MeasurementSet VLBI extensions
Kettenis M.M.
November, 2023
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 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.
Recent developments to add VLBI capabilities to CASA revealed some gaps in the metadata provided by MS
V2.0. To fill these gaps additional tables have been defined and implemented in casacore. These extension tables are
filled by the code that implements the conversion from FITS-IDI to MS. This follows the existing practice
established by the code that converts from ASDM to MS that is provided in CASA where several ASDM-specific
tables are stored in MS form with names wit an ASDM_ prefix.
The following new tables are defined:
- GAIN_CURVE: Antenna gain curves
- PHASE_CAL: Phase calibration measurements
- EARTH_ORIENTATION: Earth Orientation Parameters
2 VLBI metadata tables
This section contains a description of the VLBI metadata tables. These tables are stored as subtables of the
MS and like the standard subtables these are stored as keywords of the MS. All these subtables are
optional.
2.1 GAIN_CURVE: Antenna gain curves
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| GAIN_CURVE: Antenna gain curves
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| Name | Format | Units | Measure | Comments
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Columns |
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Keys
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| 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 |
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Data description
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| TYPE | String | | | Gain curve type |
| NUM_POLY | Int | | | # Series order |
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Data
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| GAIN | Float(,
num_poly) | | | Gain |
| SENSITIVITY | Float() | K/Jy | | Sensitivity |
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Notes:
- This sub-table contains antenna gain curves.
For further discussion of this table see casacore Issue 1029.
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ANTENNA_ID
- Antenna identifier, as indexed by ANTENNA
from MAIN.
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FEED_ID
- Feed identifier, as indexed from FEED
in MAIN. A value of -1 indicates the row is valid for all feeds.
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SPECTRAL_WINDOW_ID
- Spectral window identifier. A value of -1 indicates the row is valid for all spectral
windows.
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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.
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INTERVAL
- Time interval.
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TYPE
- Gain curve type. Reserved keywords include: (”POWER(EL)” - Power as a function of elevation;
”POWER(ZA)” - Power as a function of zenith angle; ”VOLTAGE(EL)” - Voltage as a function of elevation;
”VOLTAGE(ZA)” - Voltage as a function of zenith angle).
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NUM_POLY
- Series order for the GAIN column (number of terms in the gain curve polynomial).
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GAIN
- Coefficients of the polynomial that describes the (power or voltage) gain.
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SENSITIVITY
- Sensitivity of the antenna expressed in K/Jy. This is what AIPS calls “DPFU”.
2.2 PHASE_CAL: Phase calibration measurements
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| PHASE_CAL: Phase calibration measurements |
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| Name | Format | Units | Measure | Comments |
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Columns
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Keys
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| 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 |
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Data description
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| NUM_TONES | Int | | | # phase calibration tones |
| TONE_FREQUENCY | Double(num_tones) | Hz | | Tone frequencies |
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Data
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| PHASE_CAL | Float(,
num_tones) | | | Phase-Cal tone
measurements |
| CABLE_CAL | Double() | s | | Cable delay |
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Notes:
- This sub-table contains signal chain phase calibration measurements.
For further discussion of this table see casacore Issue 1165.
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ANTENNA_ID
- Antenna identifier, as indexed by ANTENNA
from MAIN.
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FEED_ID
- Feed identifier, as indexed from FEED
in MAIN. A value of -1 indicates the row is valid for all feeds.
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SPECTRAL_WINDOW_ID
- Spectral window identifier. A value of -1 indicates the row is valid for all spectral
windows.
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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.
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INTERVAL
- Time interval.
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NUM_TONES
- Number of phase-cal tones that are measured. This number may vary by antenna, and may vary
by spectral window as well, especially if spectral windows of varying widths are supported.
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TONE_FREQUENCY
- The sky frequencies of each measured phase-cal tone.
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PHASE_CAL
- Phase calibration measurements. These are provided as complex values that represent both the phase
and amplitude for a measured phase-cal tone. Measurements are provided as a two-dimensional array such that
separate measurements can be provided for each receptor of a feed (so separate values for each polarization)
for each of the measured tones.
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CABLE_CAL
- Cable calibration measurement. This is a measurement of the delay in the cable that provides the
reference signal to the receiver. There should be only a single reference signal per feed (even if that feed has
multiple receptors) so this is provided as a simple scalar.
2.3 EARTH_ORIENTATION: Earth orientation parameters
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| EARTH_ORIENTATION: Earth orientation parameters
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| Name | Format | Units | Measure | Comments
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Columns |
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Keys
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| TIME | Double | s | EPOCH | Time-stamp for values |
| OBSERVATION_ID | Int | | | Points to
OBSERVATION table |
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Data
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| UT1_UTC | Double | s | | UT1 - UTC |
| PM | Double(2) | rad | | Celestial pole position |
| TYPE | String | | | Type of EOP value |
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Notes:
- This sub-table contains the earth orientation parameters that were used to generate the correlator model.
For further discussion of this table see casacore Issue 1307.
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TIME
- Timestamp of the EOP values; this is expected to be the UTC time although some sources use TAI instead.
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OBSERVATION_ID
- Observation identifier (see the OBSERVATION table)
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UT1_UTC
- Difference between UT1 and UTC (UT1 - UTC) in seconds.
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PM
- Position of celestial pole; these are provided as X and Y components (in that order) in radians.
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TYPE
- Type of the EOP value; Reserved keywords include: (”PREDICTED” - for predicted parameter values;
”PRELIMINARY” - for measured parameter values that are subject to further revision in the future; ”FINAL”
- for final parameter values). If the type is not know this should be an empty string.
3 References
Kemball, A.J., Wieringa, M.H., 2000, casacore Note 229.