### Nomenclature

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

- X,Y for linear polarization in the sky frame,
- H,V for linear polarization in the antenna frame,
- R,L for circular polarization in the sky frame,
- D,S3 for circular polarization in the antenna frame.

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: , , but , ).

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:

- any polarization (linear or circular) described in any frame (sky or antenna) can be found in a UV table,
- there shall be a minimum of self-consistency, e.g. providing XX + VV is probably non-sense,
- the presence or absence of a parallactic angle column (code_uvt_para) does not modify the interpretation of the DAPS provided in the table,
- one can convert between sky and antenna frame by using the parallactic angle column, if available.