Table of contents
CSPICE_SUBPNT computes the rectangular coordinates of the sub-observer
point on a target body at a specified epoch, optionally corrected for
light time and stellar aberration.
The surface of the target body may be represented by a triaxial
ellipsoid or by topographic data provided by DSK files.
This routine supersedes cspice_subpt.
Given:
method a short string providing parameters defining the computation
method to be used.
help, method
STRING = Scalar
In the syntax descriptions below, items delimited by brackets
are optional.
`method' may be assigned the following values:
'NEAR POINT/ELLIPSOID'
The sub-observer point computation uses a
triaxial ellipsoid to model the surface of the
target body. The sub-observer point is defined
as the nearest point on the target relative to
the observer.
The word 'NADIR' may be substituted for the phrase
'NEAR POINT' in the string above.
For backwards compatibility, the older syntax
'Near point: ellipsoid'
is accepted as well.
'INTERCEPT/ELLIPSOID'
The sub-observer point computation uses a
triaxial ellipsoid to model the surface of the
target body. The sub-observer point is defined
as the target surface intercept of the line
containing the observer and the target's
center.
For backwards compatibility, the older syntax
'Intercept: ellipsoid'
is accepted as well.
'NADIR/DSK/UNPRIORITIZED[/SURFACES = <surface list>]'
The sub-observer point computation uses DSK data
to model the surface of the target body. The
sub-observer point is defined as the intercept, on
the surface represented by the DSK data, of the
line containing the observer and the nearest point
on the target's reference ellipsoid. If multiple
such intercepts exist, the one closest to the
observer is selected.
Note that this definition of the sub-observer
point is not equivalent to the "nearest point on
the surface to the observer." The phrase 'NEAR
POINT' may NOT be substituted for 'NADIR' in the
string above.
The surface list specification is optional. The
syntax of the list is
<surface 1> [, <surface 2>...]
If present, it indicates that data only for the
listed surfaces are to be used; however, data
need not be available for all surfaces in the
list. If absent, loaded DSK data for any surface
associated with the target body are used.
The surface list may contain surface names or
surface ID codes. Names containing blanks must
be delimited by double quotes, for example
'SURFACES = "Mars MEGDR 128 PIXEL/DEG"'
If multiple surfaces are specified, their names
or IDs must be separated by commas.
See the -Particulars section below for details
concerning use of DSK data.
'INTERCEPT/DSK/UNPRIORITIZED[/SURFACES = <surface list>]'
The sub-observer point computation uses DSK data
to model the surface of the target body. The
sub-observer point is defined as the target
surface intercept of the line containing the
observer and the target's center.
If multiple such intercepts exist, the one closest
to the observer is selected.
The surface list specification is optional. The
syntax of the list is identical to that for the
NADIR option described above.
Neither case nor white space are significant in
`method', except within double-quoted strings. For
example, the string ' eLLipsoid/nearpoint ' is valid.
Within double-quoted strings, blank characters are
significant, but multiple consecutive blanks are
considered equivalent to a single blank. Case is
not significant. So
"Mars MEGDR 128 PIXEL/DEG"
is equivalent to
" mars megdr 128 pixel/deg "
but not to
"MARS MEGDR128PIXEL/DEG"
target the scalar string name of the target body.
help, target
STRING = Scalar
The target body is an ephemeris object (its trajectory is given
by SPK data), and is an extended object.
The string '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.
When the target body's surface is represented by a
tri-axial ellipsoid, this routine assumes that a
kernel variable representing the ellipsoid's radii is
present in the kernel pool. Normally the kernel
variable would be defined by loading a PCK file.
et the scalar double precision epoch, expressed as seconds past
J2000 TDB, of the observer: 'et' is the epoch at which the
observer's state 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 are computed.
When aberration corrections are used, 'et' is the epoch
at which the observer's state 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 sub-observer point and the
observer, and the sign applied to 'lt' depends on the
selected correction. See the description of 'abcorr'
below for details.
fixref the name of a body-fixed reference frame centered on the target
body.
help, fixref
STRING = Scalar
`fixref' may be any such frame supported by the SPICE system,
including built-in frames (documented in the Frames Required
Reading) and frames defined by a loaded frame kernel (FK). The
string `fixref' is case-insensitive, and leading and trailing
blanks in `fixref' are not significant.
The output sub-observer point `spoint' and the
observer-to-sub-observer point vector `srfvec' will be
expressed relative to this reference frame.
abcorr the scalar string aberration correction to apply when computing
the observer-target state and the orientation of the target
body.
help, abcorr
STRING = Scalar
For remote sensing applications, where the apparent
sub-observer point seen by the observer is desired,
normally either of the corrections
'LT+S'
'CN+S'
should be used. These and the other supported options
are described below. 'abcorr' may be any of the
following:
'NONE' Apply no correction. Return the
geometric sub-observer point on the
target body.
