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.