RADAR stands for Radio Detection and Ranging. Radar was first developed in 1939 during world war II by British Scientist Watson Watt to detect and intercept German bombers attacking Britain. This was a Primary radar. So how does a primary radar works. Radar transmits Electromagnetic pulses; this pulse hits the aero planes flying in the sky; and a part of the EM pulse is reflected back from aero plane to the radar antenna receiver. Now based on the speed of EM wave (which is same as speed of light) and total time taken from transmission to reception of the pulse, distance of the target aircraft is calculated. So basic formulae to calculate the target range would be as shown on the screen.
Azimuth/Bearing of the target aero plane is same as the direction of the antenna at the point of transmission.
This is how a primary radar calculates distance and bearing of aero planes in the sky and plots on the radar scope.
2. But, there are three major limitations with primary radar.(i) Range was limited to 120 nm. To double the range, power of radar has to be increased 16 times, since signal has to travel in both directions. (ii) There was no means to identify that a particular echo pulse belongs to which aeroplane in the sky (iii) There was no height information of the target.
3. Secondary Surveillance radar is a complete solution to all the limitations of a primary radar and can give ranges of up to 300 nm without a need of increasing the transmission power. So how does secondary surveillance radar works. Secondary radar additionally transmits an interrogation pulse at a fixed frequency of 1030 MHz which is called interrogation pulse. Aero planes are equipped with an equipment called transponder. Having received interrogation pulse, after a fixed delay, transponder transmits the response pulse at freq of 1090 MHz which is received back at radar. Response signal can be coded to pass additional information automatically related to the aero plane like its identification code, call sign, height, speed or any other data as required and approved. This technique resolved all three limitations of primary radar. Range is enhanced, since now the radar interrogation pulse has to travel only one way. Identification code, Call sign, Height and other parameters is available with radar controller for ensuring identification and safe separation. The technique of Secondary surveillance radar is also called IFF (Identify Friend or Foe) in military flying, since military aero planes are allotted with special codes by Air Defense radars during war/hostilities to identify that they are part of friendly forces.
4. Now let us discuss the modes of SSR. There are 5 modes of SSR. Selection of a particular identification code on transponder is called ‘Squawk’.
Mode A: For Identification through a IFF/SSR code
Mode C: For transmitting altitude (Flight Level)
Mode S: For transmitting data/TCAS/ADS etc
5. Mode A ident codes are 4 digit codes which has to be set as allotted by ATC. Only digits 0 to 7 are allowed to be set which makes 4096 possible identity codes. Digit 8 and 9 are not allowed. Mostly aero planes always fly with Mode ‘A’ and Mode ‘C’. Mode ‘A’ for identification and mode ‘C’ for transmitting height information in terms of flight level.
6. Mode ‘S’ transmits information like call sign, speed or any other data as required for compliance and functioning of TCAS and ADS, which would be covered separately in next video.
7. Emergency SSR Codes. Normally, pilot has to select and set IFF code as allotted by ATC, but in case of emergency situation, exclusive codes are to be used. They are as follows:
In case of Radio Communication failure: 7600
In case of In fight emergency : 7700
In case of Hijack : 7500
Operating a VFR Flight in controlled airspace: 7000
Entering radar controlled airspace from oceanic control area and awaiting allotment of code : 2000
Transponder U/S :0000

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