By using the fast beam steerability of the MU radar, we have studied the characteristics of the aspect sensitivity of the VHF backscattered echo power in the troposphere and lower stratosphere. The tropopause clearly divides the altitude range into regions with and without large aspect sensitivity. In the stratosphere, large aspect sensitivity occurs in the entire altitude region and is clearly correlated with the echo power received in the vertical direction. From investigation of the Doppler spectra, it is found that the aspect sensitivity of the echo power is correlated with the zenith angle dependence of the spectral width. Fairly large aspect sensitive echoes are also received from intense scattering layers in the troposphere, although their time-height variation is large. Because of the large aspect sensitivity of the stratospheric echo power, the effective zenith angle of the antenna beam is smaller than the bore sight angle, so that the horizontal wind velocity can not be estimated correctly from the radial wind velocity measurements done at small zenith angles.
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