Table 1: FREQ_OFFSET: Frequency offset information
FREQ_OFFSET: Frequency offset information

Name

Format

Units

Measure

Comments

Columns

Key

ANTENNA1

Int

First antenna

ANTENNA2

Int

Second antenna

FEED_ID

Int

Feed id.

SPECTRAL_WINDOW_ID

Int

Spectral window id.

TIME

Double

s

EPOCH

Interval midpoint

INTERVAL

Double

s

Time interval.

FIELD_ID

Int

Field id.

Data

OFFSET

Double

Hz

Frequency offset

Notes: The table contains frequency offset information. to be added directly to the defined frequency labeling in the SPECTRAL_WINDOW  sub-table as a Measure offset. This allows bands with small time-variable, ad hoc frequency offsets to be labelled as the same SPECTRAL_WINDOW_ID , and calibrated together if required. This table could eventualy be used for the spectral windows of the radiometers if there are only frequency offsets from one device to the other. 
ANTENNAn
Antenna identifier, as indexed from ANTENNAn in MAIN  (or PWVM ).
FEED_ID
Feed identifier, as indexed from FEED  in MAIN  (or PWVM ).
SPECTRAL_WINDOW_ID
Spectral window identifier
TIME
Mid point of the time interval over which the data in the row are valid. Require to use the same TIME Measure reference as in MAIN 
INTERVAL
Time interval
OFFSET
Frequency offset to be labelled to the frequency axis for this spectral window, as defined in the SPECTRAL_WINDOW  sub-table. Required to have the same Frequency Measure reference as CHAN_FREQ  in that table.

Remark:This table needs to be baseline-based to support various peculiarities of practice in Doppler tracking, especially in VLBI. If ANTENNA1=ANTENNA2 this table fullfils the need for antenna-based offsets, as this could be required e.g. to specify the spectral response of the individual ALMA radiometers.