A system and method for enhancing detection of target signals in a passive detection system. The present invention enhances noise limited detection by combining energy from several frequencies, the fundamental and related harmonics. The present invention processes fundamental frequency signal f.sub.o and a plurality of harmonic frequency signals 2fhd o, 3f.sub.o . . . nf.sub.o that are embedded in background noise. In one embodiment these signals are summed and the summed signal is compared to a threshold to generate a target detection signal. In another embodiment, the plurality of harmonic frequency signals are processed to determine the background noise level and to determine which of the harmonic frequency signals may be harmonics of the fundamental frequency signal. The fundamental frequency signal and the harmonic signals that are determined to be harmonics of the fundamental frequency signal are summed to provide the summed signal. The summed signal is compared to a threshold level that is indicative of the background noise level to provide an enhanced target detection signal. In either embodiment, the enhanced target detection signal is displayed to an operator. If the precise location of f.sub.o is not known, several frequency bins centered around the expected frequency f.sub.o are processed along with their potential harmonic frequencies.