The contribution of this paper is to show how mission planning can be used to evaluate specifications for e. g. sensors of UAVs or to compare different UAVs by the use of simulation. This is done by introducing a risk factor for a mission and the quality specification for a optical sensor. It is also shown how a mission is specified and planned in the presence of obstacles which have to be avoided and how a priori knowledge in the form of stationary and moving objects can be accounted.