The SOUVEL binary table

Although not required by the aips++ filler, for engineering purposes the LO1 FITS file records the requested velocity. In most cases, this will be a constant value, but in the most general case, it may be an array of time, velocity and velocity first and second derivatives. These values are stored in the SOUVEL binary table. This has four columns to represent these quantities; DMJD (modified Julian date in days), VELOCITY, VDOT and VDOTDOT (velocity and derivatives in m/s, etc). There is also one keyword, VELDEF, defined as follows:

VELDEF

This keyword describes the velocity definition and reference frame used in the Doppler tracking calculations. The first four characters describe the velocity definition, followed by the rest frame. The possible values are:

• VELO - Relativistic velocity definition
• VOPT - Optical velocity definition
• VRAD - Radio velocity definition

The LO1 reference frames are:

• -TOP - observer (topographic) rest frame
• -GEO - Geocentric
• -BAR - Barycentric rest frame
• -HEL - Helocentric rest frame
• -GAL - Galactocentric rest frame
• -LSD - Dynamical LSR
• -LSR - Kinematical LSR
• -LGR - Local Group LSR
• -COB - COBE rest frame

These are described in detail in WCS Paper III, with the exception of -COB, which is a GBT-specific addition. We do not yet have precise details of how the COBE rest frame will be represented, so values of VELDEF incluidng `-COB' will not appear in FITSVER 1.1 files. WCS Paper III also allows values containing `-SOU' - Source rest frame - this is not relevant to our use and will not appear.

The ``dynamical'' LSR is a point in the vicinity of the Sun which is in a circular orbit around the Galactic centre. The Sun's motion with respect to the dynamical LSR is called the ``peculiar'' solar motion. Delhaye lists the peculiar solar motion [1] as 16.6 km/s towards Galactic coordinates L2 = 53 deg, B2 = +25 deg, or 17 49 58.7 Ra and +28 07 04 Dec (J2000).

The Kinematical LSR[2], is a point in the vicinity of the Sun which has the motion of 20 km/s towards RA 18h Dec +30d (1900).

For Galactocentric, we use the the radial velocity correction from the dynamical LSR to the galactic center which is defined as:[3] 220 km/s towards 21 12 01.1 Ra, +48 19 47 Dec (J2000).

An example header is shown below:

SOUVEL Binary Tables Extension: Header
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XTENSION= 'BINTABLE'           / binary table extension
BITPIX  =                    8 / 8-bit bytes
NAXIS   =                    2 / 2-dimensional binary table
NAXIS1  =                   32 / width of table in bytes
NAXIS2  =                    1 / number of rows in table
PCOUNT  =                    0 / size of special data area
GCOUNT  =                    1 / one data group (required keyword)
TFIELDS =                    4 / number of fields in each row
TTYPE1  = 'DMJD    '           / label for field   1
TFORM1  = '1D      '           / data format of field: 8-byte DOUBLE
TUNIT1  = 'd       '           / physical unit of field
TTYPE2  = 'VELOCITY'           / label for field   2
TFORM2  = '1D      '           / data format of field: 8-byte DOUBLE
TUNIT2  = 'm/s     '           / physical unit of field
TTYPE3  = 'VDOT    '           / label for field   3
TFORM3  = '1D      '           / data format of field: 8-byte DOUBLE
TUNIT3  = 'm/s/s   '           / physical unit of field
TTYPE4  = 'VDOTDOT '           / label for field   4
TFORM4  = '1D      '           / data format of field: 8-byte DOUBLE
TUNIT4  = 'm/s/s/s '           / physical unit of field
VELDEF  = 'VRAD-TOP'           / Velocity definition and reference frame
EXTNAME = 'SOUVEL'             / name of this binary table extension
END
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