| gfsubc |
|
Table of contents
Procedure
GFSUBC (GF, subpoint vector coordinate search )
SUBROUTINE GFSUBC ( TARGET, FIXREF, METHOD,
. ABCORR, OBSRVR, CRDSYS,
. COORD, RELATE, REFVAL,
. ADJUST, STEP, CNFINE,
. MW, NW, WORK, RESULT )
Abstract
Determine time intervals for which a coordinate of an
subpoint position vector satisfies a numerical constraint.
Required_Reading
GF
SPK
CK
TIME
WINDOWS
Keywords
COORDINATE
EVENT
GEOMETRY
SEARCH
Declarations
IMPLICIT NONE
INCLUDE 'gf.inc'
INCLUDE 'zzgf.inc'
INCLUDE 'zzabcorr.inc'
INCLUDE 'zzholdd.inc'
INTEGER LBCELL
PARAMETER ( LBCELL = -5 )
CHARACTER*(*) TARGET
CHARACTER*(*) FIXREF
CHARACTER*(*) METHOD
CHARACTER*(*) ABCORR
CHARACTER*(*) OBSRVR
CHARACTER*(*) CRDSYS
CHARACTER*(*) COORD
CHARACTER*(*) RELATE
DOUBLE PRECISION REFVAL
DOUBLE PRECISION ADJUST
DOUBLE PRECISION STEP
DOUBLE PRECISION CNFINE ( LBCELL : * )
INTEGER MW
INTEGER NW
DOUBLE PRECISION WORK ( LBCELL : MW, NW )
DOUBLE PRECISION RESULT ( LBCELL : * )
Brief_I/O
VARIABLE I/O DESCRIPTION
-------- --- --------------------------------------------------
LBCELL P SPICE Cell lower bound.
CNVTOL P Convergence tolerance.
TARGET I Name of the target body.
FIXREF I Body fixed frame associated with TARGET.
METHOD I Name of method type for subpoint calculation.
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.
CNFINE I SPICE window to which the search is confined.
MW I Workspace window size.
NW I The number of workspace windows needed for
the search.
WORK O Array of workspace windows.
RESULT I-O SPICE window containing results.
Detailed_Input
TARGET is the string 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.
FIXREF is the string name of the body-fixed, body-centered
reference frame associated with the target body TARGET.
The SPICE frame subsystem must recognize the 'fixref'
name.
METHOD is the string name of the method to use for the subpoint
calculation. The accepted values for METHOD:
'Near point: ellipsoid' The sub-observer point
computation uses a
triaxial ellipsoid to
model the surface of the
target body. The
sub-observer point is
defined as the nearest
point on the target
relative to the
observer.
'Intercept: ellipsoid' The sub-observer point
computation uses a
triaxial ellipsoid to
model the surface of the
target body. The
sub-observer point is
defined as the target
surface intercept of the
line containing the
observer and the
target's center.
The METHOD string lacks sensitivity to case, embedded,
leading and trailing blanks.
ABCORR is the string 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. See the header of
SPKEZR 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 string 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 that the observer is the
Earth.
CRDSYS is the string 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 FIXREF.
RELATE is the string or character describing the relational
operator used to define a constraint on the selected
coordinate of the subpoint 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.
The caller may indicate that the region of interest
is the set of time intervals where the quantity is
within a specified measure of an absolute extremum.
The argument ADJUST (described below) is used to
specify this measure.
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
subpoint 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, GFSUBC
finds times when the subpoint 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 subpoint 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 sin or cos
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 CNVTOL for
details.
STEP has units of TDB 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.
CNFINE must be initialized by the caller using the
SPICELIB routine SSIZED.
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.
MW is a parameter specifying the length of the SPICE
windows in the workspace array WORK (see description
below) used by this routine.
MW should be set to a number at least twice as large
as the maximum number of intervals required by any
workspace window. 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 MW must be set according to
the actual workspace requirement. A rule of thumb
for the number of intervals NINTVLS 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
MW should then be set to
2 * NINTVLS
NW is a parameter specifying the number of SPICE windows
in the workspace array WORK (see description below)
used by this routine. NW should be set to the
parameter NWMAX; this parameter is declared in the
include file gf.inc. (The reason this dimension is
an input argument is that this allows run-time
error checking to be performed.)
