COMMISSION F: WAVE PROPAGATION AND REMOTE SENSING
Edited by Yoshio Karasawa and Takeshi Manabe
F1.1 Terrestrial Fixed Radio System
A fixed wireless access (FWA) system enables to provide a broadband wireless access service
with a user bit-rate of more than 10Mbps. Propagation studies have been carried out to clarify
the multipath characteristics in several environments and frequency bands for developing thatsystem.
A 5-GHz band is expected to be available for wireless access systems and a residential area isan important service area. A new delay profile model for estimating multipath characteristics ina residential area was proposed [Kita, et al., 2001a]. It enables to calculate the delay spreadconsidering three different wave components; the front, the primary scattering, and thesecondary scattering wave components. Experimental studies using 2GHz/5GHz-dual band
antenna show new propagation models for path loss and height gain in residential area [Kita etal., 2000; Kita et al., 2001b; Kita et al., 2001c]. These studies make possible to evaluate theinfluence of the multipath interference in a region of antenna height from a few meters toaround 10m.
Urban street area is also a service area for high-speed wireless access services with over
10Mbps-transmission speed. Delay spread measurements in such areas were carried out at
5-GHz band and a relation between median value and the 90% value of the cumulative
probability of the delay spread is formulated [Kita, et al., 1999].
In addition to UHF and SHF, quasi-millimeter band has been available for a point-to-multipoint
wireless access system. In an urban area, reflected and scattered waves from buildings cause a
multipath interference. A study of delay characteristics in such an environment reports thedependency of the delay profile on the height of the base station antenna [Izuka, et al., 2001].
For calculating the delay profile, the propagation path visibility is used for taking account ofbuilding parameters.
The foliage attenuation affects radio transmission characteristics as a degradation factor in ahigh frequency region such as above quasi-millimeter band. Experimental studies of the foliage
attenuation show some compared results between 5GHz and 29GHz [Kajiwara 2000]. It isalso shown that swaying foliage in wind causes a significant fading at 29GHz, ranging over
10dB, while the fading depth at 5GHz is approximately 2dB.
The rain attenuation still has been an important parameter for a design of wireless systems andits prediction method has been improved for higher accuracy. For extending a local
estimation method to a worldwide use, a new prediction method, which considers thethunderstorm ratio, is proposed [Ito, et al., 1999a, 1999b]. This prediction method of theone-minute rain rate distribution is derived from an original databank, which is richer incontent than the ITU-R databank.
The motion of a rain area was observed by using a 12-GHz satellite-broadcasting wave
[Hayashi, et al., 2001]. Analyzing received level variations at three stations indicates the
direction, moving speed, height and width of the rain area. It is also reported that the size of therain area is less than 20km in most cases.
A scattering of the aircraft may cause interference in a long-distance radio link and abroadcasting system. Temporal variations of the interference level were simulated considering
the movement of the aircraft, positions of transmitting and receiving stations, and antenna
pattern of those stations [Takagi, et al., 2000]. Measurement was done in Thailand using a FM
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broadcasting station. The simulated result shows good agreement to the measured temporalfluctuation caused by a B747 aircraft.
(A. Sato)
F1.2 Satellite Radio System
First, in the fixed satellite radio systems, rain attenuation problems that are significant in thefrequency bands higher than 10 GHz, such as Ku and Ka bands, are extensively investigated in
the research fields of short or long distance site diversities, integration time of rainfall ratestatistics, frequency scaling techniques between Ku and Ka bands and so on.
As for site diversity characteristics, short or long distance (10-1400 km) statisticalcharacteristics are investigated among several earth stations in Sapporo, Tokyo, Osaka, andFukuoka, using VSAT networks of JSCAT-1 as a joint experiment of the Digital Satellite
Communication University Corporation Research Organization.
Each earth stationtransmitted QPSK signals from these different areas, and one in Sapporo, Hokkaido measured
their signals and processed them in order to estimate the site diversity characteristics. The
measured results are relatively in good agreement with ITU-R Rep.564-4 [Hatsuda et al.,1999].
The problems for integration time of rainfall rate statistics are studied in extremely intense rain
events in Tokyo. In such a heavy rain event with rainfall rate of larger than 100 mm/h, thecumulative distribution of clock-10 sec. rainfall rate is shown to be largely different from
log-normal or gamma distribution, while the measured rain attenuation statistics, in general,
follow the ITU-R predictions [Yamada et al., 2000]. Also, these problems are consideredusing a worldwide databank of rainfall rates with different integration time collected frommany literatures, and M distribution is proposed in order to obtain 1-min rainfall rate
distribution, which is required for radio wave propagation study, from long integration time
rainfall rates [Ito and Hosoya, 2000].
