The MU (middle and upper atmosphere) radar of Japan is a 46.5-MHz pulse-modulated monostatic Doppler radar with an active phased array system. The nominal beam width is 3.6 ø and the peak radiation power is 1 MW with maximum average power of 50 kW. The system is composed of 475 crossed three-subelement yagi antennas and an equivalent number of solid state power amplifiers (transmitter-receiver modules). Each yagi antenna is driven by a transmitter-receiver module with peak output power of 2.4 kW. This system configuration enables very fast and almost continuous beam steering that has not been realized by other mesosphere-stratosphere-troposphere radars. Also, a variety of sophisticated operations are made feasible by dividing the antenna array into several independent subarrays. A brief description of the system, particularly its antenna and power amplifiers, is presented herein. the lower atmosphere except near the planetary boundary layer (about 2-10 km). The wide-ranging results obtained to date by means of this equipment will not be restated; readers are referred to the extensive review papers Paper number 5S0523. 0048-6604/85/005S-0523508.00 radar" in reference to the middle and upper atmosphere, which are the system's principal objects of investigation.The most outstanding feature of the MU radar is an active phased array system [Fukao et al., 1980]. It is the first MST radar to employ such a system. In conventional radar systems a high-power transmitter feeds all array elements via an appropriate cascading feeding network. The MU radar system, on the other hand, does not incorporate such a passive array connected to a high-power transmitter. Instead, each element of the phased array is provided with a lowpower amplifier, and all the amplifiers are coherently driven by low-level pulses in order to produce the desired peak output power. The two systems are contrasted in Figure 1.This system configuration enables very fast and almost continuous beam steering as well as various flexible operations made possible by dividing the antenna array into independent subarrays. These capabilities, which are the result of the low signal levels at which phase shift and signal division/combination are carried out by electronic devices, are expected to fulfill the principal research requirement; that is, the MU radar enables a pioneering study of the threedimensional structures of atmospheric waves and turbulence in mesoscale and microscale. To date, this kind of study has rarely been performed in the middle atmosphere with the exception of a shortterm mesospheric observation by the SOUSY MST radar [Klosterrneyer and Raster, 1984].The outline of the MU radar system and a few 1155

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The MU (middle and upper atmosphere) radar of Japan uses an active phased array system. Each of 475 crossed three-subelement yagi antennas in a circular array is provided with a 2.4-kW peak power amplifier. This system configuration attains a very fast and almost continuous beam steerability at the total peak radiation power of 1 MW. A brief description of the in-house equipment is presented herein.

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