MeasurementSet

Introduction

The MeasurementSet version 2 [1] , is a database designed to hold radioastronomical data to be calibrated following the MeasurementEquation approach by Hamaker, Bregman, and Sault (1996).

Since its publication, the MeasurementSet (MS) design has been implemented by several software development groups, among them the CASA team and, e.g., the European VLBI Network team. CASA has also adopted the MeasurementEquation as its fundamental calibration scheme and has thus embraced the MS as its native way to store radio observations. With CASA becoming the designated analysis package for ALMA and the VLA, this means that the MS is now the default way of storing ALMA and VLA data during the actual analysis.

The ALMA and VLA raw data format, however, is not the MS but the so-called ALMA Science Data Model (ASDM) for ALMA, and the Science Data Model (SDM) for the VLA. Both of which are closely related and are discussed in a separate section. The ALMA and VLA archives hence do not store data in MS format but in (A)SDM format, and when a CASA user starts to work with this data, the first step has to be the import of the (A)SDM into the CASA MS format.

The MS is effectively a relational database which on the one hand tries to permit the storage of all imaginable radio (interferometric, single-dish) data with corresponding metadata, and on the other hand ventures to be storage-space and data-maintenance efficient by avoiding data redundancy.

The universality is achieved by offering many optional parts in the format which cover most imaginable use cases in radio astronomy. So a simple, few-antenna interferometer observing a simple object with time-independent position at just a single frequency can store its data using a small sub-set of the format while a large interferometer with antennas on time-dependent locations, observing many objects in rapid succession with time-dependent source positions using a complex, time-dependent spectral setup etc., can equally use the MS to store its data albeit using a larger subset of the possibilities of the MS.

The non-redundance of the format is achieved by simply following the standard approach of relational databases which is to put repeating pieces of information into separate database tables, the Subtables, and replacing them in the main body of the data base, the Main table, by references to the Subtables. In the case of the MS this happens in two layers of Subtables with the first layer being referenced by the Main table and the second layer being referenced by the first layer. I.e., there are some Subtables which reference other Subtables.

The Subtable referencing mechanism is defined in the original design. It works either via the line numbers of the individual Subtable ,this implies that the reference is a zero-based integer and that the removal of a line in such a Subtable requires reindexing in the referencing table(s), or via explicit references to an index column in the Subtable ,the latter is much less common.

These design principles lead to a format which puts the bulk of the data ,the interferometric visibilities and/or the single-dish total-power measurements with their timestamps, into a Main Table , and most of the metadata in the two layers of Subtables.

In the CASA MS implementation, the individual Tables are all stored in the CASA Table format, i.e. they are actually not single files on disk but directories containing several files, essentially one for each column of the table. So the entire MS is also not a single file (like, e.g., in the FITS IDI format) but a whole directory tree. For transport, the MS typically has to be turned into a single file by using the command "tar".

The Main Table contains the radio data initially in a column called DATA (interferometric data) or FLOAT_DATA (pure single-dish data). One of these two columns always has to be present.

When a calibration is applied to the DATA column, a CORRECTED_DATA column is created to contain the calibrated data leaving the original data untouched. Furthermore, a MODEL_DATA column can be required to store expectation values for the emission of calibration sources.

For large datasets these bulk data columns can require large amounts of disk space and access to them may be slow. To mitigate these problems, the CASA team is working on making the columns "virtual" as much as possible, i.e. replacing the CORRECTED_DATA and MODEL_DATA columns by parameterised versions calculated on-the-fly.

In the case of the virtual MODEL_DATA column, this is essentially a model image which is stored with the MS and converted on-the-fly to visibilities.

In the case of the virtual CORRECTED_DATA column, this is a so-called "Cal Library" which permits to calibrate the data in the DATA column on-the-fly and make the results available as if they were stored in a standard table column.

Finally, a major case of data redundance for ALMA and VLA data is of course the fact that the raw data arrive at the user in (A)SDM format but then have to be translated into MS format which creates a completely redundant copy of all raw data without any gain for the user. This problem was addressed by introducing the so-called "lazy" import of (A)SDM data. The development is not yet completely finished but is already available for ALMA interferometric data. The idea here is to also make the DATA column virtual and perform the translation from the (A)SDM format on-the-fly. This typically shrinks the MS by a factor 30 in data volume. Of course the (A)SDM raw data has to be kept on disk for access. Access speeds to a virtual DATA column are essentially the same as to a non-virtual one. They may even be a little faster since the (A)SDM data is better compressed.

MS v2.0 Layout

CASA uses the MeasurementSet Version 2 (A.J. Kemball and M.H. Wieringa, eds., 2000) as the internal working data format. The MeasurementSet set was orignially defined in AIPS++ Note 191 (Wieringa and Cornwell 1996). Reproduced below is the table structrue for the MeasurementSet as used by CASA.

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, and all defined sub-tables are tabulated below. Optional sub-tables are shown in italics and in parentheses.

Subtables
Table	Contents	Keys
ANTENNA	Antenna characteristics	ANTENNA_ID
DATA_DESCRIPTION	Data description	DATA_DESC_ID
(DOPPLER)	Doppler tracking	DOPPLER_ID, SOURCE_ID
FEED	Feed characteristics	FEED_ID, ANTENNA_ID, TIME, SPECTRAL_WINDOW_ID
FIELD	Field position	FIELD_ID
FLAG_CMD	Flag commands	TIME
(FREQ_OFFSET)	Frequency offset information	FEED_ID, ANTENNAn, FEED_ID, TIME, SPECTRAL_WINDOW_ID
HISTORY	History information	OBSERVATION_ID, TIME
OBSERVATION	Observer, Schedule, etc	OBSERVATION_ID
POINTING	Pointing information	ANTENNA_ID, TIME
POLARIZATION	Polarization setup	POLARIZATION_ID
PROCESSOR	Processor information	PROCESSOR_ID
(SOURCE)	Source information	SOURCE_ID, SPECTRAL_WINDOW_ID, TIME
SPECTRAL_WINDOW	Spectral window setups	SPECTRAL_WINDOW_ID
STATE	State information	STATE_ID
(SYSCAL)	System calibration characteristics	FEED_ID, ANTENNA_ID, TIME, SPECTRAL_WINDOW_ID
(WEATHER)	Weather info for each antenna	ANTENNA_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 FIELD, SPECTRAL_WINDOW, OBSERVATION and PROCESSOR. For the second, the key is a collection of parameters, usually including TIME. Examples are FEED, (SOURCE), (SYSCAL), and (WEATHER).