As regards the frequency scaling, rain attenuation ratio of Ka- to Ku-band satellite signals isinvestigated related to the kinds of rain fronts, on the basis of the long-term observational dataof both frequency bands obtained for the past 13 years in Osaka. A comparatively large
attenuation ratio was primarily found when the cold fronts passed over the earth station in Baiu
season. In contrast, a comparatively small ratio was primarily obtained when the earth station
was located to the south of the stationary fronts in Akisame season. A small ratio was also
found in summertime shower. This difference seems to be caused by a dominant kind ofraindrop size distribution in each season [Maekawa, 2001].
Also, the airplane effects on the earth-space path were measured by the VSAT system using
JSCAT -1 near Fukuoka Airport, Kyushu. The observation suggests that serious airplane
effects even occurs when airplanes fly over a town at a low altitude such as 800m, in addition
to the direct effects of landing and takeoff [Fujisaki et al., 2000]. Post-PARTNERS project
conducted a number of Ku-band satellite signal propagation experiments with Thailand,
Indonesia, and Philippine using JCSAT -3 and collected a large amount of precious propagation
data in the tropical or sub-tropical South-East Asia region. Some of these experiments
indicate very intense rainfall rates and rain attenuation statistics with time percentage of muchhigher than the ITU-R prediction [Igarashi et al., 2001].
In the field of broadcast satellite systems, on the other hand, some preventive countermeasures
for the rain attenuation using boosted power beams are proposed, based on the analyses of 1 h
or 10 min rainfall rate data obtained from AMeDAS (Automated Meteorological Data
Acquisition System). Computer simulations using these data suggest that with the aid of
multi-beam or beam steering antennas for the satellite radio transmission, nearly 90% of2unavailable time due to rain attenuation may be compensated even in the Ka-band (21 GHz)
advanced broadcast-satellite system [Yoshino and Ito, 1999], [Minematsu and Nomoto, 2001].
Next, in the mobile satellite radio systems, mobile communication and navigation systems arerecently proposed using quasi-zenith satellites together with geostationary satellite [Tanaka,
2000]. As for navigation systems, the accuracy analysis on internal delays of satellites andearth stations demonstrates that the Japanese domestic systems with unknown delays of a few
hundred meters may only cause a horizontal error as small as 1 m [Kawase, 2000].
Also, the urban propagation characteristics and satellite-diversity characteristics of mobile
broadcast geostationary satellite systems were studied using GPS (Global Positioning System)
signals for elevation angles of 40-50 deg. It is found from the measurements on a vehicle thata heavy signal fade is primarily caused by shadowing buildings on urban roads in east-west
direction. As regards the satellite diversity effects, more than 90% satellite visibility can beachieved in the urban city by selecting two diversity satellites with separation angles more thanabout 45 deg [Hatsuda et al., 2001].
(Y. Maekawa)F1.3 Mobile Radio Systems
A.Street micro cell systems
Many experimental and theoretical approaches have been established for investigating themobile radio propagation characteristics of street micro cells in which the antenna height ofbase station (BS) is lower than that of the buildings around the BS. Furuno et al. carried out
DOA and TOA measurements using a 2.6 GHz band directional antenna with the 3 dB
beam-width of 10 degrees and a channel sounder based on a PN correlation receiver with the
bandwidth of 60 MHz, and identified the propagation path for each received ray. The
propagation mechanisms and the effectiveness of the ray tracing calculation were then clarified[Fururno et al., 2000]. Similarly, Masui et al. carried out DOA measurements in bothline-of-sight (LOS) and non LOS (NLOS) environments by using a 8.45 GHz band directional
antenna with beamwidth of 4 degrees and a similar channel sounder with bandwidth of 100
MHz; they investigated the angle spread of arrival waves and the time delay spread [Masui etal., 2000]. Since the LOS street is the main radio zone from the BS in street micro cellsystems, most investigations considered the LOS environment, such as the measurement andmodeling of time delay characteristics [Taga et al., 1999a; Ichitsubo et al., 2000; Masui et al.,1999b; Masui et al., 1999c] and path loss measurements, a regression analysis, and channel
modeling in the 3 to 15 GHz band [Oda et al., 2000; Takahashi et al., 1999; Kage et al., 1999,
Kage et al., 2000]. Masui et al. further investigated the impact of traffic density on the breakpoint [Masui et al., 1999a, Masui et al., 2001]. Taga investigated experimentally the variation
characteristics of arrival rays as resolved from time delay spectra [Taga et al., 1999c]. An
advance system in which the time delay spread is reduced using a directional antenna at boththe BS and mobile station (MS) was also proposed and its effectiveness was investigatedexperimentally [Taga, 2000]. The time delay spread and the path loss characteristics werealso measured in LOS and NLOS environments in a suburban area, and were characterized byregression analysis [Shimizu et al., 2000].
