Monitoring insect migration with radar: the ventral‐aspect polarization pattern and its potential for target identification

Abstract

Automated vertical‐beam radars employing linear polarization and incorporating a narrow‐angle conical scan provide an effective means for season‐long monitoring of insect migratory activity at altitudes up to about 1 km. As well as detecting and quantifying migration events, these radars provide information about the identity of the targets they detect; such information may provide a means for excluding unwanted taxa from the radar‐observation summaries that are produced for pest managers and other potential users. The copolar‐linear polarization pattern (CLPP), a property of individual targets that is retrieved from the IMR data during routine processing, provides an indication of the shape of the target when viewed from below (i.e. ventrally). The form of the CLPP for insect targets, and the possible extent of its variation, is determined from scattering theory and symmetry considerations. CLPP parameters retrieved from a sample of echoes recorded at a site in inland Australia during a night when two different types of insect were migrating are consistent with the theory, but not all possible parameter combinations occurred. CLPP parameters are correlated with both target size and wingbeat frequency, and on this occasion all three characters discriminated reliably between the two target classes.

Acknowledgments

I. T. Harman made major contributions to the development of both the IMRs and the retrieval algorithms used in this study. IMR development and trials have been supported technically by the workshop staff of the School of Physical, Environmental and Mathematical Sciences and financially with grant support from the Australian Research Council. We are grateful to an anonymous reviewer for drawing our attention to the significance for this study of equation (13).