| gfilum_c |
|
Table of contents
Procedure
gfilum_c ( GF, illumination angle search )
void gfilum_c ( ConstSpiceChar * method,
ConstSpiceChar * angtyp,
ConstSpiceChar * target,
ConstSpiceChar * illmn,
ConstSpiceChar * fixref,
ConstSpiceChar * abcorr,
ConstSpiceChar * obsrvr,
ConstSpiceDouble spoint [3],
ConstSpiceChar * relate,
SpiceDouble refval,
SpiceDouble adjust,
SpiceDouble step,
SpiceInt nintvls,
SpiceCell * cnfine,
SpiceCell * result )
AbstractReturn the time window over which a specified constraint on the observed phase, solar incidence, or emission angle at a specified target body surface point is met. Required_ReadingGF FRAMES NAIF_IDS PCK SPK TIME KeywordsANGLE EPHEMERIS ILLUMINATION LIGHTING SEARCH Brief_I/O
VARIABLE I/O DESCRIPTION
-------- --- --------------------------------------------------
SPICE_GF_CNVTOL
P Convergence tolerance.
SPICE_GF_NWILUM
P Number of workspace windows for angle search.
method I Computation method.
angtyp I Type of illumination angle.
target I Name of the target body.
illmn I Name of the illumination source.
fixref I Body-fixed, body-centered target body frame.
abcorr I Aberration correction flag.
obsrvr I Name of the observing body.
spoint I Body-fixed coordinates of a target surface point.
relate I Relational operator.
refval I Reference value.
adjust I Adjustment value for absolute extrema searches.
step I Step size used for locating extrema and roots.
nintvls I Workspace window interval count.
cnfine I-O SPICE window to which the search is confined.
result O SPICE window containing results.
Detailed_Input
method is a short string providing parameters defining the
computation method to be used. Parameters include, but
are not limited to, the shape model used to represent the
surface of the target body.
The only choice currently supported is
"Ellipsoid" The illumination angle
computation uses a triaxial
ellipsoid to model the surface
of the target body. The
ellipsoid's radii must be
available in the kernel pool.
Neither case nor whitespaces are significant in `method'.
For example, the string " eLLipsoid " is valid.
angtyp is a string specifying the type of illumination angle for
which a search is to be performed. The possible values of
`angtyp' are
"PHASE"
"INCIDENCE"
"EMISSION"
When the illumination source is the sun, the incidence
angle is commonly called the "solar incidence angle."
See the -Particulars section below for a detailed
description of these angles.
Neither case nor whitespaces are significant in `angtyp'.
For example, the string " Incidence " is valid.
target is the name of a target body. The point at which the
illumination angles are defined is located on the surface
of this body.
Optionally, you may supply the integer ID code for the
object as an integer string. For example both "MOON" and
"301" are legitimate strings that indicate the moon is
the target body.
Neither case nor leading and trailing blanks are
significant in `target'. For example, the string
" Incidence " is valid. Sequences of embedded blanks
are treated as a single blank.
illmn is the name of the illumination source. This source may
be any ephemeris object. Case, blanks, and numeric values
are treated in the same way as for the input `target'.
fixref is the name of the body-fixed, body-centered reference
frame associated with the target body. The input surface
point `spoint' is expressed relative to this reference
frame, and this frame is used to define the orientation
of the target body as a function of time.
The string `fixref' is case-insensitive, and leading
and trailing blanks in `fixref' are not significant.
abcorr indicates the aberration corrections to be applied to the
observer-surface point vector, the surface point-
illumination source vector, and the target body
orientation to account for one-way light time and stellar
aberration.
Any "reception" correction accepted by spkezr_c can be used
here. See the header of spkezr_c for a detailed description
of the aberration correction options. For convenience,
the options are listed below:
"NONE" Apply no correction.
"LT" "Reception" case: correct for
one-way light time using a Newtonian
formulation.
"LT+S" "Reception" case: correct for
one-way light time and stellar
aberration using a Newtonian
formulation.
"CN" "Reception" case: converged
Newtonian light time correction.
"CN+S" "Reception" case: converged
Newtonian light time and stellar
aberration corrections.
Case and blanks are not significant in the string `abcorr'.
obsrvr is the name of an observing body. Case, blanks, and
numeric values are treated in the same way as for the
input `target'.
spoint is a surface point on the target body, expressed in
Cartesian coordinates, relative to the body-fixed target
frame designated by `fixref'.