B.Macrocell / micro cell systems
Ichitsubo et al. investigated experimentally the frequency correlation characteristics of thechannel when the MS moves from ground level to a high position like a building's roof
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[Ichitsubo et al., 1999]. Fujii et al. analyzed the number of rays available for RAKE reception
in the CDMA systems [Fujii and Imai, 1999]. These results are useful when evaluating thecoherent frequency bandwidth and the performance of RAKE receivers. Asano proposed amethod for estimating the probability density distribution of the wave arrival at the MS
antennas, and compared the predicted results with measured ones [Asano, 2000]. This
analysis seems to be a step toward modeling the arrival waves in the MS antenna. Zhang etal. proposed a deterministic model based on a ray tracing calculation method for predicting thereceived signal level in an indoor environment for the case in which the transmitted waves
come from an outer BS and pass through the windows or penetrate the wall [Zhang et al.,1999]. Taga et al. proposed a model to predict the path loss using ray tracing calculations
with a loss layer model in an open scenario over which some large buildings and vegetation are
distributed; they showed excellent agreement between the predicted and measured results [Tagaet al., 1999b].
C.Entrance systems
The BS antennas in systems beyond IMT-2000 will be mounted on the tops of buildings, notradio towers, because of the large number of BSs needed. Kitao et al. reported the time delay
characteristics observed in the entrance link between the BSs mounted on building roofs andthe time delay reduction achieved by using directional antennas to establish the entrance link
[Kato et al., 1999].
D.Formulations of mobile propagation
There has been a lot of investigations in the mobile radio communication field toward
formulating the radio propagation characteristics to evaluate the transmission quality of
wideband signals. Inoue et al. proposed a theoretical method based on an eigenvalue
decomposition technique for analyzing the received level variation characteristics of wide bandsignals in Raleigh fading environments [Inoue and Karasawa, 1999]. Yan et al. proposed atheoretical method for expressing the received level variation by using a key parameter called
equivalent received bandwidth (ERB) which is given as the product of the received bandwidth
and the maximum difference of path lengths [Yan and Kozono, 1000a; Yan and Kozono,
1999b; Yan and Kozono, 2000]. Nakabayashi et al. also used ERB in their experimental
investigation of the propagation charac teristics [Nakabayashi and Kozono, 2000; Nakabayashi
et al., 2001]. These studies appear to clarify the relation between the spatio-temporalpropagation characteristics of waves arriving at the receiving antennas, the signal's bandwidth,
and the received level of waves combined in the received antenna.
E. Indoor propagation
Several reports used the ray tracing method to predict indoor propagation characteristics andthe effectiveness of this technique has been confirmed. Imai et al. proposed algorithms for
efficiently calculating reflection and diffraction from partitions, pillars, and furniture in a room
[Imai and Fujii, 2000a]. They constructed a prediction tool using these algorithms andconfirmed their accuracy in a comparison with measurement results [Imai et al., 2000b].
Tachikawa et al. used a ray tracing calculation for estimating the time delay reduction possible
by using a tilt beam antenna in the MS when an Ad Hoc network between MSs is constructedin an indoor radio LAN environment [Tachikawa et al., 2001].
F. Spatio-temporal channel modeling and measurement method
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Recently, many research organizations have been studying and investigating spatio-temporalchannel models. Ray tracing is one candidate that yields useful predictions. Zhu et al.
presented a 2D-3D hybrid ray tracing method and calculated the TOA and DOA characteristics
for some stationary environments; they validation its effectiveness experimentally [Zhu et al.,
1999; Zu et al., 2001].
As a method of measuring TOA and DOA characteristics in mobile propagation environments,
Sekizawa et al. developed a system consisting of a linear array antenna and a PN correlation
channel sounder, and a method for obtaining the solution of rays by using 2D-DFT [Sekizawa
et al., 2000]. They carried out an experimental investigation using the equipment in an urban
street micro cell environment, and have presented their results [Sekizawa et al., 20001].
Tanabe et al. studied the DOA/TOA solution method using 2D-MUSIC [Tanabe et al., 2000],
and Mori et al. studied a DOA estimation method using a sector antenna or a DBF antenna[Mori et al., 2000; Mori et al., 2001]. Unfortunately, such antennas are not used in practice yet.
