| gffove_c |
|
Table of contents
Procedure
gffove_c ( GF, is target in FOV? )
void gffove_c ( ConstSpiceChar * inst,
ConstSpiceChar * tshape,
ConstSpiceDouble raydir [3],
ConstSpiceChar * target,
ConstSpiceChar * tframe,
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 a specified target body or ray intersects the space bounded by the field-of-view (FOV) of a specified instrument. Report progress and handle interrupts if so commanded. Required_ReadingCK FRAMES GF KERNEL NAIF_IDS PCK SPK TIME WINDOWS KeywordsEVENT FOV GEOMETRY INSTRUMENT SEARCH WINDOW Brief_I/O
VARIABLE I/O DESCRIPTION
-------- --- --------------------------------------------------
SPICE_GF_MARGIN
P Minimum complement of FOV cone angle.
SPICE_GF_CNVTOL
P Convergence tolerance.
SPICE_GF_MAXVRT
P Maximum number of FOV boundary vertices.
inst I Name of the instrument.
tshape I Type of shape model used for target body.
raydir I Ray's direction vector.
target I Name of the target body.
tframe I Body-fixed, body-centered frame for target body.
abcorr I Aberration correction flag.
obsrvr I Name of the observing body.
tol I Convergence tolerance in seconds.
udstep I Name of 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
inst is a string indicates the name of an instrument, such as
a spacecraft-mounted framing camera, the field of view
(FOV) of which is to be used for a target intersection
search: times when the specified target intersects the
region of space corresponding to the FOV are sought.
`inst' must have a corresponding NAIF ID and a frame
defined, as is normally done in a frame kernel. It must
also have an associated reference frame and a FOV shape,
boresight and boundary vertices (or reference vector and
reference angles) defined, as is usually done in an
instrument kernel.
See the header of the CSPICE routine getfov_c for a
description of the required parameters associated with an
instrument.
tshape is a string indicating the geometric model used to
represent the location and shape of the target body. The
target body may be represented by either an ephemeris
object or a ray emanating from the observer.
The supported values of `tshape' are:
"ELLIPSOID" The target is an ephemeris object.
The target's shape is represented
using 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" The target is an ephemeris object.
The body is treated as a single
point.
"RAY" The target is NOT an ephemeris
object. Instead, the target is
represented by the ray emanating
from the observer's location and
having direction vector `raydir'. The
target is considered to be visible
if and only if the ray is contained
within the space bounded by the
instrument FOV.
Case and leading or trailing blanks are not
significant in the string `tshape'.
raydir is the direction vector associated with a ray
representing the target. `raydir' is used if and only if
`tshape' (see description above) indicates the target is
modeled as a ray.
target is the name of the target body, the appearances of which
in the specified instrument's field of view are sought.
The body must be an ephemeris object.
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 `target'.
The input argument `target' is used if and only if the
target is NOT modeled as ray, as indicated by the input
argument `tshape'.
`target' may be set to a blank string if the target is
modeled as a ray.
tframe is the name of the reference frame associated with the
target. Examples of such names are "IAU_SATURN" (for
Saturn) and "ITRF93" (for the Earth).
If the target is an ephemeris object modeled as an
ellipsoid, `tframe' must designate a body-fixed reference
frame centered on the target body.
If the target is an ephemeris object modeled as a point,
`tframe' is ignored; `tframe' should be left blank.
If the target is modeled as a ray, `tframe' may designate
any reference frame. Since light time corrections are not
supported for rays, the orientation of the frame is
always evaluated at the epoch associated with the
observer, as opposed to the epoch associated with the
light-time corrected position of the frame center.
Case and leading or trailing blanks bracketing a
non-blank frame name are not significant in the string
`tframe'.
abcorr is a string indicating the aberration corrections to be
applied when computing the target's position and
orientation. The supported values of `abcorr' depend on the
target representation.
If the target is represented by a ray, the aberration
correction options are
"NONE" No correction.
"S" Stellar aberration correction, reception
case.
