fovray_c

 Procedure Abstract Required_Reading Keywords Brief_I/O Detailed_Input Detailed_Output Parameters Exceptions Files Particulars Examples Restrictions Literature_References Author_and_Institution Version Index_Entries

#### Procedure

```   void fovray_c ( ConstSpiceChar   * inst,
ConstSpiceDouble   raydir [3],
ConstSpiceChar   * rframe,
ConstSpiceChar   * abcorr,
ConstSpiceChar   * observer,
SpiceDouble      * et,
SpiceBoolean     * visible  )

```

#### Abstract

```   Determine if a specified ray is within the field-of-view (FOV) of
a specified instrument at a given time.
```

```   CK
FRAMES
KERNEL
NAIF_IDS
PCK
SPK
TIME
```

#### Keywords

```   EVENT
FOV
GEOMETRY
INSTRUMENT

```

#### 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   Body-fixed, body-centered frame for target body.
abcorr            I   Aberration correction flag.
observer          I   Name or ID code string of the observer.
et                I   Time of the observation (seconds past J2000).
visible           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 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 `observer' (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 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.

raydir     is the direction vector associated with a ray
representing a target. The ray emanates from the
location of the ephemeris object designated by the
input argument `observer' and is expressed relative to the
reference frame designated by `rframe' (see descriptions
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

Case, leading and trailing blanks are not significant
in the string.

observer   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

```   visible    is SPICETRUE if the ray is "visible", or in the
field-of-view, of `inst' at the time `et'. Otherwise,
`visible' 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 SpiceGF.h for more details.

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-6.
```

#### 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, the error will be diagnosed 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, the error will be diagnosed by routines
in the call tree of this routine.

6)  If the FOV boundary has more than SPICE_GF_MAXVRT vertices, the error
will be diagnosed by routines 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,
the error will be diagnosed 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, the error will be diagnosed 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 will be diagnosed by a routine in the call tree
of this routine.

10) If any input string argument pointer is null, the error
SPICE(NULLPOINTER) will be signaled.

11) If any input string argument other than `rframe' is empty, the
error SPICE(EMPTYSTRING) will be 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 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 CSPICE routine getfov_c 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_c 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 CSPICE routine fovtrg_c. fovtrg_c may be used to
determine if ephemeris objects such as Saturn are visible in an
instrument's FOV at a given time.
```

#### Examples

```   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.

KPL/MK

File name: fovray_ex.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
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

'naif0010.tls'
'cas00145.tsc'
'cas_v40.tf'
'cas_uvis_v06.ti'
'080428R_SCPSE_08045_08067.bsp'
'08052_08057ra.bc')

\begintext

Example code begins here.

#include <stdio.h>
#include "SpiceUsr.h"
#include "SpiceZmc.h"

int main()
{

/.
Local constants
./
#define META  "fovray_ex.tm"
#define BODLEN 32
#define TIMLEN 32
#define FRMLEN 32

/.
Local variables

The variable `time' is the observation time.
./

SpiceChar             * time = "2008-054T21:31:55.158";
SpiceChar               time_output[TIMLEN];
ConstSpiceChar        * time_format =
"YYYY-MON-DD HR:MN:SC.###::TDB (TDB)";

/.
The variables `right_asc' and `dec' are the right ascension
and declination of Epsilon CMa in degrees.
./
SpiceDouble             dec       = -28.972;
SpiceDouble             et;
SpiceDouble             raydir [3];
SpiceDouble             right_asc = 104.656;

SpiceBoolean            visible;

/.
./
furnsh_c ( META );

/.
Convert the observation time to `et'.
./
str2et_c ( time, &et );

/.
Create a unit direction vector pointing from Cassini
to the specified star. For details on corrections such
as parallax, please see the example in gfrfov_c.
./
radrec_c ( 1.0, right_asc*rpd_c(), dec*rpd_c(), raydir );

/.
Is the star in the field-of-view of Cassini's UVIS?
./
fovray_c ( "CASSINI_UVIS_FUV_OCC", raydir, "J2000",
"S", "Cassini", &et, &visible );

/.
Put the time in a specified format for output and
report the result.
./
timout_c ( et, time_format, TIMLEN, time_output );

if ( visible ) {
printf ( "Epsilon CMa was visible from the Cassini\n" );
printf ( "UVIS instrument at %s\n", time_output );
}

return (0);
}

When this program was executed on a PC/Linux/gcc platform, the
output was:

Epsilon CMa was visible from the Cassini
UVIS instrument at 2008-FEB-23 21:33:00.343 (TDB)
```

#### Restrictions

```   None.
```

```
None.
```

#### Author_and_Institution

```
S.C. Krening  (JPL)
N.J. Bachman  (JPL)
```

#### Version

```
-CSPICE Version 1.0.0, 15-FEB-2012 (SCK) (NJB)
```

#### Index_Entries

```
Ray in instrument FOV at specified time
Ray in instrument field_of_view at specified time
```
`Wed Apr  5 17:54:35 2017`