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

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.