PAPER The Integrated Ocean Observing System High-Frequency Radar Network: Status and Local, Regional, and National Applications AUTHORS ABSTRACT Jack Harlan A national high-frequency radar network has been created over the past 20 years NOAA IOOS® Program or so that provides hourly 2-D ocean surface current velocity fields in near real time Eric Terrill from a few kilometers offshore out to approximately 200 km. This preoperational net- Lisa Hazard work is made up of more than 100 radars from 30 different institutions. The Inte- Scripps Institution of Oceanography, grated Ocean Observing System efforts have supported the standards-based ingest Coastal Observing R&D Center and delivery of these velocity fields to a number of applications such as coastal search and rescue, oil spill response, water quality monitoring, and safe and efficient Carolyn Keen marine navigation. Thus, regardless of the operating institution or location of the Scripps Institution of Oceanography, radar systems, emergency response managers, and other users, can rely on a com- Institute of Geophysics and mon source and means of obtaining and using the data. Details of the history, the Planetary Physics physics, and the application of high-frequency radar are discussed with successes Donald Barrick of the integrated network highlighted. Chad Whelan CODAR Ocean Sensors, Ltd. wave height spectrum at the Bragg In the 1990s, the Office of Naval Stephan Howden wave number. Barrick was invited to Research and the National Science Stennis Space Center, University present his results at seminars in Foundation funds were used to acquire of Southern Mississippi Boulder, Colorado, as the National radars at several universities includ- Josh Kohut Oceanic and Atmospheric Administra- ing the Oregon State University, the Rutgers University tion (NOAA), and its Boulder labora- Rutgers University, the University of tories were being formed in 1970. A California-Santa Barbara, the Naval group was formed within NOAA’s Postgraduate School, the University of History and Technical new Environmental Research Labora- Rhode Island, and the University of Background for HF Radar tories to build a compact antenna sys- Connecticut. This was followed by a History tem to be used for coastal ocean surface surge in acquisition because of the Na- The present state of the U.S. na- current mapping. This was the Coastal tional Oceanographic Partnership Pro- tional high-frequency (HF) radar net- Ocean Dynamics Applications Radar gram, an NOAA/Office of Naval work has resulted from nearly 40 years (CODAR) program. After demon- Research/National Science Foundation of research and applications. HF radar strating its effectiveness, the NOAA/ program that funded coastal oceano- observations of the ocean surface truly National Ocean Service formed a Tran- graphic research at many of these same began with Crombie’s (1955) experi- sitional Engineering Program in 1978 universities. mental discovery of the mechanism to encourage development of a com- In 2002, California voters approved behind his puzzling analog sea-echo mercial version of CODAR. With funds that led to a program called spectral plots. Don Barrick (1968, only a small potential market, no exist- the Coastal Ocean Currents Moni- 1972) theoretically derived the model ing radar companies were interested in toring Program, which allowed for that indeed showed that this resonant commercializing CODAR so a small the investment of $21 million to scatter was in fact “Bragg scatter” and group left NOAA to start CODAR create a California network of HF related the echo strength to the ocean Ocean Sensors, Ltd. in the early 1980s. radars to measure ocean surface 122 Marine Technology Society Journal currents to ensure the monitoring of Data Buoy Center (http://hfradar. nications Conference in January 2012 coastal water quality. The acquisition ndbc.noaa.gov/) while data failover re- (Figure 1). began in 2005 with 40 CODAR dundancy is also provided at Rutgers radars eventually being integrated University. Data file management Physics of HF Radar with the then-existing 14 CODARs and distribution follow internation- Current Monitoring in California. ally accepted standards, for exam- Why HF radar? On a national scale, the Integrated ple, netCDF-CF file and metadata HF denotes that part of the electro- Ocean Observing System (IOOS ) formats and OpenGIS Web Coverage magnetic spectrum having frequencies Program has been facilitating the de-® Service Interface Standard® for inter- from 3 to 30 MHz, which is equivalent velopment of a national data manage- operable delivery of gridded data. to radio wavelengths of 10 to 100 m. ment and distribution system for all Nationally, an additional focus has HF radar has been shown to be the op- U.S. HF radars as well as radars operated been the effort to acquire primary timal method for coastal sea surface by the Canadian Coast Guard in Nova radio frequency licenses. To form an current mapping for a number of rea- Scotia. Presently, more than 100 HF ra- operational network, the radars