| gfposc_c |
|
Table of contents
Procedure
gfposc_c (GF, observer-target vector coordinate search)
void gfposc_c ( ConstSpiceChar * target,
ConstSpiceChar * frame,
ConstSpiceChar * abcorr,
ConstSpiceChar * obsrvr,
ConstSpiceChar * crdsys,
ConstSpiceChar * coord,
ConstSpiceChar * relate,
SpiceDouble refval,
SpiceDouble adjust,
SpiceDouble step,
SpiceInt nintvls,
SpiceCell * cnfine,
SpiceCell * result )
AbstractDetermine time intervals for which a coordinate of an observer-target position vector satisfies a numerical constraint. Required_ReadingGF NAIF_IDS SPK TIME WINDOWS KeywordsEVENT GEOMETRY SEARCH SEPARATION Brief_I/O
VARIABLE I/O DESCRIPTION
-------- --- --------------------------------------------------
SPICE_GF_CNVTOL
P Convergence tolerance.
target I Name of the target body.
frame I Name of the reference frame for coordinate
calculations.
abcorr I Aberration correction flag.
obsrvr I Name of the observing body.
crdsys I Name of the coordinate system containing `coord'.
coord I Name of the coordinate of interest.
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
target is the name of a target 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.
The target and observer define a position vector
that points from the observer to the target.
frame is the name of the reference frame in which to perform
state look-ups and coordinate calculations.
The SPICE frame subsystem must recognize the `frame'
name.
abcorr is the description of the aberration corrections
to apply to the state evaluations to account for one-way
light time and stellar aberration.
This routine accepts the same aberration corrections as
does the SPICE routine spkezr_c. 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. Returns the "true"
geometric state.
"LT" "Reception" case: correct for
one-way light time using a Newtonian
formulation.
"LT+S" "Reception" case: correct for
one-way light time and stellar
aberration using a Newtonian
formulation.
"CN" "Reception" case: converged
Newtonian light time correction.
"CN+S" "Reception" case: converged
Newtonian light time and stellar
aberration corrections.
"XLT" "Transmission" case: correct for
one-way light time using a Newtonian
formulation.
"XLT+S" "Transmission" case: correct for
one-way light time and stellar
aberration using a Newtonian
formulation.
"XCN" "Transmission" case: converged
Newtonian light time correction.
"XCN+S" "Transmission" case: converged
Newtonian light time and stellar
aberration corrections.
The `abcorr' string lacks sensitivity to case, leading
and trailing blanks.
obsrvr is the name of an observing body. Optionally, you may
supply the ID code of the object as an integer string.
For example, both "EARTH" and "399" are legitimate
strings to indicate the observer is the Earth.
crdsys is the name of the coordinate system for which the
coordinate of interest is a member.
coord is the string name of the coordinate of interest in
`crdsys'.
The supported coordinate systems and coordinate names:
crdsys coord Range
---------------- ----------------- ------------
"RECTANGULAR" "X"
"Y"
"Z"
"LATITUDINAL" "RADIUS"
"LONGITUDE" (-Pi,Pi]
"LATITUDE" [-Pi/2,Pi/2]
"RA/DEC" "RANGE"
"RIGHT ASCENSION" [0,2Pi)
"DECLINATION" [-Pi/2,Pi/2]
"SPHERICAL" "RADIUS"
"COLATITUDE" [0,Pi]
"LONGITUDE" (-Pi,Pi]
"CYLINDRICAL" "RADIUS"
"LONGITUDE" [0,2Pi)
"Z"
"GEODETIC" "LONGITUDE" (-Pi,Pi]
"LATITUDE" [-Pi/2,Pi/2]
"ALTITUDE"
"PLANETOGRAPHIC" "LONGITUDE" [0,2Pi)
"LATITUDE" [-Pi/2,Pi/2]
"ALTITUDE"
The "ALTITUDE" coordinates have a constant value of
zero +/- roundoff for ellipsoid targets.
