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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.

Figure 10: Toshiba’s phased-array weather radar (single-pol, installed at Osaka University)

This PAWR has a one-dimensional array of slot antennas aligned vertically. The antenna system is mechanically steered in the

AZ direction, while emitting electronic beams in the EL direction. These are transmitted as fan beams. On the receiver side,

pencil beams are formed with DBF (Digital Beam Forming). The basic specifications are shown in Table 2.

Table 2: Comparison between XRAIN and PAWR (X-Band)

Item XRAIN (solid-state, dual-pol) SP-PAWR

Typical Observation Range Radius of 60km Radius of 60km

Sensitivity 1mm/h at 60km or better 1mm/h at 60km or better

Temporal Resolution 5min to 10min 30sec to 60sec

Beam Shape Pencil Beam Transmit: Fan Beam

Receive: Pencil Beam (DBF)

Beam Scanning Mechanical for both AZ/EL AZ: Mechanical

EL: Electronic

Antenna Type Parabolic Active Array

Beam Width 1.2deg or less 1.0deg or less

Transmit Power (H/V) 400W 430W

2 As phased-array radar, with a purpose of precipitation observation, having real-time DBF processing which handles more than 10 angles

concurrently (according to our research, August 2012).

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As in Figure 11, dense, rapid observation features of this PAWR include:

- Observing elevation angles from 0 to 90 deg. - Transmitting wide fan beams one by one with no gaps, from the lower elevation angles upwards. - Forming received signals for multiple elevation angles simultaneously, with each having beam width of 1 deg, by means

of DBF processing.

- Performing this observation every 1 deg in the AZ direction.

Figure 11: Dense, rapid observation by fan beam and DBF

Figure 12 shows actual observation data. Cumulonimbus cloud echoes are seen within width of 3km and altitude of 8km. 3D

images like this can be obtained every 30 seconds.

Figure 12: Observation data (July 26, 2012)

Use of fan beams for PAWR makes side lobe isolation drop greater than conventional radar, which was an issue to be solved

since ground clutters in side lobe areas directly affect observation quality. We confirmed performance improvement for this by

using MMSE (Minimum Mean Square Error) Yoshikawa [2013]. The Figure 13 (a) below is the result of Fourier beamforming,

which leaves a strong ground clutter at the range of 10 to 15km, while (b) shows this was suppressed considerably due to

MMSE’s null generation for the clutter direction.

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Figure 13: Ground clutter suppression by beamforming

3.3. Dual-Pol PAWR

Since 2014, Toshiba has been developing dual-pol PAWR funded by the government of Japan, under the framework of SIP,

which will be explained in the next chapter. It is important to attain compactness and low-cost, at the same time securing

necessary performance underlying dual-polarization observation. Key techniques to this are RF-CMOS and patch antenna.

(i) Use of RF-CMOS to Reduce Cost per Channel

While the single-pol PAWR was composed of vertically aligned slot antennas, for dual polarization we chose patch

antennas, as described subsequently in (ii). Since extending the one-dimensional array for two-dimentional one in a

straightforward way would increase the number of elements by power of two, it is mandatory to reduce the cost per

channel. Tackling this issue, we adopted RF-CMOS, integrated chip technology which has been used for high frequency

radio applications, such as Bluetooth or Wi-Fi. We developed radar cells for dual-pol PAWR using RF-CMOS (Figure

14). Figure 15 is an RF-CMOS chip for Bluetooth.

Figure 14: Radar cell using RF-CMOS

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Figure 15: Bluetooth chip by Toshiba

Table 3 shows the world’s top twenty semiconductor manufacturers. Toshiba is the only weather radar manufacturer

which possesses a semiconductor manufacturing capacity of this class. This enabled Toshiba to pursue compactness

and low-cost for the radar cells making fully use of RF-CMOS technology.

Table 3: Top 20 semiconductor sales (2015), data from IHS Technology

For the developed single-pol PAWR, about 90 percent of elements were used only for reception, while only the

remaining 10 per cent also took the role of transmission. Major RF functions were implemented as discrete elements.

