1999 Volume 35 Issue 2 Pages 167-175
A simple but reliable method is proposed for detecting existence of an object within a circular area with about 4m radius, which is optimal for logging trace of small wild animals. A beacon equipped with a magnetic dipole whose field is turned on and off according to its specific serial code is placed somewhere on the ground. If a magnetic sensor carried by an animal detects the field and find that the serial bit-pattern matches to one of predetermined codes, existence of the animal within the area around the corresponding beacon is known. It is shown theoretically that the radius of detectable area is very stable against changes of sensor height, sensor inchnation, and other electronic parameters since strength of dipole field is inversely proportional to cubed distance. A 14-bit code set including 622 individual codes are devised so as to decrease theoretical probability of spurious detection by introduction of Hamming distance and exclusion of bit-patterns with very little 1s. Chance of erroneous response to impulse and burst noise is also minimized. A trace-recording system is developed. In practical system the dipole coil is driven by AC in audio frequency band. A very low-power recording unit including a tiny sensing coil, an AGC amplifier, an analog comparator, a real-time clock, a static RAM, a micro CPU and lithium batteries is developed. The system is applied to research on a male badger. 65 beacons are deposited. During 40 days in late spring, 521 successive errorless records are acquired. Ecological analysis extracted five patterns of its nightly activities and revealed that it trips around its home range in every 3 to 4 days. Thus existence detection by coded magnetic field offers an effective instrument to animal ecologists.
