Nuclear detection: phase 2

The mysterious 50 metre ring

On my first visit to Orford Ness in 2018, one of the highlights was a visit to the Bomb Ballistic Building. From its roof, looking out over the shingle towards the lighthouse, is a large concrete ring. Our National Trust guide at the time had no explanation for this structure.
A few years later, the iconic lighthouse sadly fell victim to the encroaching sea and had to be demolished. But during this time, thanks to the efforts of several dedicated individuals looking at the military history of Orford Ness, the mystery has been solved. To explain the purpose of the ring, it’s necessary to go back to the late 1950s.

Detection of nuclear explosions over large distances by electronic means

The very first nuclear explosion at the Trinity site in 1945 confirmed the expectations of scientists like Enrico Fermi that the fission bomb would have very significant electromagnetic effects.¹ At first, this fact was more of a nuisance than a useful artifact, and early American, and later British nuclear tests saw some equipment failures due to what the British referred to as “radio flash”, nowadays referred to an electromagnetic pulse, or EMP.
Despite this phenomenon being known, it took some time before creative minds decided it was possible to exploit these electromagnetic effects in order to create a remote detection system. The Americans, very keen to monitor Soviet efforts to produce a nuclear weapon, had set up a special organisation known as Air Force Material Special Weapons-One (AFMSW-1) on the16^th of February 1948. This later changed its name to AFOAT-1 and finally, AFTAC (Air Force Technical Applications Center), a name which it still holds today.
Various means were employed to monitor nuclear explosions, one of the main tools being radiological sampling. It was this technique which led to the realisation that the Soviets had managed to detonate their first nuclear weapon, nicknamed by the Western Allies as Joe-1, approximately five years earlier than the best intelligence estimates had calculated. This was a substantial shock to the Western Allies, which served to accelerate the efforts to establish a comprehensive monitoring network.²

Radar as a Sensor

By 1958, with the possibility of a nuclear testing moratorium being discussed, the pace of research and development into monitoring techniques increased still further. After all, if a moratorium did come into effect, it would be vitally important to be able to police such an agreement.
At this time, both the US and the UK were working on over-the-horizon radar (OTHR) techniques. Conventional radar is limited to line-of-sight detection, while OTHR is able to bounce radio waves off the ionosphere, theoretically enabling detection ranges right round the globe. Radio waves in the HF band, under the right conditions, are able to bounce back and forth between the ionosphere and the Earth’s surface multiple times.
In the US, a company called Engineering and Research Corporation (ERCO), which had been making the Ercoupe light aircraft from 1938 and was heavily involved in producing aircraft simulators post war, became interested in OTHR. This happened because one of the ERCO

managers, Bill Whelan, was a school friend of Dr. William J. Thaler of the Naval Research Laboratory.³ Thaler was a pioneer of over-the-horizon techniques and persuaded Whelan to look into the possibility of detecting distant missile launches (in common with nuclear explosions, a rocket motor produces significant amounts of ionisation, detectable by radar).
ERCO’s activity in OTHR led to a query from AFTAC in early 1958. AFTAC wanted to know if ERCO had detected any of the nuclear explosions from the Nevada test site.

The answer was negative, given that ERCO’s radars had not been looking in that direction. But at AFTAC request, it was agreed to set up an OTHR monitor at ERCO’s headquarters at Riverdale, MD to monitor the upcoming HARDTACK test series, and also to send another radar set out to the test location at Johnston Island, in the Pacific.⁴
The results of this exercise were successful, and AFTAC placed a contract with ACF Industries/ERCO for 8 sets of equipment. AFTAC used an alphabetical code to name their detection techniques. The “A” technique was the airborne sampling technique; “B” was the seismic technique, with many other letters covering other methods. “R” referred to the backscatter radar technique, so these radars were referred to as the R system.

