gfilum_c |

Table of contents## Proceduregfilum_c ( GF, illumination angle search ) void gfilum_c ( ConstSpiceChar * method, ConstSpiceChar * angtyp, ConstSpiceChar * target, ConstSpiceChar * illmn, ConstSpiceChar * fixref, ConstSpiceChar * abcorr, ConstSpiceChar * obsrvr, ConstSpiceDouble spoint [3], ConstSpiceChar * relate, SpiceDouble refval, SpiceDouble adjust, SpiceDouble step, SpiceInt nintvls, SpiceCell * cnfine, SpiceCell * result ) ## AbstractReturn the time window over which a specified constraint on the observed phase, solar incidence, or emission angle at a specified target body surface point is met. ## Required_ReadingGF FRAMES NAIF_IDS PCK SPK TIME ## KeywordsANGLE EPHEMERIS ILLUMINATION LIGHTING SEARCH ## Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- SPICE_GF_CNVTOL P Convergence tolerance. SPICE_GF_NWILUM P Number of workspace windows for angle search. method I Computation method. angtyp I Type of illumination angle. target I Name of the target body. illmn I Name of the illumination source. fixref I Body-fixed, body-centered target body frame. abcorr I Aberration correction flag. obsrvr I Name of the observing body. spoint I Body-fixed coordinates of a target surface point. relate I Relational operator. refval I Reference value. adjust I Adjustment value for absolute extrema searches. step I Step size used for locating extrema and roots. nintvls I Workspace window interval count. cnfine I-O SPICE window to which the search is confined. result O SPICE window containing results. ## Detailed_Inputmethod is a short string providing parameters defining the computation method to be used. Parameters include, but are not limited to, the shape model used to represent the surface of the target body. The only choice currently supported is "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. Neither case nor whitespaces are significant in `method'. For example, the string " eLLipsoid " is valid. angtyp is a string specifying the type of illumination angle for which a search is to be performed. The possible values of `angtyp' are "PHASE" "INCIDENCE" "EMISSION" When the illumination source is the sun, the incidence angle is commonly called the "solar incidence angle." See the -Particulars section below for a detailed description of these angles. Neither case nor whitespaces are significant in `angtyp'. For example, the string " Incidence " is valid. target is the name of a target body. The point at which the illumination angles are defined is located on the surface of this body. Optionally, you may supply the integer ID code for the object as an integer string. For example both "MOON" and "301" are legitimate strings that indicate the moon is the target body. Neither case nor leading and trailing blanks are significant in `target'. For example, the string " Incidence " is valid. Sequences of embedded blanks are treated as a single blank. illmn is the name of the illumination source. This source may be any ephemeris object. Case, blanks, and numeric values are treated in the same way as for the input `target'. fixref is the name of the body-fixed, body-centered reference frame associated with the target body. The input surface point `spoint' is expressed relative to this reference frame, and this frame is used to define the orientation of the target body as a function of time. The string `fixref' is case-insensitive, and leading and trailing blanks in `fixref' are not significant. abcorr indicates the aberration corrections to be applied to the observer-surface point vector, the surface point- illumination source vector, and the target body orientation to account for one-way light time and stellar aberration. Any "reception" correction accepted by spkezr_c can be used here. See the header of spkezr_c for a detailed description of the aberration correction options. For convenience, the options are listed below: "NONE" Apply no correction. "LT" "Reception" case: correct for one-way light time using a Newtonian formulation. "LT+S" "Reception" case: correct for one-way light time and stellar aberration using a Newtonian formulation. "CN" "Reception" case: converged Newtonian light time correction. "CN+S" "Reception" case: converged Newtonian light time and stellar aberration corrections. Case and blanks are not significant in the string `abcorr'. obsrvr is the name of an observing body. Case, blanks, and numeric values are treated in the same way as for the input `target'. spoint is a surface point on the target body, expressed in Cartesian coordinates, relative to the body-fixed target frame designated by `fixref'. `spoint' need not be visible from the observer's location in order for the constraint specified by `relate' and `refval' (see descriptions below) to be satisfied. The components of `spoint' have units of km. relate is a relational operator used to define a constraint on a specified illumination angle. The result window found by this routine indicates the time intervals where the constraint is satisfied. Supported