DEVELOPMENT OF A CLOUD PROFILING
FM-CW RADAR AT 94GHz
Toshiaki TAKANO (1,2,3), Yumiro SUGA(1), Kentaro TAKEI(2), Youhei KAWAMURA(2),
Tamio TAKAMURA(4), Hiroshi KUMAGAI(5), and Teruyuki NAKAJIMA(6)
(1)
Graduate School of Science and Technology, Chiba University,
Inage, Chiba 263-8522, Japan, takano@cute.te.chiba-u.ac.jp,
(2)
Faculty of Engineering, Chiba University,
(3)
Center for Frontier Electronics and Photonics, Chiba University,
(4)
Center for Environmental Remote Sensing, Chiba University,
(5)
Communications Research Laboratory, 4-2-1 Nukii Kita Machi, Koganei 184-8795,
(6)
Center for Climate System Research, The University of Tokyo, 4-6-1 Komaba, Meguro 153-8904
ABSTRACT
A cloud profiling radar transmitting frequency-modulated continuous wave (FM-CW) at 94GHz is
developed for ground-based observations. Millimeter wave at 94GHz is used to realize high sensitivity
to small cloud particles. Two 1m-diameter parabolic antennas separated by 1.4m each other are used for
transmitting and receiving the wave. The direction of the antennas is fixed at the zenith. The radar is
designed to observe clouds between 0.3 and 15 km in height with a resolution of 15 m. The system was
integrated and sensitivities and stabilities have been measured. Results of test measurements of clouds
will be presented.
INTRODUCTION
Comprehension of global environment and global change of climate is recently getting more and more important
for the human beings in order to solve severe problems such as the greenhouse warming, the drying, the ozone
holes and so on. It is necessary to know balance of solar energy coming to the Earth and cycle of water for the
comprehension. One of the most significant features to know them is cloud, which reflects and absorbs incoming
solar radiation, traps the radiation from the ground, transfers the energy in it, and radiates it outside. Information
on 3-dimensional structures of clouds, sizes and distribution of cloud particles, dependence on size of optical
characteristics of cloud particles, motions of particles in clouds, and so on are all desirable to solve role of
clouds.
Characteristics of clouds described above, however, have not been well known yet because of lack of enough
observational data to present confidential results. Observations of clouds with radars would be most powerful
method to derive the information because of following advantages: a) radio waves do not suffer from heavy
extinction such as visible light, and consequently can investigate interior of clouds, b) the radar technique, which
is an active sensing method, has great advantage of investigating interior structures of clouds to passive methods
such as total power observations of irradiance of clouds, c) Doppler measurements of clouds which have low
velocities around 10 m/sec is applicable only to radio frequency waves.
Conventional radars operated at 5GHz can detect precipitation particles but are not able to detect particles in
clouds because their sizes, less than a few tens microns, are much smaller that the wavelengths and, therefore, their
cross sections are quite small. The cross sections of particles increase rapidly with frequency in Rayleigh
scattering region. Rader observations of cloud particles at millimeter waves, which have been realized recently,
have much more sensitive [1]. Several groups have reported the development and preliminary observational results
that demonstrate powerful performances to investigate cloud particles [2],[3],[4].
We have designed and developed a cloud profiling radar at 94GHz. The purposes of the development of the
FM-CW radar are a) evaluation and verification of an FM-CW radar at 94 GHz in Japan comparing to a pulse
radar, b) obtaining millimeter wave FM-CW radar techniques and algorism of data reduction, and c) contribution to
scientific research on cloud physics. In this paper, we present design concepts and specifications of the radar as
well as results of test measurements of sensitivities, stabilities, and clouds.
DESIGNE AND CONSTRUCTION
Whole view of the developed radar is shown in Fig.1 [5].
FM-CW Radar
We adopt a frequency-modulated continuous wave (FM-CW) radar rather than a pulse radar because the former
can achieve more sensitive system than the latter if comparing with same instantaneous output power of
transmitted millimeter wave. The principle of an FM-CW radar is shown in Fig.2. The signal frequency is
modulated in the range of f0 F. Transmitted signal from one of the antennas is reflected by cloud particles,
returns, and is received by the other antenna with a delay time of t relative to the original transmitted signal.
Mixing the transmitted and received frequencies, beat frequencies fb are observed in the spectra, which are caused
by ensemble of clouds particles:
fb = 4 F r / (c Tm)
(1),
where r is the height of the clouds, Tm is the modulation interval, and c is the light velocity.
objects move in the line of sight, the frequencies of reflected signals change by fd :
fd = -2 ( f0 / c ) ( dr / dt )
(2).
Fig.1. The developed cloud profiling FM-CW radar at 94GHz.
When the
Design Concept and Requirement
Table 1. Designed parameters of antennas.
Because one of the purposes of the facility is
evaluation and verification of an FM-CW radar at
94 GHz, we design it to be a simple system so as to
develop with commercially available components
and to make maintenance and upgrade by ourselves.
We designed the facility to observe clouds
between 0.3 and 15 km in height with a resolution
of 15 m. The velocities measured as Doppler shift
should be less than 10 m/sec. The facility should
be mobile for measurement in variety of places.
Antenna Diameter
f/D ration of Antenna
Antenna Optics
Gain of Antennas
Beam Width
Antenna Separation
Direction of Antennas
Polarization
1m
0.35
Cassegrain
57 dBi
0.18 degree
1.4 m
Zenith
1 Linear
Antennas and Mounting
According with the requirements described above, we decided parameters of antenna listed in Table 1 [6].
Transmitter and Receiver Section
The block diagram of the transmitter and receiver section is shown in Fig.3 and parameters are summarized in Fig.
2. All signals including the transmitted FM-CW signal at 94GHz and local frequencies are generated from and/or
referred to two signal generators in 140 MHz range, which are synchronized each other.
Transmitted
signal
Received
fd
signal
Frequency
fo+F
fo
fo - F
f b +f d
Parameters
t
Tm
f b +f d
t : Delay time
fo : Center freqency
Tm: Modulation interval
F : Freqency modulation
fb : Beat frequency
fd : Doppler frequency
Time
Fig.2. Principle of an FM-CW radar.
f0 =
F =
T m=
Pt =
94.78 GHz
10 MHz
1 m sec
+27 dBm
Noise Figure
of Receiver 5 dB
Signal
Generator A
4.0GHz
Divider
140MHz
BPF
Modulated by
10MHz
BPF
94GHz
3.9GHz
Signal
Generator B
140MHz
Transmitting
Antenna
6
PLO
PLO
PS
PS
90GHz
108
3.9GHz
28
Receiving
Antenna
Spectrum
Analyzer
BPF
BPF:Band Pass Filter
BPF
BPF
PS:Power Splitter
PLO:Phase Locked Oscillator
Fig.3. Block diagram of the transmitter and receiver section.
Test Measurements
The integrated system has been measured on its stabilities and sensitivities in laboratories. Facilities for radar
measurements have high power and high stabilities of transmitted signals are necessary to obtain useful data. o
The final power amplifier for transmitting signal at 94GHz is cooled with a peltiert cooler to be around 50 C .
The noise figure of the pre-amplifier at 94GHz was measured to be around 5.5 dB. Long term stabilities and
sensitivities were measured and are good enough for our purpose.
After measurements of long term stabilities, we observe various kind of clouds at Chiba and compare to the total
power microwave emission, lidar observations, and so on in order accumulate examples. Algorism to derive
physical parameters from obtained data will be also developed in this process.
References
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Vol.12, No.2, p.201, 1995.
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