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

Procedure

      EV2LIN ( Evaluate "two-line" element data)
 
      SUBROUTINE EV2LIN  ( ET, GEOPHS, ELEMS, STATE )
 

Abstract

     This routine evaluates NORAD two-line element data for
     near-earth orbiting spacecraft (that is spacecraft with
     orbital periods less than 225 minutes).

Required_Reading

      None.

Keywords

       EPHEMERIS

Declarations

 
      IMPLICIT NONE
 
      DOUBLE PRECISION      ET
      DOUBLE PRECISION      GEOPHS ( * )
      DOUBLE PRECISION      ELEMS  ( * )
      DOUBLE PRECISION      STATE  ( * )
 
      INTEGER               NMODL
      PARAMETER           ( NMODL = 6 )
 

Brief_I/O

      VARIABLE  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
     NMODL       P   Parameter controlling number of buffered elements.

Detailed_Input

     ET          is the poch 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: Low altitude bound for atmospheric
                             model in km.

                 GEOPHS(6) = SO: High 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 give 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 following is the equatorial radius
                 of the earth as used by NORAD in km.

                   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 containg two-line element data
                 as prescribed below. The elements XNDD6O 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 ) = XNDT2O in radians/minute**2
                    ELEMS (  2 ) = XNDD6O in radians/minute**3
                    ELEMS (  3 ) = BSTAR
                    ELEMS (  4 ) = XINCL  in radians
                    ELEMS (  5 ) = XNODEO in radians
                    ELEMS (  6 ) = EO
                    ELEMS (  7 ) = OMEGAO in radians
                    ELEMS (  8 ) = XMO    in radians
                    ELEMS (  9 ) = XNO    in radians/minute
                    ELEMS ( 10 ) = EPOCH of the elements in seconds
                                   past ephemeris epoch J2000.

Detailed_Output

     STATE       is the state produced by evaluating the input elements
                 at the input epoch ET. Units are km and km/sec.

Parameters

      NMODL      is a parameter that controls how many element sets
                 can be buffered internally.  Since there are a lot
                 of computations that are independent of time these
                 are buffered and only computed if an unbuffered
                 model is supplied.  This value should always
                 be at least 2.  Increasing it a great deal is not
                 advised since the time needed to search the
                 buffered elements for a match increases linearly
                 with the NMODL.  Imperically, 6 seems to be a good
                 break even value for NMODL.

Exceptions

     1) No checks are made on the reasonableness of the inputs.

     2) SPICE(ITERATIONEXCEEDED) signals if the EST calculation loop
        exceds the MXLOOP value. This error should signal only for
        bad (nonphysical) TLEs.

Files

      None.

Particulars

     This routine evaluates NORAD two-line element sets for
     near-earth orbitting satellites.  Near earth is defined to
     be a satellite with an orbital period of less than 225
     minutes.  This code is an adaptation of the NORAD routine
     SGP4 to elliminate common blocks, allow buffering of models
     and intermediate parameters and double precision calculations.

Examples

     None.

Restrictions

     None.

Literature_References

      None.

Author_and_Institution

      W.L. Taber      (JPL)

Version

    SPICELIB Version 1.1.0, 15-SEP-2014 (EDW)

        Added error check to prevent infinite loop in
        calculation of EST.

    SPICELIB Version 1.0.3, 02-JAN-2008 (EDW)

        Corrected error in the calculation of the C4 term
        identified by Curtis Haase.

        Minor edit to the COEF1 declaration strictly
        identifying the constant as a double.

        From:

           COEF1  = COEF  / PSISQ**3.5

        To:

           COEF1  = COEF  / PSISQ**3.5D0

    SPICELIB Version 1.0.2, 08-JUL-2004 (EDW)

        Corrected error in the calculation of the C2 term.
        Reordered C1, C2 calculations to avoid division 
        by BSTAR.

    SPICELIB Version 1.0.1, 10-MAR-1998 (EDW)

        Corrected error in header describing the GEOPHS array.

    SPICELIB Version 1.0.0, 14-JAN-1994 (WLT)
Wed Apr  5 17:46:36 2017