A data description is defined as a frequency, bandwidth, and polarization.  This table also has a configuration id so that the ingestion process does not have to search the entire table to see if a given data description is already there.

Definition:

       Column        |       Type       | Collation | Nullable |                            Default

---------------------+------------------+-----------+----------+----------------------------------------------------------------

 data_description_id | integer          |           | not null | nextval('data_descriptions_data_description_id_seq'::regclass)

 bandwidth           | double precision |           | not null |

 frequency           | double precision |           | not null |

 polarization_id     | integer          |           | not null |

 configuration_id    | integer          |           | not null |

Indexes:

    "data_description_pk" PRIMARY KEY, btree (data_description_id)

Foreign-key constraints:

    "configurations_data_descriptions_fk" FOREIGN KEY (configuration_id) REFERENCES configurations(configuration_id) ON UPDATE CASCADE ON DELETE CASCADE

    "polarizations_data_descriptions_fk" FOREIGN KEY (polarization_id) REFERENCES polarizations(polarization_id)

Referenced by:

    TABLE "subscan_data_descriptions" CONSTRAINT "data_descriptions_subscan_data_descriptions_fk" FOREIGN KEY (data_description_id) REFERENCES data_descriptions(data_description_id)

Columns:

data_description_id: and auto-generated id.

bandwidth: the total width of the observed spectrum, in Hz.

frequency: the central frequency of the observed spectrum, in Hz.

polarization_id: the id of a polarization from the polarizations table.

configuration_id: the id of the configuration for this data description.  multiple configurations can contain data descriptions with identical freq, bandw, and pol.