Limit searches for coordinate events in the "GEODETIC"
and "PLANETOGRAPHIC" coordinate systems to `target' bodies
with axial symmetry in the equatorial plane, i.e.
equality of the body X and Y radii (oblate or prolate
spheroids).
Searches on "GEODETIC" or "PLANETOGRAPHIC" coordinates
requires body shape data, and in the case of
"PLANETOGRAPHIC" coordinates, body rotation data.
The body associated with "GEODETIC" or "PLANETOGRAPHIC"
coordinates is the center of the frame `frame'.
relate is the relational operator used to define a constraint on
the selected coordinate of the observer-target vector.
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 coordinate value is greater than the
reference value `refval'.
"=" The coordinate value is equal to the
reference value `refval'.
"<" The coordinate value is less than the
reference value `refval'.
"ABSMAX" The coordinate value is at an absolute
maximum.
"ABSMIN" The coordinate value is at an absolute
minimum.
"LOCMAX" The coordinate value is at a local
maximum.
"LOCMIN" The coordinate value is at a local
minimum.
`relate' may be used to specify an "adjusted" absolute
extremum constraint: this requires the quantity to be
within a specified offset relative to an absolute
extremum. The argument `adjust' (described below) is used
to specify this offset.
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.
The `relate' string lacks sensitivity to case, leading
and trailing blanks.
refval is the double precision reference value used together
with the argument `relate' to define an equality or
inequality to satisfy by the selected coordinate of the
observer- target vector. See the discussion of `relate'
above for further information.
The units of `refval' correspond to the type as defined
by `coord', radians for angular measures, kilometers for
distance measures.
adjust is a double precision value used to modify searches for
absolute extrema: when `relate' is set to "ABSMAX" or
"ABSMIN" and `adjust' is set to a positive value, gfposc_c
finds times when the position vector coordinate is within
`adjust' radians/kilometers of the specified extreme value.
For `relate' set to "ABSMAX", the `result' window contains
time intervals when the position vector coordinate has
values between absmax - adjust and `absmax'.
For `relate' set to "ABSMIN", the `result' window contains
time intervals when the position vector coordinate has
values between `absmin' and absmin + adjust.
`adjust' is not used for searches for local extrema,
equality or inequality conditions.
step is the double precision time step size to use in the
search.
`step' must be short enough to for a search using this step
size to locate the time intervals where coordinate
function of the position vector is monotone increasing or
decreasing. However, `step' must not be *too* short, or the
search will take an unreasonable amount of time.
For coordinates other than "LONGITUDE" and 'RIGHT
ASCENSION', the step size must be shorter than the
shortest interval, within the confinement window, over
which the coordinate is monotone increasing or
decreasing.
For "LONGITUDE" and "RIGHT ASCENSION", the step size must
be shorter than the shortest interval, within the
confinement window, over which either the sine or cosine
of the coordinate is monotone increasing or decreasing.
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 double precision 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.
In some cases the confinement window can be used to
greatly reduce the time period that must be searched
for the desired solution. See the -Particulars section
below for further discussion.
See the -Examples section below for a code example
that shows how to create a confinement window.
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 gfposc_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 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_CNVTOL has the value 1.0e-6. Units are TDB
seconds.
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. One technique to handle such a situation,
slightly contract `result' using the window routine wncond_c.
3) If the number of intervals `nintvls' is less than 1, the error
SPICE(VALUEOUTOFRANGE) is signaled.
4) If the window size of `result' is less than 2, the error
SPICE(INVALIDDIMENSION) is signaled by a routine in the call
tree of this routine.
5) If the output SPICE window `result' has insufficient capacity
to contain the number of intervals on which the specified
distance condition is met, an error is signaled
by a routine in the call tree of this routine.
6) If an error (typically cell overflow) occurs during
window arithmetic, the error is signaled by a routine
in the call tree of this routine.
