Development of a C-band pulse compression weather radar

In Japan, frequency assignment for weather radar is between 5250MHz and 5350MHz in the C-band. The problem of mutual interference among radars has already taken place with the increase in the number of weather radars. Therefore, the examination and development of the technology that decreases the interference are necessary. The National Institute of Information and Communications Technology (NICT) has developed an equipment that modulates the transmitting signal and demodulates the received signal (pulse compression function) for weather radar, and was added to the COBRA system (referred to as COBRA+). The original COBRA system uses two klystron transmitter units for polarization observations, whereas the COBRA+ system uses two TWTA transmitter units, because TWTA can transmit signal modulated by frequency and amplitude. The peak power of TWTA is 10kW. The signal/data processing subsystem of COBRA was also modified to demodulate the received signal depending on the modulated transmitted signal. The observation modes between COBRA and COBRA+ can be switched mechanically within a few seconds. COBRA+ can transmit one of the six possible polarizations, namely, horizontal polarization, vertical polarization, ±45-degree tilt linear polarizations, and right- and left-handed circular polarizations. The polarization can be switched from pulse to pulse. The return signal is measured simultaneously by two receivers, one for horizontal polarization and one for vertical polarization. Polarization parameters are also calculated. They include the differential reflectivity between the horizontal and vertical polarizations, ZDR (=ZHH/ZVV), the linear depolarization ratio, LDR (=ZHV/ZHH), the cross-correlation coefficient, ρHV(0), and the differential propagation phase between the horizontal and vertical polarizations, ΦDP (and its derivative in the range direction, KDP). In the pulse compression mode, the neighborhood of radar becomes a blind area. As a solution to this problem, dual cycle observation is carried out. In the dual cycle observation a short pulse transmitting (non-pulse compression) mode and a long pulse transmitting (pulse compression) mode are changed every beam. COBRA+ was used to observe Typhoon SONGDA. The radar reflectivities observed by the short pulse transmitting mode and by the long pulse transmitting mode agreed very well. The detail results of validation will be presented.