`spoint' need not be visible from the observer's location
in order for the constraint specified by `relate' and
`refval' (see descriptions below) to be satisfied.
The components of `spoint' have units of km.
relate is a relational operator used to define a constraint on a
specified illumination angle. The result window found by
this routine indicates the time intervals where the
constraint is satisfied. Supported values of `relate' and
corresponding meanings are shown below:
">" The angle is greater than the reference
value `refval'.
"=" The angle is equal to the reference
value `refval'.
"<" The angle is less than the reference
value `refval'.
"ABSMAX" The angle is at an absolute maximum.
"ABSMIN" The angle is at an absolute minimum.
"LOCMAX" The angle is at a local maximum.
"LOCMIN" The angle is at a local minimum.
The caller may indicate that the window of interest is
the set of time intervals where the angle is within a
specified separation from an absolute extremum. The
argument `adjust' (described below) is used to specify this
separation.
Local extrema are considered to exist only in the
interiors of the intervals comprising the confinement
window: a local extremum cannot exist at a boundary point
of the confinement window.
Case is not significant in the string `relate'.
refval is the reference value used together with the argument
`relate' to define an equality or inequality to be
satisfied by the specified illumination angle. See the
discussion of `relate' above for further information.
The units of `refval' are radians.
adjust is a parameter used to modify searches for absolute
extrema: when `relate' is set to "ABSMAX" or "ABSMIN" and
`adjust' is set to a positive value, gfilum_c will find times
when the specified illumination angle is within `adjust'
radians of the specified extreme value.
If `adjust' is non-zero and a search for an absolute
minimum is performed, the result window contains time
intervals when the specified illumination angle has
values between the absolute minimum `absmin' and
absmin + adjust radians.
If `adjust' is non-zero and the search is for an absolute
maximum, the corresponding angle is between the absolute
maximum `absmax' and absmax - adjust radians.
`adjust' is not used for searches for local extrema,
equality or inequality conditions.
step is the step size to be used in the search. `step' must be
short enough for a search using this step size to locate
the time intervals where the specified illumination angle
is monotone increasing or decreasing. However, `step' must
not be *too* short, or the search will take an
unreasonable amount of time.
The choice of `step' affects the completeness but not the
precision of solutions found by this routine; the
precision is controlled by the convergence tolerance. See
the discussion of the parameter SPICE_GF_CNVTOL for details.
`step' has units of seconds.
nintvls is an integer parameter specifying the number of intervals
that can be accommodated by each of the dynamically allocated
workspace windows used internally by this routine.
In many cases, it's not necessary to compute an accurate
estimate of how many intervals are needed; rather, the user
can pick a size considerably larger than what's really
required.
However, since excessively large arrays can prevent
applications from compiling, linking, or running properly,
sometimes `nintvls' must be set according to the actual
workspace requirement. A rule of thumb for the number of
intervals needed is
nintvls = 2*n + ( m / step )
where
n is the number of intervals in the confinement
window.
m is the measure of the confinement window, in units
of seconds.
step is the search step size in seconds.
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.
In some cases the confinement window can be used to
greatly reduce the time window that must be searched for
the desired solution. See the -Particulars section below
for further discussion.
See the -Examples section below for a code example that
shows how to create a confinement window.
In some cases the observer's state may be computed at
times outside of `cnfine' by as much as 2 seconds. See
-Particulars for details.
`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 the SPICE window of intervals, contained within the
confinement window `cnfine', on which the specified
constraint is satisfied.
`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 gfilum_c conducts its search.
The endpoints of the time intervals comprising `result' are
interpreted as seconds past J2000 TDB.
If the search is for local extrema, or for absolute
extrema with `adjust' set to zero, then normally each
interval of `result' will be a singleton: the left and
right endpoints of each interval will be identical.
If no times within the confinement window satisfy the
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
SPICE_GF_CNVTOL is the default convergence tolerance used for finding
endpoints of the intervals comprising the result
window. SPICE_GF_CNVTOL is also used for finding
intermediate results; in particular, SPICE_GF_CNVTOL is
used for finding the windows on which the specified
illumination angle is increasing or decreasing.
