(from Harlan et al, Marine Technology Society Journal, Vol. 44, No. 6, 2010)

The present state of the U.S. national high-frequency (HF) radar network has resulted from nearly 40 years of research and applications. HF radar observations of the ocean surface truly began with Crombie’s (1955) experimental discovery of the mechanism behind his puzzling analog sea-echo spectral plots. Don Barrick (1968, 1972) theoretically derived the model that indeed showed that this resonant scatter was in fact “Bragg scatter” and related the echo strength to the ocean wave height spectrum at the Bragg wave number. Barrick was invited to present his results at seminars in Boulder, Colorado, as the National Oceanic and Atmospheric Administration (NOAA), and its Boulder laboratories were being formed in 1970. A group was formed within NOAA’s new Environmental Research Laboratories to build a compact antenna system to be used for coastal ocean surface current mapping. This was the Coastal Ocean Dynamics Applications Radar (CODAR) program. After demonstrating its effectiveness, the NOAA/ National Ocean Service formed a Transitional Engineering Program in 1978 to encourage development of a commercial version of CODAR. With only a small potential market, no existing radar companies were interested in commercializing CODAR so a small group left NOAA to start CODAR Ocean Sensors, Ltd. in the early 1980s. In the 1990s, the Office of Naval Research and the National Science Foundation funds were used to acquire radars at several universities including the Oregon State University, the Rutgers University, the University of California-Santa Barbara, the Naval Postgraduate School, the University of Rhode Island, and the University of Connecticut. This was followed by a surge in acquisition because of the National Oceanographic Partnership Program, an NOAA/Office of Naval Research/National Science Foundation program that funded coastal oceanographic research at many of these same universities. In 2002, California voters approved funds that led to a program called the Coastal Ocean Currents Monitoring Program, which allowed for the investment of $21 million to create a California network of HF radars to measure ocean surface currents to ensure the monitoring of coastal water quality. The acquisition began in 2005 with 40 CODAR radars eventually being integrated with the then-existing 14 CODARs in California.  On a national scale, the Integrated Ocean Observing System (IOOS®) Program developed a national data management and distribution system for all U.S. HF radars as well as radars operated by investigators in Canada and Mexico.  Presently, approximately 150 HF radars and 30 institutions are part of the network, and their data are delivered by IOOS national data servers. The development server and data display are provided by Scripps Institution of Oceanography’s Coastal Observing Research and Development Center, and its mirror is at the NOAA National Data Buoy Center while data failover redundancy is also provided at Rutgers University. Data file management and distribution follow internationally accepted standards, for example, netCDF-CF file and metadata formats and OpenGIS® Web Coverage Service Interface Standard for interoperable delivery of gridded data.  Data delivery is accomplished using a THREDDS Data Server.

Capabilities and Description

One of the primary advantages of HF radars is the ability to see beyond the horizon when propagating over conductive sea water. This is not due to the atmosphere nor ionosphere. Rather, it is a “waveguide-type” effect. The vertically polarized electromagnetic waves attach to the mean spherical sea surface. The lower the HF frequency, the farther the signal reaches beyond the visible horizon. At 5 MHz (within IOOS, these are referred to as "long-range radars"), with modest transmit power, radar distances of 200 km are common even though the horizon for a microwave radar may be only 30 km away. Above 100 MHz, the signals become restricted to line of sight, the same as microwave and optical transmissions. However, none of the radars within the IOOS network uses transmit frequencies above 49.5 MHz.  HF radar requires that the transmitted radio signal be conducted across the sea surface. Since freshwater is inherently 5,000 times less conductive than seawater, HF signals do not travel nearly as far over freshwater. As an example, a standard 12MHz signal might travel 90 km in the coastal ocean, but only about 5 km over one of the Great Lakes.

*Maximum ranges will vary slightly depending on sea state, amount of external interference and radar transmit antenna.

**Range resolution depends on the bandwidth used by the radar's transmit waveform.  In practice, bandwidth is given by regulatory agencies and the resolutions shown here reflect typical bandwidths currently used.

• Accuracies of the radar-derived velocities, from numerous peer-reviewed studies, have been shown to be typically in the range 5-10 cm/s.
• Radar sites operate unattended and require relatively little maintenance.
• Radar site infrastructure is minimal.  Since it requires little space, the radar electronics hardware is often housed in a small portion of an existing structure or a small utility enclosure whose sophistication is mostly dependent on local climate (two examples shown below).
  

  Small enclosure used in a temperate climate.

  

  Typical enclosure used when air conditioning is necessary.

HF Radar Responsibilities

• Creating hourly gridded surface current total vector velocity data files at national servers located at the National Data Buoy Center (NDBC) HF radar data distribution and display server and Scripps Institution of Oceanography Coastal Observing Research & Development Center (CORDC) HF radar data distribution and display server.  
• Delivering near-real-time data files via THREDDS on user demand at Scripps CORDC THREDDS Data Server or at NDBC. 
• Delivering near-real-time data files to CO-OPS Tidal Data and Predictions.
• Delivering near-real-time data files to AWIPS-II in GRIB-2 format from NDBC for use at marine Weather Forecast Offices throughout US coastal areas.

IOOS HF radars are located in nearly every coastal state in the continental USA as well as Alaska, Hawaii and Puerto Rico.  

Map of HFR sites in the Continental United States, Hawaii, Puerto Rico, and Alaska (inset).

Our coverage maps illustrate the actual data returns from the sites for a given time period. In these images, the blue color indicates where data were available at least 10% of the time period. This can be thought of as the "optimal coverage" for the network. The green color indicates where data were available at least 80% of the time period. This might be thought of as a "typical obtainable coverage".

East and Gulf coast coverage for 6km resolution HFR data for FY2016

West coast coverage for 6km resolution HFR data for FY2016

Hawaii coverage for 6km resolution HFR data for FY2016

Puerto Rico coverage for 6km resolution HFR data for FY2016

For information about the proposed radar installation at Hightower Beach Park (FL), please visit this page.

A Plan to Meet the Nation’s Needs for Surface Current Mapping (updated May 2015) (pdf)

This plan presents the uses of high frequency radar, the requirements that drive the measurement of ocean surface currents, and the implementation design for a five-year, national build-out effort.

IOOS Spare Parts Process for CODAR SeaSondes (pdf), Sep 2015

HF Radar Data Archiving at NCEI (formerly NODC) (pdf), Sep 2014

Final ITU Document HF Ocean Radar (pdf), 2014

Summary of WRC-12 HF Radar Frequency Outcomes (pdf), 2012

IOOS HF Radar Data and Its Use by the U.S. Coast Guard for Search and Rescue (pdf), 2012

National HF Radar Technical Steering Team Position Paper on Use of HF Radar in Freshwater (pdf), Aug 2012

National HF Radar Technical Steering Team Position Paper on Use of HF Radar for Wave Measurement (pdf), Aug 2012

IOOS HF Radar Network Overview, Marine Technology Society Journal, Harlan et al (pdf), Nov 2010

Bibliography of HF Radar Comparisons with Other Sensors (pdf), 2007

Search and Rescue Operations Environmental Data Sources: Their Uncertainties (pdf), 2006

CODAR SeaSonde QA/QC Remote Monitoring Checklist

CODAR SeaSonde QA/QC Setup and Diagnostics