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Mystery Soviet Over-The Horizon-Tests

Wireless World   -   February 1977

It is now common knowledge that a large portion of the h.f. band of the radio frequency spectrum has been suffering for over the past few months from interference caused by a very powerful transmitter, or transmitters, located somewhere in Russia or the Ukraine. The interference became so bad that most of the communication services within the band have complained, through their respective organisations, to the Home Office and to the Frequency Registration Board of the International Telecommunication Union (ITU). Other countries (including the USA) who have been similarly affected by the transmissions, have also forwarded complaints to the ITU and the Russian Authorities.

A Home office representitive recently informed "Wireless World" that they have made a complaint direct to the Russian authorities and have been told that they are conducting tests and are taking steps to reduce the interference.

What we, the public still do not know, and are not likely to be told, either by the Russians or our defence organisations and industries, if they know, is what these tests are for. We can only speculate, and perhaps the best way to do this is to study the information at hand and then compare it with systems which we know are within the realms of our present technology, or could be feasible.

Reports indicate that the transmitter is located in the town of Gomel, an industrial town in Byelo-Russia (See Pat Hawker's comments, November issue), and this, according to a recent "Daily Mail" report, has now been confirmed by NATO direction-finders. Monitoring station engineers agree that the actual powers involved are in the tens of megawatts and Mr Dafydd Williams, chief engineer of the BBC External Broadcasting is reported to have estimated them as 20 or 40MW or more, and audible in every part of the globe.

Some American publications have claimed that the interference was first brought to the attention of the Federal Communications Commission (FCC) in July, principally by radio amateurs. Mr S. A. Cook G5XB of Intruder Watch (1) told "Wireless World" that the transmissions, which have a pulse configuration with a basic pulse-repetition frequency (prf) of ten per second, occur between about 5 and 22MHz, are widely scattered and appear to depend on the maximum-usable-frequency (m.u.f.) for propagation. For example, at dawn they can be expected between 14 and 22MHz and by 3 p.m. they may be at 14MHz or lower. When the interference first started it persisted for 10 to 12 hours at a time and, at one stage, completely obliterated the 14MHz amateur band. Another report, said to have come from the BBC, indicated that Cairo Radio had also been obliterated. However, a spokesman for the BBC monitoring station said that while this was an exaggeration the interference has been a considerable nuisance and has occasionally made monitoring impossible. Their experience of the interference was that it appeared at various times, on different frequencies and for varying durations within the the range 6 to 15MHz. A representative of the Home Office international monitoring station completed the picture by saying that the signals have been affecting frequencies from 4 to about 27MHz - almost all of the h.f. band.

Amateurs and broadcasters have not been the only ones to be affected by the interference; almost every service has been troubled - except the television services, which are on higher frequencies. Public services such as Post Office radio communications have experienced interference and so have h.f. maritime communications. It would be unrealistic to suppose that these high power signals have not had some effect on the h.f. military services too.

According to Mr Cook, the period of the 10p/s signal comprises a pulse train of up to 20 different squarewave pulses, some less than 2ms in length - an estimated pulse frequency of at least 800 pulses per second. Although the signals are very difficult to observe, even on a high-speed oscilloscope, he is convinced that there are as many as four sources all transmitting the same, or very nearly the same information, perhaps from different locations. What is equally interesting is that the signals are no longer remaining for periods of hours in one frequency band but are moving up and down the h.f. spectrum in about 100kHz steps, remaining at the chosen frequency for 30s to 10min.

The use of pulse signals suggests either over-the-horizon (o.t.h.) radar or communications. In either case a complicated system would be necessary to compensate for propagation variations, and this may involve the use of one or more source. The variations in carrier frequency could either be an attempt at remaining at the most propagatable frequency or they could be a security procedure. It is understandable that the Russians should wish to keep h.f. communications in addition to satellite communications using microwaves because, in the event of war, the satellite is very vunerable. However, it is not clear why tens of megawatts would be needed, even for communications to submarines.

Over-the-horizon radar seems to be far more probable. This is not new, (2) the USAF and the Defence Advanced Research Projects Agency (DARPA) have been actively interested in o.t.h. radar for about 15 years and distances of at least 1850km are possible.

It is interesting to note that the frequencies chosen for o.t.h radar are normally between three and 30MHz. The system would almost certainly use o.t.h.-b. (backscatter) radar which depends upon energy reflected from the target reaching a receiver antenna array via ionospheric reflections.(2) Radar of this kind is often ineffective within a certain skip distance from transmitter.(2) This may explain why an American radio amateur visiting a Soviet amateur organisation as a representative of the ARRL was told that their amateurs were unaware of any high power transmissions from their country.

Systems using more than one radar source are also in existence today. These multiradar tracking systems (3) use some of the signals received to update others so that the best estimate can be made of the target position. Although it is argued that 10p/s is too slow for tracking anything but ships, the higher frequency components within the basic pulse train could surely contain enough information for faster moving, smaller aircraft - especially using multiradar.

If the Russians are really being adventurous they could be testing a four-source system capable of detecting the actual shapes of their targets. Recent results in the study of electro-magnetic impulse response of objects (4) have indicated that the information required for the determination of the approximate shape of the objects can be contained in the low frequency range, where the wavelength is longer than the overall dimensions of the object. This means that the h.f. band could be used for targets from 120m down to 10m length or less and this would include the majority of aircraft and rockets. It is also interesteing to note that the most troubled frequency (14MHz) corresponds to 20m, about the size of an aircraft.

The study (4) showed that "below four frequencies are sufficient in most cases to provide reliable classification in the presence of substantial amounts of noise". Using only four frequencies or less, the study showed that four aircraft models (F-104, 18.24m long by 16.6m wingspan, F-4, MiG 19 and MiG 21), scaled to approximately the same size, could be reliably identified. In comparison the distinction between winged rockets and other aircraft would be a simple matter.

Is it feasible that the Russians are testing a system which incorporates both o.t.h. radar and shape recognition by radar returns? The most likely targets for such a radar would be the US B-1 swing-wing bomber and the Navy and US Airforce cruise missiles (only 4 to 6m long). Positive identification of approaching B-1 bombers and cruise missiles would be a valuable asset to the Russian forces and would probably be worth any diplomatic embarrassment caused by interference with Western radio services during preliminary trials.

REFERENCES

(1) Intruder Watch is an organisation which monitors the amateur frequency bands and reports to the Home Office any persistent intruders within those bands. In the UK there are currently 22 observers reporting an average of 100 serious intruders per month in the amateur bands alone. Throughout the world the figures are nearer 1200 per month.

(2) Janes Weapons Systems

(3) Tracking in a multiradar environment, H. W. Thomas, G. Maignan and J. T. Story, "Proc. IEE" Vol. 123, No. 3, March 1976.

(4) Identification of complex geometrical shapes by means of low frequency radar returns, Y. T. Lin and Professor A. A. Ksienski, "The Radio And Electronic Engineer" Vol. 46No. 10, October 1976.