SPICE_GF_CNVTOL is used to determine when binary
searches for roots should terminate: when a root is
bracketed within an interval of length SPICE_GF_CNVTOL,
the root is considered to have been found.
The accuracy, as opposed to precision, of roots found
by this routine depends on the accuracy of the input
data. In most cases, the accuracy of solutions will be
inferior to their precision.
The calling program can reset the convergence
tolerance; see the -Particulars section below for
further information.
SPICE_GF_NWILUM is the number of workspace windows required by
this routine.
See header file SpiceGF.h for declarations and descriptions of
parameters used throughout the GF subsystem.
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, the error SPICE(INVALIDSTEP)
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 number of intervals `nintvls' is less than 1, the error
SPICE(VALUEOUTOFRANGE) is signaled.
4) If an error (typically cell overflow) occurs while performing
window arithmetic, the error is signaled by a routine
in the call tree of this routine.
5) If the output SPICE window `result' has size less than 2, the
error SPICE(INVALIDDIMENSION) is signaled by a routine in the
call tree of this routine.
6) If the output SPICE window `result' has insufficient capacity to
hold the set of intervals on which the specified illumination
angle condition is met, an error is signaled by a routine in
the call tree of this routine.
7) If the input target body-fixed frame `fixref' is not
recognized, an error is signaled by a routine in the call
tree of this routine. A frame name may fail to be recognized
because a required frame specification kernel has not been
loaded; another cause is a misspelling of the frame name.
8) If the input frame `fixref' is not centered at the target body,
an error is signaled by a routine in the call tree of this
routine.
9) If the input argument `method' is not recognized, an error is
signaled by a routine in the call tree of this routine.
10) If the illumination angle type `angtyp' is not recognized,
an error is signaled by a routine in the call tree
of this routine.
11) If the relational operator `relate' is not recognized, an
error is signaled by a routine in the call tree of this
routine.
12) If the aberration correction specifier contains an
unrecognized value, an error is signaled by a routine in the
call tree of this routine.
13) If `adjust' is negative, an error is signaled by a routine in
the call tree of this routine.
14) If any of the input body names do not map to NAIF ID
codes, an error is signaled by a routine in the call tree of
this routine.
15) If the target coincides with the observer or the illumination
source, an error is signaled by a routine in the call tree
of this routine.
16) If required ephemerides or other kernel data are not
available, an error is signaled by a routine in the call tree
of this routine.
17) If any of the `method', `angtyp', `target', `illmn', `fixref',
`abcorr', `obsrvr' or `relate' input string pointers is null,
the error SPICE(NULLPOINTER) is signaled.
18) If any of the `method', `angtyp', `target', `illmn', `fixref',
`abcorr', `obsrvr' or `relate' input strings has zero length,
the error SPICE(EMPTYSTRING) is signaled.
19) If any the `cnfine' or `result' cell arguments has a type
other than SpiceDouble, the error SPICE(TYPEMISMATCH) is
signaled.
20) If memory cannot be allocated to create the temporary variable
required for the execution of the underlying Fortran routine,
the error SPICE(MALLOCFAILED) is signaled.
Files
Appropriate kernels must be loaded by the calling program before
this routine is called.
The following data are required:
- SPK data: ephemeris data for target, observer, and the
illumination source must be loaded. If aberration
corrections are used, the states of target, observer, and
the illumination source 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: if the target body shape is modeled as an
ellipsoid (currently no other shapes are supported),
triaxial radii for the target body must be loaded
into the kernel pool. Typically this is done by loading a
text PCK file via furnsh_c.
- Further PCK data: rotation data for the target body must be
loaded. These may be provided in a text or binary PCK file.
- Frame data: if a frame definition not built into SPICE
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 definition is
supplied by loading a frame kernel via furnsh_c.
- In some cases the observer's state may be computed at times
outside of `cnfine' by as much as 2 seconds; data required to
compute this state must be provided by loaded kernels. See
-Particulars for details.
In all cases, kernel data are normally loaded once per program
run, NOT every time this routine is called.
Particulars
This routine determines a set of one or more time intervals
within the confinement window when the specified illumination
angle satisfies a caller-specified constraint. The resulting set
of intervals is returned as a SPICE window.
