Aerospace and Electronic Systems Magazine March 2018 - 48

Feature Article:

DOI. No. 10.1109/MAES.2018.160267

HF Surface Wave Radar for Tsunami Alerting: From
System Concept and Simulations to Integration into
Early Warning Systems
Anna Dzvonkovskaya, Helzel Messtechnik GmbH, Kaltenkirchen, Germany

INTRODUCTION
An important property of the high-frequency (HF) band (3-30
MHz) is the unique ability of HF signals to propagate over long
distances and to illuminate the Earth's surface beyond the horizon.
Historically, over-the-horizon (OTH) HF radar systems were divided into two main categories: (1) those using sky wave propagation in different ionospheric layers and reaching thousands of
kilometers and (2) others using the surface (ground) wave mode
when the signals couple well to the conducting ocean surface and
hence potentially enabling propagation and observations hundreds
of kilometers from shore (depending on the radar parameters and
operating conditions). Excellent descriptions of both types of OTH
systems, including their characteristics and functionality, appear
in [1]. Originally, both types of OTH radar were intended to fulfill
military tasks of air and maritime surveillance far beyond the horizon. However, other systems, generally categorized as HF ocean
radars, have been developed for civil applications, with remote
sensing of the sea surface as a primary goal. These applications include spatial mapping of current velocity, significant wave height,
wave spectra, and surface winds. A large number of experimental and commercial HF ocean radar systems have been deployed
worldwide for remote-sensing purposes [2], [3].
A new focus in the development of ocean radar applications
occurred after Boxing Day on December 26, 2004, when a severe
tsunami was generated by a 9.1-magnitude earthquake in Northern
Sumatra. It was the deadliest tsunami in recorded human history,
causing an enormously large number of casualties because a tsunami warning was not disseminated to local governmental entities.
After this catastrophic event, the German Federal Ministry of Education and Research sponsored the German-Indonesian Tsunami
Early Warning System (TEWS) project, aimed at implementing
an operational and effective life-saving system for Indonesia, InAuthor's current address: Helzel Messtechnik GmbH, Research and Development, Carl-Benz-Strasse 9, Kaltenkirchen,
24568 Germany, E-mail: (dzvonkovskaya@helzel.com).
Manuscript received November 30, 2016, revised April 27,
2017, and September 11, 2017, and ready for publication September 18, 2017.
Review handled by D. O'Hagan.
0885/8985/18/$26.00 © 2018 IEEE
48

dia, and possibly all countries surrounding the Indian Ocean. The
main efforts in this initiative have been expended in developing
and installing a suite of ocean bottom pressure sensors by adding global positioning system technology for water level measurements to detect a tsunami offshore [4]. Additionally, the German
Aerospace Center committed to the development of novel concepts
relying on ground-based and spaceborne radar systems [5]. At this
point, the HF ocean radar was considered as a valuable component
of a system suitable for detecting a first incoming tsunami wave
far offshore and providing an early alert. Although the first basic
ideas regarding an ocean radar application for tsunami detection
were stated in 1979 [6], they were not significantly explored until
2006, because no HF radar system was specifically optimized for
tsunami monitoring in real time. However, since then, extensive
research has been conducted on how to utilize the ocean current
measurement capabilities of existing HF radar systems for tsunami
monitoring, e.g., [7]-[10]. In [7], [8], the advantages and disadvantages of HF sky wave radar are discussed. Although the potential spatial coverage of HF sky wave radar is larger than that of
surface wave radar, the ionosphere, on which sky wave propagation strongly depends, experiences large dynamical variations, and
even if signal strength is adequate, most prospective tsunami signatures are degraded by these fluctuations. Therefore, more attention has been paid to surface wave radar applicability, especially to
civil ocean radar systems installed at the coast [9], [10].
In this article, three main HF ocean radar systems are addressed
because they have delivered a significant contribution to HF surface wave radar technology for tsunami observation. Namely,
these systems are the frequency-modulated continuous wave form
(FMCW) phased array radar system with software beamforming
WERA by Helzel Messtechnik GmbH (Germany), the frequencymodulated interrupted continuous wave form (FMICW) directionfinding radar system with cross-looped antenna SeaSonde by CODAR Ocean Sensors Ltd. (United States), and the FMICW phased
array radar system with digital beamforming by Nagano Japan
Radio Corporation (NJRC; Japan).

HF RADAR SYSTEM CONCEPT FOR TSUNAMI MONITORING
Although HF ocean radar is well accepted as a valuable instrument for mapping surface currents and waves in coastal waters,

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MARCH 2018

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