  We study synthesis problems with constraints in partially observable Markov
decision processes (POMDPs), where the objective is to compute a strategy for
an agent that is guaranteed to satisfy certain safety and performance
specifications. Verification and strategy synthesis for POMDPs are, however,
computationally intractable in general. We alleviate this difficulty by
focusing on planning applications and exploiting typical structural properties
of such scenarios; for instance, we assume that the agent has the ability to
observe its own position inside an environment. We propose an abstraction
refinement framework which turns such a POMDP model into a (fully observable)
probabilistic two-player game (PG). For the obtained PGs, efficient
verification and synthesis tools allow to determine strategies with optimal
safety and performance measures, which approximate optimal schedulers on the
POMDP. If the approximation is too coarse to satisfy the given specifications,
an refinement scheme improves the computed strategies. As a running example, we
use planning problems where an agent moves inside an environment with randomly
moving obstacles and restricted observability. We demonstrate that the proposed
method advances the state of the art by solving problems several
orders-of-magnitude larger than those that can be handled by existing POMDP
solvers. Furthermore, this method gives guarantees on safety constraints, which
is not supported by the majority of the existing solvers.