"XS" Stellar aberration correction,
transmission case.
If the target is an ephemeris object, the aberration
correction options are those supported by the SPICE SPK
system. For remote sensing applications, where the
apparent position and orientation of the target seen by
the observer are desired, normally either of the
corrections
"LT+S"
"CN+S"
should be used. These and the other supported options are
described below.
Supported aberration correction options for observation
(the case where radiation is received by observer at ET)
are:
"NONE" No correction.
"LT" Light time only
"LT+S" Light time and stellar aberration.
"CN" Converged Newtonian (CN) light time.
"CN+S" CN light time and stellar aberration.
Supported aberration correction options for transmission
(the case where radiation is emitted from observer at ET)
are:
"XLT" Light time only.
"XLT+S" Light time and stellar aberration.
"XCN" Converged Newtonian (CN) light time.
"XCN+S" CN light time and stellar aberration.
For detailed information, see the geometry finder
required reading, gf.req.
Case, leading and trailing blanks are not significant
in the string `abcorr'.
obsrvr is the name of the body from which the target is
observed. The instrument designated by `inst' is treated as
if it were co-located with the observer.
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 "visible" to being "not visible" or
vice versa.
This routine relies on `udstep' returning step sizes small
enough so that state transitions within the confinement
window are not overlooked.
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 and 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 next time to check for a state transition.
`t' is a number between `t1' and `t2'. `t' is
expressed as seconds past J2000 TDB.
If a simple bisection method is desired, the CSPICE
routine gfrefn_c may be used as the refinement function.
rpt is a logical variable that 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,
"Target visibility search"
"done."
the progress report display at the end of the search will
be:
Target visibility 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 of the confinement window.
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.
gffove_c uses `udbail' only when `bail' (see above) is set to
SPICETRUE, indicating that interrupt handling is enabled.
When interrupt handling is enabled, gffove_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 gffove_c as an input argument. The
CSPICE routine
gfbail_c
may be used for this purpose.
The function `udbail' will be 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.
So `udbail' must provide a "reset" mechanism."
In the case of gfbail_c, the reset function is
gfclrh_c
If interrupt handing is not enabled, a logical
function must still be passed to gffove_c as
an input argument. The CSPICE function
gfbail_c
may be used for this purpose.
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 image
of the target body is partially or completely within
the specified instrument field of view.
`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 gffove_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'.
Parameters
All parameters described here are declared in the header file
SpiceGF.h. See that file for parameter values.
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.
SPICE_GF_MAXVRT
is the maximum number of vertices that may be used
to define the boundary of the specified instrument's
field of view.
SPICE_GF_MARGIN
is a small positive number used to constrain the
orientation of the boundary vectors of polygonal
FOVs. Such FOVs must satisfy the following constraints:
1) The boundary vectors must be contained within
a right circular cone of angular radius less
than than (pi/2) - SPICE_GF_MARGIN radians; in other
words, there must be a vector A such that all
boundary vectors have angular separation from
A of less than (pi/2)-SPICE_GF_MARGIN radians.
2) There must be a pair of boundary vectors U, V
such that all other boundary vectors lie in the
same half space bounded by the plane containing U
and V. Furthermore, all other boundary vectors
must have orthogonal projections onto a specific
plane normal to this plane (the normal plane
contains the angle bisector defined by U and V)
such that the projections have angular separation
of at least 2*SPICE_GF_MARGIN radians from the
plane spanned by U and V.
See header file SpiceGF.h for declarations and descriptions of
parameters used throughout the GF system.
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 the name of either the target or 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 specified aberration correction is not a supported
value for the target type (ephemeris object or ray), an error
is signaled by a routine in the call tree of this routine.
5) 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.
6) If the target body coincides with the observer body `obsrvr', an
error is signaled by a routine in the call tree of this
routine.
7) If the body model specifier `tshape' is not recognized, an error
is signaled by a routine in the call tree of this routine.
8) 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.
9) 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.
