The widespread availability of devices with multiple wireless interfaces and the abundance of heterogeneous positioning techniques open new market potentials for Location Based Services (LBSs), even if complicating their development. The paper claims the need for novel middleware supports capable of managing dynamically retrieved client-side positioning systems in a synergic way and depending on context, i.e., LBS requirements, user preferences, device characteristics, and overall system state. To pursue this objective, we have designed and implemented PoSIM, a contextaware middleware for the synergic exploitation and control of heterogeneous positioning systems that facilitates the development and portability of LBSs. PoSIM is translucent, i.e., it can provide LBS developers with differentiated visibility of data characteristics and control possibilities of available positioning solutions, thus dynamically adapting to application-specific deployment requirements and enabling cross-la...