Let 'lt' represent the one-way light time between the
observer and the sub-observer point (note: NOT
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
sub-observer point's location 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 sub-observer
point at the moment it emitted photons
arriving at the observer at 'et'.
The light time correction uses an
iterative solution of the light time
equation. The solution invoked by the
'LT' option uses one iteration.
Both the target position as seen by the
observer, and 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
state obtained with the 'LT' option to
account for the observer's velocity
relative to the solar system
barycenter. The result is the apparent
sub-observer point 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. Both the
position and rotation of the target
body are corrected for light time.
'CN+S' Converged Newtonian light time and
stellar aberration corrections. This
option produces a solution that is at
least as accurate at that obtainable
with the 'LT+S' option. Whether the 'CN+S'
solution is substantially more accurate
depends on the geometry of the
participating objects and on the
accuracy of the input data. In all
cases this routine will execute more
slowly when a converged solution is
computed.
The following values of 'abcorr' apply to the
"transmission" case in which photons *depart* from
the observer's location at 'et' and arrive at the
sub-observer point at the light-time corrected epoch
et+lt:
'XLT' "Transmission" case: correct for
one-way light time using a Newtonian
formulation. This correction yields the
sub-observer location at the moment it
receives photons emitted from the
observer's location at 'et'.
The light time correction uses an
iterative solution of the light time
equation. The solution invoked by the
'LT' option uses one iteration.
Both the target position as seen by the
observer, and rotation of the target
body, are corrected for light time.
'XLT+S' "Transmission" case: correct for
one-way light time and stellar
aberration using a Newtonian
formulation This option modifies the
sub-observer point obtained with the
'XLT' option to account for the
observer's velocity relative to the
solar system barycenter.
'XCN' Converged Newtonian light time
correction. This is the same as XLT
correction but with further iterations
to a converged Newtonian light time
solution.
'XCN+S' "Transmission" case: converged
Newtonian light time and stellar
aberration corrections.
obsrvr the scalar string name of the observing body.
help, obsrvr
STRING = Scalar
The observing body is an ephemeris object: it typically is a
spacecraft, the earth, or a surface point on the earth. 'obsrvr'
is case-insensitive, and leading and 'obsrvr' are not
significant. Optionally, you may trailing blanks in 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 observer.
the call:
cspice_subpnt, method, target, et, fixref, abcorr, $
obsrvr, spoint, trgepc, srfvec
returns:
spoint a double precision 3-vector defining the sub-observer point on
the target body.
help, spoint
DOUBLE = Array[3]
For target shapes modeled by ellipsoids, the
sub-observer point is defined either as the point on
the target body that is closest to the observer, or
the target surface intercept of the line from the
observer to the target's center.
For target shapes modeled by topographic data
provided by DSK files, the sub-observer point is
defined as the target surface intercept of the line
from the observer to either the nearest point on the
reference ellipsoid, or to the target's center. If
multiple such intercepts exist, the one closest to
the observer is selected.
The input argument `method' selects the target shape
model and sub-observer point definition to be used.
`spoint' is expressed in Cartesian coordinates,
relative to the body-fixed target frame designated by
`fixref'. The body-fixed target frame is evaluated at
the sub-observer epoch `trgepc' (see description below).
When light time correction is used, the duration of
light travel between `spoint' to the observer is
considered to be the one way light time.
When aberration corrections are used, `spoint' is
computed using target body position and orientation
that have been adjusted for the corrections
applicable to `spoint' itself rather than to the target
body's center. In particular, if the stellar
aberration correction applicable to `spoint' is
represented by a shift vector S, then the light-time
corrected position of the target is shifted by S
before the sub-observer point is computed.
The components of `spoint' have units of km.
trgepc the scalar double precision "sub-observer point epoch."
help, trgepc
DOUBLE = Scalar
'trgepc' is defined as follows: letting 'lt' be the one-way
light time between the observer and the sub-observer point,
'trgepc' is the epoch et-lt, et+lt, or 'et' depending on whether
the requested aberration correction is, respectively, for
received radiation, transmitted radiation, or omitted. 'lt' is
computed using the method indicated by 'abcorr'.
'trgepc' is expressed as seconds past J2000 TDB.
srfvec a double precision 3-vector defining the position vector from
the observer at 'et' to 'spoint'.
help, srfvec
DOUBLE = Array[3]
'srfvec' is expressed in the target body-fixed reference frame
designated by 'fixref', evaluated at 'trgepc'.
The components of 'srfvec' are given in units of km.
One can use the function norm to obtain the
distance between the observer and 'spoint':
dist = norm( srfvec )
The observer's position 'obspos', relative to the
target body's center, where the center's position is
corrected for aberration effects as indicated by
'abcorr', can be computed with:
obspos = spoint - srfvec
To transform the vector 'srfvec' 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
'srfvec' can be transformed to the result 'refvec' as
follows:
cspice_pxfrm2, fixref, ref, trgepc, et, xform
cspice_mxv, xform, srfvec, refvec
None.
