Procedure

     SPKE17 ( Evaluate a type 17 SPK data record)

     SUBROUTINE SPKE17 ( ET, RECIN, STATE )

Abstract

     Evaluate a single SPK data record from a segment of type 17
     (Equinoctial Elements).

Required_Reading

     SPK

Keywords

     EPHEMERIS

Declarations

     IMPLICIT NONE

     DOUBLE PRECISION      ET
     DOUBLE PRECISION      RECIN  ( * )
     DOUBLE PRECISION      STATE  ( 6 )

Brief_I/O

     VARIABLE  I/O  DESCRIPTION
     --------  ---  --------------------------------------------------
     ET         I   Target epoch.
     RECIN      I   Data record.
     STATE      O   State (position and velocity).

Detailed_Input

     ET       is a target epoch, specified as ephemeris seconds past
              J2000, at which a state vector is to be computed.

     RECIN    is a data record which, when evaluated at epoch ET,
              will give the state (position and velocity) of some
              body, relative to some center, in some inertial
              reference frame.

              The structure of RECIN is:

                 RECIN (1)  epoch of the elements in ephemeris seconds
                            past J2000.

                 RECIN (2)-RECIN (10) Equinoctial Elements:

                    RECIN (2)  is the semi-major axis (A) of the orbit.

                    RECIN (3)  is the value of H at the specified
                               epoch. ( E*SIN(ARGP+NODE) ).

                    RECIN (4)  is the value of K at the specified epoch
                               ( E*COS(ARGP+NODE) ).

                    RECIN (5)  is the mean longitude (MEAN0+ARGP+NODE)
                               at the epoch of the elements.

                    RECIN (6)  is the value of P
                               (TAN(INC/2)*SIN(NODE)) at the specified
                               epoch.

                    RECIN (7)  is the value of Q
                               (TAN(INC/2)*COS(NODE)) at the specified
                               epoch.

                    RECIN (8)  is the rate of the longitude of periapse
                               (dARGP/dt + dNODE/dt ) at the epoch of
                               the elements. This rate is assumed to
                               hold for all time.

                    RECIN (9)  is the derivative of the mean longitude
                               ( dM/dt + dARGP/dt + dNODE/dt ).  This
                               rate is assumed to be constant.

                    RECIN (10)  is the rate of the longitude of the
                                ascending node ( dNODE/dt).

                 RECIN (11) Right Ascension of the pole of the
                            orbital reference system relative to the
                            reference frame of the associated SPK
                            segment.

                 RECIN (12) Declination of the pole of the
                            orbital reference system relative to
                            the reference frame of the associated
                            SPK segment.

Detailed_Output

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

Parameters

     None.

Exceptions

     1)  If the eccentricity is greater than 0.9, the error
         SPICE(BADECCENTRICITY) is signaled.

     2)  If the semi-major axis is non-positive, the error
         SPICE(BADSEMIAXIS) is signaled.

Files

     None.

Particulars

     This routine performs a cursory examination of the elements
     of a type 17 SPK data record and then passes the equinoctial
     elements contained in that record on to the SPICE routine
     EQNCPV for evaluation.

Examples

     The SPKEnn routines are almost always used in conjunction with
     the corresponding SPKRnn routines, which read the records from
     SPK files.

     The data returned by the SPKRnn routine is in its rawest form,
     taken directly from the segment. As such, it will be meaningless
     to a user unless he/she understands the structure of the data type
     completely. Given that understanding, however, the SPKRnn
     routines might be used to examine raw segment data before
     evaluating it with the SPKEnn routines.


     C
     C     Get a segment applicable to a specified body and epoch.
     C
           CALL SPKSFS ( BODY, ET, HANDLE, DESCR, IDENT, FOUND )

     C
     C     Look at parts of the descriptor.
     C
           CALL DAFUS ( DESCR, 2, 6, DCD, ICD )
           CENTER = ICD( 2 )
           REF    = ICD( 3 )
           TYPE   = ICD( 4 )

           IF ( TYPE .EQ. 17 ) THEN

              CALL SPKR17 ( HANDLE, DESCR, ET, RECORD )
                  .
                  .  Look at the RECORD data.
                  .
              CALL SPKE17 ( ET, RECORD, STATE )
                  .
                  .  Check out the evaluated state.
                  .
           END IF

Restrictions

     None.

Literature_References

     None.

Author_and_Institution

     J. Diaz del Rio    (ODC Space)
     W.L. Taber         (JPL)

Version

    SPICELIB Version 1.1.0, 27-AUG-2021 (JDR)

        Added IMPLICIT NONE statement.

        Edited the header to comply with NAIF standard.

    SPICELIB Version 1.0.0, 08-JAN-1997 (WLT)