10) If the instrument name `inst' does not have corresponding NAIF
ID code, an error is signaled by a routine in the call
tree of this routine.
11) If the FOV parameters of the instrument are not present in
the kernel pool, an error is signaled by a routine
in the call tree of this routine.
12) If the FOV boundary has more than SPICE_GF_MAXVRT vertices, an error
is signaled by a routine in the call tree of this
routine.
13) If the instrument FOV is polygonal, and this routine cannot
find a ray R emanating from the FOV vertex such that maximum
angular separation of R and any FOV boundary vector is within
the limit (pi/2)-SPICE_GF_MARGIN radians, an error is signaled
by a routine in the call tree of this routine. If the FOV is
any other shape, the same error check will be applied with the
instrument boresight vector serving the role of R.
14) If the loaded kernels provide insufficient data to compute a
requested state vector, an error is signaled by a
routine in the call tree of this routine.
15) 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.
16) If the output SPICE window `result' has insufficient capacity
to contain the number of intervals on which the specified
visibility condition is met, an error is signaled
by a routine in the call tree of this routine.
17) 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.
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 the step size is non-positive, an error is signaled by a
routine in the call tree of this routine.
20) If the ray's direction vector is zero, 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 `inst', `tshape', `abcorr', `tframe', `target'
or `obsrvr' input string pointers is null, the error
SPICE(NULLPOINTER) is signaled.
23) If any of the `inst', `tshape' or `abcorr' 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: ephemeris data for target and observer that
describes the ephemeris of these objects for the period
defined by the confinement window, `cnfine' must be
loaded. If aberration corrections are used, the states of
target and observer relative to the solar system barycenter
must be calculable from the available ephemeris data.
Typically ephemeris data are made available by loading one
or more SPK files via furnsh_c.
- Frame data: if a frame definition is required to convert
the observer and target states to the body-fixed frame of
the target, that definition must be available in the kernel
pool. Typically the definitions of frames not already
built-in to SPICE are supplied by loading a frame kernel.
Data defining the reference frame associated with the
instrument designated by `inst' must be available in the kernel
pool. Additionally the name `inst' must be associated with an
ID code. Normally these data are made available by loading
a frame kernel via furnsh_c.
- IK data: the kernel pool must contain data such that
the CSPICE routine getfov_c may be called to obtain
parameters for `inst'. Normally such data are provided by
an IK via furnsh_c.
The following data may be required:
- PCK data: bodies modeled as triaxial ellipsoids must have
orientation data provided by variables in the kernel pool.
Typically these data are made available by loading a text
PCK file via furnsh_c.
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.
- CK data: if the instrument frame is fixed to a spacecraft,
at least one CK file will be needed to permit transformation
of vectors between that frame and both J2000 and the target
body-fixed 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).
- Since the input ray direction may be expressed in any
frame, FKs, CKs, SCLK kernels, PCKs, and SPKs may be
required to map the direction to the J2000 frame.
Kernel data are normally loaded once per program run, NOT every
time this routine is called.
ParticularsThis routine determines a set of one or more time intervals within the confinement window when a specified ray or any portion of a specified target body appears within the field of view of a specified instrument. We'll use the term "visibility event" to designate such an appearance. The set of time intervals resulting from the search is returned as a SPICE window. This routine provides the SPICE GF system's most flexible interface for searching for FOV intersection 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 either gftfov_c or gfrfov_c rather than this routine. 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 visibility events is treated as a search for state transitions: times are sought when the state of the target ray or body changes from "not visible" to "visible" 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 visibility 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 visibility state is constant: the step size should be shorter than the shortest visibility event duration and the shortest period between visibility events, within the confinement window. Having some knowledge of the relative geometry of the target and observer can be a valuable aid in picking a reasonable step size. In general, the user can compensate for lack of such knowledge by picking a very short step size; the cost is increased computation time. Note that the step size is not related to the precision with which the endpoints of the intervals of the result window are computed. That precision level is controlled by the convergence tolerance. Convergence Tolerance ===================== The times of state transitions are called ``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 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. Setting the input tolerance `tol' tighter than SPICE_GF_CNVTOL is unlikely to be useful, since the results are unlikely to be more accurate. Making the tolerance looser will speed up searches somewhat, since a few convergence steps will be omitted. However, in most cases, the step size is likely to have a much greater effect on processing time than would the convergence tolerance. The Confinement Window ====================== The simplest use of the confinement window is to specify a time interval within which a solution is sought. However, the confinement window can, in some cases, be used to make searches more efficient. Sometimes it's possible to do an efficient search to reduce the size of the time period over which a relatively slow search of interest must be performed. For an example, see the program CASCADE in the GF Example Programs chapter of the GF Required Reading, gf.req. Examples
The numerical results shown for these examples 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.