Note that all optional columns are indicated in italics and in parentheses.

MAIN table: Data, Coordinates and Flags

MAIN table: Data, coordinates and flags
Name	Format	Units	Measure	Comments
Columns
Keywords
MS_VERSION	Float			MS format version
(SORT_COLUMNS)	String			Sort columns
(SORT_ORDER)	String			Sort order
Key
TIME	Double	s	EPOCH	Integration midpoint
(TIME_EXTRA_PREC)	Double	s		extraTIME precision
ANTENNA1	Int			First antenna
ANTENNA2	Int			Second antenna
(ANTENNA3)	Int			Third antenna
FEED1	Int			Feed on ANTENNA1
FEED2	Int			Feed on ANTENNA2
(FEED3)	Int			Feed on ANTENNA3
DATA_DESC_ID	Int			Data desc. id.
PROCESSOR_ID	Int			Processor id.
(PHASE_ID)	Int			Phase id.
FIELD_ID	Int			Field id.
Non-key attributes
INTERVAL	Double	s		Sampling interval
EXPOSURE	Double	s		The effective integration time
TIME_CENTROID	Double	s	EPOCH	Time centroid
(PULSAR_BIN)	Int			Pulsar bin number
(PULSAR_GATE_ID)	Int			Pulsar gate id.
SCAN_NUMBER	Int			Scan number
ARRAY_ID	Int			Subarray number
OBSERVATION_ID	Int			Observation id.
STATE_ID	Int			State id.
(BASELINE_REF)	Bool			Reference antenna
UVW	Double(3)	m	UVW	UVW coordinates
(UVW2)	Double(3)	m	UVW	UVW (baseline 2)
Data
(DATA)	Complex(N_c, N_f)			Complex visibility matrix (synthesis arrays)
(FLOAT_DATA)	Float(N_c, N_f)			Float data matrix (single dish)
(VIDEO_POINT)	Complex(N_c)			Video point
(LAG_DATA)	Complex(N_c, N_l)			Correlation function
SIGMA	Float(N_c)			Estimated rms noise for single channel
(SIGMA_SPECTRUM)	Float(N_c, N_f^*)			Estimated rms noise
WEIGHT	Float(N_c)			Weight for whole data matrix
(WEIGHT_SPECTRUM)	Float(N_c, N_f^*)			Weight for each channel
Flag information
FLAG	Bool(N_c, N_f^*)			Cumulative data flags
FLAG_CATEGORY	Bool(N_c, N_f^*, N_cat)			Flag categories
FLAG_ROW	Bool			The row flag

Notes:: Note that N_l= number of lags, N_c= number of correlators, N_f= number of frequency channels, and N_cat= number of flag categories.
MS_VERSION: The MeasurementSet format revision number, expressed as ${major}_{revision}$ ${minor}_{revision}$. This version is 2.0.
SORT_COLUMNS: Sort indices, in the form ${index}_1$ ${index}_2$ $\cdots$, for the underlying MS. A string containing "NONE" reflects no sort order. An example might be SORT_COLUMNS="TIME ANTENNA1 ANTENNA2", to indicate sorting in in time-baseline order.
SORT_ORDER: Sort order as either "ASCENDING" or "DESCENDING".
TIME: Mid-point (not centroid) of data interval.
TIME_EXTRA_PREC: Extra time precision.
ANTENNAn: Antenna number (≥ 0), and a direct index into the ANTENNA sub-table rownr. For n > 2, triple-product data are implied.
FEEDn: Feed number ≥0). For n> 2, triple-product data are implied.
DATA_DESC_ID: Data description identifier (≥0), and a direct index into the DATA_DESCRIPTION sub-table rownr.
PROCESSOR_ID: Processor indentifier (≥0), and a direct index into the PROCESSOR sub-table rownr.
PHASE_ID: Switching phase identifier (≥0)
FIELD_ID: Field identifier (≥0).
INTERVAL: Data sampling interval. This is the nominal data interval and does not include the effects of bad data or partial integration.
EXPOSURE: Effective data interval, including bad data and partial averaging.
PULSAR_BIN: Pulsar bin number for the data record. Pulsar data may be measured for a limited number of pulse phase bins. The pulse phase bins are described in the PULSAR sub-table and indexed by this bin number.
PULSAR_GATE_ID: Pulsar gate identifier (≥0), and a direct index into the PULSAR_GATE sub-table rownr.
SCAN_NUMBER: Arbitrary scan number to identify data taken in the same logical scan. Not required to be unique.
ARRAY_ID: Subarray identifier (≥0), which identifies data in separate subarrays.
OBSERVATION_ID: Observation identifier (≥0), which identifies data from separate observations.
STATE_ID: State identifier (≥0), which identifies information relating to active reference signals or loads.
BASELINE_REF: Flag to indicate the original correlator reference antenna for baseline-based correlators (True for ANTENNA1; False for ANTENNA2).
UVW: uvw coordinates for the baseline from ANTENNE2 to ANTENNA1, i.e. the baseline is equal to the difference POSITION2 - POSITION1. The UVW given are for the TIME_CENTROID, and correspond in general to the reference type for the PHASE_DIR of the relevant field. I.e. J2000 if the phase reference direction is given in J2000 coordinates. However, any known reference is valid. Note that the choice of baseline direction and UVW definition (W towards source direction; V in plane through source and system's pole; U in direction of increasing longitude coordinate) also determines the sign of the phase of the recorded data.
UVW2: uvw coordinates for the baseline from ANTENNE3 to ANTENNA1 (triple-product data only), i.e. the baseline is equal to the difference POSITION3 - POSITION1. The UVW given are for the TIME_CENTROID, and correspond in general to the reference type for the PHASE_DIR of the relevant field. I.e. J2000 if the phase reference direction is given in J2000 coordinates. However, any known reference is valid. Note that the choice of baseline direction and UVW definition (W towards source direction; V in plane through source and system's pole; U in direction of increasing longitude coordinate) also determines the sign of the phase of the recorded data.
DATA, FLOAT_DATA, LAG_DATA: At least one of these columns should be present in a given MeasurementSet. In special cases one or more 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). If only correlation functions are stored in the MS, then N_f^* is the maximum number of lags (N_l) specified in the LAG table for this LAG_ID. If both correlation functions and frequency spectra are stored in the same MS, then N_f^* is the number of frequency channels, and the weight information refers to the frequency spectra only. The units for these columns (eg. 'Jy') specify whether the data are in flux density units or correlation coefficients.
VIDEO_POINT: The video point for the spectrum, to allow the full reverse transform.
SIGMA: The estimated rms noise for a single channel, for each correlator.
SIGMA_SPECTRUM: The estimated rms noise for each channel.
WEIGHT: The weight for the whole data matrix for each correlator, as assigned by the correlator or processor.
WEIGHT_SPECTRUM: The weight for each channel in the data matrix, as assigned by the correlator or processor. The weight spectrum should be used in preference to the WEIGHT, when available.
FLAG: An array of Boolean values with the same shape as DATA (see the DATA item above) representing the cumulative flags applying to this data matrix, as specified in FLAG_CATEGORY. Data are flagged bad if the FLAG array element is True.
FLAG_CATEGORY: An array of flag matrices with the same shape as DATA, but indexed by category. The category identifiers are specified by a keyword CATEGORY, containing an array of string identifiers, attached to the FLAG_CATEGORY column and thus shared by all rows in the MeasurementSet. The cumulative effect of these flags is reflected in column FLAG. Data are flagged bad if the FLAG array element is True. See Section 3.1.8 for further details.
FLAG_ROW: True if the entire row is flagged.