The term "illumination angles" refers to the following set of
angles:
phase angle Angle between the vectors from the
surface point to the observer and
from the surface point to the
illumination source.
incidence angle Angle between the surface normal at
the specified surface point and the
vector from the surface point to the
illumination source. When the sun is
the illumination source, this angle is
commonly called the "solar incidence
angle."
emission angle Angle between the surface normal at
the specified surface point and the
vector from the surface point to the
observer.
The diagram below illustrates the geometric relationships
defining these angles. The labels for the incidence, emission,
and phase angles are "inc.", "e.", and "phase".
*
illumination source
surface normal vector
._ _.
|\ /| illumination
\ phase / source vector
\ . . /
. .
\ ___ /
. \/ \/
_\ inc./
. / \ /
. | e. \ /
* <--------------- * surface point on
viewing vector target body
location to viewing
(observer) location
Note that if the target-observer vector, the target normal vector
at the surface point, and the target-illumination source vector
are coplanar, then phase is the sum of the incidence and emission
angles. This rarely occurs; usually
phase angle < incidence angle + emission angle
All of the above angles can be computed using light time
corrections, light time and stellar aberration corrections, or no
aberration corrections. In order to describe apparent geometry as
observed by a remote sensing instrument, both light time and
stellar aberration corrections should be used.
The way aberration corrections are applied by this routine
is described below.
Light time corrections
======================
Observer-target surface point vector
------------------------------------
Let `et' be the epoch at which an observation or remote
sensing measurement is made, and let et - lt (`lt' stands
for "light time") be the epoch at which the photons
received at `et' were emitted from the surface point `spoint'.
Note that the light time between the surface point and
observer will generally differ from the light time between
the target body's center and the observer.
Target body's orientation
-------------------------
Using the definitions of `et' and `lt' above, the target body's
orientation at et - lt is used. The surface normal is
dependent on the target body's orientation, so the body's
orientation model must be evaluated for the correct epoch.
Target body -- illumination source vector
-----------------------------------------
The surface features on the target body near `spoint' will
appear in a measurement made at `et' as they were at et-lt.
In particular, lighting on the target body is dependent on
the apparent location of the illumination source as seen
from the target body at et-lt. So, a second light time
correction is used to compute the position of the
illumination source relative to the surface point.
Stellar aberration corrections
==============================
Stellar aberration corrections are applied only if
light time corrections are applied as well.
Observer-target surface point body vector
-----------------------------------------
When stellar aberration correction is performed, the
observer-to-surface point direction vector, which we'll
call SRFVEC, is adjusted so as to point to the apparent
position of `spoint': considering `spoint' to be an ephemeris
object, SRFVEC points from the observer's position at `et' to
the light time and stellar aberration
corrected position of `spoint'.
Target body-illumination source vector
--------------------------------------
The target body-illumination source vector is the apparent
position of the illumination source, corrected for light
time and stellar aberration, as seen from the surface point
`spoint' at time et-lt.
Below we discuss in greater detail aspects of this routine's
solution process that are relevant to correct and efficient
use of this routine in user applications.
The Search Process
==================
Regardless of the type of constraint selected by the caller, this
routine starts the search for solutions by determining the time
periods, within the confinement window, over which the specified
illumination angle is monotone increasing and monotone decreasing.
Each of these time periods is represented by a SPICE window.
Having found these windows, all of the illumination angle's local
extrema within the confinement window are known. Absolute extrema
then can be found very easily.
Within any interval of these "monotone" windows, there will be at
most one solution of any equality constraint. Since the boundary
of the solution set for any inequality constraint is contained in
the union of
- the set of points where an equality constraint is met
- the boundary points of the confinement window
the solutions of both equality and inequality constraints can be
found easily once the monotone windows have been found.
Step Size
=========
The monotone windows (described above) are found via a two-step
search process. Each interval of the confinement window is
searched as follows: first, the input step size is used to
determine the time separation at which the sign of the rate of
change of the illumination angle will be sampled. Starting at the
left endpoint of an interval, samples will be taken at each step.
If a change of sign is found, a root has been bracketed; at that
point, the time at which the rate of change of the selected
illumination angle is zero can be 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 illumination angle is monotone:
the step size should be shorter than the shortest of these
intervals (within the confinement window).
