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Nomenclature

We introduce the following letters in order to identify the various polarization states which can be encountered:

We distiguish here the polarizations in the sky frame or in the antenna frame4. The conversion between the sky and antenna frames is possible knowing the parallactic angle. However, some observatories deliver natively polarizations (or Stokes products) in one frame while other observatories deliver them in the other frame. As we can not ensure the parallactic angle is always available for the conversion, the polarization descriptors listed above allow to store the polarizations in their native frame.
Note that the circular polarizations are also defined in a reference frame, as their Stokes cross-products differ depending on the frame (namely: $RR=DD$, $LL=SS$, but $RL \neq DS$, $LR \neq SD$).

At this stage, we can define the polarization codes and Stokes product codes5 (see Tables 3 and 4). Note that the values for the Stokes products I, Q, U, V, RR, LL, RL, LR, XX, YY, XY, YX match the Conventional Stokes Values from the FITS standard. The code 0 for NONE can be used in the data format to indicate that no polarimetry data is present in the UV table (see next section).




Table 3: Polarization code names and values.
Name Value
code_polar_h 1
code_polar_v 2
code_polar_x 3
code_polar_y 4
code_polar_r 5
code_polar_l 6
code_polar_d 7
code_polar_s 8




Table 4: Stokes products code names and values.
Name Value Name Value
code_stokes_none 0    
code_stokes_i 1    
code_stokes_q 2    
code_stokes_u 3    
code_stokes_v 4    
code_stokes_rr -1 code_stokes_dd -9
code_stokes_ll -2 code_stokes_ss -10
code_stokes_rl -3 code_stokes_ds -11
code_stokes_lr -4 code_stokes_sd -12
code_stokes_xx -5 code_stokes_hh -13
code_stokes_yy -6 code_stokes_vv -14
code_stokes_xy -7 code_stokes_hv -15
code_stokes_yx -8 code_stokes_vh -16

Then the following rules apply:


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Gildas manager 2023-06-01