ANTENNA: Antenna Characteristics

ANTENNA: Antenna characteristics
Name	Format	Units	Measure	Comments
Columns
Data
NAME	String			Antenna name
STATION	String			Station name
TYPE	String			Antenna type
MOUNT	String			Mount type:alt-az, equatorial, X-Y, orbiting, bizarre
POSITION	Double(3)	m	POSITION	Antenna X,Y,Z phase reference positions
OFFSET	Double(3)	m	POSITION	Axes offset of mount to FEED REFERENCE point
DISH_DIAMETER	Double	m		Diameter of dish
(ORBIT_ID)	Int			Orbit id.
(MEAN_ORBIT)	Double(6)			Mean Keplerian elements
(PHASED_ARRAY_ID)	Int			Phased array id.
Flag information
FLAG_ROW	Bool			Row flag

Notes:

This sub-table contains the global antenna properties for each antenna in the MS. It is indexed directly from MAIN via ANTENNAn.

NAME

Antenna name (e.g. "NRAO_140")

STATION

Station name (e.g. "GREENBANK")

TYPE

Antenna type. Reserved keywords include: ("GROUND-BASED" - conventional antennas; "SPACE-BASED" - orbiting antennas; "TRACKING-STN" - tracking stations).

MOUNT

Mount type of the antenna. Reserved keywords include: ("EQUATORIAL" - equatorial mount; "ALT-AZ" - azimuth-elevation mount; "X-Y" - x-y mount; "SPACE-HALCA" - specific orientation model.)

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.

OFFSET

Axes offset of mount to feed reference point.

DISH_DIAMETER

Nominal diameter of dish, as opposed to the effective diameter.

ORBIT_ID

Orbit identifier. Index used in ORBIT sub-table if ANTENNA_TYPE is "SPACE_BASED".

MEAN_ORBIT

Mean Keplerian orbital elements, using the standard convention (Flatters 1998):

0: Semi-major axis of orbit (a) in m.
1: Ellipticity of orbit (e).
2: Inclination of orbit to the celestial equator (i) in deg.
3: Right ascension of the ascending node (Ω) in deg.
4: Argument of perigee (ω ) in deg.
5: Mean anomaly (M) in deg.

PHASED_ARRAY_ID

Phased array identifier. Points to a PHASED_ARRAY sub-table which points back to multiple entries in the ANTENNA sub-table and contains information on how they are combined.

FLAG_ROW

Boolean flag to indicate the validity of this entry. Set to True for an invalid row. This does not imply any flagging of the data in MAIN, but is necessary as the ANTENNA index in MAIN points directly into the ANTENNA sub-table. Thus FLAG_ROW can be used to delete an antenna entry without re-ordering the ANTENNA indices throughout the MS.

DATA_DESCRIPTION: Data Description Table

DATA_DESCRIPTION: Data description table
Name	Format	Units	Measure	Comments
Columns
Data
SPECTRAL_WINDOW_ID	Int			Spectral window id.
POLARIZATION_ID	Int			Polarization id.
(LAG_ID)	Int			Lag fn. id.
Flags
FLAG_ROW	Bool			Row flag.

Notes:: This table define the shape of the associated DATA array in MAIN, and in indexed directly by DATA_DESC_ID.
SPECTRAL_WINDOW_ID: Spectral window identifier.
POLARIZATION_ID: Polarization identifier (≥0); direct index into the POLARIZATION sub-table.
LAG_ID: Lag function identifier (≥0), and a direct index into the LAG sub-table rownr.
FLAG_ROW: True if the row does not contain valid data; does not imply flagging in MAIN.

DOPPLER: Doppler Tracking Information

DOPPLER: Doppler tracking information
Name	Format	Units	Measure	Comments
Columns
Key
DOPPLER_ID	Int			Doppler tracking id.
SOURCE_ID	Int			Source id.
Data
TRANSITION_ID	Int			Transition id.
VELDEF	Double	m/s	Doppler	Velocity definition of Doppler shift.