Takeuchi et al. developed a channel sounder using PN correlation detection with high TOA
resolution of 0.5 [ns] [Takeuchi et al., 2001]. Multi-path propagation structure, coupled DOA,
TOA and relative field strength, in the urban mobile communication environment of 25GHz
band was measured by means of super resolution techniques [Kuwahara et al., 1999]. These
detailed investigation of the propagation mechanism is continuing to advance thespatio-temporal channel model.
G. Others (Tunnel, ITS)
Several reports considered ray tracing methods for predicting the propagation characteristics intunnels. Uchida et al. clarified the conditions for achieving sufficiently accurate ray tracing
calculations, which radius of tunnel curvature is larger than 20 wavelengths [Uchida et al.,1999]. Fujimori et al. used ray tracing to determine the optimum beamwidth of BS antenna inorder to realize a radio zone in a tunnel efficiently [Fujimori and Araki, 1999].
For the intelligent transport system (ITS), Suwa et al. presented the time delay characteristics
in the propagation environment established when the MS antenna is mounted on a car and the
BS antenna set up at the road side with and without shadowing due to others car, and the timedelay reduction effect achieved by using beam antenna at both MS and BS [Suwa et al., 1999].
Karasawa used ray tracing to estimate the impact of the ground reflected rays on car-to-car
communication in the 60 GHz band [Karasawa, 2000].
Arai et al. presented a propagation simulator to evaluate the performance of portable handsets
in a quasi practical field [Arai et al., 2000].
(T. Taga)
F2.1 Atmosphere
In this section, remote sensing of atmosphere from the ground to the altitude of about 100 km isreviewed.
A.GPS meteorology
GPS (Global Positioning System) meteorology is a novel sounding technique of the atmosphere,by measuring propagation delay time of radio waves transmitted by GPS satellites. It provides
us with information of electron density in the ionosphere, temperature in the stratosphere andhumidity (water vapor) in the troposphere. A ground-based GPS receiver network by
Geographical Survey Institute of Japan can provide precipitable water vapor (PWV) with ahorizontal resolution of about 50 km every 12 hours, which compares well with objective
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analysis data [Iwabuchi et al., 2000]. With the receiver on board the LEO (Low Earth Orbit)
satellites, the global measurement and a better vertical resolution could be obtained. From
GPS/LEO observations, temperature profiles between the upper troposphere and stratospherehave been analyzed and global distribution of atmospheric gravity waves, which is a major
phenomena of transporting momentum and energy from troposphere to upper middle
atmosphere, has been presented [Tsuda et al., 2000a]. Global distribution of tropopause height
was also reported by Nishida and Tsuda [2000]. Global distribution of sporadic E layer andionospheric irregularity have also been observed using GPS/LEO [Hocke and Tsuda, 2001;
Hocke et al., 2001]. It is noteworthy that holographic analysis of GPS/LEO data could provide
high resolution profiling and detailed structure in D-region ionosphere (60-95km) with 300-500m resolution has been reported by Igarashi et al. [2000].
B. Various techniques of observation in troposphere with MST radars(MST = Mesosphere Stratosphere Troposphere)1) Spatial domain interferometer (SDI)
Among interferometric technique in atmospheric radar, SDI has a longest history. The MU
radar of Kyoto Univ. is suitable to carry out different setup of SDI method because of its
flexible system design. Horizontal wind velocity could be obtained accurately even in thepresence of high aspect sensitivity using multi-beam SDI method [Kawano and Fukao, 2001].
Tilt of turbulence layer due to meteorological disturbance has also been clarified using such atechnique [Kawano et al., 2001]
2) Frequency domain interferometer (FDI)
FDI using two (or multiple) radar frequency has been developed significantly in this three years,
using the MU radar. About the estimation of layer thickness using FDI method, biases due tomultiple scattering layer [Luce et al. 1999], limited horizontal extent [Luce et al., 2000a], tiltsof the scattering layers [Luce et al., 2000b] are carefully discussed. Furthermore, FII(Frequency domain interferometric imaging) technique by means of multiple radar frequency
has been executed and complex structure associated with Kelvin Helmholtz instability has been
observed in detail [Luce et al., 2001a, b]3) Many beam method
By using the rapid beam steerability of the MU radar, many beam observations with beam
numbers between several tens to hundreds have been carried out. Such observations could
detect detailed structure of the echo intensity and wind velocity fields. Significance of thevertical shear of horizontal wind to the echo power is suggested, which is associated withgravity waves and affects the measurement of vertical wind velocity in atmospheric radar[Worthington et al., 1999a, b]4) Application to the meteorological observation
Wind structures around Baiu front [Shigaraki et al., 2000] and gravity waves associated withthe front [Ogino et al., 1999] have been reported by using MU radar and other observation
techniques such as radiosondes, C/Ku band radars etc. Structure of a typhoon has also studied
using MU and L-band boundary layer radar [Teshiba et al., 2001].
