Index of Functions: A B C D E F G H I J K L M N O P Q R S T U V W X
Index Page getelm_c

Table of contents

Procedure Abstract Required_Reading Keywords Brief_I/O Detailed_Input | Detailed_Output Parameters Exceptions Files Particulars Examples | Restrictions Literature_References Author_and_Institution Version Index_Entries

#### Procedure

       getelm_c ( Get the components from two-line elements )

       void getelm_c ( SpiceInt         frstyr,
                       SpiceInt         lineln,
                       const void     * lines,
                       SpiceDouble    * epoch,
                       SpiceDouble    * elems   )

#### Abstract

       Parse the "lines" of a two-line element set, returning the
       elements in units suitable for use in SPICE software.

#### Required_Reading

       None.

#### Keywords

       PARSING

#### Brief_I/O

       VARIABLE  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_Input

       frstyr      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_Output

       epoch       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.

#### Parameters

       None.

#### Exceptions

       1)  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.

#### Files

       You must have loaded a SPICE leapseconds kernel into the
       kernel pool prior to calling this routine.

#### Particulars

       This 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

#### Examples

       The 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
             getelm_c to perform time conversions. Use a metakernel for
             convenience.
             ./
             furnsh_c ( "getelm_ex1.tm" );

             /.
             Retrieve the data from the kernel, and place it on
             the `consts' array.
             ./
             for ( i = 0; i < 8; i++ )
             {
                bodvcd_c ( CENTER, noadpn[i], 1, &n, consts+i );
             }

             /.
             Convert the Two Line Elements lines to the
             element sets.
             ./
             for ( i = 0; i < TLESSZ; i++ )
             {
                getelm_c ( 1950, TLELLN, tle[i*2], epochs+i, elems+(i*10) );
             }

             /.
             Define the beginning and end of the segment to be
             -/+ 12 hours from the first and last epochs,
             respectively.
             ./
             first = epochs[0] - 0.5 * spd_c();
             last  = epochs[TLESSZ-1] + 0.5 * spd_c();

             /.
             `ncomch' is the number of characters to reserve for the
             kernel's comment area. This example doesn't write
             comments, so set to zero.
             ./
             ncomch = 0;

             /.
             Internal file name and segment ID.
             ./
             ifname = "Test for type 10 SPK internal file name";
             segid  = "SPK type 10 test segment";

             /.
             Open a new SPK file.
             ./
             spkopn_c ( SPK10, ifname, ncomch, &handle );

             /.
             Now add the segment.
             ./
             spkw10_c ( handle, BODY,   CENTER, FRMNAM, first,  last,
                        segid,  consts, TLESSZ, elems,  epochs );

             /.
             Close the SPK file.
             ./
             spkcls_c ( handle );

             return ( 0 );
          }

          When this program is executed, no output is presented on
          screen. After run completion, a new SPK type 10 exists in
          the output directory.

       2) 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 evsgp4_c to propagate a state to
          an epoch of interest.

          Use the meta-kernel from the previous example to load the
          required SPICE kernels.

          Example code begins here.

          /.
             Program getelm_ex2
          ./
          #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 evsgp4_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 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
             getelm_c to perform time conversions. Use a meta-kernel for
             convenience.
             ./
             furnsh_c ( "getelm_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 evsgp4_c to the epoch of
             interest.
             ./
             str2et_c ( TIMSTR, &et );
             evsgp4_c ( et, geophs, elems, state );

             /.
             Display the results.
             ./
             printf( "Epoch   : %s\n", TIMSTR );
             printf( "Position: %15.8f %15.8f %15.8f\n",
                            state[0], state[1], state[2] );
             printf( "Velocity: %15.8f %15.8f %15.8f\n",
                            state[3], state[4], state[5] );

             return ( 0 );
          }

          When this program was executed on a PC/Linux/gcc/64-bit
          platform, the output was:

          Epoch   : 2020-05-26 02:25:00
          Position:  -4644.60403398  -5038.95025539   -337.27141116
          Velocity:     -0.45719025      0.92884817     -7.55917355

#### Restrictions

       1)  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_Institution

       N.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 getelm_c call lacked
           the 'lineln' argument, the use of 'et' implied a pointer
           rather than a value.

       -CSPICE Version 1.0.0, 06-AUG-1999 (NJB) (WLT)

#### Index_Entries

       Parse two-line elements

#### Link to routine getelm_c source file getelm_c.c

    Fri Dec 31 18:41:07 2021