ENVIRONMENT & RESOURCE

Development of High-Resolution Meteorological Radar
Katsuyuki IMAI *, Yasuhiko URA, Takao NAKAGAWA, Tomoo USHIO and Zenichiro KAWASAKIRainfall observation using weather radars has a major advantage that it is possible to observe precipitation over awide area in a short time. However, the precipitation data observed by weather radars often do not correspond to
those obtained by ground-based rain gauges. One of the causes of this disagreement is the non-uniformity ofrainfall distribution in a radar scattering volume. Another cause is that most types of radar cannot receive radar
echo at low altitude, because the earth’s curvature causes radar beam to overshoot several kilometers above theground surface, or because the receiver in a vertical pointing mode is turned off during pulse transmission. For thepurpose of identifying how the reflectivity profile changes near the ground and estimating precisely the causes ofreflectivity measurement errors, the authors developed a high-resolution meteorological radar that is capable ofobserving radar echo near the ground with a high temporal and spatial resolution and a high accuracy.

1. Introduction
In recent years, weather observation instruments
are being developed and installed for early detection ofabnormal weather. There are already more than 20
units of real-time weather radars for rainfall observationin Japan, and more number of high-performance weather radars with Doppler detection or dual polarization
capability are being installed each year. The observation
area of this kind of weather radar covers 100 to 300 km
in radius of the radar. Therefore, it may seem as if there
already exists in Japan a weather radar network covering
the whole country. However, rainfall data observed by aweather radar does not necessarily correspond to thatmeasured by an instrument such as a ground-based raingauge. There are often cases where it is shown on theweb that a radar echo is received when no actual rainfall
is observed. Such incidents occur because the earth has
a curved surface. Due to the earth’s curvature, a radarthat emits radio waves horizontally observes far up in
the sky at a point 100 to 200 km from its installation
location. Thus, this brings about a problem known by
the name of “sensing gap,” which is the existence ofmany unobservable areas around the boundary layer
with the existing weather radar network. Also, for example, even when an azimuth resolution is about 0.1
degree, the spatial extent at a point 200 km ahead ismore than 300 m, so a weather radar can observe only
the average field of the space. Moreover, a range resolution needs to be more than several tens of meters due toa limited occupied bandwidth, and therefore thenonuniformity of rainfall density is theoretically unobservable. This is the reason for data discrepancy. Forearly prediction and detection of regional extreme
weather such as tornadoes and torrential rains, the cellsof the existing weather radar network should be further
subdivided, and it is indispensable to install many medium-range radars that can provide high temporal andspatial resolution data. Based on this idea, the authors
successfully developed a high-resolution meteorologicalradar (hereafter referred to as HRMR) in cooperation

with Osaka University's Graduate School of Engineering.
In this paper, the authors report on the technologicalaspect of this HRMR (Photo 1) and the initial observation results. The evaluation of actual observation of thedeveloped radar system was carried out at theTanegashima site of the Japan Aerospace Exploration
Agency (JAXA).

Photo 1. HRMR (at JAXA’s HRMR site)

2. System outline
Table 1 and Fig. 1 show the specifications and configuration of the developed HRMR system, respectively.
This HRMR system is composed of antenna units, atransmitter/receiver unit, a signal processing unit, and adata processing unit (a PC). The basic design conceptsof these constituent units will be described in the following sections.
2-1 Frequency
For observing the nonuniformity of rainfall distribution and for analyzing the structure of tornadoes, aSEI TECHNICAL REVIEW · NUMBER 67 · OCTOBER 2008 · 47

Table 1. HRMR system specifications

Items

Specifications

Remarks

Operational Frequenc 15.75GHz
Operational Mode Spiral , Conical , Fix

SystemBand Width

80MHz (max)

15.71GHz - 15.79GHz

Modulation

FM charp

CoverageAz : 360˚ / El : 90˚Resolution (Az/El) 3˚(min)Resolution (Range) 5m (min)variable

Resolution (Time) 1min./scan

Antenna

Antenna Gain36dBi

Beam Width3˚Polarization

LinearCross Polarization 25dB (min)Antenna Noise Temp. 75K (typ.)Transmitted Power 10W (max)Transmitter
Duty Ratio
0%~100%ReceiverNoise Figure
2dB (max)
D/AA/DSignal
Processing Range Gate

170MHz - 14bit

IQ 2ch170MHz - 14bit

IQ 2ch32k (max)

IPP

variable

Data
OSProcessing

Windows XP*1*1 Windows and Windows XP are either trademarks or registeredtrademarks of Microsoft Corporation in the United States and/or
other countries.

Data Processing Unit
PC(Windows XP)

Sequencer
ControlHigh Power
Amplifier

RS-232C

Digital Signal Processing

13.75GHz
mix.

mix.

