| fovtrg_c |
|
Table of contents
Procedure
fovtrg_c ( Is target in FOV at time? )
void fovtrg_c ( ConstSpiceChar * inst,
ConstSpiceChar * target,
ConstSpiceChar * tshape,
ConstSpiceChar * tframe,
ConstSpiceChar * abcorr,
ConstSpiceChar * obsrvr,
SpiceDouble * et,
SpiceBoolean * visibl )
AbstractDetermine if a specified ephemeris object is within the field-of-view (FOV) of a specified instrument at a given time. Required_ReadingCK FRAMES KERNEL NAIF_IDS PCK SPK TIME KeywordsEVENT FOV GEOMETRY INSTRUMENT Brief_I/OVARIABLE I/O DESCRIPTION -------- --- ------------------------------------------------- inst I Name or ID code string of the instrument. target I Name or ID code string of the target. tshape I Type of shape model used for the target. tframe I Body-fixed, body-centered frame for target body. 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 (SPICETRUE/SPICEFALSE). 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 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 CSPICE routine getfov_c 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 is the name of the target body. 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 is a string indicating the geometric model used to
represent the shape of the target body. 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 is the name of the body-fixed, body-centered reference
frame associated with the target body. Examples of
such names are 'IAU_SATURN' (for Saturn) and 'ITRF93'
(for the 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.
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 is the name of the body from which the target is
observed. 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 is the observation time in seconds past the J2000
epoch.
Detailed_Output
visibl is SPICETRUE if `target' is fully or partially in the
field-of-view of `inst' at the time `et'. Otherwise,
`visibl' is SPICEFALSE.
Parameters
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. See the header file SpiceGF.h for details.
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 (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.
Exceptions
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 `inst', `target', `tshape', `abcorr', `obsrvr'
or `tframe' input string pointers is null, the error
SPICE(NULLPOINTER) is signaled.
15) If any of the `inst', `target', `tshape', `abcorr' or `obsrvr'
input strings has zero length, the error SPICE(EMPTYSTRING) is
signaled.
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 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 CSPICE routine getfov_c 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 furnsh_c once per program run,
NOT every time this routine is called.
ParticularsTo treat the target as a ray rather than as an ephemeris object, use the higher-level CSPICE routine fovray_c. fovray_c 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. 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) 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 CASSINI SPICE PDS archive.
End of meta-kernel
Example code begins here.
/.
Program fovtrg_ex1
./
#include <stdio.h>
#include "SpiceUsr.h"
#include "SpiceZmc.h"
int main()
{
/.
Local constants
./
#define META "fovtrg_ex1.tm"
#define BODLEN 32
#define TIMLEN 32
#define FRMLEN 32
/.
Local variables
The variable `sclk' is the spacecraft clock time of the image.
./
SpiceChar body[BODLEN];
SpiceChar frame_name[FRMLEN];
SpiceChar * sclk = "1665078907.122";
SpiceChar time[TIMLEN];
ConstSpiceChar * time_format =
"YYYY-MON-DD HR:MN:SC.###::TDB (TDB)";
SpiceDouble et;
SpiceInt body_id;
SpiceInt cassini_id;
SpiceInt frame_code;
SpiceBoolean found;
SpiceBoolean visible;
/.
Load kernels.
./
furnsh_c ( META );
/.
Retrieve Cassini's NAIF ID.
./
bodn2c_c ( "Cassini", &cassini_id, &found );
if ( !found )
{
printf ( "Could not find ID code for Cassini." );
return (1);
}
/.
Convert the image tag SCLK to ET.
./
scs2e_c ( cassini_id, sclk, &et );
/.
Convert the ET to a string format for the output.
./
timout_c ( et, time_format, TIMLEN, time );
/.
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.
./
printf ( "At time %s the following were\n", time );
printf ( "in the field of view of CASSINI_ISS_NAC\n" );
for ( body_id = 600; body_id < 700; body_id++ )
{
/.
Check to see if the `body_id' has a translation.
./
bodc2n_c ( body_id, BODLEN, body, &found );
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.
./
cidfrm_c ( body_id, FRMLEN, &frame_code, frame_name,
&found );
if ( found )
{
/.
Is this body in the field-of-view of Cassini's
ISS narrow-angle camera?
./
fovtrg_c ( "CASSINI_ISS_NAC", body, "Ellipsoid",
frame_name, "CN+S", "Cassini", &et,
&visible );
if ( visible )
{
printf ( " %s\n", body );
}
}
}
}
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/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.
Restrictions
1) The reference frame associated with `inst' must be centered at the
observer or must be inertial. No check is done to ensure this.
Literature_ReferencesNone. Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) S.C. Krening (JPL) Version
-CSPICE Version 1.0.1, 13-AUG-2021 (JDR)
Edited header to comply with NAIF standard. Changed the
argument name "visible" to "visibl" for consistency with other
functions.
-CSPICE Version 1.0.0, 15-FEB-2012 (SCK) (NJB)
Index_EntriesTarget in instrument FOV at specified time Target in instrument field_of_view at specified time |
Fri Dec 31 18:41:06 2021