Notes:: This sub-table contains frame information for different Doppler tracking modes. It is indexed from the SPECTRAL_WINDOW_ID sub-table (with SOURCE_ID as a secondary index) and thus allows the specification of a source-dependent Doppler tracking reference for each SPECTRAL_WINDOW. This model allows multiple possible transitions per source per spectral window, but only one reference at any given time.
DOPPLER_ID: Doppler identifier, as used in the SPECTRAL_WINDOW sub-table.
SOURCE_ID: Source identifier (as used in the SOURCE sub-table).
TRANSITION_ID: This index selects the appropriate line from the list of transitions stored for each SOURCE_ID in the SOURCE table.
VELDEF: Velocity definition of the Doppler shift, e.g., RADIO or OPTICAL velocity in m/s.

FEED: Feed Characteristics

FEED: Feed characteristics
Name	Format	Units	Measure	Comments
Columns
Key
ANTENNA_ID	Int			Antenna id
FEED_ID	Int			Feed id
SPECTRAL_WINDOW_ID	Int			Spectral window id.
TIME	Double	s	EPOCH	Interval midpoint
INTERVAL	Double	s		Time interval
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).
(FOCUS_LENGTH)	Double	m		Focus length
(PHASED_FEED_ID)	Int			Phased feed
POLARIZATION_TYPE	String (NUM_RECEPTORS)			Type of polarization to which a given RECEPTOR responds.
POL_RESPONSE	Complex (NUM_RECEPTORS, NUM_RECEPTORS)			Feed polzn. response
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.

Notes:: A feed is a collecting element on an antenna, such as a single horn, that shares joint physical properties and makes sense to calibrate as a single entity. It is an abstraction of a generic antenna feed and is considered to have one or more RECEPTORs that respond to different polarization states. A FEED may have a time-variable beam and polarization response. Feeds are numbered from 0 on each separate antenna for each SPECTRAL_WINDOW_ID. Consequently, 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.
ANTENNA_ID: Antenna number, as indexed from ANTENNAn in MAIN.
FEED_ID: Feed identifier, as indexed from FEEDn in MAIN.
SPECTRAL_WINDOW_ID: Spectral window identifier. A value of -1 indicates the row is valid for all spectral windows.
TIME: Mid-point of time interval for which the feed parameters in this row are valid. The same Measure reference used for the TIME column in MAIN must be used.
INTERVAL: Time interval.
NUM_RECEPTORS: Number of receptors on this feed. See POLARIZATION_TYPE for further information.
BEAM_ID: Beam identifier. Points to an optional BEAM sub-table defining the primary beam and polarization response for this FEED. A value of -1 indicates that no associated beam response is defined.
BEAM_OFFSET: Beam position offset, as defined on the sky but in the antenna reference frame.
FOCUS_LENGTH: Focus length. As defined along the optical axis of the antenna.
PHASED_FEED_ID: Phased feed identifier. Points to a PHASED_FEED sub-table which in turn points back to multiple entries in the FEED table, and specifies the manner in which they are combined.
POLARIZATION_TYPE: Polarization type to which each receptor responds (e.g. "R","L","X" or "Y"). This is the receptor polarization type as recorded in the final correlated data (e.g. "RR"); i.e. as measured after all polarization combiners.
POL_RESPONSE: Polarization response at the center of the beam for this feed. Expressed in a linearly polarized basis ($ \bf\vec e_x$, $ \bf\vec e_y$) using the IEEE convention.
POSITION: Offset of feed relative to the feed reference position for this antenna (see ANTENNA sub-table).
RECEPTOR_ANGLE: Polarization reference angle. Converts into parallactic angle in the sky domain.

FIELD: Field Positions for Each Source

FIELD: Field positions for each source
Name	Format	Units	Measure	Comments
Columns
Key
Data
NAME	String			Name of field
CODE	String			Special characteristics of field
TIME	Double	s	EPOCH	Time origin for the directions and rates
NUM_POLY	Int			Series order
DELAY_DIR	Double(2, NUM_POLY+1)	rad	DIRECTION	Direction of delay center.
PHASE_DIR	Double(2, NUM_POLY+1)	rad	DIRECTION	Phase center.
REFERENCE_DIR	Double(2, NUM_POLY+1)	rad	DIRECTION	Reference center
SOURCE_ID	Int			Index in Source table
(EPHEMERIS_ID)	Int			Ephemeris id.
Flags
FLAG_ROW	Bool			Row flag

Notes:: The FIELD table defines a field position on the sky. For interferometers, this is the correlated field position. For single dishes, this is the nominal pointing direction.
NAME: Field name; user specified.
CODE: Field code indicating special characteristics of the field; user specified.
TIME: Time reference for the directions and rates. Required to use the same TIME Measure reference as in MAIN.
NUM_POLY: Series order for the *_DIR columns.
DELAY_DIR: Direction of delay center; can be expressed as a polynomial in time. Final result converted to the defined Direction Measure type.
PHASE_DIR: Direction of phase center; can be expressed as a polynomial in time. Final result converted to the defined Direction Measure type.
REFERENCE_DIR: Reference center; can be expressed as a polynomial in time. Final result converted to the defined Direction Measure type. Used in single-dish to record the associated reference direction if position-switching has already been applied. For interferometric data, this is the original correlated field center, and may equal DELAY_DIR or PHASE_DIR.
SOURCE_ID: Points to an entry in the optional SOURCE subtable, a value of -1 indicates there is no corresponding source defined.
EPHEMERIS_ID: Points to an entry in the EPHEMERIS sub-table, which defines the ephemeris used to compute the field position. Useful for moving, near-field objects, where the ephemeris may be revised over time.
FLAG_ROW: True if data in this row are invalid, else False. Does not imply flagging in MAIN.

