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Subsections

• MAIN table: Coordinates, Data, pointers and Flags
• ANTENNA: antenna characteristics
• ARRAY: Array characteristics
• FEED: Feed characteristics
• FIELD: Field positions for each source
• OBSERVATION: Observation information
• OBS_LOG: Observation log information
• SOURCE: Source information
• SPECTRAL_WINDOW: Spectral window setups
• SYSCAL: System calibration
• WEATHER: weather station information

MS layout

There is a MAIN table containing a number of data columns and keys into various subtables. There is at most one of each subtable. The subtables are stored as keywords of the MS.

Subtables
Table	Contents	Keys
ANTENNA	Antenna characteristics	ANTENNA_ID, ARRAY_ID
ARRAY	Array characteristics	ARRAY_ID
CORRELATOR	Correlator setup	CORRELATOR_ID
FEED	Feed characteristics	FEED_ID, ANTENNA_ID, ARRAY_ID, TIME, SPECTRAL_WINDOW_ID
FIELD	Position etc for each pointing.	FIELD_ID
FLAGGING_LOG	Log of flagging operations	ANTENNA_ID, ARRAY_ID, FEED_ID, SPECTRAL_WINDOW_ID, FIELD_ID, TIME, INTERVAL
OBSERVATION	Observer, Schedule, etc	OBSERVATION_ID
OBS_LOG	Log from on-line system	OBSERVATION_ID, TIME
SOURCE	Positions, etc for each source	SOURCE_ID, SPECTRAL_WINDOW_ID, TIME
SPECTRAL_WINDOW	Spectral window setups	SPECTRAL_WINDOW_ID
SYSCAL	System calibration characteristics	FEED_ID, ANTENNA_ID, ARRAY_ID, TIME, SPECTRAL_WINDOW_ID
WEATHER	Weather info for each antenna	ANTENNA_ID, ARRAY_ID, TIME

Note that there are two types of subtables. For the first, simpler type, the key (ID) is the row number in the subtable. Examples are ARRAY, FIELD, SPECTRAL_WINDOW, OBSERVATION and CORRELATOR. For the second, the key is a collection of parameters, usually including TIME. Examples are ANTENNA, FEED, SOURCE, SYSCAL, WEATHER. We think the Calabretta interpolation test is a good one: CORRELATOR setups cannot be sensibly interpolated and thus a different ID is required for each setup. For the interpolable tables, one needs to decide what value is actually to be used. We think this belongs in access routines independent of the MS.

Notes:

• All ID columns are zero-based, a value of -1 indicates that there is no corresponding subtable present.
• All required columns should be filled with suitable defaults if not actually used.
• For time dependent tables, a value of DBL_MAX (defined by including aips/Mathematics/Constants.h) for interval can be used to specify non time dependent entries.
• The layout of the CORRELATOR and FLAGGING_LOG table is currently unspecified. We need to gain some experience with specific implementations of these before specifying a general format.

MAIN table: Coordinates, Data, pointers and Flags

MAIN table: Coordinates, Data, pointers and Flags
Name	Format	Units	Measure	Comments
Columns
Coordinate information
ANTENNA1	Int			First antenna
ANTENNA2	Int			Second antenna
EXPOSURE	Double	s		The effective integration time
FEED1	Int			Feed on ANTENNA1
FEED2	Int			Feed on ANTENNA2
INTERVAL	Double	s		Sampling interval
PULSAR_BIN	Int			Pulsar bin number
SCAN_NUMBER	Int			Scan Number
TIME	Double	s	EPOCH	Midpoint of the integration.
TIME_EXTRA_PREC	Double	s		add to TIME for extra precision (optional)
UVW	Double(3)	m	UVW	UVW coordinates
Pointers into subtables
ARRAY_ID	Int
CORRELATOR_ID	Int
FIELD_ID	Int
OBSERVATION_ID	Int
PULSAR_ID	Int
SPECTRAL_WINDOW_ID	Int
Data
DATA	Complex(NUM_CORR, NUM_CHAN)			Complex visibility matrix (synthesis arrays, optional)
FLOAT_DATA	Float(NUM_CORR, NUM_CHAN)			Float data matrix (single dish, optional)
SIGMA	Float(NUM_CORR)			Estimated rms noise for single channel
SIGMA_SPECTRUM	Float(NUM_CORR, NUM_CHAN)			Estimated rms noise (optional)
WEIGHT	Float			Weight for whole data matrix
WEIGHT_SPECTRUM	Float(NUM_CHAN)			Weight for each channel (optional)
Flag information
FLAG	Bool(NUM_CORR, NUM_CHAN)			The data flags
FLAG_HISTORY	Bool (NUM_CORR, NUM_CHAN, NUM_HIS)			History of flags (optional)
FLAG_ROW	Bool			The row flag