Any numerical results shown for these examples may differ between
platforms as the results depend on the SPICE kernels used as input
and the machine specific arithmetic implementation.
1) Find the sub-Earth point on Mars for a specified time.
Compute the sub-Earth points using both triaxial ellipsoid
and topographic surface models. Topography data are provided by
a DSK file. For the ellipsoid model, use both the "intercept"
and "near point" sub-observer point definitions; for the DSK
case, use both the "intercept" and "nadir" definitions.
Display the locations of both the Earth and the sub-Earth
point relative to the center of Mars, in the IAU_MARS
body-fixed reference frame, using both planetocentric and
planetographic coordinates.
The topographic model is based on data from the MGS MOLA DEM
megr90n000cb, which has a resolution of 4 pixels/degree. A
triangular plate model was produced by computing a 720 x 1440
grid of interpolated heights from this DEM, then tessellating
the height grid. The plate model is stored in a type 2 segment
in the referenced DSK file.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File: subpnt_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
--------- --------
de430.bsp Planetary ephemeris
mar097.bsp Mars satellite ephemeris
pck00010.tpc Planet orientation and
radii
naif0011.tls Leapseconds
megr90n000cb_plate.bds Plate model based on
MEGDR DEM, resolution
4 pixels/degree.
\begindata
KERNELS_TO_LOAD = ( 'de430.bsp',
'mar097.bsp',
'pck00010.tpc',
'naif0011.tls',
'megr90n000cb_plate.bds' )
\begintext
End of meta-kernel
Example code begins here.
PRO subpnt_ex1
;;
;; Load kernel files via the meta-kernel.
;;
cspice_furnsh, 'subpnt_ex1.tm'
;;
;; Convert the UTC request time to ET (seconds past
;; J2000, TDB).
;;
cspice_str2et, '2008 aug 11 00:00:00', et
;;
;; Look up the target body's radii. We'll use these to
;; convert Cartesian to planetodetic coordinates. Use
;; the radii to compute the flattening coefficient of
;; the reference ellipsoid.
;;
cspice_bodvrd, 'MARS', 'RADII', 3, radii
;;
;; Let RE and RP be, respectively, the equatorial and
;; polar radii of the target.
;;
re = radii[0]
rp = radii[2]
f = ( re-rp)/re
;;
;; Compute sub-observer point using light time and stellar
;; aberration corrections. Use both ellipsoid and DSK
;; shape models, and use all of the "near point,"
;; "intercept," and "nadir" sub-observer point definitions.
;;
method = [ 'Intercept: ellipsoid', 'Near point: ellipsoid', $
'Intercept/DSK/Unprioritized', 'Nadir/DSK/Unprioritized' ]
for i=0,3 do begin
cspice_subpnt, method[i], 'MARS', et, 'IAU_MARS', 'LT+S', $
'EARTH', spoint, trgepc, srfvec
;;
;; Compute the observer's distance from SPOINT.
;;
odist = norm(srfvec);
;;
;; Convert the sub-observer point's rectangular coordinates
;; to planetographic longitude, latitude and altitude.
;; Convert radians to degrees.
;;
cspice_recpgr, 'mars', spoint, re, f, spglon, spglat, spgalt
spglon = spglon * cspice_dpr();
spglat = spglat * cspice_dpr();
;;
;; Convert sub-observer point's rectangular coordinates to
;; planetocentric radius, longitude, and latitude. Convert
;; radians to degrees.
;;
cspice_reclat, spoint, spcrad, spclon, spclat
spclon = spclon * cspice_dpr()
spclat = spclat * cspice_dpr()
;;
;; Compute the observer's position relative to the center of the
;; target, where the center's location has been adjusted using
;; the aberration corrections applicable to the sub-point.
;; Express the observer's location in geodetic coordinates.
;;
obspos = spoint - srfvec;
cspice_recpgr, 'mars', obspos, re, f, opglon, opglat, opgalt
opglon = opglon * cspice_dpr()
opglat = opglat * cspice_dpr()
;;
;; Convert the observer's rectangular coordinates to
;; planetocentric longitude, latitude and altitude. Convert
;; radians to degrees.