FLAG_CMD: Flag Commands

FLAG_CMD: Flag commands
Name	Format	Units	Measure	Comments
Columns
Key
TIME	Double	s	EPOCH	Mid-point of interval
INTERVAL	Double	s		Time interval
Data
TYPE	String			FLAG or UNFLAG
REASON	String			Flag reason
LEVEL	Int			Flag level
SEVERITY	Int			Severity code
APPLIED	Bool			True if applied in MAIN
COMMAND	String			Flag command

Notes:: The FLAG_CMD sub-table defines global flagging commands which apply to the data in MAIN, as described in Section 3.1.8.
TIME: Mid-point of the time interval to which this flagging command applies. Required to use the same TIME Measure reference as used in MAIN.
INTERVAL: Time interval.
TYPE: Type of flag command, representing either a flagging ("FLAG") or un-flagging ("UNFLAG") operation.
REASON: Flag reason; user specified.
LEVEL: Flag level (≥0); reflects different revisions of flags which have the same REASON.
SEVERITY: Severity code for the flag, on a scale of 0-10 in order of increasing severity; user specified.
APPLIED: True if this flag has been applied to MAIN, and update in FLAG_CATEGORY and FLAG. False if this flag has not been applied to MAIN.
COMMAND: Global flag command, expressed in the standard syntax for data selection, as adopted within the project as a whole.

FREQ_OFFSET: Frequency Offset Information

FREQ_OFFSET: Frequency offset information
Name	Format	Units	Measure	Comments
Columns
Key
ANTENNA1	Int			Antenna 1.
ANTENNA2	Int			Antenna 2.
FEED_ID	Int			Feed id.
SPECTRAL_WINDOW_ID	Int			Spectral window id.
TIME	Double	s	EPOCH	Interval midpoint
INTERVAL	Double	s		Time interval
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 labeled as the same SPECTRAL_WINDOW_ID, and calibrated together if required.
ANTENNAn: Antenna identifier, as indexed from ANTENNAn in MAIN.
FEED_ID: Antenna identifier, as indexed from FEEDn in MAIN.
SPECTRAL_WINDOW_ID: Spectral window identifier.
TIME: Mid-point of the time interval for which this offset is valid. Required to use the same TIME Measure reference as used in MAIN.
INTERVAL: Time interval.
OFFSET: Frequency offset to be added 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.

HISTORY: History Information

HISTORY: History information
Name	Format	Units	Measure	Comments
Columns
Key
TIME	Double	s	EPOCH	Time-stamp for message
OBSERVATION_ID	Int			Points to OBSERVATION table
Data
MESSAGE	String			Log message
PRIORITY	String			Message priority
ORIGIN	String			Code origin
OBJECT_ID	String			Originating ObjectID
APPLICATION	String			Application name
CLI_COMMAND	String(*)			CLI command sequence
APP_PARAMS	String(*)			Application paramters

Notes:: This sub-table contains associated history information for the MS.
TIME: Time-stamp for the history record. Required to have the same TIME Measure reference as used in MAIN.
OBSERVATION_ID: Observation identifier (see the OBSERVATION table)
MESSAGE: Log message.
PRIORITY: Message priority, with allowed types: ("DEBUGGING", "WARN", "NORMAL", or "SEVERE").
ORIGIN: Source code origin from which message originated.
OBJECT_ID: Originating ObjectID, if available, else blank.
APPLICATION: Application name.
CLI_COMMAND: CLI command sequence invoking the application.
APP_PARAMS: Application parameter values, in the adopted project-wide format.

OBSERVATION: Observation Information

OBSERVATION: Observation information
Name	Format	Units	Measure	Comments
Columns
Data
TELESCOPE_NAME	String			Telescope name
TIME_RANGE	Double(2)	s	EPOCH	Start, end times
OBSERVER	String			Name of observer(s)
LOG	String(*)			Observing log
SCHEDULE_TYPE	String			Schedule type
SCHEDULE	String(*)			Project schedule
PROJECT	String			Project identification string.
RELEASE_DATE	Double	s	EPOCH	Target release date
Flags
FLAG_ROW	Bool			Row flag.

Notes:: This table contains information specifying the observing instrument or epoch. See the discussion in Section 3.3 for details. It is indexed directly from MAIN via OBSERVATION_ID.
TELESCOPE_NAME: Telescope name (e.g. "WSRT" or "VLBA").
TIME_RANGE: The start and end times of the overall observing period spanned by the actual recorded data in MAIN. Required to use the same TIME Measure reference as in MAIN.
OBSERVER: The name(s) of the observer(s).
LOG: The observing log, as supplied by the telescope or instrument.
SCHEDULE_TYPE: The schedule type, with current reserved types ("VLBA-CRD", "VEX", "WSRT", "ATNF").
SCHEDULE: Unmodified schedule file, of the type specified, and as used by the instrument.
PROJECT: Project code (e.g. "BD46")
RELEASE_DATE: Project release date. This is the date on which the data may become public.
FLAG_ROW: Row flag. True if data in this row is invalid, but does not imply any flagging in MAIN.

POINTING: Antenna Pointing Information

POINTING: Antenna pointing information
Name	Format	Units	Measure	Comments
Columns
Key
ANTENNA_ID	Int			Antenna id.
TIME	Double	s	EPOCH	Interval midpoint
INTERVAL	Double	s		Time interval
Data
NAME	String			Pointing position desc.
NUM_POLY	Int			Series order
TIME_ORIGIN	Double	s	EPOCH	Origin for the polynomial
DIRECTION	Double(2, NUM_POLY+1)	rad	DIRECTION	Antenna pointing direction
TARGET	Double(2, NUM_POLY+1)	rad	DIRECTION	Target direction
(POINTING_OFFSET)	Double(2, NUM_POLY+1)	rad	DIRECTION	A priori pointing correction
(SOURCE_OFFSET)	Double(2, NUM_POLY+1)	rad	DIRECTION	Offset from source
(ENCODER)	Double(2)	rad	DIRECTION	Encoder values
(POINTING_MODEL_ID)	Int			Pointing model id.
TRACKING	Bool			True if on-position
(ON_SOURCE)	Bool			True if on-source
(OVER_THE_TOP)	Bool			True if over the top