The optimal step size is *not* necessarily related to the lengths
of the intervals comprising the result window. For example, if
the shortest monotone interval has length 10 days, and if the
shortest result window interval has length 5 minutes, a step size
of 9.9 days is still adequate to find all of the intervals in the
result window. In situations like this, the technique of using
monotone windows yields a dramatic efficiency improvement over a
state-based search that simply tests at each step whether the
specified constraint is satisfied. The latter type of search can
miss solution intervals if the step size is longer than the
shortest solution interval.
Having some knowledge of the relative geometry of the target,
observer, and illumination source can be a valuable aid in
picking a reasonable step size. In general, the user can
compensate for lack of such knowledge by picking a very short
step size; the cost is increased computation time.
Note that the step size is not related to the precision with which
the endpoints of the intervals of the result window are computed.
That precision level is controlled by the convergence tolerance.
Convergence Tolerance
=====================
As described above, the root-finding process used by this routine
involves first bracketing roots and then using a search process
to locate them. "Roots" are both times when local extrema are
attained and times when the illumination angle is equal to a
reference value. All endpoints of the intervals comprising the
result window are either endpoints of intervals of the
confinement window or roots.
Once a root has been bracketed, a refinement process is used to
narrow down the time interval within which the root must lie.
This refinement process terminates when the location of the root
has been determined to within an error margin called the
"convergence tolerance." The convergence tolerance used by this
routine is set via the parameter SPICE_GF_CNVTOL.
The value of SPICE_GF_CNVTOL is set to a "tight" value so that the
tolerance doesn't become the limiting factor in the accuracy of
solutions found by this routine. In general the accuracy of input
data will be the limiting factor.
The user may change the convergence tolerance from the default
SPICE_GF_CNVTOL value by calling the routine gfstol_c, e.g.
gfstol_c ( tolerance value in seconds );
Call gfstol_c prior to calling this routine. All subsequent
searches will use the updated tolerance value.
Searches over time windows of long duration may require use of
larger tolerance values than the default: the tolerance must be
large enough so that it, when added to or subtracted from the
confinement window's lower and upper bounds, yields distinct time
values.
Setting the tolerance tighter than SPICE_GF_CNVTOL is unlikely to be
useful, since the results are unlikely to be more accurate.
Making the tolerance looser will speed up searches somewhat,
since a few convergence steps will be omitted.
The Confinement Window
======================
The simplest use of the confinement window is to specify a time
interval within which a solution is sought. However, the
confinement window can, in some cases, be used to make searches
more efficient. Sometimes it's possible to do an efficient search
to reduce the size of the time period over which a relatively
slow search of interest must be performed.
Certain types of searches require the state of the observer,
relative to the solar system barycenter, to be computed at times
slightly outside the confinement window `cnfine'. The time window
that is actually used is the result of "expanding" `cnfine' by a
specified amount "T": each time interval of `cnfine' is expanded by
shifting the interval's left endpoint to the left and the right
endpoint to the right by T seconds. Any overlapping intervals are
merged. (The input argument `cnfine' is not modified.)
The window expansions listed below are additive: if both
conditions apply, the window expansion amount is the sum of the
individual amounts.
- If a search uses an equality constraint, the time window
over which the state of the observer is computed is expanded
by 1 second at both ends of all of the time intervals
comprising the window over which the search is conducted.
- If a search uses stellar aberration corrections, the time
window over which the state of the observer is computed is
expanded as described above.
When light time corrections are used, expansion of the search
window also affects the set of times at which the light time-
corrected state of the target is computed.
In addition to the possible 2 second expansion of the search
window that occurs when both an equality constraint and stellar
aberration corrections are used, round-off error should be taken
into account when the need for data availability is analyzed.
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) Determine time intervals over which the MER-1 ("Opportunity")
rover's location satisfies certain constraints on its
illumination and visibility as seen from the Mars
Reconnaissance Orbiter (MRO) spacecraft.
In this case we require the emission angle to be less than
20 degrees and the solar incidence angle to be less than
60 degrees.
