evsgp4 |

Table of contents## ProcedureEVSGP4 ( Evaluate "two-line" element data ) SUBROUTINE EVSGP4 ( ET, GEOPHS, ELEMS, STATE ) ## AbstractEvaluate NORAD two-line element data for earth orbiting spacecraft. This evaluator uses algorithms as described in Vallado 2006 [4]. This routine supersedes SPICELIB routines EV2LIN and DPSPCE. ## Required_ReadingNone. ## KeywordsEPHEMERIS ## DeclarationsIMPLICIT NONE INCLUDE 'zzsgp4.inc' DOUBLE PRECISION ET DOUBLE PRECISION GEOPHS ( 8 ) DOUBLE PRECISION ELEMS ( 10 ) DOUBLE PRECISION STATE ( 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 ## Detailed_InputET is the epoch 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: High altitude bound for atmospheric model in km. GEOPHS(6) = SO: Low 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 define 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 ER value is the equatorial radius in km of the Earth as used by NORAD. 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 containing two-line element data as prescribed below. The elements NDD6O 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 ) = NDT20 in radians/minute**2 ELEMS ( 2 ) = NDD60 in radians/minute**3 ELEMS ( 3 ) = BSTAR ELEMS ( 4 ) = INCL in radians ELEMS ( 5 ) = NODE0 in radians ELEMS ( 6 ) = ECC ELEMS ( 7 ) = OMEGA in radians ELEMS ( 8 ) = M0 in radians ELEMS ( 9 ) = N0 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 relative to the TEME reference frame. ## ParametersAFSPC set the SGP4 propagator to use the original Space Track #3 GST algorithm as described in Hoots [1]; value defined in zzsgp4.inc. ## Exceptions1) No checks are made on the reasonableness of the inputs. 2) If a problem occurs when evaluating the elements, an error is signaled by a routine in the call tree of this routine. ## FilesNone. ## ParticularsThis routine evaluates any NORAD two-line element sets for near-earth orbiting satellites using the algorithms described in Vallado 2006 [4]. ## ExamplesThe numerical results shown for this example 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) Suppose you have a set of two-line elements for the LUME 1 cubesat. This example shows how you can use this routine together with the routine GETELM to propagate a state to an epoch of interest. Use the meta-kernel shown below to load the required SPICE kernels. KPL/MK File name: evsgp4_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 --------- ------------------------------------ naif0012.tls Leapseconds geophysical.ker geophysical constants for evaluation of two-line element sets. The geophysical.ker is a PCK file that is provided with the SPICE toolkit under the "/data" directory. \begindata KERNELS_TO_LOAD = ( 'naif0012.tls', 'geophysical.ker' ) \begintext End of meta-kernel Example code begins here. PROGRAM EVSGP4_EX1 IMPLICIT NONE C C Local parameters. C CHARACTER*(*) TIMSTR PARAMETER ( TIMSTR = '2020-05-26 02:25:00' ) INTEGER PNAMLN PARAMETER ( PNAMLN = 2 ) INTEGER TLELLN PARAMETER ( TLELLN = 69 ) C C The LUME-1 cubesat is an Earth orbiting object; set C the center ID to the Earth ID. C INTEGER CENTER PARAMETER ( CENTER = 399 ) C C Local variables. C CHARACTER*(PNAMLN) NOADPN ( 8 ) CHARACTER*(TLELLN) TLE ( 2 ) DOUBLE PRECISION ELEMS ( 10 ) DOUBLE PRECISION EPOCH DOUBLE PRECISION ET DOUBLE PRECISION GEOPHS ( 8 ) DOUBLE PRECISION STATE ( 6 ) INTEGER I INTEGER N C C These are the variables that will hold the constants C required by ## RestrictionsNone. ## Literature_References[1] F. Hoots and R. Roehrich, "Spacetrack Report #3: Models for Propagation of the NORAD Element Sets," U.S. Air Force Aerospace Defense Command, Colorado Springs, CO, 1980. [2] F. Hoots, "Spacetrack Report #6: Models for Propagation of Space Command Element Sets," U.S. Air Force Aerospace Defense Command, Colorado Springs, CO, 1986. [3] F. Hoots, P. Schumacher and R. Glover, "History of Analytical Orbit Modeling in the U. S. Space Surveillance System," Journal of Guidance, Control, and Dynamics. 27(2):174-185, 2004. [4] D. Vallado, P. Crawford, R. Hujsak and T. Kelso, "Revisiting Spacetrack Report #3," paper AIAA 2006-6753 presented at the AIAA/AAS Astrodynamics Specialist Conference, Keystone, CO., August 21-24, 2006. ## Author_and_InstitutionM. Costa Sitja (JPL) E.D. Wright (JPL) ## VersionSPICELIB Version 1.0.0, 02-NOV-2021 (EDW) (MCS) |

Fri Dec 31 18:36:21 2021