evsgp4_c |

Table of contents## Procedureevsgp4_c ( Evaluate "two-line" element data ) void evsgp4_c ( SpiceDouble et, ConstSpiceDouble geophs [8], ConstSpiceDouble elems [10], SpiceDouble state [6] ) ## AbstractEvaluate NORAD two-line element data for earth orbiting spacecraft. This evaluator uses algorithms as described in Vallado 2006 [4]. ## Required_ReadingNone. ## KeywordsEPHEMERIS ## 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[0] = J2 gravitational harmonic for Earth. geophs[1] = J3 gravitational harmonic for Earth. geophs[2] = J4 gravitational harmonic for Earth. These first three constants are dimensionless. geophs[3] = KE: Square root of the GM for Earth where GM is expressed in Earth radii cubed per minutes squared. geophs[4] = QO: High altitude bound for atmospheric model in km. geophs[5] = SO: Low altitude bound for atmospheric model in km. geophs[6] = RE: Equatorial radius of the earth in km. geophs[7] = AE: Distance units/earth radius (normally 1). Below are currently recommended values for these items: J2 = 1.082616e-3 J3 = -2.53881e-6 J4 = -1.65597e-6 The next item is the square root of GM for the Earth given in units of earth-radii**1.5/Minute KE = 7.43669161e-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 = 120.0e0 SO = 78.0e0 The ER value is the equatorial radius in km of the Earth as used by NORAD. ER = 6378.135e0 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 = 1.0e0 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[ 0 ] = NDT20 in radians/minute**2 elems[ 1 ] = NDD60 in radians/minute**3 elems[ 2 ] = BSTAR elems[ 3 ] = INCL in radians elems[ 4 ] = NODE0 in radians elems[ 5 ] = ECC elems[ 6 ] = OMEGA in radians elems[ 7 ] = M0 in radians elems[ 8 ] = N0 in radians/minute elems[ 9 ] = 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. ## ParametersNone. ## 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_c 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 CSPICE 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 ./ #include <stdio.h> #include <string.h> #include "SpiceUsr.h" int main( ) { /. Local parameters. ./ #define TIMSTR "2020-05-26 02:25:00" #define PNAMLN 3 #define TLELLN 70 /. The LUME-1 cubesat is an Earth orbiting object; set the center ID to the Earth ID. ./ #define CENTER 399 /. Local variables. ./ SpiceDouble elems [10]; SpiceDouble epoch; SpiceDouble et; SpiceDouble geophs [8]; SpiceDouble state [6]; SpiceInt i; SpiceInt n; /. These are the variables that will hold the constants required by evspg4_c. These constants are available from the loaded PCK file, which provides the actual values and units as used by NORAD propagation model. Constant Meaning -------- ------------------------------------------ J2 J2 gravitational harmonic for Earth. J3 J3 gravitational harmonic for Earth. J4 J4 gravitational harmonic for Earth. KE Square root of the GM for Earth. QO High altitude bound for atmospheric model. SO Low altitude bound for atmospheric model. ER Equatorial radius of the Earth. AE Distance units/earth radius. ./ SpiceChar noadpn [8][PNAMLN] = { "J2", "J3", "J4", "KE", "QO", "SO", "ER", "AE" }; /. Define the Two-Line Element set for LUME-1. ./ SpiceChar tle [2][TLELLN] = { "1 43908U 18111AJ 20146.60805006 .00000806" " 00000-0 34965-4 0 9999", "2 43908 97.2676 47.2136 0020001 220.6050 " "139.3698 15.24999521 78544" }; /. Load the MK file that includes the PCK file that provides the geophysical constants required for the evaluation of the two-line elements sets and the LSK, as it is required by getelm_c to perform time conversions. ./ furnsh_c ( "evsgp4_ex1.tm" ); /. Retrieve the data from the kernel, and place it on the `geophs' array. ./ for ( i = 0; i < 8; i++ ) { bodvcd_c ( CENTER, noadpn[i], 1, &n, geophs+i ); } /. Convert the Two Line Elements lines to the element sets. Set the lower bound for the years to be the beginning of the space age. ./ getelm_c ( 1957, TLELLN, tle, &epoch, elems ); /. Now propagate the state using ## 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) ## Version-CSPICE Version 1.0.0, 05-NOV-2021 (MCS) ## Index_EntriesEvaluate NORAD two-line element data using SGP4. |

Fri Dec 31 18:41:06 2021