Important Questions For AAI ATC Interview | Article 2/5

I am here with the 2nd article of the exclusive AAI ATC Interview Series which primarily focuses on the preparation of the JE (ATC) Interview. The series is consisted of Five Articles which cover non-technical as well technical questions that I suppose can be asked in the ATC interview.

The credit for inventing the radar goes to Robert Watson Watt. Initially Radar was used by British Royal Air Force during World War 2 as an early warning system for aerial attack by enemy. But later on Radar found applications in various fields like Civil Aviation and  Meteorology. For a long period, Radar has been an integral part of Air Traffic Control and is used for the surveillance and navigation purpose. Considering its importance in ATC, I have written this post which aims at covering the important questions related to Radar that might be asked in interview.

Ques. 27: What is RADAR and how does it work?

Ans. RADAR is an acronym for RAdio Detecting And Ranging.

Radar transmits the electromagnetic energy pulses in space. These pulses get reflected from an object e.g. an aircraft. This received pulse which has very low power as compared to transmitted pulse is called Echo. The Radar system uses this echo signal to calculate the distance and direction of the reflecting object.

A simple formula to calculate the distance of the object is as:

R= c*t/2radar atc

R: distance of the object.

t = total time taken by the signal to return to Radar

c= speed of light (approx. 3,00,000kms/sec)

Ques. 28: What are the various RADAR bands?

Ans. Radar operates on UHF and SHF because:

a) These frequencies are free from disturbance.

b) Higher frequency, shorter wavelength, RADAR more effective (as shorter wavelengths are reflected more efficiently.)

Various RADAR bands are as:

radar bands

Ques. 29: What is PRF and PRT?

Ans. PRF: The Pulse Repetition Frequency (PRF) of the radar system is the number of pulses that are transmitted per second.

PRT: The time between the beginning of one pulse and the start of the next pulse is called pulse-repetition time (PRT) and is equal to the reciprocal of PRF as follows:

PRT =     1/PRF

Targets at a range equivalent to the pulse width from the radar are not detected.

Ques. 30: What is the unambiguous range of the Radar?

Ans.The Unambiguous range is the maximum range at which a target can be located so as to guarantee that the leading edge of the echo pulse from that target is received before transmission begins for the next pulse. The pulse-repetition frequency (PRF) determines this maximum unambiguous range of given radar before ambiguities start to occur. This range can be determined by using the following equations:

Rmax =    c * ( PRT – PW )/2

Here c= speed of light; PRT=1/PRF

Ques. 31: What is the duty cycle?

Ans. Duty cycle basically represents the fraction of time that a system is in active state or in other words, it is the percentage of one period in which a signal is active.

  • For example, if a system transmits a pulse for 1 second and then waits for 9 seconds to transmit next pulse, then we cans say that system is active (ON) for 1 sec. and inactive (OFF) for next 9 sec. So the total time= 10 seconds. Hence duty cycle is 0.1 (ON-time/Total time).
  • For a RADAR system, Duty cycle can be found out by multiplying pulse width and pulse-repetition frequency.

Ques. 32: What is the Radar range equation?

Ans.The radar range equation is used to determine the maximum theoretical range of the Radar. It is basically a mathematical expression that represents the physical dependence of Radar’s range with the transmit power. It also includes effect of various important factors such as Gain of the antenna, wavelength of the transmitted pulse and Radar Cross-section. Radar Range Equation: 

radar range equation

σ = radar cross section [m2]
PE = received power [W]
G = antenna gain
PS = transmitted power
R = Range

Ques. 33: What is Primary Radar?

Ans. Primary Surveillance Radar (PSR) transmits a high power signal, some of which is reflected by the aircraft back to the radar. The radar determines the aircraft’s position in a range from the elapsed time between transmission and reception of the reflection. PSR does not provide the identity or altitude of the aircraft (disadvantage). However, PSR does not require any specific equipment (transponder) on the aircraft and can display non equipped/faulty or non-co-operative aircraft (advantage of PSR).  Application: Aerodrome surface surveillance (e.g. as Surface Movement Radar).primary radar

Ques. 34: What is Transponder?

Ans. Transponder can be summed up as a combination of TRANSmitter+resPONDER. A transponder is a device used in wireless communications, monitoring, or control device. It receives the incoming signal and automatically responds to this signal.

In Air navigation, a transponder is fitted in the aircraft which responds (by providing aircraft identification and other useful data such as speed, direction, altitude of the aircraft) to the interrogating signal sent by Ground-based secondary radar.

