A pdf version of this note is available.

Contents

1 Introduction

The MeasurementSet (MS) defines a format in which interferometer visibilities and single-dish data are stored. It is implemented in software packages using casa/casacore code. Version 2.0 (Note 229; Kemball and Wieringa 2000) of the MeasurementSet has been in use since 2000 in different packages and at different telescopes. This revision is to correct or help in common usages by adding missing data that was felt needed for data reduction. In V2.0 we feel that sometimes schema structure was defined to help with efficiency issues that was forseen (e.g definition of a FLOAT_DATA column for single dish). In this revision we try to not do that. We realize that a given data structure can be implemented in different ways in software to achieve maximum efficiency or speed. This is the reason why we have tried to keep the information redundancy to a minimum and focussed on completeness(although we may attach an addendum in the future, to this document, to describe implementation choices especially with regards to IO rates and large data issues) .

The changes are not backward compatible; so converters will be needed between v2.0 and v3.0 and vice versa. Some of the changes are to help common needs and complaints like column versioning (removing the need of CORRECTED_DATA or FLAG_VERSION) others are expansion for VLBI and non-dish arrays (e.g LOFAR).

Here are some of the major changes from MS V2.0 to MS V3.0:

• Explicit keys One of the major complaints of ms v2.0 is the use of row ids in subtables as implicit keys. In v3.0 we are moving to explicit keys; e.g ANTENNA subtable will have a column ANTENNA_ID so that when making a subset MS re-indexing is not needed in the main table. This makes it easier to make a subset of an MS without the need to reindex the data in the main table or any subtables.
• Single-dish processing: FLOAT_DATA is replaced by DATA unifying it with interferometer. It is left to the software column implementation to store floats efficiently rather than have multiple columns defined for different data types. We do not need to define in the schematic how the data is to be stored and accessed for efficiency purposes. The implementation can decide to compress data or keep as float or integers but on request a given DATA column cell, for e.g, single precision complex numbers are served.
• Synthesis calibration: The need for calibrations table as part the MeasurementSet has been felt by several telescopes and those using v2.0 (e.g ALMA and EVLA) have been using non standard tables to carry calibration type information.
• Data,weight, flag versioning Non standard columns (e.g CORRECTED_DATA) or kludges have been used by casa (for e.g) to deal with having version of data, flag or weight. These three columns have the same shapes in every row of an MS V3.0. Whenever changes are made, e.g. in flagging or calibration, and a new version is needed a new triplet of columns will be made. It is left to software implementation to do the smart thing (not making unnecessary copies). E.g if flagging only is done and need to be saved in a new version of FLAG column, the new version of DATA an WEIGHT are just virtual columns pointing back to the previous version.
• Data DescriptionThe concept of data description is being deprecated. It saved a column of integers but users found it confusing. SPECTRAL_WINDOW_ID and POLARIZATION_ID are explicit columns in the main table.
• VLBI data reduction: From v2.0 added an optional subtable INTERFEROMETER_MODEL
• Phased Array interferometers: A new optional subtable, PHASED_ARRAY, which will be defining which antennas form a phased array station.
• BEAM, EPHEMERIDES and SCAN subtables: In MS V2.0 the BEAM (BEAM_ID), EPHEMERIDES (EPHEMERIS_ID) and SCAN (SCAN_NUMBER) subtables are referred to but were not defined. Here we explicitly define these optional subtables.

Some features and reasons for some changes made and not made:

• Changes: The changes proposed here are designed to be as incremental as possible and taking into account what usage at different telescopes and software packages has found lacking.
• Compatibility: Some of the changes proposed here are not backwards compatible but converters can be written to move data between version 2.0 and 3.0. As there is more information in version 3.0 going to version 2.0 may be lossy.
• Calibration information: Many of the direct and peripheral information stored at data collection can be stored as calibration terms (Jones or Mueller matrices). Therefore MS v3.0 will carry in its definition optional calibration tables which may carry monitoring information that can be used to modify the data, weights and flags. The calibration tables definition will be added to this document once it is finalized
• Storage: A future document will provide results and suggestion for data storage structures for some commonly used access patterns and storage system in use.
• Multi measurement sets: The reason for Multi Measurement Sets (MMS) was to bypass an issue in the impementation in parallel access subsection of an ms could not be locked separately. We therefore postpone the need to define the MMS format if no implementation of multi-lockable Measurementsets is do-able.