Notes:

DATA, FLOAT_DATA

At least one of these columns should be present in any MeasurementSet. In special cases both could be present (e.g., single dish data used in synthesis imaging or a mix of auto and crosscorrelations on a multi-feed single dish)

ANTENNA1, ANTENNA2

For Single dish ANTENNA1=ANTENNA2

PULSAR_BIN

For a pulsar the correlations are assumed to be measured for a limited number of pulse phase bins. This is the particular bin for which this data was measured.

FLAG

An array of bools with the same shape as DATA. Data is flagged bad if FLAG is True.

FLAG_HISTORY

A set of NUM_HIS flags for each data point, each for a different category of flagging (e.g. on-line, calibration, interactive, etc.)

Next we describe the various subtables. The columns are categorized into Keys, Data description, Data, and Flags.

ANTENNA: antenna characteristics

ANTENNA: antenna characteristics
Name	Format	Units	Measure	Comments
Columns
Coordinate information
ANTENNA_ID	Int			Key
ARRAY_ID	Int			Key
Pointers
ORBIT_ID	Int			Orbit parameters
PHASED_ARRAY_ID	Int			Phased array
Data
DISH_DIAMETER	Double	m		Diameter of dish
MOUNT	String			Mount type:alt-az, equatorial, X-Y, orbiting, bizarre
NAME	String			Antenna name
OFFSET	Double(3)	m	POSITION	Axes offset of mount to FEED REFERENCE point
POSITION	Double(3)	m	POSITION	Antenna X,Y,Z phase reference positions
STATION	String			Station name

Notes:

POSITION

In a right-handed frame, X towards the intersection of the equator and the Greenwich meridian, Z towards the pole. The exact frame should be specified in the MEASURE_REFERENCE keyword (ITRF or WGS84). The reference point is the point on the az or ha axis closest to the el or dec axis.

ARRAY: Array characteristics

ARRAY: Array characteristics
Name	Format	Units	Measure	Comments
Data
NAME	String			Name of array (e.g. 'VLAA', 'EVN', 'VLBA-PT'). Informational only
POSITION	Double(3)	m	POSITION	Array reference position (optional), same frame as antenna positions.

FEED: Feed characteristics

FEED: Feed characteristics
Name	Format	Units	Measure	Comments
Columns
Keys
ANTENNA_ID	Int			Antenna id
ARRAY_ID	Int			Array id
FEED_ID	Int			Feed id
INTERVAL	Double	s		Interval
SPECTRAL_WINDOW_ID	Int			Spectral Window id
TIME	Double	s	EPOCH	Midpoint of validity range of parameters
Data description
NUM_RECEPTORS	Int			# receptors on this feed
Data
BEAM_ID	Int			Beam model
BEAM_OFFSET	Double(2, NUM_RECEPTORS)	rad	DIRECTION	Beam position offset (on sky but in antenna reference frame).
PHASED_FEED_ID	Int			Phased feed
POLARIZATION_TYPE	String (NUM_RECEPTORS)			Type of polarization to which a given RECEPTOR responds. Probably R, L or X, Y.
POL_RESPONSE	Complex (NUM_RECEPTORS, NUM_RECEPTORS)			D-matrix i.e. leakage between two receptors
POSITION	Double(3)	m	POSITION	Position of feed relative to feed reference position for this antenna
RECEPTOR_ANGLE	Double (NUM_RECEPTORS)	rad		The reference angle for polarization. Converts into Parallactic angle in the Sky domain.