C. Radio Acoustic Sounding System (RASS)RASS is a technique of tropospheric and stratospheric temperature sounding using acoustic
wave sources and a radar. By measuring Doppler velocities of the radar waves scattered byacoustic wave front, temperature can be derived. Using this technique, turbulence energy
dissipation and vertical eddy diffusivity in the troposphere has been studied in detail by the MU
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radar [Hermawan and Tsuda, 1999]. Furthermore, estimation method of energy dissipation rate
has been carefully investigated and it has been found that the previous measurement using
radar echo intensity could be biased due to humidity profile, and advantage of spectral width
method has been proved using
MU radar/RASS experiment [Furumoto and Tsuda, 2001]. It should also be noted that humidity
profiles can be derived by combining RASS and turbulent echo intensity observation with the
VHF radar, which is a novel technique of sounding humidity with high time resolution [Tsudaet al., 2001].
D. Mesosphere and lower thermosphere (MLT)
MLT region is the atmosphere of 50 - 150 km altitude. This region has recently become of greatinterest because it is a boundary region between the neutral atmosphere and the upper
atmosphere. The former is changed with the meteorological variation and climate change of thelower atmosphere, and the latter varies under a strong effect of solar activity. For example,
temperature of MLT region is considered to be cooled largely corresponding to the greenhouseeffect due to the change of radiative balance in the middle atmosphere. Therefore, the precise
measurement of MLT region and clarification of variation and trend of the physical parametersof this region, as well as understanding of the dynamical process there, is of great importance.
Continuous monitoring of this region has been carried out using meteor radars and MF(medium frequency) radars, and wind velocities and various atmospheric waves have beenstudied. MST (Mesosphere Stratosphere Troposphere) radars have also contributed especiallyto the observation of short period (down to 5 min in period) gravity waves. Mean wind and its
variation have been extensively observed with MF radars in Wakkanai (45 deg N) andYamagawa (31 deg N) [Namboothiri et al., 2000; Kishore et al., 2000]. On the other hand, as
an MST radar the MU radar in Shigaraki (35 deg N) has been observing the mesosphere for along time since 1986, and interannual variability of mean winds has been closely analyzed[Namboothiti et al., 1999; Gavrilov et al., 1999]. Signals of QBO (quasi-biennial oscillation)and longer term variability have been found. Atmospheric waves which transport energy andmomentum in the middle atmosphere have also been studied extensively with these radars.
These studies includes intensity and momentum fluxes of gravity waves with the MU radarlong term observations [Gavrilov et al., 2000a, b; Gavrilov and Fukao, 1999], and semidiurnaland diurnal tides by MF radars [Hocke and Igarashi, 1999; Igarashi et al., 1999].
It is also notable that Japanese observation in equatorial region, especially in Indonesia hascontributed to the understanding of equatorial atmospheric dynamics. Atmospheric tides andultra fast Kelvin waves have been intensively studied with Jakarta meteor radar (6 deg S),
Pontianak MF radar (0 deg N) and international collaboration with other radars [Tsuda et al.,
1999; Yoshida et al., 1999].
It should also be stressed that coordinated observation of radar and optical measurement (lidar,
airglow imager, photometer) has been extremely promoted in the last three years [Tsuda et al.,2000a]. Cooperative sodium lidars and MU radar observation clarified the association between
dynamics and atomic sodium layer/sporadic sodium layer [Miyagawa et al., 1999; Kobayashi
et al., 1999]. Airglow imaging observation has contributed to the studies of small scale gravity
waves, especially on the propagation characteristics in the middle atmosphere [Nakamura et al.,
1999; Ejiri et al., 2001; Shiokawa et al., 1999]. Rotational temperature measurement using OH
airglow and cooperative observation with radar-derived winds has provided important
information on the structure of gravity waves in the MLT region [Takahashi et al., 1998, 1999;
Buriti et al., 2000].
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Important technical advances in this three years are 1) high altitude (up to 120 km) windobservation using MF radar meteor echoes [Tsutsumi et al., 1999], 2) evaluation of bias inwind measurement due to signal saturation in MF radar [Yamazaki et al., 2000], 3) new
analysis method to improve height range of mesosphere observation with an MST radar[Takeda et al., 2001].