RESULT is a double precision SPICE window which will contain
the search results. RESULT must be declared and
initialized with sufficient size to capture the full
set of time intervals within the search region on which
the specified condition is satisfied.
RESULT must be initialized by the caller via the
SPICELIB routine SSIZED.
If RESULT is non-empty on input, its contents will be
discarded before GFSUBC conducts its search.
Detailed_Output
WORK is an array used to store workspace windows.
This array should be declared by the caller as shown:
INCLUDE 'gf.inc'
...
DOUBLE PRECISION WORK ( LBCELL : MW, NWMAX )
where MW is a constant declared by the caller and
NWMAX is a constant defined in the SPICELIB INCLUDE
file gf.inc. See the discussion of MW above.
WORK need not be initialized by the caller.
WORK is modified by this routine. The caller should
re-initialize this array before attempting to use it for
any other purpose.
RESULT is the SPICE window of intervals, contained within the
confinement window CNFINE, on which the specified
constraint is satisfied.
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.
Parameters
LBCELL is the integer value defining the lower bound for
SPICE Cell arrays (a SPICE window is a kind of cell).
CNVTOL is the convergence tolerance used for finding
endpoints of the intervals comprising the result
window. CNVTOL is also used for finding intermediate
results; in particular, CNVTOL is used for finding the
windows on which the specified coordinate is increasing
or decreasing. CNVTOL is used to determine when binary
searches for roots should terminate: when a root is
bracketed within an interval of length 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.
See INCLUDE file gf.inc 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. One technique to handle such a situation,
slightly contract RESULT using the window routine WNCOND.
3) If the window size MW is less than 2 or not an even value,
the error SPICE(INVALIDDIMENSION) is signaled.
4) If the window size of RESULT is less than 2, the error
SPICE(INVALIDDIMENSION) is signaled.
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 size of the workspace WORK is too small, an error is
signaled by a routine in the call tree of this routine.
9) If the aberration correction specifier contains an
unrecognized value, an error is signaled by a routine in the
call tree of this routine.
10) If ADJUST is negative, an error is signaled by a routine in
the call tree of this routine.
11) 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.
12) If required ephemerides or other kernel data are not
available, an error is signaled by a routine in the call tree
of this routine.
13) 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.
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.
- 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.
Particulars
This routine provides a simpler, but less flexible interface
than does the routine GFEVNT for conducting searches for
subpoint 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 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 subpoint 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 CNVTOL (defined
in gf.inc).
The value of CNVTOL is set to a "tight" value so that the
tolerance doesn't become the limiting factor in the accuracy of
solutions found by this routine. In general the accuracy of input
data will be the limiting factor.
The user may change the convergence tolerance from the default
CNVTOL value by calling the routine GFSTOL, e.g.
CALL GFSTOL( tolerance value )
Call GFSTOL prior to calling this routine. All subsequent
searches will use the updated tolerance value.
Setting the tolerance tighter than CNVTOL is unlikely to be
useful, since the results are unlikely to be more accurate.
Making the tolerance looser will speed up searches somewhat,
since a few convergence steps will be omitted. However, in most
cases, the step size is likely to have a much greater effect
on processing time than would the convergence tolerance.
The Confinement Window
======================
The simplest use of the confinement window is to specify a time
interval within which a solution is sought. However, the
confinement window can, in some cases, be used to make searches
more efficient. Sometimes it's possible to do an efficient search
to reduce the size of the time period over which a relatively
slow search of interest must be performed.
