High-frequency Doppler radar (HFDR) has been used by the coastal oceanography community for several decades to map ocean currents and waves several hundred kilometers from shore.1 HFDR information can be crucial for both search-and-rescue missions and tracking of pollution plumes from accidental spills. In addition, new research is promoting the technology as an effective means to track large ships, small boats, and low-flying aircraft around coastal waters, which is essential for the effective surveillance of entire coastlines.
Low-cost, low-power, autonomous HFDR stations are needed for operations in island and other remote environments (see Figure 1). In recent years, data from these stations have increasingly been integrated into maritime-domain awareness (MDA) programs. Islands present special challenges in this context because of their extensive, convoluted coastlines and remote locations. A mountainous island only 30km across may possess more than 90km of coastline, yet might require four or five HFDR installations to cover all approaches. Many islands also lack the modern infrastructure necessary to power such stations and transfer data for real-time processing.
Figure 1. Coastal high-frequency Doppler-radar installation.
Similar problems exist for locations such as Alaska's North Slope. Alaska accounts for roughly half of the US coastline and large areas are affected by the presence of seasonal Arctic sea ice. The thinning and retreating ice continues to expose larger stretches of Alaska's ice-locked coast to industrial activities and maritime traffic. Oil and gas development is advancing into coastal waters, which introduces significant risks of oil spills in one of the most extreme and challenging ocean environments in the world.
The National Center for Island, Maritime, and Extreme Environment Security (CIMES) was established by the Department of Homeland Security in June 2008. Based at the University of Hawaii at Manoa (UHM), CIMES is a partnership between UHM, the University of Alaska at Fairbanks (UAF), and the University of Puerto Rico, Mayaguez.
CIMES' mission is to develop a leading, multi-institution research and education program in MDA. The center's initial research portfolio includes remote sensing, underwater acoustics, decision-support systems, and HFDR. The latter project has focused on three aspects: data analysis and processing for MDA applications, low-cost, low-power electronics and antennas, and support systems to both power the radars and retrieve data from remote installations.
HFDR exploits ducting (or tunneling) of radio waves along the conductive seawater surface. Ducting occurs when sudden changes in atmospheric properties make microwave, ultrahigh- and very-high-frequency signals propagate hundreds of kilometers beyond the normal radio horizon (and on rare occasions even further). A typical system broadcasts a frequency-modulated electromagnetic wave through an array of transmitting antennas and listens to the signals reflected by ocean waves through an array of receiving antennas.2 The amplitudes, phases, and Doppler shifts of these signals carry information from which surface currents, the surface-wave spectrum, and wind direction can be inferred. Mobile man-made targets are easily detected through their large Doppler shifts, which clearly stand out from the background noise.
HFDRs use surface-ducted electromagnetic ground waves that follow the Earth's curvature, which allows data to be retrieved from beyond the horizon, out of the antennas' line of sight. Unlike sky waves, which are affected by unpredictable ionospheric conditions, ground waves are a reliable means of propagation of up to hundreds of kilometers over seawater. Signal attenuation limits the maximum range typically to 60 and 120km at 30 and 15MHz, respectively. An absolute limit of 250–300km is imposed by spurious signal reflections from the ionospheric E layer, which blanks out more distant ocean echoes.
Semi-permanent HFDR stations are now being set up on the south side of the island of Oahu (Hawai'i) to monitor approaching commercial boat traffic (see Figure 2). Simultaneously, engineers at UHM have successfully tested the prototype of a new low-cost, low-power radar receiver and measured performance exceeding that of commercially available receivers. After all three stations will have been established on south Oahu (completion due in late 2009), additional HFDRs will be configured for rapid deployment, to be used in support of the US Coast Guard's Search and Rescue Optimal Planning System. Portable systems will also be used to explore mountain-top configurations.
Figure 2. Computer-generated 3D topography of the south coast of the island of Oahu (Hawai'i). Black dots on the inset map correspond to the radar locations indicated by yellow crosses on the main image.
Researchers based at UAF are currently developing a remote HFDR system for the Arctic coast, which will allow a year-round presence even during times of continuous darkness. Synergy between the island and Arctic efforts will continue to improve our understanding of advanced surveillance techniques in remote regions.
Department of Oceanography
Pierre Flament is a professor. He is the lead investigator of CIMES' coastal-radar effort and a recognized authority in the study of coastal oceanography and instrument development.
Roy Wilkens is a senior research scientist at the Hawaii Institute of Geophysics and Planetology and the director of CIMES. His research interests include mapping seafloor properties, paleoceanography, and marine instrumentation.