Notes:

FEED

A FEED is e.g. a single horn. A FEED can have one or two RECEPTORs that respond to different polarization states. Feeds are numbered from 0 on each separate antenna and for each SPECTRAL_WINDOW_ID. So FEED_ID should be non-zero only in the case of feed-arrays, i.e., multiple simultaneous beams on the sky at the same frequency and polarization.

SPECTRAL_WINDOW_ID

A value of -1 indicates the row is valid for all spectral windows.

BEAM_ID

Points to an optional BEAM subtable with parameters (or image) for the primary beam for this antenna with this feed. No format specified yet for the BEAM table.

FIELD: Field positions for each source

FIELD: Field positions for each source
Name	Format	Units	Measure	Comments
Columns
Keys
FIELD_ID	Int			Field Id
Data
CODE	String			Special characteristics of field
DELAY_DIR	Double(2)	rad	DIRECTION	Direction of delay center.
DELAY_DIR_RATE	Double(2)	rad/s		Rate of change of direction of delay direction
NAME	String			Name of field
PHASE_DIR	Double(2)	rad	DIRECTION	Phase center.
PHASE_DIR_RATE	Double(2)	rad/s		Rate of change of direction of phase center.
POINTING_DIR	Double(2)	rad	DIRECTION	Pointing center
POINTING_DIR_RATE	Double(2)	rad/s		Rate of change of direction of pointing center.
REFERENCE_DIR	Double(2)	rad	DIRECTION	Reference center
REFERENCE_DIR_RATE	Double(2)	rad/s		Rate of change of direction of reference center.
SOURCE_ID	Int			Index in Source table
TIME	Double	s	EPOCH	Time origin for the directions and rates

Notes:

SOURCE_ID

Points to an entry in the optional SOURCE subtable, a value of -1 indicates there is no corresponding source defined.

OBSERVATION: Observation information

OBSERVATION: Observation information
Name	Format	Units	Measure	Comments
Columns
Data
CORR_SCHEDULE	String(*)			Correlator script
OBSERVER	String			Name of observer(s)
OBS_SCHEDULE	String(*)			Project Schedule
PROJECT	String			Project identification string.
Flags

Notes:

OBSERVATION

This contains information about the observation process. In the first instance, this would be a record only. Eventually we may want to be able to interpret Schedules for processing hints e.g. CALIBRATION_GROUPs. The OBSERVATION_ID is the row number.

OBS_LOG: Observation log information

OBS_LOG: Observation log information
Name	Format	Units	Measure	Comments
Columns
Keys
OBSERVATION_ID	Int			Points to OBSERVATION table
TIME	Double	s	EPOCH	Timestamp for message
Data
MESSAGE	String			log message

SOURCE: Source information

SOURCE: Source information
Name	Format	Units	Measure	Comments
Columns
Keys
INTERVAL	Double	s		Interval
SOURCE_ID	Int			Source id
SPECTRAL_WINDOW_ID	Int			Spectral Window id
TIME	Double	s	EPOCH	Midpoint of time for which this set of parameters is accurate
Data
CALIBRATION_GROUP	Int			# grouping for calibration purpose
CODE	String			Special characteristics of source, e.g. Bandpass calibrator
DIRECTION	Double(2)	rad	DIRECTION	Direction (e.g. RA, DEC)
NAME	String			Name of source as given during observations
POSITION	Double(3)	m	POSITION	Position (e.g. for solar system objects)
PROPER_MOTION	Double(2)	rad/s		Proper motion
SYSVEL	Double	m/s	RADIAL VELOCITY	Systemic velocity at reference

