Aerospace and Electronic Systems Magazine March 2018 - 53

Dzvonkovskaya
dar system was installed at the eastern
coast of the Kii Channel, at a range of
about 1,000 km from the epicenter near
Honshu Island. The radar consisted of
one transmitting (three-element Yagi
type) and eight receiving antennas and
digital beamforming provided a bearing resolution of 7.5° in the region of
±45°.The radar system used FMICW
to transmit with a center frequency of
24.5 MHz and a sweep bandwidth of
100 kHz, resulting in a spatial resolution of 1.5 km. In the tsunami monitoring mode, the radar was continuously
transmitting, producing radial current
velocities every 2-3 minutes. A timedistance diagram of the band-passed
(9-200 minutes) radial velocity along
the main beam reveals the tsunami
Figure 4.
wave train propagating from the contiMeasured water level at the tide gauge in Lebu, Chile, on March 12, 2011, and water level data from the
nental shelf slope to the inner channel,
NOAA tsunami propagation model (data courtesy of D. Figueroa, University of Concepcion, Chile, and
as the first three progressive waves,
C. Moore, NOAA, United States).
and then natural oscillations of 30-40
minutes, were excited by the waves.
The observational data on currents
pened and indicated one wave period less in the tsunami train.
from a 16-MHz WERA radar system with 12 receive antenna eleMoreover, the modeled water elevation is half as large as that meaments deployed on the south shore of Oahu, Hawaii, were studied
sured; therefore, the predicted tsunami current velocity is correin [25] and revealed oscillatory radial currents after 7.5 hours
spondingly smaller.
from the 2011 Tohoku tsunami. Over Penguin Bank, a 50-m shalPursuing the Japan tsunami event in 2011, direction-finding
low water area extending west from Molokai, currents were sepaCODAR SeaSonde radar systems recorded strong tsunami signals.
rated into two distinct groups, one showing 0.27 m/s currents at
Several radars on two continents 8,200 km apart obtained the disa 43-minute period and lasting 6 hours and the other indicating
tinctive tsunami signatures in the 42-MHz radar system in Hok0.14 m/s currents at a 27-minute period and lasting 2 hours.
kaido, Japan, and 13- and 5-MHz systems in California [22].
The direction-finding systems of [22] use FMICW, which is
essentially a FMCW signal that is broken up into pulses, creating
ALGORITHMS FOR REAL-TIME TSUNAMI ALERTING
a 50% duty cycle. The signal is received by two loop antennas
(positioned at a 90° angle to each other) and a monopole. The softCertainly, one of the main challenges for HF radar signal analysis
ware determines the direction of the signal by the multiple signal
is determining how to distinguish a tsunami pattern among differclassification algorithm computing signal bearings every 5°. Traent ocean waves while monitoring in real time. In this section, the
ditionally, these systems use only one receive antenna pole and
techniques for tsunami detection by HF radar are considered.
have less spatial resolution, which explains why a system CIT of
at least 30 minutes is required to obtain a current velocity map.
This fact degrades the system capability for monitoring tsunami
THE DETECTION OF TSUNAMIS ON THE BASIS OF A
events in real time, although the collected measurements can be
DIRECT RETRIEVAL OF TSUNAMI CURRENT VELOCITY
used for further research. In [22], [23], the components of the radial velocity were averaged over 2-km circular bands, ranging up
The detection of tsunamis by HF radar is first considered by usto 8 km from shore. Several analysis methods revealed the onset of
ing a direct inversion of tsunami currents from the radar Doppler
the distinctive ∼40-minute period oscillations that were confirmed
spectra. This technique is typically limited to areas where such
in all cases by nearby tide gauges. Arrival times measured by the
currents are large enough, e.g., at least 5 cm/s greater than backradars preceded those at neighboring tide gauges by an average of
ground current velocity at the continental shelf. For the WERA
19 minutes in Japan and 15 minutes in the United States. In this
ocean radar system, the tsunami detection technique is always
work, it was found that tsunami arrival within the radar coverage
based on a statistical approach, which suggests using an alert
area can be announced 8 minutes after its first appearance.
scale as the probability of detecting a tsunami. The estimation at
Signals from the tsunami waves induced by the 2011 eartha particular time is based on residuals of measured radial velocity
quake and from subsequent resonances were also detected as radial
separated from tidal components, which is done by applying a
velocity variability by an NJRC HF ocean radar [24]. The HF ramoving polynomial regression spanning the previous 45-60 minMARCH 2018

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Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine March 2018