getelm_c |

Table of contents## Proceduregetelm_c ( Get the components from two-line elements ) void getelm_c ( SpiceInt frstyr, SpiceInt lineln, const void * lines, SpiceDouble * epoch, SpiceDouble * elems ) ## AbstractParse the "lines" of a two-line element set, returning 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. lineln is the length of the strings in the `lines' array. This length shall be sufficient to store a complete line of the "two-line element" set, including the null terminator character. Since each line of the "two-line element" set is 69 characters, `lineln' shall be set to 70. 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). See -Particulars for a detailed description of the format. ## 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 SPICE routines EV2LIN and spkw10_c. Also note that the elements NDD6O and BSTAR incorporate the exponential factor present in the input two-line elements in `lines'. (See -Particulars below). 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. ## ParametersNone. ## Exceptions1) If an error occurs while trying to parse the two-line element set, the error SPICE(BADTLE) is signaled by a routine in the call tree of this routine and a description of the detected issue in the "two-line element" set is reported on the long error message. 2) If any of the `frstyr' input string pointer or the `lines' input array pointer are null, the error SPICE(NULLPOINTER) is signaled. 3) If the `frstyr' input string pointer or the `lines' input array pointer have zero length, the error SPICE(EMPTYSTRING) is signaled. 4) If the `lines' input array strings have length less than two characters, the error SPICE(STRINGTOOSHORT) is signaled. 5) If memory cannot be allocated to create the temporary variable required for the execution of the underlying Fortran routine, the error SPICE(MALLOCFAILED) is signaled. 6) If the conversion of `lines' from C to Fortran string array fails, the error SPICE(STRINGCONVERROR) is signaled. ## FilesYou must have loaded a SPICE leapseconds kernel into the kernel pool prior to calling 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 are provided in two-lines in accordance with the format required by the two-line element sets available from Space Command (formerly NORAD). See [1] and [2] for details. Each of these lines is 69 characters long. The following table define each of the individual fields for lines 1 and 2. Line Column Type Description ---- ------ ---- ------------------------------------------ 1 01 N Line number of Element Data (always 1) 1 03-07 N Satellite number (NORAD catalog number) 1 08 A Classification (U: Unclassified; S: Secret) 1 10-11 N International designator (last two digits of launch year). 1 12-14 N International designator (launch number of the year). 1 15-17 A International designator (piece of the launch) 1 19-20 N Epoch year (last two digits of year). 1 21-32 N Epoch (day of the year and portion of the day) 1 34-43 N NDT20: first time derivative of Mean Motion 1 45-52 N NDD60: Second time derivative of Mean Motion (decimal point assumed) 1 54-61 N BSTAR drag term (decimal point assumed) 1 63 N Ephemeris type 1 65-68 N Element number 1 69 N Checksum. 2 01 N Line number of Element Data (always 2) 2 03-07 N Satellite number (must be the same as in line 1) 2 09-16 N INCL: Inclination, in degrees 2 18-25 N NODE0: Right Ascension of the Ascending Node, in degrees 2 27-33 N ECC: Eccentricity (decimal point assumed) 2 35-42 N OMEGA: Argument of Perigee, in degrees 2 44-51 N M0: Mean Anomaly, in degrees 2 53-63 N N0: Mean Motion (revolutions per day) 2 64-68 N Revolution number at epoch 2 69 N Checksum The column type A indicates "characters A-Z", the type N means "numeric." Column refers to the substring within the line, e.g. 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 ^ 123456789012345678901234567890123456789012345678901234567890123456789 1 2 3 4 5 6 In this example, the satellite number (column 03-07) is 22076. The "raw" elements used by this routine in the first lines are described in detail below as in several instances exponents and decimal points are implied. Note that the input units are degrees, degrees/day**n and revolutions/day. The epoch (column 19-32; line 1) has a format NNNNN.NNNNNNNN, where: Fraction DOY of day --- -------- NNNNN.NNNNNNNN -- Year An epoch of 00001.00000000 corresponds to 00:00:00 UTC on 2000 January 01. The first derivative of Mean Motion (column 34-43, line 1), has a format +.NNNNNNNN, where the first character could be either a plus sign, a minus sign or a space. The second derivative of Mean Motion (column 45-52, line 1) and the BSTAR drag term (see [1] for details -- column 54-61, line 1) have a format +NNNNN-N, where the first character could be either a plus sign, a minus sign or a space, the decimal point is assumed, and the exponent is marked by the sign (+/-) at character 6 (column 51 and 60 for the second derivative and BSTAR drag term respectively). The "raw" elements in the second line consists primarily of mean elements calculated