Table of contents
CSPICE_FOVTRG determines if a specified ephemeris object is within
the field-of-view (FOV) of a specified instrument at a given time.
Given:
inst indicates the name of an instrument, such as a
spacecraft-mounted framing camera.
[1,c1] = size(inst); char = class(inst)
or
[1,1] = size(inst); cell = class(inst)
The field of view (FOV) of the instrument will be used to
determine if the target 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)-SPICE_GF_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 Mice routine cspice_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.
target the name of the target body.
[1,c2] = size(target); char = class(target)
or
[1,1] = size(target); cell = class(target)
This routine determines if the target body appears in the
instrument's field of view.
Both object names and NAIF IDs are accepted. For
example, both 'Moon' and '301' are accepted. Case and
leading or trailing blanks are not significant in the
string.
tshape a string indicating the geometric model used to
represent the shape of the target body.
[1,c3] = size(tshape); char = class(tshape)
or
[1,1] = size(tshape); cell = class(tshape)
The supported options are:
'ELLIPSOID' Use a triaxial ellipsoid model,
with radius values provided via the
kernel pool. A kernel variable
having a name of the form
'BODYnnn_RADII'
where nnn represents the NAIF
integer code associated with the
body, must be present in the kernel
pool. This variable must be
associated with three numeric
values giving the lengths of the
ellipsoid's X, Y, and Z semi-axes.
'POINT' Treat the body as a single point.
Case and leading or trailing blanks are not
significant in the string.
tframe the name of the body-fixed, body-centered reference
frame associated with the target body.
[1,c4] = size(tframe); char = class(tframe)
or
[1,1] = size(tframe); cell = class(tframe)
Examples of such names are 'IAU_SATURN' (for Saturn) and
'ITRF93' (for Earth).
If the target body is modeled as a point, `tframe'
is ignored and should be left blank. (Ex: ' ').
Case and leading or trailing blanks bracketing a
non-blank frame name are not significant in the string.
abcorr indicates the aberration corrections to be applied
when computing the target's position and orientation.
[1,c5] = size(abcorr); char = class(abcorr)
or
[1,1] = size(abcorr); cell = class(abcorr)
For remote sensing applications, where the apparent
position and orientation of the target seen by the
observer are desired, normally either of the
corrections:
'LT+S'
'CN+S'
should be used. These and the other supported options
are described below.
Supported aberration correction options for
observation (the case where radiation is received by
observer at `et') are:
'NONE' No correction.
'LT' Light time only
'LT+S' Light time and stellar aberration.
'CN' Converged Newtonian (CN) light time.
'CN+S' CN light time and stellar aberration.
Supported aberration correction options for
transmission (the case where radiation is emitted from
observer at `et') are:
'XLT' Light time only.
'XLT+S' Light time and stellar aberration.
'XCN' Converged Newtonian (CN) light time.
'XCN+S' CN light time and stellar aberration.
Case, leading and trailing blanks are not significant
in the string.
obsrvr the name of the body from which the target is
observed.
[1,c6] = size(obsrvr); char = class(obsrvr)
or
[1,1] = size(obsrvr); cell = class(obsrvr)
The instrument `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 the observation time(s) in seconds past the J2000
epoch.
[1,n] = size(et); double = class(et)
the call:
[visibl] = cspice_fovtrg( inst, target, tshape, ...
tframe, abcorr, obsrvr, et )
returns:
visibl true if `target' is fully or partially in the
field-of-view of `inst' at the time `et'.
[1,n] = size(visibl); logical = class(visibl)
Otherwise, `visibl' is false.
`visibl' returns with the same vectorization measure, N,
as `et'.
SPICE_GF_MAXVRT
is the maximum number of vertices that may be used
to define the boundary of the specified instrument's
field of view.
SPICE_GF_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) - 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 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*SPICE_GF_MARGIN radians from the plane spanned
by U and V.
SPICE_GF_MARGIN is currently set to 1.e-12.
See Mice header file MiceGF.m for declarations and descriptions of
parameters used throughout the GF system.
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.