2nd LOA/D A/DI2ch Rotary-J
Az/El
Controller
Amplifire

LANLow Noise
AmplifierAmplifire

StatusMonitor

Antenna / Transmitter / Receiver UnitQ

14 bitD/A D/A170MHz
IQ

StatusDemodulator

LOand the cost for further radar network expansion, it isdesirable that the current weather radar network is complement by installing medium-range radars having a
range coverage around 20 km. In this development, Kuband (15.75 GHz) was selected as the center frequency,
because the output power of semiconductor powerdevices is not high enough with Ka-band although theyare recently becoming increasingly higher in output.
2-2 Antenna and beam scanning method
A mechanical azimuth/elevation (Az/El) drive system typically used in parabolic antennas is presently
common among most weather radars. Recently, the useof a mechanical drive system for Az rotation only and aphased array antenna for El scanning is being proposed.
However, in order to compensate the earlier described
problem of surface gap, full-volume scanning including
the zenith direction is necessary, so it is technically difficult for a phased array antenna whose practical scanning range is ±45 degrees, and it is also difficult in termsof costs. Regarding temporal resolution, it is desirable
that full-volume scanning takes less than one minute, so
the speed of rotation in the Az direction needs to be 20
to 40rpm. Although this is not a rotation speed that is
technically difficult to achieve, it must withstand continuous running for 24 hours, 365 days, and small and lightantennas are required. In consideration of theserequirements, the authors adopted Luneberg lensantenna that is used in Sumitomo Electric’s conventional weather observation radar (wind profiler radar) andwhose performance is proven. With the use of Luneberg
lens, the authors realized a method of high-speed beamscanning using a compact primary radiator.
The principle of Luneberg lens is shown in Fig. 2. A
Luneberg lens is a type of dielectric lens proposed by R.
K. Luneberg in 1944. The permittivity changes depending on the distance from the center of the lens, and theincident plane wave is concentrated into one focal pointon the surface symmetrical to the lens. Because the lensis of spherical shape, each of the radio waves comingfrom every direction has its own focal point, and therefore all points on the lens surface can become focalpoints, and radio waves from arbitrary directions can bereceived independently. Conversely, a radio wave from a
focal point is radiated as a plane wave after passing

Modulator

2GHz

Signal Processing Unit
Radio waves arriving fromarbitrary directions haverespective focal points and canbe picked up independently
and simultaneously.

Fig. 1. HRMR system configuration
Luneberg Lens

Radio Wave

48 · Development of High-Resolution Meteorological Radar

Focal Point
Dielectric Constantrange resolution of at least 2 m is required. With common pulse modulation and/or frequency modulation
taken into consideration, the frequency bandwidth necessary for this 2-m range resolution is 80MHz and higher. Therefore, either of Ku-band and Ka-band, which
are relatively easy to obtain certification among the frequency bands assigned to radar systems, is suitable forthis HRMR. Moreover, considering the scale of cumulonimbus clouds, mutual interference among radars,

2.0
1.8
1.6
1.4
1.2
1.0r

εr = 2 – –2R

0 0.2 0.4 0.6 0.8 1CenterSurface

Relative Radiusεr : Dielectric Constantr : Radius
R : lens RadiusFig. 2. Principle of Luneberg lensAntenna Gain [dBi]through this lens. These characteristics suggest that theLuneberg lens functions as a multi-beam transmission/reception antenna. In the HRMR development, the authors
used this Luneberg lens as a high-speed beam scanningantenna. In the developed system, a pair of Luneberg
lenses (diameter: 450 mm, antenna gain: 36 dBi) arebeing used, respectively, as a transmitter and a receiver.
Figure 3 shows the antenna pattern of a Luneberg lens
at a frequency of 15.75 GHz.

4035302520151050-5-10H-plane
E-plane-90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90

Angle [deg]Fig. 3. Antenna pattern of 450 mm dia. Luneberg lens antenna

2-3 Radome
Because of its structure, a standard parabolic antenna radar can be used without a cover. However, in thecase of a weather radar to which absolute received signal
strength is important, a radome is indispensable
because the antenna gain is greatly affected by raindrops flowing on the reflector.
The problems of a polyhedral radome made of aflat panel are that it tends to be large in size, and that its
performance may be insufficient for use with dual polarization that demands a cross polarization of more than
25 dB. Therefore, the radome adopted in the developed
radar system is of an integrated-type honeycomb sandwich structure that features excellent strength and transmission/reflection characteristics.
2-4 Transmitter
Although klystrons and other types of vacuum tubes
are used in existing weather radars and other equipment, solid-state components are seen as indispensable
to the transmitters of future radars from the viewpointsof lifetime and handling ease. Because a range coverageof 20 km (at 20 dBZ) is the design value in this development, an output of 10 W was realized by the parallel
composition of GaAs power devices that deliver several
watts each. In frequencies beyond Ku-band, the maximum output power of a GaAs power device is several W.
Therefore, instead of GaAs power devices, which are themainstream so far, GaN power devices are recently getting a lot of attention. GaN power devices have alreadybeen put to practical use in applications such as cellular
phone base stations. GaN devices of several watts for Xband applications are already on the market, and what
is anticipated for the future is the model change to GaNtransmitter for Ku-band radars.

