Table of contents
CSPICE_EDTERM computes a set of points on the umbral or penumbral
terminator of a specified target body, where the target shape is modeled
as an ellipsoid.
Given:
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
ellipsoid 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.
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, where `lt' is the one-way light time
between the target body's center and the observer.
See the description of `abcorr' below for details.
fixref the name of the reference frame relative to which the output
terminator points are expressed.
help, fixref
STRING = Scalar
This must be 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 `fixref'.
`fixref' 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 `fixref' 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_edterm, trmtyp, source, target, et, fixref, abcorr, $
obsrvr, npts, trgepc, obspos, trmpts
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
`fixref', 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 call
xvec = trmpts[*,i] - obspos
To transform the vector `obspos' from a reference frame
`fixref' at time `trgepc' to a time-dependent reference
frame REF at time `et', the routine cspice_pxfrm2 should be
called. Let `xform' be the 3x3 matrix representing the
rotation from the reference frame `fixref' at time
`trgepc' to the reference frame `ref' at time `et'. Then
`obspos' can be transformed to the result `refvec' as
follows:
cspice_pxfrm2, fixref, ref, trgepc, et, xform
refvec = xform * obspos
trmpts an array of points on the umbral or penumbral terminator of the
ellipsoid, as specified by the input argument `trmtyp'.
help, trmpts
DOUBLE = Array[3,N]
The ith point is contained in the array elements
trmpts[*,i], j = 0, 1, 2
Each terminator point 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
the target center to source center vector. The
magnitude of the separation in longitude between the
tangency points on the light source is
2*pi / npts
If the target is spherical, the terminator points
also are uniformly distributed in longitude in the
cylindrical system described above. If 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 `fixref'. Units are km.
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 the
Moon. Perform a consistency check using the solar incidence
angle at each point. 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 center of the Sun, the solar incidence angle at an umbral
terminator point should exceed 90 degrees by approximately the
angular radius of the Sun, while the angle at a penumbral
terminator points should be less than 90 degrees by that amount.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: edterm_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
--------- --------
de421.bsp Planetary ephemeris
pck00010.tpc Planet orientation and
radii
naif0010.tls Leapseconds
\begindata
KERNELS_TO_LOAD = ( 'de421.bsp',
'pck00010.tpc',
'naif0010.tls' )
\begintext
End of meta-kernel
Example code begins here.
PRO edterm_ex1
META = 'edterm_ex1.tm'
NPTS = 3L
first = 1L
trmtyps = [ 'UMBRAL', 'PENUMBRAL' ]
s = [ -1d, 1d]
;;
;; Load meta-kernel.
;;
cspice_furnsh, META
;;
;; Set observation time.
;;
utc = '2007 FEB 3 00:00:00.000'
cspice_str2et, utc, et
;;
;; Set participating objects, frame, and aberration
;; corrections.
;;
obsrvr = 'EARTH';
target = 'MOON';
source = 'SUN';
fixref = 'IAU_MOON';
abcorr = 'LT+S';
;;
;; Look up the radii of the sun.
;;
cspice_bodvrd, source, 'RADII', 3, srcrad
;;
;; Compute terminator points.
;;
for trmidx=0, 1 do begin
cspice_edterm, trmtyps[trmidx], source, target, $
et, fixref, abcorr, $
obsrvr, NPTS, trgepc, $
obspos, trmpts
;;
;; Validate terminator points.
;;
;; Look up the target-sun vector at the light-time
;; corrected target epoch.
;;
if ( first ) then begin
cspice_spkpos, source, trgepc, fixref, abcorr, target, $
srcpos, ltime
first = 0L
endif
print, ' Terminator type: ', trmtyps[trmidx]
for i = 0, NPTS - 1 do begin
;;
;; Convert the ith terminator point to latitudinal
;; coordinates. Display the point.
;;
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()
;;
;; Find the illumination angles at the
;; ith terminator point.
;;
cspice_ilumin, 'Ellipsoid', target, et, fixref, $
abcorr, obsrvr, trmpts[*,i], trgepc, $
srfvec, phase, solar, emissn
print, ' Solar incidence angle (deg): ', $
solar * cspice_dpr()
;;
;; Find the angular radius of the Sun as seen from
;; the terminator point.
;;
angrad = asin ( srcrad[0]/cspice_vdist(srcpos, trmpts[*,i]) );
;;
;; Display the solar incidence angle after
;; adjusting the angular radius of the Sun
;; as seen from the terminator point.The
;; result should be approximately 90 degrees.
