  Future CMB experiments will require exquisite control of systematics in order
to constrain the $B$-mode polarisation power spectrum. One class of systematics
that requires careful study is instrumental systematics. The potential impact
of such systematics is most readily understood by considering analysis
pipelines based on pair differencing. In this case, any differential gain,
pointing or beam ellipticity between the two detectors in a pair can result in
intensity leakage into the $B$-mode spectrum, which needs to be controlled to a
high precision due to the much greater magnitude of the total intensity signal
as compared to the $B$-mode signal. One well known way to suppress such
systematics is through careful design of the scan-strategy, in particular
making use of any capability to rotate the instrument about its pointing
(boresight) direction. Here, we show that the combination of specific choices
of such partial boresight rotation angles with redundancies present in the scan
strategy is a powerful approach for suppressing systematic effects. This
mitigation can be performed in analysis in advance of map-making and, in
contrast to other approaches (e.g. deprojection or filtering), results in no
signal loss. We demonstrate our approach explicitly with time ordered data
simulations relevant to next-generation ground-based CMB experiments, using
deep and wide scan strategies appropriate for experiments based in Chile. These
simulations show a reduction of multiple orders of magnitude in the spurious
$B$-mode signal arising from differential gain and differential pointing
systematics.
