Calibrating data containing XX and YY is somewhat simpler. You have
insufficient information to solve for instrumental polarisation and
the calibrator Q and U. You have no choice but to assume that these are
zero. This means that we can skip a number of steps in the calibration
All these assumptions will result in some error in the calibration - this
unavoidable if only XX and YY are measured. Provided the secondary
calibrator is weakly polarised, the error is unlikely to be too substantial.