The structure of the Measurement Set allows independent set up for the frequency and polarization description of the data9 . E.g., it is valid to have data at 1420MHz, with some fraction of the data recorded with only parallel hand polarizations (e.g. ”RR” and ”LL”) and some fraction of the data recorded in full polarization mode (e.g. ”RR”, ”LL”, ”RL”, and ”LR”). Note however that a polarization set up describes a spectral window and not the individual frequency channels in a spectral window.
Consequently, polarization selection expressions may need to be qualified with the spectral window and polarization ID specification as well. Therefore, current the polarization selection expressions are of the type [SPW:]POL10 . The SPW part of such expressions follow the syntax of the Spectral Window Selection expression (see Section 5.1) – except that the channel specifications are not allowed here.
As with other expressions, the polarization selection expression also consists of comma separated list of full specifications. The separator for lists for the individual parts of the specification (i.e. SPW, POL, etc.) is semi-colon. However note that if the optional qualifiers are omitted (i.e., SPW etc.), either of the separators can be used (comma or semi-colon). Thus "RR;LL" is equivalent to "RR,LL".
The POL specification is a semi-colon separated list of polarization products (i.e., it is a baseline based selection). E.g. POL can be "RR" or "LL" or "RR;LR" or "RR;LL;LR;RL", etc. Without the qualifiers, comma can also be used as a separater. Polarization specifications in the list can be in any order. A number of [SPW:]POL specifications can be given as a comma separated list. E.g.