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.

As can be seen from the map this ROTHR can cover a vast area of
1.6 million nautical miles, straddling the whole Caribbean. The scan
area stretches from the coast of Colombia in South America up through
Nicaragua and Honduras to Florida (on its west boundary) and then
south