Search for times when Saturn's satellite Phoebe is within
the FOV of the Cassini narrow angle camera (CASSINI_ISS_NAC).
To simplify the problem, restrict the search to a short time
period where continuous Cassini bus attitude data are
available.
Use a step size of 1 second to reduce chances of missing
short visibility events.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: gffove_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
----------------------------- ----------------------
naif0012.tls Leapseconds
pck00010.tpc Satellite orientation
and radii
041014R_SCPSE_01066_04199.bsp CASSINI, planetary and
Saturn satellite
ephemeris
cas_v40.tf Cassini FK
04161_04164ra.bc Cassini bus CK
cas00071.tsc Cassini SCLK kernel
cas_iss_v10.ti Cassini IK
\begindata
KERNELS_TO_LOAD = ( 'naif0012.tls',
'pck00010.tpc',
'041014R_SCPSE_01066_04199.bsp',
'cas_v40.tf',
'04161_04164ra.bc',
'cas00071.tsc',
'cas_iss_v10.ti' )
\begintext
End of meta-kernel
Example code begins here.
/.
Program gffove_ex1
./
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
/.
Local constants
./
#define META "gffove_ex1.tm"
#define TIMFMT "YYYY-MON-DD HR:MN:SC.######::TDB (TDB)"
#define TIMLEN 41
#define MAXWIN 10000
#define TIMTOL 1.e-6
/.
Local variables
./
SPICEDOUBLE_CELL ( cnfine, MAXWIN );
SPICEDOUBLE_CELL ( result, MAXWIN );
SpiceBoolean bail;
SpiceBoolean rpt;
SpiceChar * abcorr;
SpiceChar * inst;
SpiceChar * obsrvr;
SpiceChar * target;
SpiceChar * tframe;
SpiceChar timstr [2][ TIMLEN ];
SpiceChar * tshape;
SpiceDouble endpt [2];
SpiceDouble et0;
SpiceDouble et1;
SpiceDouble raydir [3];
SpiceInt i;
SpiceInt j;
SpiceInt n;
/.
Load kernels.
./
furnsh_c ( META );
/.
Insert search time interval bounds into the
confinement window.
./
str2et_c ( "2004 JUN 11 06:30:00 TDB", &et0 );
str2et_c ( "2004 JUN 11 12:00:00 TDB", &et1 );
wninsd_c ( et0, et1, &cnfine );
/.
Initialize inputs for the search.
./
inst = "CASSINI_ISS_NAC";
target = "PHOEBE";
tshape = "ELLIPSOID";
tframe = "IAU_PHOEBE";
abcorr = "LT+S";
obsrvr = "CASSINI";
/.
Select a 1-second step. We'll ignore any target
appearances lasting less than 1 second.
./
gfsstp_c ( 1.0 );
printf ( "\n"
"Instrument: %s\n"
"Target: %s\n",
inst,
target );
/.
Turn on interrupt handling and progress reporting.
./
bail = SPICETRUE;
rpt = SPICETRUE;
/.
Perform the search.
