Table of contents
Deprecated: This routine has been superseded by the Mice routine
cspice_subslr. This routine is supported for purposes of backward
compatibility only.
CSPICE_SUBSOL_PL02 returns the rectangular coordinates of the sub-solar
point on a target body in the body-fixed frame associated with the body,
at a particular epoch, optionally corrected for light time and stellar
aberration. In addition, it returns return the observer's distance from
the sub-solar point.
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.
[1,1] = size(handle); int32 = class(handle)
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 the target body.
[SPICE_DLA_DSCSIZ,1] = size(dladsc); int32 = class(dladsc)
method a short string specifying the computation method
to be used.
[1,c1] = size(method); char = class(method)
or
[1,1] = size(method); cell = class(method)
The choices are:
'Intercept' The sub-solar point is defined as
the plate model surface intercept
of the ray starting at the Sun and
passing through the target's
center.
'Ellipsoid
near point' The sub-solar point is defined as
the plate model surface intercept
of the ray starting at the Sun and
passing through the nearest point
to the observer on a reference
ellipsoid associated with the
target body.
This option requires that the
reference ellipsoid's radii be
available in the kernel pool.
Neither case nor white space are significant in the
string `method'. For example, the string
' ellipsoidNEARPOINT'
is valid.
target the name of the target body.
[1,c2] = size(target); char = class(target)
or
[1,1] = size(target); cell = class(target)
`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 the target body's surface is
represented by a plate model, and that a DSK file
containing the plate model has been loaded via cspice_dasopr.
et the epoch, represented as seconds past J2000 TDB, at
which the sub-solar point on the target body is to be
computed.
[1,1] = size(et); double = class(et)
When aberration corrections are used, `et' refers to the epoch
at which radiation is received at the observer.
abcorr indicates the aberration corrections to be applied to
the position and orientation of the target body and the
position of the Sun to account for one-way light time
and stellar aberration.
[1,c3] = size(abcorr); char = class(abcorr)
or
[1,1] = size(abcorr); cell = class(abcorr)
See the discussion in the -Particulars section for
recommendations on how to choose aberration corrections.
`abcorr' may be any of the following:
'NONE' Apply no correction. Use the geometric
positions of the Sun and target body
relative to the observer; evaluate the
target body's orientation at `et'.
The following values of `abcorr' apply to the
"reception" case in which photons depart from the
target'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
uses the position and orientation of the
target at the moment it emitted photons
arriving at the observer at `et'. The
position of the Sun relative to the
target is corrected for the one-way light
time from the Sun to the target.
The light time correction uses an
iterative solution of the light time
equation (see -Particulars for details).
The solution invoked by the 'LT' option
uses one iteration.
'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 (note the
target plays the role of "observer" in the
computation of the aberration-corrected
target-Sun vector). The result is the
sub-solar point computed using apparent
position and orientation of the target as
seen by the observer and the apparent
position of the Sun as seen by the target.
'CN' Converged Newtonian light time correction.
In solving the light time equation, the
'CN' correction iterates until the
solution converges (three iterations on
all supported platforms).
'CN+S' Converged Newtonian light time
and stellar aberration corrections.
obsrvr the name of the observing body.
[1,c4] = size(obsrvr); char = class(obsrvr)
or
[1,1] = size(obsrvr); cell = class(obsrvr)
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.
the call:
[spoint, dist, pltid] = cspice_subsol_pl02( handle, dladsc, ...
method, target, ...
et, abcorr, ...
obsrvr )
returns:
spoint the sub-solar point on the target body expressed
relative to the body-fixed reference frame of the target
body.
[3,1] = size(spoint); double = class(spoint)
The definition of sub-solar point depends on the
selected computation method. See the description of the
input argument `method' for details.
The target body-fixed frame, which is time-dependent, is
evaluated at `et' if `abcorr' is 'NONE'; otherwise the
frame is evaluated at et-lt, where `lt' is the one-way
light time from target to observer.
The position and orientation of the target body and the
position of the Sun are corrected for aberration as
specified by `abcorr'; the corrected positions and
orientation are used in the computation of `spoint'.
dist the distance between the observer and the sub-solar
point.
[1,1] = size(dist); double = class(dist)
The observer is presumed to be outside the
target body, so `dist' is always non-negative.
pltid the integer ID code of the plate on which the
sub-solar point is located.
