| gfocce_c |
|
Table of contents
Procedure
gfocce_c ( GF, occultation event )
void gfocce_c ( ConstSpiceChar * occtyp,
ConstSpiceChar * front,
ConstSpiceChar * fshape,
ConstSpiceChar * fframe,
ConstSpiceChar * back,
ConstSpiceChar * bshape,
ConstSpiceChar * bframe,
ConstSpiceChar * abcorr,
ConstSpiceChar * obsrvr,
SpiceDouble tol,
void ( * udstep ) ( SpiceDouble et,
SpiceDouble * step ),
void ( * udrefn ) ( SpiceDouble t1,
SpiceDouble t2,
SpiceBoolean s1,
SpiceBoolean s2,
SpiceDouble * t ),
SpiceBoolean rpt,
void ( * udrepi ) ( SpiceCell * cnfine,
ConstSpiceChar * srcpre,
ConstSpiceChar * srcsuf ),
void ( * udrepu ) ( SpiceDouble ivbeg,
SpiceDouble ivend,
SpiceDouble et ),
void ( * udrepf ) ( void ),
SpiceBoolean bail,
SpiceBoolean ( * udbail ) ( void ),
SpiceCell * cnfine,
SpiceCell * result )
AbstractDetermine time intervals when an observer sees one target occulted by another. Report progress and handle interrupts if so commanded. The surfaces of the target bodies may be represented by triaxial ellipsoids or by topographic data provided by DSK files. Required_ReadingFRAMES GF KERNEL NAIF_IDS SPK TIME WINDOWS KeywordsEVENT GEOMETRY SEARCH WINDOW Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- 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-O SPICE window to which the search is restricted. result 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 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'.
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 empty or blank.
Case and leading or trailing blanks bracketing a
non-blank frame name 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 empty or 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_c 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 prototype for `udstep' is
void ( * udstep ) ( SpiceDouble et,
SpiceDouble * 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.
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. Units are TDB seconds.
If a constant step size is desired, the CSPICE routine
gfstep_c
may be used as the step size function. If gfstep_c is used,
the step size must be set by calling gfsstp_c 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
prototype for `udrefn' is:
void ( * udrefn ) ( SpiceDouble t1,
SpiceDouble t2,
SpiceBoolean s1,
SpiceBoolean s2,
SpiceDouble * 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'.
s2 is the visibility state at time `t2'.
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 CSPICE
routine gfrefn_c may be used.
rpt is a logical variable which controls whether progress
reporting is enabled. When `rpt' is SPICETRUE, progress
reporting is enabled and the routines `udrepi', `udrepu', and
`udrepf' (see descriptions below) are used to report
progress.
udrepi is a user-defined routine that initializes a progress
report. When progress reporting is enabled, `udrepi' is
called at the start of a search. The prototype for
`udrepi' is
void ( * udrepi ) ( SpiceCell * cnfine,
ConstSpiceChar * srcpre,
ConstSpiceChar * 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 CSPICE routine gfrepi_c may be used as the actual
argument corresponding to `udrepi'. If so, the CSPICE
routines gfrepu_c and gfrepf_c must be the actual arguments
corresponding to `udrepu' and `udrepf'.
udrepu is a user-defined routine that updates the progress
report for a search. The prototype of `udrepu' is
void ( * udrepu ) ( SpiceDouble ivbeg,
SpiceDouble ivend,
SpiceDouble 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 (that is, the sum of the
lengths of the intervals) of the confinement window
`cnfine'.
`et' is the current time reached in the search for an event.
`et' must lie in the interval
ivbeg : ivend
inclusive. The input values of `et' for a given interval
need not form an increasing sequence.
The CSPICE routine gfrepu_c may be used as the actual
argument corresponding to `udrepu'. If so, the CSPICE
routines gfrepi_c and gfrepf_c must be the actual arguments
corresponding to `udrepi' and `udrepf'.
udrepf is a user-defined routine that finalizes a
progress report. `udrepf' has no arguments.