;;
cspice_reclat, obspos, opcrad, opclon, opclat
opclon = opclon * cspice_dpr()
opclat = opclat * cspice_dpr()
print, 'Computational Method = ', method[i]
print, ' '
print, FORMAT='(A,F21.9)', $
' Observer altitude (km) = ', opgalt
print, FORMAT='(A,F21.9)', $
' Length of SRFVEC (km) = ', odist
print, FORMAT='(A,F21.9)', $
' Sub-observer point altitude (km) = ', spgalt
print, FORMAT='(A,F21.9)', $
' Sub-observer planetographic longitude (deg) = ', spglon
print, FORMAT='(A,F21.9)', $
' Observer planetographic longitude (deg) = ', opglon
print, FORMAT='(A,F21.9)', $
' Sub-observer planetographic latitude (deg) = ', spglat
print, FORMAT='(A,F21.9)', $
' Observer planetographic latitude (deg) = ', opglat
print, FORMAT='(A,F21.9)', $
' Sub-observer planetocentric longitude (deg) = ', spclon
print, FORMAT='(A,F21.9)', $
' Observer planetocentric longitude (deg) = ', opclon
print, FORMAT='(A,F21.9)', $
' Sub-observer planetocentric latitude (deg) = ', spclat
print, FORMAT='(A,F21.9)', $
' Observer planetocentric latitude (deg) = ', opclat
print, ' '
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:
Computational Method = Intercept: ellipsoid
Observer altitude (km) = 349199089.540938914
Length of SRFVEC (km) = 349199089.577634573
Sub-observer point altitude (km) = -0.000000000
Sub-observer planetographic longitude (deg) = 199.302305032
Observer planetographic longitude (deg) = 199.302305032
Sub-observer planetographic latitude (deg) = 26.262401237
Observer planetographic latitude (deg) = 25.994936751
Sub-observer planetocentric longitude (deg) = 160.697694968
Observer planetocentric longitude (deg) = 160.697694968
Sub-observer planetocentric latitude (deg) = 25.994934171
Observer planetocentric latitude (deg) = 25.994934171
Computational Method = Near point: ellipsoid
Observer altitude (km) = 349199089.540930629
Length of SRFVEC (km) = 349199089.540930629
Sub-observer point altitude (km) = -0.000000000
Sub-observer planetographic longitude (deg) = 199.302305032
Observer planetographic longitude (deg) = 199.302305032
Sub-observer planetographic latitude (deg) = 25.994936751
Observer planetographic latitude (deg) = 25.994936751
Sub-observer planetocentric longitude (deg) = 160.697694968
Observer planetocentric longitude (deg) = 160.697694968
Sub-observer planetocentric latitude (deg) = 25.729407227
Observer planetocentric latitude (deg) = 25.994934171
Computational Method = Intercept/DSK/Unprioritized
Observer altitude (km) = 349199089.541009188
Length of SRFVEC (km) = 349199091.785398543
Sub-observer point altitude (km) = -2.207669751
Sub-observer planetographic longitude (deg) = 199.302305002
Observer planetographic longitude (deg) = 199.302305002
Sub-observer planetographic latitude (deg) = 26.262576677
Observer planetographic latitude (deg) = 25.994936751
Sub-observer planetocentric longitude (deg) = 160.697694998
Observer planetocentric longitude (deg) = 160.697694998
Sub-observer planetocentric latitude (deg) = 25.994934171
Observer planetocentric latitude (deg) = 25.994934171
Computational Method = Nadir/DSK/Unprioritized
Observer altitude (km) = 349199089.540999591
Length of SRFVEC (km) = 349199091.707164228
Sub-observer point altitude (km) = -2.166164622
Sub-observer planetographic longitude (deg) = 199.302305004
Observer planetographic longitude (deg) = 199.302305003
Sub-observer planetographic latitude (deg) = 25.994936751
Observer planetographic latitude (deg) = 25.994936751
Sub-observer planetocentric longitude (deg) = 160.697694996
Observer planetocentric longitude (deg) = 160.697694997
Sub-observer planetocentric latitude (deg) = 25.729237570
Observer planetocentric latitude (deg) = 25.994934171
2) Use cspice_subpnt to find the sub-spacecraft point on Mars for the
Mars Reconnaissance Orbiter spacecraft (MRO) at a specified time,
using both the 'Ellipsoid/Near point' computation method and an
ellipsoidal target shape, and the 'DSK/Unprioritized/Nadir'
method and a DSK-based shape model.
Use both LT+S and CN+S aberration corrections to illustrate
the differences.
Convert the spacecraft to sub-observer point vector obtained from
cspice_subpnt into the MRO_HIRISE_LOOK_DIRECTION reference frame at
the observation time. Perform a consistency check with this
vector: compare the Mars surface intercept of the ray emanating
from the spacecraft and pointed along this vector with the
sub-observer point.
Perform the sub-observer point and surface intercept computations
using both triaxial ellipsoid and topographic surface models.
For this example, the topographic model is based on the MGS MOLA
DEM megr90n000eb, which has a resolution of 16 pixels/degree.
Eight DSKs, each covering longitude and latitude ranges of 90
degrees, were made from this data set. For the region covered by
a given DSK, a grid of approximately 1500 x 1500 interpolated
heights was produced, and this grid was tessellated using
approximately 4.5 million triangular plates, giving a total plate
count of about 36 million for the entire DSK set.