values of `relate' and corresponding meanings are shown below: ">" The angle is greater than the reference value `refval'. "=" The angle is equal to the reference value `refval'. "<" The angle is less than the reference value `refval'. "ABSMAX" The angle is at an absolute maximum. "ABSMIN" The angle is at an absolute minimum. "LOCMAX" The angle is at a local maximum. "LOCMIN" The angle is at a local minimum. The caller may indicate that the window of interest is the set of time intervals where the angle is within a specified separation from an absolute extremum. The argument `adjust' (described below) is used to specify this separation. Local extrema are considered to exist only in the interiors of the intervals comprising the confinement window: a local extremum cannot exist at a boundary point of the confinement window. Case is not significant in the string `relate'. refval is the reference value used together with the argument `relate' to define an equality or inequality to be satisfied by the specified illumination angle. See the discussion of `relate' above for further information. The units of `refval' are radians. adjust is a parameter used to modify searches for absolute extrema: when `relate' is set to "ABSMAX" or "ABSMIN" and `adjust' is set to a positive value, ## Detailed_Outputcnfine is the input confinement window, updated if necessary so the control area of its data array indicates the window's size and cardinality. The window data are unchanged. result is the SPICE window of intervals, contained within the confinement window `cnfine', on which the specified constraint is satisfied. `result' must be declared and initialized with sufficient size to capture the full set of time intervals within the search region on which the specified condition is satisfied. If `result' is non-empty on input, its contents will be discarded before ## ParametersSPICE_GF_CNVTOL is the default convergence tolerance used for finding endpoints of the intervals comprising the result window. SPICE_GF_CNVTOL is also used for finding intermediate results; in particular, SPICE_GF_CNVTOL is used for finding the windows on which the specified illumination angle is increasing or decreasing. SPICE_GF_CNVTOL is used to determine when binary searches for roots should terminate: when a root is bracketed within an interval of length SPICE_GF_CNVTOL, the root is considered to have been found. The accuracy, as opposed to precision, of roots found by this routine depends on the accuracy of the input data. In most cases, the accuracy of solutions will be inferior to their precision. The calling program can reset the convergence tolerance; see the -Particulars section below for further information. SPICE_GF_NWILUM is the number of workspace windows required by this routine. See header file SpiceGF.h for declarations and descriptions of parameters used throughout the GF subsystem. ## Exceptions1) In order for this routine to produce correct results, the step size must be appropriate for the problem at hand. Step sizes that are too large may cause this routine to miss roots; step sizes that are too small may cause this routine to run unacceptably slowly and in some cases, find spurious roots. This routine does not diagnose invalid step sizes, except that if the step size is non-positive, the error SPICE(INVALIDSTEP) is signaled by a routine in the call tree of this routine. 2) Due to numerical errors, in particular, - Truncation error in time values - Finite tolerance value - Errors in computed geometric quantities it is *normal* for the condition of interest to not always be satisfied near the endpoints of the intervals comprising the result window. The result window may need to be contracted slightly by the caller to achieve desired results. The SPICE window routine wncond_c can be used to contract the result window. 3) If the number of intervals `nintvls' is less than 1, the error SPICE(VALUEOUTOFRANGE) is signaled. 4) If an error (typically cell overflow) occurs while performing window arithmetic, the error is signaled by a routine in the call tree of this routine. 5) If the output SPICE window `result' has size less than 2, the error SPICE(INVALIDDIMENSION) is signaled by a routine in the call tree of this routine. 6) If the output SPICE window `result' has insufficient capacity to hold the set of intervals on which the specified illumination angle condition is met, an error is signaled by a routine in the call tree of this routine. 7) If the input target body-fixed frame `fixref' is not recognized, an error 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. 8) If the input frame `fixref' is not centered at the target body, an error is signaled by a routine in the call tree of this routine. 9) If the input argument `method' is not recognized, an error is signaled by a routine in the call tree of this routine. 10) If the illumination angle type `angtyp' is not recognized, an error is signaled by a routine in the call tree of this routine. 