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java.lang.Objectspice.basic.IlluminationAngles
public class IlluminationAngles
Class IlluminationAngles supports illumination angle computations.
Illumination angles =================== The term "illumination angles" refers to following set of angles: phase angle Angle between the vectors from the surface point to the observer and from the surface point to the illumination source. incidence angle Angle between the surface normal at the specified surface point and the vector from the surface point to the illumination source. emission angle Angle between the surface normal at the specified surface point and the vector from the surface point to the observer. The diagram below illustrates the geometric relationships defining these angles. The labels for the incidence, emission, and phase angles are "inc.", "e.", and "phase". illumination source surface normal vector ._ _. |\ /| illumination \ phase / source vector \ . . / . . \ ___ / . \/ \/ _\ inc./ . / \ / . | 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-illumination source vector are coplanar, then phase is the sum of the incidence and emission angles. This rarely occurs; usually phase angle < 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. In order to describe apparent geometry as observed by a remote sensing instrument, both light time and stellar aberration corrections should be used. The way aberration corrections are applied by the constructors of this class is described below. Light time corrections ====================== Observer-target surface point 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 surface point `spoint'. Note that the light time between the surface point and observer will generally differ from the light time between the target body's center and the observer. 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 -- illumination source vector ----------------------------------------- The surface features on the target body near `spoint' will appear in a measurement made at `et' as they were at et-lt. In particular, lighting on the target body is dependent on the apparent location of the illumination source as seen from the target body at et-lt. So, a second light time correction is used to compute the position of the illumination source relative to the surface point. Stellar aberration corrections ============================== Stellar aberration corrections are applied only if light time corrections are applied as well. Observer-target surface point body vector ----------------------------------------- When stellar aberration correction is performed, the direction vector `srfvec' is adjusted so as to point to the apparent position of `spoint': considering `spoint' to be an ephemeris object, `srfvec' points from the observer's position at `et' to the light time and stellar aberration corrected position of `spoint'. Target body-illumination source vector -------------------------------------- The target body-illumination source vector is the apparent position of the illumination source, corrected for light time and stellar aberration, as seen from the target body at time et-lt. Using DSK data ============== DSK loading and unloading ------------------------- DSK files providing data used by this class are loaded by callingKernelDatabase.load(java.lang.String)
and can be unloaded by callingKernelDatabase.unload(java.lang.String)
orKernelDatabase.clear()
See the documentation ofKernelDatabase.load(java.lang.String)
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 a set 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 class, 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 "DSK//UNPRIORITIZED" "DSK/UNPRIORITIZED/ " "UNPRIORITIZED/ /DSK" The simplest form of the `method' argument specifying use of DSK data is one that lacks a surface list, for example: "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 string suffices. 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 the constructors of this class 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\"" Escaped 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 "DSK/UNPRIORITIZED/SURFACES = \"Mars MEGDR 64 PIXEL/DEG\", 3" Aberration corrections using DSK data ------------------------------------- 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.
The 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 phase, solar incidence, and emission angles at the sub-solar and sub-spacecraft points on Mars as seen from the Mars Global Surveyor spacecraft at a specified UTC time. Use both an ellipsoidal Mars shape model and topographic data provided by a DSK file. For both surface points, use the "near point" and "nadir" definitions for ellipsoidal and DSK shape models, respectively. Use converged Newtonian light time and stellar aberration corrections. 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: IlluminationAnglesEx1.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 mgs_ext12_ipng_mgs95j.bsp MGS ephemeris 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', 'mgs_ext12_ipng_mgs95j.bsp', 'megr90n000cb_plate.bds' ) \begintext Example code begins here. // // IlluminationAngles example 1 // // // Import JNISpice API classes. // import spice.basic.*; // // Import the conversion factor DPR. // import static spice.basic.AngularUnits.