./
gffove_c ( inst, tshape, raydir, target, tframe,
abcorr, obsrvr, TIMTOL, 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
{
n = wncard_c ( &result );
if ( n == 0 )
{
printf ( "No FOV intersection found.\n" );
}
else
{
printf ( " Visibility start time Stop time\n" );
for ( i = 0; i < n; i++ )
{
wnfetd_c ( &result, i, endpt, endpt+1 );
for ( j = 0; j < 2; j++ )
{
timout_c ( endpt[j], TIMFMT, TIMLEN, timstr[j] );
}
printf ( " %s %s\n", timstr[0], timstr[1] );
}
}
printf ( "\n" );
}
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Instrument: CASSINI_ISS_NAC
Target: PHOEBE
Target visibility search 100.00% done.
Visibility start time Stop time
2004-JUN-11 07:35:27.066980 (TDB) 2004-JUN-11 08:48:03.954696 (TDB)
2004-JUN-11 09:02:56.580045 (TDB) 2004-JUN-11 09:35:04.038509 (TDB)
2004-JUN-11 09:49:56.476397 (TDB) 2004-JUN-11 10:22:04.242879 (TDB)
2004-JUN-11 10:36:56.283771 (TDB) 2004-JUN-11 11:09:04.397165 (TDB)
2004-JUN-11 11:23:56.020645 (TDB) 2004-JUN-11 11:56:04.733536 (TDB)
Note that the progress report has the format shown below:
Target visibility search 2.66% done.
The completion percentage was updated approximately once per
second.
2) A variation of example (1): search the same confinement
window for times when a selected background star is visible.
We use the FOV of the Cassini ISS wide angle camera
(CASSINI_ISS_WAC) to enhance the probability of viewing the
star.
The star we'll use has catalog number 6000 in the Hipparcos
Catalog. The star's J2000 right ascension and declination,
proper motion, and parallax are taken from that catalog.
Use the meta-kernel from the first example.
Example code begins here.
/.
Program gffove_ex2
./
#include <stdio.h>
#include <math.h>
#include "SpiceUsr.h"
int main()
{
/.
Local constants
./
#define META "gffove_ex1.tm"
#define TIMFMT "YYYY-MON-DD HR:MN:SC.######::TDB (TDB)"
#define TIMLEN 41
#define MAXWIN 10000
#define TIMTOL 1.e-6
#define AU 149597870.693
/.
Local variables
./
SPICEDOUBLE_CELL ( cnfine, MAXWIN );
SPICEDOUBLE_CELL ( result, MAXWIN );
SpiceBoolean bail;
SpiceBoolean rpt;
SpiceChar * abcorr;
SpiceChar * inst;
SpiceChar * obsrvr;
SpiceChar * rframe;
SpiceChar * target;
SpiceChar timstr [2][ TIMLEN ];
SpiceChar * tshape;
SpiceDouble dec;
SpiceDouble decdeg;
SpiceDouble decdg0;
SpiceDouble decepc;
SpiceDouble decpm;
SpiceDouble dtdec;
SpiceDouble dtra;
SpiceDouble endpt [2];
SpiceDouble et0;
SpiceDouble et1;
SpiceDouble lt;
SpiceDouble parlax;
SpiceDouble plxdeg;
SpiceDouble pos [3];
SpiceDouble pstar [3];
SpiceDouble ra;
SpiceDouble radeg0;
SpiceDouble radeg;
SpiceDouble raepc;
SpiceDouble rapm;
SpiceDouble raydir [3];
SpiceDouble rstar;
SpiceDouble t;
SpiceInt catno;
SpiceInt i;
SpiceInt j;
SpiceInt n;
/.
Load kernels.
./
furnsh_c ( META );
/.
Insert search time interval bounds into the
confinement window.
./
str2et_c ( "2004 JUN 11 06:30:00 TDB", &et0 );
str2et_c ( "2004 JUN 11 12:00:00 TDB", &et1 );
wninsd_c ( et0, et1, &cnfine );
/.
Initialize inputs for the search.
./
inst = "CASSINI_ISS_WAC";
target = " ";
tshape = "RAY";
/.
Create a unit direction vector pointing from
observer to star. We'll assume the direction
is constant during the confinement window, and
we'll use et0 as the epoch at which to compute the
direction from the spacecraft to the star.
