| fovray |
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Table of contents
Procedure
FOVRAY ( Is ray in FOV at time? )
SUBROUTINE FOVRAY ( INST, RAYDIR, RFRAME, ABCORR,
. OBSRVR, ET, VISIBL )
Abstract
Determine if a specified ray is within the field-of-view (FOV) of
a specified instrument at a given time.
Required_Reading
CK
FRAMES
KERNEL
NAIF_IDS
PCK
SPK
TIME
Keywords
EVENT
FOV
GEOMETRY
INSTRUMENT
Declarations
IMPLICIT NONE
INCLUDE 'gf.inc'
CHARACTER*(*) INST
DOUBLE PRECISION RAYDIR ( 3 )
CHARACTER*(*) RFRAME
CHARACTER*(*) ABCORR
CHARACTER*(*) OBSRVR
DOUBLE PRECISION ET
LOGICAL VISIBL
Brief_I/O
VARIABLE I/O DESCRIPTION
-------- --- -------------------------------------------------
INST I Name or ID code string of the instrument.
RAYDIR I Ray's direction vector.
RFRAME I Reference frame of ray's direction vector.
ABCORR I Aberration correction flag.
OBSRVR I Name or ID code string of the observer.
ET I Time of the observation (seconds past J2000).
VISIBL O Visibility flag (.TRUE./.FALSE.).
Detailed_Input
INST indicates the name of an instrument, such as a
spacecraft-mounted framing camera. The field of view
(FOV) of the instrument will be used to determine if
the direction from the observer to a target,
represented as a ray, is visible with respect to the
instrument.
The position of the instrument INST is considered to
coincide with that of the ephemeris object OBSRVR (see
description below).
The size of the instrument's FOV is constrained by the
following: There must be a vector A such that all of
the instrument's FOV boundary vectors have an angular
separation from A of less than (pi/2)-MARGIN radians
(see description below). For FOVs that are circular or
elliptical, the vector A is the boresight. For FOVs
that are rectangular or polygonal, the vector A is
calculated.
See the header of the SPICELIB routine GETFOV for a
description of the required parameters associated with
an instrument.
Both object names and NAIF IDs are accepted. For
example, both 'CASSINI_ISS_NAC' and '-82360' are
accepted. Case and leading or trailing blanks are not
significant in the string.
RAYDIR is the direction vector defining a ray of interest.
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 description below).
RFRAME is the name of the reference frame associated with
the input ray's direction vector RAYDIR. Note: RFRAME
does not need to be the instrument's reference frame.
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, leading and trailing blanks are not significant
in the string.
ABCORR indicates the aberration corrections to be applied
when computing the ray's direction.
The supported aberration correction options are:
'NONE' No correction.
'S' Stellar aberration correction,
reception case.
'XS' Stellar aberration correction,
transmission case.
For detailed information, see the geometry finder
required reading, gf.req.
Case, leading and trailing blanks are not significant
in the string.
OBSRVR is the name of 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.
Both object names and NAIF IDs are accepted. For
example, both 'CASSINI' and '-82' are accepted. Case
and leading or trailing blanks are not significant in
the string.
ET is the observation time in seconds past the J2000
epoch.
Detailed_Output
VISIBL is .TRUE. if the ray is "visible", or in the
field-of-view, of INST at the time ET. Otherwise,
VISIBL is .FALSE.
Parameters
MAXVRT is the maximum number of vertices that may be used
to define the boundary of the specified instrument's
field of view.
MARGIN 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 than (pi/2) - 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)-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 specific plane normal to
this plane (the normal plane contains the
angle bisector defined by U and V) such that
the projections have angular separation of at
least 2*MARGIN radians from the plane spanned
by U and V.
MARGIN is currently set to 1.D-12.
See INCLUDE file gf.inc for declarations and descriptions of
parameters used throughout the GF system.
Exceptions
1) If the observer's name cannot be mapped to a NAIF ID code, the
error SPICE(IDCODENOTFOUND) is signaled.
2) If the aberration correction flag calls for light time
correction, the error SPICE(INVALIDOPTION) is signaled.
3) If the ray's direction vector is zero, the error
SPICE(ZEROVECTOR) is signaled.
4) If the instrument name INST does not have corresponding NAIF
ID code, an error is signaled by a routine in the call
tree of this routine.
5) If the FOV parameters of the instrument are not present in
the kernel pool, an error is signaled by a routine
in the call tree of this routine.
6) If the FOV boundary has more than MAXVRT vertices, an error
is signaled by a routine in the call tree of this
routine.
7) If the instrument FOV shape is a polygon or rectangle, and
this routine cannot find a ray R emanating from the FOV vertex
such that maximum angular separation of R and any FOV boundary
vector is within the limit (pi/2)-MARGIN radians, an error is
signaled by a routine in the call tree of this routine. If the
FOV is any other shape, the same error check will be applied
with the instrument boresight vector serving the role of R.
8) If the loaded kernels provide insufficient data to compute a
requested state vector, an error is signaled by a
routine in the call tree of this routine.
9) If an error occurs while reading an SPK or other kernel file,
the error is signaled by a routine in the call tree
of this routine.