E. Spaceborne Sensors
National Space Development Agency (NASDA) and Communications Research Laboratory arecollaborating to develop a spaceborne submillimeter-wave limb sounder called
Superconducting Submillimeter-Wave Limb-Emission Sounder (JEM/SMILES) to be aboard
the Japanese Experiment Module (JEM) of the International Space Station. JEM/SMILES isdesigned to observe global three dimensional distributions of stratosphericozone-depletion-related gases such as O3, ClO, BrO, HCl, HNO3, and HO2 in
submillimeter-wave bands around 640 GHz by a highly sensitive SIS receiver operating at 4.5
K. Simulation studies based on the detailed instrumental design have demonstratated its
capability of retrieving stratospheric minor constituents. JEM/SMILES is scheduled to be
attached to the Exposed Facility of JEM in 2005 [Seta, et al., 2000; SMILES Mission Team,2001].
(T. Nakamura)F2.2 Hydrometeors and Other Particles
Measurements of prec ipitation and cloud have been among major remote sensing researchareas in Japan, a part of which is a continuation and expansion from rain-attenuation studies for
Earth-space communication links using ground-based radars. The other expertise comes fromradar meteorology and hydrology for weather forecast and disaster prevention, and related
atmospheric science studies. In 1997, the Tropical Rainfall Measuring Mission (TRMM)
satellite [Kummerow et al. 2000] was launched as a joint project between the U.S.A. and Japanthat carries the first spaceborne rain radar [Kozu, et al. 2001]. Because of the rapid progress inspaceborne sensor technology, many studies on space-based remote sensing have also been
performed in recent years.
In the following, recent research activities in Japan are reviewed by classifying into
ground-based and space-based remote sensing techniques and observation results.
A. Ground-based remote sensing studies
The main target of weather radars has been precipitation particles. They have tw o functions,one of which is to observe the amount of precipitation (including the liquid equivalent offrozen hydrometeors). The other function is to observe wind velocity field through the Doppler
effect of the scattered electromagnetic wave from moving particles.
Recently, radars with shorter wavelength have appeared to observe the non-precipitating,floating particles. Their main targets are cloud and fog.
For the observations of precipitating particles, many researches have been made to translate theecho intensity into the amount of precipitation. This relation is called Z-R relationship. It is
well known fact among radar meteorologists that this translation is not unique. While the echo
intensity is proportional to the distribution of 6th power of diameter of falling particles, theamount of precipitation is proportional to the distribution of 3rd power of diameter and falling
velocity of the particles.
Moreover estimation of the rain rate by echo intensity has non-negligible error for an echo8region that lies behind a strong echo region due to attenuation of scattered wave through thestrongly precipitating region.
To overcome these problems, many efforts have been made. One type of approach is tocompare echo intensity with the drop size distribution by using disdrometer [Maki et al. 2001]
or wind profiler [Kobayashi and Adachi 2001] and to establish new Z-R relationships
depending on the types of precipitations (convective type or stratiform type etc.). The othertype of approach is to use dual-polarization Doppler radar that has many parameters todetermine rainfall rate without using echo intensity [Timothy et al. 1999].
As the Doppler velocity is a component of a velocity vector along the beam direction, it is often
unsatisfactory for meteorologists who want to know the real wind vector field. Recently new
Doppler radar system called bistatic Doppler radar system was developed. This system iscomposed of a traditional Doppler radar and single or multiple receivers that can determine thewind velocity vector with high precision because a single Doppler radar illuminates the targets.
As the receivers catch obliquely scattered electromagnetic wave, geometrical aspect of signal
processing is much more complicated than the usual Doppler radars that use only backward
scattering. Satoh and Wurman [1999, 2001] discussed this geometrical issue with observed data.
Takaya and Nakazato [2001] theoretically argued the precision of the determined velocity
vector by a bistatic Doppler radar system.
Fog observation by scanning weather radar was first realized with Ka band (35GHz) radar.
Akaeda et al. (2001) observed marine advection fog to reveal that there are three types of fog;
stratiform, roll and open cell. Hamazu et al. [2001] compared their wind data by Ka-band
Doppler radar with the data by the MU radar (50 MHz), to confirm the proper acquisition of
Doppler velocity by their radar. Applying it to marine advection fog, they made clear that the
movement of the fog is governed by the wind just above the fog layer. An observation wasmade by a W-band (95GHz) polarimetric Doppler radar to observe the internal structure ofdeep stratiform ice cloud [Fujiyoshi et al. 1999]. The cloud showed convective cell structure atits upper level, streaks at midlevel and thin stratiform structure at lower level.
More sophisticated multi-parameter radar (dual polarization and dual wave length) was
developed to observe a precipitation system from its formation stage that is a cloud system
without precipitation [Iwanami et al. 2001, Wakayama et al. 2001]. It is also reported that UHF
wind profilers can even be used to observe cirrus cloud properties [Kobayashi et al. 2001]. It isadvantageous to observe the cloud “downward” to avoid severe attenuation by rain. A
multi-parameter (multi-polarization and Doppler) airborne cloud radar (SPIDER) was
developed [Horie et al. 2000] for scientific studies and as a test-bed for future spaceborne cloudradar.
