  Cosmology is one of the four science pillars of LSST, which promises to be
transformative for our understanding of dark energy and dark matter. The LSST
Dark Energy Science Collaboration (DESC) has been tasked with deriving
constraints on cosmological parameters from LSST data. Each of the cosmological
probes for LSST is heavily impacted by the choice of observing strategy. This
white paper is written by the LSST DESC Observing Strategy Task Force (OSTF),
which represents the entire collaboration, and aims to make recommendations on
observing strategy that will benefit all cosmological analyses with LSST. It is
accompanied by the DESC DDF (Deep Drilling Fields) white paper (Scolnic et
al.). We use a variety of metrics to understand the effects of the observing
strategy on measurements of weak lensing, large-scale structure, clusters,
photometric redshifts, supernovae, strong lensing and kilonovae. In order to
reduce systematic uncertainties, we conclude that the current baseline
observing strategy needs to be significantly modified to result in the best
possible cosmological constraints. We provide some key recommendations: moving
the WFD (Wide-Fast-Deep) footprint to avoid regions of high extinction, taking
visit pairs in different filters, changing the 2x15s snaps to a single exposure
to improve efficiency, focusing on strategies that reduce long gaps (>15 days)
between observations, and prioritizing spatial uniformity at several intervals
during the 10-year survey.