For a radar system to obtain enough observation
range (observation coverage), the loss in feed systems(from transmitter to antenna and from antenna toreceiver) need to be restrained. Therefore, in this development the authors divided a receiver-transmitter into
two parts, and installed a second local oscillator (13.75GHz) at the top of the rotary joint. By this it has become
possible to keep the loss in feed systems at less than 0.5
dB because a signal is transmitted at 2 GHz to the rotary
joint and the frequency is up-converted to 15.75 GHz
just under the lens antenna and then amplified by ahigh-power amplifier.
Because the system is assumed to be used for continuous observation, each device’s status monitoring signals (DC power fail, internal temperature anomaly,transmitting frequency, transmitter output power, etc.)
are sent to the indoor device through the slip ring, andthen warning messages are displayed on the PC screen.
2-5 Signal Processing Unit
In order to cover an observation range of 20 kmwith a range resolution of 2 m, an inter-pulse period(IPP: 1/PRF) has to be composed of 10K or more sampling points. When pulse compression is done by FFT
techniques such as matched filtering, the required processing ability is 16K or 32K FFTs per IPP. In this system,
parallel processing is performed by using 32 units ofDSPs. A configuration where general-purpose DSPs arecascade connected to FPGAs has flexibility, cost advantages, and high versatility. In initial observations, linear
frequency chirp signals were used for modulation. For
future optimization of range side-lobe and S/N deterioration, the system employs dual-channel (IQ) A/D and
D/A converters whose sampling rate is 170 MHz anddynamic range 14 bits.
Because the capacity of data obtained at above
range resolution and temporal resolution is about 360
MB in one minute, issues of data quality management
and real-time generation/storage of lighter processeddata are to be discussed in the future.

3. Initial observation results
The initial observation results obtained by this system at the HRMR site of the Japan Aerospace
Exploration Agency (JAXA) are shown in the following.
All data are rainfall observation results.
Figure 4 shows a Doppler spectrum in a certain
direction. The horizontal axis shows the Doppler velocity of the target. In this figure, positive velocity represents target approaching toward the radar, and negative
velocity represents target moving away from the radar.
The figure shows that wind velocity varies altitudinallyand that this is observed at a very high range resolution.
Figures 5 and 6 respectively show the radar echo
strength and the radial Doppler velocity observed by aradar scanning in the Az direction at a fixed elevation
angle of 3 degrees. The horizontal axis shows the distance in the east–west direction and the vertical axis
shows the distance in the north–south direction. TheSEI TECHNICAL REVIEW · NUMBER 67 · OCTOBER 2008 · 49

Velocity[m/s] Operation_200802222330

202014108meridional [km]outline of Tanegashima Island is also plotted in thebackground. The developed radar system is located at
the center of this figure.
The radar echo strength and the radial Doppler
velocity shown respectively in these figures are indicatedby the color gradation. Positive velocity represents adirection moving toward the radar, and negative velocity
represents a direction moving away from the radar. Thismeans that it can be understood from Fig. 6 that thewind was from the west in the area concerned during
the observation. These data obtained at an unprecedentedly high range resolution show that non-uniformity rainfall distribution was observed.

20-2-8-10-14-20-20-20

Velocity and Doppler Spectrum-1001020zonal [km]8000

Fig. 6. Radial Doppler velocity

6000

height [m]4. Conclusion
4000

2000

0-20-1001020

Power [dB]-15-2

1024velocity [m/s]Fig. 4. Range profile of the Doppler Spectrum

Reflectivity[dB] Operation_200802222330

20787010meridional [km]625304536-102820-20-20-1001020zonal [km]Fig. 5. Radar echo strength

50 · Development of High-Resolution Meteorological Radar

The authors developed a high-resolution meteorological radar (HRMR) and the initial observation wasconducted in Tanegashima island. The initial observation shows a fire structure of precipitation which is difficult to be obtained by the conventional weather radar,demonstrating the usefulluess of this radar. As was stated in the Introduction section, the original purpose ofthis development was to subdivide each cell of the existing weather radar network and increase the network
density by installing a large number of the developed
radar. Obtaining more detailed initial data is extremely
effective not only in improving the accuracy of weather
prediction but also in making early detection and prediction of localized extreme weather such as tornadoes
and localized torrential downpours. The future development subjects are to develop a method for multilateralobservation of tornado occurrence by radar covering
the cell concerned and a tracking algorithm for intercell movement of tornadoes.

Contributors (The lead author is indicated by an asterisk (*)).

K. IMAI*• SEI Hybrid Products, Inc.

Y. URA• SEI Hybrid Products, Inc.

T. NAKAGAWA• Sumitomo Electric System Solutions, Ltd.

T. USHIO• PH.D, Associate Professor, Department of Information and Communications Technology, Osaka University

Z. KAWASAKI• PH.D, Professor, Department of Information and Communications Technology, Osaka University

SEI TECHNICAL REVIEW · NUMBER 67 · OCTOBER 2008 · 51