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
longitude. Round-off error may cause solutions near these branches
to cross the branch. Use of the SPICE routine WNCOND 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 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) Find the time during 2007 for which the subpoint position
vector of the Sun on Earth in the IAU_EARTH frame lies within
a geodetic latitude-longitude "box" defined as
16 degrees <= latitude <= 17 degrees
85 degrees <= longitude <= 86 degrees
This problem requires four searches, each search on one of the
box restrictions. The user needs also realize the temporal
behavior of latitude greatly differs from that of the
longitude. The sub-observer point latitude varies between
approximately 23.44 degrees and -23.44 degrees during the
year. The sub-observer point longitude varies between -180
degrees and 180 degrees in one day.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: gfsubc_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
--------- --------
de414.bsp Planetary ephemeris
pck00008.tpc Planet orientation and
radii
naif0008.tls Leapseconds
\begindata
KERNELS_TO_LOAD = ( 'de414.bsp',
'pck00008.tpc',
'naif0008.tls' )
\begintext
End of meta-kernel
Example code begins here.
PROGRAM GFSUBC_EX1
IMPLICIT NONE
C
C Include GF parameter declarations:
C
INCLUDE 'gf.inc'
C
C SPICELIB functions
C
DOUBLE PRECISION SPD
DOUBLE PRECISION DPR
DOUBLE PRECISION RPD
INTEGER WNCARD
C
C Local parameters
C
INTEGER LBCELL
PARAMETER ( LBCELL = -5 )
C
C Create 50 windows.
C
INTEGER MAXWIN
PARAMETER ( MAXWIN = 1000 )
C
C One window consists of two intervals.
C
INTEGER NINTRVL
PARAMETER ( NINTRVL = MAXWIN *2 )
INTEGER STRLEN
PARAMETER ( STRLEN = 28 )
C
C Local variables
C
CHARACTER*(STRLEN) TIMFMT
CHARACTER*(STRLEN) BEGSTR
CHARACTER*(STRLEN) ENDSTR
CHARACTER*(STRLEN) TARGET
CHARACTER*(STRLEN) OBSRVR
CHARACTER*(STRLEN) ABCORR
CHARACTER*(STRLEN) METHOD
CHARACTER*(STRLEN) FIXREF
CHARACTER*(STRLEN) CRDSYS
CHARACTER*(STRLEN) COORD
CHARACTER*(STRLEN) RELATE
DOUBLE PRECISION STEP
DOUBLE PRECISION CNFINE ( LBCELL : 2 )
DOUBLE PRECISION RESULT1 ( LBCELL : NINTRVL )
DOUBLE PRECISION RESULT2 ( LBCELL : NINTRVL )
DOUBLE PRECISION RESULT3 ( LBCELL : NINTRVL )
DOUBLE PRECISION RESULT4 ( LBCELL : NINTRVL )
DOUBLE PRECISION WORK ( LBCELL : NINTRVL, NWMAX )
DOUBLE PRECISION BEGTIM
DOUBLE PRECISION ENDTIM
DOUBLE PRECISION LEFT
DOUBLE PRECISION RIGHT
DOUBLE PRECISION REFVAL
DOUBLE PRECISION ADJUST
DOUBLE PRECISION RAD ( 2 )
DOUBLE PRECISION LON ( 2 )
DOUBLE PRECISION LAT ( 2 )
DOUBLE PRECISION TRGEPC
DOUBLE PRECISION LPOS ( 3 )
DOUBLE PRECISION RPOS ( 3 )
DOUBLE PRECISION SRFVEC ( 3 )
INTEGER COUNT
INTEGER I
C
C Saved variables
C
C The confinement, workspace and result windows CNFINE,
C WORK, RESULT1, RESULT2, RESULT3 and RESULT4 are saved
C because this practice helps to prevent stack overflow.
C
SAVE CNFINE
SAVE RESULT1
SAVE RESULT2
SAVE RESULT3
SAVE RESULT4
SAVE WORK
C
C Load kernels.
C
CALL FURNSH ('gfsubc_ex1.tm')
TIMFMT = 'YYYY-MON-DD HR:MN:SC.###### ::TDB ::RND'
C
C Initialize windows RESULT and CNFINE.