SPECTRAL_WINDOW: Spectral window setups

SPECTRAL_WINDOW: Spectral window setups
Name	Format	Units	Measure	Comments
Columns
Data description columns
CORR_TYPE	Int(NUM_CORR)			Polarization of correlation
CORR_PRODUCT	Int(2, NUM_CORR)			(see below)
IF_CONV_CHAIN	Int			The IF conversion chain
NUM_CHAN	Int			# spectral channels
NUM_CORR	Int			# correlations
Data
CHAN_FREQ	Double(NUM_CHAN)	Hz	FREQUENCY	Center frequencies for each channel in the data matrix.
MOLECULE	String			Molecule observed (optional)
REF_FREQUENCY	Double	Hz	FREQUENCY	The reference frequency.
RESOLUTION	Double(NUM_CHAN)	Hz		The effective noise bandwidth of spectral channels
TOTAL_BANDWIDTH	Double	Hz		total bandwidth for this window
TRANSITION	String			Transition of molecule (optional)
REST_FREQUENCY	Double	Hz	FREQUENCY	The rest frequency (spectral line)

Notes:

CHAN_FREQ

Can be non-linear to allow for e.g. AOS

CORR_PRODUCT

Pair of integers for each correlation product, specifying the receptors from which the signal originated. To get the meaning of the values (0 or 1), see the POLARIZATION_TYPE in the FEED table. An example would be (0,0), (0,1), (1,0), (1,1) for all correlations between whatever the receptors on FEED1 and FEED2 are measuring

CORR_TYPE

An integer for each correlation product indicating the Stokes type as defined in the Stokes class enumeration.

IF_CONV_CHAIN

Identification of the electronic signal path for the case of multiple (simultaneous) IFs. (e.g. VLA: AC=0, BD=1, ATCA: Freq1=0, Freq2=1)

NUM_CORR

RR would be 1, RR, LL would be 2, XX,YY,XY,YX would be 4, etc.

RESOLUTION

The effective noise bandwidth of spectral channels (the frequency resolution of each channel), this may differ from the channel spacing (difference in frequency between the centers of adjacent channels). The Vector nature allows for variable-width channels.

SYSCAL: System calibration

SYSCAL: System calibration
Name	Format	Units	Measure	Comments
Columns
Keys
ANTENNA_ID	Int			Antenna id
ARRAY_ID	Int			Array id
FEED_ID	Int			Feed id
INTERVAL	Double	s		Interval
SPECTRAL_WINDOW_ID	Int			Spectral window id
TIME	Double	s	EPOCH	Midpoint of time for which this set of parameters is accurate
Data description
NUM_RECEPTORS	Int			# receptors on this feed (1 or 2)
Data
PHASE_DIFF	Float	rad		Phase difference between receptor 2 and receptor 1
TCAL	Float (NUM_RECEPTORS)	K		Calibration temp
TRX	Float (NUM_RECEPTORS)	K		Receiver temperature
TSYS	Float (NUM_RECEPTORS)	K		System temp
TCAL_SPECTRUM	Float (NUM_RECEPTORS, NUM_CHAN)	K		Calibration temp (optional)
TRX_SPECTRUM	Float (NUM_RECEPTORS, NUM_CHAN)	K		Receiver temperature (optional)
TSYS_SPECTRUM	Float (NUM_RECEPTORS, NUM_CHAN)	K		System temp (optional)
Flags
PHASE_DIFF_FLAG	Bool			Flag for PHASE_DIFF
TCAL_FLAG	Bool			Flag for TCAL
TRX_FLAG	Bool			Flag for TRX
TSYS_FLAG	Bool			Flag for TSYS

WEATHER: weather station information

WEATHER: weather station information
Name	Format	Units	Measure	Comments
Columns
Keys
ANTENNA_ID	Int			Antenna number
ARRAY_ID	Int			Array id
INTERVAL	Double	s		Interval over which data is relevant
TIME	Double	s	EPOCH
Data
H2O	Float	m-2		Average column density of water
IONOS_ELECTRON	Float	m-2		Average column density of electrons
PRESSURE	Float	Pa		Ambient atmospheric pressure
REL_HUMIDITY	Float			Ambient relative humidity
TEMPERATURE	Float	K		Ambient Air Temperature for an antenna
WIND_DIRECTION	Float	rad		Average wind direction
WIND_SPEED	Float	m/s		Average wind speed