All DSKs in the set use the surface ID code 499001, so there is
no need to specify the surface ID in the `method' strings passed
to cspice_sincpt and cspice_subpnt.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: subpnt_ex2.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
--------- --------
de430.bsp Planetary ephemeris
mar097.bsp Mars satellite ephemeris
pck00010.tpc Planet orientation and
radii
naif0011.tls Leapseconds
mro_psp4_ssd_mro95a.bsp MRO ephemeris
mro_v11.tf MRO frame specifications
mro_sclkscet_00022_65536.tsc MRO SCLK coefficients
parameters
mro_sc_psp_070925_071001.bc MRO attitude
megr90n000eb_*_plate.bds Plate model DSKs based
on MEGDR DEM, resolution
16 pixels/degree.
\begindata
KERNELS_TO_LOAD = (
'de430.bsp',
'mar097.bsp',
'pck00010.tpc',
'naif0011.tls',
'mro_psp4_ssd_mro95a.bsp',
'mro_v11.tf',
'mro_sclkscet_00022_65536.tsc',
'mro_sc_psp_070925_071001.bc',
'megr90n000eb_LL000E00N_UR090E90N_plate.bds'
'megr90n000eb_LL000E90S_UR090E00S_plate.bds'
'megr90n000eb_LL090E00N_UR180E90N_plate.bds'
'megr90n000eb_LL090E90S_UR180E00S_plate.bds'
'megr90n000eb_LL180E00N_UR270E90N_plate.bds'
'megr90n000eb_LL180E90S_UR270E00S_plate.bds'
'megr90n000eb_LL270E00N_UR360E90N_plate.bds'
'megr90n000eb_LL270E90S_UR360E00S_plate.bds' )
\begintext
End of meta-kernel
Example code begins here.
PRO subpnt_ex2
;;
;; Local constants
;;
META = 'subpnt_ex2.tm'
NCORR = 2
NMETH = 2
abcorr = [ 'LT+S', 'CN+S' ]
fixref = 'IAU_MARS'
sinmth = [ 'Ellipsoid', 'DSK/Unprioritized' ]
submth = [ 'Ellipsoid/Near point', 'DSK/Unprioritized/Nadir' ]
;;
;; Load kernel files via the meta-kernel.
;;
cspice_furnsh, META
;;
;; Convert the TDB request time string to seconds past
;; J2000, TDB.
;;
cspice_str2et, '2007 SEP 30 00:00:00 TDB', et
;;
;; Compute the sub-spacecraft point using each method.
;; Compute the results using both LT+S and CN+S aberration
;; corrections.
;;
for i=0, NMETH - 1L do begin
print, ''
print, 'Sub-observer point computation method = ', submth(i)
for j=0, NCORR - 1L do begin
cspice_subpnt, submth(i), 'mars', et, fixref, abcorr(j), $
'mro', spoint, trgepc, srfvec
;;
;; Compute the observer's altitude above `spoint'.
;;
alt = cspice_vnorm( srfvec )
;;
;; Express `srfvec' in the MRO_HIRISE_LOOK_DIRECTION
;; reference frame at epoch `et'. Since `srfvec' is expressed
;; relative to the IAU_MARS frame at `trgepc', we must call
;; cspice_pxfrm2 to compute the position transformation matrix
;; from IAU_MARS at `trgepc' to the MRO_HIRISE_LOOK_DIRECTION
;; frame at time `et'.
;;
;; To make code formatting a little easier, we'll store
;; the long MRO reference frame name in a variable:
;;
hiref = 'MRO_HIRISE_LOOK_DIRECTION'
cspice_pxfrm2, 'iau_mars', hiref, trgepc, et, xform
cspice_mxv, xform, srfvec, mrovec
;;
;; Convert sub-observer point rectangular coordinates to
;; planetocentric latitude and longitude. Convert radians to
;; degrees.
;;
cspice_reclat, spoint, radius, lon, lat
lon *= cspice_dpr( )
lat *= cspice_dpr( )
;;
;; Write the results.
;;
print, ''
print, ' Aberration correction = ', abcorr(j)
print, ''
print, ' MRO-to-sub-observer vector in'
print, ' MRO HIRISE look direction frame'
print, format='(A,F22.9)', ' X-' + $
'component (km) =', $
mrovec[0]
print, format='(A,F22.9)', ' Y-' + $
'component (km) =', $
mrovec[1]
print, format='(A,F22.9)', ' Z-' + $
'component (km) =', $
mrovec[2]
print, format='(A,F22.9)', ' Sub-observer point' + $
' radius (km) =', radius
print, format='(A,F22.9)', ' Planetocentric' + $
' latitude (deg) =', lat
print, format='(A,F22.9)', ' Planetocentric' + $
' longitude (deg) =', lon
print, format='(A,F22.9)', ' Observer' + $
' altitude (km) =', alt
;;
;; Consistency check: find the surface intercept on
;; Mars of the ray emanating from the spacecraft and having
;; direction vector `mrovec' in the MRO HIRISE look direction
;; reference frame at `et'. Call the intercept point
;; `xpoint'. `xpoint' should coincide with `spoint', up to a
;; small round-off error.
