Table of contents
CSPICE_GFSNTC determines the time intervals for which a coordinate of a
surface intercept position vector satisfies a numerical constraint.
Given:
target the scalar string naming the target body.
help, target
STRING = Scalar
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.
On calling cspice_gfsntc, the kernel pool must contain the
radii data corresponding to `target'.
fixref the scalar string naming the body-fixed, body-centered reference
frame associated with the target body `target'.
help, fixref
STRING = Scalar
The SPICE frame subsystem must recognize the `fixref' name.
method the scalar string naming the method to use for the surface
intercept calculation.
help, method
STRING = Scalar
The accepted values for method:
'Ellipsoid' The intercept computation uses
a triaxial ellipsoid to model
the surface of the target body.
The ellipsoid's radii must be
available in the kernel pool.
The `method' string lacks sensitivity to case, and to leading
and trailing blanks.
abcorr the scalar string indicating the aberration corrections to apply
to the state evaluations to account for one-way light time and
stellar aberration.
help, abcorr
STRING = Scalar
This routine accepts the same aberration corrections as does
the Icy routine cspice_spkezr is accepted here. See the
header of cspice_spkezr 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.
'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, and to embedded,
leading and trailing blanks.
*Note*
When using a reference frame defined as a dynamic frame,
the user should realize defining an aberration correction
for the search different from that in the frames
definition will affect the search results.
In general, use the same aberration correction for
intercept point searches as used in the definition of a
dynamic frame (if applicable).
obsrvr the scalar string naming the observing body.
help, obsrvr
STRING = Scalar
Optionally, you may supply the ID code of the object as an
integer string. For example, both 'EARTH' and '399' are
legitimate strings to supply to indicate the observer is Earth.
dref the scalar string naming the reference frame corresponding to
`dvec'.
help, dref
STRING = Scalar
The `dref' string lacks sensitivity to case, and to leading
and trailing blanks.
dvec the pointing or boresight 3-vector from the observer.
help, dvec
DOUBLE = Array[3]
The intercept of this vector and target is the event of
interest.
crdsys the scalar string naming the coordinate system for which the
coordinate of interest is a member.
help, crdsys
STRING = Scalar
coord the scalar string naming the coordinate of interest in `crdsys'.
help, coord
STRING = Scalar
The supported coordinate systems and coordinate names are:
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).
relate the scalar string or character describing the relational
operator used to define a constraint on the selected coordinate
of the surface intercept vector.
help, relate
STRING = Scalar
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:
'>' Separation is greater than the reference
value refval.
'=' Separation is equal to the reference
value refval.
'<' Separation is less than the reference
value refval.
'ABSMAX' Separation is at an absolute maximum.
'ABSMIN' Separation is at an absolute minimum.
'LOCMAX' Separation is at a local maximum.
'LOCMIN' Separation 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, and to leading
and trailing blanks.
refval the scalar double precision reference value used together with
argument `relate' to define an equality or inequality to satisfy
by the selected coordinate of the surface intercept vector.
help, refval
DOUBLE = Scalar
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 a scalar 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, cspice_gfsntc finds times
when the intercept vector coordinate is within `adjust'
radians/kilometers of the specified extreme value.
help, adjust
DOUBLE = Scalar
For `relate' set to ABSMAX, the `result' window contains
time intervals when the intercept vector coordinate has
values between ABSMAX - adjust and ABSMAX.
For `relate' set to ABSMIN, the `result' window contains
time intervals when the intercept vector coordinate has
values between ABSMIN and ABSMIN + adjust.
`adjust' is not used for searches for local extrema,
equality or inequality conditions.
step the scalar double precision time step size to use in the search.
help, step
DOUBLE = Scalar
Selection of the time step for surface intercept geometry
requires consideration of the mechanics of a surface
intercept event. In most cases, two distinct searches
will be needed, one to determine the windows when the
boresight vector intercepts the surface and then the
search based on the user defined constraints within those
windows. The boresight of nadir pointing instrument may
continually intercept a body, but an instrument scanning
across a disc will have configurations when the
boresight does not intercept the body.
The step size must be smaller than the shortest interval
within the confinement window over which the intercept
exists and also smaller than the shortest interval over
which the intercept does not exist.