;;
print, ' Solar incidence angle adjusted for'
print, $
FORMAT ='(" sun''s angular radius (deg): ", F18.9)', $
( solar + ( s[trmidx]*angrad ) ) * cspice_dpr()
print, ''
endfor
endfor
;;
;; 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, the output was:
Terminator type: UMBRAL
Terminator point : 0
Radius (km): 1737.4000
Planetocentric longitude (deg): -95.084553
Planetocentric latitude (deg): 0.0040527629
Solar incidence angle (deg): 90.269766
Solar incidence angle adjusted for
sun's angular radius (deg): 90.000000125
Terminator point : 1
Radius (km): 1737.4000
Planetocentric longitude (deg): 84.228092
Planetocentric latitude (deg): 59.995756
Solar incidence angle (deg): 90.269766
Solar incidence angle adjusted for
sun's angular radius (deg): 90.000000019
Terminator point : 2
Radius (km): 1737.4000
Planetocentric longitude (deg): 87.216418
Planetocentric latitude (deg): -59.979551
Solar incidence angle (deg): 90.269766
Solar incidence angle adjusted for
sun's angular radius (deg): 90.000000043
Terminator type: PENUMBRAL
Terminator point : 0
Radius (km): 1737.4000
Planetocentric longitude (deg): 84.914101
Planetocentric latitude (deg): -0.0040730470
Solar incidence angle (deg): 89.730234
Solar incidence angle adjusted for
sun's angular radius (deg): 90.000000122
Terminator point : 1
Radius (km): 1737.4000
Planetocentric longitude (deg): -95.769216
Planetocentric latitude (deg): -59.995785
Solar incidence angle (deg): 89.730234
Solar incidence angle adjusted for
sun's angular radius (deg): 90.000000021
Terminator point : 2
Radius (km): 1737.4000
Planetocentric longitude (deg): -92.780892
Planetocentric latitude (deg): 59.979499
Solar incidence angle (deg): 89.730234
Solar incidence angle adjusted for
sun's angular radius (deg): 90.000000044
This routine models the boundaries of shadow regions on an
ellipsoidal target body "illuminated" by a spherical light
source. Light rays are assumed to travel along straight lines;
refraction is not modeled.
Points on the target body's surface are classified according to
their illumination as follows:
- A target surface point X for which no vector from X to any
point in the light source intersects the target, except at
X, is considered to be "completely illuminated."
- A target surface point X for which each vector from X to a
point in the light source intersects the target at points
other than X is considered to be "in total shadow."
- All other target points are considered to be in partial
shadow.
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.
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(NOTRANSLATION) is
signaled by a routine in the call tree of this routine.
3) 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.
4) If the terminator type is not recognized, an error
is signaled by a routine in the call tree of
this routine.
5) If the terminator point count `npts' is not at least 1, an error
is signaled by a routine in the call tree of this routine.
6) If the light source has non-positive radius, an error
is signaled by a routine in the call tree of
this routine.
7) 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.
8) 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.
9) If radii are available but either body does not have three
radii, an error is signaled by a routine in the call tree of
this routine.
10) If any of the radii is less-than or equal to zero, an error is
signaled by a routine in the call tree of this routine.
11) If any SPK look-up fails, an error is signaled by
a routine in the call tree of this routine.
12) If any of the input arguments, `trmtyp', `source', `target',
`et', `fixref', `abcorr', `obsrvr' or `npts', is undefined, an
error is signaled by the IDL error handling system.
13) If any of the input arguments, `trmtyp', `source', `target',
`et', `fixref', `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.
14) If any of the output arguments, `trgepc', `obspos' or
`trmpts', is not a named variable, an error is signaled by the
Icy interface.
Appropriate SPK, PCK, and frame kernels must be loaded by the
calling program before this routine is called.
The following data are required:
- SPK data: ephemeris data for the 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
FRAMES.REQ
PCK.REQ
SPK.REQ
TIME.REQ
None.
N.J. Bachman (JPL)
J. Diaz del Rio (ODC Space)
-Icy Version 1.1.0, 01-NOV-2021 (JDR)
Changed argument names "fixfrm" and "termpts" to "fixref" and
"trmpts" for consistency with other routines. Changed the
variable name used for light time to "ltime" in the header
comments.
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.
-Icy Version 1.0.0, 14-JUN-2012 (NJB)
find terminator on ellipsoid
find umbral terminator on ellipsoid
find penumbral terminator on ellipsoid
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