11) If the relational operator `relate' is not recognized, an error is signaled by a routine in the call tree of this routine. 12) If the aberration correction specifier contains an unrecognized value, an error is signaled by a routine in the call tree of this routine. 13) If `adjust' is negative, an error is signaled by a routine in the call tree of this routine. 14) If any of the input body names do not map to NAIF ID codes, an error is signaled by a routine in the call tree of this routine. 15) If the target coincides with the observer or the illumination source, an error is signaled by a routine in the call tree of this routine. 16) If required ephemerides or other kernel data are not available, an error is signaled by a routine in the call tree of this routine. 17) If any of the `method', `angtyp', `target', `illmn', `fixref', `abcorr', `obsrvr' or `relate' input string pointers is null, the error SPICE(NULLPOINTER) is signaled. 18) If any of the `method', `angtyp', `target', `illmn', `fixref', `abcorr', `obsrvr' or `relate' input strings has zero length, the error SPICE(EMPTYSTRING) is signaled. 19) If any the `cnfine' or `result' cell arguments has a type other than SpiceDouble, the error SPICE(TYPEMISMATCH) is signaled. 20) If memory cannot be allocated to create the temporary variable required for the execution of the underlying Fortran routine, the error SPICE(MALLOCFAILED) is signaled. ## FilesAppropriate kernels must be loaded by the calling program before this routine is called. The following data are required: - SPK data: ephemeris data for target, observer, and the illumination source must be loaded. If aberration corrections are used, the states of target, observer, and the illumination source 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: if the target body shape is modeled as an ellipsoid (currently no other shapes are supported), triaxial radii for the target body must be loaded into the kernel pool. Typically this is done 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. - Frame data: if a frame definition not built into SPICE 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 some cases the observer's state may be computed at times outside of `cnfine' by as much as 2 seconds; data required to compute this state must be provided by loaded kernels. See -Particulars for details. In all cases, kernel data are normally loaded once per program run, NOT every time this routine is called. ## ParticularsThis routine determines a set of one or more time intervals within the confinement window when the specified illumination angle satisfies a caller-specified constraint. The resulting set of intervals is returned as a SPICE window. The term "illumination angles" refers to the 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. When the sun is the illumination source, this angle is commonly called the "solar incidence angle." 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 this routine 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 observer-to-surface point direction vector, which we'll call 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 surface point `spoint' at time et-lt. Below we discuss in greater detail aspects of this routine's solution process that are relevant to correct and efficient use of this routine in user applications. The Search Process ================== Regardless of the type of constraint selected by the caller, this routine starts the search for solutions by determining the time periods, within the confinement window, over which the specified illumination angle is monotone increasing and monotone decreasing. Each of these time periods is represented by a SPICE window. Having found these windows, all of the illumination angle's local extrema within the confinement window are known. Absolute extrema then can be found very easily. Within any interval of these "monotone" windows, there will be at most one solution of any equality constraint. Since the boundary of the solution set for any inequality constraint is contained in the union of - the set of points where an equality constraint is met - the boundary points of the confinement window the solutions of both equality and inequality constraints can be found easily once the monotone windows have been found. Step Size ========= The monotone windows (described above) are found via a two-step search process. Each interval of the confinement window is searched as follows: first, the input step size is used to determine the time separation at which the sign of the rate of change of the illumination angle will be sampled. Starting at the left endpoint of an interval, samples will be taken at each step. If a change of sign is found, a root has been bracketed; at that point, the time at which the rate of change of the selected illumination angle is zero can be found by a refinement process, for example, via binary search. Note that the optimal choice of step size depends on the lengths of the intervals over which the