Ascent and descent rates of high-flying insect migrants determined with a non-coherent vertical-beam entomological radar

ABSTRACT Vertical-beam radars employing non-coherent transmission and reception provide an effective means of detecting insects migrating at heights of hundreds of metres. Current units can determine the insects’ horizontal velocities, orientations, and some characteristics indicative of the targets’ identities; however, they are not able to detect the vertical component of the targets’ motion. It has generally been presumed that more sophisticated radar technology, employing Doppler processing and requiring coherent operation, would be needed to observe ascent and descent of targets, but we show here that with modern, high-speed, data-acquisition technology it is possible to determine these quantities from the echo signals produced by current non-coherent transceivers. A specially developed data-analysis algorithm that extracts precise target ranges from each of the radar’s pulses and calculates averages over multiple samples forms an essential component of this new capability. An upgraded Insect Monitoring Radar (‘IMRU’) incorporating these elements can estimate target heights with a precision of 0.13 m up to heights of 1 km (and 0.2 m from 1 to 2.5 km), and it does so every 0.13 s. This allows ascent and descent rates to be estimated to a precision of 0.4 m s‒1 at lower altitudes and to 0.1 m s‒1 at the greatest heights, where the broader beam produces echoes with longer duration. Brief case studies are presented that show IMRU observations of ascending and descending insects in daytime convective thermals, around dusk (when nocturnal migration commences with a take-off flight), during an established nocturnal migration, and at the leading edge of a mesofront. Some limitations and potential biases of the method are identified, and its utility relative to alternative methods is considered.

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