oscelt_c |

Table of contents## Procedureoscelt_c ( Determine conic elements from state ) void oscelt_c ( ConstSpiceDouble state[6], SpiceDouble et, SpiceDouble mu, SpiceDouble elts[8] ) ## 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 ## 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 by a routine in the call tree of this routine. 2) If the specific angular momentum vector derived from `state' is the zero vector, the error SPICE(DEGENERATECASE) is signaled by a routine in the call tree of this routine. 3) If the position or velocity vectors derived from `state' is the zero vector, the error SPICE(DEGENERATECASE) is signaled by a routine in the call tree of this routine. 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 eccentricity 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 CSPICE routine conics_c is the inverse of this routine: conics_c maps a set of osculating elements and a time to a state vector. ## ExamplesThe numerical results shown for these examples 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) Determine the osculating elements of Phobos with respect to Mars at some arbitrary time in the J2000 inertial reference frame. Use the meta-kernel shown below to load the required SPICE kernels. KPL/MK File name: oscelt_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 --------- -------- mar097.bsp Mars satellite ephemeris gm_de431.tpc Gravitational constants naif0012.tls Leapseconds \begindata KERNELS_TO_LOAD = ( 'mar097.bsp', 'gm_de431.tpc', 'naif0012.tls' ) \begintext End of meta-kernel Example code begins here. /. Program oscelt_ex1 ./ #include <stdio.h> #include "SpiceUsr.h" int main( ) { /. Local variables. ./ SpiceDouble elts [8]; SpiceDouble et; SpiceDouble lt; SpiceDouble mu [1]; SpiceDouble state [6]; SpiceInt dim; /. Load the meta kernel listing the needed SPK, LSK and PCK with gravitational parameters kernels. ./ furnsh_c ( "oscelt_ex1.tm" ); /. Convert the time string to ephemeris time ./ str2et_c( "Dec 25, 2007", &et ); /. Retrieve the state of Phobos with respect to Mars in J2000. ./ spkezr_c ( "PHOBOS", et, "J2000", "NONE", "MARS", state, < ); /. Read the gravitational parameter for Mars. ./ bodvrd_c ( "MARS", "GM", 1, &dim, mu ); /. Convert the state 6-vector to the elts 8-vector. Note: the bodvrd_c returns data as arrays, so to access the gravitational parameter (the only value in the array), we use mu[0]. ./ ## 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 CSPICE routine conics_c and compare the resulting state vector with the one supplied to this routine. ## Literature_References[1] R. Bate, D. Mueller, and J. White, "Fundamentals of Astrodynamics," Dover Publications Inc., 1971. ## Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) E.D. Wright (JPL) ## Version-CSPICE Version 1.0.3, 10-AUG-2021 (JDR) Edited the header to comply with NAIF standard. Added complete code examples to -Examples section. -CSPICE Version 1.0.2, 27-DEC-2007 (NJB) Updated -Index_Entries header section to use keywords "osculating" and "convert." Updated -Particulars header section to refer to conics_c. -CSPICE Version 1.0.1, 17-NOV-2005 (NJB) The -Exceptions and -Restrictions header sections were filled in. Some corrections were made to the code example. -CSPICE Version 1.0.0, 16-APR-1999 (EDW) ## Index_Entriesconic elements from state osculating elements from state convert state to osculating elements |

Fri Dec 31 18:41:10 2021