Table of contents
Deprecated: This routine has been superseded by the Icy routine
cspice_termpt. This routine is supported for purposes of backward
compatibility only.
CSPICE_TERM_PL02 computes a set of points on the umbral or penumbral
terminator of a specified target body, where the target body's surface
is represented by a triangular plate model contained in a type 2 DSK
segment.
Given:
handle the DAS file handle of a DSK file open for read access.
help, handle
LONG = Scalar
This kernel must contain a type 2 segment that provides a plate
model representing the entire surface of the target body.
dladsc the DLA descriptor of a DSK segment representing the surface of
a target body.
help, dladsc
LONG = Array[SPICE_DLA_DSCSIZ]
trmtyp a string indicating the type of terminator to compute: umbral or
penumbral.
help, trmtyp
STRING = Scalar
The umbral terminator is the boundary of the portion of the
target surface in total shadow. The penumbral terminator is the
boundary of the portion of the surface that is completely
illuminated. Note that in astronomy references, the unqualified
word "terminator" refers to the umbral terminator. Here, the
unqualified word refers to either type of terminator.
To compute the terminator points, this routine first
computes a set of points on the terminator of the
indicated type on the surface of a reference ellipsoid
for the target body. Each such point defines the
direction of a ray emanating from the target center and
associated with a terminator point on the actual surface
defined by the plate model. The outermost surface
intercept of each such ray is a considered to be a
terminator point of the surface defined by the plate model.
Possible values of `trmtyp' are
'UMBRAL'
'PENUMBRAL'
Case and leading or trailing blanks in `trmtyp' are
not significant.
source the name of the body acting as a light source.
help, source
STRING = Scalar
`source' is case-insensitive, and leading and trailing blanks
in `target' are not significant. Optionally, you may supply a
string containing the integer ID code for the object. For
example both 'SUN' and '10' are legitimate strings that indicate
the Sun is the light source.
This routine assumes that a kernel variable
representing the light source's radii is present in
the kernel pool. Normally the kernel variable would
be defined by loading a PCK file.
The shape of the light source is always modeled as a
sphere, regardless of whether radii defining a
triaxial ellipsoidal shape model are available in the
kernel pool. The maximum radius of the body is used
as the radius of the sphere.
target the name of the target body.
help, target
STRING = Scalar
`target' is case-insensitive, and leading and trailing blanks
in `target' are not significant. Optionally, you may supply a
string containing the integer ID code for the object. For
example both 'MOON' and '301' are legitimate strings that
indicate the moon is the target body.
This routine assumes that a kernel variable representing
the target's radii is present in the kernel pool.
Normally the kernel variable would be defined by loading
a PCK file.
et the epoch of participation of the observer, expressed as
ephemeris seconds past J2000 TDB: `et' is the epoch at which the
observer's position is computed.
help, et
DOUBLE = Scalar
When aberration corrections are not used, `et' is also
the epoch at which the position and orientation of the
target body and position of the light source are computed.
When aberration corrections are used, `et' is the epoch
at which the observer's position relative to the solar
system barycenter is computed; in this case the
position and orientation of the target body are
computed at et-lt or et+lt, where `lt' is the one-way
light time between the target body's center and the
observer, and the sign applied to `lt' depends on the
selected correction. See the description of `abcorr'
below for details.
fixfrm the name of the reference frame relative to which the output
terminator points are expressed.
help, fixfrm
STRING = Scalar
This must a body-centered, body-fixed frame associated with the
target. The frame's axes must be compatible with the triaxial
ellipsoidal shape model associated with the target body
(normally provide via a PCK): this routine assumes that the
first, second, and third axis lengths correspond, respectively,
to the x, y, and z-axes of the frame designated by `fixfrm'.
`fixfrm' may refer to a built-in frame (documented in
the Frames Required Reading) or a frame defined by a
loaded frame kernel (FK).
The orientation of the frame designated by `fixfrm' is
evaluated at epoch of participation of the target
body. See the descriptions of `et' and `abcorr' for details.
abcorr indicates the aberration correction to be applied when computing
the observer-target position, the orientation of the target
body, and the target-source position vector.
help, abcorr
STRING = Scalar
`abcorr' may be any of the following.
