A method and receiver for detecting a signal wherein the signal is encoded the signal with a pair of spaced predetermined frequencies and wherein energy is detected in each of the pair of predetermined frequencies and in a predetermined frequency between the pair of predetermined frequencies. Energy detected in each of the pair of predetermined frequencies is compared with the energy detected in the predetermined frequency between the pair of predetermined frequencies. The detecting includes passing the encoded signal through narrow band filters, each one of the filters being tuned to a corresponding one of the pair of predetermined frequencies and the predetermined frequency between the pair of predetermined frequencies.