7) If the relational operator `relate' is not recognized, an
error is signaled by a routine in the call tree of this
routine.
8) If the aberration correction specifier contains an
unrecognized value, an error is signaled by a routine in the
call tree of this routine.
9) If `adjust' is negative, an error is signaled by a routine in
the call tree of this routine.
10) If either 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.
11) If required ephemerides or other kernel data are not
available, an error is signaled by a routine in the call tree
of this routine.
12) If the search uses GEODETIC or PLANETOGRAPHIC coordinates, and
the center body of the reference frame has unequal equatorial
radii, an error is signaled by a routine in the call tree of
this routine.
13) If any of the `target', `frame', `abcorr', `obsrvr', `crdsys',
`coord' or `relate' input string pointers is null, the error
SPICE(NULLPOINTER) is signaled.
14) If any of the `target', `frame', `abcorr', `obsrvr', `crdsys',
`coord' or `relate' input strings has zero length, the error
SPICE(EMPTYSTRING) is signaled.
15) If any the `cnfine' or `result' cell arguments has a type
other than SpiceDouble, the error SPICE(TYPEMISMATCH) is
signaled.
16) 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 SPK and PCK 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, observer, and any intermediate objects in
a chain connecting the targets and observer that cover the
time period specified by the window `cnfine'. 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 using
furnsh_c.
- If non-inertial reference frames are used, then PCK
files, frame kernels, C-kernels, and SCLK kernels may be
needed.
- 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.
Such kernel data are normally loaded once per program
run, NOT every time this routine is called.
ParticularsThis routine provides a simpler, but less flexible interface than does the routine gfevnt_c for conducting searches for observer-target position vector coordinate value events. Applications that require support for progress reporting, interrupt handling, non-default step or refinement functions, or non-default convergence tolerance should call gfevnt_c rather than this routine. This routine determines a set of one or more time intervals within the confinement window when the selected coordinate of the observer-target position vector satisfies a caller-specified constraint. 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 ================== 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 coordinate function is monotone increasing and monotone decreasing. Each of these time periods is represented by a SPICE window. Having found these windows, all of the coordinate function'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 using 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 coordinate 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 time derivative of the coordinate is zero can be found by a refinement process, for example, using a binary search. Note that the optimal choice of step size depends on the lengths of the intervals over which the coordinate function 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 and observer can be a valuable aid in picking a reasonable step size. In general, the user can compensate for lack of such knowledge by picking a very short step size; the cost is increased computation time. Note that the step size is not related to the precision with which the endpoints of the intervals of the result window are computed. That precision level is controlled by the convergence tolerance. Convergence Tolerance ===================== As described above, the root-finding process used by this routine involves first bracketing roots and then using a search process to locate them. "Roots" are both times when local extrema are attained and times when the coordinate function is equal to a reference value. All endpoints of the intervals comprising the result window are either endpoints of intervals of the confinement window or roots. Once a root has been bracketed, a refinement process is used to narrow down the time interval within which the root must lie. This refinement process terminates when the location of the root has been determined to within an error margin called the "convergence tolerance." The default convergence tolerance used by this routine is set by the parameter SPICE_GF_CNVTOL (defined in 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. 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 ); Call gfstol_c prior to calling this routine. All subsequent searches will use the updated tolerance value. Setting the tolerance tighter than SPICE_GF_CNVTOL is unlikely to be useful, since the results are unlikely to be more accurate. Making the tolerance looser will speed up searches somewhat, since a few convergence steps will be omitted. However, in most cases, the step size is likely to have a much greater 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. Practical use of the coordinate search capability would likely consist of searches over multiple coordinate constraints to find time intervals that satisfies the constraints. An effective technique to accomplish such a search is to use the result window from one search as the confinement window of the next. 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. Longitude and Right Ascension ============================= The cyclic nature of the longitude and right ascension coordinates produces branch cuts at +/- 180 degrees longitude and 0-360 right ascension. Round-off error may cause solutions near these branches to cross the branch. Use of the SPICE routine wncond_c will contract solution windows by some epsilon, reducing the measure of the windows and eliminating the branch crossing. A one millisecond contraction will in most cases eliminate numerical round-off caused branch crossings. 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) Find the time during 2007 for which the latitude of the
Earth-Sun vector in IAU_EARTH frame has the maximum value,
i.e. the latitude of the Tropic of Cancer.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: gfposc_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
pck00009.tpc Planet orientation and
radii
naif0009.tls Leapseconds
\begindata
KERNELS_TO_LOAD = ( 'de421.bsp',
'pck00009.tpc',
'naif0009.tls' )
\begintext
End of meta-kernel
Example code begins here.
