returns a time step.
UDREFN I Name of the routine that computes a refined time.
RPT I Progress report flag.
UDREPI I Function that initializes progress reporting.
UDREPU I Function that updates the progress report.
UDREPF I Function that finalizes progress reporting.
BAIL I Logical indicating program interrupt monitoring.
UDBAIL I Name of a routine that signals a program interrupt.
CNFINE I SPICE window to which the search is restricted.
RESULT I-O SPICE window containing results.
Detailed_Input
OCCTYP indicates the type of occultation that is to be found.
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 an 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.
FRONT is the name of the target body that occults --- that is,
passes in front of --- the other. 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.
Case and leading or trailing blanks are not significant
in the string FRONT.
FSHAPE is a string indicating the geometric model used to
represent the shape of the front target body. 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 is the name of the body-fixed, body-centered reference
frame associated with the front target body. 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 blank.
Case and leading or trailing blanks are not
significant in the string FFRAME.
BACK is the name of the target body that is occulted by ---
that is, passes in back of --- the other. 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.
Case and leading or trailing blanks are not
significant in the string BACK.
BSHAPE is the shape specification for the body designated by
BACK. The supported options are those for FSHAPE. See the
description of FSHAPE above for details.
BFRAME is the name of the body-fixed, body-centered reference
frame associated with the "back" target body. See the
description of FFRAME above for details. 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 blank.
Case and leading or trailing blanks bracketing a
non-blank frame name are not significant in the string
BFRAME.
ABCORR indicates the aberration corrections to be applied to the
state of the target body to account for one-way light
time. Stellar aberration corrections are ignored if
specified, since these corrections don't improve the
accuracy of the occultation determination.
See the header of the SPICE routine SPKEZR for a detailed
description of the aberration correction options. For
convenience, the options supported by this routine are
listed below:
'NONE' Apply no correction.
'LT' "Reception" case: correct for
one-way light time using a Newtonian
formulation.
'CN' "Reception" case: converged
Newtonian light time correction.
'XLT' "Transmission" case: correct for
one-way light time using a Newtonian
formulation.
'XCN' "Transmission" case: converged
Newtonian light time correction.
Case and blanks are not significant in the string
ABCORR.
OBSRVR is the name of the body from which the occultation is
observed. Optionally, you may supply the integer NAIF
ID code for the body as a string.
Case and leading or trailing blanks are not
significant in the string OBSRVR.
TOL is a tolerance value used to determine convergence of
root-finding operations. TOL is measured in TDB seconds
and must be greater than zero.
UDSTEP is an externally specified routine that computes a
time step used to find transitions of the state being
considered. A state transition occurs where the state
changes from being "in occultation" to being "not in
occultation" or vice versa.
This routine relies on UDSTEP returning step sizes small
enough so that state transitions within the confinement
window are not overlooked. There must never be two roots
A and B separated by less than STEP, where STEP is the
minimum step size returned by UDSTEP for any value of ET;
in the interval [A, B].
The calling sequence for UDSTEP is:
CALL UDSTEP ( ET, STEP )
where:
ET is the input start time from which the
algorithm is to search forward for a state
transition. ET is expressed as seconds past
J2000 TDB. ET is a DOUBLE PRECISION number.
STEP is the output step size. STEP indicates
how far to advance ET so that ET and
ET+STEP may bracket a state transition and
definitely do not bracket more than one
state transition. STEP is a DOUBLE
PRECISION number. Units are TDB seconds.
If a constant step size is desired, the SPICELIB routine
GFSTEP
may be used as the step size function. If GFSTEP is used,
the step size must be set by calling GFSSTP prior to
calling this routine.
UDREFN is the name of the externally specified routine that
refines the times that bracket a transition point. In
other words, once a pair of times, T1 and T2, that
bracket a state transition have been found, UDREFN
computes an intermediate time T such that either [T1, T]
or [T, T2] contains the time of the state transition. The
calling sequence for UDREFN is:
CALL UDREFN ( T1, T2, S1, S2, T )
where the inputs are:
T1 is a time when the visibility state is S1. T1
is expressed as seconds past J2000 TDB.
T2 is a time when the visibility state is S2. T2 is
expressed as seconds past J2000 TDB. T2 is
assumed to be larger than T1.