The reader can verify that the observation start times of the
MRO HIRISE images
Product ID Image start time
---------- ----------------
TRA_000873_1780_RED 2006-10-03T12:44:13.425
PSP_001414_1780_RED 2006-11-14T15:39:55.373
PSP_001612_1780_RED 2006-11-30T01:38:34.390
are contained within the result window found by the
example program shown below.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File: gfilum_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
pck00010.tpc Planet orientation
and radii
naif0010.tls Leapseconds
mer1_surf_rover_ext10_v1.bsp MER-1 ephemeris
mer1_surf_rover_ext11_v1.bsp MER-1 ephemeris
mer1_ls_040128_iau2000_v1.bsp MER-1 landing site
ephemeris
mro_psp1.bsp MRO ephemeris
mer1_v10.tf MER-1 frame kernel
\begindata
KERNELS_TO_LOAD = ( 'de421.bsp',
'pck00010.tpc',
'naif0010.tls',
'mro_psp1.bsp',
'mer1_surf_rover_ext10_v1.bsp',
'mer1_surf_rover_ext11_v1.bsp',
'mer1_ls_040128_iau2000_v1.bsp',
'mro_psp1.bsp',
'mer1_v10.tf' )
\begintext
Example code begins here.
/.
Program gfilum_ex1
./
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
/.
Output time format:
./
#define TIMFMT "YYYY MON DD HR:MN:SC.### UTC"
/.
Meta-kernel name:
./
#define META "gfilum_ex1.tm"
/.
Maximum number of intervals in the windows
used in this program:
./
#define MAXIVL 1000
#define MAXWIN ( 2 * MAXIVL )
/.
Maximum length of time string:
./
#define TIMLEN 41
/.
Local variables
./
SPICEDOUBLE_CELL ( cnfine, MAXWIN );
SPICEDOUBLE_CELL ( result, MAXWIN );
SPICEDOUBLE_CELL ( wnsolr, MAXWIN );
SpiceChar * abcorr;
SpiceChar * fixref;
SpiceChar * illmn;
SpiceChar * method;
SpiceChar * obsrvr;
SpiceChar * target;
SpiceChar timstr [ TIMLEN ];
SpiceChar * utcbeg;
SpiceChar * utcend;
SpiceDouble adjust;
SpiceDouble emissn;
SpiceDouble et0;
SpiceDouble et1;
SpiceDouble finish;
SpiceDouble phase;
SpiceDouble refval;
SpiceDouble rovlt;
SpiceDouble rovpos [ 3 ];
SpiceDouble solar;
SpiceDouble srfvec [ 3 ];
SpiceDouble start;
SpiceDouble step;
SpiceDouble trgepc;
SpiceInt i;
/.
Load kernels:
./
furnsh_c ( META );
/.
Set the search interval:
./
utcbeg = "2006 OCT 02 00:00:00 UTC";
str2et_c ( utcbeg, &et0 );
utcend = "2006 NOV 30 12:00:00 UTC";
str2et_c ( utcend, &et1 );
wninsd_c ( et0, et1, &cnfine );
/.
Set observer, target, aberration correction, and the
Mars body-fixed, body-centered reference frame. The
lighting source is the sun.
Aberration corrections are set for remote observations.
./
illmn = "sun";
obsrvr = "mro";
target = "mars";
abcorr = "cn+s";
fixref = "iau_mars";
/.
Use the rover position at the start of
the search interval as the surface point.
./
spkpos_c ( "MER-1", et0, fixref,
"NONE", target, rovpos, &rovlt );
/.
Initialize the adjustment value for absolute
extremum searches. We're not performing
such searches in this example, but this input
to GFILUM must still be set.
./
adjust = 0.0;
/.
The computation uses an ellipsoidal model for the
target body shape.
./
method = "Ellipsoid";
/.
Set the reference value to use for the solar
incidence angle search.
./
refval = 60.0 * rpd_c();
/.
Since the period of the solar incidence angle
is about one Martian day, we can safely use 6 hours
as the search step.
./
step = 21600.0;
/.
Search over the confinement window for times
when the solar incidence angle is less than
the reference value.
./
gfilum_c ( method, "INCIDENCE", target, illmn,
fixref, abcorr, obsrvr, rovpos,
"<", refval, adjust, step,
MAXIVL, &cnfine, &wnsolr );
/.
Set the reference value for the emission angle search.
./
refval = 20.0 * rpd_c();
/.
We'll use 15 minutes as the search step. This step
is small enough to be suitable for Mars orbiters.
Units are seconds.
./
step = 900.0;
/.
Search over the previous result window for times when the
emission angle is less than the reference value.