/.
Program gfposc_ex1
./
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "SpiceUsr.h"
#define MAXWIN 750
#define TIMFMT "YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND"
#define TIMLEN 41
int main( )
{
/.
Create the needed windows. Note, one window
consists of two values, so the total number
of cell values to allocate is twice
the number of intervals.
./
SPICEDOUBLE_CELL ( result, 2*MAXWIN );
SPICEDOUBLE_CELL ( cnfine, 2 );
SpiceDouble begtim;
SpiceDouble endtim;
SpiceDouble step;
SpiceDouble adjust;
SpiceDouble refval;
SpiceDouble beg;
SpiceDouble end;
SpiceChar begstr [ TIMLEN ];
SpiceChar endstr [ TIMLEN ];
SpiceChar * relate = "ABSMAX";
SpiceChar * crdsys = "LATITUDINAL";
SpiceChar * coord = "LATITUDE";
SpiceChar * targ = "SUN";
SpiceChar * obsrvr = "EARTH";
SpiceChar * frame = "IAU_EARTH";
SpiceChar * abcorr = "NONE";
SpiceInt count;
SpiceInt i;
/.
Load kernels.
./
furnsh_c( "gfposc_ex1.tm" );
/.
Store the time bounds of our search interval in
the cnfine confinement window.
./
str2et_c( "2007 JAN 01", &begtim );
str2et_c( "2008 JAN 01", &endtim );
wninsd_c ( begtim, endtim, &cnfine );
/.
The latitude varies relatively slowly, ~46 degrees during the
year. The extrema occur approximately every six months.
Search using a step size less than half that value (180 days).
For this example use ninety days (in units of seconds).
./
step = (90.)*spd_c();
adjust = 0.;
refval = 0;
/.
List the beginning and ending points in each interval
if result contains data.
./
gfposc_c ( targ,
frame,
abcorr,
obsrvr,
crdsys,
coord,
relate,
refval,
adjust,
step,
MAXWIN,
&cnfine,
&result );
count = wncard_c( &result );
/.
Display the results.
./
if (count == 0 )
{
printf ( "Result window is empty.\n\n" );
}
else
{
for ( i = 0; i < count; i++ )
{
/.
Fetch the endpoints of the Ith interval
of the result window.
./
wnfetd_c ( &result, i, &beg, &end );
if ( beg == end )
{
timout_c ( beg, TIMFMT, TIMLEN, begstr );
printf ( "Event time: %s\n", begstr );
}
else
{
timout_c ( beg, TIMFMT, TIMLEN, begstr );
timout_c ( end, TIMFMT, TIMLEN, endstr );
printf ( "Interval %d\n", i + 1);
printf ( "From : %s \n", begstr );
printf ( "To : %s \n", endstr );
printf( " \n" );
}
}
}
kclear_c();
return( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Event time: 2007-JUN-21 17:54:13.172475 (TDB)
2) A minor modification of the program listed in Example 1; find
the time during 2007 for which the latitude of the Earth-Sun
vector in IAU_EARTH frame has the minimum value, i.e. the
latitude of the Tropic of Capricorn.
