Various systems and methods for estimating the position of a radiation source in three-dimensional space, together with sensors for use in such systems are described. In some embodiments, the systems include a plurality of radiation sensors. The three-dimensional position of the radiation source is estimated relative to each sensor using an aperture that casts shadows on a radiation detector as a function of the incident angle of the incoming radiation. In some embodiments, the ratio of a reference radiation intensity to a measured radiation intensity is used to estimate direction of the radiation source relative to the sensor. When the angular position of the radiation source is estimated relative to two sensors, the position of the radiation source in three dimensions can be triangulated based on the known relative positions of the two sensors.