Wideband ambiguity function of South-China FM bat sonar signals

Echolocating insectivorous bats, such as the South-China FM bats Pipitrellus Abramus Temmink, detect and track their moving prey by means of echolocation. Despite a series of papers published on bat signal analysis, their discussion concentrated on the 1D FFT spectrum. The wideband ambiguity function (WAF) is developed to describe the range-velocity estimation performance of the bats´ sonar signals. Twenty-one signals emitted by the South-China FM bat, during natural hunting of an insect were analyzed via time, frequency and the 3D wideband ambiguity diagram (WAD). The evolution of the main signal parameters (pulse interval, pulse duration, bandwidth, dominant frequency, WAD, and the elliptical contours of ambiguity) from a search to a capture of an insect are summarized. The result suggests that the search signal emitted by the FM bat are best used to extract coarse and fine information from different frequency components of one echo. In a number of hunting sequences the authors found that the terminal signals contained 2nd and 3rd harmonics. The dominant frequency of the terminal signals in almost all the sequences went down suddenly from 50 kHz to 30 kHz. This is to reduce the delay error caused by Doppler velocity coupling. In the terminal phase, using shorter duration combining with depressing frequency, enables bats to reduce the coupling delay error to zero and the long axis of ellipse coincidence with the velocity axis.<>