Instead, dual-pol PAWR radar cells offer a one chip solution for this, by realizing these functions of the receiver

front end as IC. Process is based on 180nm, keeping its bare chip size as minimum as 3mm x 3mm.

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(ii) Use of Patch Antenna Optimized for Dual-Polarization Antenna elements are important as they directly affect the characteristics of polarization and beam scanning. As the

slot antenna structure used in the single-pol PAWR would be difficult to be applied for dual-pol, we adopted

polarized shared-aperture slot-coupled patch antennas. Various types of composition are possible depending on how

power feeding and the displacement of slots are done. We chose the best layout in a way that both cross polarization

suppression and H/V port isolation were optimized to attain high accuracy of dual-pol observation. A patch antenna

is composed of 4x1 elements to do analogue synthesis, resulting in low-cost as there is no need for implementing

RF-CMOS front ends and A/D converters for every channel. This way we succeeded in reducing the channel per cost

of the system. A prototype patch antenna is shown in Figure 16.

Figure 16: X-Band 4x1 patch antenna prototype (69.82mm x 15mm. Diameter of the coin is 23.5 mm)

Radiation pattern measurements were performed using a 4x1 patch neighboring other twelve antenna elements. The

results confirmed cross polarization discrimination of more than 30dB in the principal direction. Applying advanced

signal processing, we aim at even higher discrimination in order to attain a generally accepted Zdr bias of 0.1dB or

less. Based upon the preliminary results, the developed patch antenna proved to be suitable for the dual-pol PAWR

antenna structure. Currently we are further developing a large scale array antenna system.

4. Field Trial, How to Reduce Disaster

4.1. SIP (Cross-ministerial Strategic Innovation Promotion Program)

SIP is a national project for science, technology and innovation, spearheaded by the Council for Science, Technology and

Innovation. It identified 10 themes that will address the most important social problems facing Japan. Dual-pol PAWR is

related to the program “Enhancement of Societal Resiliency against Natural Disasters”, with its mission being as “Developing

a real-time data sharing system of information related to major earthquakes, tsunamis, heavy rains, tornado, and other natural

disasters and utilizing the latest science and technologies, hence improving the capacity of disaster prevention and response in

our society and citizens.”3

Figure 17 shows the framework we envision for disaster reduction. Weather radar systems are deployed for rainfall

measurement, especially exploiting rapid full 3-dim observation capabilities of dual-pol PAWR in order to estimate heavy

rainfall accurately. Measurements are then made use of to give forecasts based upon numerical meteorological models.

Additionally, water gauges and flowmeters are put into operation to measure water level in rivers and flow in sewer pipes,

respectively, with their data output being integrated at a data center. The integrated data are further to be passed to runoff

analysis algorithms to generate notification information for various users. Presupposed applications are flooding forecast for

river management bureaus, and inundation forecast for municipalities or railway companies. As a schedule, Toshiba will finish

the development of dual-pol PAWR and conduct field tests by 2018.

3 “What is the Cross-ministerial Strategic Innovation Promotion Program?”, from the Cabinet Office Web Site,

http://www8.cao.go.jp/cstp/panhu/sip_english/sip_en.html

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Figure 17: Disaster reduction example within SIP framework

4.2. Early Warning With VIL

In 2015, we conducted field tests for single-pol PAWR in the Kansai region in Japan using an evaluation system Toshiba and

Osaka University developed with the support from Osaka Prefecture, as a preparation for the planned field tests of dual-pol

PAWR. While PAWR observes the upper sky to predict localized heavy rainfall, XRAIN observes rainfall on the ground level.

We used VIL (Vertically Integrated Liquid) as an indicator to estimate potentials of rainfall in the sky. Figure 18 shows how

we expected the system would work with VIL in issuing alarms earlier than in the case of just observing the ground surface.

