Conventional spectrum sensing schemes can detect active transmitters but not passive receivers, which have to be nevertheless protected from excessive interference whenever their bands are reused. In this paper, a resource allocation scheme for underlay cognitive radios is formulated, taking into account uncertainty of both propagation gains and locations of incumbent receivers. The performance of orthogonal access by secondary users is maximized under average interference constraints, using channel statistics and maps that pin-point areas where primary receivers are likely to reside. These maps are tracked using a Bayesian approach, based on a 1-bit message sent by the primary system whenever a communication disruption occurs due to interference.