Systematic survey of space debris with the MU radar

针对甚高频有源相控阵雷达（MU），提出了一种特殊的空间碎片观测模式，用于测量雷达截面（RCS）的时变。由于使用超长波长，小碎片的RCS减小，但由于天线孔径大，输出功率高，因此该雷达在空间碎片探测方面显示出与美国SPACECOM用于空间碎片常规监测的雷达相当的灵敏度。该模式充分利用了多目标雷达的快速波束可操纵性，通过比较重叠波束的回波功率比可以精确地确定目标的瞬时位置。该模式的高度分辨率约为200m，角分辨率约为0.1‰，比雷达的原始分辨率提高了几十倍。由于对大多数空间碎片的MU雷达信号散射分析采用简单的低频近似，因此可以根据观测到的RCS变化幅度估计每个目标的有效轴比。初步统计研究表明，RCS较小的碎片的RCS变化比较大的碎片的RCS变化更大，这表明它们的形状可能是细长的，而不是撞击估计中通常假设的球形。一系列假设椭球形状的数值模拟表明，观测到的小碎片的体积估计不到具有相同RCS的球体体积的一半。这些结果有望为碎片防护设计提供有价值的信息。

A special observation mode for space debris was developed for a VHF active phased-array radar(MU radar)in order to measure the time variation of the radar cross section(RCS). The reduced RCS of small debris due to the use of a very long wavelength is compensated for by the large antenna aperture and the high output power, so that the radar has shown comparable sensitivity in space debris detection to those employed by US SPACECOM for routine monitoring of space debris. The mode makes full use of the fast beam-steerability of the MU radar, so that the instantaneous location of a target can be determined precisely by comparing the echo power ratio of overlapping beams. The height and angular resolutions of this mode are about 200 m and 0.1ﾟ, respectively, which are several tens of times better than the raw resolutions of the radar. Since a simple low-frequency approximation holds for the analysis of scattering of the MU radar signal from most of space debris, it is feasible to estimate the effective axial ratio of each target from the magnitude of observed RCS variations. Preliminary statistical study showed that debris with smaller RCS has larger RCS variations than larger debris, suggesting that they may have elongated shape rather than spherical shape as usually assumed in the impact estimation. A series of numerical simulations assuming an ellipsoidal shape has shown that the volume of small observed debris is estimated to be less than half of that of a sphere with the same RCS. These results are expected to serve as valuable information in the debris shield design.