Use the meta-kernel from the first example.
Example code begins here.
/.
Program gfposc_ex2
./
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "SpiceUsr.h"
#define MAXWIN 750
#define TIMFMT "YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND"
#define TIMLEN 41
int main( )
{
/.
Create the needed windows. Note, one window
consists of two values, so the total number
of cell values to allocate is twice
the number of intervals.
./
SPICEDOUBLE_CELL ( result, 2*MAXWIN );
SPICEDOUBLE_CELL ( cnfine, 2 );
SpiceDouble begtim;
SpiceDouble endtim;
SpiceDouble step;
SpiceDouble adjust;
SpiceDouble refval;
SpiceDouble beg;
SpiceDouble end;
SpiceChar begstr [ TIMLEN ];
SpiceChar endstr [ TIMLEN ];
SpiceChar * relate = "ABSMIN";
SpiceChar * crdsys = "LATITUDINAL";
SpiceChar * coord = "LATITUDE";
SpiceChar * targ = "SUN";
SpiceChar * obsrvr = "EARTH";
SpiceChar * frame = "IAU_EARTH";
SpiceChar * abcorr = "NONE";
SpiceInt count;
SpiceInt i;
/.
Load kernels.
./
furnsh_c( "gfposc_ex1.tm" );
/.
Store the time bounds of our search interval in
the cnfine confinement window.
./
str2et_c( "2007 JAN 01", &begtim );
str2et_c( "2008 JAN 01", &endtim );
wninsd_c ( begtim, endtim, &cnfine );
/.
The latitude varies relatively slowly, ~46 degrees during the
year. The extrema occur approximately every six months.
Search using a step size less than half that value (180 days).
For this example use ninety days (in units of seconds).
./
step = (90.)*spd_c();
adjust = 0.;
refval = 0;
/.
List the beginning and ending points in each interval
if result contains data.
./
gfposc_c ( targ,
frame,
abcorr,
obsrvr,
crdsys,
coord,
relate,
refval,
adjust,
step,
MAXWIN,
&cnfine,
&result );
count = wncard_c( &result );
/.
Display the results.
./
if (count == 0 )
{
printf ( "Result window is empty.\n\n" );
}
else
{
for ( i = 0; i < count; i++ )
{
/.
Fetch the endpoints of the Ith interval
of the result window.
./
wnfetd_c ( &result, i, &beg, &end );
if ( beg == end )
{
timout_c ( beg, TIMFMT, TIMLEN, begstr );
printf ( "Event time: %s\n", begstr );
}
else
{
timout_c ( beg, TIMFMT, TIMLEN, begstr );
timout_c ( end, TIMFMT, TIMLEN, endstr );
printf ( "Interval %d\n", i + 1);
printf ( "From : %s \n", begstr );
printf ( "To : %s \n", endstr );
printf( " \n" );
}
}
}
kclear_c();
return( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Event time: 2007-DEC-22 06:04:32.635539 (TDB)
3) Find the time during 2007 for which the Z component of the
Earth-Sun vector in IAU_EARTH frame has value 0, i.e. crosses
the equatorial plane (this also defines a zero latitude).
The search should return two times, one for an ascending
passage and one for descending.
Use the meta-kernel from the first example.
Example code begins here.
/.
Program gfposc_ex3
./
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "SpiceUsr.h"
#define MAXWIN 750
#define TIMFMT "YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND"
#define TIMLEN 41
int main( )
{
/.
Create the needed windows. Note, one window
consists of two values, so the total number
of cell values to allocate is twice
the number of intervals.