Files
Appropriate SPICE kernels must be loaded by the calling program
before this routine is called.
The following data are required:
- SPK data: ephemeris data for the observer at the time
ET. If aberration corrections are used, the state of the
observer relative to the solar system barycenter
must be calculable from the available ephemeris data.
- Data defining the reference frame in which the instrument's
FOV is defined must be available in the kernel pool.
Additionally the name INST must be associated with an ID
code.
- IK data: the kernel pool must contain data such that
the SPICELIB routine GETFOV may be called to obtain
parameters for INST.
The following data may be required:
- CK data: if the frame in which the instrument's FOV is
defined is fixed to a spacecraft, at least one CK file will
be needed to permit transformation of vectors between that
frame and the J2000 frame.
- SCLK data: if a CK file is needed, an associated SCLK
kernel is required to enable conversion between encoded SCLK
(used to time-tag CK data) and barycentric dynamical time
(TDB).
- Since the input ray direction may be expressed in any
frame, additional FKs, CKs, SCLK kernels, PCKs, and SPKs
may be required to map the direction to the J2000 frame.
Kernel data are normally loaded via FURNSH once per program run,
NOT every time this routine is called.
Particulars
To treat the target as an ephemeris object rather than a ray, use
the higher-level SPICELIB routine FOVTRG. FOVTRG may be used to
determine if ephemeris objects such as Saturn are visible in an
instrument's FOV at a given time.
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) The Cassini Ultraviolet Imaging Spectrograph (UVIS)
has been used to measure variations in starlight as
rings and moons occult Cassini's view of the stars.
One of these events happened at 2008-054T21:31:55.158 UTC.
Let's verify that Epsilon CMa (Adhara) was in the
Cassini UVIS field-of-view at the observation time.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: fovray_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
--------- --------
naif0010.tls Leapseconds
cpck26Jan2007.tpc Satellite orientation and
radii
cas00145.tsc Cassini SCLK
cas_v40.tf Cassini frames
cas_uvis_v06.ti Cassini UVIS instrument
080428R_SCPSE_08045_08067.bsp Merged spacecraft,
planetary, and satellite
ephemeris
08052_08057ra.bc Orientation for Cassini
\begindata
KERNELS_TO_LOAD = ( 'cpck26Jan2007.tpc'
'naif0010.tls'
'cas00145.tsc'
'cas_v40.tf'
'cas_uvis_v06.ti'
'080428R_SCPSE_08045_08067.bsp'
'08052_08057ra.bc')
\begintext
Example code begins here.
PROGRAM FOVRAY_EX1
IMPLICIT NONE
C
C SPICELIB functions
C
C Returns radians per degree.
C
DOUBLE PRECISION RPD
C
C Local parameters
C
CHARACTER*(*) META
PARAMETER ( META = 'fovray_ex1.tm' )
CHARACTER*(*) TIMFMT
PARAMETER ( TIMFMT =
. 'YYYY-MON-DD HR:MN:SC.##::TDB (TDB)' )
C
C This is the UTC time of the observation.
C
CHARACTER*(*) TIME
PARAMETER ( TIME = '2008-054T21:31:55.158' )
C
C Local variables
C
CHARACTER*(30) TIMSTR
DOUBLE PRECISION DEC
DOUBLE PRECISION ET
DOUBLE PRECISION RA
DOUBLE PRECISION RAYDIR ( 3 )
LOGICAL VISIBL
C
C RA and DEC are the right ascension and declination
C of Epsilon CMa in degrees.
C
RA = 104.656
DEC = -28.972
C
C Load the kernels.
C
CALL FURNSH ( META )
C
C Convert the observation time from UTC to ET.
C
CALL STR2ET ( TIME, ET )
C
C Create a unit direction vector pointing from Cassini
C to the specified star. For details on corrections such
C as parallax, please see the example in GFRFOV.
C
CALL RADREC ( 1.D0, RA*RPD(), DEC*RPD(), RAYDIR )
C
C Is the star in the field-of-view of Cassini's UVIS?
C
CALL FOVRAY ( 'CASSINI_UVIS_FUV_OCC', RAYDIR,
. 'J2000', 'S', 'CASSINI', ET, VISIBL )
C
C Put the time in a specified format for output.
C
CALL TIMOUT ( ET, TIMFMT, TIMSTR )
IF ( VISIBL ) THEN
WRITE(*,*) 'Epsilon CMa was visible from the ',
. 'Cassini UVIS instrument at '
WRITE(*,*) TIMSTR
END IF
END
When this program was executed on a Mac/Intel/gfortran/64-bit
platform, the output was:
Epsilon CMa was visible from the Cassini UVIS instrument at
2008-FEB-23 21:33:00.34 (TDB)
Restrictions
None.
Literature_References
None.
Author_and_Institution
N.J. Bachman (JPL)
J. Diaz del Rio (ODC Space)
S.C. Krening (JPL)
Version
SPICELIB Version 1.0.1, 03-JUL-2021 (JDR)
Edited header to comply with NAIF standard. Corrected the
value of MARGIN in the $Parameters section.
SPICELIB Version 1.0.0, 15-FEB-2012 (SCK) (NJB)
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Fri Dec 31 18:36:22 2021