ev2lin |

## ProcedureEV2LIN ( Evaluate "two-line" element data) SUBROUTINE EV2LIN ( ET, GEOPHS, ELEMS, STATE ) ## AbstractThis routine evaluates NORAD two-line element data for near-earth orbiting spacecraft (that is spacecraft with orbital periods less than 225 minutes). ## Required_ReadingNone. ## KeywordsEPHEMERIS ## DeclarationsIMPLICIT NONE DOUBLE PRECISION ET DOUBLE PRECISION GEOPHS ( * ) DOUBLE PRECISION ELEMS ( * ) DOUBLE PRECISION STATE ( * ) INTEGER NMODL PARAMETER ( NMODL = 6 ) ## Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- ET I Epoch in seconds past ephemeris epoch J2000. GEOPHS I Geophysical constants ELEMS I Two-line element data STATE O Evaluated state NMODL P Parameter controlling number of buffered elements. ## Detailed_InputET is the poch in seconds past ephemeris epoch J2000 at which a state should be produced from the input elements. GEOPHS is a collection of 8 geophysical constants needed for computing a state. The order of these constants must be: GEOPHS(1) = J2 gravitational harmonic for earth GEOPHS(2) = J3 gravitational harmonic for earth GEOPHS(3) = J4 gravitational harmonic for earth These first three constants are dimensionless. GEOPHS(4) = KE: Square root of the GM for earth where GM is expressed in earth radii cubed per minutes squared. GEOPHS(5) = QO: Low altitude bound for atmospheric model in km. GEOPHS(6) = SO: High altitude bound for atmospheric model in km. GEOPHS(7) = RE: Equatorial radius of the earth in km. GEOPHS(8) = AE: Distance units/earth radius (normally 1) Below are currently recommended values for these items: J2 = 1.082616D-3 J3 = -2.53881D-6 J4 = -1.65597D-6 The next item is the square root of GM for the earth given in units of earth-radii**1.5/Minute KE = 7.43669161D-2 The next two items give the top and bottom of the atmospheric drag model used by the type 10 ephemeris type. Don't adjust these unless you understand the full implications of such changes. QO = 120.0D0 SO = 78.0D0 The following is the equatorial radius of the earth as used by NORAD in km. ER = 6378.135D0 The value of AE is the number of distance units per earth radii used by the NORAD state propagation software. The value should be 1 unless you've got a very good understanding of the NORAD routine SGP4 and the affect of changing this value.. AE = 1.0D0 ELEMS is an array containg two-line element data as prescribed below. The elements XNDD6O and BSTAR must already be scaled by the proper exponent stored in the two line elements set. Moreover, the various items must be converted to the units shown here. ELEMS ( 1 ) = XNDT2O in radians/minute**2 ELEMS ( 2 ) = XNDD6O in radians/minute**3 ELEMS ( 3 ) = BSTAR ELEMS ( 4 ) = XINCL in radians ELEMS ( 5 ) = XNODEO in radians ELEMS ( 6 ) = EO ELEMS ( 7 ) = OMEGAO in radians ELEMS ( 8 ) = XMO in radians ELEMS ( 9 ) = XNO in radians/minute ELEMS ( 10 ) = EPOCH of the elements in seconds past ephemeris epoch J2000. ## Detailed_OutputSTATE is the state produced by evaluating the input elements at the input epoch ET. Units are km and km/sec. ## ParametersNMODL is a parameter that controls how many element sets can be buffered internally. Since there are a lot of computations that are independent of time these are buffered and only computed if an unbuffered model is supplied. This value should always be at least 2. Increasing it a great deal is not advised since the time needed to search the buffered elements for a match increases linearly with the NMODL. Imperically, 6 seems to be a good break even value for NMODL. ## Exceptions1) No checks are made on the reasonableness of the inputs. 2) SPICE(ITERATIONEXCEEDED) signals if the EST calculation loop exceds the MXLOOP value. This error should signal only for bad (nonphysical) TLEs. ## FilesNone. ## ParticularsThis routine evaluates NORAD two-line element sets for near-earth orbitting satellites. Near earth is defined to be a satellite with an orbital period of less than 225 minutes. This code is an adaptation of the NORAD routine SGP4 to elliminate common blocks, allow buffering of models and intermediate parameters and double precision calculations. ## ExamplesNone. ## RestrictionsNone. ## Literature_ReferencesNone. ## Author_and_InstitutionW.L. Taber (JPL) ## VersionSPICELIB Version 1.1.0, 15-SEP-2014 (EDW) Added error check to prevent infinite loop in calculation of EST. SPICELIB Version 1.0.3, 02-JAN-2008 (EDW) Corrected error in the calculation of the C4 term identified by Curtis Haase. Minor edit to the COEF1 declaration strictly identifying the constant as a double. From: COEF1 = COEF / PSISQ**3.5 To: COEF1 = COEF / PSISQ**3.5D0 SPICELIB Version 1.0.2, 08-JUL-2004 (EDW) Corrected error in the calculation of the C2 term. Reordered C1, C2 calculations to avoid division by BSTAR. SPICELIB Version 1.0.1, 10-MAR-1998 (EDW) Corrected error in header describing the GEOPHS array. SPICELIB Version 1.0.0, 14-JAN-1994 (WLT) |

Wed Apr 5 17:46:36 2017