Doppler radar signatures of migrating birds

Biological targets such as birds are a rather common example of clutter affecting weather radars operation, especially during seasonal migrations. From the other side, the identification of echoes from birds is of great interest for ornithologists and helps to reduce a potential hazard for aircraft security. Birds movements were monitored by an S-band Doppler radar working in the "fixed-beam" mode of operation: the antenna was pointed in a suitable direction with respect to the preferential routes of nocturnal migrants flying over the Pianura Padana (Northern Italy); resolution volumes in 75 m steps up to about 13 km ahead were enlightened by the radar beam for a period of several seconds, long enough to detect the signal modulation superimposed by the continuous change of birds scattering cross section during flight. Off-line processing of amplitude and Doppler signals allowed : a) to determine the statistical properties of the echoes from migrating birds; b) to derive a set of flight parameters (wingbeat frequency, duration of the intervals of flapping and gliding flight, velocity, etc.) that may prove useful in view of species identification.