need sons. First, the targets required to pro- dars and 30 institutions are part of the to operate at dedicated radio frequen- duce coherent sea echo using HF are network, and their data are delivered cies, which requires the approval of surface gravity waves, typically of sev- by IOOS national data servers. The the International Telecommunications eral to a few tens of meters wavelength, development server and data display Union as well as U.S. agencies. The which are well understood and nearly are provided by Scripps Institution of process to acquire those frequencies always present in the open ocean. Sec- Oceanography’s Coastal Observing Re- has been supported by NOAA IOOS ond, vertically polarized HF waves can search and Development Center (http:// for nearly 5 years, with the expec- propagate over conductive seawater cordc.ucsd.edu/projects/mapping/), tation that the final approvals will be via coupling to the mean spherical sea and its mirror is at the NOAA National given at the World Radiocommu- surface, producing measurement ranges FIGURE 1 Montage of U.S. HF radar site locations. Green sites are sending data on schedule. Red sites have delayed data. (Color versions of figures available online at: http://www.ingentaconnect.com/content/mts/mtsj/2010/00000044/00000006.) November/December 2010 Volume 44 Number 6 123 beyond line of sight, out to 200 km Doppler proportional to the relative apriorito properly remove the Bragg or more offshore. Third, Doppler sea speed of the ocean wave traveling di- wave phase speed: echo at HF, under most wave condi- rectly toward or away from the radar. sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi tions, has a well-defined signal from The transmit frequency of the radar gλo 2πD wave–current interactions that is easily determines the radar wavelength and, c ¼ tanh 2π λo distinguishable from wave–wave pro- hence, determines the length of ocean cesses. This allows for robust extraction waves from which the radar wave will of current velocities. It is primarily backscatter. Because attenuation in- Range and Velocity Determination these three features, along with the creases as frequency increases, the result All HF radar systems currently used spatial resolutions that are possible is that higher frequency radars (shorter for ocean measurements use some due to the frequency modulation dis- radar wavelength) have a shorter maxi- form of frequency-modulated contin- cussed below, which place the HF mum range. Approximately one third uous wave (FMCW) waveform for band in a unique status for coastal of the radars in the United States are range determination. FMCW has the current monitoring. in the 4- to 5-MHz band, which can benefitofmuchlowermaximum achieve 200 km or more, depending power requirements to achieve the on conditions. Another third operates same average power and, therefore, Physics of HF Sea Scattering in the 12- to 14-MHz band and can range performance as older time- The two environmental conditions achieve an approximately 90-km gated pulsed radars (Barrick, 1973). necessary for HF current mapping are range. Approximately one quarter For closely spaced or colocated trans- conductive surface water and the pres- of the radars operate in the 24- to mit and receive antennas, a pulsed ence of surface gravity waves of suffi- 27-MHz band and achieve ranges of and gated FMCW (or FMiCW, “i” = cient length and height. Conductivity approximately 45 km. At the higher interrupted) waveform is desirable of water is primarily determined by frequencies, it is possible to obtain whereby the transmit signal is cycled salinity, which is typically 32–37 PSU greater radio spectrum bandwidth on and off and radar echo received in in the open ocean. As salinity decreases, that in turn allows for higher range res- opposition over a period determined so does the strength of the sea echo and, olution. The resolutions vary from less by the system’s achievable range. therefore, range of measurement. Since than 1 km to approximately 6 km. Re- This is done to prevent saturation of freshwater is inherently 5,000 times less gardless of the operating frequency, the the electronics as well as the received conductive than seawater, HF signals do physics is the same. Assuming a sta- echo by the much stronger transmit not travel nearly as far (e.g. Fernandez tionary radar, the relative wave speed signal. For both cases, the fundamen- et al., 2000). It has been observed in is comprised of the phase speed of the tal range determination is the same bays and around river mouths that dur- Bragg wave plus any underlying cur- (Barrick, 1973). ing times of high freshwater discharge rent. For deep water, the phase speed A continuous linear frequency ranges can be significantly reduced forsurfacewavesiswellknownasa sweep (or chirp) over a fixed band- λ (e.g., Long et al., 2006). function of o: width and pulse repetition frequency The ocean surface, at any given mo- rffiffiffiffiffiffiffi is generated in the receiver and ampli- ment, contains a random structure of gλ fied for transmit.