8. Over-the-horizon or Ionospheric HF Radar--OTHR As mentioned briefly in Chapter 1, it was in April 1976 that the then Soviet Union first unleashed a diabolical noise on the HF bands which caused widespread interference to all broadcasting and telecommunication services between 6 and 20 MHz. On the first day the "knock-knock-knock" went on continuously for over ten hours. Radio amateurs, who were among the services that suffered from the interference, soon came to call this noise "the woodpecker". By rotating their beams when tuned to the 14 MHz band they established that the transmissions appeared to originate from the vicinity of the town of Gomel in the U.S.S.R. The governments of many countries world-wide immediately protested to Moscow, and all they got in reply was a brief statement that the U.S.S.R. was carrying out "an experiment". The reason for the very strong on/off pulses was probably because, at first, the Russians were using existing radar antennas which permit the transmitting and receiving functions to share the same antenna. Modern OTHR installations have different transmitting and receiving sites, often located many miles apart. From the early 1950s pulsed oblique ionosphere sounders had shown that the normal ionosphere is much more stable than had previously been thought to be. The physical reason for this is that the incredibly tenuous ionized gas which does the reflecting has a molasses-like viscosity. Of course, there are daily and seasonal changes, but over limited periods of half an hour or so, the F layer at a given location is actually quite well-behaved. It bounces back signals in a nearly constant direction and with nearly constant amplitude--just what is required for good radar performance. Over-the-horizon HF radars use the ionosphere as a kind of mirror to "see" around the curvature of the earth. They have a variety of uses, both military and civilian. And they have the advantage over line-of-sight microwave radars of being able to cover enormous areas with much less power and at a fraction of the cost of the latter. A "relocatable" OTHR system can track aircraft targets right down to ground level. In an early experiment operators were puzzled by the sudden disappearance from their screen of an aircraft they had been tracking as it taxied along the ground. They found out later that the reason for the disappearance was that the aircraft had gone into a metal hangar which did not show on the screen because it was not in motion, as explained below. In 1979 the United States Air Force began experimenting with an OTHR system at a site near Bangor, Maine. Because HF frequencies were being used the power was kept very low to minimize interference to other services during the early tests. At the time of writing (1989) it is believed that a full-power relocatable OTHR system situated in Virginia is being used in the anti-drug war.