"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.

Futhermore, 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.