S1 is the visibility state at time T1. S1 is a
LOGICAL value.
S2 is the visibility state at time T2. S2 is a
LOGICAL value.
The output is:
T is the next time to check for a state
transition. T is expressed as seconds past
J2000 TDB and is between T1 and T2.
If a simple bisection method is desired, the SPICELIB
routine GFREFN may be used.
RPT is a logical variable which controls whether progress
reporting is enabled. When RPT is .TRUE., progress
reporting is enabled and the routines UDREPI, UDREPU, and
UDREPF (see descriptions below) are used to report
progress.
UDREPI is a user-defined subroutine that initializes a progress
report. When progress reporting is enabled, UDREPI is
called at the start of a search. The calling sequence of
UDREPI is
UDREPI ( CNFINE, SRCPRE, SRCSUF )
DOUBLE PRECISION CNFINE ( LBCELL : * )
CHARACTER*(*) SRCPRE
CHARACTER*(*) SRCSUF
where
CNFINE
is the confinement window specifying the time period over
which a search is conducted, and
SRCPRE
SRCSUF
are prefix and suffix strings used in the progress
report: these strings are intended to bracket a
representation of the fraction of work done. For example,
when the CSPICE progress reporting functions are used, if
srcpre and srcsuf are, respectively,
"Occultation/transit search"
"done."
the progress report display at the end of the
search will be:
Occultation/transit search 100.00% done.
The SPICELIB routine GFREPI may be used as the actual
argument corresponding to UDREPI. If so, the SPICELIB
routines GFREPU and GFREPF must be the actual arguments
corresponding to UDREPU and UDREPF.
UDREPU is a user-defined subroutine that updates the progress
report for a search. The calling sequence of UDREPU is
UDREPU ( IVBEG, IVEND, ET )
DOUBLE PRECISION IVBEG
DOUBLE PRECISION IVEND
DOUBLE PRECISION ET
Here IVBEG, IVEND are the bounds of an interval that is
contained in some interval belonging to the confinement
window. The confinement window is associated with some
root finding activity. It is used to determine how much
total time is being searched in order to find the events
of interest.
ET is an epoch belonging to the interval
[IVBEG, IVEND].
In order for a meaningful progress report to be
displayed, IVBEG and IVEND must satisfy the following
constraints:
- IVBEG must be less than or equal to IVEND.
- The interval [ IVBEG, IVEND ] must be contained in
some interval of the confinement window. It can be
a proper subset of the containing interval; that
is, it can be smaller than the interval of the
confinement window that contains it.
- Over a search, the sum of the differences
IVEND - IVBEG
for all calls to this routine made during the search
must equal the measure of the confinement window.
The SPICELIB routine GFREPU may be used as the actual
argument corresponding to UDREPU. If so, the SPICELIB
routines GFREPI and GFREPF must be the actual arguments
corresponding to UDREPI and UDREPF.
UDREPF is a user-defined subroutine that finalizes a
progress report. UDREPF has no arguments.
The SPICELIB routine GFREPF may be used as the actual
argument corresponding to UDREPF. If so, the SPICELIB
routines GFREPI and GFREPU must be the actual arguments
corresponding to UDREPI and UDREPU.
BAIL is a logical variable indicating whether or not interrupt
handling is enabled. When BAIL is set to .TRUE., the
input function UDBAIL (see description below) is used to
determine whether an interrupt has been issued.
UDBAIL is the name of a user defined logical function that
indicates whether an interrupt signal has been issued
(for example, from the keyboard). UDBAIL has no
arguments and returns a LOGICAL value. The return value
is .TRUE. if an interrupt has been issued; otherwise the
value is .FALSE.
GFOCCE uses UDBAIL only when BAIL (see above) is set to
.TRUE., indicating that interrupt handling is enabled.
When interrupt handling is enabled, GFOCCE and routines
in its call tree will call UDBAIL to determine whether to
terminate processing and return immediately.
If interrupt handing is not enabled, a logical function
must still be passed to GFOCCE as an input argument. The
SPICELIB function
GFBAIL
may be used for this purpose.
CNFINE is a SPICE window that confines the time period over
which the specified search is conducted. CNFINE may
consist of a single interval or a collection of
intervals.