1) A spectacular picture was taken by Cassini's
narrow-angle camera on Oct. 6, 2010 that shows
six of Saturn's moons. Let's verify that the moons
in the picture are Epimetheus, Atlas, Daphnis, Pan,
Janus, and Enceladus.
To see this picture, visit:
http://photojournal.jpl.nasa.gov/catalog/PIA12741
or go to the PDS Image Node's Image Atlas at
http://pds-imaging.jpl.nasa.gov/search/search.html.
Select Cassini as the mission, ISS as the instrument,
and enter 1_N1665078907.122 as the Product ID in the
Product tab. Note: these directions may change as the
PDS Imaging Node changes.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: fovtrg_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
cpck*.tpc Satellite orientation and
radii
pck00010.tpc Planet orientation and
radii
cas_rocks_v18.tf FK for small satellites
around Saturn
cas_v40.tf Cassini FK
cas_iss_v10.ti Cassini ISS IK
cas00149.tsc Cassini SCLK
*.bsp Ephemeris for Cassini,
planets, and satellites
10279_10284ra.bc Orientation for Cassini
\begindata
KERNELS_TO_LOAD = ( 'naif0010.tls'
'cpck14Oct2010.tpc'
'cpck_rock_21Jan2011_merged.tpc'
'pck00010.tpc'
'cas_rocks_v18.tf'
'cas_v40.tf'
'cas_iss_v10.ti'
'cas00149.tsc'
'110317AP_RE_90165_18018.bsp'
'110120BP_IRRE_00256_25017.bsp'
'101210R_SCPSE_10256_10302.bsp'
'10279_10284ra.bc' )
\begintext
For project meta-kernels similar to the one shown
here, please see the CASSINI SPICE PDS archive.
End of meta-kernel
Example code begins here.
function fovtrg_ex1()
%
% Load the meta kernel.
%
cspice_furnsh( 'fovtrg_ex1.tm' );
%
% Retrieve Cassini's NAIF ID.
%
[cassini_id, found] = cspice_bodn2c( 'CASSINI' );
if (~found)
fprintf( 'Could not find ID code for Cassini.\n' )
return
end
%
% Convert the image tag SCLK to ET.
%
et = cspice_scs2e( cassini_id, '1665078907.122' );
%
% Convert `et' to a string format for the output.
%
time_format = 'YYYY-MON-DD HR:MN:SC.###::TDB (TDB)';
time = cspice_timout( et, time_format );
%
% Search through all of Saturn's moons to see if each
% satellite was in the ISS NAC's field-of-view at
% the image time. We're going to take advantage of the
% fact that all Saturn's moons have a NAIF ID of 6xx.
%
fprintf( 'At time %s the following were\n', time )
fprintf( 'in the field of view of CASSINI_ISS_NAC\n' )
for body_id = 600:699
%
% Check to see if the `body_id' has a translation.
%
[body, found] = cspice_bodc2n( body_id );
if (found)
%
% Check to see if a body-fixed frame for this ID exists.
% If the frame is not in the kernel pool, we cannot
% perform the visibility test. The main cause of a
% failure is a missing kernel.
%
[frame_code, frame_name, found] = cspice_cidfrm( body_id );
if (found)
%
% Is this body in the field-of-view of Cassini's
% ISS narrow-angle camera?
%
visibl = cspice_fovtrg( 'CASSINI_ISS_NAC', body, ...
'Ellipsoid', frame_name, ...
'CN+S', 'CASSINI', et );
if ( visibl )
fprintf( ' %s\n', body )
end
end
end
end
%
% It's always good form to unload kernels after use,
% particularly in Matlab due to data persistence.
%
cspice_kclear
When this program was executed on a Mac/Intel/Octave5.x/64-bit
platform, the output was:
At time 2010-OCT-06 17:09:45.346 (TDB) the following were
in the field of view of CASSINI_ISS_NAC
ENCELADUS
JANUS
EPIMETHEUS
ATLAS
PAN
DAPHNIS
ANTHE
Note: there were actually 7 of Saturn's satellites in the
field-of-view of Cassini's narrow-angle camera. However, Anthe
is very small and was probably obscured by other objects or
shadow.
