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.
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
southwards through Puerto Rico, to Trinidad & Tobago and the northern
coast of Venezuela.
But this vast area is not covered continuously; the system
operator can provide surveillance in a number of sectors known as DIRs
(dwell information regions). Each one of the 176 DIRs can be
"illuminated" for only a few seconds at a time. Small aircraft and
small vessels can be detected by an ingenious method, only when they
move. This is how it is done:
At the receiving site of the ROTHR system a very large antenna
stretches out over a distance of 8,400 feet. It consists of 372
dual-monopole vertical elements each 19 feet high, backed by a huge
reflector screen which makes the antenna substantially unidirectional.
Each pair of vertical elements has its own receiver which digitizes
the incoming signals. All the digitized signals are then fed through a
fibre-optic link to a master signal processor. The main receiver can
be programmed to pass on "returns" from one particular region while
eliminating most of the other returns as unwanted noise or clutter.
But because the wanted target is moving, while the clutter is not, a
filtering system based on the Doppler Shift principle (even when the
echo is only one or two Hertz different) will lock on to it and track
it as long as it stays in motion.
Furthermore, the ROTHR system has its own built-in automatic
management & assessment function and does not have to depend on
external sounding data. It measures the ionosphere height continuously
and instantly selects the most appropriate frequency to use to scan
the target area, ideally in one hop.
This automatic function uses a quasi-vertical incidence sounder
(QVI) to measure the height of the ionosphere near the transmitting
and receiving sites, which as mentioned earlier can be miles apart,
and a radar backscatter sounder to measure the height of the
ionosphere downrange 500 to 1,800 nautical miles away. The incoming
real-time data from these soundings are compared with data stored in
computer memory. Once real-time data are matched to a model of the
ionosphere, the model can be used to operate the system for the best
results, based on the prevailing propagation conditions. The data for
the ionospheric models take up more than 200 megabytes of computer
storage space. Operators thus know when and where to expect degraded
performance. Of course, strong solar activity can virtually make
over-the-horizon HF radar unusable.
A Spectrum Analyser display shows all the frequencies between 5
and 28 MHz. In order to avoid possible interference to other services,
those frequencies which are known to be permanently allocated to fixed
broadcasting and telecommunication stations are locked out, as well as
frequencies which happen to be used at any instant so that they can
also be avoided by the OTHR transmitter.
GLOSSARY for non-technical readers.
A.M. A mode of modulation (amplitude).
A.R.R.L. Amateur Radio Relay League (U.S.A.).
Beacon Transmitter radiating identification signal.
C.Q. General call, to any station.
C.R.T. Cathode ray tube (like TV screen).
C.W. Continuous wave (mode of sending telegraphy).
Callsign Station identification (letters & numbers).
Coherer A device for making radio frequencies audible.
DE Morse abbreviation for 'from' (French).
DX Communication over a long distance.
Detector Any device for making radio frequencies audible.
Doppler shift Change in pitch (of sound) or frequency of a (radio) wave
E.D.E.S. Initials of a war-time Greek guerrilla organisation.
E.E.R. Equivalent Greek initials for R.A.A.G. (q.v.)
E.L.A.S. Initials of a war-time Greek guerrilla organisation.
E.L.F. Extremely Low Frequency.
E.M.E. Earth-moon-earth. Also Moonbounce q.v.
H.H.M.S. His Hellenic Majesty's Ship.
Gasfet A type of transistor.
KHz Kilohertz--international unit for kilocycle.
M.U.F. Maximum usable frequency.
MHz Megahertz--international unit for megacycle.
Moonbounce Communication by reflection from the moon.
OTHR Over-the-horizon radar.
Q code Abbreviations used when communicating by telegraphy.
Q1 Unreadable.
Q2 Barely readable--only some words.
Q3 Readable with considerable difficulty.
Q4 Readable with practically no difficulty.
Q5 Perfectly readable.
QRO High power.
QRP Low power.
QRT "Stop sending". Frequently used for "shut up".
QSO Two-way communication.
QST Call to all stations. Also title of journal of the A.R.R.L.
QTH Location or address of a station.
R.A.A.G. Radio Amateur Association of Greece.
R.F. Radio frequency.
R.S.G.B. Radio Society of Great Britain.
RST System of reporting readability, strength & tone of a signal.
RX Receiver.
S unit Unit for reporting strength of received signal.
S.I. unit International system of definitions.
SSB Single side-band--a mode of modulation.
SWL Room where radio equipment is set up.
Shack Room where radio equipment is set up.
Silent key Deceased radio amateur.
Sporadic E. Propagation via the E layer of the ionosphere.
T.E.P. Transequatorial propagation.
TX Transmitter.
Troposcatter Propagation via the troposphere.
U.H.F. Ultra high frequency.
V.H.F. Very high frequency.
W.A.C. Worked (contacted) all continents.
XYL Wife of a radio amateur.
YL Young lady operator.
73 Morse abbreviation for "best regards".
Yagi A type of antenna designed by a Japanese of that name.