/* dvhat.f -- translated by f2c (version 19980913).
   You must link the resulting object file with the libraries:
	-lf2c -lm   (in that order)
*/

#include "f2c.h"

/* $Procedure DVHAT ( Derivative and unit vector "V-hat" of a state) */
/* Subroutine */ int dvhat_(doublereal *s1, doublereal *sout)
{
    /* System generated locals */
    doublereal d__1;

    /* Local variables */
    extern /* Subroutine */ int vequ_(doublereal *, doublereal *), vperp_(
	    doublereal *, doublereal *, doublereal *), unorm_(doublereal *, 
	    doublereal *, doublereal *);
    doublereal length;
    extern /* Subroutine */ int vsclip_(doublereal *, doublereal *);

/* $ Abstract */

/*     Find the unit vector corresponding to a state vector and the */
/*     derivative of the unit vector. */

/* $ Disclaimer */

/*     THIS SOFTWARE AND ANY RELATED MATERIALS WERE CREATED BY THE */
/*     CALIFORNIA INSTITUTE OF TECHNOLOGY (CALTECH) UNDER A U.S. */
/*     GOVERNMENT CONTRACT WITH THE NATIONAL AERONAUTICS AND SPACE */
/*     ADMINISTRATION (NASA). THE SOFTWARE IS TECHNOLOGY AND SOFTWARE */
/*     PUBLICLY AVAILABLE UNDER U.S. EXPORT LAWS AND IS PROVIDED "AS-IS" */
/*     TO THE RECIPIENT WITHOUT WARRANTY OF ANY KIND, INCLUDING ANY */
/*     WARRANTIES OF PERFORMANCE OR MERCHANTABILITY OR FITNESS FOR A */
/*     PARTICULAR USE OR PURPOSE (AS SET FORTH IN UNITED STATES UCC */
/*     SECTIONS 2312-2313) OR FOR ANY PURPOSE WHATSOEVER, FOR THE */
/*     SOFTWARE AND RELATED MATERIALS, HOWEVER USED. */

/*     IN NO EVENT SHALL CALTECH, ITS JET PROPULSION LABORATORY, OR NASA */
/*     BE LIABLE FOR ANY DAMAGES AND/OR COSTS, INCLUDING, BUT NOT */
/*     LIMITED TO, INCIDENTAL OR CONSEQUENTIAL DAMAGES OF ANY KIND, */
/*     INCLUDING ECONOMIC DAMAGE OR INJURY TO PROPERTY AND LOST PROFITS, */
/*     REGARDLESS OF WHETHER CALTECH, JPL, OR NASA BE ADVISED, HAVE */
/*     REASON TO KNOW, OR, IN FACT, SHALL KNOW OF THE POSSIBILITY. */

/*     RECIPIENT BEARS ALL RISK RELATING TO QUALITY AND PERFORMANCE OF */
/*     THE SOFTWARE AND ANY RELATED MATERIALS, AND AGREES TO INDEMNIFY */
/*     CALTECH AND NASA FOR ALL THIRD-PARTY CLAIMS RESULTING FROM THE */
/*     ACTIONS OF RECIPIENT IN THE USE OF THE SOFTWARE. */

/* $ Required_Reading */

/*     None. */

/* $ Keywords */

/*     VECTOR */
/*     DERIVATIVE */
/*     MATH */

/* $ Declarations */
/* $ Brief_I/O */

/*     VARIABLE  I/O  DESCRIPTION */
/*     --------  ---  -------------------------------------------------- */
/*     S1        I     State to be normalized. */
/*     SOUT      O     Unit vector S1 / |S1|, and its time derivative. */

/* $ Detailed_Input */

/*     S1       This 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_Output */

/*     SOUT     SOUT 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. */

/* $ Parameters */

/*     None. */

/* $ Exceptions */

/*     Error 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. */

/* $ Files */

/*     None. */

/* $ Particulars */

/*     Let 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. */

/* $ Examples */

/*   Any numerical results shown for this example may differ between */
/*   platforms as the results depend on the SPICE kernels used as input */
/*   and the machine specific arithmetic implementation. */