The endpoints of the time intervals comprising CNFINE
are interpreted as seconds past J2000 TDB.
See the $Examples section below for a code example
that shows how to create a confinement window.
CNFINE must be initialized by the caller via the
SPICELIB routine SSIZED.
RESULT is a double precision SPICE window which will contain
the search results. 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 condition is satisfied.
RESULT must be initialized by the caller via the
SPICELIB routine SSIZED.
If RESULT is non-empty on input, its contents will be
discarded before GFOCCE conducts its search.
Detailed_Output
RESULT is a SPICE window representing the set of time intervals,
within the confinement period, when the specified
occultation occurs.
The endpoints of the time intervals comprising RESULT are
interpreted as seconds past J2000 TDB.
If no times within the confinement window satisfy the
search criteria, RESULT will be returned with a
cardinality of zero.
Parameters
LBCELL is the SPICE cell lower bound.
Exceptions
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
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, the error SPICE(INVALIDSHAPECOMBO)
is signaled.
9) 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.
10) 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.
11) If the loaded kernels provide insufficient data to compute the
requested state vector, an error is signaled by a routine in
the call tree of this routine.
12) 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.
13) 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.
14) 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.
15) If the result window has size less than 2, the error
SPICE(WINDOWTOOSMALL) is signaled.
16) If the occultation type OCCTYP is invalid, an error is
signaled by a routine in the call tree of this routine.
17) If the aberration correction specification ABCORR is invalid,
an error is signaled by a routine in the call tree of this
routine.
18) If the convergence tolerance size is non-positive, the error
SPICE(INVALIDTOLERANCE) is signaled.
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 operation of this routine is interrupted, the output result
window will be invalid.
Files
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 target, 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
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 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.
Particulars
This routine provides the SPICE GF system's most flexible
interface for searching for occultation events.
Applications that require do not require support for progress
reporting, interrupt handling, non-default step or refinement
functions, or non-default convergence tolerance normally should
call GFOCLT rather than this routine.
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, via 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
=====================
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 high-level GF routines that
call this routine is set via the parameter CNVTOL, which is
declared in the INCLUDE file gf.inc. The value of 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.
Setting the input tolerance TOL tighter than 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. For an example, see
the program CASCADE in the GF Example Programs chapter of the GF
Required Reading, gf.req.
Using DSK data
==============
DSK loading and unloading
-------------------------
DSK files providing data used by this routine are loaded by
calling FURNSH and can be unloaded by calling UNLOAD or
KCLEAR. See the documentation of 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 sets 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
FSHAPE and BSHAPE, when DSK shape models are specified, 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 a target 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", 499003'
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) Conduct a search using the default GF progress reporting
capability.
The program will use console I/O to display a simple
ASCII-based progress report.
The program will find occultations of the Sun by the Moon as
seen from the center of the Earth over the month December,
2001.
We 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.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: gfocce_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.
PROGRAM GFOCCE_EX1
IMPLICIT NONE
EXTERNAL GFSTEP
EXTERNAL GFREFN
EXTERNAL GFREPI
EXTERNAL GFREPU
EXTERNAL GFREPF
INTEGER WNCARD
LOGICAL GFBAIL
EXTERNAL GFBAIL
C
C Local parameters
C
CHARACTER*(*) TIMFMT
PARAMETER ( TIMFMT =
. 'YYYY MON DD HR:MN:SC.###### ::TDB (TDB)' )
DOUBLE PRECISION CNVTOL
PARAMETER ( CNVTOL = 1.D-6 )
INTEGER MAXWIN
PARAMETER ( MAXWIN = 2 * 100 )
INTEGER TIMLEN
PARAMETER ( TIMLEN = 40 )
INTEGER LBCELL
PARAMETER ( LBCELL = -5 )
C
C Local variables
C
CHARACTER*(TIMLEN) WIN0
CHARACTER*(TIMLEN) WIN1
CHARACTER*(TIMLEN) BEGSTR
CHARACTER*(TIMLEN) ENDSTR
DOUBLE PRECISION CNFINE ( LBCELL : 2 )
DOUBLE PRECISION ET0
DOUBLE PRECISION ET1
DOUBLE PRECISION LEFT
DOUBLE PRECISION RESULT ( LBCELL : MAXWIN )
DOUBLE PRECISION RIGHT
INTEGER I
LOGICAL BAIL
LOGICAL RPT
C
C Saved variables
C
C The confinement and result windows CNFINE and RESULT are
C saved because this practice helps to prevent stack
C overflow.
