oscelt |

## ProcedureOSCELT ( Determine conic elements from state ) SUBROUTINE OSCELT ( STATE, ET, MU, ELTS ) IMPLICIT NONE ## AbstractDetermine the set of osculating conic orbital elements that corresponds to the state (position, velocity) of a body at some epoch. ## Required_ReadingNone. ## KeywordsCONIC EPHEMERIS ## DeclarationsDOUBLE PRECISION STATE ( 6 ) DOUBLE PRECISION ET DOUBLE PRECISION MU DOUBLE PRECISION ELTS ( 8 ) ## Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- STATE I State of body at epoch of elements. ET I Epoch of elements. MU I Gravitational parameter (GM) of primary body. ELTS O Equivalent conic elements. ## Detailed_InputSTATE is the state (position and velocity) of the body at some epoch. Components are x, y, z, dx/dt, dy/dt, dz/dt. STATE must be expressed relative to an inertial reference frame. Units are km and km/sec. ET is the epoch of the input state, in ephemeris seconds past J2000. 3 2 MU is the gravitational parameter (GM, km /sec ) of the primary body. ## Detailed_OutputELTS are equivalent conic elements describing the orbit of the body around its primary. The elements are, in order: RP Perifocal distance. ECC Eccentricity. INC Inclination. LNODE Longitude of the ascending node. ARGP Argument of periapsis. M0 Mean anomaly at epoch. T0 Epoch. MU Gravitational parameter. The epoch of the elements is the epoch of the input state. Units are km, rad, rad/sec. The same elements are used to describe all three types (elliptic, hyperbolic, and parabolic) of conic orbit. ## ParametersNone. ## Exceptions1) If MU is not positive, the error SPICE(NONPOSITIVEMASS) is signaled. 2) If the specific angular momentum vector derived from STATE is the zero vector, the error SPICE(DEGENERATECASE) is signaled. 3) If the position or velocity vectors derived from STATE is the zero vector, the error SPICE(DEGENERATECASE) is signaled. 4) If the inclination is determined to be zero or 180 degrees, the longitude of the ascending node is set to zero. 5) If the eccentricity is determined to be zero, the argument of periapse is set to zero. 6) If the eccentricy of the orbit is very close to but not equal to zero, the argument of periapse may not be accurately determined. 7) For inclinations near but not equal to 0 or 180 degrees, the longitude of the ascending node may not be determined accurately. The argument of periapse and mean anomaly may also be inaccurate. 8) For eccentricities very close to but not equal to 1, the results of this routine are unreliable. 9) If the specific angular momentum vector is non-zero but "close" to zero, the results of this routine are unreliable. 10) If STATE is expressed relative to a non-inertial reference frame, the resulting elements are invalid. No error checking is done to detect this problem. ## FilesNone. ## ParticularsThe SPICELIB routine CONICS is the inverse of this routine: CONICS maps a set of osculating elements and a time to a state vector. ## ExamplesLet VINIT contain the initial state of a spacecraft relative to the center of a planet at epoch ET, and let GM be the gravitation parameter of the planet. The call CALL ## Restrictions1) The input state vector must be expressed relative to an inertial reference frame. 2) Osculating elements are generally not useful for high-accuracy work. 3) Accurate osculating elements may be difficult to derive for near-circular or near-equatorial orbits. Osculating elements for such orbits should be used with caution. 4) Extracting osculating elements from a state vector is a mathematically simple but numerically challenging task. The mapping from a state vector to equivalent elements is undefined for certain state vectors, and the mapping is difficult to implement with finite precision arithmetic for states near the subsets of R6 where singularities occur. In general, the elements found by this routine can have two kinds of problems: - The elements are not accurate but still represent the input state accurately. The can happen in cases where the inclination is near zero or 180 degrees, or for near-circular orbits. - The elements are garbage. This can occur when the eccentricity of the orbit is close to but not equal to 1. In general, any inputs that cause great loss of precision in the computation of the specific angular momentum vector or the eccentricity vector will result in invalid outputs. For further details, see the Exceptions section. Users of this routine should carefully consider whether it is suitable for their applications. One recommended "sanity check" on the outputs is to supply them to the SPICELIB routine CONICS and compare the resulting state vector with the one supplied to this routine. ## Literature_References[1] Roger Bate, Fundamentals of Astrodynamics, Dover, 1971. ## Author_and_InstitutionN.J. Bachman (JPL) K.R. Gehringer (JPL) I.M. Underwood (JPL) E.D. Wright (JPL) ## VersionSPICELIB Version 1.3.1, 28-FEB-2008 (NJB) Updated Index_Entries header section to use keywords "osculating" and "convert." Updated Particulars header section to refer to CONICS. Fixed typo in in-line comments. SPICELIB Version 1.3.0, 17-NOV-2005 (NJB) Updated to remove non-standard use of duplicate arguments in VSCL call. The Exceptions and Restrictions header sections were updated. SPICELIB Version 1.2.0, 28-JAN-2003 (NJB) (EDW) Bug fixes: routine previously didn't correctly compute the argument of periapse or mean anomaly for some cases. Also, the arguments of the ACOS and DACOSH functions were able to go out of range, causing floating-point exceptions. The computations of M0 and INC were re-coded for improved accuracy. Also, added error checks for non-positive MU, zero position, velocity, and specific angular momentum vectors. SPICELIB Version 1.1.0, 29-FEB-1996 (KRG) The declaration for the SPICELIB function PI is now preceded by an EXTERNAL statement declaring PI to be an external function. This removes a conflict with any compilers that have a PI intrinsic function. SPICELIB Version 1.0.2, 6-APR-1995 (WLT) A typo was fixed in the description of the node vector in the comments of the routine. SPICELIB Version 1.0.1, 10-MAR-1992 (WLT) Comment section for permuted index source lines was added following the header. SPICELIB Version 1.0.0, 31-JAN-1990 (IMU) |

Wed Apr 5 17:46:59 2017