difference between primary and secondary radar systems

primary radar systems:

1. Passive System
2.  A radio beam transmitted, hitting a target and then the radio waves being reflected back to a receiver .
3.  Doesn't Contains transponder (Transmitter Responder)

The advantage of Primary Surveillance Radar (PSR) 

• it operates totally independently of the target aircraft - that is, no action from the aircraft is required for it to provide a radar return.

The disadvantages of PSR are that, 

• firstly, enormous amounts of power must be radiated to ensure returns from the target. This is especially true if long range is desired. 
• Secondly, because of the small amount of energy returned at the receiver, returns may be easily disrupted due to such factors as changes of target attitude or signal attenuation due to heavy rain. This may cause the displayed target to 'fade'. 
• Thirdly, correlation of a particular radar return with a particular aircraft requires an identification process. When PSR was the only type of radar available, this was typically achieved by the Controller instructing an aircraft to turn and observing same on their display, or by correlating a DME distance report by the aircraft with the position of a particular return along a known track.

secondary radar systems:

1. Active System
2.  The Secondary Surveillance Radar SSR transmits an all call and any aircraft with a transponder will respond with a minimum of an ID code some will have encoding altimeters and the aircraft altitude will also be transmitted back
3. Contains transponder (Transmitter Responder)
4. target aircraft's transponder responds to interrogation by the ground station by transmitting a coded reply signal.
5. Advantage of Secondary Radar 

• I)because the reply signal is transmitted from the aircraft it is much stronger when received at the ground station, thus giving the possibility of much greater range and reducing the problems of signal attenuation; 
• the transmitting power required of the ground station for a given range is much reduced, thus providing considerable economy
•  because the signals in each direction are electronically coded the possibility is offered to transmit additional information between the two stations.

The disadvantage of SSR is that it requires a target aircraft to carry an operating transponder.

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