./
SPICEDOUBLE_CELL ( result, 2*MAXWIN );
SPICEDOUBLE_CELL ( cnfine, 2 );
SpiceDouble begtim;
SpiceDouble endtim;
SpiceDouble step;
SpiceDouble adjust;
SpiceDouble refval;
SpiceDouble beg;
SpiceDouble end;
SpiceChar begstr [ TIMLEN ];
SpiceChar endstr [ TIMLEN ];
SpiceChar * relate = "=";
SpiceChar * crdsys = "RECTANGULAR";
SpiceChar * coord = "Z";
SpiceChar * targ = "SUN";
SpiceChar * obsrvr = "EARTH";
SpiceChar * frame = "IAU_EARTH";
SpiceChar * abcorr = "NONE";
SpiceInt count;
SpiceInt i;
/.
Load kernels.
./
furnsh_c( "gfposc_ex1.tm" );
/.
Store the time bounds of our search interval in
the cnfine confinement window.
./
str2et_c( "2007 JAN 01", &begtim );
str2et_c( "2008 JAN 01", &endtim );
wninsd_c ( begtim, endtim, &cnfine );
/.
The latitude varies relatively slowly, ~46 degrees during the
year. The extrema occur approximately every six months.
Search using a step size less than half that value (180 days).
For this example use ninety days (in units of seconds).
./
step = (90.)*spd_c();
adjust = 0.;
refval = 0;
/.
List the beginning and ending points in each interval
if result contains data.
./
gfposc_c ( targ,
frame,
abcorr,
obsrvr,
crdsys,
coord,
relate,
refval,
adjust,
step,
MAXWIN,
&cnfine,
&result );
count = wncard_c( &result );
/.
Display the results.
./
if (count == 0 )
{
printf ( "Result window is empty.\n\n" );
}
else
{
for ( i = 0; i < count; i++ )
{
/.
Fetch the endpoints of the Ith interval
of the result window.
./
wnfetd_c ( &result, i, &beg, &end );
if ( beg == end )
{
timout_c ( beg, TIMFMT, TIMLEN, begstr );
printf ( "Event time: %s\n", begstr );
}
else
{
timout_c ( beg, TIMFMT, TIMLEN, begstr );
timout_c ( end, TIMFMT, TIMLEN, endstr );
printf ( "Interval %d\n", i + 1);
printf ( "From : %s \n", begstr );
printf ( "To : %s \n", endstr );
printf( " \n" );
}
}
}
kclear_c();
return( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Event time: 2007-MAR-21 00:01:25.500673 (TDB)
Event time: 2007-SEP-23 09:46:39.579484 (TDB)
4) Find the times between Jan 1, 2007 and Jan 1, 2008
corresponding to the apoapsis on the Moon's orbit around the
Earth (note, the gfdist_c routine can also perform this search).
Use the meta-kernel from the first example.
Example code begins here.
/.
Program gfposc_ex4
./
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "SpiceUsr.h"
#define MAXWIN 750
#define TIMFMT "YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND"
#define TIMLEN 41
int main( )
{
/.
Create the needed windows. Note, one window
consists of two values, so the total number
of cell values to allocate is twice
the number of intervals.
./
SPICEDOUBLE_CELL ( result, 2*MAXWIN );
SPICEDOUBLE_CELL ( cnfine, 2 );
SpiceDouble begtim;
SpiceDouble endtim;
SpiceDouble step;
SpiceDouble adjust;
SpiceDouble refval;
SpiceDouble beg;
SpiceDouble end;
SpiceChar begstr [ TIMLEN ];
SpiceChar endstr [ TIMLEN ];
SpiceChar * relate = "LOCMAX";
SpiceChar * crdsys = "SPHERICAL";
SpiceChar * coord = "RADIUS";
SpiceChar * targ = "MOON";
SpiceChar * obsrvr = "EARTH";
SpiceChar * frame = "J2000";
SpiceChar * abcorr = "NONE";
SpiceInt count;
SpiceInt i;
/.
Load kernels.
./
furnsh_c( "gfposc_ex1.tm" );
/.
Store the time bounds of our search interval in
the cnfine confinement window.