'NONE' Apply no correction. Compute the
terminator points using the position
of the light source and target, and
the orientation of the target, at `et'.
Let `lt' represent the one-way light time between the
observer and the target body's center. The following
values of `abcorr' apply to the "reception" case in
which photons depart from the target body's center at
the light-time corrected epoch et-lt and *arrive* at
the observer's location at `et':
'LT' Correct for one-way light time (also
called "planetary aberration") using a
Newtonian formulation. This correction
yields the location of the terminator
points at the approximate time they
emitted photons arriving at the
observer at `et' (the difference between
light time to the target center and
light time to the terminator points
is ignored).
The light time correction uses an
iterative solution of the light time
equation. The solution invoked by the
'LT' option uses one iteration.
The target position as seen by the
observer, the position of the light
source as seen from the target at
et-lt, and the rotation of the target
body, are corrected for light time.
'LT+S' Correct for one-way light time and
stellar aberration using a Newtonian
formulation. This option modifies the
positions obtained with the 'LT' option
to account for the observer's velocity
relative to the solar system
barycenter. This correction also
applies to the position of the light
source relative to the target. The
result is the apparent terminator as
seen by the observer.
'CN' Converged Newtonian light time
correction. In solving the light time
equation, the 'CN' correction iterates
until the solution converges. The
position and rotation of the target
body and the position of the light
source relative to the target are
corrected for light time.
'CN+S' Converged Newtonian light time
and stellar aberration corrections.
obsrvr the name of the observing body.
help, obsrvr
STRING = Scalar
This is typically a spacecraft, the Earth, or a surface point
on the Earth. `obsrvr' is case-insensitive, and leading and
trailing blanks in `obsrvr' are not significant. Optionally, you
may supply a string containing the integer ID code for the
object. For example both 'EARTH' and '399' are legitimate
strings that indicate the Earth is the observer.
npts the number of terminator points to compute.
help, npts
LONG = Scalar
the call:
cspice_term_pl02, handle, dladsc, trmtyp, source, target, $
et, fixfrm, abcorr, obsrvr, npts, $
trgepc, obspos, trmpts, pltids
returns:
trgepc the "target epoch."
help, trgepc
DOUBLE = Scalar
`trgepc' is defined as follows: letting `lt' be the one-way
light time between the target center and observer, `trgepc' is
either the epoch et-lt or `et' depending on whether the
requested aberration correction is, respectively, for received
radiation or omitted. `lt' is computed using the method
indicated by `abcorr'.
`trgepc' is expressed as seconds past J2000 TDB.
obspos the vector from the center of the target body at epoch `trgepc'
to the observer at epoch `et'.
help, obspos
DOUBLE = Array[3]
`obspos' is expressed in the target body-fixed reference frame
`fixfrm', which is evaluated at `trgepc'.
`obspos' is returned to simplify various related
computations that would otherwise be cumbersome. For
example, the vector `xvec' from the observer to the
Ith terminator point can be calculated via the
expression
xvec = trmpts[*,i] - obspos
The components of `obspos' are given in units of km.
trmpts an array of points on the umbral or penumbral terminator of the
target, as specified by the input argument `trmtyp'.
help, trmpts
DOUBLE = Array[3,npts]
The ith point is contained in the array elements
trmpts[*,i]
As described above, each terminator point lies on a ray
emanating from the center of the target and passing
through a terminator point on the target's reference
ellipsoid. Each terminator point *on the reference
ellipsoid* is the point of tangency of a plane that is
also tangent to the light source. These associated
points of tangency on the light source have uniform
distribution in longitude when expressed in a
cylindrical coordinate system whose Z-axis is `obspos'.
The magnitude of the separation in longitude between the
tangency points on the light source is
2*Pi / npts
If the reference ellipsoid for the target is spherical,
the terminator points also are uniformly distributed in
longitude in the cylindrical system described above. If
the reference ellipsoid of the target is non-spherical,
the longitude distribution of the points generally is
not uniform.
