Polar Mode

All the cross products needed for measuring the four Stokes parameters (see Chapter 15) are measured in this mode. The number of channels available per baseline is again restricted to 128 and further one sideband from all 30 antennas is processed. Thus the maximum possible bandwidth in the Polar Mode is 16 MHz, as opposed to 32 MHz in the Indian Polar mode (which measures Stokes I for unpolarized sources), and the spectral resolution is also half of the maximum possible in the Indian Polar Mode.

Figure 25.8: Signal flow in the GMRT correlator in the Polar Mode. Signal names preceded by a $T$ indicate time series and angular brackets denote time average. See the caption of Figure 25.6 for other details.

The delay-DPC multiplexer is configured so that the data flow will be as shown in Fig 25.8. The data from one side band for both polarizations (in this case $R_{Ui}$, $L_{Ui}$) is multiplexed to get the required data sequences. The MAC is configured in the polar mode such that it measures the cross product of the two polarizations in addition to the cross products of a polarization with itself. Adjacent channels of the cross product of one of the polarizations (eg: $R_{Ui} \times R_{Ui}^{'}$) are averaged and integrated in 128 accumulators of the MAC. Unlike in the Indian-Polar mode, the second set of 128 accumulators integrate the cross product of the two polarizations (eg: $R_{Ui} \times L_{Ui}^{'}$). Similar measurements of the second polarization (i.e. in this case, $L_{Ui} \times L_{Ui}^{'}$ and $L_{Ui} \times R_{Ui}^{'}$) are made in the second MAC. Thus all required cross products are measured, from which, as described in Chapter 15 all four Stokes parameters of the source can be computed.