illumination angle is monotone: the step size should be shorter than the shortest of these intervals (within the confinement window). The optimal step size is *not* necessarily related to the lengths of the intervals comprising the result window. For example, if the shortest monotone interval has length 10 days, and if the shortest result window interval has length 5 minutes, a step size of 9.9 days is still adequate to find all of the intervals in the result window. In situations like this, the technique of using monotone windows yields a dramatic efficiency improvement over a state-based search that simply tests at each step whether the specified constraint is satisfied. The latter type of search can miss solution intervals if the step size is longer than the shortest solution interval. Having some knowledge of the relative geometry of the target, observer, and illumination source can be a valuable aid in picking a reasonable step size. In general, the user can compensate for lack of such knowledge by picking a very short step size; the cost is increased computation time. Note that the step size is not related to the precision with which the endpoints of the intervals of the result window are computed. That precision level is controlled by the convergence tolerance. Convergence Tolerance ===================== As described above, the root-finding process used by this routine involves first bracketing roots and then using a search process to locate them. "Roots" are both times when local extrema are attained and times when the illumination angle is equal to a reference value. All endpoints of the intervals comprising the result window are either endpoints of intervals of the confinement window or roots. Once a root has been bracketed, a refinement process is used to narrow down the time interval within which the root must lie. This refinement process terminates when the location of the root has been determined to within an error margin called the "convergence tolerance." The convergence tolerance used by this routine is set via the parameter SPICE_GF_CNVTOL. The value of SPICE_GF_CNVTOL is set to a "tight" value so that the tolerance doesn't become the limiting factor in the accuracy of solutions found by this routine. In general the accuracy of input data will be the limiting factor. The user may change the convergence tolerance from the default SPICE_GF_CNVTOL value by calling the routine gfstol_c, e.g. gfstol_c ( tolerance value in seconds ); Call gfstol_c prior to calling this routine. All subsequent searches will use the updated tolerance value. Searches over time windows of long duration may require use of larger tolerance values than the default: the tolerance must be large enough so that it, when added to or subtracted from the confinement window's lower and upper bounds, yields distinct time values. Setting the tolerance tighter than SPICE_GF_CNVTOL is unlikely to be useful, since the results are unlikely to be more accurate. Making the tolerance looser will speed up searches somewhat, since a few convergence steps will be omitted. The Confinement Window ====================== The simplest use of the confinement window is to specify a time interval within which a solution is sought. However, the confinement window can, in some cases, be used to make searches more efficient. Sometimes it's possible to do an efficient search to reduce the size of the time period over which a relatively slow search of interest must be performed. Certain types of searches require the state of the observer, relative to the solar system barycenter, to be computed at times slightly outside the confinement window `cnfine'. The time window that is actually used is the result of "expanding" `cnfine' by a specified amount "T": each time interval of `cnfine' is expanded by shifting the interval's left endpoint to the left and the right endpoint to the right by T seconds. Any overlapping intervals are merged. (The input argument `cnfine' is not modified.) The window expansions listed below are additive: if both conditions apply, the window expansion amount is the sum of the individual amounts. - If a search uses an equality constraint, the time window over which the state of the observer is computed is expanded by 1 second at both ends of all of the time intervals comprising the window over which the search is conducted. - If a search uses stellar aberration corrections, the time window over which the state of the observer is computed is expanded as described above. When light time corrections are used, expansion of the search window also affects the set of times at which the light time- corrected state of the target is computed. In addition to the possible 2 second expansion of the search window that occurs when both an equality constraint and stellar aberration corrections are used, round-off error should be taken into account when the need for data availability is analyzed. ## 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) Determine time