The terminator points are expressed in the body-fixed
reference frame designated by `fixfrm'. Units are km.
pltids an array of integer ID codes of the plates on which the
terminator points are located.
help, pltids
LONG = Array[npts]
The ith plate ID corresponds to the ith terminator point. These
ID codes can be use to look up data associated with the plate,
such as the plate's vertices or outward normal vector.
None.
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) Compute sets of umbral and penumbral terminator points on Phobos
as seen from Mars. Perform a consistency check using the solar
incidence angle at each point, where the solar incidence angle
is computed using both a reference ellipsoid and the actual
plate model surface and surface normal. We expect to see a
solar incidence angle of approximately 90 degrees. Since the
solar incidence angle is measured between the local outward
normal and the direction to the Sun, the solar incidence angle
at an umbral or penumbral terminator point should be,
respectively, greater than or less than 90 degrees by
approximately the angular radius of the Sun as seen from each
terminator point.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File: term_pl02_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
--------- --------
mar097.bsp Mars satellite ephemeris
pck00010.tpc Planet orientation and
radii
naif0010.tls Leapseconds
\begindata
KERNELS_TO_LOAD = ( 'mar097.bsp',
'pck00010.tpc',
'naif0010.tls' )
\begintext
End of meta-kernel
Use the DSK kernel below to provide the plate model representation
of the surface of Phobos.
phobos_3_3.bds
Example code begins here.
PRO term_pl02_ex1, meta, dsknam
;;
;; Constants
;;
NPOINTS = 3
NTYPES = 2
TIMLEN = 40
ILUM_METHOD = 'ELLIPSOID'
TOL = 1.d-12
UTCSTR = '2007 FEB 9 00:00:00 UTC'
;;
;; Initial values
;;
target = 'Phobos'
fixfrm = 'IAU_PHOBOS'
abcorr = 'CN+S'
fixfrm = 'IAU_PHOBOS'
obsrvr = 'Mars'
trmtypes = [ 'Umbral', 'Penumbral' ]
utcstr = '2007 FEB 9 00:00:00 UTC'
;;
;; Load the meta kernel.
;;
cspice_furnsh, meta
;;
;; Open the DSK file for read access.
;; We use the DAS-level interface for
;; this function.
;;
cspice_dasopr, dsknam, handle
;;
;; Begin a forward search through the
;; kernel, treating the file as a DLA.
;; In this example, it's a very short
;; search.
;;
cspice_dlabfs, handle, dladsc, found
if ( ~found ) then begin
;;
;; We arrive here only if the kernel
;; contains no segments. This is
;; unexpected, but we're prepared for it.
;;
cspice_kclear
message, 'SPICE(NOSEGMENT): No segment found in file '+ dsknam
return
endif
;;
;; If we made it this far, `dladsc' is the
;; DLA descriptor of the first segment.
;;
;; Now compute sub-points using both computation
;; methods. We'll vary the aberration corrections
;; and the epochs.
;;
cspice_str2et, UTCSTR, et
cspice_timout, et, 'YYYY-MON-DD HR:MN:SC.### ::TDB(TDB)', $
TIMLEN, timstr
print
print, 'Observer: ', obsrvr
print, 'Target: ', target
print, 'Observation epoch: ', timstr
print, 'Aberration correction: ', abcorr
print, 'Body-fixed frame: ', fixfrm
print
;;
;; Look up the radii of the Sun. We'll use these as
;; part of a computation to check the solar incidence
;; angles at the terminator points.
;;
cspice_bodvrd, 'SUN', 'RADII', 3, sunRadii
;;
;; Now compute grids of terminator points using both
;; terminator types.
;;
for typidx = 0, NTYPES-1 do begin
;;
;; Select the terminator type.
;;
trmtyp = trmtypes[ typidx ]
print, 'Terminator type: ', trmtyp
print
;;
;; Compute the terminator point set.
;;
cspice_term_pl02, handle, dladsc, $
trmtyp, 'Sun', target, $
et, fixfrm, abcorr, $
obsrvr, NPOINTS, trgepc, $
obpos, trmpts, pltids
;;
;; Display the terminator points.