;;
cspice_sincpt, sinmth(i), 'mars', et, 'iau_mars', $
abcorr(j), 'mro', hiref, mrovec, $
xpoint, xepoch, xvec, found
if ( not found ) then begin
print, 'Bug: no intercept'
endif else begin
;;
;; Report the distance between `xpoint' and `spoint'.
;;
print, format='(A,F22.9)', ' Intercept' + $
' comparison error (km) =', $
cspice_vdist( xpoint, spoint )
print, ''
endelse
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:
Sub-observer point computation method = Ellipsoid/Near point
Aberration correction = LT+S
MRO-to-sub-observer vector in
MRO HIRISE look direction frame
X-component (km) = 0.286933229
Y-component (km) = -0.260425939
Z-component (km) = 253.816326385
Sub-observer point radius (km) = 3388.299078378
Planetocentric latitude (deg) = -38.799836378
Planetocentric longitude (deg) = -114.995297227
Observer altitude (km) = 253.816622175
Intercept comparison error (km) = 0.000002144
Aberration correction = CN+S
MRO-to-sub-observer vector in
MRO HIRISE look direction frame
X-component (km) = 0.286933107
Y-component (km) = -0.260426683
Z-component (km) = 253.816315915
Sub-observer point radius (km) = 3388.299078376
Planetocentric latitude (deg) = -38.799836382
Planetocentric longitude (deg) = -114.995297449
Observer altitude (km) = 253.816611705
Intercept comparison error (km) = 0.000000001
Sub-observer point computation method = DSK/Unprioritized/Nadir
Aberration correction = LT+S
MRO-to-sub-observer vector in
MRO HIRISE look direction frame
X-component (km) = 0.282372596
Y-component (km) = -0.256289313
Z-component (km) = 249.784871247
Sub-observer point radius (km) = 3392.330239436
Planetocentric latitude (deg) = -38.800230156
Planetocentric longitude (deg) = -114.995297338
Observer altitude (km) = 249.785162334
Intercept comparison error (km) = 0.000002412
Aberration correction = CN+S
MRO-to-sub-observer vector in
MRO HIRISE look direction frame
X-component (km) = 0.282372464
Y-component (km) = -0.256290075
Z-component (km) = 249.784860121
Sub-observer point radius (km) = 3392.330239564
Planetocentric latitude (deg) = -38.800230162
Planetocentric longitude (deg) = -114.995297569
Observer altitude (km) = 249.785151209
Intercept comparison error (km) = 0.000000001
For ellipsoidal target bodies, there are two different popular
ways to define the sub-observer point: "nearest point on the
target to the observer" or "target surface intercept of the line
containing observer and target." These coincide when the target
is spherical and generally are distinct otherwise.
For target body shapes modeled using topographic data provided by
DSK files, the "surface intercept" notion is valid, but the
"nearest point on the surface" computation is both inefficient to
execute and may fail to yield a result that is "under" the
observer in an intuitively clear way. The NADIR option for DSK
shapes instead finds the surface intercept of a ray that passes
through the nearest point on the target reference ellipsoid. For
shapes modeled using topography, there may be multiple
ray-surface intercepts; the closest one to the observer is
selected.
The NADIR definition makes sense only if the target shape is
reasonably close to the target's reference ellipsoid. If the
target is very different---the nucleus of comet
Churyumov-Gerasimenko is an example---the intercept definition
should be used.
This routine computes light time corrections using light time
between the observer and the sub-observer point, as opposed to
the center of the target. Similarly, stellar aberration
corrections done by this routine are based on the direction of
the vector from the observer to the light-time corrected
sub-observer point, not to the target center. This technique
avoids errors due to the differential between aberration
corrections across the target body. Therefore it's valid to use
aberration corrections with this routine even when the observer
is very close to the sub-observer point, in particular when the
observer to sub-observer point distance is much less than the
observer to target center distance.
When comparing sub-observer point computations with results from
sources other than SPICE, it's essential to make sure the same
geometric definitions are used.
Using DSK data
==============
DSK loading and unloading
-------------------------
DSK files providing data used by this routine are loaded by
calling cspice_furnsh and can be unloaded by calling cspice_unload or
cspice_kclear. See the documentation of cspice_furnsh for limits on
numbers of loaded DSK files.
For run-time efficiency, it's desirable to avoid frequent
loading and unloading of DSK files. When there is a reason to
use multiple versions of data for a given target body---for
example, if topographic data at varying resolutions are to be
used---the surface list can be used to select DSK data to be
used for a given computation. It is not necessary to unload
the data that are not to be used. This recommendation presumes
that DSKs containing different versions of surface data for a
given body have different surface ID codes.