The data below are for the star with catalog
number 6000 in the Hipparcos catalog. Angular
units are degrees; epochs have units of Julian
years and have a reference epoch of J1950.
The reference frame is J2000.
./
catno = 6000;
plxdeg = 0.000001056;
radeg0 = 19.290789927;
rapm = -0.000000720;
raepc = 41.2000;
decdg0 = 2.015271007;
decpm = 0.000001814;
decepc = 41.1300;
rframe = "j2000";
/.
Correct the star's direction for proper motion.
The argument t represents et0 as Julian years
past J1950.
./
t = ( et0 / jyear_c() )
+ ( j2000_c()- j1950_c() ) / 365.25;
dtra = t - raepc;
dtdec = t - decepc;
radeg = radeg0 + dtra * rapm;
decdeg = decdg0 + dtdec * decpm;
ra = radeg * rpd_c();
dec = decdeg * rpd_c();
radrec_c ( 1.0, ra, dec, pstar );
/.
Correct star position for parallax applicable at
the Cassini orbiter's position. (The parallax effect
is negligible in this case; we're simply demonstrating
the computation.)
./
parlax = plxdeg * rpd_c();
rstar = AU / tan(parlax);
/.
Scale the star's direction vector by its distance from
the solar system barycenter. Subtract off the position
of the spacecraft relative to the solar system barycenter;
the result is the ray's direction vector.
./
vscl_c ( rstar, pstar, pstar );
spkpos_c ( "cassini", et0, "j2000", "none",
"solar system barycenter", pos, < );
vsub_c ( pstar, pos, raydir );
/.
Correct the star direction for stellar aberration when
we conduct the search.
./
abcorr = "S";
obsrvr = "CASSINI";
/.
Select a 1-second step. We'll ignore any target
appearances lasting less than 1 second.
./
gfsstp_c ( 1.0 );
/.
Turn on interrupt handling and progress reporting.
./
bail = SPICETRUE;
rpt = SPICETRUE;
printf ( "\n"
"Instrument: %s\n"
"Star's catalog number: %d\n",
inst,
(int)catno );
/.
Perform the search.
./
gffove_c ( inst, tshape, raydir, target, rframe,
abcorr, obsrvr, TIMTOL, 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
{
n = wncard_c ( &result );
if ( n == 0 )
{
printf ( "No FOV intersection found.\n" );
}
else
{
printf ( " Visibility start time Stop time\n" );
for ( i = 0; i < n; i++ )
{
wnfetd_c ( &result, i, endpt, endpt+1 );
for ( j = 0; j < 2; j++ )
{
timout_c ( endpt[j], TIMFMT, TIMLEN, timstr[j] );
}
printf ( " %s %s\n", timstr[0], timstr[1] );
}
}
printf ( "\n" );
}
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Instrument: CASSINI_ISS_WAC
Star's catalog number: 6000
Target visibility search 100.00% done.
Visibility start time Stop time
2004-JUN-11 06:30:00.000000 (TDB) 2004-JUN-11 12:00:00.000000 (TDB)
Restrictions
1) The kernel files to be used by gffove_c must be loaded (normally via
the CSPICE routine furnsh_c) before gffove_c is called.
Literature_ReferencesNone. Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) E.D. Wright (JPL) Version
-CSPICE Version 1.0.2, 06-AUG-2021 (JDR)
Edited the header to comply to comply with NAIF standard.
Updated Examples' kernels set to use PDS archived data.
Updated the description of "cnfine" and "result" arguments.
Added entries #17 and #24 in -Exceptions section.
-CSPICE Version 1.0.1, 12-JUL-2016 (EDW)
Edit to example program to use "%d" with explicit casts
to int for printing SpiceInts with printf.
-CSPICE Version 1.0.0, 15-APR-2009 (NJB) (EDW)
Index_EntriesGF low-level target in instrument FOV search |
Fri Dec 31 18:41:07 2021