ducrss_c |

## Procedurevoid ducrss_c ( ConstSpiceDouble s1 [6], ConstSpiceDouble s2 [6], SpiceDouble sout[6] ) ## AbstractCompute the unit vector parallel to the cross product of two 3-dimensional vectors and the derivative of this unit vector. ## Required_ReadingNone. ## KeywordsVECTOR DERIVATIVE ## Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- s1 I Left hand state for cross product and derivative. s2 I Right hand state for cross product and derivative. sout O Unit vector and derivative of the cross product. ## Detailed_Inputs1 This may be any state vector. Typically, this might represent the apparent state of a planet or the Sun, which defines the orientation of axes of some coordinate system. s2 Any state vector. ## Detailed_Outputsout This variable represents the unit vector parallel to the cross product of the position components of 's1' and 's2' and the derivative of the unit vector. If the cross product of the position components is the zero vector, then the position component of the output will be the zero vector. The velocity component of the output will simply be the derivative of the cross product of the position components of 's1' and 's2'. 'sout' may overwrite 's1' or 's2'. ## ParametersNone. ## ExceptionsError free. 1) If the position components of 's1' and 's2' cross together to give a zero vector, the position component of the output will be the zero vector. The velocity component of the output will simply be the derivative of the cross product of the position vectors. 2) If 's1' and 's2' are large in magnitude (taken together, their magnitude surpasses the limit allowed by the computer) then it may be possible to generate a floating point overflow from an intermediate computation even though the actual cross product and derivative may be well within the range of double precision numbers. ## FilesNone. ## Particulars
## ExamplesOne often constructs non-inertial coordinate frames from apparent positions of objects. However, if one wants to convert states in this non-inertial frame to states in an inertial reference frame, the derivatives of the axes of the non-inertial frame are required. For example consider an Earth meridian frame defined as follows. The z-axis of the frame is defined to be the vector normal to the plane spanned by the position vectors to the apparent Sun and to the apparent body as seen from an observer. Let 'sun' be the apparent state of the Sun and let 'body' be the apparent state of the body with respect to the observer. Then the unit vector parallel to the z-axis of the Earth meridian system and its derivative are given by the call: ## RestrictionsNo checking of 's1' or 's2' is done to prevent floating point overflow. The user is required to determine that the magnitude of each component of the states is within an appropriate range so as not to cause floating point overflow. In almost every case there will be no problem and no checking actually needs to be done. ## Literature_ReferencesNone. ## Author_and_InstitutionN.J. Bachman (JPL) W.L. Taber (JPL) E.D. Wright (JPL) ## Version-CSPICE Version 1.0.0, 23-NOV-2009 (EDW) ## Index_EntriesCompute a unit cross product and its derivative |

Wed Apr 5 17:54:32 2017