DSK data priority
-----------------
A DSK coverage overlap occurs when two segments in loaded DSK
files cover part or all of the same domain---for example, a
given longitude-latitude rectangle---and when the time
intervals of the segments overlap as well.
When DSK data selection is prioritized, in case of a coverage
overlap, if the two competing segments are in different DSK
files, the segment in the DSK file loaded last takes
precedence. If the two segments are in the same file, the
segment located closer to the end of the file takes
precedence.
When DSK data selection is unprioritized, data from competing
segments are combined. For example, if two competing segments
both represent a surface as sets of triangular plates, the
union of those sets of plates is considered to represent the
surface.
Currently only unprioritized data selection is supported.
Because prioritized data selection may be the default behavior
in a later version of the routine, the UNPRIORITIZED keyword is
required in the `method' argument.
Syntax of the `method' input argument
-------------------------------------
The keywords and surface list in the `method' argument
are called "clauses." The clauses may appear in any
order, for example
'NADIR/DSK/UNPRIORITIZED/<surface list>'
'DSK/NADIR/<surface list>/UNPRIORITIZED'
'UNPRIORITIZED/<surface list>/DSK/NADIR'
The simplest form of the `method' argument specifying use of
DSK data is one that lacks a surface list, for example:
'NADIR/DSK/UNPRIORITIZED'
'INTERCEPT/DSK/UNPRIORITIZED'
For applications in which all loaded DSK data for the target
body are for a single surface, and there are no competing
segments, the above strings suffice. This is expected to be
the usual case.
When, for the specified target body, there are loaded DSK
files providing data for multiple surfaces for that body, the
surfaces to be used by this routine for a given call must be
specified in a surface list, unless data from all of the
surfaces are to be used together.
The surface list consists of the string
'SURFACES = '
followed by a comma-separated list of one or more surface
identifiers. The identifiers may be names or integer codes in
string format. For example, suppose we have the surface
names and corresponding ID codes shown below:
Surface Name ID code
------------ -------
"Mars MEGDR 128 PIXEL/DEG" 1
"Mars MEGDR 64 PIXEL/DEG" 2
"Mars_MRO_HIRISE" 3
If data for all of the above surfaces are loaded, then
data for surface 1 can be specified by either
'SURFACES = 1'
or
'SURFACES = "Mars MEGDR 128 PIXEL/DEG"'
Double quotes are used to delimit the surface name
because it contains blank characters.
To use data for surfaces 2 and 3 together, any
of the following surface lists could be used:
'SURFACES = 2, 3'
'SURFACES = "Mars MEGDR 64 PIXEL/DEG", 3'
'SURFACES = 2, Mars_MRO_HIRISE'
'SURFACES = "Mars MEGDR 64 PIXEL/DEG", Mars_MRO_HIRISE'
An example of a `method' argument that could be constructed
using one of the surface lists above is
'NADIR/DSK/UNPRIORITIZED/SURFACES= "Mars MEGDR 64 PIXEL/DEG",3'
Aberration corrections
----------------------
For irregularly shaped target bodies, the distance between the
observer and the nearest surface intercept need not be a
continuous function of time; hence the one-way light time
between the intercept and the observer may be discontinuous as
well. In such cases, the computed light time, which is found
using iterative algorithm, may converge slowly or not at all.
In all cases, the light time computation will terminate, but
the result may be less accurate than expected.
1) If the specified aberration correction is unrecognized, an
error is signaled by a routine in the call tree of this
routine.
2) If either the target or observer input strings cannot be
converted to an integer ID code, the error
SPICE(IDCODENOTFOUND) is signaled by a routine in the call
tree of this routine.
3) If `obsrvr' and `target' map to the same NAIF integer ID code, the
error SPICE(BODIESNOTDISTINCT) is signaled by a routine in the
call tree of this routine.
4) If the input target body-fixed frame `fixref' is not recognized,
the error SPICE(NOFRAME) is signaled by a routine in the call
tree of this routine. A frame name may fail to be recognized
because a required frame specification kernel has not been
loaded; another cause is a misspelling of the frame name.
5) If the input frame `fixref' is not centered at the target body,
the error SPICE(INVALIDFRAME) is signaled by a routine in the
call tree of this routine.
6) If the input argument `method' is not recognized, the error
SPICE(INVALIDMETHOD) is signaled by this routine, or, the
error is signaled by a routine in the call tree of this
routine.
7) If the sub-observer point type is not specified or is not
recognized, the error SPICE(INVALIDSUBTYPE) is signaled by a
routine in the call tree of this routine.
8) If the target and observer have distinct identities but are at
the same location (for example, the target is Mars and the
observer is the Mars barycenter), the error
SPICE(NOSEPARATION) is signaled by a routine in the call tree
of this routine.
