According to the invention, a PIR sensor system (100) comprises a first PIR sensor (1) associated with a first sensing region (11) and a second PIR sensor (2) associated with a second sensing region (12). The first and second sensing regions partially overlap and are divided into detection cells (46a-46f) and (45a-45d). Each detection cell is represented by predetermined characteristics of first and second output signals, which signals are based on input signals from first and second sensing elements (4-7) of each PIR sensor. Thereby different detection cells are encoded by a certain combination of signal characteristics (amplitude and sign) of the first and second output signals. Further, the PIR sensor system comprises a processing unit (15) configured to associate the characteristics of the first and second output signals with one of the detection cells for determining in which of the detection cells a heat source is positioned. The invention is advantageous in that production costs of the PIR sensor system are reduced. A calibration method for identifying the correspondence between the detection cells and the output signals at installation is also provided.