dvhat_c |

Table of contents## Proceduredvhat_c ( Derivative and unit vector "V-hat" of a state) void dvhat_c ( ConstSpiceDouble s1 [6], SpiceDouble sout[6] ) ## AbstractFind the unit vector corresponding to a state vector and the derivative of the unit vector. ## Required_ReadingNone. ## KeywordsDERIVATIVE MATH VECTOR ## Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- s1 I State to be normalized. sout O Unit vector s1 / |s1|, and its time derivative. ## Detailed_Inputs1 is any double precision state. If the position component of the state is the zero vector, this routine will detect it and will not attempt to divide by zero. ## Detailed_Outputsout is a state containing the unit vector pointing in the direction of position component of `s1' and the derivative of the unit vector with respect to time. `sout' may overwrite `s1'. ## ParametersNone. ## ExceptionsError free. 1) If `s1' represents the zero vector, then the position component of `sout' will also be the zero vector. The velocity component will be the velocity component of `s1'. ## FilesNone. ## ParticularsLet `s1' be a state vector with position and velocity components P and V respectively. From these components one can compute the unit vector parallel to P, call it `u' and the derivative of `u' with respect to time, `du'. This pair (u,du) is the state returned by this routine in `sout'. ## 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 `state' gives the apparent state of a body with respect to an observer. This routine can be used to compute the instantaneous angular rate of the object across the sky as seen from the observers vantage. Use the meta-kernel shown below to load the required SPICE kernels. KPL/MK File name: dvhat_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 pck00008.tpc Planet orientation and radii naif0009.tls Leapseconds \begindata KERNELS_TO_LOAD = ( 'de421.bsp', 'pck00008.tpc', 'naif0009.tls' ) \begintext End of meta-kernel Example code begins here. /. Program dvhat_ex1 ./ #include <stdio.h> #include <math.h> #include "SpiceUsr.h" int main() { SpiceDouble et; SpiceDouble lt; SpiceDouble omega; SpiceDouble state [6]; SpiceDouble ustate [6]; SpiceChar * epoch = "Jan 1 2009"; SpiceChar * target = "MOON"; SpiceChar * frame = "J2000"; SpiceChar * abcorr = "LT+S"; SpiceChar * obsrvr = "EARTH BARYCENTER"; /. Load SPK, PCK, and LSK kernels, use a meta kernel for convenience. ./ furnsh_c ( "dvhat_ex1.tm" ); /. Define an arbitrary epoch, convert the epoch to ephemeris time. ./ str2et_c ( epoch, &et ); /. Calculate the state of the moon with respect to the earth-moon barycenter in J2000, corrected for light time and stellar aberration at `et'. ./ spkezr_c ( target, et, frame, abcorr, obsrvr, state, < ); /. Calculate the unit vector of `state' and the derivative of the unit vector. ./ ## RestrictionsNone. ## Literature_ReferencesNone. ## Author_and_InstitutionJ. Diaz del Rio (ODC Space) E.D. Wright (JPL) ## Version-CSPICE Version 1.0.2, 02-JUL-2021 (JDR) Edits to header to comply with NAIF standard. Added meta-kernel to the example. -CSPICE Version 1.0.1, 06-MAY-2010 (EDW) Reordered header sections to proper NAIF convention. Minor edit to code comments eliminating typo. -CSPICE Version 1.0.0, 07-JUL-1999 (EDW) ## Index_EntriesState of a unit vector parallel to a state vector |

Fri Dec 31 18:41:05 2021