The synchronization of wireless sensor networks is a crucial task, especially for wide area networks. In case of many applications state-of-the-art techniques such as GPS or network based timing protocols are not suitable. In this paper we present a new concept that allows for the efficient synchronization of sensor networks using the time stamps of so-called Signals of Opportunity. We explain how these time stamps can be examined on the example of Digital Audio Broadcasting and show measurement results on the achievable performance.