xarray-ms

---

xarray-ms presents a Measurement Set v4 view (MSv4) over CASA Measurement Sets (MSv2). It provides access to MSv2 data via the xarray API, allowing MSv4 compliant applications to be developed on well-understood MSv2 data.

>>> import xarray_ms
>>> import xarray
>>> ds = xarray.open_dataset("/data/L795830_SB001_uv.MS/",
                             chunks={"time": 2000, "baseline": 1000})
>>> ds
  <xarray.Dataset> Size: 70GB
  Dimensions:                     (time: 28760, baseline: 2775, frequency: 16,
                                  polarization: 4, uvw_label: 3)
  Coordinates:
      antenna1_name               (baseline) object 22kB dask.array<chunksize=(1000,), meta=np.ndarray>
      antenna2_name               (baseline) object 22kB dask.array<chunksize=(1000,), meta=np.ndarray>
      baseline_id                 (baseline) int64 22kB dask.array<chunksize=(1000,), meta=np.ndarray>
    * frequency                   (frequency) float64 128B 1.202e+08 ... 1.204e+08
    * polarization                (polarization) <U2 32B 'XX' 'XY' 'YX' 'YY'
    * time                        (time) float64 230kB 1.601e+09 ... 1.601e+09
  Dimensions without coordinates: baseline, uvw_label
  Data variables:
      EFFECTIVE_INTEGRATION_TIME  (time, baseline) float64 638MB dask.array<chunksize=(2000, 1000), meta=np.ndarray>
      FLAG                        (time, baseline, frequency, polarization) uint8 5GB dask.array<chunksize=(2000, 1000, 16, 4), meta=np.ndarray>
      TIME_CENTROID               (time, baseline) float64 638MB dask.array<chunksize=(2000, 1000), meta=np.ndarray>
      UVW                         (time, baseline, uvw_label) float64 2GB dask.array<chunksize=(2000, 1000, 3), meta=np.ndarray>
      VISIBILITY                  (time, baseline, frequency, polarization) complex64 41GB dask.array<chunksize=(2000, 1000, 16, 4), meta=np.ndarray>
      WEIGHT                      (time, baseline, frequency, polarization) float32 20GB dask.array<chunksize=(2000, 1000, 16, 4), meta=np.ndarray>
  Attributes:
      antenna_xds:          <xarray.Dataset> Size: 4kB\nDimensions:           (...
      version:              0.0.1
      creation_date:        2024-09-10T14:29:22.587984+00:00
      data_description_id:  0

Measurement Set v4

NRAO/SKAO are developing a new xarray-based Measurement Set v4 specification. While there are many changes some of the major highlights are:

• xarray is used to define the specification.
• MSv4 data consists of Datasets of ndarrays on a regular time-channel grid. MSv2 data is tabular and, while in many instances the time-channel grid is regular, this was not guaranteed, especially after MSv2 datasets had been transformed by various tasks.

xarray Datasets are self-describing and they are therefore easier to reason about and work with. Additionally, the regularity of data will make writing MSv4-based software less complex.

xradio

casangi/xradio provides a reference implementation that converts CASA v2 Measurement Sets to Zarr v4 Measurement Sets using the python-casacore package.

Why xarray-ms?

• By developing against an MSv4 xarray view over MSv2 data, developers can develop applications on well-understood data, and then seamlessly transition to newer formats. Data can also be exported to newer formats (principally zarr) via xarray's native I/O routines. However, the xarray view of either format looks the same to the software developer.
• xarray-ms builds on xarray's backend API: Implementing a formal CASA MSv2 backend has a number of benefits:
  • xarray's internal I/O routines such as open_dataset and open_datatree can dispatch to the backend to load data.
  • Similarly xarray's lazy loading mechanism dispatches through the backend.
  • Automatic access to any chunked array types supported by xarray including, but not limited to dask.
  • Arbitrary chunking along any xarray dimension.
• xarray-ms uses arcae, a high-performance backend to CASA Tables implementing a subset of python-casacore's interface.
• Some limited support for irregular MSv2 data via padding.

Work in Progress

The Measurement Set v4 specification is currently under active development. xarray-ms is currently under active development and does not yet have feature parity with xradio.

Most measures information and many secondary sub-tables are currently missing. However, the most important parts of the MAIN tables, as well as the ANTENNA, POLARIZATON and SPECTRAL_WINDOW sub-tables are implemented and should be sufficient for basic algorithm development.