wards 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