Procedure

     SPKW15 ( SPK, write a type 15 segment )

     SUBROUTINE SPKW15 ( HANDLE, BODY,  CENTER, FRAME,  FIRST, LAST,
    .                    SEGID,  EPOCH, TP,     PA,     P,     ECC,
    .                    J2FLG,  PV,    GM,     J2,     RADIUS      )

Abstract

     Write an SPK segment of type 15 given a type 15 data record.

Required_Reading

     SPK

Keywords

     EPHEMERIS

Declarations

     IMPLICIT NONE

     INTEGER               HANDLE
     INTEGER               BODY
     INTEGER               CENTER
     CHARACTER*(*)         FRAME
     DOUBLE PRECISION      FIRST
     DOUBLE PRECISION      LAST
     CHARACTER*(*)         SEGID
     DOUBLE PRECISION      EPOCH
     DOUBLE PRECISION      TP     ( 3 )
     DOUBLE PRECISION      PA     ( 3 )
     DOUBLE PRECISION      P
     DOUBLE PRECISION      ECC
     DOUBLE PRECISION      J2FLG
     DOUBLE PRECISION      PV     ( 3 )
     DOUBLE PRECISION      GM
     DOUBLE PRECISION      J2
     DOUBLE PRECISION      RADIUS

Brief_I/O

     VARIABLE  I/O  DESCRIPTION
     --------  ---  --------------------------------------------------
     HANDLE     I   Handle of an SPK file open for writing.
     BODY       I   Body code for ephemeris object.
     CENTER     I   Body code for the center of motion of the body.
     FRAME      I   The reference frame of the states.
     FIRST      I   First valid time for which states can be computed.
     LAST       I   Last valid time for which states can be computed.
     SEGID      I   Segment identifier.
     EPOCH      I   Epoch of the periapse.
     TP         I   Trajectory pole vector.
     PA         I   Periapsis vector.
     P          I   Semi-latus rectum.
     ECC        I   Eccentricity.
     J2FLG      I   J2 processing flag.
     PV         I   Central body pole vector.
     GM         I   Central body GM.
     J2         I   Central body J2.
     RADIUS     I   Equatorial radius of central body.

Detailed_Input

     HANDLE   is the file handle of an SPK file that has been
              opened for writing.

     BODY     is the NAIF ID for the body whose states are
              to be recorded in an SPK file.

     CENTER   is the NAIF ID for the center of motion associated
              with BODY.

     FRAME    is the reference frame that states are referenced to,
              for example 'J2000'.

     FIRST    are the bounds on the ephemeris times, expressed as
     LAST     seconds past J2000.

     SEGID    is the segment identifier. An SPK segment identifier
              may contain up to 40 characters.

     EPOCH    is the epoch of the orbit elements at periapse
              in ephemeris seconds past J2000.

     TP       is a vector parallel to the angular momentum vector
              of the orbit at epoch expressed relative to FRAME. A
              unit vector parallel to TP will be stored in the
              output segment.

     PA       is a vector parallel to the position vector of the
              trajectory at periapsis of EPOCH expressed relative
              to FRAME. A unit vector parallel to PA will be
              stored in the output segment.

     P        is the semi-latus rectum--- p in the equation:

                 r = p/(1 + ECC*COS(Nu))

     ECC      is the eccentricity.

     J2FLG    is the J2 processing flag describing what J2
              corrections are to be applied when the orbit is
              propagated.

              All J2 corrections are applied if the value of J2FLG
              is not 1, 2 or 3.

              If the value of the flag is 3 no corrections are
              done.

              If the value of the flag is 1 no corrections are
              computed for the precession of the line of apsides.
              However, regression of the line of nodes is
              performed.

              If the value of the flag is 2 no corrections are
              done for the regression of the line of nodes.
              However, precession of the line of apsides is
              performed.

              Note that J2 effects are computed only if the orbit
              is elliptic and does not intersect the central body.

     PV       is a vector parallel to the north pole vector of the
              central body expressed relative to FRAME. A unit
              vector parallel to PV will be stored in the output
              segment.

     GM       is the central body GM.

     J2       is the central body J2 (dimensionless).

