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OVER-THE-HORIZON RADAR (OTHR)

Ground-Based Ocean Remote Sensing on Basin-Wide Scales

Overview

More recently, our efforts using military over-the-horizon radars for measuring ocean currents and wind directions have led to the award of a Small Business Innovative Research grant for the development of a more compact, transportable OTH radar for NOAA, academic and commercial use. Unlike the enormous antennas seen above (2 miles in length), this system will employ a superdirective antenna which will span an area roughly the size of a soccer field.

How does over-the-horizon radar work?

What ocean properties can OTH radar measure?

In order of increasing difficulty,

• Surface wind (Bragg-resonant wave) direction
• Radial surface currents (current vectors with two radars)
• Sea state (eg., rms wave height)
• Surface wind speed
• Dominant wave period
• Dominant wave direction
• Non-directional (scalar) ocean wave spectrum
• Combined swell and wind-wave spectrum

How far can OTH radars see?

Using one ionospheric reflection, an OTH radar normally covers the range interval between about 500 and 3,500 km. This experimental surface wind-direction map, which covers about 40-million square kilometers of the Pacific Ocean, was made in about three hours with the three Air Force OTH-B radars on the California-Oregon border. Only direction is indicated here; wind speed was not measured. Even so, mesoscale structure of surface cyclones, anticyclones, and fronts is clearly resolved.

Synoptic and mesoanalysis over the tropical Atlantic

Can OTH radars track tropical storms and hurricanes?

This surface wind direction map was made in less than one hour with the Air Force OTH-B radar in Maine, just as Hurricane Andrew was heading toward its devastating encounter with the South Florida coast.

Tracking Hurricane Claudette

The OTH-B radar in Maine captured five images of the surface wind-direction pattern associated with 1993 Hurricane Claudette, which just missed Bermuda. Eye locations derived from these radar images are compared here with the official National Hurricane Center track for the storm. See [Georges and Harlan, 1993] for more details.

Surface currents in the Florida Straits

This map of the radial component of ocean surface currents was made using the U.S. Navy Relocatable Over-the-Horizon Radar (ROTHR) to the north in Virginia. Currents flowing toward the radar (that is, nearly northward) are colored red and orange. Currents flowing away from the radar are colored green and blue.

1997 vector current maps in the Florida Straits.
1998 vector current maps in the Yucatan.

Surface currents in the Western Caribbean and Gulf of Mexico

This map of radial currents made by the Texas ROTHR (to the northwest) shows details of the Atlantic Western Boundary Current as it flows through the Yucatan Passage, makes a 180-deg turn in the Gulf of Mexico (the Loop Current) and flows eastward into the Florida Straits. Bifurcations and eddies in the western Caribbean reveal the influence of bottom topography. See our Radio Science paper on validation of ROTHR current measurements in the Gulf of Mexico.

The first dual-OTH-radar surface current map

This test, performed by simultaneously illuminating the Florida Straits with both the U.S. Navy ROTHR radars in Texas and Virginia, reveals the structure of the Florida Current, and ancilliary flows, with 10-km resolution [Georges, Harlan and Lematta, 1996]

Can OTH radar data be merged with satellite data?

The color strips show 24 hours of ERS-1 scatterometer coverage over the North Atlantic. Wind speed is color coded, with orange indicating highest speeds. The OTH-B wind directions for the same day are superimposed, filling in the gaps in the satellite data [">Georges et al., 1996]. Data merged by Paul Chang at NOAA/NESDIS.

What OTH radar data are available now?

NOAA's tests with the Air Force and Navy radars have so far been for research and demonstration purposes only. Therefore, only archival data are available. The Air Force has deactivated its OTH-B system and now maintains the six East Coast and West Coast OTH-B radars in a state called warm storage, which preserves the physical and electrical integrity of the system and permits recall, should a need arise.

The two U.S. Navy ROTHRs in Virginia and Texas are presently in full-time use for drug interdiction surveillance. Our oceanographic research using OTHR has not been permitted since October, 1999. Other OTH radars in France, Australia, and Russia could be adapted to environmental monitoring.

Other applications of OTH radar

• Hurricane Tracking and Forecasting
• Numerical Weather Prediction
• Space Weather Forecasts
• Marine Weather Forecasts
• Pollution Control
• Fisheries Oceanography
• Air-traffic control
• Search and Rescue
• Calibration and Validation of Satellite Ocean Sensors
• Shortwave Radio Forecasting
• Climate Change
• Marine Weather Research
• Coastal Wave Forecasting
• Ionospheric Research
• Iceberg Tracking

So what?

We have researched ways to exploit the unused oceanographic potential of existing military over-the-horizon radars. So far, we've mapped ocean-surface wind directions over areas large enough to reveal the structure and progress of tropical waves that spawn hurricanes. We've also found new ways to map ocean surface currents, with detail not available from any other sensor, existing or planned. The cost of this research is a tiny fraction of what taxpayers have already paid for these radars.

Research like this spawns practical benefits to society only after integrated regional ocean-observing systems combine new kinds of data with ocean-circulation models and make usable products readily available to customers.

Who are the customers? For example, accurate maps of ocean surface currents, both in coastal zones and in the open ocean, are required by Navy Fleet operations, by Coast Guard search-and-rescue activities, by fisheries managers, by offshore drilling operations, by shipping interests, by coastal ecosystem managers, by oil-spill response teams, by global-climate-change researchers, and by the general public. Keeping such customers in mind, even at the research stage, helps us design a better product.

What next?

The purpose of piggybacking our research on military OTH radars has been to demonstrate the ocean-monitoring potential of this technology, not a long-term monitoring solution. The next step is to design a low-cost skywave radar dedicated to oceanographic applications and let potential users decide whether the cost is justified by the kinds of services we have demonstrated. A possible installation would focus on the Gulf of Mexico, the Caribbean Sea, and the hurricane approaches to the U.S. East Coast and Gulf Coast. See our paper: "The case for building a current-mapping over-the-horizon radar".