intervals over which the MER-1 ("Opportunity") rover's location satisfies certain constraints on its illumination and visibility as seen from the Mars Reconnaissance Orbiter (MRO) spacecraft. In this case we require the emission angle to be less than 20 degrees and the solar incidence angle to be less than 60 degrees. The reader can verify that the observation start times of the MRO HIRISE images Product ID Image start time ---------- ---------------- TRA_000873_1780_RED 2006-10-03T12:44:13.425 PSP_001414_1780_RED 2006-11-14T15:39:55.373 PSP_001612_1780_RED 2006-11-30T01:38:34.390 are contained within the result window found by the example program shown below. Use the meta-kernel shown below to load the required SPICE kernels. KPL/MK File: gfilum_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 --------- -------- de421.bsp Planetary ephemeris pck00010.tpc Planet orientation and radii naif0010.tls Leapseconds mer1_surf_rover_ext10_v1.bsp MER-1 ephemeris mer1_surf_rover_ext11_v1.bsp MER-1 ephemeris mer1_ls_040128_iau2000_v1.bsp MER-1 landing site ephemeris mro_psp1.bsp MRO ephemeris mer1_v10.tf MER-1 frame kernel \begindata KERNELS_TO_LOAD = ( 'de421.bsp', 'pck00010.tpc', 'naif0010.tls', 'mro_psp1.bsp', 'mer1_surf_rover_ext10_v1.bsp', 'mer1_surf_rover_ext11_v1.bsp', 'mer1_ls_040128_iau2000_v1.bsp', 'mro_psp1.bsp', 'mer1_v10.tf' ) \begintext Example code begins here. /. Program gfilum_ex1 ./ #include <stdio.h> #include "SpiceUsr.h" int main() { /. Output time format: ./ #define TIMFMT "YYYY MON DD HR:MN:SC.### UTC" /. Meta-kernel name: ./ #define META "gfilum_ex1.tm" /. Maximum number of intervals in the windows used in this program: ./ #define MAXIVL 1000 #define MAXWIN ( 2 * MAXIVL ) /. Maximum length of time string: ./ #define TIMLEN 41 /. Local variables ./ SPICEDOUBLE_CELL ( cnfine, MAXWIN ); SPICEDOUBLE_CELL ( result, MAXWIN ); SPICEDOUBLE_CELL ( wnsolr, MAXWIN ); SpiceChar * abcorr; SpiceChar * fixref; SpiceChar * illmn; SpiceChar * method; SpiceChar * obsrvr; SpiceChar * target; SpiceChar timstr [ TIMLEN ]; SpiceChar * utcbeg; SpiceChar * utcend; SpiceDouble adjust; SpiceDouble emissn; SpiceDouble et0; SpiceDouble et1; SpiceDouble finish; SpiceDouble phase; SpiceDouble refval; SpiceDouble rovlt; SpiceDouble rovpos [ 3 ]; SpiceDouble solar; SpiceDouble srfvec [ 3 ]; SpiceDouble start; SpiceDouble step; SpiceDouble trgepc; SpiceInt i; /. Load kernels: ./ furnsh_c ( META ); /. Set the search interval: ./ utcbeg = "2006 OCT 02 00:00:00 UTC"; str2et_c ( utcbeg, &et0 ); utcend = "2006 NOV 30 12:00:00 UTC"; str2et_c ( utcend, &et1 ); wninsd_c ( et0, et1, &cnfine ); /. Set observer, target, aberration correction, and the Mars body-fixed, body-centered reference frame. The lighting source is the sun. Aberration corrections are set for remote observations. ./ illmn = "sun"; obsrvr = "mro"; target = "mars"; abcorr = "cn+s"; fixref = "iau_mars"; /. Use the rover position at the start of the search interval as the surface point. ./ spkpos_c ( "MER-1", et0, fixref, "NONE", target, rovpos, &rovlt ); /. Initialize the adjustment value for absolute extremum searches. We're not performing such searches in this example, but this input to GFILUM must still be set. ./ adjust = 0.0; /. The computation uses an ellipsoidal model for the target body shape. ./ method = "Ellipsoid"; /. Set the reference value to use for the solar incidence angle search. ./ refval = 60.0 * rpd_c(); /. Since the period of the solar incidence angle is about one Martian day, we can safely use 6 hours as the search step. ./ step = 21600.0; /. Search over the confinement window for times when the solar incidence angle is less than the reference value. ./ ## Restrictions1) The kernel files to be used by this routine must be loaded (normally using the CSPICE routine furnsh_c) before this routine is called. 2) This routine has the side effect of re-initializing the illumination angle utility package. Callers may need to re-initialize the package after calling this routine. ## Literature_ReferencesNone. ## Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) B.V. Semenov (JPL) E.D. Wright (JPL) ## Version-CSPICE Version 1.1.0, 01-NOV-2021 (JDR) Updated short error messages for consistency within CSPICE wrapper interface: MALLOCFAILURE -> MALLOCFAILED, and INVALIDDIMENSION -> VALUEOUTOFRANGE. Updated header to describe use of expanded confinement window. Edited the header to comply with NAIF standard. Changed code example for the solution to fit within the -Examples section without modifications. Updated the description of "nintvls", "cnfine" and "result" arguments. Added entry #20 in -Exceptions section. -CSPICE Version 1.0.0, 27-FEB-2014 (NJB) (BVS) (EDW) ## Index_Entriessolve for illumination_angle constraints solve for phase_angle constraints solve for solar_incidence_angle constraints solve for incidence_angle constraints solve for emission_angle constraints search using illumination_angle constraints search using lighting_angle constraints |

Fri Dec 31 18:41:07 2021