Phased-array technology is primarily employed in atmospheric and wind profiling radars for meteorological remote sensing. As a novel avenue of advancement in phased-array technology, the Multiple-Input Multiple-Output (MIMO) technique, originally developed for communication systems, has been applied to radar systems. A MIMO radar system can be used to create a virtual receive antenna aperture plane with transmission freedom. The MIMO technique requires orthogonal waveforms on each transmitter to identify the transmit signals using multiple receivers; various methods have been developed to realize the orthogonality. In this study, we focus on the Doppler Division Multiple Access (DDMA) MIMO technique by using slightly different frequencies for the transmit waveforms, which can be separated by different receivers in the Doppler frequency domain. The Middle and Upper atmosphere (MU) radar is a VHF-band phased array atmospheric radar with multi-channel receivers. Additional configurations are necessary, requiring the inclusion of multi-channel transmitters to enable its operation as a MIMO radar. In this study, a comparison between the brightness distribution of the beamformer, utilizing echoes reflected from the moon, and the antenna pattern obtained through calculations revealed a high degree of consistency, which means that the MU radar functions effectively as a MIMO radar. Furthermore, it is demonstrated that the simultaneous application of MIMO and Capon techniques has a mutually enhancing effect.

In order to assess the possible impacts of meteors with spacecraft, which is among major hazard in the space environment, it is essential to establish an accurate statistics of their mass and velocity. We developed a radar-optical combined system for detecting faint meteors consisting of a powerful VHF Doppler radar and an ICCD video camera. The Doppler pulse compression scheme is used to enhance the S/N ratio of the radar echoes with very large Doppler shifts, as well as to determine their range with a resolution of 200 m. A very high sensitivity of more than 14 magnitude and 9 magnitude for radar and optical sensors, respectively, has been obtained. Instantaneous direction of meteor body observed by the radar is determined with the interferometry technique. We examined the optimum way of the receiving antenna arrangements, and also of the signal processing. Its absolute accuracy was confirmed by the optical observations with background stars as a reference. By combining the impinging velocity of meteor bodies derived by the radar with the absolute visual magnitude determined by the video camera simultaneously, the mass of each meteor body was estimated. The developed observation system will be used to create a valuable data base of the mass and velocity information of faint meteors, on which very little is known so far. The data base is expected to play a vital role in our understanding of the space environment needed for designing large space structures.

Meteor storms and showers are now considered as potential hazard in the space environment. Radar observations of meteors has an advantage of a much higher sensitivity over optical observations. The MU radar of Kyoto University, Japan has a unique capability of very fast beam steerability as well as a high sensitivity to the echoes from ionization around the meteors. We developed a special observation scheme which enables us to determine the orbit of individual meteors. The direction of the target is determined by comparing the echo intensity at three adjacent beams. The Doppler pulse compression technique is applied to improve the signal-to-noise ratio of the echoes from the very fast target, and also to determine the range accurately. The developed scheme was applied to the observation made during the Leonid meteor storm on November 18, 1998 (JST). Estimated orbital distribution seems to suggest that the very weak meteors detected by the MU radar are dominated by sporadic meteors rather than the stream meteors associated with the Leonids storm.

The design and performance evaluation is presented of a partially adaptive array that suppresses clutter from low elevation angles in atmospheric radar observations. The norm-constrained and directionally constrained minimization of power (NC-DCMP) algorithm has been widely used to suppress clutter in atmospheric radars, because it can limit the signal-to-noise ratio (SNR) loss to a designated amount, which is the most important design factor for atmospheric radars. To suppress clutter from low elevation angles, adding supplemental antennas that have high response to the incoming directions of clutter has been considered to be more efficient than to divide uniformly the high-gain main array. However, the proper handling of the gain differences of main and sub-arrays has not been well studied. We performed numerical simulations to show that using the proper gain weighting, the sub-array configuration has better clutter suppression capability per unit SNR loss than the uniformly divided arrays of the same size. The method developed is also applied to an actual observation dataset from the MU radar at Shigaraki, Japan. The properly gain-weighted NC-DCMP algorithm suppresses the ground clutter sufficiently with an average SNR loss of about 1 dB less than that of the uniform-gain configuration.

An analytic expression of the Capon spectrum is derived for two uncorrelated incident signals. On the basis of this theoretical formulation, we discuss the effect of a factor arising from the inner product of mode vectors with respect to the incident angles, which compromises the resolution. We show numerical examples to demonstrate the effect that the inner product of mode vectors has on the shape of the Capon spectrum.

For phased and adaptive arrays of antennas, an optimal arrangement of antenna elements is essential to avoid grating lobes in the visible angular region of the array. Large sidelobes cause degradation in signal-to-noise ratio; grating lobes, in the worst case, cause malfunctions. One method of evaluating sidelobe level is square integration. For a given set element positions, evaluation by square integration of the sidelobes involves Fourier transform and numerical integration. For faster evaluation, we developed an equivalent transform algorithm that requires no numerical Fourier transform or integration. Using this new algorithm, we introduced a fast trial-and-error algorithm that iteratively applies random perturbation to the array, evaluates the function, and minimizes it. A number of separate runs of this algorithm have been conducted under the constraint of 3-fold rotational symmetry for stability. The optimal output, for which the function is minimized, is a uniformly spaced equilateral-triangular-type arrays that, unfortunately, has unwanted grating lobes. However the algorithm also yields variations trapped at local minima, some of which do not have grating lobes and whose sidelobe peaks are sufficiently low within a wide angular region. For the case N=12, a characteristic triagular-rectangular-type array often arises, which has not only better sidelobe properties as evaluated by square-integration and peak sidelobe, but also sufficient element-to-element clearance. For the case N=36, one of the results achieves a peak-sidelobe level of -8 dB, with a minimum element-to-element separation of 0.76 wavelength.