The R system is designed and deployed

The design which was settled on was very distinctive. It consisted of a helical transmission antenna of monstrous proportions (the largest helical antenna in the world at the time). This helix was wound around a wooden box frame, and the whole thing could be rotated, the supports running on a single rail track mounted on a concrete circle. The receiving arrays were monopoles, placed in front of curtain-like reflecting screens.
The prototype R system was running in Muirkirk, MD, by mid-1960, and some months later the first operational R system was erected within the Walker Air Force Base Defense Area at Roswell, New Mexico.

Further R systems were installed at Point Barrow, Alaska (in this case, the helix was not used due to the danger of ice accretion), Hilo (Hawaii), Wheelus Air Force Base (Tripoli, Libya), Ramey Air Force Base (Puerto Rico) and Karachi.

The R system on Orford Ness

The eighth R system was originally destined for the south Pacific, but was never deployed there.⁵ Instead, it remained at the USAF logistics hub at McClellan Air Force Base, near Sacramento, CA. In the early 1960s, the UK and the US were embarking on an ambitious programme called CLEAR SKY, which was intended to police the upcoming Partial Test Ban Treaty which would be signed in Moscow in October 1963. CLEAR SKY envisaged many different reporting techniques (Acoustic, telluric currents, geomagnetic, atmospheric fluorescence, VLF phase anomalies, broadband EMP, cosmic noise, seismic, air sampling, and finally, backscatter radar). AFTAC wanted the UK to help provide additional stations in the Commonwealth, and offered to provide a complete R system.⁶

Orford Ness had already been the site of OTHR radars: in 1960 the French atmospheric test at Reggane in the Sahara was detected by a home-grown OTHR system set up by AWRE staff.⁷ (refer to the IRGON article here). Another British-developed OTHR system called ZINNIA was also deployed on the Ness, probably a couple of years later.⁸
The location on the vegetated shingle spit had been identified as being ideal for such electronic intelligence gathering, due to several reasons, i.e. secure location; being able to “look out” towards the USSR with an unobstructed path to the sea horizon; the fact that it was a radio quiet zone, being distant from major population centres; and last but not least, excellent ground conductivity.⁹ Maintaining a suitable earth potential in relation to the aerial systems is important for sensitive radio and radar installations.
So, it came to be that the R system was shipped (probably air freighted in 3 consignments from McClellan AFB) and installed at Orford Ness during 1964. Sometimes referred to by an AWRE codename POUND NOTE¹⁰, it was operated 24/7 from March 1965 to March 1967 by a Joint Services Detachment. Its primary role was continuous monitoring for atmospheric and outer-space tests in breach of the Partial Test Ban Treaty.

After 1967, it seems that AWRE retained the R system for research purposes. IRGON has not yet established exactly when it was dismantled.

R system basic specifications

 Transmitting power: ~ 135 kW

 Frequency: variable between ~ 5-25 MHz

 Pulse repetition frequency: variable between 10 and 20

 Length of helix: 110 ft. (33.5 m)

 Diameter of circular track: 167 ft. (50.9 m)

References

1. Trinity, Report LA-6300-H, K. T. Bainbridge, Los Alamos Scientific Laboratory, May 1976
2. A Fifty Year Commemorative History of Long Range Detection. Air Force Technical Applications Center, HQ, Patrick AFB, September 1997
3. Personal communication with Edwin Lyon
4. Old Radios and HANEs by Ed Lyon, Radio Age, January 2018
5. Two Radar Hits and a Miss – at Orford Ness by Ed Lyon. Radio Age, July 2020
6. US/UK agreement on Project CLEAR SKY: Understandings reached in US/UK technical discussions of September 1964. Office of the Assistant secretary of Defense, 12th October, 1964.
7. Echoes and Reflections, Keith Wood, Serendipity Press, 2004, p. 136
8. Book of the Bricks, Farnborough Air Sciences Trust. Web edition January 2023, p. 124
9. Memorandum to I. Maddock from S.D. Abercrombie, Superintendent of Electronics Field Experiments, AWRE Aldermaston., reference 621.396.969.106/1, 1st October 1963
10. Orfordness – Secret Site. Gordon Kinsey, Terence Dalton Ltd., 1981. p. 119