;;
for i = 0, NPOINTS-1 do begin
cspice_reclat, trmpts[*,i], radius, lon, lat
print, 'Terminator point: ', i
print, 'Radius (km): ', radius
print, 'Planetocentric longitude (deg): ', $
lon * cspice_dpr()
print, 'Planetocentric latitude (deg): ', $
lat * cspice_dpr()
print, 'Plate ID: ', pltids[i]
;;
;; Compute the angular radius of the Sun as seen from
;; the current terminator point. Subtracting (adding)
;; this value from (to) the solar incidence angle for
;; umbral (penumbral) terminator points should yield a
;; value close to 90 degrees. This provides a sanity
;; check on the locations of the terminator points.
;;
;; First find the position of the Sun relative to the
;; target's center at the light time corrected epoch
;; trgepc.
;;
cspice_spkpos, 'Sun', trgepc, fixfrm, $
abcorr, target, sunPos, ltime
sunVec = sunPos - trmpts[*,i]
sunAngRad = asin( sunRadii[0] / cspice_vnorm(sunVec) )
;;
;; Compute the delta by which we adjust the solar
;; incidence angles.
;;
if ( typidx eq 0 ) then begin
;;
;; Umbral
;;
delta = -sunAngRad
endif else begin
;;
;; Penumbral
;;
delta = sunAngRad
endelse
;;
;; Compute the illumination angles using an ellipsoidal
;; representation of the target's surface. The role of
;; this representation is to provide an outward surface
;; normal.
;;
cspice_ilumin, ILUM_METHOD, target, et, $
fixfrm, abcorr, obsrvr, $
trmpts[*,i], trgepc, srfvec, $
phase, solar, emissn
print, 'Solar incidence angle derived using'
print, '- an ellipsoidal reference surface (deg): ', $
solar * cspice_dpr()
print, ' > adjusted for Solar angular ' +$
'surface radius (deg): ', $
(solar+delta) * cspice_dpr()
;;
;; Compute the illumination angles at the terminator point
;; using the actual plate model surface normal.
;;
cspice_illum_pl02, handle, dladsc, target, $
et, abcorr, obsrvr, $
trmpts[*,i], phase, $
solar, emissn
print, '- plate model''s surface and ' +$
'normal vector (deg): ', $
solar * cspice_dpr()
print
endfor
endfor
;;
;; Close the DSK file. Unload all other kernels as well.
;;
cspice_dascls, handle
;;
;; It's always good form to unload kernels after use,
;; particularly in IDL due to data persistence.
;;
cspice_kclear
END
When this program was executed on a Mac/Intel/IDL8.x/64-bit
platform, with the following variables as inputs
meta = 'term_pl02_ex1.tm'
dsknam = 'phobos_3_3.bds'
the output was:
Observer: Mars
Target: Phobos
Observation epoch: 2007-FEB-09 00:01:05.184 (TDB)
Aberration correction: CN+S
Body-fixed frame: IAU_PHOBOS
Terminator type: Umbral
Terminator point: 0
Radius (km): 12.111257
Planetocentric longitude (deg): 34.584501
Planetocentric latitude (deg): -0.0012984655
Plate ID: 200400
Solar incidence angle derived using
- an ellipsoidal reference surface (deg): 90.182028
> adjusted for Solar angular surface radius (deg): 89.999999
- plate model's surface and normal vector (deg): 90.240660
Terminator point: 1
Radius (km): 9.7746648
Planetocentric longitude (deg): -143.65994
Planetocentric latitude (deg): 43.397190
Plate ID: 156958
Solar incidence angle derived using
- an ellipsoidal reference surface (deg): 90.182028
> adjusted for Solar angular surface radius (deg): 90.000000
- plate model's surface and normal vector (deg): 87.138686
Terminator point: 2
Radius (km): 11.500619
Planetocentric longitude (deg): -146.12815
Planetocentric latitude (deg): -43.082379
Plate ID: 25552
Solar incidence angle derived using
- an ellipsoidal reference surface (deg): 90.182028
> adjusted for Solar angular surface radius (deg): 90.000000
- plate model's surface and normal vector (deg): 91.404206
Terminator type: Penumbral
Terminator point: 0
Radius (km): 12.859785
Planetocentric longitude (deg): -145.41550
Planetocentric latitude (deg): 0.0012985114
Plate ID: 86763
Solar incidence angle derived using
- an ellipsoidal reference surface (deg): 89.817971
> adjusted for Solar angular surface radius (deg): 90.000000
- plate model's surface and normal vector (deg): 89.055489
Terminator point: 1
