Retrotechtacular: Cover Your CONUS With OTH-B Radar

If you’re a ham, you already know that the ionosphere is a great backboard for bouncing HF signals around the globe.  It’s also useful for over-the-horizon backscatter (OTH-B (PDF)) radar applications, which the United States Air Force’s Rome Laboratory experimented with during the Cold War.

During the trial program, transmit and receive sites were set up ninety miles apart inside the great state of Maine. The 1/2 mile-long transmit antenna was made up of four arrays of twelve dipole elements and operated at 1MW. An antenna back screen and ground screen further expanded the signal’s range. Transmission was most often controlled by computers within the transmit building, but it could also be manually powered and adjusted.

The receive site had 50-ft. antenna elements stretching 3900 feet, and a gigantic ground screen covering nearly eight acres. Signals transmitted from the dipole array at the transmit site bounced off of the ionosphere and down to the receive site. Because of step-scanning, the system was capable of covering a 180° arc. OTH-B radar systems across the continental United States were relegated to storage at the end of the Cold War, but could be brought back into service given enough time and money.

Thanks for the tip, [Jacek].

Retrotechtacular is a weekly column featuring hacks, technology, and kitsch from ages of yore. Help keep it fresh by sending in your ideas for future installments.

13 thoughts on “Retrotechtacular: Cover Your CONUS With OTH-B Radar

  1. OTH-B radar sites were the bane of amateur radio operators all over the world. The Russians had one not-so-affectionately called the “Woodpecker” that blanked out entire bands for hours at a time, from what I’ve heard. I wasn’t a ham then, so I have had no personal experience with it, but I’ve heard plenty about it from older hams who have.

    1. I was just about to chime in about the Woodpecker. I was a ham on HF in the late 80s and early 90s and remember it well. On the one hand, it meant that you knew that 40 meters was open. On the other hand, it also meant that most of the band was useless.

  2. Google wiki for Jindalee (Jindalee Operational Radar Network) system in Australia. A brief excerpt:

    The JORN network is operated by No. 1 Radar Surveillance Unit RAAF (1RSU). Data from the JORN sites is fed to the JORN Coordination Centre at RAAF Base Edinburgh where it is passed on to other agencies and military units. Officially the system allows the Australian Defence Force to observe air and sea activity north of Australia to distances up to 4000 km.[22] This encompasses all of Java, Irian Jaya, Papua New Guinea and the Solomon Islands, and may include Singapore.[23]

    *******Actually in 1997 the prototype was able to detect missile launches by China[24] over 5,500 kilometres (3,400 mi) away.******************* (my emphasis)

    JORN is so sensitive it is able to track planes as small as a Cessna 172 taking off and landing in East Timor 2600 km away[citation needed]. Current research is anticipated to increase its sensitivity by a factor of ten beyond this level[citation needed].

    It is also reportedly able to detect stealth aircraft, as typically these are designed only to avoid detection by microwave radar.[6] Project DUNDEE[25] was a cooperative research project, with American missile defence research, into using JORN to detect missiles.[26] The JORN was anticipated to play a role in future Missile Defense Agency initiatives, detecting and tracking missile launches in Asia.[27]

    As JORN is reliant on the interaction of signals with the ionosphere (‘bouncing’), disturbances in the ionosphere adversely effect performance. The most significant factor influencing this is solar changes, which include sunrise, sunset and solar disturbances. The effectiveness of JORN is also reduced by extreme weather, including lightning and rough seas.[28]

    As JORN uses the Doppler principle to detect objects, it cannot detect objects moving at a tangent to the system, or objects moving at a similar speed to their surroundings.[28]

    1. The most significant factor influencing JORN is the on/off switch, and that is why they missed flight MH370 completely, otherwise they would have known the exact coordinates where it hit the sea surface.

      1. Agreed. And the Aus government will still not say whether it was on or off, although apparently is only turned on for exercises or specific needs such as detecting illegal immigrant boats during peacetime.

        1. The irony is that if they funded it to run 24/7 it would have cost them less than what they have wasted on the ongoing sonar search. They didn’t even need to say they spotted it with JORN, they could have just made it look like they had a lucky break with where they started the sonar search.

  3. Google Earth shows it still there, in Maine. 3 arrays each, of TX and RX. Looks like they’re still usable.

    And re: The Woodpecker — I remember hearing it on the ham bands. Hams hated it, and the USSR kept denying they were responsible. Hams finally figured out that if it heard stuff that seemed to interfere (same pulse pattern, synchronized), it would move. So, they started programming their automatic keyers to the same rate and whenever the Woodpecker showed up on the bands, it was met with mimicking transmissions, which caused it to move. Not too long afterwards, the Woodpecker went silent.

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