TD-16070 1 CIMO-TECO 2016 DEVELOPMENT OF PHASED-ARRAY WEATHER RADAR:FIELD TRIAL, DUAL-POL, AND HOW IT REDUCES DISASTER M. Wada 1 , H. Yonekubo 1 , T. Ushio 2 , S. Satoh 3 , A. Adachi 4 , S. Tsuchiya 5 1 TOSHIBA Corporation, Tokyo, Japan 2 Osaka University, Osaka, Japan 3 National Institute of Information and Communication Technology (NICT), Tokyo, Japan 4 MRI, Japan Meteorological Agency (JMA), Tsukuba, Japan 5 NILIM, Ministry of Land, Infrastructure, Transport and Tourism (MLIT), Tsukuba, Japan SESSION 2A: Developments in Observing Technologies and Systems 1. Introduction Recently, we see increasing demands for prediction techniques aimed at a growing number of sporadic, localized weather disasters such as heavy rainfalls and tornados. Localized heavy rainfalls are caused by the rapid development of cumulonimbus clouds. Cumulonimbus clouds develop in the vertical direction with its lifecycle being just 30 to 60 minutes, bringing heavy rainfalls of more than 100 mm/h within a narrow area. In order to predict heavy rainfalls it is important to observe rainfall potentials up to 15 km in clouds under development. Corresponding to the demands for prediction, the research and development of phased-array weather radar have been quite high for recent years. (Bluestein[2010], Isom [2013], Wu[2014], Hopf[2015] ) On the other hand, importance is given more than ever, amid an increasing frequency of extreme weathers, to the stability of radar operations and the easiness of system maintenance in order to observe weather phenomena without interruption. Conventional electron tube based radar systems cannot satisfy these demands, because they impose high operational costs on users. Toshiba has been leading the manufacturing of weather radar systems from early times. Not only weather radar systems, it has also supplied a great many of defense and air traffic control (ATC) equipment to both the domestic and global markets. Possessing world-top class technology of semiconductor manufacturing, Toshiba has promoted adopting “solid-state” (using semiconductor) transmitters for defense and ATC equipment. One of the greatest achievements was Airport Surveillance Radar (ASR). For ASR, solid-state technology is now widely prevailing all over the world. This brings stability high enough to operate radar systems for 24 hours a day. Moreover, solid-state systems do not require periodic replacement of devices, unlike in the case of electron tubes, therefore keep running cost reasonable. The price of products itself is also becoming less expensive compared with electron tube radar. What is more, based upon the stable solid-state transmitter techniques, Toshiba succeeded in the development of Phased-Array Weather Radar (PAWR: Figure 1), in 2012, which enabled rapid observation of growing cumulonimbus clouds. As of 2016, there are four of PAWR used in Japan. This type of radar usually costs more than ten times as high as conventional dish-type radar, but owing to advanced core techniques such as dense integration of devices, the manufacturing cost is gradually approaching that of conventional ones.
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TD-16070
1
CIMO-TECO 2016
DEVELOPMENT OF PHASED-ARRAY WEATHER RADAR:FIELD TRIAL,
DUAL-POL, AND HOW IT REDUCES DISASTER
M. Wada1, H. Yonekubo
1, T. Ushio
2 , S. Satoh
3 , A. Adachi
4 ,
S. Tsuchiya
5
1 TOSHIBA Corporation, Tokyo, Japan
2 Osaka University, Osaka, Japan
3 National Institute of Information and Communication Technology (NICT), Tokyo, Japan
4 MRI, Japan Meteorological Agency (JMA), Tsukuba, Japan
5 NILIM, Ministry of Land, Infrastructure, Transport and Tourism (MLIT), Tsukuba, Japan
SESSION 2A: Developments in Observing Technologies and Systems
1. Introduction
Recently, we see increasing demands for prediction techniques aimed at a growing number of sporadic, localized weather
disasters such as heavy rainfalls and tornados.