The CSPICE routine gfrepf_c may be used as the actual
argument corresponding to `udrepf'. If so, the CSPICE
routines gfrepi_c and gfrepu_c 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 SPICETRUE, 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). The prototype of `udbail'
is
SpiceBoolean ( * udbail ) ( void )
The return value is SPICETRUE if an interrupt has
been issued; otherwise the value is SPICEFALSE.
gfocce_c uses `udbail' only when `bail' (see above) is set to
SPICETRUE, indicating that interrupt handling is enabled.
When interrupt handling is enabled, gfocce_c 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_c as an input argument. The
CSPICE function
gfbail_c
may be used for this purpose.
The function `udbail' will usually be tested multiple times by
the GF system between the time an interrupt is issued and the
time when control is returned to the calling program, so
`udbail' must continue to return SPICETRUE until explicitly
reset by the calling application. Therefore `udbail' must
provide a "reset" mechanism." In the case of gfbail_c, the reset
function is
gfclrh_c
See the -Examples header section below for a complete code
example demonstrating use of the CSPICE interrupt handling
capability.
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 declared as a double precision SpiceCell.
CSPICE provides the following macro, which declares and
initializes the cell
SPICEDOUBLE_CELL ( cnfine, CNFINESZ );
where CNFINESZ is the maximum capacity of `cnfine'.
Detailed_Output
cnfine is the input confinement window, updated if necessary so the
control area of its data array indicates the window's size
and cardinality. The window data are unchanged.
result is a SPICE window representing the set of time intervals,
within the confinement period, when the specified
occultation occurs.
`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.
If `result' is non-empty on input, its contents will be
discarded before gfocce_c conducts its search.
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.
`result' must be declared as a double precision SpiceCell.
CSPICE provides the following macro, which declares and
initializes the cell
SPICEDOUBLE_CELL ( result, RESULTSZ );
where RESULTSZ is the maximum capacity of `result'.
ParametersNone. 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_c 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 by a routine in the call tree of this routine.
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 by a routine in the call
tree of this routine.
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 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 operation of this routine is interrupted, the output result
window will be invalid.
22) If any of the `occtyp', `front', `fshape', `back', `bshape',
`abcorr', `obsrvr', `bframe' or `fframe' input string pointers
is null, the error SPICE(NULLPOINTER) is signaled.
23) If any of the `occtyp', `front', `fshape', `back', `bshape',
`abcorr' or `obsrvr' input strings has zero length, the error
SPICE(EMPTYSTRING) is signaled.
24) If any the `cnfine' or `result' cell arguments has a type
other than SpiceDouble, the error SPICE(TYPEMISMATCH) is
signaled.
25) If any attempt to change the handler for the interrupt signal
SIGINT fails, the error SPICE(SIGNALFAILED) is signaled.
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 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 furnsh_c.
- 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_c.
- 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.
ParticularsThis 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_c 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 SPICE_GF_CNVTOL, which is declared in the header file SpiceGF.h. 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. Making 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. 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_c and can be unloaded by calling unload_c or kclear_c. See the documentation of furnsh_c 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\"" Escaped 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" 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 default GF progress reporting
and interrupt handling capabilities.
The program will use console I/O to display a simple
ASCII-based progress report.
The program will trap keyboard interrupts (on most systems,
generated by typing the "control C" key combination). This
feature can be used in non-trivial applications to allow
the application to continue after a search as been interrupted.
The program will find occultations of the Sun by the Moon 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 20 seconds, which implies we ignore
occultation events lasting less than 20 seconds, if any exist.
Given this step size and the length of the search interval, the
user has time to interrupt the computation. In an interactive
setting, the user might speed up the search by lengthening the
step size or shortening the search interval, as long as these
adjustments don't prevent the search from finding the correct
solution.
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
./
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
/.
Constants
./
#define TIMFMT "YYYY MON DD HR:MN:SC.###### ::TDB (TDB)"
#define CNVTOL 1.e-6
#define MAXWIN 200
#define TIMLEN 41
/.
