Solid-state weather radar which reached the practical use stage

1. Introduction

With a rush of disasters in recent years caused by unusual weather, especially localized torrential downpours and gusts, weather radar plays a more and more important role in weather observation.

To deal with these phenomena, Toshiba has developed the latest model of weather radar and installed it under management of MLIT (Ministry of Land, Infrastructure, Transport and tourism) in 2010. This is the first X-band solid-state weather radar in service in Japan, which adopted new technologies including multi-parameter observation and solid-state transmitter to enable very accurate observation of precipitation and to achieve drastic reduction of its size and life cycle cost. This year, Toshiba supplied 9 units of X-band solid-state weather radars.

The new weather radar will provide us with useful weather observation data and contribute significantly to improvement of safety and security of our society.

2. Solid-state transmitter

Fig. 1 shows the process of evolution of weather radar in Japan. The weather radar in the initial stage started its operation as an analog radar which displayed the intensity of back scattering of radio wave from rain as the intensity of brightness on a display. Later, the observation values were digitalized to enable the observation of rainfall quantitatively. In the 1990's, the weather radar in Japan was divided into two radar types: Doppler radar and dual polarization radar. The transmitter type changed from magnetron to klystron.

Today, MLIT tries to deploy dual polarization radars. Solid-state transmitters are employed for most of these radars.

Fig.1 Evolution of Weather Radar in Japan

3. X-band Solid-state weather radar

3.1 Development of X-band solid-state weather radar

The X-band solid-state weather radar which was first put into service in Japan employed solid-state transmitter and small and high-performance digital signal processor to enable wide-area and high-precision observation with small power.

Fig. 2 shows the first solid-state weather radar and Fig. 3 shows the major components. The major components were accommodated in the radome at the top of a steel tower by making them compact and light weight. Thus, the attenuation of radio wave by the length of wave guide was minimized to allow sufficient observation performance with small power. The size was reduced by approximately 1/4 (based on Toshiba products) compared with the conventional weather radars and the consumption power was reduced by approximately 1/10 (based on Toshiba products). Table 1 shows the major specification of this weather radar.

Fig. 2 X-band solid-state weather radar (outward appearance)

Fig. 3 X-band solid-state weather radar (equipment installed in a radome)

Table 1 Major specification of X-band solid-state weather radar

3.2 Frequency separation of short and long pulses

The pulse compression is performed by giving linear FM modulation to the long transmitting pulse of 32µs in this weather radar to achieve the range resolution of 150 m. As radars cannot transmit while receiving, the observation in a short range is not possible while transmitting long pulses. To solve this problem and to enable full coverage from short range to long range, the observation by short pulse for short range observation is also performed. Fig. 4 shows the time sequence of transmitting pulse.

According to this system, the reflection wave of long pulse from precipitation area which is outside the observation range is also received at the same time and the reflection waves of short and long pulses are mixed in the radar of its own station. To avoid this problem, the frequencies of short and long pulses are detuned in this radar as shown in Fig. 5.

Fig. 4 Transmission and receiving schedule of solid-state radar

Fig. 5 Separation of frequencies of long and short pulses

Fig. 6 shows the calculation result of received level when the frequencies of transmitting and receiving waves of long and short pulses are detuned (when the frequencies are separated). For example, if the frequencies of transmitting and receiving waves are detuned by 2.5 MHz, the received level is reduced by 35 dB for short pulse and the received level is reduced by 50 dB for long pulse.

Fig. 6 Suppression of received level by frequency detuning

Fig. 7 shows the relationship between the intensity of the mutually interfered secondary echo and the intensity of the primary echo. If the frequency is not detuned, it is observed that the received power is reduced approximately by 15 dB when the secondary echo of long pulse interfered in the short pulse area. The received power is reduced by approximately 35 dB when the short pulse interfered in the long pulse area. As the received level is further reduced by 50 dB for long pulse and by 35 dB for short pulse when the long and short pulses are detuned by 2.5 MHz, the precipitation intensity of the secondary echo is reduced to 0.01 mm/h which is a negligible level.

Fig. 7 Effect of interference suppression

3.3 Distribution of observation data through the web

The high-precision observation data obtained by this weather radar is distributed to public tentatively to be used for evacuation and disaster prevention activities when torrential rainfall occurs after going through a variety of processing together with observation data of other X-band multi-parameter radars provided by MLIT (Fig. 8).

Fig.8  Example of Rainfall Observation detected by X-band MP Radar

 (provided by Ministry of Land, Infrastructure, Transport and Tourism)

*The title of this example should read "X-band MP radar precipitation information Under test operation" This information is distributed under test operation to find out the most appropiriate operating condition for the area and to improve the precipitation calculation method. This observation data shows the precipitation condition of Osaka area in Kinki District of Japan.

4. Conclusion

Toshiba has delivered 10 units of solid-state weather radars for actual operation in Japan and the service is started for general public. Satisfactory results which are comparable to electronic tube radars are obtained. In addition, C-band solid-state radar has already been developed as well as S-band solid-state power amplifier.

In presentation, We will show the comparison data between Solid-state and Klystron.