The ACE ancillary data file ACE_ANCIL.HDF contains two kinds of data. The data
are organized into two data structures (VGroups) in the HDF file.

The first kind of data are the ACE spacecraft attitude and orbit parameters. They
are contained in  a VGroup called VG_attorb_entry. The contents of the VGroup are
described below.

The second kind of data are parameters used to convert the S/C clock values
into seconds since the S/C EPOCH, which was a short time after ACE launch. They
are contained in a VGroup called CG_ccr_entry, also described below.


VG_attorb_entry - ACE spacecraft attitude and orbit parameters
--------------------------------------------------------------

    uint32   sctime;      /* time of measurement in Spacecraft clock ticks */

    int32    istp_time[2]; /* time of measurement in ISTP format. */
                           /* First entry is YEARDAYOFYEAR, eg: 1998201 for */
			   /* day 201 of 1998. Second entry is the millisecond of */
			   /* the day */

    char8    UT_time[13];  /* A character string containing UT time of */
                           /* the measurement */

    float64  pri2bdy[3][3];    /* Primary to body rotation matrix */
                               /* NOT IMPLEMENTED */

    uint8    state;    /* state status.  {Beg/end}_{pass/manuever} */
                       /* The data is collected under one of the following */
		       /* four conditions: */
                       /* state = 1 - Beginning of DSN telemetry pass */
                       /* state = 2 - End of DSN telemetry pass */
                       /* state = 3 - Beginning of spacecraft maneuver */
                       /* state = 4 - End of spacecraft maneuver */

    /* attitude data. Attitude vectors are normalized to unity */
    /* See ASC web page for definitions of coordinate systems */
    float64  att_J2GCI[3]; /* attitude direction cosines in J2000 */
    float64  att_RTN[3]; /* attitude direction cosines in RTN */
    float64  att_GSE[3]; /* attitude direction cosines in GSE */
    float64  att_GSM[3]; /* attitude direction cosines in GSM */
    float64  att_HSEb[3]; /* attitude direction cosines in HSEb */
    float64  att_HSEa[3]; /* attitude direction cosines in HSEa */
    float64  att_HS[3]; /* attitude direction cosines in HS */

    /* spin-rate data */
    float64  spin_J2GCI[3]; /* spin rate vector in J2000, rad/sec */
    float64  spin_RTN[3]; /* spin rate vector in RTN, rad/sec */
    float64  spin_GSE[3]; /* spin rate vector in GSE, rad/sec */
    float64  spin_GSM[3]; /* spin rate vector in GSM, rad/sec */
    float64  spin_HSEb[3]; /* spin rate vector in HSEb, rad/sec */
    float64  spin_HSEa[3]; /* spin rate vector in HSEa, rad/sec */
    float64  spin_HS[3]; /* spin rate vector in HS, rad/sec */

    /* spacecraft position data */
    float64  pos_J2GCI[3]; /* S/C position in J2000, km */
    float64  pos_GSE[3]; /* S/C position in GSE, km */
    float64  pos_GSM[3]; /* S/C position in GSM, km */
    float64  pos_HSEb[3]; /* S/C position in HSEb, km */
    float64  pos_HSEa[3]; /* S/C position in HSEa, km NOT IMPLEMENTED */
    float64  pos_HS[3]; /* S/C position in HS, km */

    /* spacecraft velocity data */
    float64  vel_J2GCI[3]; /* S/C velocity in J2000, km/s */
    float64  vel_GSE[3]; /* S/C velocity in GSE, km/s */
    float64  vel_GSM[3]; /* S/C velocity in GSM, km/s */
    float64  vel_HSEb[3]; /* S/C velocity in HSEb, km/s NOT IMPLEMENTED */
    float64  vel_HSEa[3]; /* S/C velocity in HSEa, km/s NOT IMPLEMENTED */
    float64  vel_HS[3]; /* S/C velocity in HS, km/s NOT IMPLEMENTED */


VG_ccr_entry 0 ACE Spacecraft Clock Calibration Parameters
----------------------------------------------------------

/* The values here are used to convert the S/C clock values into seconds */
/* since the S/C EPOCH, which was a short time after ACE launch */
/* S/C_EPOCH_TIME = SC_clock * SLOPE + OFFSET */

/* The offset between S/C_EPOCH and ACE_EPOCH is 52069704.604 seconds */
/* ACE_EPOCH is seconds since Jan 1, 1996 */

/* the start and end variables indicate the range of validity of the */
/* slope and offset data. Generally, the ACE Flight Dynamics team will */
/* generate a new set of parameters for each 24-hour period */

    uint32   start;    /* Start of range in S/C clock ticks */
    uint32   end;      /* End of range in S/C clock ticks */
    float64  slope;    /* slope for linear fit */
    float64  offset;   /* offset for linear fit */
    uint8    cdh;      /* =1: C&DH A, =2: C&DH B, error otherwise */
    uint16   year;     /* 4 digit year for product */
    uint16   doy;      /* Day of Year for product */