*; public class IlluminationAnglesEx1 { static{ System.loadLibrary( "JNISpice" ); } public static void main( String[] args ) { // // Local constants // final String META = "IlluminationAngles.tm"; final int NMETH = 2; // // Local variables // // // `method' strings for illumination angle computations. // String[] ilumth = { "Ellipsoid", "DSK/Unprioritized" }; // // `method' strings for sub-spacecraft point and // sub-solar point computations. // String[] submth = { "Near Point/Ellipsoid", "DSK/Nadir/Unprioritized" }; int i; try { // // Load kernels. // KernelDatabase.load( META ); // // Convert the UTC request time string to seconds past // J2000 TDB. // String utc = "2003 OCT 13 06:00:00 UTC"; TDBTime et = new TDBTime( utc ); System.out.format ( "%n" + "UTC epoch is %s%n", utc ); // // Assign observer, target, and illumination source // names. The acronym MGS indicates Mars Global // Surveyor. See NAIF_IDS for a list of names // recognized by SPICE. // // Also set the target body-fixed frame and // the aberration correction flag. // Body target = new Body( "Mars" ); Body obsrvr = new Body( "MGS" ); Body ilusrc = new Body( "Sun" ); ReferenceFrame fixref = new ReferenceFrame( "IAU_MARS"); AberrationCorrection abcorr = new AberrationCorrection( "CN+S" ); for ( i = 0; i < NMETH; i++ ) { // // Find the sub-solar point on Mars as // seen from the MGS spacecraft at `et'. Use the // "near point" style of sub-point definition // when the shape model is an ellipsoid, and use // the "nadir" style when the shape model is // provided by DSK data. This makes it easy to // verify the solar incidence angle when // the target is modeled as an ellipsoid. // SubSolarRecord ssolrec = new SubSolarRecord( submth[i], target, et, fixref, abcorr, obsrvr ); Vector3 ssolpt = ssolrec.getSubPoint(); // // Now find the sub-spacecraft point. // SubObserverRecord sscrec = new SubObserverRecord( submth[i], target, et, fixref, abcorr, obsrvr ); Vector3 sscpt = sscrec.getSubPoint(); // // Find the phase, solar incidence, and emission // angles at the sub-solar point on Mars as // seen from MGS at time `et'. // IlluminationAngles solAngles = new IlluminationAngles( ilumth[i], target, ilusrc, et, fixref, abcorr, obsrvr, ssolpt ); // // Do the same for the sub-spacecraft point. // IlluminationAngles sscAngles = new IlluminationAngles( ilumth[i], target, ilusrc, et, fixref, abcorr, obsrvr, sscpt ); // // Obtain the illumination angles and flags. // Convert the angles to degrees and write // the outputs. // double sslphs = solAngles.getPhaseAngle() * DPR; double sslemi = solAngles.getEmissionAngle() * DPR; double sslinc = solAngles.getIncidenceAngle() * DPR; boolean sslvis = solAngles.isVisible(); boolean ssllit = solAngles.isLit(); double sscphs = sscAngles.getPhaseAngle() * DPR; double sscemi = sscAngles.getEmissionAngle() * DPR; double sscinc = sscAngles.getIncidenceAngle() * DPR; boolean sscvis = sscAngles.isVisible(); boolean ssclit = sscAngles.isLit(); System.out.format( "%n" + " IlluminationAngle method: %s%n" + " SubObserverRecord method: %s%n" + " SubSolarRecord method: %s%n" + "%n" + " Illumination angles at the " + "sub-solar point:%n" + "%n" + " Phase angle (deg): %15.9f%n" + " Solar incidence angle (deg): %15.9f%n" + " Emission angle (deg): %15.9f%n" + " Visible: %b%n" + " Lit: %b%n", ilumth[i], submth[i], submth[i], sslphs, sslinc, sslemi, sslvis, ssllit ); if ( i == 0 ) { System.out.format( "%n" + " The solar incidence angle " + "should be 0.%n" + " The emission " + "and phase angles should be equal.%n" ); } System.out.format( "%n" + " Illumination angles at the " + "sub-s/c point:%n" + "%n" + " Phase angle (deg): %15.9f%n" + " Solar incidence angle (deg): %15.9f%n" + " Emission angle (deg): %15.9f%n" + " Visible: %b%n" + " Lit: %b%n", sscphs, sscinc, sscemi, sscvis, ssclit ); if ( i == 0 ) { System.out.format( "%n" + " The emission angle " + "should be 0.%n" + " The solar incidence " + "and phase angles should be equal.%n" ); } } System.out.println( " " ); } catch ( SpiceException exc ) { exc.printStackTrace(); } } } When this program was executed on a PC/Linux/gcc/64-bit/Java 1.5 platform, the output was: UTC epoch is 2003 OCT 13 06:00:00 UTC IlluminationAngle method: Ellipsoid SubObserverRecord method: Near Point/Ellipsoid SubSolarRecord method: Near Point/Ellipsoid Illumination angles at the sub-solar point: Phase angle (deg): 138.370270685 Solar incidence angle (deg): 0.000000000 Emission angle (deg): 138.370270685 Visible: false Lit: true The solar incidence angle should be 0. The emission and phase angles should be equal. Illumination angles at the sub-s/c point: Phase angle (deg): 101.439331040 Solar incidence angle (deg): 101.439331041 Emission angle (deg): 0.000000002 Visible: true Lit: false The emission angle should be 0. The solar incidence and phase angles should be equal. IlluminationAngle method: DSK/Unprioritized SubObserverRecord method: DSK/Nadir/Unprioritized SubSolarRecord method: DSK/Nadir/Unprioritized Illumination angles at the sub-solar point: Phase angle (deg): 138.387071677 Solar incidence angle (deg): 0.967122745 Emission angle (deg): 137.621480599 Visible: false Lit: true Illumination angles at the sub-s/c point: Phase angle (deg): 101.439331359 Solar incidence angle (deg): 101.555993667 Emission angle (deg): 0.117861156 Visible: true Lit: false
Upgraded to support DSK usage.
Now supports "isLit" and "isVisible" methods. These indicate whether the illumination source and observer, respectively, are visible from the surface point.