To treat the target as a ray rather than as an ephemeris object,
use the higher-level Mice routine cspice_fovray. cspice_fovray may be used
to determine if distant target objects such as stars are visible
in an instrument's FOV at a given time, as long as the direction
from the observer to the target can be modeled as a ray.
1) If the name of either the target or observer cannot be
translated to a NAIF ID code, an error is signaled by
a routine in the call tree of this routine.
2) If the specified aberration correction is an unrecognized
value, an error is signaled by a routine
in the call tree of this routine.
3) If the radii of a target body modeled as an ellipsoid cannot
be determined by searching the kernel pool for a kernel
variable having a name of the form
'BODYnnn_RADII'
where nnn represents the NAIF integer code associated with
the body, an error is signaled by a routine in the
call tree of this routine.
4) If the target and observer bodies are the same, an error is
signaled by a routine in the call tree of this routine.
5) If the body model specifier `tshape' is invalid, an error is
signaled by either this routine or a routine in the call tree
of this routine.
6) If a target body-fixed reference frame associated with a
non-point target is not recognized, an error is signaled by a
routine in the call tree of this routine.
7) If a target body-fixed reference frame is not centered at the
corresponding target body, an error is signaled by a routine
in the call tree of this routine.
8) If the instrument name `inst' does not have a corresponding NAIF
ID code, an error is signaled by a routine in the call tree of
this routine.
9) 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.
10) If the FOV boundary has more than SPICE_GF_MAXVRT vertices, an error
is signaled by a routine in the call tree of this
routine.
11) 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)-SPICE_GF_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.
12) 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.
13) 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.
14) If any of the input arguments, `inst', `target', `tshape',
`tframe', `abcorr', `obsrvr' or `et', is undefined, an error
is signaled by the Matlab error handling system.
15) If any of the input arguments, `inst', `target', `tshape',
`tframe', `abcorr', `obsrvr' or `et', is not of the expected
type, or it does not have the expected dimensions and size, an
error is signaled by the Mice interface.
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 target and observer that
describe the ephemerides of these objects at the time `et'.
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.
- Frame data: if a frame definition is required to convert
the observer and target states to the body-fixed frame of
the target, that definition must be available in the kernel
pool. Typically the definitions of frames not already
built-in to SPICE are supplied by loading a frame kernel.
- 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 Mice routine cspice_getfov may be called to obtain
parameters for `inst'.
The following data may be required:
- PCK data: bodies modeled as triaxial ellipsoids must have
orientation data provided by variables in the kernel pool.
Bodies modeled as triaxial ellipsoids must have radii
lengths provided by variables in the kernel pool.
- 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 both J2000 and the target body-fixed 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).
Kernel data are normally loaded via cspice_furnsh once per program run,
NOT every time this routine is called.
1) The reference frame associated with `inst' must be centered at
the observer or must be inertial. No check is done to ensure
this.
CK.REQ
FRAMES.REQ
KERNEL.REQ
MICE.REQ
NAIF_IDS.REQ
PCK.REQ
SPK.REQ
TIME.REQ
None.
J. Diaz del Rio (ODC Space)
S.C. Krening (JPL)
E.D. Wright (JPL)
-Mice Version 1.1.0, 24-AUG-2021 (EDW) (JDR)
Added square brackets to output argument.
Added -Parameters, -Exceptions, -Files, -Restrictions,
-Literature_References and -Author_and_Institution sections. Corrected
the value and changed the name of the parameter "MARGIN" to
"SPICE_GF_MARGIN".
Edited the header to comply with NAIF standard. Completed the
list of required readings.
Changed argument names "instrument", "target_shape",
"target_frame", "observer" and "visible" to "inst", "tshape",
"tframe", "obsrvr" and "visibl" for consistency with other
routines.
Eliminated use of "lasterror" in rethrow.
Removed reference to the function's corresponding CSPICE header from
-Required_Reading section.
-Mice Version 1.0.1, 13-APR-2015 (EDW)
Edit to correct typos in "Usage" string.
-Mice Version 1.0.0, 16-FEB-2012 (SCK)
Target in instrument FOV at specified time
Target in instrument field_of_view at specified time
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