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 TDB seconds.
nintvls a scalar integer value specifying the number of intervals in the
the internal workspace array used by this routine.
help, nintvls
LONG = Scalar
`nintvls' should be at least as large as the number of
intervals within the search region on which the specified
observer-target vector coordinate function is monotone
increasing or decreasing. It does no harm to pick a value of
`nintvls' larger than the minimum required to execute the
specified search, but if chosen too small, the search will fail.
cnfine a scalar double precision window that confines the time period
over which the specified search is conducted.
help, cnfine
STRUCT = cspice_celld(2*N)
`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.
the call:
cspice_gfsntc, target, fixref, method, abcorr, obsrvr, dref, $
dvec, crdsys, coord, relate, refval, $
adjust, step, nintvls, cnfine, result
returns:
result the scalar double precision window of intervals, contained
within the confinement window `cnfine', on which the specified
constraint is satisfied.
help, result
STRUCT = cspice_celld(2*R)
If `result' is non-empty on input, its contents
will be discarded before cspice_gfsntc conducts its
search.
`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 constraint
is satisfied.
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
constraint, `result' will be returned with a
cardinality of zero.
SPICE_GF_CNVTOL
is the 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 coordinate
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.
SPICE_GF_NWMAX
is the number of workspace windows required by
this routine.
See Icy include file IcyeGF.pro for declarations and descriptions of
parameters used throughout the GF system.
Any numerical results shown for these examples may differ between
platforms as the results depend on the SPICE kernels used as input
and the machine specific arithmetic implementation.
1) Find the time during 2007 for which the latitude of the
intercept point of the vector pointing from the sun towards
the earth in the IAU_EARTH frame equals zero i.e. the intercept
point crosses the equator.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: gfsntc_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 = ( 'naif0008.tls'
'de414.bsp'
'pck00008.tpc' )
\begintext
End of meta-kernel
Use the kernel shown below to define a dynamic frame,
Sun-Earth Motion.
KPL/FK
File name: gfsntc_sem.tf
The Sun-Earth Motion frame is defined by the sun-to-earth
direction vector as the X axis. The Y axis in the earth
orbital plane, and Z completing the right hand system.
\begindata
FRAME_SEM = 10100000
FRAME_10100000_NAME = 'SEM'
FRAME_10100000_CLASS = 5
FRAME_10100000_CLASS_ID = 10100000
FRAME_10100000_CENTER = 10
FRAME_10100000_RELATIVE = 'J2000'
FRAME_10100000_DEF_STYLE = 'PARAMETERIZED'
FRAME_10100000_FAMILY = 'TWO-VECTOR'
FRAME_10100000_PRI_AXIS = 'X'
FRAME_10100000_PRI_VECTOR_DEF = 'OBSERVER_TARGET_POSITION'
FRAME_10100000_PRI_OBSERVER = 'SUN'
FRAME_10100000_PRI_TARGET = 'EARTH'
FRAME_10100000_PRI_ABCORR = 'NONE'
FRAME_10100000_SEC_AXIS = 'Y'
FRAME_10100000_SEC_VECTOR_DEF = 'OBSERVER_TARGET_VELOCITY'
FRAME_10100000_SEC_OBSERVER = 'SUN'
FRAME_10100000_SEC_TARGET = 'EARTH'
FRAME_10100000_SEC_ABCORR = 'NONE'
FRAME_10100000_SEC_FRAME = 'J2000'
\begintext
End of frames kernel
Example code begins here.
PRO gfsntc_ex1
MAXWIN = 1000
TIMFMT = 'YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND'
TIMLEN = 41
DVEC = [ 1.D, 0.D, 0.D ]
;;
;; Load kernels.
;;
cspice_furnsh, 'gfsntc_ex1.tm'
cspice_furnsh, 'gfsntc_sem.tf'
;;
;; Store the time bounds of our search interval in
;; the cnfine confinement window.
;;
cspice_str2et, [ '2007 JAN 01', '2008 JAN 01'], et
cnfine = cspice_celld( 2 )
cspice_wninsd, et[0], et[1], 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 = cspice_spd()*90.D
adjust = 0.D0
refval = 0.D0
target = 'EARTH'
obsrvr = 'SUN'
dref = 'SEM'
method = 'Ellipsoid'
fixref = 'IAU_EARTH'
crdsys = 'LATITUDINAL'
coord = 'LATITUDE'
relate = '='
;;
;; Use the same aberration correction flag as that in the SEM frame
;; definition.
;;
abcorr = 'NONE'
result = cspice_celld( MAXWIN*2)
cspice_gfsntc, target, fixref, method, abcorr, obsrvr, dref, $
dvec, crdsys, coord, relate, refval, $
adjust, step, MAXWIN, cnfine, result
;;
;; List the beginning and ending times in each interval
;; if result contains data.