2 Summary of changes

This section contains a description of the changes proposed for each table in the MS definition. A full definition of the v3.0 MS format is given in Section 5.

All the subtables not mentioned here will have an explicit ID column; in v3.0 implicit row number being an ID is no longer valid. For example ANTENNA subtable will have a ANTENNA_ID column added as a key.

2.1 MAIN table

• removal of FLAG_CATEGORY

  This column has not been used effectively. With the versioning of FLAG below it is redundant.

• Removal of SIGMA WEIGHT being always considered to be $\frac{1}{{\sigma }^2}$ this a redundant column.
• Removal of FLAG_ROW The purpose of this column was to avoid having to renumber rows when e.g. removing an antenna. This is not necessary with explicit row numbers anymore.
• Redefinition of WEIGHT to be WEIGHT_SPECTRUM With most of contemporary inteferometers being spectral machines with possibility of distinctive weights per channel, usage of MSv2.0 WEIGHT is low w.r.t WEIGHT_SPECTRUM. In this version WEIGHT column will have the same shape as the data.
• Versioning of (DATA, WEIGHT, FLAG) triplet These three columns will have the same shapes. There may be many versions them in an MS. But the active version will be refered to as DATA, WEIGHT and FLAG. The versions need not be copies. For e.g if only DATA is corrected and a new version is made. The active DATA will refer to DATA1 column while the active WEIGHT, FLAG (thus WEIGHT1 and FLAG1) will refer to the to WEIGHT0 and FLAG0 respectively. We leave it to the software to implement these in the most optimal fashion without making duplicate copies. This removes the need for non-standard column like CORRECTED_DATA or FLAG_VERSION etc.
• Merging of DATA and FLOAT_DATA It is unnecessary to have explicit DATA and FLOAT_DATA. The software implementation can do the optimal storage in the presence of float data only and serve it as complex.
• Replacement of DATA_DESC_ID This unique ID has caused more confusion than the extra column of integers is worth. In this version we are reverting to having explicit POLARIZATION_ID and SPECTRAL_WINDOW_ID which refers to keys in the POLARIZATION and SPECTRAL_WINDOW subtables directly. DATA_DESC_ID column is removed.

2.2 ARRAY table

• ARRAY_ID An explicit ID column is newly defined.
• ARRAY_CENTER A new column which will give the full Measures position of where the array center is assumed for each ARRAY_ID

2.3 FIELD table

Changes applicable to the FIELD table are discussed in this section.

An explicit FIELD_ID

2.3.1 Direction information

The PHASE_DIR information is to be derived from the ephemeris subtable if the EPHEMERIS_ID is not -1.

A new column optional DURATION column is added; when present in combination with TIME (time origin) it defines the time range for when the information for direction for a given FIELD_ID. In this fashion step function phasecentering that happens in some correlators

2.4 FLAG_CMD

FLAG_CMD table is made an optional subtable.

2.5 OBSERVATION table

Apart from the explicit OBSERVATION_ID column version 3.0 adds the ARRAY_CENTER: A new column which will give the full Measures position of where the array center is assumed for each ARRAY_ID

2.6 SOURCE table

The SOURCE table already had a SOURCE_ID column. In this version it it proposed to generalize the SOURCE_MODEL. Apart from a TableRecord it can take a URL which will point to catalogue style source list or even images representing the model associated with a SOURCE_ID and SPECTRAL_WINDOW_ID.

2.7 SPECTRAL_WINDOW table

Apart from the explicit SPECTRAL_WINDOW_ID column; an optional column LO_FREQUENCY which (can be vector of frequencies) gives the Local Oscillator frequencies in the chain of frequency conversion.

3 New sub-tables

New sub-tables added to MS v3.0 are included in this Section.

3.0.1 BEAM subtable

This table was proposed in v2.0 but we explicitly define it here. It is referenced from the FEED table or from the newly proposed PHASED_ARRAY table here.

3.0.2 EPHEMERIDES

This optional table is explicitly defined and is referred to from the FIELD table via the EPHEMERIS_ID column.