Localized heavy rainfalls are caused by the rapid development of cumulonimbus clouds. Cumulonimbus clouds develop in the
vertical direction with its lifecycle being just 30 to 60 minutes, bringing heavy rainfalls of more than 100 mm/h within a
narrow area. In order to predict heavy rainfalls it is important to observe rainfall potentials up to 15 km in clouds under
development. Corresponding to the demands for prediction, the research and development of phased-array weather radar have
been quite high for recent years. (Bluestein[2010], Isom [2013], Wu[2014], Hopf[2015])
On the other hand, importance is given more than ever, amid an increasing frequency of extreme weathers, to the stability of
radar operations and the easiness of system maintenance in order to observe weather phenomena without interruption.
Conventional electron tube based radar systems cannot satisfy these demands, because they impose high operational costs on
users.
Toshiba has been leading the manufacturing of weather radar systems from early times. Not only weather radar systems, it has
also supplied a great many of defense and air traffic control (ATC) equipment to both the domestic and global markets.
Possessing world-top class technology of semiconductor manufacturing, Toshiba has promoted adopting “solid-state” (using
semiconductor) transmitters for defense and ATC equipment. One of the greatest achievements was Airport Surveillance Radar
(ASR). For ASR, solid-state technology is now widely prevailing all over the world. This brings stability high enough to
operate radar systems for 24 hours a day. Moreover, solid-state systems do not require periodic replacement of devices, unlike
in the case of electron tubes, therefore keep running cost reasonable. The price of products itself is also becoming less
expensive compared with electron tube radar.
What is more, based upon the stable solid-state transmitter techniques, Toshiba succeeded in the development of Phased-Array
Weather Radar (PAWR: Figure 1), in 2012, which enabled rapid observation of growing cumulonimbus clouds. As of 2016,
there are four of PAWR used in Japan. This type of radar usually costs more than ten times as high as conventional dish-type
radar, but owing to advanced core techniques such as dense integration of devices, the manufacturing cost is gradually
approaching that of conventional ones.
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Figure 1: Observation by phased-array weather radar
Regarding observation performance, a field test was conducted in 2015 using a PAWR installed at Osaka University, to
confirm its high potential for detecting heavy rainfall in real-time. This PAWR was actually a single-pol radar. However,
Toshiba has also been developing dual-pol PAWR and conducts field tests by 2018 to demonstrate its great capabilities as
operating weather radar towards the 2020 Tokyo Olympics with the support from the government of Japan.
In this paper we describe the development of our solid-state weather radar systems, and present new techniques to detect
localized severe weather with high accuracy. Based upon development results, we will further discuss how to make use of
them in order to reduce damages and/or loss of lives due to natural disasters, and how the next generation weather radar
systems we lead will change the way weather phenomena are observed in the near future.
This paper is organized as follows:
Chapter 2 looks back at the history of weather radar to give an overview of the related technology.
Chapter 3 explains three types of new radar from Toshiba, namely solid-state dual-pol weather radar, single-pol PAWR, and
dual-pol PAWR.
Chapter 4 explains possibilities of applying our radar technologies to disaster reduction, based upon field test results from the
single-pol PAWR.
Chapter 5 gives a proposal on an ideal deployment strategy of future weather radar networks.
Chapter 6 presents conclusions.
2. Evolution of Weather Radar
Figure 2 shows the evolution of weather radar.
In 1950s, weather radar began with systems that detected azimuth and range to rain regions, and estimated rainfall rate
qualitatively from received signal power. As element technology, these radar systems used self-oscillation magnetron devices
for transmitters, and analog logarithmic amplifiers for receivers.
In 1970s, the advancement of digital IC technology enabled quantitative precipitation estimation (QPE).
In 1990s, radars with the Doppler capability emerged. This type of radar observed radial velocity of hydrometeors, which
enabled airflow estimation in addition to rainfall intensity.
Klystron amplifiers became the mainstream for transmitters with a view to obtaining stable phase information. For receivers,
linear amplifiers and digital IQ technologies were adopted. Then on the side of the Doppler radar, dual polarization weather
radar, which used both horizontal and vertical polarizations, began to be adopted just around the same period. Observing with
two orthogonal polarized waves paved the way for real-time, highly accurate precipitation estimation.
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In 2000s, Doppler and dual-pol radar technologies were unified as one radar system. For transmitters, the replacement of
electron tubes by microwave semiconductors (solid-state devices) started as element technology.
