getelm_c |

## Procedurevoid getelm_c ( SpiceInt frstyr, SpiceInt lineln, const void * lines, SpiceDouble * epoch, SpiceDouble * elems ) ## AbstractGiven a the "lines" of a two-line element set, parse the lines and return the elements in units suitable for use in SPICE software. ## Required_ReadingNone. ## KeywordsPARSING ## Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- frstyr I Year of earliest representable two-line elements. lineln I Length of strings in lines array. lines I A pair of "lines" containing two-line elements. epoch O The epoch of the elements in seconds past J2000. elems O The elements converted to SPICE units. ## Detailed_Inputfrstyr is the first year possible for two line elements. Since two line elements allow only two digits for the year, some conventions must be followed concerning which century the two digits refer to. frstyr is the year of the earliest representable elements. The two-digit year is mapped to the year in the interval from frstyr to frstyr + 99 that has the same last two digits as the two digit year in the element set. For example if frstyr is set to 1960 then the two digit years are mapped as shown in the table below: Two-line Maps to element year 00 2000 01 2001 02 2002 . . . . . . 58 2058 59 2059 -------------------- 60 1960 61 1961 62 1962 . . . . . . 99 1999 Note that if Space Command should decide to represent years in 21st century as 100 + the last two digits of the year (for example: 2015 is represented as 115) instead of simply dropping the first two digits of the year, this routine will correctly map the year as long as you set frstyr to some value between 1900 and 1999. lines is a pair of lines of text that comprise a Space command ``two-line element'' set. lines should be declared SpiceChar lines[2][lineln]; These text lines should be the same as they are presented in the two-line element files available from Space Command (formerly NORAD). Below is an example of a two-line set for TOPEX. TOPEX 1 22076U 92052A 97173.53461370 -.00000038 00000-0 10000-3 0 594 2 22076 66.0378 163.4372 0008359 278.7732 81.2337 12.80930736227550 ## Detailed_Outputepoch is the epoch of the two line elements supplied via the input array lines. Epoch is returned in TDB seconds past J2000. elems is an array containing the elements from the two line set supplied via the array lines. The elements are in units suitable for use by the CSPICE routine ev2lin_. Also note that the elements XNDD6O and BSTAR incorporate the exponential factor present in the input two line elements in LINES. (See particulars below. ELEMS [ 0 ] = XNDT2O in radians/minute**2 ELEMS [ 1 ] = XNDD6O in radians/minute**3 ELEMS [ 2 ] = BSTAR ELEMS [ 3 ] = XINCL in radians ELEMS [ 4 ] = XNODEO in radians ELEMS [ 5 ] = EO ELEMS [ 6 ] = OMEGAO in radians ELEMS [ 7 ] = XMO in radians ELEMS [ 8 ] = XNO in radians/minute ELEMS [ 9 ] = EPOCH of the elements in seconds past ephemeris epoch J2000. ## ParametersNone. ## ExceptionsNo checking of the inputs is performed in this routine. However, this routine does call other CSPICE routines. If one of these routines detects an error it will diagnose it and signal an error. ## FilesYou must have loaded a SPICE leapseconds kernel into the kernel pool prior to caling this routine. ## ParticularsThis routine parses a Space Command Two-line element set and returns the orbital elements properly scaled and in units suitable for use by other SPICE software. Input elements look like the following ## ExamplesSuppose you have a set of two-line elements and an array containing the related geophysical constants necessary to evaluate a state. The example below shows how you can use this routine together with the routine EV2LIN to propagate a state to an epoch of interest. #include <string.h> #include <stdio.h> #include "SpiceUsr.h" SpiceDouble et; SpiceDouble epoch; SpiceInt frstyr; . . . /. The parameters below will make it easier to make assignments to the array GEOPHS required by EV2LIN. J2 --- location of J2 J3 --- location of J3 J4 --- location if J4 KE --- location of KE = sqrt(GM) in eart-radii**1.5/MIN QO --- location of upper bound of atmospheric model in KM SO --- location of lower bound of atmospheric model in KM ER --- location of earth equatorial radius in KM. AE --- location of distance units/earth radius ./ #define J2 0 #define J3 1 #define J4 2 #define KE 3 #define QO 4 #define SO 5 #define ER 6 #define AE 7 /. We set the lower bound for the years to be the beginning of the space age. ./ frstyr = 1957; /. Read in the next two lines from the text file that contains the two-line elements. We assume that file has been opened properly and that we have set the ``file pointer'' to the correct location for reading the next set of elements. ./ for ( i = 0; i < 2; i++ ) { fgets ( line[i], lineln, textfile ); line[i][ strlen(line[i]) ] = '\0'; } ## RestrictionsThe format of the two-line elements suffer from a "millenium" problem---only two digits are used for the year of the elements. It is not clear how Space Command will deal with this problem as the year 2000 comes and goes. We hope that by adjusting the input frstyr you should be able to use this routine well into the 21st century. However, since we can't predict how others will resolve the millenium problem we can't be sure that our approach will be addequate to deal with the problem. The approach taken to mapping the two-digit year to the full year is given by the code below. Here, yr is the integer obtained by parsing the two-digit year from the first line of the elements. begyr = (frstyr/100)*100; year = begyr + yr; if ( year < frstyr ) { year += 100; } This mapping will be changed if future two-line element representations make this method of computing the full year inaccurate. ## Literature_ReferencesNone. ## Author_and_InstitutionN.J. Bachman (JPL) W.L. Taber (JPL) ## Version-CSPICE Version 1.0.1, 15-NOV-2007 (EDW) Minor edits to example section; the ## Index_EntriesParse two-line elements |

Wed Apr 5 17:54:35 2017