C
CALL SSIZED ( NINTRVL, RESULT1 )
CALL SSIZED ( NINTRVL, RESULT2 )
CALL SSIZED ( NINTRVL, RESULT3 )
CALL SSIZED ( NINTRVL, RESULT4 )
CALL SSIZED ( 2, CNFINE )
C
C Store the time bounds of our search interval in
C the CNFINE confinement window.
C
CALL STR2ET ( '2007 JAN 01', BEGTIM )
CALL STR2ET ( '2008 JAN 01', ENDTIM )
CALL WNINSD ( BEGTIM, ENDTIM, CNFINE )
C
C The latitude varies relatively slowly (46 degrees) during
C the year. The extrema occur approximately every six
C months. Search using a step size less than half that
C value (180 days). For this example use ninety days (in
C units of seconds).
C
STEP = SPD()*90.D0
C
C Perform four searches to determine the times when the
C latitude- longitude box restriction conditions apply to
C the subpoint vector.
C
C Use geodetic coordinates.
C
ADJUST = 0.D0
TARGET = 'EARTH'
OBSRVR = 'SUN'
METHOD = 'Near point: ellipsoid'
FIXREF = 'IAU_EARTH'
CRDSYS = 'GEODETIC'
ABCORR = 'NONE'
C
C Perform the searches such that the result window of a
C search serves as the confinement window of the
C subsequent search.
C
C Since the latitude coordinate varies slowly and is well
C behaved over the time of the confinement window, search
C first for the windows satisfying the latitude
C requirements, then use that result as confinement for
C the longitude search.
C
COORD = 'LATITUDE'
REFVAL = 16.D0 * RPD()
RELATE = '>'
CALL GFSUBC ( TARGET, FIXREF,
. METHOD, ABCORR, OBSRVR,
. CRDSYS, COORD,
. RELATE, REFVAL,
. ADJUST, STEP, CNFINE,
. NINTRVL, NWMAX, WORK, RESULT1 )
REFVAL = 17.D0 * RPD()
RELATE = '<'
CALL GFSUBC ( TARGET, FIXREF,
. METHOD, ABCORR, OBSRVR,
. CRDSYS, COORD,
. RELATE, REFVAL,
. ADJUST, STEP, RESULT1,
. NINTRVL, NWMAX, WORK, RESULT2 )
C
C Now the longitude search.
C
COORD = 'LONGITUDE'
C
C Reset the step size to something appropriate for the 360
C degrees in 24 hours domain. The longitude shows near
C linear behavior so use a step size less than half the
C period of twelve hours. Ten hours will suffice in this
C case.
C
STEP = SPD() * (10.D0/24.D0)
REFVAL = 85.D0 * RPD()
RELATE = '>'
CALL GFSUBC ( TARGET, FIXREF,
. METHOD, ABCORR, OBSRVR,
. CRDSYS, COORD,
. RELATE, REFVAL,
. ADJUST, STEP, RESULT2,
. NINTRVL, NWMAX, WORK, RESULT3 )
C
C Contract the endpoints of each window to account
C for possible round-off error at the -180/180 degree
C branch.
C
C A contraction value of a millisecond should eliminate
C any round-off caused branch crossing.
C
CALL WNCOND ( 1D-3, 1D-3, RESULT3 )
REFVAL = 86.D0 * RPD()
RELATE = '<'
CALL GFSUBC ( TARGET, FIXREF,
. METHOD, ABCORR, OBSRVR,
. CRDSYS, COORD,
. RELATE, REFVAL,
. ADJUST, STEP, RESULT3,
. NINTRVL, NWMAX, WORK, RESULT4 )
C
C Check the number of intervals in the result window.
C
COUNT = WNCARD(RESULT4)
C
C List the beginning and ending points in each interval
C if RESULT contains data.
C
IF ( COUNT .EQ. 0 ) THEN
WRITE(*, '(A)') 'Result window is empty.'
ELSE
WRITE(*, '(A)') ' Time (TDB) '
. // ' LAT (deg) LON (deg)'
WRITE(*, '(A)') ' ---------------------------'
. // ' ----------- -----------'
DO I = 1, COUNT
C
C Fetch the endpoints of the Ith interval
C of the result window.
