This study demonstrates the ability of a portable X-band Doppler weather radar (XDR) to measure Doppler velocity (Vd). Existing portable X-band weather radars are housed in a container and hence have to be carried by a truck. Therefore they have limitations in their installation places. For installations at small areas where the existing X-band weather radars cannot be installed (e.g., rooftop area of small building), XDR is designed to be carried by a cart. Components of the outdoor unit (a parabolic antenna with a diameter of 1.2 m, magnetron transmitter, and radio frequency (RF) and intermediate frequency (IF) analog components) are housed in a compact body with a weight less than 300 kg. The radar operation, IF digital processing, and data storage are carried out by a desktop computer having a commercial IF digital receiver. In order to attain the required portability and reduced purchase and running costs, XDR uses a magnetron transmitter. Because XDR is the first that utilizes an IF digital receiver for the signal processing specific to magnetron transmitters (i.e., the phase correction of received signals due to the randomness of the transmitted pulse phase), Vd measured by XDR (hereafter VdXDR) was assessed. Using the dataset collected from 25 to 26 October 2009 at the Shigaraki MU Observatory (3451'N, 13606'E), the equivalent radar reflectivity factor (Ze) and VdXDR were assessed using Ze and Vd measured by a Micro Rain Radar and a L-band Doppler radar named LQ-7. The results using correlation coefficients and regression lines demonstrate that XDR measured Ze and Vd accurately. The results also show that IF digital receivers are useful for providing magnetron weather radars with the function of Vd measurement, and further suggest that a combination of IF digital receiver and magnetron transmitter contributes to future development of Doppler weather radars, because high cost performance is strongly required for a precipitation monitoring radar network.

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.

A 5.3-GHz klystron has been recently designed and fabricated. In many countries, the transmitting frequency of 5.6 GHz (5,600 to 5,650 MHz) is commonly used for C-band meteorological radars. However, 5.3 GHz is generally used in Japan. To detect low-level wind shears by a Doppler radar, it is essential to use a MOPA (Master Oscillator and Power Amplifier) that generates stable coherent microwaves. The klystron is most suitable for this purpose. However, there are no commercially available klystrons in C-band that operate at 5.3 GHz. We developed a klystron for this band, making use of a simulation technique originally devised for S- and X-bands. The klystron operates at frequencies between 5,250 and 5,350 MHz. The typical operating parameters are a peak output power of 200 kW, a pulse width of 1 µs, and an RF duty cycle of 0.002. The klystron, including the electromagnet for focusing the magnetic field, is approximately 67 cm long with a diameter of 40 cm and a weight of 162 kg. Phase modulation is suppressed below 20% of the phase change required for the minimum resolution of Doppler velocity measurement by the radar for which this klystron is employed. The klystron shows favorable performance for Doppler radars operated in major airports in Japan.