CSPICE_GFRFOV determine the time intervals when a specified ray intersects
the space bounded by the field-of-view (FOV) of a specified instrument.
For important details concerning this module's function, please refer to
the CSPICE routine gfrfov_c.
All parameters described here are declared in the header file
SpiceGF.h. See that file for parameter values.
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.
is the maximum number of vertices that may be used
to define the boundary of the specified instrument's
field of view.
is a small positive number used to constrain the
orientation of the boundary vectors of polygonal
FOVs. Such FOVs must satisfy the following constraints:
1) The boundary vectors must be contained within
a right circular cone of angular radius less
than (pi/2) - SPICE_GF_MARGIN radians; in other
words, there must be a vector A such that all
boundary vectors have angular separation from
A of less than (pi/2)-SPICE_GF_MARGIN radians.
2) There must be a pair of boundary vectors U, V
such that all other boundary vectors lie in
the same half space bounded by the plane
containing U and V. Furthermore, all other
boundary vectors must have orthogonal
projections onto a plane normal to this plane
such that the projections have angular
separation of at least 2*SPICE_GF_MARGIN radians
from the plane spanned by U and V.
inst the scalar string naming the instrument, such as a
spacecraft-mounted framing camera, the field of view
(FOV) of which is to be used for an target intersection
search: the direction from the observer to a target
is represented by a ray, and times when the specified
ray intersects the region of space bounded by the FOV
The position of the instrument designated by 'inst' is
considered to coincide with that of the ephemeris
object designated by the input argument 'obsrvr' (see
'inst' must have a corresponding NAIF ID and a frame
defined, as is normally done in a frame kernel. It
must also have an associated reference frame and a FOV
shape, boresight and boundary vertices (or reference
vector and reference angles) defined, as is usually
done in an instrument kernel.
See the header of the CSPICE routine getfov_c for a
description of the required parameters associated with
raydir a double precision 3-vector describing a ray pointing
toward a target. The ray emanates from the
location of the ephemeris object designated by the
input argument 'obsrvr' and is expressed relative to the
reference frame designated by 'rframe' (see descriptions
rframe the scalar string naming the reference frame associated with
the input ray's direction vector 'raydir'.
Since light time corrections are not supported for
rays, the orientation of the frame is always evaluated
at the epoch associated with the observer, as opposed
to the epoch associated with the light-time corrected
position of the frame center.
Case and leading or trailing blanks bracketing a non-blank
frame name are not significant in the string 'rframe'.
abcorr the scalar string indicating the aberration corrections
to apply when computing the 'raydir' direction.
The supported aberration correction options are
'NONE' No correction.
'S' Stellar aberration correction,
'XS' Stellar aberration correction,
For detailed information, see the geometry finder required
Case, leading and trailing blanks are not significant
in the string 'abcorr'.
obsrvr the scalar string naming the body from which the target
represented by 'raydir' is observed. The instrument designated
by 'inst' is treated as if it were co-located with the observer.
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
step the scalar double precision time step size to use in the search.
'step' must be short enough for a search using this step
size to locate the time intervals where coordinate function of
the surface intercept vector is monotone increasing or
decreasing. However, step must not be *too* short, or the search
will take an unreasonable amount of time.
The choice of step affects the completeness but not the precision
of solutions found by this routine; the precision is controlled
by the convergence tolerance.
'step' has units of TDB seconds.
cnfine a scalar double precision 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
See the Examples section below for a code example
that shows how to create a confinement window.
cspice_gfrfov, inst, raydir, rframe, abcorr, obsrvr, step, $
result the scalar double precision window of intervals, contained
within the confinement window 'cnfine', on which the specified
constraint is satisfied.
If 'result' is non-empty on input, its contents
will be discarded before cspice_gfrfov conducts its
'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
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.
Any numerical results shown for this example may differ between
platforms as the results depend on the SPICE kernels used as input
and the machine specific arithmetic implementation.
This example is an extension of the example in the
The problem statement for that example is
Search for times when Saturn's satellite Phoebe is within the
FOV of the Cassini narrow angle camera (CASSINI_ISS_NAC). To
simplify the problem, restrict the search to a short time
period where continuous Cassini bus attitude data are
Use a step size of 10 seconds to reduce chances of missing
short visibility events.
Here we search the same confinement window for times when a
selected background star is visible. We use the FOV of the
Cassini ISS wide angle camera (CASSINI_ISS_WAC) to enhance the
probability of viewing the star.
The star we'll use has catalog number 6000 in the Hipparcos
Catalog. The star's J2000 right ascension and declination,
proper motion, and parallax are taken from that catalog.
Use the meta-kernel from the cspice_gftfov example:
File name: gftfov_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
cpck05Mar2004.tpc Satellite orientation and
981005_PLTEPH-DE405S.bsp Planetary ephemeris
020514_SE_SAT105.bsp Satellite ephemeris
030201AP_SK_SM546_T45.bsp Spacecraft ephemeris
cas_v37.tf Cassini FK
04135_04171pc_psiv2.bc Cassini bus CK
cas00084.tsc Cassini SCLK kernel
cas_iss_v09.ti Cassini IK
KERNELS_TO_LOAD = ( 'naif0009.tls',
MAXWIN = 1000
TIMFMT = 'YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND'
TIMLEN = 41
AU = 149597870.693D
;; Load kernels.