[1,1] = size(pltid); int32 = class(pltid)
This ID code 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) Find the sub-solar point on Phobos as seen from Earth for a
specified sequence of times. Perform the computation twice,
using both the "intercept" and "ellipsoid near point"
options. Compute the corresponding sub-solar point values
using an ellipsoidal surface for comparison.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File: subsol_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.
function subsol_pl02_ex1( meta, dsknam )
%
% Constants
%
NCORR = 2;
NSAMP = 3;
NMETHOD = 2;
TIMLEN = 40;
FIXREF = 'IAU_PHOBOS';
TOL = 1.d-12;
%
% Initial values
%
abcorrs = { 'NONE', 'CN+S' };
emethods = { 'Intercept: ellipsoid', 'Near point: ellipsoid' };
plmethods = { 'Intercept', 'Ellipsoid near point' };
obsrvr = 'Earth';
target = 'Phobos';
%
% Load the metakernel.
%
cspice_furnsh( meta )
%
% Open the DSK file for read access.
% We use the DAS-level interface for
% this function.
%
handle = cspice_dasopr( dsknam );
%
% Begin a forward search through the
% kernel, treating the file as a DLA.
% In this example, it's a very short
% search.
%
[dladsc, found] = cspice_dlabfs( handle );
if ~found
%
% We arrive here only if the kernel
% contains no segments. This is
% unexpected, but we're prepared for it.
%
fprintf( 'No segments found in DSK file %s\n', dsknam )
return
end
%
% If we made it this far, `dladsc' is the
% DLA descriptor of the first segment.
%
% Now compute sub-solar points using both computation
% methods. We'll vary the aberration corrections
% and the epochs.
%
et0 = 0.0;
stepsize = 1.d6;
for i = 0:(NSAMP-1)
%
% Set the computation time for the ith sample.
%
et = et0 + i * stepsize;
timstr = cspice_timout( et, ...
'YYYY-MON-DD HR:MN:SC.### ::TDB(TDB)' );
fprintf( '\n\nObservation epoch: %s\n', timstr )
for coridx = 1:NCORR
abcorr = abcorrs( coridx );
fprintf( '\n abcorr = %s\n', char(abcorr) );
for midx = 1:NMETHOD
%
% Select the computation method.
%
method = plmethods( midx );
fprintf( '\n Method = %s\n ', char(method) )
%
% Compute the sub-solar point using a plate
% model representation of the target's surface.
%
[xpt, dist, plid] = ...
cspice_subsol_pl02( handle, dladsc, method, ...
target, et, abcorr, ...
obsrvr );
%
% Represent the surface point in latitudinal
% coordinates.
%
[ xr, xlon, xlat] = cspice_reclat( xpt );
fprintf( ...
'\n Sub-solar point on plate model surface:\n' )
fprintf( ' Planetocentric Longitude (deg): %f\n', ...
xlon * cspice_dpr() )
fprintf( ' Planetocentric Latitude (deg): %f\n', ...
xlat * cspice_dpr() )
fprintf( ' Radius (km): %f\n', ...
xr )
fprintf( ' Observer distance (km): %f\n', ...
dist )
fprintf( ' ID of surface point plate: %d\n', ...
plid )
%
% Compute the sub-solar point using an ellipsoidal
% representation of the target's surface.
%
method = emethods( midx );
[xpt, trgepc, srfvec] = cspice_subslr( method, target, ...
et, FIXREF, ...
abcorr, obsrvr );
%
% Represent the surface point in latitudinal
% coordinates.
%
[xr, xlon, xlat] = cspice_reclat( xpt );
fprintf( ' Sub-solar point on ellipsoidal surface:\n' )
fprintf( ' Planetocentric Longitude (deg): %f\n', ...
xlon * cspice_dpr() )
fprintf( ' Planetocentric Latitude (deg): %f\n', ...
xlat * cspice_dpr() )
fprintf( ' Radius (km): %f\n', ...
xr )
fprintf( ' Observer distance (km): %f\n', ...
cspice_vnorm(srfvec) )
end
end
end
%
% Close the DSK file. Unload all other kernels as well.