Local variables
./
SpiceBoolean bail;
SpiceBoolean rpt;
SpiceChar * win0;
SpiceChar * win1;
SpiceChar begstr [ TIMLEN ];
SpiceChar endstr [ TIMLEN ];
SPICEDOUBLE_CELL ( cnfine, MAXWIN );
SPICEDOUBLE_CELL ( result, MAXWIN );
SpiceDouble et0;
SpiceDouble et1;
SpiceDouble left;
SpiceDouble right;
SpiceInt i;
/.
Load kernels.
./
furnsh_c ( "gfocce_ex1.tm" );
/.
Obtain the TDB time bounds of the confinement
window, which is a single interval in this case.
./
win0 = "2001 DEC 10 00:00:00 TDB";
win1 = "2002 JAN 01 00:00:00 TDB";
str2et_c ( win0, &et0 );
str2et_c ( win1, &et1 );
/.
Insert the time bounds into the confinement
window.
./
wninsd_c ( et0, et1, &cnfine );
/.
Select a twenty-second step. We'll ignore any occultations
lasting less than 20 seconds.
./
gfsstp_c ( 20.0 );
/.
Turn on interrupt handling and progress reporting.
./
bail = SPICETRUE;
rpt = SPICETRUE;
/.
Perform the search.
./
gfocce_c ( "ANY",
"MOON", "ellipsoid", "IAU_MOON",
"SUN", "ellipsoid", "IAU_SUN",
"LT", "EARTH", CNVTOL,
gfstep_c, gfrefn_c, rpt,
gfrepi_c, gfrepu_c, gfrepf_c,
bail, gfbail_c, &cnfine,
&result );
if ( gfbail_c() )
{
/.
Clear the CSPICE interrupt indication. This is
an essential step for programs that continue
running after an interrupt; gfbail_c will
continue to return SPICETRUE until this step
has been performed.
./
gfclrh_c();
/.
We've trapped an interrupt signal. In a realistic
application, the program would continue operation
from this point. In this simple example, we simply
display a message and quit.
./
printf ( "\nSearch was interrupted.\n\nThis message "
"was written after an interrupt signal\n"
"was trapped. By default, the program "
"would have terminated \nbefore this message "
"could be written.\n\n" );
}
else
{
if ( wncard_c(&result) == 0 )
{
printf ( "No occultation was found.\n" );
}
else
{
for ( i = 0; i < wncard_c(&result); i++ )
{
/.
fetch and display each occultation interval.
./
wnfetd_c ( &result, i, &left, &right );
timout_c ( left, TIMFMT, TIMLEN, begstr );
timout_c ( right, TIMFMT, TIMLEN, endstr );
printf ( "Interval %d\n", (int)i );
printf ( " Start time: %s\n", begstr );
printf ( " Stop time: %s\n", endstr );
}
}
}
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Occultation/transit search 100.00% done.
Interval 0
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_c, the caller must set the step
size by calling the entry point gfsstp_c before
calling gfocce_c. The call syntax for gfsstp_c is
gfsstp_c ( step );
Literature_ReferencesNone. Author_and_InstitutionN.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
-CSPICE Version 2.0.1, 06-AUG-2021 (JDR)
Edited the header to comply with NAIF standard.
Renamed example's meta-kernel.
Updated the description of "cnfine" and "result" arguments.
Added entries #15 and #24 in -Exceptions section.
-CSPICE Version 2.0.0, 29-FEB-2016 (NJB) (EDW)
Edit to example program to use "%d" with explicit casts
to int for printing SpiceInts with printf.
Updated to support use of DSKs.
-CSPICE Version 1.0.0, 15-APR-2009 (NJB) (LSE) (WLT) (IMU) (EDW)
Index_EntriesGF mid-level occultation search Link to routine gfocce_c source file gfocce_c.c |
Fri Dec 31 18:41:07 2021