There is disclosed a method of determining one or more candidate frequencies for a carrier signal in a received signal, which method comprises: generating a narrowband spectrum of the received signal; detecting one or more peaks in the narrowband spectrum; generating a candidate frequency list, each frequency at which a peak occurs being included in the candidate frequency list. The method further comprises: removing the detected one or more peaks from the narrowband spectrum to generate a modified narrowband spectrum; detecting one or more further peaks in the modified narrowband spectrum; and modifying the candidate frequency list in dependence on the one or more further peaks.