C
SAVE CNFINE
SAVE RESULT
C
C Load kernels.
C
CALL FURNSH ( 'gfocce_ex1.tm' )
C
C Initialize the confinement and result windows.
C
CALL SSIZED ( 2, CNFINE )
CALL SSIZED ( MAXWIN, RESULT )
C
C Obtain the TDB time bounds of the confinement
C window, which is a single interval in this case.
C
WIN0 = '2001 DEC 01 00:00:00 TDB'
WIN1 = '2002 JAN 01 00:00:00 TDB'
CALL STR2ET ( WIN0, ET0 )
CALL STR2ET ( WIN1, ET1 )
C
C Insert the time bounds into the confinement
C window.
C
CALL WNINSD ( ET0, ET1, CNFINE )
C
C Select a 20 second step. We'll ignore any occultations
C lasting less than 20 seconds.
C
CALL GFSSTP ( 20.D0 )
C
C Turn on progress reporting; turn off interrupt
C handling.
C
RPT = .TRUE.
BAIL = .FALSE.
C
C Perform the search.
C
CALL GFOCCE ( 'ANY',
. 'MOON', 'ellipsoid', 'IAU_MOON',
. 'SUN', 'ellipsoid', 'IAU_SUN',
. 'LT', 'EARTH', CNVTOL,
. GFSTEP, GFREFN, RPT,
. GFREPI, GFREPU, GFREPF,
. BAIL, GFBAIL, CNFINE, RESULT )
IF ( WNCARD(RESULT) .EQ. 0 ) THEN
WRITE (*,*) 'No occultation was found.'
ELSE
DO I = 1, WNCARD(RESULT)
C
C Fetch and display each occultation interval.
C
CALL WNFETD ( RESULT, I, LEFT, RIGHT )
CALL TIMOUT ( LEFT, TIMFMT, BEGSTR )
CALL TIMOUT ( RIGHT, TIMFMT, ENDSTR )
WRITE (*,*) 'Interval ', I
WRITE (*,*) ' Start time: '//BEGSTR
WRITE (*,*) ' Stop time: '//ENDSTR
END DO
END IF
END
When this program was executed on a Mac/Intel/gfortran/64-bit
platform, the output was:
Occultation/transit search 100.00% done.
Interval 1
Start time: 2001 DEC 14 20:10:14.195952 (TDB)
Stop time: 2001 DEC 14 21:35:50.317994 (TDB)
Note that the progress report has the format shown below:
Occultation/transit search 6.02% done.
The completion percentage was updated approximately once per
second.
When the program was interrupted at an arbitrary time,
the output was:
Occultation/transit search 13.63% done.
Search was interrupted.
This message was written after an interrupt signal
was trapped. By default, the program would have terminated
before this message could be written.
Restrictions
1) If the caller passes in the default, constant step
size routine, GFSTEP, the caller must set the step
size by calling the entry point GFSSTP before
calling GFOCCE. The call syntax for GFSSTP is
CALL GFSSTP ( STEP )
Literature_References
None.
Author_and_Institution
N.J. Bachman (JPL)
J. Diaz del Rio (ODC Space)
L.S. Elson (JPL)
W.L. Taber (JPL)
I.M. Underwood (JPL)
E.D. Wright (JPL)
Version
SPICELIB Version 2.0.1, 27-AUG-2021 (JDR)
Edited the header to comply with NAIF standard.
Added note on program interruption in $Examples section.
Renamed example's meta-kernel. Added SAVE statements for CNFINE
and RESULT variables in code example.
Updated description of UDSTEP, UDREPI and RESULT arguments.
Added entries #15 and #21 to the $Exceptions section.
Corrected reporting message in UDREPI description.
SPICELIB Version 2.0.0, 24-FEB-2016 (NJB)
Now supports DSK target shapes.
Updated lengths of saved shape variables to accommodate
DSK "method" specifications.
SPICELIB Version 1.0.0, 15-APR-2009 (NJB) (LSE) (WLT) (IMU) (EDW)
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Fri Dec 31 18:36:24 2021