MHO_MixedPolYShift

Purpose

MHO_MixedPolYShift applies a static -90 degree phase offset to the Y-polarization component of mixed linear/circular polarization products (e.g., RY or YR). This correction ensures that mixed-polarization cross-products (XR+YR or RX+RY) sum coherently. This operator is primarily intended for use during mixed VGOS-SX observations.

Control File Trigger

  • Keyword: mixed_pol_yshift90

  • Category: calibration

  • Priority: 3.5

Parameters for mixed_pol_yshift90

Parameter

Type

Description

mixed_pol_yshift90

boolean

When true, enables the Y-pol -90 degree phase shift for mixed linear/circular polarization products; when false, the operator is not created.

Input Data

This operator acts on the visibility_type container in-place.

Algorithm

The operator has no Initialize method; all work occurs in ExecuteInPlace. The phase offset is fixed at -90 degrees (configurable via SetPhaseOffset, though the builder always uses -90 degrees).

Execution (``ExecuteInPlace``):

  1. Iterate over both stations: reference (st_idx = 0) and remote (st_idx = 1).

  2. For each station, retrieve the station identifier from the visibility container tags.

  3. Iterate over all polarization products on the POLPROD_AXIS:

    1. Check if the polarization product is a mixed linear/circular product by calling IsMixedLinCirc, which returns true when the product contains both a circular label (R or L) and a linear label (X or Y).

    2. Check if the IsApplicable condition is met: the product must contain Y at the position corresponding to the current station index (index 0 for reference, index 1 for remote).

    3. If applicable, compute the base phasor:

      \[\Phi_{\rm shift} = \exp\!\left(i \cdot \phi_Y \cdot \frac{\pi}{180}\right) = \exp\!\left(-i \cdot \frac{\pi}{2}\right)\]

      where \(\phi_Y\) (the fYPolPhaseOffset member) defaults to -90.0 degrees.

    4. For the reference station (st_idx = 0), complex-conjugate the phasor:

      \[\Phi_{\rm shift,\,ref} = \overline{\Phi_{\rm shift}} = \exp\!\left(i \cdot \frac{\pi}{2}\right)\]

    5. For each frequency channel, determine the net sideband from the channel axis label net_sideband. For LSB channels (net_sideband = L), conjugate the phasor again to account for the sideband sign flip:

      \[\Phi_{\rm LSB} = \overline{\Phi_{\rm shift}}\]

    6. Apply the final phasor by multiplying the visibility sub-view for that (pol-product, channel) pair:

      \[V[p, ch, :, :] \leftarrow V[p, ch, :, :] \cdot \Phi_{\rm applied}\]

The net effect depends on station and sideband:

  • Remote + USB: multiply by \(\exp(-i\pi/2) = -i\)

  • Remote + LSB: multiply by \(\exp(+i\pi/2) = +i\) (conjugated for LSB)

  • Reference + USB: multiply by \(\exp(+i\pi/2) = +i\) (conjugated for reference)

  • Reference + LSB: multiply by \(\exp(-i\pi/2) = -i\) (conjugated twice: reference + LSB)

Effect on Data

For each mixed linear/circular polarization product containing a Y polarization, the operator multiplies the entire channel’s visibility data by a +/-90 degree phase phasor. The sign of the phase rotation depends on both which station carries the Y-polarization (reference vs remote) and the channel’s net sideband (USB vs LSB).