Notes:: This table contains information concerning the primary pointing direction of each antenna as a function of time. Note that the pointing offsets for inidividual feeds on a given antenna are specified in the FEED sub-table with respect to this pointing direction.
ANTENNA_ID: Antenna identifier, as specified by ANTENNAn in MAIN.
TIME: Mid-point of the time interval for which the information in this row is valid. Required to use the same TIME Measure reference as in MAIN.
INTERVAL: Time interval.
NAME: Pointing direction name; user specified.
NUM_POLY: Series order for the polynomial expressions in DIRECTION and POINTING_OFFSET.
TIME_ORIGIN: Time origin for the polynomial expansions.
DIRECTION: Antenna pointing direction, optionally expressed as polynomial coefficients. The final result is interpreted as a Direction Measure using the specified Measure reference.
TARGET: Target pointing direction, optionally expressed as polynomial coefficients. The final result is interpreted as a Direction Measure using the specified Measure reference. This is the true expected position of the source, including all coordinate corrections such as precession, nutation etc.
POINTING_OFFSET: The a priori pointing corrections applied by the telescope in pointing to the DIRECTION position, optionally expressed as polynomial coefficients. The final result is interpreted as a Direction Measure using the specified Measure reference.
SOURCE_OFFSET: The commanded offset from the source position, if offset pointing is being used.
ENCODER: The current encoder values on the primary axes of the mount type for the antenna, expressed as a Direction Measure.
TRACKING: True if tracking the nominal pointing position.
ON-SOURCE: True if the nominal pointing direction coincides with the source, i.e. offset-pointing is not being used.
OVER-THE-TOP: True if the antenna was driven to this position "over the top" (az-el mount).

POLARIZATION: Polarization Setup Information

POLARIZATION: Polarization setup information
Name	Format	Units	Measure	Comments
Columns
Data description columns
NUM_CORR	Int			# correlations
Data
CORR_TYPE	Int(NUM_CORR)			Polarization of correlation
CORR_PRODUCT	Int(2, NUM_CORR)			Receptor cross-products
Flags
FLAG_ROW	Bool			Row flag

Notes:: This table defines the polarization labeling of the DATA array in MAIN, and is directly indexed from the DATA_DESCRIPTION table via POLARIZATION_ID.
NUM_CORR: The number of correlation polarization products. For example, for (RR) this value would be 1, for (RR, LL) it would be 2, and for (XX,YY,XY,YX) it would be 4, etc.
CORR_TYPE: An integer for each correlation product indicating the Stokes type as defined in the Stokes class enumeration.
CORR_PRODUCT: Pair of integers for each correlation product, specifying the receptors from which the signal originated. The receptor polarization is defined in the POLARIZATION_TYPE column in the FEED table. An example would be (0,0), (0,1), (1,0), (1,1) to specify all correlations between two receptors.
FLAG_ROW: Row flag. True is the data in this row are not valid, but does not imply the flagging of any DATA in MAIN.

PROCESSOR: Processor Information

PROCESSOR: Processor information
Name	Format	Units	Measure	Comments
Columns
Data
TYPE	String			Processor type
SUB_TYPE	String			Processor sub-type
TYPE_ID	Int			Processor type id.
MODE_ID	Int			Processor mode id.
(PASS_ID)	Int			Processor pass number
Flags
FLAG_ROW	Bool			Row flag

Notes:: This table holds summary information for the back-end processing device used to generate the basic data in the MAIN table. Such devices include correlators, radiometers, spectrometers, pulsar-timers, amongst others. See Section 4.0.4 for further details.
TYPE: Processor type; reserved keywords include ("CORRELATOR" - interferometric correlator; "SPECTROMETER" - single-dish correlator; "RADIOMETER" - generic detector/integrator; "PULSAR-TIMER" - pulsar timing device).
SUB_TYPE: Processor sub-type, e.g. "GBT" or "JIVE".
TYPE_ID: Index used in a specialized sub-table named as subtype_type, which contains time-independent processor information applicable to the current data record (e.g. a JIVE_CORRELATOR sub-table). Time-dependent information for each device family is contained in other tables, dependent on the device type.
MODE_ID: Index used in a specialized sub-table named as subtype_type_mode, containing information on the processor mode applicable to the current data record. (e.g. a GBT_SPECTROMETER_MODE sub-table).
PASS_ID: Pass identifier; this is used to distinguish data records produced by multiple passes through the same device, where this is possible (e.g. VLBI correlators). Used as an index into the associated table containing pass information.
FLAG_ROW: Row flag. True if data in the row is not valid, but does not imply flagging in MAIN.

SOURCE: Source Information

SOURCE: Source information
Name	Format	Units	Measure	Comments
Columns
Key
SOURCE_ID	Int			Source id
TIME	Double	s	EPOCH	Midpoint of time for which this set of parameters is accurate
INTERVAL	Double	s		Interval
SPECTRAL_WINDOW_ID	Int			Spectral Window id
Data description
NUM_LINES	Int			Number of spectral lines
Data
NAME	String			Name of source as given during observations
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)
(POSITION)	Double(3)	m	POSITION	Position (e.g. for solar system objects)
PROPER_MOTION	Double(2)	rad/s		Proper motion
(TRANSITION)	String(NUM_LINES)			Transition name
(REST_FREQUENCY)	Double(NUM_LINES)	Hz	FREQUENCY	Line rest frequency
(SYSVEL)	Double(NUM_LINES)	m/s	RADIAL VELOCITY	Systemic velocity at reference
(SOURCE_MODEL)	TableRecord			Default csm
(PULSAR_ID)	Int			Pulsar id.

Notes:: This table contains time-variable source information, optionally associated with a given FIELD_ID.
SOURCE_ID: Source identifier (≥ 0), as specified in the FIELD sub-table.
TIME: Mid-point of the time interval for which the data in this row is valid. Required to use the same TIME Measure reference as in MAIN.
INTERVAL: Time interval.
SPECTRAL_WINDOW_ID: Spectral window identifier. A -1 indicates that the row is valid for all spectral windows.
NUM_LINES: Number of spectral line transitions associated with this source and spectral window id. combination.
NAME: Source name; user specified.
CALIBRATION_GROUP: Calibration group number to which this source belongs; user specified.
CODE: Source code, used to describe any special characteristics f the source, such as the nature of a calibrator. Reserved keyword, including ("BANDPASS CAL").
DIRECTION: Source direction at this TIME.
POSITION: Source position (x, y, z) at this TIME (for near-field objects).
PROPER_MOTION: Source proper motion at this TIME.
TRANSITION: Transition names applicable for this spectral window (e.g. "v=1, J=1-0, SiO").
REST_FREQUENCY: Rest frequencies for the transitions.
SYSVEL: Systemic velocity for each transition.
SOURCE_MODEL: Reference to an assigned component source model table.
PULSAR_ID: An index used in the PULSAR sub-table to define further pulsar-specific properties if the source is a pulsar.

