An apparatus, method and computer program is described comprising: determining, using a first passive module, whether one or more of a first plurality of frequency ranges is occupied, wherein each of the first plurality of frequency ranges has one of a first set of one or more bandwidths; determining, using a second passive module, whether one or more of a second plurality of frequency ranges is occupied, wherein each of the second plurality of frequency ranges has one of a second set of one or more bandwidths, and wherein a smallest bandwidth amongst the second set of bandwidths is larger than a largest bandwidth amongst the first set of bandwidths; and triggering an active module in the event that one or more of the first plurality of frequency ranges is determined to be occupied and none of said second plurality of frequency ranges is determined to be occupied, wherein the active module comprises a radio detector for detecting a target waveform.