;;
count = cspice_wncard( result )
if ( count eq 0 ) then begin
print, 'Result window is empty.'
endif else begin
for i= 0L, (count - 1L ) do begin
cspice_wnfetd, result, i, left, right
cspice_timout, [left, right], TIMFMT, TIMLEN, timstr
if ( left eq right ) then begin
print, 'Event time: ', timstr[0]
endif else begin
print, 'From : ', timstr[0]
print, 'To : ', timstr[1]
print
endelse
endfor
endelse
;;
;; It's always good form to unload kernels after use,
;; particularly in IDL due to data persistence.
;;
cspice_kclear
END
When this program was executed on a Mac/Intel/IDL8.x/64-bit
platform, the output was:
Event time: 2007-MAR-21 00:01:25.495120 (TDB)
Event time: 2007-SEP-23 09:46:39.574124 (TDB)
2) Find the time during 2007 for which the intercept point on the
earth of the sun-to-earth vector as described in Example 1 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 intercept latitude varies between approximately
23.44 degrees and -23.44 degrees during the year. The intercept
longitude varies between -180 degrees and 180 degrees in one
day.
Use the meta-kernel and the frames kernel from the first
example.
Example code begins here.
PRO gfsntc_ex2
MAXWIN = 1000
TIMFMT = 'YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND'
TIMLEN = 41
DVEC = [ 1.D, 0.D, 0.D ]
;;
;; Load kernels.
;;
cspice_furnsh, 'gfsntc_ex1.tm'
cspice_furnsh, 'gfsntc_sem.tf'
;;
;; Store the time bounds of our search interval in
;; the cnfine confinement window.
;;
cspice_str2et, [ '2007 JAN 01', '2008 JAN 01'], et
cnfine = cspice_celld( 2 )
cspice_wninsd, et[0], et[1], 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 = cspice_spd()*90.D
;;
;; Perform four searches to determine the times when the
;; latitude-longitude box restriction conditions apply to the surface
;; intercept vector.
;;
;; Use geodetic coordinates.
;;
adjust = 0.D0
target = 'EARTH'
obsrvr = 'SUN'
dref = 'SEM'
method = 'Ellipsoid'
fixref = 'IAU_EARTH'
crdsys = 'GEODETIC'
;;
;; Use the same aberration correction flag as that in the SEM frame
;; definition.
;;
abcorr = 'NONE'
;;
;; Perform the searches such that the result window of a search
;; serves as the confinement window of the subsequent search.
;;
;;
;; Since the latitude coordinate varies slowly and is well behaved
;; over the time of the confinement window, search first for the
;; windows satisfying the latitude requirements, then use that result
;; as confinement for the longitude search.
;;
coord = 'LATITUDE'
refval = 16.D * cspice_rpd()
relate = '>'
;;
;; Perform this search using the geometric position
;; of the bodies; set the aberration correction to 'NONE'.
;;
result1 = cspice_celld( MAXWIN*2)
cspice_gfsntc, target, fixref, method, abcorr, obsrvr, dref, $
dvec, crdsys, coord, relate, refval, $
adjust, step, MAXWIN, cnfine, result1
refval = 17.D * cspice_rpd()
relate = '<'
result2 = cspice_celld( MAXWIN*2)
cspice_gfsntc, target, fixref, method, abcorr, obsrvr, dref, $
dvec, crdsys, coord, relate, refval, $
adjust, step, MAXWIN, result1, result2
;;
;; Now the longitude search.
;;
coord = 'LONGITUDE'
;;
;; Reset the step size to something appropriate for the 360
;; degrees in 24 hours domain. The longitude shows near
;; linear behavior so use a step size less than half the period
;; of twelve hours. Ten hours will suffice in this case.
;;
step = cspice_spd() * (10.D/24.D)
refval = 85.D * cspice_rpd()
relate = '>'
result3 = cspice_celld( MAXWIN*2)
cspice_gfsntc, target, fixref, method, abcorr, obsrvr, dref, $
dvec, crdsys, coord, relate, refval, $
adjust, step, MAXWIN, result2, result3
;;
;; Contract the endpoints of each window to account
;; for possible round-off error at the -180/180 degree branch.
;;
;; A contraction value of a millisecond should eliminate
;; any round-off caused branch crossing.