SPECTRAL_WINDOW: Spectral Window Description

SPECTRAL_WINDOW: Spectral window description
Name	Format	Units	Measure	Comments
Columns
Data description columns
NUM_CHAN	Int			# spectral channels
Data
NAME	String			Spectral window name
REF_FREQUENCY	Double	Hz	FREQUENCY	The reference frequency.
CHAN_FREQ	Double(NUM_CHAN)	Hz	FREQUENCY	Center frequencies for each channel in the data matrix.
CHAN_WIDTH	Double(NUM_CHAN)	Hz		Channel width for each channel in the data matrix.
MEAS_FREQ_REF	Int			FREQUENCY Measure ref.
EFFECTIVE_BW	Double(NUM_CHAN)	Hz		The effective noise bandwidth of each spectral channel
RESOLUTION	Double(NUM_CHAN)	Hz		The effective spectral resolution of each channel
TOTAL_BANDWIDTH	Double	Hz		total bandwidth for this window
NET_SIDEBAND	Int			Net sideband
(BBC_NO)	Int			Baseband converter no.
(BBC_SIDEBAND)	Int			BBC sideband
IF_CONV_CHAIN	Int			The IF conversion chain
(RECEIVER_ID)	Int			Receiver id.
FREQ_GROUP	Int			Frequency group
FREQ_GROUP_NAME	String			Freq. group name
(DOPPLER_ID)	Int			Doppler id.
(ASSOC_SPW_ID)	Int(*)			Associated spw_id.
(ASSOC_NATURE)	String(*)			Nature of association
Flags
FLAG_ROW	Bool

Notes:: This table describes properties for each defined spectral window. A spectral window is both a frequency label for the associated DATA array in MAIN, but also represents a generic frequency conversion chain that shares joint physical properties and makes sense to calibrate as a single entity.
NUM_CHAN: Number of spectral channels.
NAME: Spectral window name; user specified.
REF_FREQUENCY: The reference frequency. A frequency representative of this spectral window, usually the sky frequency corresponding to the DC edge of the baseband. Used by the calibration system if a fixed scaling frequency is required or in algorithms to identify the observing band.
CHAN_FREQ: Center frequencies for each channel in the data matrix. These can be frequency-dependent, to accommodate instruments such as acousto-optical spectrometers. Note that the channel frequencies may be in ascending or descending frequency order.
CHAN_WIDTH: Nomical channel width of each spectral channel. Although these can be derived from CHAN_FREQ by differencing, it is more efficient to keep a separate reference to this information.
MEAS_FREQ_REF: Frequency Measure reference for CHAN_FREQ. This allows a row-based reference for this column in order to optimize the choice of Measure reference when Doppler tracking is used. Modified only by the MS access code.
EFFECTIVE_BW: The effective noise bandwidth of each spectral channel.
RESOLUTION: The effective spectral resolution of each channel.
TOTAL_BANDWIDTH: The total bandwidth for this spectral window.
NET_SIDEBAND: The net sideband for this spectral window.
BBC_NO: The baseband converter number, if applicable.
BBC_SIDEBAND: The baseband converter sideband, is applicable.
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)
RECEIVER_ID: Index used to identify the receiver associated with the spectral window. Further state information is planned to be stored in a RECEIVER sub-table.
FREQ_GROUP: The frequency group to which the spectral window belongs. This is used to associate spectral windows for joint calibration purposes.
FREQ_GROUP_NAME: The frequency group name; user specified.
DOPPLER_ID: The Doppler identifier defining frame information for this spectral window.
ASSOC_SPW_ID: Associated spectral windows, which are related in some fashion (e.g. "channel-zero").
ASSOC_NATURE: Nature of the association for ASSOC_SPW_ID; reserved keywords are ("CHANNEL-ZERO" - channel zero; "EQUAL-FREQUENCY" - same frequency labels; "SUBSET" - narrow-band subset).
FLAG_ROW: True if the row does not contain valid data.

STATE: State Information

STATE: State information
Name	Format	Units	Measure	Comments
Columns
Data
SIG	Bool			Signal
REF	Bool			Reference
CAL	Double	K		Noise calibration
LOAD	Double	K		Load temperature
SUB_SCAN	Int			Sub-scan number
OBS_MODE	String			Observing mode
Flags
FLAG_ROW	Bool			Row flag

Notes:: This table defines the state parameters for a particular data record as they refer to external loads, calibration sources or references, and also characterizes the observing mode of the data record, as an aid to defining the scheduling heuristics. It is indexed directly via STATE_ID in MAIN.
SIG: True if the source signal is being observed.
REF: True for a reference phase.
CAL: Noise calibration temperature (zero if not added).
LOAD: Load temperature (zero if no load).
SUB_SCAN: Sub-scan number (≥ 0), relative to the SCAN_NUMBER in MAIN. Used to identify observing sequences.
OBS_MODE: Observing mode; defined by a set of reserved keywords characterizing the current observing mode (e.g. "OFF-SPECTRUM"). Used to define the schedule strategy.
FLAG_ROW: True if the row does not contain valid data. Does not imply flagging in MAIN.