%
cspice_dascls( handle )
cspice_kclear
When this program was executed on a Mac/Intel/Octave6.x/64-bit
platform, with the following variables as inputs
meta = 'subsol_pl02_ex1.tm';
dsknam = 'phobos_3_3.bds';
the output was:
Observation epoch: 2000-JAN-01 12:00:00.000 (TDB)
abcorr = NONE
Method = Intercept
Sub-solar point on plate model surface:
Planetocentric Longitude (deg): 102.413905
Planetocentric Latitude (deg): -24.533127
Radius (km): 11.612325
Observer distance (km): 276700026.580116
ID of surface point plate: 164811
Sub-solar point on ellipsoidal surface:
Planetocentric Longitude (deg): 102.413905
Planetocentric Latitude (deg): -24.533127
Radius (km): 10.922580
Observer distance (km): 276700027.168434
Method = Ellipsoid near point
Sub-solar point on plate model surface:
Planetocentric Longitude (deg): 105.857346
Planetocentric Latitude (deg): -16.270558
Radius (km): 11.645162
Observer distance (km): 276700027.058857
ID of surface point plate: 192093
Sub-solar point on ellipsoidal surface:
Planetocentric Longitude (deg): 105.973365
Planetocentric Latitude (deg): -15.976232
Radius (km): 11.249340
Observer distance (km): 276700027.400706
abcorr = CN+S
Method = Intercept
Sub-solar point on plate model surface:
Planetocentric Longitude (deg): 114.623420
Planetocentric Latitude (deg): -24.533628
Radius (km): 11.411417
Observer distance (km): 276710249.789163
ID of surface point plate: 170492
Sub-solar point on ellipsoidal surface:
Planetocentric Longitude (deg): 114.623420
Planetocentric Latitude (deg): -24.533628
Radius (km): 11.046740
Observer distance (km): 276710250.099485
Method = Ellipsoid near point
Sub-solar point on plate model surface:
Planetocentric Longitude (deg): 120.870428
Planetocentric Latitude (deg): -15.247903
Radius (km): 11.350346
Observer distance (km): 276710250.680859
ID of surface point plate: 205719
Sub-solar point on ellipsoidal surface:
Planetocentric Longitude (deg): 120.795481
Planetocentric Latitude (deg): -15.366726
Radius (km): 11.494153
Observer distance (km): 276710250.555135
Observation epoch: 2000-JAN-13 01:46:40.000 (TDB)
abcorr = NONE
Method = Intercept
Sub-solar point on plate model surface:
Planetocentric Longitude (deg): 4.432684
Planetocentric Latitude (deg): -24.281966
Radius (km): 12.888491
Observer distance (km): 286106845.772886
ID of surface point plate: 122143
Sub-solar point on ellipsoidal surface:
Planetocentric Longitude (deg): 4.432684
Planetocentric Latitude (deg): -24.281966
Radius (km): 11.980161
Observer distance (km): 286106846.561862
Method = Ellipsoid near point
Sub-solar point on plate model surface:
Planetocentric Longitude (deg): 3.418663
Planetocentric Latitude (deg): -12.568166
Radius (km): 12.783152
Observer distance (km): 286106846.122051
ID of surface point plate: 149271
Sub-solar point on ellipsoidal surface:
Planetocentric Longitude (deg): 3.411488
Planetocentric Latitude (deg): -12.479982
Radius (km): 12.689005
Observer distance (km): 286106846.205591
abcorr = CN+S
Method = Intercept
[...]
Warning: incomplete output. Only 100 out of 189 lines have been
provided.
cspice_subsol_pl02 computes the sub-solar point on a target body.
cspice_subsol_pl02 also determines the distance of the observer from the
sub-solar point.
Sub-point Definitions
=====================
This routine offers two ways of defining the sub-solar point:
- The 'intercept' method. In general, this definition
calls for defining a ray emanating from the Sun and
passing through the center of the target body. The intercept
on the first plate (the one closest to the observer) hit by this
ray is the sub-point.
- The 'ellipsoid near point' method. When a target's surface is
modeled by a set of triangular plates, the notion of "dropping
a perpendicular segment to the surface," which makes sense for
convex surfaces, becomes problematic: there need not be any
plate whose normal vector is parallel to a segment from the Sun
to some point on that plate, or there could be more than one
such plate. If such a plate exists, it might be located
anywhere on the visible surface---not necessarily "below" the
Sun.
To work around these problems, the ellipsoid near point method
uses a reference ellipsoid to define a preliminary sub-solar
point: this is the unique point on the ellipsoid's surface at
which the outward surface normal points toward the Sun. Then
the plate model sub-solar point is defined as the plate
intercept closest to the Sun of a ray emanating from the Sun
and passing through the preliminary sub-solar point on the
ellipsoid.
For a large target such as Mars, or for any target whose reference
ellipsoid deviates significantly from spherical, the results
obtained using the two sub-point definitions can be quite different.
The example program provided below demonstrates this fact; Phobos is
the target body in this case. Some analysis on the user's part will
be needed to select the "best" definition for a given application.
When comparing sub-solar point computations with results from
sources other than SPICE, it's essential to make sure the same
geometric definitions are used.
