pxfrm2_c |

Table of contents## Procedurepxfrm2_c ( Position Transform Matrix, Different Epochs ) void pxfrm2_c ( ConstSpiceChar * from, ConstSpiceChar * to, SpiceDouble etfrom, SpiceDouble etto, SpiceDouble rotate[3][3] ) ## AbstractReturn the 3x3 matrix that transforms position vectors from one specified frame at a specified epoch to another specified frame at another specified epoch. ## Required_ReadingFRAMES ## KeywordsFRAMES TRANSFORM ## Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- from I Name of the frame to transform from. to I Name of the frame to transform to. etfrom I Evaluation time of `from' frame. etto I Evaluation time of `to' frame. rotate O A position transformation matrix from frame `from' to frame `to'. ## Detailed_Inputfrom is the name of a reference frame recognized by CSPICE that corresponds to the input `etfrom'. to is the name of a reference frame recognized by CSPICE that corresponds to the desired output at `etto'. etfrom is the epoch in ephemeris seconds past the epoch of J2000 (TDB) corresponding to the `from' reference frame. etto is the epoch in ephemeris seconds past the epoch of J2000 (TDB) that corresponds to the `to' reference frame. ## Detailed_Outputrotate is the transformation matrix that relates the reference frame `from' at epoch `etfrom' to the frame `to' at epoch `etto'. If (x, y, z) is a position relative to the reference frame `from' at time `etfrom' then the vector ( x', y', z') is the same position relative to the frame `to' at epoch `etto'. Here the vector ( x', y', z' ) is defined by the equation: - - - - - - | x' | | | | x | | y' | = | rotate | | y | | z' | | | | z | - - - - - - ## ParametersNone. ## Exceptions1) If sufficient information has not been supplied via loaded SPICE kernels to compute the transformation between the two frames, an error is signaled by a routine in the call tree of this routine. 2) If either frame `from' or `to' is not recognized, the error SPICE(UNKNOWNFRAME) is signaled by a routine in the call tree of this routine. 3) If any of the `from' or `to' input string pointers is null, the error SPICE(NULLPOINTER) is signaled. 4) If any of the `from' or `to' input strings has zero length, the error SPICE(EMPTYSTRING) is signaled. ## FilesAppropriate kernels must be loaded by the calling program before this routine is called. Kernels that may be required include SPK files, PCK files, frame kernels, C-kernels, and SCLK kernels. Such kernel data are normally loaded once per program run, NOT every time this routine is called. ## Particulars
## 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) Suppose that MGS has taken a picture of Mars at time `etrec' with the MOC narrow angle camera. We want to know the latitude and longitude associated with two pixels projected to Mars' surface: the boresight and one along the boundary of the field of view (FOV). Due to light time, the photons taken in the picture left Mars at time `etemit', when Mars was at a different state than at time `etrec'. In order to solve this problem, we could use the sincpt_c routine for both pixels, but this would be slow. Instead, we will assume that the light time for each pixel is the same. We will call sincpt_c once to get the light time and surface point associated with the boresight. Then, we will rotate one of the FOV boundary vectors from the camera frame at `etrec' to the body-fixed Mars frame at `etemit', and call the faster routine surfpt_c to retrieve the surface point for one of the FOV boundary vectors. This example problem could be extended to find the latitude and longitude associated with every pixel in an instrument's field of view, but this example is simplified to only solve for two pixels: the boresight and one along the boundary of the field of view. Assumptions: 1) The light times from the surface points in the camera's field of view to the camera are equal. 2) The camera offset from the center of gravity of the spacecraft is zero. If the data are more accurate and precise, this assumption can be easily discarded. 3) An ellipsoid shape model for the target body is sufficient. 4) The boundary field of view vector returned from getfov_c is associated with a boundary field of view pixel. If this example were extended to include a geometric camera model, this assumption would not be needed since the direction vectors associated with each pixel would be calculated from the geometric camera model. Use the meta-kernel shown below to load the required SPICE kernels. KPL/MK File name: pxfrm2_ex1.tm This is the meta-kernel file for the example problem for the subroutine PXFRM2. These kernel files can be found in the NAIF archives. 