     RADIUS   is the equatorial radius of the central body.

     Units are radians, km, seconds.

Detailed_Output

     None. A type 15 segment is written to the file attached
     to HANDLE.

Parameters

     None.

Exceptions

     1)  If the eccentricity is less than zero, the error
         SPICE(BADECCENTRICITY) is signaled.

     2)  If the semi-latus rectum is 0, the error
         SPICE(BADLATUSRECTUM) is signaled.

     3)  If the pole vector, trajectory pole vector or periapsis vector
         have zero length, the error SPICE(BADVECTOR) is signaled.

     4)  If the trajectory pole vector and the periapsis vector are
         not orthogonal, the error SPICE(BADINITSTATE) is signaled.
         The test for orthogonality is very crude. The routine simply
         checks that the dot product of the unit vectors parallel
         to the trajectory pole and periapse vectors is less than
         0.00001. This check is intended to catch blunders, not to
         enforce orthogonality to double precision capacity.

     5)  If the mass of the central body is non-positive, the error
         SPICE(NONPOSITIVEMASS) is signaled.

     6)  If the radius of the central body is negative, the error
         SPICE(BADRADIUS) is signaled.

     7)  If the segment identifier has more than 40 non-blank
         characters, the error SPICE(SEGIDTOOLONG) is signaled.

     8)  If the segment identifier contains non-printing characters,
         the error SPICE(NONPRINTABLECHARS) is signaled.

     9)  If there are inconsistencies in the BODY, CENTER, FRAME or
         FIRST and LAST times, an error is signaled by
         a routine in the call tree of this routine.

Files

     A new type 15 SPK segment is written to the SPK file attached
     to HANDLE.

Particulars

     This routine writes an SPK type 15 data segment to the open SPK
     file according to the format described in the type 15 section of
     the SPK Required Reading. The SPK file must have been opened with
     write access.

     This routine is provided to provide direct support for the MASL
     precessing orbit formulation.

Examples

     Suppose that at time EPOCH you have the J2000 periapsis
      state of some object relative to some central body and would
      like to create a type 15 SPK segment to model the motion of
      the object using simple regression and precession of the
      line of nodes and apsides. The following code fragment
      illustrates how you can prepare such a segment. We shall
      assume that you have in hand the J2000 direction of the
      central body's pole vector, its GM, J2 and equatorial
      radius. In addition we assume that you have opened an SPK
      file for write access and that it is attached to HANDLE.

     (If your state is at an epoch other than periapse the
      fragment below will NOT produce a "correct" type 15 segment
      for modeling the motion of your object.)

      C
      C     First we get the osculating elements.
      C
            CALL OSCELT ( STATE, EPOCH, GM, ELTS )

      C
      C     From these collect the eccentricity and semi-latus rectum.
      C
            ECC = ELTS ( 2 )
            P   = ELTS ( 1 ) * ( 1.0D0 + ECC )
      C
      C     Next get the trajectory pole vector and the
      C     periapsis vector.
      C
            CALL UCRSS ( STATE(1), STATE(4), TP )
            CALL VHAT  ( STATE(1),           PA )

      C
      C     Enable both J2 corrections.
      C

           J2FLG = 0.0D0

      C
      C     Now add the segment.
      C

            CALL SPKW15 ( HANDLE, BODY,  CENTER, FRAME,  FIRST, LAST,
            .              SEGID,  EPOCH, TP,     PA,    P,     ECC,
            .              J2FLG,  PV,    GM,     J2,    RADIUS      )

Restrictions

     None.

Literature_References

     None.

Author_and_Institution

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

Version

    SPICELIB Version 2.1.0, 03-JUN-2021 (JDR)

        Added IMPLICIT NONE statement.

        Edited the header to comply with NAIF standard.

    SPICELIB Version 2.0.0, 29-MAY-2012 (NJB)

        Input vectors that nominally have unit length
        are mapped to local copies that actually do
        have unit length. The applicable inputs are TP, PA,
        and PV. The Detailed Input header section was updated
        to reflect the change.

        Some typos in error messages were corrected.

    SPICELIB Version 1.0.0, 28-NOV-1994 (WLT)