;;
cspice_wncond, 1D-3, 1D-3, result3
refval = 86.D0 * cspice_rpd()
relate = '<'
result4 = cspice_celld( MAXWIN*2)
cspice_gfsntc, target, fixref, method, abcorr, obsrvr, dref, $
dvec, crdsys, coord, relate, refval, $
adjust, step, MAXWIN, result3, result4
;;
;; List the beginning and ending times in each interval
;; if result contains data.
;;
result = result4
count = cspice_wncard( result )
if ( count eq 0 ) then begin
print, 'Result window is empty.'
endif else begin
for i= 0L, (count - 1L ) do begin
cspice_wnfetd, result, i, left, right
cspice_timout, [left, right], TIMFMT, TIMLEN, timstr
if ( left eq right ) then begin
print, 'Event time: ', timstr[0]
endif else begin
print, 'From : ', timstr[0]
print, 'To : ', timstr[1]
print
endelse
endfor
endelse
;;
;; It's always good form to unload kernels after use,
;; particularly in IDL due to data persistence.
;;
cspice_kclear
END
When this program was executed on a Mac/Intel/IDL8.x/64-bit
platform, the output was:
From : 2007-MAY-05 06:14:04.637735 (TDB)
To : 2007-MAY-05 06:18:03.621906 (TDB)
From : 2007-MAY-06 06:13:59.583483 (TDB)
To : 2007-MAY-06 06:17:58.569239 (TDB)
From : 2007-MAY-07 06:13:55.102940 (TDB)
To : 2007-MAY-07 06:17:54.090298 (TDB)
From : 2007-AUG-06 06:23:17.282927 (TDB)
To : 2007-AUG-06 06:27:16.264009 (TDB)
From : 2007-AUG-07 06:23:10.545441 (TDB)
To : 2007-AUG-07 06:27:09.524924 (TDB)
From : 2007-AUG-08 06:23:03.233996 (TDB)
To : 2007-AUG-08 06:27:02.211888 (TDB)
From : 2007-AUG-09 06:22:55.351256 (TDB)
To : 2007-AUG-09 06:26:54.327565 (TDB)
This routine determines a set of one or more time intervals
within the confinement window when the selected coordinate of
the surface intercept 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
=====================
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 IcyGF.pro).
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 cspice_gfstol, e.g.
cspice_gfstol, tolerance value
Call cspice_gfstol 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
longitude. Round-off error may cause solutions near these branches
to cross the branch. Use of the Icy routine cspice_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.
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 cspice_wncond.
3) If the number of intervals `nintvls' is less than 1, the error
SPICE(VALUEOUTOFRANGE) is signaled by a routine in the call
tree of this routine.
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 input arguments, `target', `fixref', `method',
`abcorr', `obsrvr', `dref', `dvec', `crdsys', `coord',
`relate', `refval', `adjust', `step', `nintvls', `cnfine' or
`result', is undefined, an error is signaled by the IDL error
handling system.
14) If any of the input arguments, `target', `fixref', `method',
`abcorr', `obsrvr', `dref', `dvec', `crdsys', `coord',
`relate', `refval', `adjust', `step', `nintvls', `cnfine' or
`result', is not of the expected type, or it does not have the
expected dimensions and size, an error is signaled by the Icy
interface.
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
cspice_furnsh.
- PCK data: bodies modeled as triaxial ellipsoids must have
semi-axis lengths provided by variables in the kernel pool.
Typically these data are made available by loading a text
PCK file using cspice_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.
1) The kernel files to be used by this routine must be loaded
(normally using the Icy routine cspice_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.
CK.REQ
GF.REQ
ICY.REQ
SPK.REQ
TIME.REQ
WINDOWS.REQ
None.
J. Diaz del Rio (ODC Space)
E.D. Wright (JPL)
-Icy Version 1.0.2, 03-NOV-2021 (JDR)
Updated header to describe use of expanded confinement window.
Added -Parameters, -Exceptions, -Files, -Restrictions,
-Literature_References and -Author_and_Institution sections.
Edited the header to comply with NAIF standard. Corrected typos in
header. Renamed example's meta-kernel.
Removed reference to the routine's corresponding CSPICE header from
-Abstract section.
Added arguments' type and size information in the -I/O section.
-Icy Version 1.0.1, 05-SEP-2012 (EDW)
Edit to comments to correct search description.
Edits to and corrections of argument descriptions and
header.
Header updated to describe use of cspice_gfstol.
-Icy Version 1.0.0, 15-APR-2009 (EDW)
GF surface intercept coordinate search
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