In measuring rain with a millimeter wave dual-polarization radar, multiple scattering effects
can be significant in polarization signatures and care has to be taken in the interpretation of theobserved result. A basic theoretical and experimental study for the multiple scattering
properties was done by Tazaki et al. [2000].
B. Space-based remote sensing studies
Studies on precipitation and cloud remote sensing from space in Japan may be categorized in
the following four aspects; (i) system study and development, (2) algorithm development andrelated precipitation modeling, (3) validation of space-based measurements, and (4) basic
studies for next-generation spaceborne radars.
Major activities in the category (1) are to summarize the development of TRMM Precipitation
Radar (PR) based on in-orbit verifications [Okamoto 2000; Kozu et al. 2000; Kozu et al. 2001],9

studies for the TRMM-follow-on mission concepts and system studies [IHAS and ESTO 1998],
and studies for a cloud observation space mission with a millimeter-wave radar [CRL and
ESTO 2000]. It should be noted that the TRMM PR still functions normally and keeps almost
the same performance after 4 years in orbit, which enhances the value of long-term global
distribution of rainfall and 3D rain structure database.
There have been many studies regarding the category (2). “Standard algorithms” used for the
TRMM PR have been developed in joint efforts by Japan and the U.S.A. [Okamoto, 2000] andstill are under-improvement, especially after the TRMM altitude was raised to 400 km from theoriginal 350 km in order to extend the life time. Since the radar reflectivity data from the
TRMM PR are well calibrated [Kozu et al. 2001], interfaces between algorithms for “Level-1”(i.e. received power and reflectivity) and “Level-2” (rain rate, surface radar cross-section andrainfall-type) products are well defined. However, there are various difficulties in making theLevel-2 algorithms stable for various surface conditions and rainfall properties, in particular for
off-nadir observations (greater than about 10 degrees). It has been reported that the rain ratefrom the TRMM PR is somewhat lower than those from other TRMM products andground-based raingauge measurements [Kummerow et al. 2000]. Outlines of the PR Level-2
algorithms and improvement efforts are described in Meneghini et al. [2000], Iguchi et al.
[2000]; and Awaka [2000].
In addition to the “standard algorithms”, Aonashi and Liu [2000] have been developing andimproving passive microwave algorithms utilizing the simultaneous active and passive
measurements by TRMM. They concluded that the inclusion of 10-GHz channel into thepassive microwave retrieval is effective to improve the retrieval accuracy and that a general
way to make correction for the inhomogeneity of rain within a radiometer field of view(especially for 10-GHz channel) has to be developed. The beam filling error correction is also
crucial even in the PR measurement that has much finer spatial resolution (4.3 km) than TMI[Kozu and Iguchi 1999; Iguchi et al., 2000].
The information of rain height and vertical structure is important because it is closely related to
the vertical profile of latent-heating, to improving both active and passive rainfall retrieval
accuracies, and to improving the accuracy of attenuation prediction for satellite-Earth paths.
The TRMM PR made it possible to globally measure the rain height and vertical structure withhigh reliability. Short and Nakamura [2000] made a comprehensive statistical analysis of
TRMM PR-derived rain height and found that a distinct “bimodal” distribution exists over theoceans (not over land areas), with the lower and upper modes of about 2 km and 5 km,
respectively. Thurai and Iguchi [2000] compared the 0 degree isotherm height model given by
the ITU-R Rec.839 with the bright band height statistics (zonal mean) obtained from the
TRMM PR, and found that they agree very well considering that the bright band “peak” (theheight of peak reflectivity) generally appears several hundreds meters lower than the 0 degree
isotherm height. Comparisons classified into continental and oceanic areas may be necessary tovalidate the ITU-R model in more detail.
The validation of TRMM algorithms and products have been among the major efforts after the
TRMM launch. Such efforts are also described in the papers by algorithm developers[Meneghini et al. 2000; Iguchi et al. 2000; Awaka 2000]. In addition, Ohsaki [2001] andOhsaki et al. [2001] validated the detection of rain area with the TRMM PR, and examined thescan-angle dependence of the mean storm height obtained from the TRMM PR by using
ground-based radar data as well as theoretical considerations. Sato and Kishore [1999] made aseries of very careful comparisons between radar reflectivities obtained from the TRMM PR
and the Middle and Upper atmosphere (MU) radar at Shigaraki, Japan, and found that they10

agree within about 1 dB, indicating the validity of calibration accuracy of the TRMM PR
evaluated in Kozu et al. [2001].
