Backscatter Radar using HF

One of the things that surprised me was that the frequencies used by the Reagan "Star Wars" backscatter radar early warning alert system were in what Amateur Radio operators call the high frequency or "HF" region of the electromagnetic spectrum. They used these frequencies for the same reason that radio amateurs like them; because there are times when the ionosphere favors long range radio transmission and reception at these frequencies.

The rest of the information on this page was compiled from several websites; Wikepedia, http://www.craigsrailroadpages.com/oth-b/ and http://www.interceptradio.com/wiki/index.php/OTHB_Radar. Just a few months ago an OTH-B site was fFeatured in the "Daily Google Earth"; on May 7, 2011: http://dailygoogleearth.com/2011/05/07/trapezoid-in-the-high-desert/

"Backscatter Radar"

Given certain conditions in the atmosphere, radio signals broadcast up towards the ionosphere will be reflected back towards the ground. After reflection off the atmosphere, a small amount of the signal will reflect off the ground back towards the sky, and an even smaller proportion of that, will go back towards the broadcaster.

Since the signal reflected from the ground, or sea, will be very large compared to the signal reflected from a "target", some system needs to be used to distinguish the targets from the background noise. The easiest way to do this is to use the Doppler effect, which uses frequency shift created by moving objects to measure their velocity. By filtering out all the backscatter signal close to the original transmitted frequency, moving targets become visible.

The largest radar system in the world.

The U.S. Air Force's over-the-horizon-backscatter (OTH-B) air defense radar system is by several criteria the largest radar system in the world. Six one million watt OTH radars see far beyond the range of conventional microwave radars. OTH-B radar was developed to detect approaching Soviet bombers and cruise missiles while they were still thousands of miles out over the Pacific Ocean. It was developed over 25 years at a cost of $1.5 billion to warn against Soviet bomber attacks when the planes were still thousands of miles from US air space. Here is a map showing the region of coverage provided by the Maine and Oregon sites. The Alaskan site was never completed, and would have been "watching" beyond the Northern horizon, over the North Pole.

In order to achieve a beamwidth of 1/2 degree at HF, an antenna array several kilometers long is required.

Utilizing three 65 ft. high x 8,000 ft. long backscreens with 549 supporting towers and framework. The wire screens is like hogwire fence stretched across three, 1-1/2 mile long, 65 foot tall fences.

A section of the transmitter array in Oregon.

Antenna Types: Horizontal Linear Phased Arrays
Maximum CW Radiated Power: 1,000,000 W (12 transmitters)
Effective Radiated Power: 80 dBW (100 MegaWatts)
Transmit Array Design: Canted Dipoles, Vertical Backscreen

With the end of the cold war, the two so-far completed western sectors and the eastern ones were turned off and placed in "warm storage," allowing them to be used again if needed.

At some time after the military application was moth-balled, one of these radar systems was used to detect and measure the ocean currents carrying massive amounts of debris in the Ocean. For this application, they used the part of the signal that was filtered out during the intended "backscatter" operation.

A final decision was made to remove all radar equipment at the west coast sector's transmitter site outside Christmas Valley, Oregon and its receiver site near Tulelake, California. This work was completed by July 2007.

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