./
gfilum_c ( method, "EMISSION", target, illmn,
fixref, abcorr, obsrvr, rovpos,
"<", refval, adjust, step,
MAXIVL, &wnsolr, &result );
/.
Display the result window. Show the solar incidence
and emission angles at the window's interval
boundaries.
./
printf( "\n" );
if ( wncard_c( &result ) == 0 )
{
printf( " Window is empty: condition "
"is not met.\n" );
}
else
{
printf ( " "
" Solar Incidence Emission\n"
" "
" (deg) (deg)\n"
"\n" );
for ( i = 0; i < wncard_c( &result ); i++ )
{
wnfetd_c ( &result, i, &start, &finish );
/.
Compute the angles of interest at the boundary
epochs.
./
timout_c ( start, TIMFMT, TIMLEN, timstr );
ilumin_c ( method, target, start, fixref,
abcorr, obsrvr, rovpos, &trgepc,
srfvec, &phase, &solar, &emissn );
printf ( "Start: %s %13.8f %13.8f\n",
timstr, solar*dpr_c(), emissn*dpr_c() );
timout_c ( finish, TIMFMT, TIMLEN, timstr );
ilumin_c ( method, target, finish, fixref,
abcorr, obsrvr, rovpos, &trgepc,
srfvec, &phase, &solar, &emissn );
printf ( "Stop: %s %13.8f %13.8f\n",
timstr, solar*dpr_c(), emissn*dpr_c() );
printf ( "\n" );
}
}
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Solar Incidence Emission
(deg) (deg)
Start: 2006 OCT 03 12:43:46.949 UTC 56.10415019 20.00000019
Stop: 2006 OCT 03 12:44:42.288 UTC 56.29996181 20.00000015
Start: 2006 OCT 08 16:03:33.956 UTC 56.48955485 20.00000021
Stop: 2006 OCT 08 16:04:29.495 UTC 56.68754510 19.99999997
Start: 2006 OCT 13 19:23:24.634 UTC 56.88741059 19.99999988
Stop: 2006 OCT 13 19:24:12.492 UTC 57.05931857 20.00000017
Start: 2006 OCT 18 22:43:21.631 UTC 57.30924467 20.00000012
Stop: 2006 OCT 18 22:43:47.966 UTC 57.40457272 20.00000004
Start: 2006 NOV 14 15:39:44.153 UTC 54.32875839 19.99999994
Stop: 2006 NOV 14 15:40:10.446 UTC 54.42668077 19.99999990
Start: 2006 NOV 19 18:59:10.190 UTC 54.63096111 20.00000007
Stop: 2006 NOV 19 18:59:54.776 UTC 54.79840753 19.99999985
Start: 2006 NOV 24 22:18:38.342 UTC 54.94960000 19.99999982
Stop: 2006 NOV 24 22:19:30.964 UTC 55.14883883 20.00000003
Start: 2006 NOV 30 01:38:07.309 UTC 55.28054784 19.99999983
Stop: 2006 NOV 30 01:39:03.296 UTC 55.49418925 19.99999999
Restrictions
1) The kernel files to be used by this routine must be loaded
(normally using the CSPICE routine furnsh_c) before this
routine is called.
2) This routine has the side effect of re-initializing the
illumination angle utility package. Callers may
need to re-initialize the package after calling this routine.
Literature_ReferencesNone. Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) B.V. Semenov (JPL) E.D. Wright (JPL) Version
-CSPICE Version 1.1.0, 01-NOV-2021 (JDR)
Updated short error messages for consistency within CSPICE wrapper
interface: MALLOCFAILURE -> MALLOCFAILED, and INVALIDDIMENSION ->
VALUEOUTOFRANGE.
Updated header to describe use of expanded confinement window.
Edited the header to comply with NAIF standard.
Changed code example for the solution to fit within the -Examples
section without modifications.
Updated the description of "nintvls", "cnfine" and "result" arguments.
Added entry #20 in -Exceptions section.
-CSPICE Version 1.0.0, 27-FEB-2014 (NJB) (BVS) (EDW)
Index_Entriessolve for illumination_angle constraints solve for phase_angle constraints solve for solar_incidence_angle constraints solve for incidence_angle constraints solve for emission_angle constraints search using illumination_angle constraints search using lighting_angle constraints |
Fri Dec 31 18:41:07 2021