Figure 18: Heavy rainfall detection in the upper air with VIL

Firstly, Figure 19 shows the display results of both the sky and ground observations at 15:00, on August 8, 2015. While

potentials of heavy rainfall were detected in the upper sky (the outer part is blue, whereas the inner part is red, indicating high

intensity), only slight level of rainfall was detected near the ground (green-colored). From the 3D image it is visually clear that

PAWR detected an increase of VIL at altitude of up to 10km.

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Figure 19: Result of field trial (comparison on display)

Next, Figure 20 shows the rainfall comparison between the sky and ground surface. At 14:56, the evaluation system judged

that cumulonimbus clouds had formed as a precursor for possible heavy rainfall (VIL, represented by light blue line, exceeding

a threshold). A caution alarm was immediately issued. It was 15:25 when the system detected heavy rainfall after PAWR had

grasped a sudden increase of VIL. A warning alarm was immediately issued. At 15:42 it was reported that traffic regulations

had been issued at the Kinki Expressway Matsubara JCT because of flooding. The results indicate that the evaluation system

can notify alarms at least 30 minutes before an actual damage takes place on the ground level. This way we verified the

effectiveness of our early warning systems against heavy rainfall which could affect rivers, sewages, and roads.

Figure 20: Result of field trial (comparison with VIL / rainfall rate graph)

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5. Weather Radar Deployment Strategy

Solid-state weather radar brings high maintainability and stability without losing observation performance. Phased-array

weather radar provides rapid observation capabilities highly useful for disaster reduction.

New technologies offer users a wide range of choices regarding weather radar deployment. However, there should be an ideal

strategy to realize optimal deployment of weather radar systems when we think of a particular country. This is expressed in 3

steps as shown in Figure 21.

Figure 21: Deployment strategy

Step 1) Firstly, the whole land region will be covered by S or C-Band systems which have long observation range. Although

some of recent X-Band radar systems have relatively long observation range, X-Band radar suffers from severe sensitivity to

attenuation. In addition to the well-known attenuation by intervening precipitation, there is also the attenuation due to a wet

radome; when rain clouds exist right above the radar, the radome surface is covered with water films which absorb radio wave.

Absorption is the severest for X-Band, which has a shorter wavelength than S or C Band. Therefore, S or C Bands are suitable

for long range coverage. Ideally, observation areas will be overlapped by neighboring radar systems, the advantage of which is,

when a particular radar system fails this can be covered up by one of the neighboring ones. Japan has taken this deployment

approach. Because a multiple number of radar systems will be deployed all over the whole country, it is highly important to

increase the reliability of each system, while reducing the total cost of maintenance. (Thus, solid-state radar)

Step 2) In addition to S/C -Band systems, X-Band dual-pol radar systems (with traditional parabolic antennas) or single-pol

PAWR will be deployed. Although S/C-Band radar has an advantage of long range coverage, it also has a disadvantage of not

being able to observe for low altitude in far areas, due to increasing distance between radar systems and to the earth curvature

effect. As is known well, radar beams tend to spread as they cover long distance, and the earth curvature effect becomes

dominant accordingly, making a sampling volume of the radar “overshoot” against target precipitation (Figure 22).

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Figure 22: Beam spread and earth curvature

Taking an example of NEXRAD in the US, it was reported that its S-Band radar network covered less than 70 per cent of the

troposphere below AGL (Above Ground Level) 1km. McLaughlin [2009]. Figure 23 shows an example of the NEXRAD

coverage for different altitudes. Coverage gaps get bigger as altitude gets lower from 3km down to 1km.

Figure 23: Example NEXRAD coverage at (a) 3 and (b) 1 km AGL. Data from McLaughlin [2009]

In order to avoid this issue, radar systems as gap filler are necessary. Since the objective of gap filler radar is observation

within near distance, X-Band radar is generally the best choice in terms of economy. Mainly they are installed at relatively low

altitude on top of buildings or steel towers on the plain ground.

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On the other hand, there is another usage of X-Band radar complementing the S/C radar network: Nowcast / Forecast. This is

where PAWR fully exerts its capabilities. The field tests described in Chapter 4 proved that the detection of rainfall potentials

in the upper sky could serve well to the prediction of 30 minutes ahead (Nowcast). By analyzing wind observation data based

upon numerical meteorological models, further prediction information can be obtained looking into several hours ahead

(Forecast).

