Woodpeckers and Over the Horizon Radar
Christopher Columbus sailed west across the Atlantic Ocean hoping to find a new course to the East Indies. Because of the curvature of the earth, he could only see as far ahead as was visible before it curved out of sight.
This magical disappearing point, known as the horizon, depends on how far above the ground or the water’s surface one is located. A rule of thumb for this distance in meters is given by:
1000 * sqrt(14.81 * your height) = distance to horizon
For example, if you were 10 meters above the surface of the water on Columbus’ ship, then your horizon was about 12,170 meters, or 12 kilometers. You can see the horizon clearly marked in the attached diagram.
How far can radars see? Most radar systems really require a “line of sight” view to their targets. We put radar transmitters and receivers on high towers or on airborne platforms to increase the distance to their horizons. But there’s more to the story. Even with a clear line of sight, the distance R to which a radar can see is limited by its electronics. This distance is given by the radar range equation:
Pt Gt s Ae
R4 = ————-
Pr (4π)2
where
Pr = power of signal received by receiving antenna
Pt = power of signal transmitted by antenna
Gt = gain of the transmitting antenna
s = radar cross section of the target
Ae = effective area of receiving antenna
The effective area Ae tells how much power the antenna can capture from the returned radar. It is related to the gain of the receiving antenna Gr. The antenna gain Gt and Gr tells us how efficiently an antenna transmits or receives power compared to a reference antenna. The radar cross section of the target s describes how detectable a target is to radar.
One could argue that Columbus could “see” beyond the horizon because he used the stars to navigate. Can radar ever see beyond the horizon?
Yes, it can. While line-of-sight microwave (1 GHz – 100+ GHz) radars can detect targets hundreds of kilometers away, HF (3-30 MHz) Over The Horizon Radars (OTHR) can actually detect targets thousands of kilometers away. They do this by using skywave propagation. The radar bounces off the atmosphere’s ionosphere to a far distant point on the surface of the earth, and bounces back again, as shown in the diagram. There are also HF radars that use ground wave propagation to follow the curvature of the earth beyond the horizon and back again.
OTHR was a post-World War II technology first deployed in the 1950s. Many countries performed research on OTHR and developed operational systems, including Australia, Canada, China, France, Italy, Japan, Russia, Ukraine, the UK, and the US (see https://apps.dtic.mil/dtic/tr/fulltext/u2/a474069.pdf). Most details about OTHR technologies remain hidden behind classified doors.
But there is one story we can tell. This is the story of the mysterious signal known as the WOODPECKER. It was so named because of the “rat tat tat tat” sound it made. This signal was first heard in the United States in 1976. The Woodpecker signal operated on frequencies from 3 MHz to 30 MHz, regularly interfering on frequencies licensed for amateur, commercial, and international communications. The source of the signal was eventually traced to the Soviet Union. Of course, the Soviets never acknowledged it was theirs.
The Woodpecker was actually an OTHR system known as Duga which the Soviets started building in 1972. One reliable source reported that the program was undertaken to “”to detect the nuclear attack on the USSR in the first two-three minutes after the launch of the ballistic missiles.” The Woodpecker stopped pecking in 1989 after the fall of the Soviet Union.
There were three different Duga installations. There was a radar receiver near Chernobyl in Ukraine, and an extremely powerful (10 megawatt!) transmitter about 60 kilometers away. A third Duga installation was placed on the Russian Pacific coast, near the island of Sakhalnsk.
While factual details about the Duga remain sketchy even today, its Ukrainian radar receiver has become a tourist attraction. It stands 150 meters high and 700 meters long, a gigantic wall of metal, antennas, and turbines. If you’re curious, look at:
https://www.cnn.com/travel/article/duga-radar-chernobyl-ukraine/index.html
https://www.amusingplanet.com/2017/12/the-russian-woodpecker.html
People have always wanted to see what’s just beyond the horizon. Christopher Columbus used the stars, and over the horizon radars use skywaves and ground waves.
Next week, we’re going to talk about a complementary technology to OTHR known as HF direction finding (HFDF). Come on back, and we’ll even discuss why some people mistook some HFDF antenna systems for elephant cages!