using the sgp4/sdp4 orbital model (See [1]). The Inclination, the Right Ascension of the Ascending Node, the Argument of Perigee and the Mean Anomaly have units of degrees and can range from 0 up to 360 degrees, except for the Inclination that ranges from 0 to 180 degrees. The Eccentricity value is provided with an assumed leading decimal point. For example, a value of 9790714 corresponds to an eccentricity of 0.9790714. The Mean motion is measured in revolutions per day and its format is NN.NNNNNNN. This routine extracts these values, "inserts" the implied decimal points and exponents and then converts the inputs to units of radians, radians/minute, radians/minute**2, and radians/minute**3 ## 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 that you have collected the two-line element data for a spacecraft with NORAD ID 18123. The following example code demonstrates how you could go about creating a type 10 SPK segment. Use the meta-kernel shown below to load the required SPICE kernels. KPL/MK File name: getelm_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 getelm_ex1 ./ #include <string.h> #include "SpiceUsr.h" int main( ) { /. Local parameters. ./ #define PNAMLN 3 #define SPK10 "getelm_ex1.bsp" #define TLELLN 70 /. The SPK type 10 segment will contain 18 two-line elements sets for the NORAD spacecraft 18123 with respect to the Earth (ID 399) in the J2000 reference frame. As stated in the naif_ids required reading, for Earth orbiting spacecraft lacking a DSN identification code, the NAIF ID is derived from the tracking ID assigned to it by NORAD via: NAIF ID = -100000 - NORAD ID code ./ #define TLESSZ 9 #define BODY -118123 #define CENTER 399 #define FRMNAM "J2000" /. Local variables. ./ SpiceChar * ifname; SpiceChar * segid; SpiceDouble consts [8]; SpiceDouble elems [10*TLESSZ]; SpiceDouble epochs [TLESSZ]; SpiceDouble first; SpiceDouble last; SpiceInt handle; SpiceInt i; SpiceInt n; SpiceInt ncomch; /. These are the variables that will hold the constants required by SPK type 10. 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 sets. ./ SpiceChar tle [2*TLESSZ][TLELLN] = { "1 18123U 87 53 A 87324.61041692 -.00000023" " 00000-0 -75103-5 0 00675", "2 18123 98.8296 152.0074 0014950 168.7820 " "191.3688 14.12912554 21686", "1 18123U 87 53 A 87326.73487726 .00000045" " 00000-0 28709-4 0 00684", "2 18123 98.8335 154.1103 0015643 163.5445 " "196.6235 14.12912902 21988", "1 18123U 87 53 A 87331.40868801 .00000104" " 00000-0 60183-4 0 00690", "2 18123 98.8311 158.7160 0015481 149.9848 " "210.2220 14.12914624 22644", "1 18123U 87 53 A 87334.24129978 .00000086" " 00000-0 51111-4 0 00702", "2 18123 98.8296 161.5054 0015372 142.4159 " "217.8089 14.12914879 23045", "1 18123U 87 53 A 87336.93227900 -.00000107" " 00000-0 -52860-4 0 00713", "2 18123 98.8317 164.1627 0014570 135.9191 " "224.2321 14.12910572 23425", "1 18123U 87 53 A 87337.28635487 .00000173" " 00000-0 10226-3 0 00726", "2 18123 98.8284 164.5113 0015289 133.5979 " "226.6438 14.12916140 23475", "1 18123U 87 53 A 87339.05673569 .00000079" " 00000-0 47069-4 0 00738", "2 18123 98.8288 166.2585 0015281 127.9985 " "232.2567 14.12916010 24908", "1 18123U 87 53 A 87345.43010859 .00000022" " 00000-0 16481-4 0 00758", "2 18123 98.8241 172.5226 0015362 109.1515 " "251.1323 14.12915487 24626", "1 18123U 87 53 A 87349.04167543 .00000042" " 00000-0 27370-4 0 00764", "2 18123 98.8301 176.1010 0015565 100.0881 " "260.2047 14.12916361 25138" }; /. Load the PCK file that provides the geophysical constants required for the evaluation of the two-line elements sets. Load also an LSK, as it is required by ## Restrictions1) The format of the two-line elements suffer from a "millennium" problem --- only two digits are used for the year of the elements. It is not clear how Space Command will deal with this problem. NAIF hopes that by adjusting the input `frstyr' you should be able to use this routine well into the 21st century. 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 = year + 100; } This mapping will be changed if future two-line element representations make this method of computing the full year inaccurate. ## 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] "SDC/SCC Two Card Element Set - Transmission Format," ADCOM/DO Form 12. [3] F. Hoots, "Spacetrack Report #6: Models for Propagation of Space Command Element Sets," U.S. Air Force Aerospace Defense Command, Colorado Springs, CO, 1986. [4] 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. [5] 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_InstitutionN.J. Bachman (JPL) M. Costa Sitja (JPL) J. Diaz del Rio (ODC Space) W.L. Taber (JPL) E.D. Wright (JPL) ## Version-CSPICE Version 1.0.2, 06-NOV-2021 (JDR) (MCS) Edited the header to comply with NAIF standard. Added complete code example. Updated -Particulars to describe in detail the TLE format and Literature_Reference to point to the sources of the detailed documentation. -CSPICE Version 1.0.1, 15-NOV-2007 (EDW) Minor edits to example section; the ## Index_EntriesParse two-line elements |

Fri Dec 31 18:41:07 2021