It may be argued that S or C Band PAWR are effective for wider coverage, but actually X-Band is the best operating frequency

in terms of economy, for one thing, and observation density, for the other.

For instance, we can think of general effective range for X, C, and S Band radar as 75km, 150km, and 300km, respectively,

while assuming the same beam width (1deg) among them. When we observe cumulonimbus clouds up to the altitude of 15km,

the number of beam positions which fulfill a particular range becomes fewer as we try to observe at a far distance with lower

frequency (Figure 24, Table 4).

For X-Band, the number of positions is forty four at the range of 10km, and twelve at 75km, indicating that radar can perform

observation very finely divided in the vertical direction. For C and S Band, the number is six at 150km and two at 300km,

respectively, which means that 3-D observation density is fairly limited.

Figure 24: Number of beam positions among different operating frequencies

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Table 4: Number of positions below altitude of 15km at different ranges

This is the reason why the X-Band PAWR is the optimal to grasp the detailed three-dimensional structure of cumulonimbus

clouds.

At present in Japan, X-Band dual-pol parabolic radar and single-pol PAWR are deployed in a complementary way, with each

taking a separate role of gap filler and forecast. A radar network, where these two types coexist, is still in a transient

deployment phase.

Step 3) X-Band dual-pol parabolic radar and single-pol PAWR will be functionally integrated as dual-pol PAWR. While

covering vast land regions with S or C-Band radar systems, the radar network will be complemented by X-Band radar fulfilling

both functions of gap filler and forecast. This will be the time when a country completes an ideal radar network observing

weather phenomena all through the land, from the ground surface to the sky, with rapidness and density never attained before.

Range(km) Positions Remarks10 4425 3250 1875 12 X-band100 9125 7150 6 C-band200 4250 3300 2 S-band

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6. Conclusion

With a growing number of world disasters caused by localized heavy rainfall as background, we are constantly working on

new technologies that can bring innovation to the way weather radar performs observation. Based upon advanced

semiconductor technologies, we developed solid-state weather radar very early, which had high maintainability, stability, and

power efficiency. At the second stage, we developed single-pol PAWR, featuring rapid and dense observation due to DBF

processing, which enabled nowcast / forecast of localized heavy rainfall. At the third stage, we are developing dual-pol PAWR,

the key technologies of which include RF-CMOS and patch antenna array structure. This makes systems compact and low-cost,

while keeping high accuracy required for dual polarization observation. We will finish the development of dual-pol PAWR by

2017, its field tests by 2018, to demonstrate its full capabilities towards the 2020 Tokyo Olympics. As its preliminary phase,

we already verified the capabilities of single-pol PAWR to predict heavy rainfall.

Based upon these results, we proposed how the ideal deployment of the next generation weather radar networks should be. The

S/C network comes first, covering the whole land of a country, followed by an X-Band network having both features of gap

filler and nowcast / forecast to provide dense, rapid observation from the ground surface to the sky.

With its leading innovation technologies, we are going to contribute to the disaster reduction of the world.

ACKNOWLEDGMENTS:

The authors would like to thank people and organizations concerned with supporting our R&D activities.

Solid-state weather radar was supported by Ministry of Internal Affairs and Communications as a national project.

The research results of single-pol phased-array weather radar were achieved as "R&D of a next-generation Doppler radar", the

Commissioned Research of National Institute of Information and Communications Technology (NICT) , JAPAN.

The R&D of dual-pol phased-array weather radar has been supported by Council for Science, Technology and Innovation

(CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Enhancement of societal resiliency against natural

disasters”(Funding agency :JST).

TD-16070

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CIMO-TECO 2016

REFERENCES:

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Adachi, A., T. Kobayashi, H. Yamauchi, and S. Onogi, 2013: Detection of potentially hazardous convective clouds with a

dual-polarized C-band radar, Atmos.Meas.Tech., 6, 2741-2760.

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