Aberration Corrections
======================
Below, we indicate the aberration corrections to use for some
common applications:
1) Compute the sub-solar point using the apparent direction
and orientation of a target. This is the most common case for
a remote-sensing observation. When the observer's altitude
is more than one target radius above the surface:
Use 'LT+S': apply both light time and stellar
aberration corrections.
Note that when the observer is close to the target surface,
this choice may yield inaccurate results, since light time is
measured between the observer and the target center. When the
observer has altitude of less than one target radius above the
surface, aberration corrections should be omitted, so in this
case abcorr should be set to:
'NONE'
Note that this selection calls for using the geometric position
of the Sun.
2) Use a geometric position vector and uncorrected target
orientation as low-accuracy estimates for an application where
execution speed is critical.
Use 'NONE'.
See the header of the CSPICE routine spkezr_c for a detailed
discussion of aberration corrections.
If any of the listed errors occur, the output arguments are
left unchanged.
1) If the input argument `method' is not recognized, the error
SPICE(DUBIOUSMETHOD) is signaled by a routine in the call
tree of this routine.
2) If either of the input body names `target' or `obsrvr' cannot be
mapped to NAIF integer codes, 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 codes, the
error SPICE(BODIESNOTDISTINCT) is signaled by a routine in the call
tree of this routine.
4) If frame definition data enabling the evaluation of the state
of the target relative to the observer in target body-fixed
coordinates have not been loaded prior to calling cspice_subsol_pl02,
an error is signaled by a routine in the call tree of this
routine.
5) If the specified aberration correction is not recognized, an
error is signaled by a routine in the call tree of this
routine.
6) If insufficient ephemeris data have been loaded prior to
calling cspice_subsol_pl02, an error is signaled by a
routine in the call tree of this routine.
7) If a DSK providing a DSK type 2 plate model has not been
loaded prior to calling cspice_subsol_pl02, an error is signaled by a
routine in the call tree of this routine.
8) If the computation method is "near point" and radii of the
target body have not been loaded into the kernel pool, an
error is signaled by a routine in the call tree of this
routine.
9) If PCK data supplying a rotation model for the target body
have not been loaded prior to calling cspice_subsol_pl02, an error is
signaled by a routine in the call tree of this routine.
10) If the segment associated with the input DLA descriptor does not
contain data for the designated target, the error
SPICE(TARGETMISMATCH) is signaled by a routine in the call tree
of this routine.
11) 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.
12) If the sub-solar point cannot be computed because the ray from
the observer to the aim point designated by `method' fails to
intersect the target surface as defined by the plate model, the
error SPICE(NOINTERCEPT) is signaled by a routine in the call
tree of this routine.
13) Use of transmission-style aberration corrections is not
permitted. If abcorr specified such a correction, the
error SPICE(NOTSUPPORTED) is signaled by a routine in the call
tree of this routine.
14) The observer is presumed to be outside the target body; no
checks are made to verify this.
15) If any of the input arguments, `handle', `dladsc', `method', `target'
`et', `abcorr' or `obsrvr', is undefined, an error is signaled by the
Matlab error handling system.
16) If any of the input arguments, `handle', `dladsc', `method', `target'
`et', `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 Mice interface.
Appropriate DSK, SPK, PCK, and frame data must be available to
the calling program before this routine is called. Typically
the data are made available by loading kernels; however the
data may be supplied via subroutine interfaces if applicable.
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 Sun 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: triaxial radii for the target body must be loaded
into the kernel pool if the "Near Point" method is selected.
Typically these data are made available 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.
Either type of file may be loaded via cspice_furnsh.
- 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.
In all cases, kernel data are normally loaded once per program
run, NOT every time this routine is called.
1) This routine assumes that the origin of the body-fixed reference
frame associated with the target body is located in the interior
of that body.
MICE.REQ
DSK.REQ
PCK.REQ
SPK.REQ
TIME.REQ
None.
N.J. Bachman (JPL)
J. Diaz del Rio (ODC Space)
E.D. Wright (JPL)
-Mice Version 1.1.0, 26-OCT-2021 (EDW) (JDR)
Edited the header to comply with NAIF standard. Changed the output
argument name "plateID" to "pltid" for consistency with other
functions.
Added -Parameters, -Exceptions, -Files, -Restrictions,
-Literature_References and -Author_and_Institution sections.
Eliminated use of "lasterror" in rethrow.
Removed reference to the function's corresponding CSPICE header from
-Required_Reading section.
Index lines now state that this routine is deprecated.
-Mice Version 1.0.0, 25-JUL-2016 (NJB) (EDW)
DEPRECATED sub-solar point using plate_model
DEPRECATED sub-solar point using type_2 DSK
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