illum_pl02 |

Table of contents## Procedureillum_pl02 (illumination angles using DSK type 2 plate model) void illum_pl02 ( SpiceInt handle, ConstSpiceDLADescr * dladsc, ConstSpiceChar * target, SpiceDouble et, ConstSpiceChar * abcorr, ConstSpiceChar * obsrvr, SpiceDouble spoint [3], SpiceDouble * phase, SpiceDouble * solar, SpiceDouble * emissn ) ## AbstractDeprecated: This routine has been superseded by the CSPICE routines ilumin_c, illumg_c and illumf_c. This routine is supported for purposes of backward compatibility only. Compute 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. ## Required_ReadingFRAMES PCK SPK TIME ## KeywordsGEOMETRY ## Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- handle I DSK handle. dladsc I DLA descriptor of target body segment. target I Name of target body. et I Epoch in ephemeris seconds past J2000 TDB. abcorr I Aberration correction. obsrvr I Name of observing body. spoint I Body-fixed coordinates of a target surface point. phase O Phase angle at the surface point. solar O Solar incidence angle at the surface point. emissn O Emission angle at the surface point. ## Detailed_Inputhandle is 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 is the DLA descriptor of a DSK segment representing the surface of the target body. target is 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 dasopr_c. et is 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). The "CN" correction typically does not substantially improve accuracy because the errors made by ignoring relativistic effects may be larger than the improvement afforded by obtaining convergence of the light time solution. The "CN" correction computation also requires a significantly greater number of CPU cycles than does the one-iteration light time correction. "CN+S" Converged Newtonian light time and stellar aberration corrections. obsrvr is 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 is 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'. ## Detailed_OutputFor all of the angles below, if `spoint' does not lie on one of the *exterior* plates comprising the DSK type 2 surface representation, the "intercept" style "sub-observer point" corresponding to `spoint' is used in the illumination angle computations in place of `spoint'. The selected point will always be on the *outermost* plate intersected by a ray emanating from the target body's center and passing through `spoint'. See the header of subpt_pl02 for details concerning the definition of the sub-observer point. In all cases, the normal vector is taken from the plate on which the sub-point corresponding to `spoint' lies. If this sub-point lies on an edge or vertex, a normal vector for one of the bordering plates is selected. phase is the phase angle at `spoint', as seen from `obsrvr' at time `et'. This is the angle between the spoint-obsrvr vector and the spoint-sun vector. Units are radians. The range of `phase' is [0, pi]. solar is the solar incidence angle at `spoint', as seen from `obsrvr' at time `et'. This is the angle between the surface normal vector at `spoint' and the spoint-sun vector. Units are radians. The range of `solar' is [0, pi]. emissn is the emission angle at `spoint', as seen from `obsrvr' at time `et'. This is the angle between the surface normal vector at `spoint' and the spoint-observer vector. Units are radians. The range of `emissn' is is [0, pi]. See Particulars below for a detailed discussion of the definitions of these angles. ## ParametersNone. ## ExceptionsIf any of the listed errors occur, the output arguments are left unchanged. 1) If `spoint' is the zero vector, the error SPICE(ZEROVECTOR) is signaled. 2) If either of the input body names `target' or `obsrvr' cannot be mapped to NAIF integer codes, the error SPICE(IDCODENOTFOUND) is signaled. 3) If `obsrvr' and `target' map to the same NAIF integer ID codes, the error SPICE(BODIESNOTDISTINCT) is signaled. 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 ## FilesAppropriate 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 furnsh_c. - 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 furnsh_c. - 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 furnsh_c. - 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 furnsh_c. In all cases, kernel data are normally loaded once per program run, NOT every time this routine is called. ## 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. ## ExamplesThe numerical results shown for this example may differ across platforms. The results depend on the SPICE kernels used as input, the compiler and supporting libraries, and the machine specific arithmetic implementation. 