Abstract: A method and an apparatus for surveilling RF signals, the apparatus comprising a receiver for receiving RF signals at a selectively variable reception frequency and converting any signal received at that frequency to a predetermined IF signal, a sampling circuit for sampling the IF signal and providing output signals representative of a plurality of predetermined properties of the received signal; a circuit for comparing the representative output signals against a plurality of sets of reference signals each of which is representative of a particular modulation type and providing a first output indicative of the type of modulation of the received signal when the representative signals substantially match the set of reference values of one of the modulation types and a second output indicative of an unidentified received signal when the representative signals do not match any of the sets of reference signals.