SYSCAL: System Calibration

SYSCAL: System calibration
Name	Format	Units	Measure	Comments
Columns
Key
ANTENNA_ID	Int			Antenna id
FEED_ID	Int			Feed id
SPECTRAL_WINDOW_ID	Int			Spectral window id
TIME	Double	s	EPOCH	Midpoint of time for which this set of parameters is accurate
INTERVAL	Double	s		Interval
Data
(PHASE_DIFF)	Float	rad		Phase difference between receptor 0 and receptor 1
(TCAL)	Float (N_r)	K		Calibration temp
(TRX)	Float (N_r)	K		Receiver temperature
(TSKY)	Float (N_r)	K		Sky temperature
(TSYS)	Float (N_r)	K		System temp
(TANT)	Float (N_r)	K		Antenna temperature
(TANT_TSYS)	Float(N_r)			$ {{T_{ant}}\over{T_{sys}}}$
(TCAL_SPECTRUM)	Float (N_r, N_f)	K		Calibration temp
(TRX_SPECTRUM)	Float (N_r, N_f)	K		Receiver temperature
(TSKY_SPECTRUM)	Float (N_r, N_f)	K		Sky temperature spectrum
(TSYS_SPECTRUM)	Float (N_r, N_f)	K		System temp
(TANT_SPECTRUM)	Float (N_r, N_f)	K		Antenna temperature spectrum
(TANT_TSYS_SPECTRUM)	Float (N_r,N_f)			$ {{T_{ant}}\over{T_{sys}}}$ spectrum
Flags
(PHASE_DIFF_FLAG)	Bool			Flag for PHASE_DIFF
(TCAL_FLAG)	Bool			Flag for TCAL
(TRX_FLAG)	Bool			Flag for TRX
(TSKY_FLAG)	Bool			Flag for TSKY
(TSYS_FLAG)	Bool			Flag for TSYS
(TANT_FLAG)	Bool			Flag for TANT
(TANT_TSYS_FLAG)	Bool			Flag for ${{T_{ant}}\over{T_{sys}}}$

Notes:: This table contains time-variable calibration measurements for each antenna, as indexed on feed and spectral window. Note that N_r= number of receptors, and N_f= number of frequency channels.
ANTENNA_ID: Antenna identifier, as indexed by ANTENNAn in MAIN.
FEED_ID: Feed identifier, as indexed by FEEDn in MAIN.
SPECTRAL_WINDOW_ID: Spectral window identifier.
TIME: Mid-point of the time interval for which the data in this row are valid. Required to use the same TIME Measure reference as that in MAIN.
INTERVAL: Time interval.
PHASE_DIFF: Phase difference between receptor 0 and receptor 1.
TCAL: Calibration temperature.
TRX: Receiver temperature.
TSKY: Sky temperature.
TSYS: System temperature.
TANT: Antenna temperature.
TANT_TSYS: Antenna temperature over system temperature.
TCAL_SPECTRUM: Calibration temperature spectrum.
TRX_SPECTRUM: Receiver temperature spectrum.
TSKY_SPECTRUM: Sky temperature spectrum.
TSYS_SPECTRUM: System temperature spectrum.
TANT_SPECTRUM: Antenna temperature spectrum.
TANT_TSYS_SPECTRUM: Antenna temperature over system temperature spectrum.
PHASE_DIFF_FLAG: True if PHASE_DIFF flagged.
TCAL_FLAG: True if TCAL flagged.
TRX_FLAG: True if TRX flagged.
TSKY_FLAG: True if TSKY flagged.
TSYS_FLAG: True if TSYS flagged.
TANT_FLAG: True if TANT flagged.
TANT_TSYS_FLAG: True if TANT_TSYS flagged.

WEATHER: Weather Station Information

WEATHER: weather station information
Name	Format	Units	Measure	Comments
Columns
Key
ANTENNA_ID	Int			Antenna number
TIME	Double	s	EPOCH	Mid-point of interval
INTERVAL	Double	s		Interval over which data is relevant
Data
(H2O)	Float	m^-2		Average column density of water
(IONOS_ELECTRON)	Float	m^-2		Average column density of electrons
(PRESSURE)	Float	hPa		Ambient atmospheric pressure
(REL_HUMIDITY)	Float			Ambient relative humidity
(TEMPERATURE)	Float	K		Ambient air temperature for an antenna
(DEW_POINT)	Float	K		Dew point
(WIND_DIRECTION)	Float	rad		Average wind direction
(WIND_SPEED)	Float	m/s		Average wind speed
Flags
(H2O_FLAG)	Bool			Flag for H2O
(IONOS_ELECTRON_FLAG)	Bool			Flag for IONOS_ELECTRON
(PRESSURE_FLAG)	Bool			Flag for PRESSURE
(REL_HUMIDITY_FLAG)	Bool			Flag for REL_HUMIDITY
(TEMPERATURE_FLAG)	Bool			Flag for TEMPERATURE
(DEW_POINT_FLAG)	Bool			Flag for DEW_POINT
(WIND_DIRECTION_FLAG)	Bool			Flag for WIND_DIRECTION
(WIND_SPEED_FLAG)	Bool			Flag for WIND_SPEED

Notes:: This table contains mean external atmosphere and weather information.
ANTENNA_ID: Antenna identifier, as indexed by ANTENNAn from MAIN.
TIME: Mid-point of the time interval over which the data in the row are valid. Required to use the same TIME Measure reference as in MAIN.
INTERVAL: Time interval.
H2O: Average column density of water.
IONOS_ELECTRON: Average column density of electrons.
PRESSURE: Ambient atmospheric pressure.
REL_HUMIDITY: Ambient relative humidity.
TEMPERATURE: Ambient air temperature.
DEW_POINT: Dew point temperature.
WIND_DIRECTION: Average wind direction.
WIND_SPEED: Average wind speed.
H2O_FLAG: Flag for H2O.
IONOS_ELECTRON_FLAG: Flag for IONOS_ELECTRON.
PRESSURE_FLAG: Flag for PRESSURE.
REL_HUMIDITY_FLAG: Flag for REL_HUMIDITY.
TEMPERATURE_FLAG: Flag for TEMPERATURE.
DEW_POINT_FLAG: Flag for DEW_POINT.
WIND_DIRECTION_FLAG: Flag for DEW_POINT.
WIND_SPEED_FLAG: Flag for DEW_POINT.

Citation Number	1
Citation Text	Kemball & Wieringa 2000