As for the basic studies on the dual-frequency radar for the TRMM-follow-on mission andspaceborne cloud radars, comprehensive presentations were made in IHAS and ESTO [1998]
and ESTO [2000]. In addition, Sato et al. [2001] made a laboratory experiment of adigital-analog pulse compression system with a very low range sidelobe level that is important
to improve the sensitivity of spaceborne rain/cloud radar in near future, because a “full” digitalpulse compression would be beyond the capability of near-future onboard real-time processing.
Their result is encouraging but still needs efforts to compensate the waveform distortion due tothe saturation in a high-power radar transmitter.
(T. Kozu and Y. Takaya)
F2.3 Ocean and Ice
Ocean wave direction wad determined by newly proposed split-look processing of the synthetic
aperture radar (SAR) from the JERS-1 SAR data [Ouchi, 1999]. Concerning with the analysis,
the theory of ocean wave backscattering was proposed [Ouchi, 2000].
Sea ice in the Okhotsk sea were investigated by using spaceborne and airborne SAR andmicrowave radiometer. Ice type classification were carried out by the X- an L-bandpolarimetric airborne SAR (Pi-SAR) and resulted that the dual frequency polarimetry is useful
to detect the ice characteristics in very thin ice [Matsuoka, 2001]. Also, such polarimetrictechnique applied for saline lake (Saroma lake) solved the ice thickness [Wakabayashi, 2001].
Methane bubbles in the lake ice were detected by satellite and airborne SAR[Matsuoka, 1999].
(S. Uratsuka)
F2.4 Land, Vegetation and Others
Fully polarimetric and synthetic aperture radar sensing as well as newly developed polarimetric
SAR interferometry have been attracting attention [Boerner et al., 2000] from theory to data
analysis, in various areas including land, vegetation, and others, during these three years.
The basic principle of radar polarimetry is devoted to find optimum polarization states fortarget enhancement and for retrieving target information [Yang, et al., 1999, 2000, 2001].
Compound scattering matrix is examined by Kitayama [2001]
Polarimetric and Interferometric airborne SAR (Pi-SAR) system has demonstrated its potential
performance with high resolution (1.5 by 1.5 m at the X-band, 3 by 3 m at the L-band), fullypolarimetric (HH, HV, VH, and VV, both in the X- and L-bands) and simultaneousinterferometric data acquisition function at the X-band [Kobayashi, et al., 2000]. The image
acquired with Pi-SAR includes forest, city, mountains, ocean, and volcano activities [Uratsuka,
et al., 2001], with top-of-the world quality, and it attracted a great reputation among radar
experts in the world. This Pi-SAR data will also provide valuable information for
ALOS-PALSAR satellite system to be launched in 2004.
There were several implementations of fully polarimetric radar systems. One is a real-timepolarimetric FM-CW radar system, developed at Niigata University [Nakamura, 1999]. The
radar is especially suited for short range sensing such as snowpack application and subsurface
sensing [Moriyama, et al, 1999, 2000].The other is a polarimetric borehole radar system fordetecting cracks and anomalies within underground structure developed at Tohoku University[Miwa, et al., 1999, 2000]. It utilizes circular/conformal array antennas for directionalpolarimetric scattering matrix measurement. From the scattering matrix information together
with super -resolution technique, the detailed ground structure and fracture were retrieved
11

[Sato, et al., 2000; Ebihara, et al., 2000].
For obtaining ideal polarimetric data, calibration is mandatory. Using an active calibrator with
30dB isolation together with polarization selective dihedral reflectors developed at CRL, Fujita
et al. [1988] succeeded in the calibration of SIR-C data and Pi-SAR data.
As far as SAR image analysis is concerned, Kurosu, et al. [1999] showed textural feature forland cover classification based on continuous observations with JERS-1 SAR. Polarimetric data
are well classified with textural feature [Hoshi, et al., 1999], wavelet textural feature [Fukuda,
et al., 1999], polarimetric index derived from scattering matrix elements [Yoshioda, et al.,1999], and scattering mechanisms [Hosokawa, et al., 2000].
Polarimetric response of decorative trees is examined in a laboratory by Murase, et al., [2000],
and analyzed by several polarimetric indices [Yamaguchi, et al., 2001].
A super-resolution method applied to polarimetric coherency optimization yielded an
accurate estimation of tree height [Yamada, et al., 2001]. The novel technique is resulted froma combination of polarimetry and interferometry (Pol InSAR) and locates accurate scattering
centers corresponding to ground and canopy. This method is successfully applied to Briatia(Russia) data acquired with SIR-C/X-SAR [Sato, et al., 2001] and estimated forest height.
(Y. Yamaguchi)
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