 How does over-the-horizon radar work?
Over-the-horizon (OTH) radars were developed to detect military targets far beyond the optical horizon. They use 5-28-MHz radio waves, which reflect from the ionosphere, reaching up to 3,500 km in one "hop." Properties of the ocean surface are extracted from the minute amount of energy scattered by the sea surface back to the radar. To find out how we extract ocean-surface properties from what the military discards as "sea clutter," see our THEORY PAGE.
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
NOAA adapted the Air Force OTH-B radars in Maine to map surface wind streamlines in the hurricane breeding grounds of the tropical Atlantic. Tropical waves crossing the Atlantic can be seen in detail not available with any other sensor. Under favorable conditions, such waves develop into tropical storms and hurricanes. Working with NOAA's Hurricane Research Division, we provided these experimental maps to the National Hurricane Center for 40 days during the 1994 hurricane season.
Take a look at a 19-day MOVIE LOOP of OTH-B streamline maps. (192K GIF) This is not a model -- it's real data! See Young, Harlan and Georges [1996]
 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.
ROTHR Surface Wind and Current Maps of Hurricanes Hortense, Bonnie, and Georges.
 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 (The following were written in 1996 by T.M. Georges)
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".
Questions? Comments? Suggestions? E-mail to:
Jack Harlan ( Jack.Harlan@colorado.edu )
-- or see these publications
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