9) If insufficient ephemeris data have been loaded prior to
calling cspice_subpnt, an error is signaled by a
routine in the call tree of this routine. Note that when
light time correction is used, sufficient ephemeris data must
be available to propagate the states of both observer and
target to the solar system barycenter.
10) If the computation method specifies an ellipsoidal target
shape and triaxial radii of the target body have not been
loaded into the kernel pool prior to calling cspice_subpnt, an error
is signaled by a routine in the call tree of this routine.
11) The target must be an extended body, and must have a shape
for which a sub-observer point can be defined.
If the target body's shape is modeled by DSK data, the shape
must be such that the specified sub-observer point
definition is applicable. For example, if the target shape
is a torus, both the NADIR and INTERCEPT definitions might
be inapplicable, depending on the relative locations of the
observer and target.
12) If PCK data specifying the target body-fixed frame orientation
have not been loaded prior to calling cspice_subpnt, an error is
signaled by a routine in the call tree of this routine.
13) If `method' specifies that the target surface is represented by
DSK data, and no DSK files are loaded for the specified
target, an error is signaled by a routine in the call tree
of this routine.
14) If `method' specifies that the target surface is represented by
DSK data, and the ray from the observer to the sub-observer
point doesn't intersect the target body's surface, the error
SPICE(SUBPOINTNOTFOUND) is signaled by a routine in the call
tree of this routine.
15) If the surface intercept on the target body's reference
ellipsoid of the observer to target center vector cannot not
be computed, the error SPICE(DEGENERATECASE) is signaled by a
routine in the call tree of this routine. Note that this is a
very rare case.
16) If radii for `target' are not found in the kernel pool, an error
is signaled by a routine in the call tree of this routine.
17) If the size of the `target' body radii kernel variable is not
three, an error is signaled by a routine in the call tree of
this routine.
18) If any of the three `target' body radii is less-than or equal to
zero, an error is signaled by a routine in the call tree of
this routine.
19) If any of the input arguments, `method', `target', `et',
`fixref', `abcorr' or `obsrvr', is undefined, an error is
signaled by the IDL error handling system.
20) If any of the input arguments, `method', `target', `et',
`fixref', `abcorr' or `obsrvr', is not of the expected type,
or it does not have the expected dimensions and size, an error
is signaled by the Icy interface.
21) If any of the output arguments, `spoint', `trgepc' or
`srfvec', is not a named variable, an error is signaled by the
Icy interface.
Appropriate 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 must be
loaded. 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.
Typically ephemeris data are made available by loading one
or more SPK files via cspice_furnsh.
- PCK data: rotation data for the target body must be
loaded. These may be provided in a text or binary PCK file.
- Shape data for the target body:
PCK data:
If the target body shape is modeled as an ellipsoid,
triaxial radii for the target body must be loaded into
the kernel pool. Typically this is done by loading a
text PCK file via cspice_furnsh.
Triaxial radii are also needed if the target shape is
modeled by DSK data, but the DSK NADIR method is
selected.
DSK data:
If the target shape is modeled by DSK data, DSK files
containing topographic data for the target body must be
loaded. If a surface list is specified, data for at
least one of the listed surfaces must be loaded.
The following data may be required:
- 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 definition is supplied by loading a
frame kernel via cspice_furnsh.
- Surface name-ID associations: if surface names are specified
in `method', the association of these names with their
corresponding surface ID codes must be established by
assignments of the kernel variables
NAIF_SURFACE_NAME
NAIF_SURFACE_CODE
NAIF_SURFACE_BODY
Normally these associations are made by loading a text
kernel containing the necessary assignments. An example
of such an assignment is
NAIF_SURFACE_NAME += 'Mars MEGDR 128 PIXEL/DEG'
NAIF_SURFACE_CODE += 1
NAIF_SURFACE_BODY += 499
In all cases, kernel data are normally loaded once per program
run, NOT every time this routine is called.
None.
ICY.REQ
DSK.REQ
NAIF_IDS.REQ
PCK.REQ
SPK.REQ
TIME.REQ
None.
N.J. Bachman (JPL)
J. Diaz del Rio (ODC Space)
S.C. Krening (JPL)
E.D. Wright (JPL)
-Icy Version 2.0.1, 01-NOV-2021 (JDR)
Edited the header to comply with NAIF standard. Added example's
meta-kernel. Updated example #1 to use DSK data. Added second example.
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 2.0.0, 04-APR-2017 (EDW) (NJB)
Updated to support use of DSKs.
-Icy Version 1.0.2, 15-NOV-2011 (SCK)
References to the new cspice_pxfrm2 routine were added
to the 'I/O returns' section. A problem description was
added to the -Examples section.
-Icy Version 1.0.1, 12-APR-2011 (EDW)
Corrected typo in example program comments.
-Icy Version 1.0.0, 01-FEB-2008 (EDW)
find sub-observer point on target body
find sub-spacecraft point on target body
find nearest point to observer on target body
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