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 pck00009.tpc Planet orientation and radii naif0009.tls Leapseconds mgs_ext12_ipng_mgs95j.bsp MGS ephemeris mgs_moc_v20.ti MGS MOC instrument parameters mgs_sclkscet_00061.tsc MGS SCLK coefficients mgs_sc_ext12.bc MGS s/c bus attitude \begindata KERNELS_TO_LOAD = ( 'de421.bsp', 'pck00009.tpc', 'naif0009.tls', 'mgs_ext12_ipng_mgs95j.bsp', 'mgs_moc_v20.ti', 'mgs_sclkscet_00061.tsc', 'mgs_sc_ext12.bc' ) \begintext End of meta-kernel. Example code begins here. /. Program pxfrm2_ex1 ./ #include <stdio.h> #include "SpiceUsr.h" int main() { /. Constants ABCORR is the desired light time and stellar aberration correction setting. METAKR is the name of the meta-kernel. ./ #define ABCORR "CN+S" #define METAKR "pxfrm2_ex1.tm" #define FRMNLN 32 #define NCORNR 4 #define SHPLEN 80 /. Local variables ./ SpiceBoolean found; /. MGS_MOC_NA is the name of the camera that took the picture being analyzed. ./ SpiceChar *camera = "MGS_MOC_NA"; /. The variable `obsref' is the observer reference frame on MGS. ./ SpiceChar obsref [FRMNLN] ; SpiceChar shape [SHPLEN] ; SpiceDouble bounds [NCORNR][3]; SpiceDouble bndvec [3]; SpiceDouble bsight [3]; SpiceDouble dist; /. The variable `etemit' is the time at which the photons were emitted from Mars, and `etrec' is the time at which the picture was taken by MGS. ./ SpiceDouble etemit; SpiceDouble etrec; /. The variables `lat' and `lon' and the latitude and longitude associated with one of the boundary FOV vectors. ./ SpiceDouble lat; SpiceDouble lon; /. The variable `pmgsmr' is the opposite of the apparent position of Mars with respect to MGS. ./ SpiceDouble pmgsmr [3]; /. The variable `radii' is a vector of the semi-axes of Mars. ./ SpiceDouble radii [3]; SpiceDouble radius; /. The variable `rotate' is a position transformation matrix from the camera frame at `etrec' to the IAU_MARS frame at `etemit'. ./ SpiceDouble rotate [3][3]; SpiceDouble spoint [3]; SpiceDouble srfvec [3]; SpiceDouble tmp [3]; SpiceInt camid; SpiceInt dim; SpiceInt n; /. ------------------ Program Setup ------------------ Load kernels. ./ furnsh_c ( METAKR ); /. Convert the time the picture was taken from a UTC time string to seconds past J2000, TDB. ./ str2et_c ( "2003 OCT 13 06:00:00 UTC", &etrec ); /. Assume the one-way light times from different surface points on Mars to MGS within the camera's FOV are equal. This means the photons that make up different pixels were all emitted from Mars at `etemit' and received by the MGS MOC camera at `etrec'. It would be slow to process images using `sincpt_c' for every pixel. Instead, we will use `sincpt_c' on the boresight pixel and use `surfpt_c' for one of the FOV boundary pixels. If this example program were extended to include all of the camera's pixels, `surfpt_c' would be used for the remaining pixels. Get the MGS MOC Narrow angle camera (MGS_MOC_NA) ID code. Then look up the field of view (FOV) parameters by calling `getfov_c'. ./ bodn2c_c ( camera, &camid, &found ); if ( !found ) { setmsg_c ("Could not find ID code for instrument #." ); errch_c ("#", camera ); sigerr_c ("SPICE(NOTRANSLATION)"); } /. `getfov_c' will return the name of the camera-fixed frame in the string `obsref', the camera boresight vector in the array `bsight', and the FOV corner vectors in the array `bounds'. ./ getfov_c ( camid, NCORNR, SHPLEN, FRMNLN, shape, obsref, bsight, &n, bounds ); printf( "Observation Reference Frame: %s\n", obsref ); /. ----------- Boresight Surface Intercept ----------- Retrieve the time, surface intercept point, and vector from MGS to the boresight surface intercept point in IAU_MARS coordinates. ./ sincpt_c ( "Ellipsoid", "Mars", etrec, "IAU_MARS", ABCORR, "MGS", obsref, bsight, spoint, &etemit, srfvec, &found ); if ( !found ) { setmsg_c("Intercept not found for the boresight vector."); sigerr_c("SPICE(NOINTERCEPT)"); } /. Convert the intersection point of the boresight vector and Mars from rectangular into latitudinal coordinates. Convert radians to degrees. ./ reclat_c ( spoint, &radius, &lon, &lat ); lon *= dpr_c(); lat *= dpr_c(); printf( "Boresight surface intercept coordinates:\n" " Radius (km) : %f\n" " Latitude (deg): %f\n" " Longitude (deg): %f\n", radius, lat, lon ); /.---- A Boundary FOV Surface Intercept (surfpt_c) ----- Now we will transform one of the FOV corner vectors into the IAU_MARS frame so the surface intercept point can be calculated using surfpt_c, which is faster than subpnt_c. If this example program were extended to include all of the pixels in the camera's FOV, a few steps, such as finding the rotation matrix from the camera frame to the IAU_MARS frame, looking up the radii values for Mars, and finding the position of MGS with respect to Mars could be done once and used for every pixel. Find the rotation matrix from the ray's reference frame at the time the photons were received (etrec) to IAU_MARS at the time the photons were emitted (etemit). ./ ## RestrictionsNone. ## Literature_ReferencesNone. ## Author_and_InstitutionJ. Diaz del Rio (ODC Space) S.C. Krening (JPL) W.L. Taber (JPL) ## Version-CSPICE Version 1.0.1, 13-AUG-2021 (JDR) Updated -Exceptions section with description of missing exceptions and its format to comply with NAIF standard. Edited the header to comply with NAIF standard. Fixed reference to CSPICE API in code example output. Changed boundary vector used in code example for consistency with other examples. -CSPICE Version 1.0.0, 01-FEB-2012 (SCK) (WLT) ## Index_EntriesPosition transformation matrix for different epochs |

Fri Dec 31 18:41:11 2021