A radar system has a transmitter that emits radio waves
called radar signals in predetermined directions. When these come into contact
with an object they are usually reflected or scattered in many directions.
Radar signals are reflected especially well by materials of considerable
electrical conductivity—especially
by most metals, by sea water and by wet lands. The radar signals that are reflected back towards
the transmitter are the desirable ones that make radar work. If the object is
moving either toward or away from the transmitter, there is a slight equivalent
change in the frequency of the radio waves, caused by the Doppler effect.
If electromagnetic waves traveling through one material meet
another, having a very different dielectric constant or diamagnetic constant
from the first, the waves will reflect or scatter or refract from the boundary between the
materials. This means that a solid object in air or in a vacuum, or a
significant change in atomic density between the object and what is surrounding
it, will usually scatter radio waves from its surface.
Radar receivers are usually, but not always, in the same
location as the transmitter. Although the reflected radar signals captured by
the receiving antenna are usually very weak, they can be strengthened by
electronic amplifiers.
There are basically two types of radar:
1) Primary radar
2) Secondary radar
Primary radar has lots of limitations. It works best with
large all-metal aircraft, not so well on small, composite aircraft, and not at
all with some of the new "stealth" technology. Its range is limited by
terrain and precipitation. It's rather indiscriminite about what it detects:
airplanes, trucks, hills, trees. And it only reports a target's and
range, not its altitude (only 2D).
Secondary radar was invented to overcome these limitations.
It depends on a transponder in the aircraft to respond to
interrogations (type of signal) from the ground station. Depending on the type of interrogation,
the transponder sends back an identification code or altitude
information.
Radar ground stations:
It consist of
three separate antennas. The biggest one is the primary radar
antenna, which looks like a parabolic dish that goes round and round . This antenna transmits powerful pulses and then listens for echoes. It
is used to detect aircraft skin paint and also can detect weather
to some degree.
The second ground station antenna, called the directional
antenna, is used to send interrogations to airborne transponders and to receive
replies from those transponders, providing secondary radar capability. It is a
bar-shaped that is usually perched atop the primary radar antenna and
rotates along with it. It's called directional because, like the
primary radar antenna, it is designed to beam the interrogations and to receive
the replies only from the direction it is pointed.
However, the directional antenna is less than perfectly
directional. To design a perfectly directional antenna, we have to make it
infinitely large and thus not practical . Real-world directional antennas have weaker side lobes in
addition to the main lobe. The side lobes are too weak to be a
problem for distant aircraft, but for aircraft close to the antenna site they
are a big problem. Unless something was done about them, the side lobes would
cause a close-in aircraft to show up as three or four different targets on the
controller's screen, causing confusion.
Side lobe supression:
That's where the third antenna comes in. It's called the
omnidirectional antenna because it radiates equally in all directions. Every
time the directional antenna sends out an interrogation (which consists of a
pair of pulses), the omnidirectional antenna sends out its
own pulse . The signal from the omnidirectional antenna is designed
to be much weaker than the main lobe of the directional antenna, but stronger
than its side lobes.
When the transponder receives an interrogation, it compares
the strength of the three pulses it receives and according to the strength of the signals it provides information.