An intrusion detection system includes a microwave transceiver detecting motion in a protected space. The microwave transceiver generates a first signal. A first infrared sensor detects a source of infrared energy in a plurality of upper detection zones within the protected space. The first infrared sensor generates an upper sensor signal. A second infrared sensor detects a source of infrared energy in a plurality of lower detection zones positioned below the upper detection zones within the protected space and intersecting a floor surface within the protected space. The second infrared sensor generates a lower sensor signal. A processor receives the first signal, the upper sensor signal and the lower sensor signal. The processor generates an alarm signal in response to the first signal exceeding a threshold value. The threshold value is varied in response a relationship between the lower sensor signal and the upper sensor signal.