Since the first units were placed along the Gulf Coast in the 1950s, ornithologists and birders have become increasingly aware of the power of using radar as a tool for understanding bird migration. In addition to detecting and depicting meteorological phenomena, this radar network can be used to watch and to track the movements of birds. In this feature we will provide some basics for how to interpret radar data, in particular how to understand the movements of birds on Weather Surveillance Radar – 1988D (WSR-88D). A second installment will discuss challenges in identifying biological targets and some locally interesting patterns visible on radar.

Radar (an acronym for radio detection and ranging) was originally developed and employed leading up to World War II, in particular to detect aircraft. However, from almost the earliest days of radar surveillance, movements of birds were known to appear in radar imagery; the term “angels” was originally applied to the patterns of birds on radar, before solid ground-truthing confirmed these patterns as birds. Over the last 70-80 years, technological advances in computing, electronics, and physics have produced a wonderful array of developments that make radars a powerful tool for studying aeroecology, in particular detecting density, location, direction, and speed of biological targets, such as birds, insects, and bats.

Simple basics of radar

A WSR-88D unit (hereafter radar) emits a pulse of electromagnetic radiation; the antenna emitting and receiving this pulse is stationed at one of several angles of elevation above the ground, so the energy gradually travels higher and higher above the surface of the earth under typical conditions as it moves away from the radar. This is important, so remember it for the next paragraph. As the radar scans, this beam of pulsed energy moves away from the radar until something interferes with it – a target. This target may be a bird, a rain drop, insects, or smoke particles – regardless, some of the energy of the initial pulse bounces back to the radar, representing the relative magnitude of the target/s scattering the return energy. When the radar receives this return energy, the radar has a location of the target relative to the radar as well as a degree of reflectivity in terms of how much scattered energy returns to the radar (base reflectivity image). Additionally, radar provides information on the direction and speed of target movements relative to the station itself (base velocity image). With these data, we can say something about the magnitude, position, extent, and speed of the targets detected on radar.