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.

海洋监视雷达常被应用于雷达鸟类学研究领域，但此类设备鲜有经过校准的情况。这一缺陷使得研究人员无法精准测定研究对象鸟类的雷达散射截面（RCS）。此外，若鸟类集群密度过高，雷达无法对单个个体进行分辨，则无法将体积极反射率转换为数值化的种群密度。
本研究采用已知RCS的标准球形靶标，对一台商用现货海洋监视雷达完成校准。校准完成后，利用该雷达在燕鸥的繁殖地——美国纽约大古尔岛（Great Gull Island）的陆基观测站中，对普通燕鸥（Sterna hirundo L.）和粉红燕鸥（Sterna dougallii Montagu）进行追踪，并测定其RCS。同时，我们对燕鸥群的回波进行积分求和，将得到的总群散射截面与雷达同步拍摄照片的目视统计数量进行对比。
该雷达的校准参数测定误差为1%。经校准后的RCS测量结果预计可达到±2 dB的测量精度。燕鸥的平均RCS经估算为-28 dBsm（即相对于1平方米的分贝数），其幅值与简单理论模型的计算结果相符。当鸟类姿态角处于垂直于雷达波束的侧视方向时，其RCS较迎头或尾追方向高出3~4 dB。群回波总散射截面与鸟类种群数量呈线性相关，该线性拟合的斜率（即RCS的独立估算值）为-32 dBsm，与单个鸟类的RCS估算值相差不超过一个数量级，且处于RCS值频率分布的中位数附近。
上述研究结果表明，经过校准的海洋监视雷达可通过回波积分法对集群鸟类进行数量统计。对海洋雷达开展野外校准具备实际可行性，可支持有效的观测测量工作，因此此类校准应更为频繁地进行。