3.1 INTERFEROMETER_MODEL

This optional subtable contains information necessary for VLBI arrays

3.1.1 Multi element station based antenna (PHASED_ARRAY subtable)

Information about elements that makes a station antenna, combination scheme etc

3.1.2 SCAN subtable

This will carry the information about the scan intent and information necessary to tie back information from the online system

3.1.3 Associated tables (CAL_TABLES)

A lot of data processing or online correction information are carried as non standard subtables. The formal existance of calibration tables associated with a given MS will satisfy the needs of many of the non-standard subtables.

4 MS v3.0 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, and all defined sub-tables are tabulated below. Optional sub-tables are shown in italics.

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

4.1 MAIN table: Data, coordinates and flags

4.2 MAIN table: continued

Notes:

Note that $Nl$= number of lags, $Nc$= number of correlators, $Nf$= number of frequency channels, and $Ncat$= number of flag categories.

MS_VERSION

The MeasurementSet format revision number, expressed as $major\text{\_}revision.minor\text{\_}revision$. This version is 2.0.

SORT_ORDER

Sort order as either ”ASCENDING” or ”DESCENDING”.

TIME

Mid-point (not centroid) of data interval.

TIME_EXTRA_PREC

Extra time precision.

ANTENNA$n$

Antenna number ($\ge 0$), and a direct key index into the ANTENNA sub-table. For $n>2$, triple-product data are implied.

FEED$n$

Feed number ($\ge 0$). For $n>2$, triple-product data are implied.

SPECTRAL_WINDOW_ID

Spectral window identifier ($\ge 0$), and a direct key index into the SPECTRAL_WINDOW sub-table.

POLARIZATION_ID

Polarization identifier ($\ge 0$), and a direct key index into the POLARIZATION sub-table.

PROCESSOR_ID

Processor indentifier ($\ge 0$), and a direct key index into the PROCESSOR sub-table.

PHASE_ID

Switching phase identifier ($\ge 0$)

FIELD_ID

Field identifier ($\ge 0$) a direct key index into the FIELD sub-table.

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 ($\ge 0)$, 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; ($\ge 0$) a direct key index into the SCAN sub-table.

ARRAY_ID

Subarray identifier $\left(\ge 0\right)$, which identifies data in separate subarrays, as defined in Section 3.3.

OBSERVATION_ID

Observation identifier $\left(\ge 0\right)$, which identifies data from separate observations, as defined in Section 3.3.

STATE_ID

State identifier $\left(\ge 0\right)$, as defined in Section 3.1.5.

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, 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 $N\ast f$ is the maximum number of lags ($Nl$) specified in the LAG table for this LAG_ID. If both correlation functions and frequency spectra are stored in the same MS, then $N\ast f$ 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.

WEIGHT

The weight for each channel, with the same shape as DATA, as assigned by the correlator or processor.

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. Data are flagged bad if the FLAG array element is True.

4.3 ANTENNA: Antenna characteristics

Notes:

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

ANTENNA_ID

ID of the antenna.

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 ($\Omega$) in $deg.$
• 4: Argument of perigee ($\omega$) 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.

4.4 BEAM: Beam information

Notes:

This sub-table contains Beam information. Referred to from the FEED and PHASED_ARRAY subtables.

BEAM_ID

Beam identifier

TYPE

The way the beam information is expressed. Fixed set of strings possible (POLYNOMIAL, AIRY, NUMERIC, IMAGE, GAUSSIAN,INVERSEPOLYNOMIAL, COSPOLYNOMIAL, ZERNIKE)

COEFFICIENTS

The coefficients for the expressed type. All the beams are defined at 1GHz; exceptio for IMAGE: it can have the spectral coordinates expressed in the image

BEAM_ROTATION

sign of beam rotation angle with respect to parallatic angle.

ALTERNATE_URL

Information on how to access beams that are expressed as images (from some telescope archive for e.g).

4.5 CORRELATOR_TYPE: Doppler tracking information

Notes:

This optional sub-table contains correlator information referred to from the PROCESSOR subtable.

4.6 DOPPLER: Doppler tracking information

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_INDEX

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.

4.7 EPHEMERIDES: Ephemerides information

Notes:

This sub-table contains ephemeris information as referred to from FIELD table.

Either multiple rows of information for a given EPHEMERIS_ID is given in this table with time or a URL is provided to get an ephemeris table of known format (e.g the one used by Measures in casacore)

EPHEMERIS_ID

identifier referenced in FIELD table

TIME

Epoch of observation of object

DIRECTION

Measures direction of object at given time and seen from OBSLOC position.

