Difference between revisions of "Polarization Calibration"

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(Linear to Circular Conversion)
(Linear to Circular Conversion)
Line 34: Line 34:
 
Stokes \, U = XY^* - YX^* \\
 
Stokes \, U = XY^* - YX^* \\
 
P_{linear} = \sqrt{U^2 + Q^2} \\
 
P_{linear} = \sqrt{U^2 + Q^2} \\
\Theta = \frac{1}{2}\tan^{-1}{\frac{U}{Q}} \\
+
\Theta = \frac{1}{2}\tan^{-1}{\frac{U}{Q}}
 
\end{align}
 
\end{align}
 
</math>
 
</math>

Revision as of 20:23, 24 September 2016

Linear to Circular Conversion

At EOVSA’s linear feeds, in the electric field the linear polarization, X and Y, relates to RCP and LCP (R and L) as:

In terms of autocorrelation powers, we have the 4 polarization products XX*, YY*, XY* and YX*, where the * denotes complex conjugation. The quantities RR* and LL* are then

One problem is that there is generally a non-zero delay in Y with respect to X. This creates phase slopes in XY* and YX* from which we can determine the delay very accurately. As a check,

For completeness:

When I plot the quantities I, V, R and L as measured (Figure 1) for geosynchronous satellite Ciel-2, the results look reasonable, except that there are parts of the band where R and L are mis-assigned, and others where they do not separate well.

The problem is that residual phase slope of Y with respect to X, caused by a difference in delay between the two channels. This can be seen in the upper panel of Figure 2, which shows the uncorrected phases of XY* and YX*. To correct the phases, the RCP phase was fit by a linear least-squares routine, and then the phases were offset by π/2 for both XY* and YX* according to:

Polarization Mixing Correction

Due to relative feed rotation between az-al mounted antennas and equatorial mounted antennas