1) 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.) Use the meta-kernel shown below to load the required SPICE kernels. KPL/MK File: illum_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. /. Program illum_pl02_ex1 ./ #include <stdio.h> #include <math.h> #include "SpiceUsr.h" int main() { /. Local parameters ./ #define FILSIZ 256 #define NCORR 2 #define NSAMP 3 #define NMETHOD 2 #define TOL ( 1.e-12 ) #define CORLEN 15 #define METHLEN 81 #define TIMLEN 41 /. Local variables ./ SpiceBoolean found; SpiceChar * abcorr; SpiceChar * abcorrs[ NCORR ] = { "NONE", "LT+S" }; SpiceChar dsk [ FILSIZ ]; SpiceChar * emethod; SpiceChar * emethods[ NMETHOD ] = { "Intercept", "Near point" }; SpiceChar meta [ FILSIZ ]; SpiceChar * method; SpiceChar * methods [ NMETHOD ] = { "Intercept", "Ellipsoid near point" }; SpiceChar * obsrvr = "Mars"; SpiceChar * target = "Phobos"; SpiceChar timstr [ TIMLEN ]; SpiceDLADescr dladsc; SpiceDouble alt; SpiceDouble dist; SpiceDouble emissn; SpiceDouble et0; SpiceDouble et; SpiceDouble phase; SpiceDouble solar; SpiceDouble stepsize ; SpiceDouble xlat; SpiceDouble xlon; SpiceDouble xpt [3]; SpiceDouble xr; SpiceInt coridx; SpiceInt handle; SpiceInt i; SpiceInt midx; SpiceInt plid; /. Prompt for the name of a meta-kernel specifying all of the other kernels we need. Load the metakernel. ./ prompt_c ( "Enter meta-kernel name > ", FILSIZ, meta ); furnsh_c ( meta ); /. Prompt for the name of the DSK to read. ./ prompt_c ( "Enter DSK name > ", FILSIZ, dsk ); /. Open the DSK file for read access. We use the DAS-level interface for this function. ./ dasopr_c ( dsk, &handle ); /. Begin a forward search through the kernel, treating the file as a DLA. In this example, it's a very short search. ./ dlabfs_c ( handle, &dladsc, &found ); if ( !found ) { /. We arrive here only if the kernel contains no segments. This is unexpected, but we're prepared for it. ./ setmsg_c ( "No segments found in DSK file #."); errch_c ( "#", dsk ); sigerr_c ( "SPICE(NODATA)" ); } /. 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 = 0.0; stepsize = 1.e6; for ( i = 0; i < NSAMP; i++ ) { /. Set the computation time for the ith sample. ./ et = et0 + i*stepsize; timout_c ( et, "YYYY-MON-DD " "HR:MN:SC.### ::TDB(TDB)", TIMLEN, timstr ); printf ( "\n\nObservation epoch: %s\n", timstr ); for ( coridx = 0; coridx < NCORR; coridx++ ) { /. Select the aberration correction. ./ abcorr = abcorrs[coridx]; printf ( "\n" " abcorr = %s\n", abcorr ); for ( midx = 0; midx < NMETHOD; midx++ ) { /. Select the computation method. ./ method = methods [midx]; emethod = emethods[midx]; printf ( "\n" " Method = %s\n", method ); /. Compute the sub-observer point using a plate model representation of the target's surface. ./ subpt_pl02 ( handle, &dladsc, method, target, et, abcorr, obsrvr, xpt, &alt, &plid ); /. Compute the illumination angles at the sub-observer point. ./ ## Restrictions1) 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. 2) This routine does not compute illumination angles for surface points on interior plates, for example plates representing the interior of a cave or tunnel. See the -Detailed_Output section. 3) Illumination angles on an irregular target body surface may differ greatly from those on a reference ellipsoid for the same surface, as illustrated by the example program shown above. Users may want to consider using the ellipsoid formulation of this algorithm, which is implemented in the CSPICE routine illum_c. ## Literature_ReferencesNone. ## Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) ## Version-CSPICE Version 2.1.0, 26-OCT-2021 (JDR) (NJB) Bug fix: call to ljust_ is now followed by call to F2C_ConvertStr. Edited the Examples section to comply with NAIF standard. Index lines now state that this routine is deprecated. -CSPICE Version 2.0.0, 22-FEB-2017 (NJB) Bug fix: the DSK segment's surface ID code is no longer required to match that of the target. The segment's center ID must match. Include file references have been updated. Now calls zzdsksgr_ instead of dsksgr_. Now calls pltnrm_ instead of pltrec_. Added failed_c calls. -Beta Version 1.3.0, 30-APR-2014 (NJB) (BVS) Now includes dsk_proto.h. Added failed_c check after dskgd_ call. Last update was 07-APR-2014 (BVS) Changed FRAME to FRAMES in the Required_Reading section. -Beta Version 1.2.0, 14-MAY-2010 (NJB) Updated for compatibility with new DSK design. Updated kernels used in example program. -Beta Version 1.1.0, 09-FEB-2007 (NJB) Bug fix: type of local variable fDLADescr was changed to SpiceInt. -Beta Version 1.0.0, 06-NOV-2006 (NJB) ## Index_EntriesDEPRECATED illumination angles using DSK type 2 plate model DEPRECATED lighting angles using DSK type 2 plate model DEPRECATED phase angle using DSK type 2 plate model DEPRECATED emission angle using DSK type 2 plate model DEPRECATED solar incidence angle using DSK type 2 |

Fri Dec 31 18:41:08 2021