RADIAL_VELOCITY

Velocity of object

SHAPE

Apparent Elliptical shape of object

ALTERNATE_URL

Location of a ephemeris table in documented format.

4.8 FEED: Feed characteristics

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 ANTENNA$n$ in MAIN.

FEED_ID

Feed identifier, as indexed from FEED$n$ 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 $\left({\overrightarrow{e}}_x,{\overrightarrow{e}}_y\right)$ 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.

4.9 FIELD: Field positions for each source

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.

FIELD_ID

Field id.

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.

4.10 FLAG_CMD: Flag commands

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 $\left(\ge 0\right)$; 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.

4.11 FREQ_OFFSET: Frequency offset information

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.

ANTENNA$n$

Antenna identifier, as indexed from ANTENNA$n$ in MAIN.

FEED_ID

Antenna identifier, as indexed from FEED$n$ 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.

4.12 HISTORY: History information

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.

4.13 INTERFEROMETER_MODEL: VLBI Interferometer information

Notes:

This sub-table contains information associated for VLBI

TIME

Time-stamp as origin for all time based polynomials model.

FIELD_ID

Field id, FIELD subtable

ANTENNA_ID

antenna id, antenna subtable

SPECTRAL_WINDOW_ID

Spectral window id.

PHASE_DELAY

phase delay modelled as a time polynomial for each receptor.

PHASE_RATE

rate of change for phase delay .

GROUP_DELAY

Group delay.

GROUP_RATE

Group delay rate.

DISP_DELAY

Dispersive delay

DISP_DELAY_RATE

Dispersive delay rate

CLOCK_ERROR

CLOCK_ERROR_RATE

4.14 OBSERVATION: Observation information

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.

OBSERVATION_ID

Observation id. key

TELESCOPE_NAME

Telescope name (e.g. ”WSRT” or ”VLBA”).

ARRAY_CENTER

Reference position used by the correlator for e.g.

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.

4.15 PHASED_ARRAY: phased array station information

Notes:

The table contains information about phased array information. It refers to antenna ids in ANTENNA table, FEED_ID for feed used in forming this phased array.

ANTENNA_ID

Index into the ANTENNA table to show to which ANTENNA this phased array belongs. Note that this is an n-to-1 mapping: one ANTENNA can consist of multiple PHASED_ARRAYs.

POSITION

Position of the antenna field in absolute ITRF coordinates

COORDINATE_SYSTEM

(cartesian) direction vectors in ITRF (or measure defined) describing the local field coordinate system. This defines the antenna field plane, and the ’up’ direction, the normal direction to the antenna field plane. Note that in general this is not the zenith direction. The coordinate system is necessary to evaluate the beam pattern directions. The coordinate system can also be used for describing polarization alignment.

ELEMENT_OFFSET

Relative offsets of each element from POSITION

ELEMENT_FLAG

flag for invalid antennas in array not used.

BEAM_ID

Beam id as defined in BEAM table.

4.16 POINTING: Antenna pointing information

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 ANTENNA$n$ 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).

4.17 POLARIZATION: Polarization setup information

Notes:

This table defines the polarization labeling of the DATA array in MAIN, and is directly indexed by 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.

4.18 PROCESSOR: Processor information

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 $subtype\text{\_}type$, 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 $subtype\text{\_}type\text{\_}mode$, 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.

4.19 SCAN: Scan information

Notes:

This sub-table contains associated scan information for the MS.

SCAN_NUMBER

Scan number as referred from main table

SCAN_INTENT

Intent for the scan (an scan number may have multiple intents). Fixed set of string for a telescope e.g CAL_POINTING or TARGET

EXECBLOCK_ID

A number that is unique to the observation execution block. Used to get more info from the online system of some telescopes.

TIME

Time of start for that scan

TIME_INTERVAL

interval for which this scan is observed from TIME

4.20 SOURCE: Source information

Notes:

This table contains time-variable source information, optionally associated with a given FIELD_ID.

SOURCE_ID

Source identifier ($\ge 0)$, 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 $\left(x,y,z\right)$ 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 or a URL to access source model

PULSAR_ID

An index used in the PULSAR sub-table to define further pulsar-specific properties if the source is a pulsar.

4.21 SPECTRAL_WINDOW: Spectral window description