./
str2et_c( "2007 JAN 01", &begtim );
str2et_c( "2008 JAN 01", &endtim );
wninsd_c ( begtim, endtim, &cnfine );
/.
This search requires a change in the step size since the
Moon's orbit about the earth (earth-moon barycenter) has
a twenty-eight day period. Use a step size something
less than half that value. In this case, we use twelve
days.
./
step = 12.*spd_c();
adjust = 0.;
refval = 0;
/.
List the beginning and ending points in each interval
if result contains data.
./
gfposc_c ( targ,
frame,
abcorr,
obsrvr,
crdsys,
coord,
relate,
refval,
adjust,
step,
MAXWIN,
&cnfine,
&result );
count = wncard_c( &result );
/.
Display the results.
./
if (count == 0 )
{
printf ( "Result window is empty.\n\n" );
}
else
{
for ( i = 0; i < count; i++ )
{
/.
Fetch the endpoints of the Ith interval
of the result window.
./
wnfetd_c ( &result, i, &beg, &end );
if ( beg == end )
{
timout_c ( beg, TIMFMT, TIMLEN, begstr );
printf ( "Event time: %s\n", begstr );
}
else
{
timout_c ( beg, TIMFMT, TIMLEN, begstr );
timout_c ( end, TIMFMT, TIMLEN, endstr );
printf ( "Interval %d\n", i + 1);
printf ( "From : %s \n", begstr );
printf ( "To : %s \n", endstr );
printf( " \n" );
}
}
}
kclear_c();
return( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Event time: 2007-JAN-10 16:26:18.784521 (TDB)
Event time: 2007-FEB-07 12:39:35.055710 (TDB)
Event time: 2007-MAR-07 03:38:07.308330 (TDB)
Event time: 2007-APR-03 08:38:55.191516 (TDB)
Event time: 2007-APR-30 10:56:49.819340 (TDB)
Event time: 2007-MAY-27 22:03:28.834302 (TDB)
Event time: 2007-JUN-24 14:26:23.617432 (TDB)
Event time: 2007-JUL-22 08:43:50.113902 (TDB)
Event time: 2007-AUG-19 03:28:33.515939 (TDB)
Event time: 2007-SEP-15 21:07:13.940711 (TDB)
Event time: 2007-OCT-13 09:52:30.791223 (TDB)
Event time: 2007-NOV-09 12:32:50.039258 (TDB)
Event time: 2007-DEC-06 16:54:31.199770 (TDB)
5) Find times between Jan 1, 2007 and Jan 1, 2008 when the
latitude (elevation) of the observer-target vector between
DSS 17 and the Moon, as observed in the DSS 17 topocentric
(station) frame, exceeds 83 degrees.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: gfposc_ex5.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
pck00009.tpc Planet orientation and
radii
naif0009.tls Leapseconds
earthstns_itrf93_050714.bsp SPK for DSN Station
Locations
earth_topo_050714.tf Topocentric DSN stations
frame definitions
earth_latest_high_prec.bpc High precision earth PCK
\begindata
KERNELS_TO_LOAD = ( 'de421.bsp',
'pck00009.tpc',
'naif0009.tls',
'earthstns_itrf93_050714.bsp',
'earth_topo_050714.tf',
'earth_latest_high_prec.bpc' )
\begintext
End of meta-kernel
Example code begins here.
/.
Program gfposc_ex5
./
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "SpiceUsr.h"
#define MAXWIN 750
#define TIMFMT "YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND"
#define TIMLEN 41
int main( )
{
/.
Create the needed windows. Note, one window
consists of two values, so the total number
of cell values to allocate is twice
the number of intervals.