Since 2010s, solid-state weather radar has been generally accepted in Japan. Based on this, Toshiba developed single-pol
PWAR. It has an active array of solid-state transmit elements, and major RF functions are implemented on discrete elements.
For the dual-pol PAWR which Toshiba is now developing, these functions are implemented on a single IC, attaining dual
polarization capabilities. The system is comparable in size with traditional radar.
The next chapter explains details of the three types of new radar which bring a huge step forward to weather observation.
Figure 2: Evolution of weather radar
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3. Three New Types of Toshiba Weather Radar
3.1. Dual-Pol Solid-State Radar
As part of contract research from Ministry of Internal Affairs and Communications, Toshiba developed the world-first
solid-state C-Band operational weather radar1 in 2007 for MRI (Meteorological Research Institute). Afterwards, it has
delivered more than ten X-Band and five C-Band solid-state weather radar systems to MLIT (Ministry of Land, Infrastructure,
Transport and Tourism). In total it has a supply record of more than twenty-five solid-state weather radar systems to a number
of customers as of 2016.
(i) High Power Output Technology
GaN HEMT (Gallium Nitride High Electron Mobility Transistor) is used as a solid-state device. Power output of one
device is not sufficient for an operation of radar. Therefore a multiple number of solid-state devices are synthesized
within a power amplifier unit, or PA Unit (Figure 3).
Figure 3: GaN HEMT device / Power Amp.Unit
Using four or eight of this PA Unit, for the horizontal and vertical polarization respectively, desired high transmit
power is obtained with little signal loss of synthesis (Figure 4). Peak power is 6kW/12kW for C-Band, 10kW/20kW
for S-Band, in H/V total.
Figure 4: Power Amp. Unit / Solid-state transmitter
1 As radar, with a purpose of precipitation observation, realized by solid-state transmitter having operating frequency of 5.3GHz, and output
power of more than 3.5kW (according to our research, April 2008).
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(ii) Pulse Compression Technique
Figure 5 shows the basic principle of pulse compression.
Figure 5: Pulse compression
Conventional weather radar uses electron tubes such as magnetron or klystron in order to amplify transmit power.
Peak transmit power of several hundreds of kW is obtained but the pulse width is limited to a few micro seconds.
With solid-state radar, peak transmit power is as small as a few kW, but a pulse width of up to a few hundred micro
seconds can be transmitted. In terms of average power, this attains transmit energy of equivalent to or even more than
that of electron tube radar. Usually a long pulse entails degraded range resolution, but pulse compression secures
range resolution as fine as that obtained with conventional radar.
With LFM (Linear Frequency Modulation), the most general form of pulse compression, radar transmits a pulse after
applying linear frequency modulation to transmit frequency. Then by passing the received signals through a filter with
frequency versus delay characteristics, frequency components scattered within a pulse are concentrated to one point,
thus called pulse compression. There is also NLFM (Non-Linear Frequency Modulation) as further advanced
technique.
Techniques mentioned in (i) and (ii) bring the following advantages.
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High Observation Accuracy
Since its first delivery in 2007, MRI has been leading studies on dual-polarimetric observations with solid-state C-Band
weather radar in Japan. Yamauchi [2012], Adachi[2013], Adachi[2015]
As mentioned previously, the radar of this type uses long pulses with pulse compression technique to increase the average
power. Because radar cannot observe in the vicinity of the antenna with long-pulse observations, this radar alternately
transmits short and long pulses to cover the blind region associated with the long-pulse observations. Figure 6 shows that
solid-state radar has very high quality data with no gap between the long (>20 km) and short pulse regions.
Figure 6: Sample observation data from solid-state C-Band dual-pol Doppler radar at MRI (May 10, 2016)
Figure 7 shows the distribution of HV, correlation coefficient, observed under stratiform precipitation conditions with SNR of
more than 20dB. In general, the number of samples required for dual-pol observations with high reliability is larger than that
for single-pol observations, resulting in a coarser time resolution. This is not the case for solid-state weather radar as shown in
the figure; only 20 samples are sufficient to get values of HV as high as 0.998 for long-pulse and 0.992 for short pulse
observations, respectively. Reasons for the higher HV of long pulse observations than that of short pulse observations may
include that firstly, the solid-state transmitters are very stable, and secondly, SNR is higher for the long pulse region. It could
also be said that, while target echoes are fluctuating within a pulse duration of 100us, for example, pulse compression piles up
averaged echoes, making the correlation coefficient converge to unity faster than a short pulse.