Field Summary | |
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static java.lang.String |
ELLIPSOID
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Constructor Summary | |
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IlluminationAngles(java.lang.String method,
Body target,
Body ilusrc,
Time t,
ReferenceFrame fixref,
AberrationCorrection abcorr,
Body observer,
Vector3 spoint)
Find the illumination angles (phase, solar incidence, and emission) at a specified surface point on a target body, using a specified illumination source; create a record containing the result. |
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IlluminationAngles(java.lang.String method,
Body target,
Time t,
ReferenceFrame fixref,
AberrationCorrection abcorr,
Body observer,
Vector3 spoint)
Find the illumination angles (phase, solar incidence, and emission) at a specified surface point on a target body, using the sun as the illumination source; create a record containing the result. |
Method Summary | |
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double |
getEmissionAngle()
Return the emission angle. |
double |
getIncidenceAngle()
Return the illumination source incidence angle. |
double |
getPhaseAngle()
Return the phase angle. |
double |
getSolarIncidenceAngle()
Return the solar incidence angle. |
Vector3 |
getSurfaceVector()
Return the observer to surface point vector. |
TDBTime |
getTargetEpoch()
Return the target epoch. |
boolean |
isLit()
Return the illumination flag. |
boolean |
isVisible()
Return the visibility flag. |
Methods inherited from class java.lang.Object |
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clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
Field Detail |
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public static final java.lang.String ELLIPSOID
Constructor Detail |
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public IlluminationAngles(java.lang.String method, Body target, Body ilusrc, Time t, ReferenceFrame fixref, AberrationCorrection abcorr, Body observer, Vector3 spoint) throws SpiceException
Values and meanings of the `method' argument are described below.
method is a short string providing parameters defining the computation method to be used. In the syntax descriptions below, items delimited by brackets are optional. `method' may be assigned the following values: "ELLIPSOID" The illumination angle computation uses a triaxial ellipsoid to model the surface of the target body. The ellipsoid's radii must be available in the kernel pool. "DSK/UNPRIORITIZED[/SURFACES =]" The illumination angle computation uses topographic data to model the surface of the target body. These data must be provided by loaded DSK files. The surface list specification is optional. The syntax of the list is [, ...] 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 escaped 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. Neither case nor white space are significant in `method', except within double-quoted strings representing surface names. For example, the string " eLLipsoid " is valid. Within double-quoted strings representing surface names, 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\"
SpiceException
public IlluminationAngles(java.lang.String method, Body target, Time t, ReferenceFrame fixref, AberrationCorrection abcorr, Body observer, Vector3 spoint) throws SpiceException
See the discussion of the argument `method' in the detailed
documentation of IlluminationAngles(String,Body,
Body,Time,ReferenceFrame,AberrationCorrection,Body,Vector3)
.
SpiceException
Method Detail |
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public double getPhaseAngle()
Let `phase' refer to the phase angle at `spoint', as seen from `obsrvr' at time `et'. This is the angle between the spoint-obsrvr vector and the spoint-illumination source vector. Units are radians. The range of `phase' is [0, pi]. See Particulars above for a detailed discussion of the definition.
public double getIncidenceAngle()
Let `incdnc' refer to the illumination source 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-illumination source vector. Units are radians. The range of `incdnc' is [0, pi]. See Particulars above for a detailed discussion of the definition.
public double getSolarIncidenceAngle()
This method is deprecated and is retained for
backward compatibility. Use
getIncidenceAngle()
in place of this method.
public double getEmissionAngle()
Let `emissn' refer to 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 [0, pi]. See Particulars above for a detailed discussion of the definition.
public TDBTime getTargetEpoch()
Let `trgepc' refer to "target surface point epoch." `trgepc' is defined as follows: letting `lt' be the one-way light time between the observer and the input surface point `spoint', `trgepc' is either the epoch et-lt or `et' depending on whether the requested aberration correction is, respectively, for received radiation or omitted. `lt' is computed using the method indicated by `abcorr'.
public Vector3 getSurfaceVector()
Let `srfvec' refer to this vector; it is the vector from the observer's position at `et' to the aberration-corrected (or optionally, geometric) position of `spoint', where the aberration corrections are specified by `abcorr'. `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 Vector3.norm()
to obtain the
distance between the observer and `spoint':
dist = srfvec.norm();
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 via the call:
obspos = spoint.sub(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 method
ReferenceFrame.getPositionTransformation(ReferenceFrame,Time,Time)
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:
xform = fixref.getPositionTransformation( ref, trgepc, et ); refvec = xform.mxv( srfvec );
public boolean isVisible()
Let `visible' refer to this flag, which indicates whether the surface point is visible to the observer. `visible' takes into account whether the target surface occults `spoint', regardless of the emission angle at `spoint'. `visible' is returned with the value true if `spoint' is visible; otherwise it is false.
public boolean isLit()
Let `lit' refer to this logical flag, which indicates whether the surface point is illuminated; the point is considered to be illuminated if the vector from the point to the center of the illumination source doesn't intersect the target surface. `lit' takes into account whether the target surface casts a shadow on `spoint', regardless of the incidence angle at `spoint'. `lit' is returned with the value true if `spoint' is illuminated; otherwise it is false.
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