cspice_illum_pl02 |
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## AbstractDeprecated: This routine has been superseded by the CSPICE routines cspice_ilumin, cspice_illumg and cspice_illumf. This routine is supported for purposes of backward compatibility only. CSPICE_ILLUM_PL02 returns the illumination angles---phase, solar incidence, and emission---at a specified point on a target body at a particular epoch, optionally corrected for light time and stellar aberration. The target body's surface is represented by a triangular plate model contained in a type 2 DSK segment. For important details concerning this module's function, please refer to the CSPICE routine illum_pl02. ## I/OGiven: handle the DAS file handle of a DSK file open for read access. 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. target the name of the target body. `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 using 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 illumination angles are to be computed. 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. 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 that the illumination angles are 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. 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. spoint a surface point on the target body, expressed in rectangular body-fixed (body equator and prime meridian) coordinates. `spoint' need not be visible from the observer's location at time `et'. the call: ## ExamplesAny 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. Find the illumination angles at both the sub-observer point and sub-solar point on Phobos as seen from Mars for a specified sequence of times. Perform each computation twice, using both the "intercept" and "ellipsoid near point" options for the sub-observer point and sub-solar point computations. Compute the corresponding illumination angles using an ellipsoidal surface for comparison. (Note that the surface points on the plate model generally will not lie on the ellipsoid's surface, so the emission and solar incidence angles won't generally be zero at the sub-observer and sub-solar points, respectively.) In the following example program, the file phobos_3_3.bds is a DSK file containing a type 2 segment that provides a plate model representation of the surface of Phobos. The file mar097.bsp is a binary SPK file containing data for Phobos, Mars, and the Sun for a time interval starting at the date 2000 JAN 1 12:00:00 TDB. pck00010.tpc is a planetary constants kernel file containing radii and rotation model constants. naif0010.tls is a leapseconds kernel. All of the kernels other than the DSK file should be loaded via a meta-kernel. An example of the contents of such a kernel is: KPL/MK File name: illum.tm \begindata KERNELS_TO_LOAD = ( 'naif0010.tls' 'pck00010.tpc' 'mar097.bsp' ) \begintext PRO ILLUM_PL02_T, meta, dsk ;; ;; Constants ;; NCORR = 2 NSAMP = 3 NMETHOD = 2 TIMLEN = 40 FIXREF = 'IAU_PHOBOS' ILUM_METHOD = 'ELLIPSOID' TOL = 1.d-12 ;; ;; Initial values ;; abcorrs = [ 'NONE', 'CN+S' ] methods = [ 'Intercept', 'Ellipsoid near point' ] obsrvr = 'Mars' target = 'Phobos' ;; ;; 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, dsk, 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 '+ dsk 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. ;; et0 = 0.d stepsize = 1.d6 for i = 0, (NSAMP-1) do begin ;; ;; Set the computation time for the ith sample. ;; et = et0 + i * stepsize cspice_timout, et, $ 'YYYY-MON-DD HR:MN:SC.### ::TDB(TDB)', $ TIMLEN, timstr print print, 'Observation epoch: ', timstr for coridx = 0, NCORR-1 do begin abcorr = abcorrs( coridx ) print print, ' abcorr = ' + abcorr for midx = 0, NMETHOD-1 do begin ;; ;; Select the computation method. ;; method = methods[ midx ] print print, ' Method = ' + method ;; ;; Compute the sub-observer point using a plate ;; model representation of the target's surface. ;; cspice_subpt_pl02, handle, dladsc, method, $ target, et, abcorr, $ obsrvr, xpt, alt, plid ;; ;; Compute the illumination angles at the ;; sub-observer point. ;; ## ParticularsThe term "illumination angles" refers to following set of angles: solar incidence angle Angle between the surface normal at the specified surface point and the vector from the surface point to the Sun. emission angle Angle between the surface normal at the specified surface point and the vector from the surface point to the observer. phase angle Angle between the vectors from the surface point to the observing body and from the surface point to the Sun. The diagram below illustrates the geometric relationships defining these angles. The labels for the solar incidence, emission, and phase angles are "s.i.", "e.", and "phase". * Sun surface normal vector ._ _. |\ /| Sun vector \ phase / \ . . / . . \ ___ / . \/ \/ _\ s.i./ . / \ / . | e. \ / * <--------------- * surface point on viewing vector target body location to viewing (observer) location Note that if the target-observer vector, the target normal vector at the surface point, and the target-sun vector are coplanar, then phase is the sum of incidence and emission. This is rarely true; usually phase angle < solar incidence angle + emission angle All of the above angles can be computed using light time corrections, light time and stellar aberration corrections, or no aberration corrections. The way aberration corrections are used is described below. Care must be used in computing light time corrections. The guiding principle used here is "describe what appears in an image." We ignore differential light time; the light times from all points on the target to the observer are presumed to be equal. Observer-target body vector --------------------------- Let `et' be the epoch at which an observation or remote sensing measurement is made, and let et - lt ("lt" stands for "light time") be the epoch at which the photons received at `et' were emitted from the body (we use the term "emitted" loosely here). The correct observer-target vector points from the observer's location at `et' to the target body's location at et - lt. The target-observer vector points in the opposite direction. Since light time corrections are not symmetric, the correct target-observer vector CANNOT be found by computing the light time corrected position of the observer as seen from the target body. Target body's orientation ------------------------- Using the definitions of `et' and `lt' above, the target body's orientation at et - lt is used. The surface normal is dependent on the target body's orientation, so the body's orientation model must be evaluated for the correct epoch. Target body -- Sun vector ------------------------- All surface features on the target body will appear in a measurement made at `et' as they were at the target at epoch et-lt. In particular, lighting on the target body is dependent on the apparent location of the Sun as seen from the target body at et-lt. So, a second light time correction is used in finding the apparent location of the Sun. Stellar aberration corrections, when used, are applied as follows: Observer-target body vector --------------------------- In addition to light time correction, stellar aberration is used in computing the apparent target body position as seen from the observer's location at time `et'. This apparent position defines the observer-target body vector. Target body-Sun vector ---------------------- The target body-Sun vector is the apparent position of the Sun, corrected for light time and stellar aberration, as seen from the target body at time et-lt. Note that the target body's position is not affected by the stellar aberration correction applied in finding its apparent position as seen by the observer. Once all of the vectors, as well as the target body's orientation, have been computed with the proper aberration corrections, the element of time is eliminated from the computation. The problem becomes a purely geometric one, and is described by the diagram above. ## Required ReadingICY.REQ ABCORR.REQ DSK.REQ PCK.REQ SPK.REQ TIME.REQ ## Version-Icy Version 1.0.0, 14-DEC-2016, ML (JPL), EDW (JPL) ## Index_Entriesillumination angles using dsk triangular plate_model lighting angles using dsk triangular plate_model illumination angles using dsk type_2 plate_model lighting angles using dsk type_2 plate_model phase angle using dsk triangular plate_model emission angle using dsk triangular plate_model solar incidence angle using dsk triangular plate_model phase angle using dsk type_2 plate_model emission angle using dsk type_2 plate_model solar incidence angle using dsk type_2 plate_model |

Wed Apr 5 17:58:02 2017