Data from: Quantitative ornithology with a commercial marine radar: standard-target calibration, target detection and tracking, and measurement of echoes from individuals and flocks

• Main Authors: Urmy, Samuel S., Warren, Joseph D.
• Language: unknown
• Published: 2016
• Subjects: Life sciences; medicine and health care; Gull Island; Montagu; Sterna hirundo
• Online Access: http://nbn-resolving.org/urn:nbn:nl:ui:13-7v-ywr9; https://easy.dans.knaw.nl/ui/datasets/id/easy-dataset:96037

Marine surveillance radars are commonly used for radar ornithology, but they are rarely calibrated. This prevents them from measuring the radar cross-sections (RCS) of the birds under study. Furthermore, if the birds are aggregated too closely for the radar to resolve them individually, the bulk volume reflectivity cannot be translated into a numerical density. We calibrated a commercial off-the-shelf marine radar using a standard spherical target of known RCS. Once calibrated, the radar was used to measure the RCS of common and roseate terns (Sterna hirundo L. and Sterna dougallii Montagu) tracked from a land-based installation at their breeding colony on Great Gull Island, NY, USA. We also integrated echoes from flocks of terns, comparing these total flock cross-sections with visual counts from photos taken at the same time as the radar measurements. The radar's calibration parameters were determined with 1% error. RCS measurements made after calibration were expected to be accurate within ±2 dB. Mean tern RCS was estimated at -28 dB relative to one square meter (dBsm), agreeing in magnitude with a simple theoretical model. RCS was 3-4 dB higher when birds’ aspect angles were broadside to the radar beam compared with head- or tail-on. Integrated flock cross-section was linearly related to the number of birds. The slope of this line, an independent estimate of RCS, was -32 dBsm, within an order of magnitude of the estimate from individual birds, and near the middle of the frequency distribution of RCS values. These results indicate that a calibrated marine radar can count the birds in an aggregation via echo integration. Field calibration of marine radars is practical, enables useful measurements, and should be done more often.