Up-link frequency of Secondary radar: 1030MHz, Down-link frequency of Transponder: 1090 MHz.

Ques. 35: What is a Secondary Radar?

Ans. Secondary radar system is basically a cooperative target identification system consisting of two main elements, a ground based interrogator and an aircraft transponder. The Radar system (functioning as an interrogator) transmits an encoded signal to the aircraft. The on-board transponder interprets the encoded signal and transmits an encoded reply (with the requested information) back to the interrogator. The reply message received from the transponder is decoded/ interpreted by the Radar’s receiver system. But it is to be noted here that working of secondary radar requires positive cooperation from the target end.

secondary radar

Ques. 36: What are the advantages and disadvantages of secondary radar?

Ans. Advantages:

  • Reply signal transmitted by Transponder is very strong (unlike Echo signal in case of Radar), this provides good detection capability independent of clutter and weather.
  • The transmitting power required of the ground station for a given range is much reduced, thus providing considerable economy.
  • The signal received from the Transponder offers a lot of details of the target such as Speed, Direction, Altitude, identification of the target etc.


  • It depends on aircraft avionics, so the aircraft/target with no transponder cannot utilize the benefits of secondary radar.
  • If the target is not cooperating then Secondary radar will not be able to provide any information. Just like in 09/11 attacks on America when terrorists switched off the transponders of hijacked aircraft and it was impossible for ATC to trace those aircraft. So Secondary radar needs to be used in conjunction with the primary radar in order to avoid such situations getting repeated.

Ques. 37: What is the difference between primary and secondary radar?

Ans. Primary Radar is used to identify targets by transmitting high-frequency signals toward it. The transmitted pulses (also called Echo) are reflected by the target and then received by the same radar. This echo signal is processed to extract target information.

On the other hand, Secondary Radar relies on the active signals transmitted by the transponder unit on the airborne target (e.g. an aircraft). Transponder is a radio receiver that receives the 1030 MHz signals (interrogation signal transmitted by Radar) and transmits 1090 MHz pulse (reply message with target details) back to the Radar.

 In the context of ATC: Primary Radar is both an advantage and a disadvantage. With Primary radar, even if the target is not replying the interrogation signals or does not even have transponder unit, ATC Unit can trace/locate it on the RADAR although target identification is not possible (Disadvantage). Primary radar is still used by ATC today as a backup/complementary system to secondary radar, although its coverage and information is more limited.

Secondary Radar provides various details (like object identification code, altitude, direction, speed etc.) of the target which are transmitted back by a strong response signal by the transponder.

Ques. 38: What are the applications of RADAR in ATC?

Ans.Various types of Radars used in Air Traffic Control are as:

  • Air Route Surveillance Radar (ARSR): As the name suggests ARSR (an application of SSR) is long range radar with a typical range of 300 NM. It operates in L-band and is used in Area Control Center to keep surveillance of en-route aircraft.
  • Surface Movement Radar (SMR): SMR (basically a Primary Surveillance Radar) is used to provide clear display of all the aircraft (and other well-equipped vehicles) on the ground as an aid to the Tower and Ground Controllers to manage the traffic safely and efficiently on the ground.
  • Precision Approach Radar (PAR): It is basically an SSR which assists the Air Traffic Controllers working in Approach Control position. PAR provides information on identification, air speed, direction and altitude of aircraft in the air within the vicinity of the airport.

Edit: Read the latest post about important definitions and terms like ADS-B, SSR, SBAS, GBAS, Runway, Flight Level, etc for ATC Interview Preparation. Link is:
Important Terms and Definitions for JE-ATC Interview

I hope that you will get benefitted from the above information about Radar. Please share this post if you enjoyed and do subscribe to get the latest ATC related news and articles directly in your mailbox.


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13 thoughts on “Important Questions For AAI ATC Interview | Article 2/5”

        1. sudhir sheoran

          AAI is large organization and There might be various reasons behind this which are beyond our imagination.

  1. pratikshya borthakur

    hello sir, while converting my cgpa to percentage i did a mistake in the form. will it be a problem in the interview?

    1. These questions have been suggested to keep in mind for the interview of ATC, for which you are going to appear. So, in my opinion, answer to your question is affirmative.

  2. I am an ece graduate so do i need to prepare more of my core subjects for the post of ATC JE because it is related in a way to my subjects or a general study of whatever has been provided here will do?

    1. In interview they are generally asking about yourself, ATC and it’s functioning and some technical questions related to ATC communication. So prepare about all these.

    1. You can enhance your knowledge about field you are going to work in. But selection will be based on your performance in written test only.

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