Radius (km): 10.327413
Planetocentric longitude (deg): 36.340069
Planetocentric latitude (deg): -43.397192
Plate ID: 76977
Solar incidence angle derived using
- an ellipsoidal reference surface (deg): 89.817971
> adjusted for Solar angular surface radius (deg): 90.000000
- plate model's surface and normal vector (deg): 77.351956
Terminator point: 2
Radius (km): 10.086025
Planetocentric longitude (deg): 33.871859
Planetocentric latitude (deg): 43.082380
Plate ID: 282136
Solar incidence angle derived using
- an ellipsoidal reference surface (deg): 89.817971
> adjusted for Solar angular surface radius (deg): 90.000000
- plate model's surface and normal vector (deg): 88.997322
In this routine, we use the term "umbral terminator" to denote
the curve usually called the "terminator": this curve is the
boundary of the portion of the target body's surface that lies in
total shadow. We use the term "penumbral terminator" to denote
the boundary of the completely illuminated portion of the
surface.
Boundaries of illuminated regions on an arbitrary surface are often
complicated point sets: boundaries of shadows of mountains and
craters, if present, all contribute to the overall set. To make the
terminator computation tractable, we simplify the problem by using a
reference ellipsoid for guidance. We compute a set of terminator
points on the reference ellipsoid for the target body, then use
those points to define the latitudes and longitudes of terminator
points on the surface defined by the specified triangular shape
model. As such, the set of terminator points found by this routine
is just an approximation.
Below we discuss the computation of terminator points on the target
body's reference ellipsoid.
This routine assumes a spherical light source. Light rays are
assumed to travel along straight lines; refraction is not modeled.
Points on the reference ellipsoid at which the entire cap of
the light source is visible are considered to be completely
illuminated. Points on the ellipsoid at which some portion
(or all) of the cap of the light source are blocked are
considered to be in partial (or total) shadow.
In general, the terminator on an ellipsoid is a more complicated
curve than the limb (which is always an ellipse). Aside from
various special cases, the terminator does not lie in a plane.
However, the condition for a point X on the ellipsoid to lie on
the terminator is simple: a plane tangent to the ellipsoid at X
must also be tangent to the light source. If this tangent plane
does not intersect the vector from the center of the ellipsoid to
the center of the light source, then X lies on the umbral
terminator; otherwise X lies on the penumbral terminator.
1) If the input frame name `fixref' cannot be mapped
to a frame ID code, the error SPICE(NOTRANSLATION) is
signaled by a routine in the call tree of this routine.
2) If the target name `target' cannot be mapped to a body ID code,
the error SPICE(IDCODENOTFOUND) is signaled by a routine in the
call tree of this routine.
3) If the source name `source' cannot be mapped to a body ID
code, an error is signaled by a routine in the call tree of
this routine.
4) If the frame designated by `fixref' is not centered
on the target, the error SPICE(INVALIDFIXREF) is
signaled by a routine in the call tree of this routine.
5) If the terminator type is not recognized, an error
is signaled by a routine in the call tree of
this routine.
6) If the set size `npts' is not at least 1, an error
is signaled by a routine in the call tree of
this routine.
7) If any of the reference ellipsoid's semi-axis lengths is
non-positive, an error is signaled by a routine in the
call tree of this routine.
8) If the light source has non-positive radius, an error
is signaled by a routine in the call tree of
this routine.
9) If the light source intersects the smallest sphere centered at
the origin and containing the ellipsoid, an error is signaled
by a routine in the call tree of this routine.
