RADAR IN OKINAWA ISLAND

       Makoto Satake, Yukari Shusse, Katsuhiro Nakagawa, Shoichiro Kojima, and Shinsuke Sato

               National Institute of Information and Communications Technology, Japan


We had developed a C-band multi-parameter weather radar in National Institute of Information and
Communications Technology (NICT), Japan. It is a full-polarimetric radar with Doppler measurement
capability. Its location is on the top of a mountain in the center of the Okinawa Island, southernmost
part of Japan. Since 1992 the radar has been operated to observe precipitations in sub-tropical maritime
climate region, in order to precise estimate rainfall rates and to classify precipitation particles. However,
we also would like to make use of the radar for another application other than precipitation
measurements. Okinawa Island is relatively small and plain, roughly 100 km in south to north, 10 km in
east to west, and its highest part is about 500 m above sea level. Located at 360 m above sea level, the
radar has good view to the surrounding ocean. Therefore, we would apply the weather radar to ocean
surface observation.

The radar has two Klystron transmitters of 250 kW, so that it can change transmitting polarization
(horizontal, vertical, and so on) pulse to pulse. Its center frequency is 5.34 GHz (C-band). The
backscattering echo from the targets is received in both horizontal and vertical polarizations
simultaneously. The radar has typically 200 km coverage with 0.9 degree beamwidth, transmitting
2.0/1.0/0.5 sec pulses. The antenna is a parabolic one of 4.5 m diameter. The spatial resolution,
therefore, is a few kilometers in azimuth and a few hundreds meters in range.

Objective of our ocean surface observation are following: (1) to investigate variation of radar cross-
section in HH, HV, VV polarization channels depending on wind speeds and directions, and (2) to
examine polarization characteristics of backscattering from ocean surface and floating objects including
ships. (HV stand for transmitting in horizontal polarization and receiving in vertical polarization.) For
those objectives, we have had the radar observe ocean surfaces, in different wind conditions, since
February 2007. We take a Plan Position Indicator (PPI) scan every 3 minutes with –0.5 or –1.0 degrees
(downward) of antenna elevation angle, transmitting alternative H and V polarization signals. When
looking downward, the coverage of radar at the sea level changes according to the elevation angles. With
0.9 degree beamwidth, –0.5 elevation looking has rather good coverage, from 20 km to 100 km at sea level.

The preliminary results show that the radar cross-section is larger when the wind speed is larger.
Downwind side of the island shows smaller radar cross-sections, as the island seems to block the wind.
We present more detailed analysis of dependency of the radar cross-section on winds. Also we try to
spot ships from the radar data, as there are several ferry boat services from the island to nearby islands.
Polarimetric characteristics of backscattering signals from the ships and from the combination of the ship
and the ocean surface are of our great interest.

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