Radar



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 conductivityespecially 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.