./
SPICEDOUBLE_CELL ( result, 2*MAXWIN );
SPICEDOUBLE_CELL ( cnfine, 2 );
SpiceDouble begtim;
SpiceDouble endtim;
SpiceDouble step;
SpiceDouble adjust;
SpiceDouble refval;
SpiceDouble beg;
SpiceDouble end;
SpiceChar begstr [ TIMLEN ];
SpiceChar endstr [ TIMLEN ];
SpiceChar * relate = ">";
SpiceChar * crdsys = "LATITUDINAL";
SpiceChar * coord = "LATITUDE";
SpiceChar * targ = "MOON";
SpiceChar * obsrvr = "DSS-17";
SpiceChar * frame = "DSS-17_TOPO";
SpiceChar * abcorr = "NONE";
SpiceInt count;
SpiceInt i;
/.
Load kernels.
./
furnsh_c( "gfposc_ex5.tm" );
/.
Store the time bounds of our search interval in
the cnfine confinement window.
./
str2et_c( "2007 JAN 01", &begtim );
str2et_c( "2008 JAN 01", &endtim );
wninsd_c ( begtim, endtim, &cnfine );
/.
This search uses a step size of four hours since the
time for all declination zero-to-max-to-zero passes
within the search window exceeds eight hours.
The example uses an 83 degree elevation because of its
rare occurrence and short duration.
./
step = (4./24.)*spd_c();
adjust = 0.;
refval = 83. * rpd_c();
/.
List the beginning and ending points in each interval
if result contains data.
./
gfposc_c ( targ,
frame,
abcorr,
obsrvr,
crdsys,
coord,
relate,
refval,
adjust,
step,
MAXWIN,
&cnfine,
&result );
count = wncard_c( &result );
/.
Display the results.
./
if (count == 0 )
{
printf ( "Result window is empty.\n\n" );
}
else
{
for ( i = 0; i < count; i++ )
{
/.
Fetch the endpoints of the Ith interval
of the result window.
./
wnfetd_c ( &result, i, &beg, &end );
if ( beg == end )
{
timout_c ( beg, TIMFMT, TIMLEN, begstr );
printf ( "Event time: %s\n", begstr );
}
else
{
timout_c ( beg, TIMFMT, TIMLEN, begstr );
timout_c ( end, TIMFMT, TIMLEN, endstr );
printf ( "Interval %d\n", i + 1);
printf ( "From : %s \n", begstr );
printf ( "To : %s \n", endstr );
printf( " \n" );
}
}
}
kclear_c();
return( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Interval 1
From : 2007-FEB-26 03:18:48.229281 (TDB)
To : 2007-FEB-26 03:31:29.734931 (TDB)
Interval 2
From : 2007-MAR-25 01:12:38.550572 (TDB)
To : 2007-MAR-25 01:23:53.909469 (TDB)
Restrictions
1) The kernel files to be used by this routine must be loaded
(normally via the CSPICE routine furnsh_c) before this routine
is called.
2) This routine has the side effect of re-initializing the
coordinate quantity 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) E.D. Wright (JPL) Version
-CSPICE Version 1.1.0, 03-NOV-2021 (JDR) (EDW)
Added use of ALLOC_CHECK_INTRA to check net null effect on
alloc count.
Updated header to describe use of expanded confinement window.
Edited the header to comply with NAIF standard.
Added entries #3, #4, #11 and #14 in -Exceptions section.
Updated the description of "nintvls", "cnfine" and "result" arguments.
-CSPICE Version 1.0.2, 31-JUL-2014 (EDW)
Edit to header, replaced ' character with character " to indicate
C strings.
Edit to header, correct Required Reading entry eliminating ".REQ"
suffix.
-CSPICE Version 1.0.1, 28-FEB-2013 (NJB) (EDW)
Header was updated to discuss use of gfstol_c.
Edit to comments to correct search description.
Edits to Example section, proper description of "standard.tm"
meta kernel.
-CSPICE Version 1.0.1, 26-AUG-2009 (EDW)
Correction of several typos.
-CSPICE Version 1.0.0, 10-FEB-2009 (NJB) (EDW)
Index_EntriesGF position coordinate search |
Fri Dec 31 18:41:07 2021