C
CALL WNFETD ( RESULT4, I, LEFT, RIGHT )
CALL TIMOUT ( LEFT, TIMFMT, BEGSTR )
CALL TIMOUT ( RIGHT, TIMFMT, ENDSTR )
C
C Determine the latitude and longitude of the
C subpoint at the event interval boundaries.
C
CALL SUBPNT ( METHOD, TARGET, LEFT,
. FIXREF, ABCORR, OBSRVR,
. LPOS, TRGEPC, SRFVEC )
CALL RECLAT ( LPOS, RAD(1), LON(1), LAT(1) )
CALL SUBPNT ( METHOD, TARGET, RIGHT,
. FIXREF, ABCORR, OBSRVR,
. RPOS, TRGEPC, SRFVEC )
CALL RECLAT ( RPOS, RAD(2), LON(2), LAT(2) )
WRITE(*,'(2A,2F14.8)') 'From : ', BEGSTR,
. LAT(1)*DPR(), LON(1)*DPR()
WRITE(*,'(2A,2F14.8)') 'To : ', ENDSTR,
. LAT(2)*DPR(), LON(2)*DPR()
WRITE(*,*) ' '
END DO
END IF
END
When this program was executed on a Mac/Intel/gfortran/64-bit
platform, the output was:
Time (TDB) LAT (deg) LON (deg)
--------------------------- ----------- -----------
From : 2007-MAY-05 06:12:59.452307 16.05435608 86.00000000
To : 2007-MAY-05 06:16:59.436479 16.05514776 85.00000417
From : 2007-MAY-06 06:12:54.398070 16.33714720 86.00000000
To : 2007-MAY-06 06:16:54.383826 16.33792651 85.00000417
From : 2007-MAY-07 06:12:49.917541 16.61544356 86.00000000
To : 2007-MAY-07 06:16:49.904901 16.61621026 85.00000417
From : 2007-MAY-08 06:12:46.017221 16.88916258 86.00000000
To : 2007-MAY-08 06:16:46.006200 16.88991646 85.00000417
From : 2007-AUG-06 06:22:12.099776 16.68071740 86.00000000
To : 2007-AUG-06 06:26:12.080859 16.67996165 85.00000417
From : 2007-AUG-07 06:22:05.362314 16.40641076 86.00000000
To : 2007-AUG-07 06:26:05.341799 16.40564259 85.00000417
From : 2007-AUG-08 06:21:58.050893 16.12767782 86.00000000
To : 2007-AUG-08 06:25:58.028786 16.12689748 85.00000417
Restrictions
1) The kernel files to be used by this routine must be loaded
(normally using the SPICELIB routine FURNSH) 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_References
None.
Author_and_Institution
N.J. Bachman (JPL)
J. Diaz del Rio (ODC Space)
E.D. Wright (JPL)
Version
SPICELIB Version 1.2.0, 27-OCT-2021 (JDR) (NJB)
Edited the header to comply with NAIF standard.
Added initialization of QCPARS(10) to pacify Valgrind.
Modified code example's output to comply with maximum line
length of header comments. Added SAVE statements for CNFINE,
WORK, RESULT1, RESULT2, RESULT3 and RESULT4 variables in code
example.
Added entries #5 and #9 in $Exceptions section.
Updated description of WORK and RESULT arguments in $Brief_I/O,
$Detailed_Input and $Detailed_Output. Extended description of
COORD argument.
Updated header to describe use of expanded confinement window.
SPICELIB Version 1.1.0, 05-SEP-2012 (EDW)
Edit to comments to correct search description.
Implemented use of ZZHOLDD to allow user to alter convergence
tolerance.
Removed the STEP > 0 error check. The GFSSTP call includes
the check.
SPICELIB Version 1.0.1, 22-AUG-2009 (EDW)
Edited argument descriptions.
Edit to Example description, replaced "intercept" with
"sub-observer point."
SPICELIB Version 1.0.0, 17-FEB-2009 (NJB) (EDW)
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Fri Dec 31 18:36:25 2021