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Figure 7: Correlation coefficient (MRI)
On the other hand, MLIT is conducting ground level observation with twenty six C-Band radar systems and thirty nine X-Band
radar systems all over the land. Observation data from the X-Band radar network are disclosed to the public under the name of
“XRAIN”. The dual-pol Doppler radar systems for XRAIN are densely deployed while maintaining resolution equivalent to
S/C-Band systems, leading to very high quality observation data.
National Institute for Land and Infrastructure Management (NILIM), which is MLIT’s research institute, conducted accuracy
evaluation of solid-state weather radar in 2011. Figure 8 shows the observation comparison between a ground-set rain gauge
and one of XRAIN radar systems (at Okayama Prefecture, August 12, 2011). Correspondence is clearly seen.
Figure 8: Comparison between ground-set rain gauge and XRAIN observations (NILIM)
As further statistical verification, Table 1 shows the observation results for eleven sets of solid-state X-Band radar systems
Toshiba delivered. Taking ground-set rain gauges within radius of 60km as target, rainfall rates for 60 minutes were compared.
Three evaluation indices were used, namely, correlation coefficient, root-mean-square error, and total rainfall ratio, calculated
as follows, setting x and y as rainfall rate of rain gauge and radar, respectively (with N as sample number):
0
2
4
6
8
10
0.980 0.985 0.990 0.995 1.000
hv
Freq
uenc
y of
occ
urre
nce
(%)
N=20N=40N=100N=20N=40N=100
Long Pulse t = 129ms
Short Pulse t = 1ms
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Correlation Coefficient: r
N
i
i
N
i
i
N
i
ii
xxyy
xxyy
r
1
2
1
2
1
)()(
))((
Root-Mean-Square Error: RMSE
N
i
ii xyN
RMSE1
2)(1
Total Rainfall Ratio: s
N
i
i
N
i
i
x
y
s
1
1
For the correlation coefficient, the results show very high accuracy. Correlation over 0.9 was obtained except for two sites.
Table 1: Rainfall Comparison between solid-state X-Band dual-pol radar (Toshiba) and rain gauge, 0 to 60km, 60 min (Data
from NILIM)
SitesCorrelation
CoefficientRMSE Total Rainfall Ratio
Kanto 0.93 2.66 1.23
Jubu-san 0.90 2.01 1.23
Tsune-yama 0.94 2.32 1.03
Kuma-yama 0.93 2.11 1.11
Nogaibara 0.93 3.08 1.49
Ushio-yama 0.91 2.99 1.58
Kusenbu 0.94 2.70 1.36
Suga-dake 0.94 2.33 1.22
Furutsuki-yama 0.92 3.03 1.16
Kazashi-yama 0.88 2.63 1.21
Sakura-jima 0.88 3.33 1.07
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Maintainability
Electron tubes need to be replaced at least once in two years, causing temporary suspensions of observation and increase of
running cost. Furthermore, power output characteristics of reserved spare parts may degrade with aging. On the contrary,
solid-state devices have a much longer life span, which reduces running cost drastically. Toshiba’s solid-state transmitter
synthesizes typically eight modules of PA Unit, and even if one module of H or V channel fails, reliable observations can be
continued with slightly decreased output power of one channel, leading to stable operations. Furthermore, the failed module
can be replaced with a spare module while the system is operating, as shown in Figure 9.
Figure 9: Solid-state weather radar (compact, easy for maintenance)
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3.2. Single-Pol PAWR
Figure 10 shows the single-pol PAWR developed in a joint collaboration of Toshiba, Osaka University and NICT (National
Institute of Information and Communication Technology) through 2009 to 2012 and installed on top of the Osaka University
campus. This is the world-first phased-array weather radar which realizes rapid three-dimensional observation by scanning
multiple angles concurrently2. Four of the same type have been installed in Japan as of 2016.