;; Store the time bounds of our search interval in
;; the cnfine confinement window.
cspice_str2et, [ '2004 JUN 11 06:30:00 TDB', $
'2004 JUN 11 12:00:00 TDB' ], et
cnfine = cspice_celld( 2 )
cspice_wninsd, et, et, cnfine
;;Initialize inputs for the search.
inst = 'CASSINI_ISS_WAC'
;; Create a unit direction vector pointing from observer to star.
;; We'll assume the direction is constant during the confinement
;; window, and we'll use 'et' as the epoch at which to compute the
;; direction from the spacecraft to the star.
;; The data below are for the star with catalog number 6000
;; in the Hipparcos catalog. Angular units are degrees; epochs
;; have units of Julian years and have a reference epoch of J1950.
;; The reference frame is J2000.
parallax_deg = 0.000001056D
ra_deg_0 = 19.290789927D
ra_pm = -0.000000720D
ra_epoch = 41.2000D
dec_deg_0 = 2.015271007D
dec_pm = 0.000001814D
dec_epoch = 41.1300D
rframe = 'J2000'
result = cspice_celld( MAXWIN*2)
;; Correct the star's direction for proper motion.
;; The argument 't' represents 'et' as Julian years past J1950.
t = et/cspice_jyear() $
+ ( cspice_j2000()- cspice_j1950() )/365.25D
dtra = t - ra_epoch
dtdec = t - dec_epoch
ra_deg = ra_deg_0 + dtra * ra_pm
dec_deg = dec_deg_0 + dtra * dec_pm
ra = ra_deg * cspice_rpd()
dec = dec_deg * cspice_rpd()
cspice_radrec, 1.D, ra, dec, starpos
;; Correct star position for parallax applicable at
;; the Cassini orbiter's position. (The parallax effect
;; is negligible in this case; we're simply demonstrating
;; the computation.)
parallax = parallax_deg * cspice_rpd()
stardist = AU/tan(parallax)
;; Scale the star's direction vector by its distance from
;; the solar system barycenter. Subtract off the position
;; of the spacecraft relative to the solar system barycenter;
;; the result is the ray's direction vector.
starpos = stardist * starpos
cspice_spkpos, 'cassini', et, 'J2000', 'NONE', $
'solar system barycenter', pos, ltime
raydir = starpos - pos
;; Correct the star direction for stellar aberration when
;; we conduct the search.
abcorr = 'S'
obsrvr = 'CASSINI'
stepsz = 10.D0
cspice_gfrfov, inst, raydir, rframe, abcorr, $
obsrvr, stepsz, cnfine, result
;; List the beginning and ending points 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
endif else begin
print, 'From : ', timstr
print, 'To : ', timstr
;; It's always good form to unload kernels after use,
;; particularly in IDL due to data persistence.
From : 2004-JUN-11 06:30:00.000000 (TDB)
To : 2004-JUN-11 12:00:00.000000 (TDB)
This routine determines a set of one or more time intervals when
the specified ray in contained within the field of view of a
specified instrument. We'll use the term "visibility event" to
designate such an appearance. The set of time intervals resulting
from the search is returned as a SPICE window.
The Search Process
The search for visibility events is treated as a search for state
transitions: times are sought when the state of the ray
changes from "not visible" to "visible" or vice versa.
Each interval of the confinement window is searched as follows:
first, the input step size is used to determine the time
separation at which the visibility state will be sampled.
Starting at the left endpoint of an interval, samples will be
taken at each step. If a state change is detected, a root has
been bracketed; at that point, the "root"--the time at which the
state change occurs---is found by a refinement process, for
example, by a binary search.
Note that the optimal choice of step size depends on the lengths
of the intervals over which the visibility state is constant:
the step size should be shorter than the shortest visibility event
duration and the shortest period between visibility events, within
the confinement window.
Having some knowledge of the relative geometry of the ray 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
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.
Once a root has been bracketed, a refinement process is used to
narrow down the time interval within which the root must lie.
This refinement process terminates when the location of the root
has been determined to within an error margin called the
"convergence tolerance." The convergence tolerance used by this
routine is set by the parameter SPICE_GF_CNVTOL.
The value of SPICE_GF_CNVTOL is set to a "tight" value so that the
tolerance doesn't become the limiting factor in the accuracy of
solutions found by this routine. In general the accuracy of input
data will be the limiting factor.
The user may change the convergence tolerance from the default
SPICE_GF_CNVTOL value by calling the routine cspice_gfstol, e.g.
cspice_gfstol, tolerance value in seconds
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 affect 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.
-Icy Version 1.0.1, 14-MAY-2012, EDW (JPL)
Minor edit to code comments eliminating typo.
Header updated to describe use of cspice_gfstol.
-Icy Version 1.0.0, 15-APR-2009, EDW (JPL)
GF ray in instrument FOV search