Aerospace and Electronic Systems Magazine March 2018 - 50

HF Surface Wave Radar for Tsunami Alerting

Figure 2.

Measured radial ocean surface current map with the superimposed simulated tsunami currents using a radar CIT of about 9 minutes (left) and 2 minutes
(right) [16].

of being detected offshore. As may be observed from Figure 1,
optimal velocity resolution for surface tsunami currents should be
better than 5 cm/s. This potentially allows observations beyond
the shelf edge, with a water depth of about 200 m. The velocity
resolution is improved if the CIT is increased; however, considering a tsunami wave periodicity of 10 to 60 minutes, an optimum
sampling interval of 0.5-4 minutes is a firm requirement for the
detection of tsunami events.

LONG-RANGE COVERAGE
To provide sufficient warning time, it is obvious that tsunami signatures must be observed as far from the shore as possible. Hence,
it is highly desirable to make measurements at the edge of the
continental shelf or even further to capture the dynamical changes
caused by the tsunami as it crosses the shelf edge [12]. Long-range
HF coastal ocean radar systems are well able to observe surface
current velocities beyond the horizon. Thus, if the shelf is 50 to 200
km wide, there is a potential tsunami warning time of 30 minutes
to 2 hours, respectively. Using HF radar technology, there is always a trade-off between the precision of the surface current speed
measurements and range coverage, with the latter simultaneously
depending on operating frequency, transmit power, antenna gain,
water salinity, and sea state.
Due to its ability to resolve spatially complex current patterns, the large-aperture phased array beamforming radar system is a good choice for tsunami observation. In [13], it was
shown that, to achieve range performance comparable to that of
phased array systems, a direction-finding system must operate
at a lower frequency and have a significantly longer CIT. If the
CIT is shortened to accommodate the expected period of 10-60
50

minutes for tsunami fluctuations, then the range of observation
is correspondingly shortened. One of the reasons for better range
coverage by phased array systems is that spatial processing by
beamforming has higher signal-to-noise ratio and greater directivity. The latter results in better angle resolution for velocity
components within the radar coverage. This is especially important because these components can have significant spatial variability due to the variation in bathymetry within the coverage
area.

SHORT INTEGRATION TIME AND FAST UPDATE RATE
To evaluate the formation of a tsunami on the shallow shelf, the
sampling interval needs to be as short as possible to capture the
highly changing currents at the shelf edge. An update interval
criterion of 0.5-1 minute is desirable to capture sinusoidal oscillations in surface current velocity in a tsunami wave train. Some
improvements to the NJRC radar system were suggested in [14],
where it was proposed to implement a short-time Fourier transform with a zero-padding technique to enhance resolutions in
both time and current velocity. Alternatively, the parallel time architecture already implemented in the WERA ocean radar system
allows a tsunami current estimation procedure with a 2-minute
CIT and a 30-second update rate similar to a vessel-tracking application [15].
Figure 2 shows an example of how the CIT influences the mapping of measured currents. Note that the shorter integration time
results in decreased sensitivity to detect small changes in velocity
caused by a tsunami event as follows from formula (2). Integrating
more than a half of the tsunami wave period leads to loss of information regarding the rapidly changing currents in space and time.

IEEE A&E SYSTEMS MAGAZINE

MARCH 2018

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