A dual sensing intrusion detection system has a microwave channel and a PIR channel. The signal from the microwave channel is amplified by an amplifier having a substantially constant gain, and is digitized and converted into a first digital signal. The signal from the PIR detector is amplified by amplifier having a substantial constant gain and is digitized and converted into a second digital signal. The first and second digital signals are compared to a microwave threshold signal and a PIR threshold signal respectively to generate an alarm signal. The system has a thermistor to detect the ambient temperature. The PIR threshold signal can be adjusted based upon changes in static conditions from the output of the thermistor. In addition, the microwave threshold signal can be changed statically based upon long term trend of the first digitized signal. Finally, the PIR threshold signal and the microwave threshold signal can be changed based upon detection from the other respective channel due to dynamic changes in the environment.