/*   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. */

/*           PROGRAM DVHAT_T */
/*           IMPLICIT NONE */

/*           DOUBLE PRECISION      ET */
/*           DOUBLE PRECISION      LT */
/*           DOUBLE PRECISION      OMEGA */
/*           DOUBLE PRECISION      STATE  (6) */
/*           DOUBLE PRECISION      USTATE (6) */

/*           DOUBLE PRECISION      VNORM */

/*           CHARACTER*(32)        EPOCH */
/*           CHARACTER*(32)        TARGET */
/*           CHARACTER*(32)        FRAME */
/*           CHARACTER*(32)        ABCORR */
/*           CHARACTER*(32)        OBSRVR */

/*     C */
/*     C     Load SPK, PCK, and LSK kernels, use a meta kernel for */
/*     C     convenience. */
/*     C */
/*           CALL FURNSH ( 'standard.tm' ) */

/*     C */
/*     C     Define an arbitrary epoch, convert the epoch to ephemeris */
/*     C     time. */
/*     C */
/*           EPOCH = 'Jan 1 2009' */
/*           CALL STR2ET ( EPOCH, ET ) */

/*     C */
/*     C     Calculate the state of the moon with respect to the */
/*     C     earth-moon barycenter in J2000, corrected for light time */
/*     C     and stellar aberration at ET. */
/*     C */
/*           TARGET   = 'MOON' */
/*           FRAME    = 'J2000' */
/*           ABCORR   = 'LT+S' */
/*           OBSRVR   = 'EARTH BARYCENTER' */

/*           CALL SPKEZR ( TARGET, ET, FRAME, ABCORR, OBSRVR, STATE, LT ) */

/*     C */
/*     C     Calculate the unit vector of STATE and the derivative of the */
/*     C     unit vector. */
/*     C */
/*           CALL DVHAT ( STATE, USTATE ) */

/*     C */
/*     C     Calculate the instantaneous angular velocity from the */
/*     C     magnitude of the derivative of the unit vector. */
/*     C */
/*     C          v = r x omega */
/*     C */
/*     C          ||omega|| = ||v||  for  r . v = 0 */
/*     C                      ----- */
/*     C                      ||r|| */
/*     C */
/*     C          ||omega|| = ||v||  for  ||r|| = 1 */
/*     C */
/*           OMEGA = VNORM( USTATE(4) ) */

/*           WRITE(*,*) 'Instantaneous angular velocity, rad/sec', OMEGA */

/*           END */

/*   The program outputs: */

/*       Instantaneous angular velocity, rad/sec  2.48106658E-06 */

/* $ Restrictions */

/*     None. */

/* $ Literature_References */

/*     None. */

/* $ Author_and_Institution */

/*     N.J. Bachman    (JPL) */
/*     W.L. Taber      (JPL) */

/* $ Version */

/* -    SPICELIB Version 1.1.1, 06-MAY-2010 (EDW) */

/*        Expanded the code example into a complete program. */

/*        Reordered header sections to proper NAIF convention. */
/*        Removed Revision section, it listed a duplication of a */
/*        Version section entry. */

/* -    SPICELIB Version 1.1.0, 02-SEP-2005 (NJB) */

/*        Updated to remove non-standard use of duplicate arguments */
/*        in VPERP and VSCL calls. */

/* -    SPICELIB Version 1.0.0, 15-JUN-1995 (WLT) */

/* -& */
/* $ Index_Entries */

/*     State of a unit vector parallel to a state vector */

/* -& */

/*     Get the position portion of the output state and the length of */
/*     the input position. */

    unorm_(s1, sout, &length);
    if (length == 0.) {

/*        If the length of the input position is zero, just copy */
/*        the input velocity to the output velocity. */

	vequ_(&s1[3], &sout[3]);
    } else {

/*        Otherwise the derivative of the unit vector is just the */
/*        component of the input velocity perpendicular to the input */
/*        position, scaled by the reciprocal of the length of the */
/*        input position. */

	vperp_(&s1[3], sout, &sout[3]);
	d__1 = 1. / length;
	vsclip_(&d__1, &sout[3]);
    }
    return 0;
} /* dvhat_ */

