Systematic survey of space debris with the MU radar

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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.