The Performance of Bat Sonar Systems

Abstract

Echolocating bats emit orientation sounds and analyze the returning echoes, in order to detect the presence, and to characterize the location and nature of the reflecting target. The operational principles of bat echolocation systems are similar to those of technical systems, e.g. radar and sonar systems. Like those, biological systems consist of a transmitter which produces and radiates a particular type of signal, and a receiver which picks up and analyzes the returning echoes. In order to detect a target, a bat has to decide whether an echo of its own sound is present or not. The detection is only useful when combined with further information processing. The distance of a target is determined by the time delay between signal emission and echo reception. The direction or angular position of a target is determined by interaural echo differences. The relative velocity between bat and target is encoded in the Doppler-shifted echoes. It can also be determined by range tracking. Fluttering movements of the target, e.g. wing movements of insects, produce amplitude and frequency modulations in the echoes. Further target features such as size, shape and surface properties are encoded in a complex spectral composition and the temporal structure of the whole echo field. The detection, localization, and characterization of targets is restricted in the presence of interference factors such as external and internal noise, clutter echoes, and spectral echo changes due to the frequency-dependent directionality of the transmitting and receiving antennae and due to atmospheric influences. An echolocation system with an ideally designed type of signal and receiver for the extraction of all information with maximal accuracy and for a maximal suppression of all possible interference is not realizable.