The HiveSentry and HeartSentry exploit low-cost, low-power Doppler radar technology which is commonly used in automotive collision avoidance systems. In a collision avoidance system, the radar only needs to issue an alarm when a target is detected. In the present applications the detected signal is examined in more detail.
Although the HiveSentry and HeartSentry have a very similar continuous-wave Doppler radar architecture, signals returned by the radar are quite different.
The HiveSentry examines the Doppler frequency shift produced by flying bees arriving and departing from the beehive entrance. Here the frequency of the radar return is radar carrier frequency dependent. The power in the Doppler frequency spectrum is proportional to the level of flying activity.
The HeartSentry examines the phase modulation in the radar return due to heart-beat induced chest wall motion. The frequency of the modulation only depends on the heart beat. It is independent of the radar carrier frequency.
In both cases, the radar return signals are weak and noisy. They must be significantly amplified and filtered. In addition, in the case of the radar stethoscope, the signal must be frequency translated to make it comparable to a conventional acoustic stethoscope.