  Ground-based Cosmic Microwave Background (CMB) experiments (except those
located at the South Pole) take advantage of sky rotation to achieve multiple
crossing angles (i.e. scans across each pixel in multiple directions). This
crossing-angle coverage is important for controlling the magnitude of some
systematics, and for the removal of 1/f noise during the map-making process.
Typically, these experiments scan the sky using constant elevation scans, which
puts strong constraints on the crossing-angle coverage that is achievable, and
therefore on the advantage gained from sky rotation. In this work we elucidate
the relationship between scanning elevation and crossing angle, and derive some
general "rules of thumb" for scheduling constant elevation scans to maximise
the crossing-angle coverage. We derive some bounds on the quality of
crossing-angle coverage that can be achieved independently of detailed
scheduling choices, which are an important consideration for map-making
algorithms if upcoming CMB surveys are to achieve their aims, and we discuss
how these results might relate to field selection. These bounds can be used to
forecast the effect of scan-coupled systematics for upcoming surveys. We also
show quantitatively how some simple choices of boresight rotation improve the
possible systematics mitigation from purely sky rotation. Our results are
relevant for other surveys that perform constant elevation scans and may have
scan-coupled systematics, such as intensity mapping surveys.