10) If radii for the target body or light source are not
available in the kernel pool, an error is signaled by
a routine in the call tree of this routine.
11) If radii are available but either body does not have three
radii, the error SPICE(INVALIDCOUNT) is signaled by a routine
in the call tree of this routine.
12) If any SPK look-up fails, an error is signaled by
a routine in the call tree of this routine.
13) If a DSK providing a DSK type 2 plate model has not been
loaded prior to calling term_pl02, an error is signaled by a
routine in the call tree of this routine.
14) If the segment associated with the input DLA descriptor is not
of data type 2, the error SPICE(WRONGDATATYPE) is signaled by
a routine in the call tree of this routine.
15) If a surface point cannot be computed because the ray
corresponding to a longitude/latitude pair fails to intersect
the target surface as defined by the plate model, an error is
signaled by a routine in the call tree of this routine.
16) If the DSK segment identified by `dladsc' is not for the
body identified by `target', the error SPICE(DSKTARGETMISMATCH)
is signaled by a routine in the call tree of this routine.
17) If any of the input arguments, `handle', `dladsc', `trmtyp', `source',
`target', `et', `fixfrm', `abcorr', `obsrvr', or `npts', is
undefined, an error is signaled by the IDL error handling system.
18) If any of the input arguments, `handle', `dladsc', `trmtyp', `source',
`target', `et', `fixfrm', `abcorr', `obsrvr', or `npts', is not of
the expected type, or it does not have the expected dimensions and
size, an error is signaled by the Icy interface.
19) If any of the output arguments, `trgepc', `obspos', `trmpts', or
`pltids' is not a named variable, an error is signaled by the Icy
interface.
Appropriate DSK, SPK, PCK, and frame kernels must be loaded by the
calling program before this routine is called.
The following data are required:
- DSK data: a DSK file containing a plate model representing the
target body's surface must be loaded. This kernel must contain
a type 2 segment that contains data for the entire surface of
the target body.
- SPK data: ephemeris data for target, observer, and light
source must be loaded. If aberration corrections are used,
the states of all three objects relative to the solar system
barycenter must be calculable from the available ephemeris
data. Typically ephemeris data are made available by loading
one or more SPK files via cspice_furnsh.
- PCK data: triaxial radii for the target body and
the light source must be loaded into the kernel pool.
Typically this is done by loading a text PCK file via
cspice_furnsh.
- Further PCK data: rotation data for the target body must
be loaded. These may be provided in a text or binary PCK
file.
- Frame data: if a frame definition is required to convert
the observer and target states to the target body-fixed
frame designated by `fixref', 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.
In all cases, kernel data are normally loaded once per program
run, NOT every time this routine is called.
1) This routine models light paths as straight lines.
ICY.REQ
ABCORR.REQ
DSK.REQ
PCK.REQ
SPK.REQ
TIME.REQ
None.
J. Diaz del Rio (ODC Space)
M. Liukis (JPL)
E.D. Wright (JPL)
-Icy Version 1.1.0, 01-JUN-2021 (JDR)
Changed argument names "npoints", "termpts" and "plateids" to "npts",
"trmpts" and "pltids" for consistency with other routines.
Edited the header to comply with NAIF standard.
Added -Parameters, -Exceptions, -Files, -Restrictions,
-Literature_References and -Author_and_Institution sections.
Removed reference to the routine's corresponding CSPICE header from
-Abstract section.
Added arguments' type and size information in the -I/O section.
Index lines now state that this routine is deprecated.
-Icy Version 1.0.0, 16-DEC-2016 (EDW) (ML)
DEPRECATED find terminator on plate model
DEPRECATED find terminator on triangular shape model
DEPRECATED find terminator on DSK type_2 shape model
DEPRECATED find umbral terminator on plate model
DEPRECATED find umbral terminator on shape model
DEPRECATED find umbral terminator on DSK type_2 shape
DEPRECATED find penumbral terminator on plate model
DEPRECATED find penumbral terminator on shape model
DEPRECATED find penumbral terminator on DSK type_2 shape
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