The development of electronic communications over a period of time has been a big advantage for the aviation industry.The Airbus believes in Fly , Navigate and Communicate . With the help of electronic communication, the emphasis on verbal communication between the pilots and the air traffic controller has reduced severely .This helps aviators to concentrate on flying the aircraft which is of utmost importance.

Transponders and squawk codes help in reducing verbal communication and help maintain a silent cockpit.They assist the air traffic controller in knowing the aircraft position on their radars.



A transponder (XPDR) as the name suggest is a transmitter and receiver. It is an electronic device that produces a response to an interrogation signal sent by the air traffic controller.

It was initially used in the military to identify aircraft’s . It was termed as ‘Identification of Friend or Foe’ as a military aircraft sent interrogation signals to another aircraft to find out if they are their friends or foes .

However , in commercial operations, as we only have friends we do not use these terms . The Air traffic controller assigns each aircraft a squawk code which enable them to identify the aircraft on their radar and other aircraft’s collision avoidance system.

Conventional Transponder


The ground based equipment transmits interrogation pulse signals on a frequency of 1030 MHz (Megahertz) and receives on 1090 MHz.While the Aircraft transponder , transmits on 1090 MHz and receives on 1030 MHz .

Signals from the ground transmitter are transmitted in pair of pulses that are coded and each code is known as a MODE.There are a few modes that will be discussed down below. The replies from the aircraft are however in all directions.

The ground receiver then decodes the reply from the aircraft and displays the necessary information such as aircraft call sign, altitude , speed etc on the radar.


The different modes of transponder help us gain different information of the aircraft .

  • MODE A:This type of transponder provides an identification code only .
  • MODE C: In Mode C, along with identification code , aircraft pressure altitude is provided as well .
  • MODE S: In Mode S (selective) ,there are a number of details that can be provided along with aircraft identification code and altitude. For example, aircraft ground speed , destination of the aircraft , desired track etc.

A Mode C transponder is commonly found in general aviation aircraft’s where as the commercial jets are equipped with Mode S transponder. In the flight plan that is needed to be file before a flight , it is necessary to mention the type of surveillance equipment (transponder) installed on board. In the ICAO flight plan, the necessary details are included in box 10,that is , Equipment ( more on flight plan in a separate post).



Once the ground receiver has decoded the information, the radar displays the necessary information depending upon the mode of transponder. The different aircraft’s are shown as a blip or a trace on the radar screen.In the figure down below , the example shows the aircraft is equipped with a MODE S transponder.



A-320 Transponder

STBY: The Stand By function powers up the transponder and makes it available for operation.

ON : In the ON position, it will send primary information to the radar, that is , it will work like a MODE A transponder.

ALT:If the ALT RPTG is in the ON position , the transponder will send altitude data and will work like a MODE C/ MODE S transponder.

IDENT: All modes (A,C,S) include an ident button.It reveals the identity of the aircraft to the ATC on their radar and helps them locate the aircraft too. For example, when the ATC requests the aircraft to ‘SQUAWK IDENT’ , the pilots need to press the ident button which leads to the aircraft blip on the radar to flash and enables the controller to easily identify the aircraft among many other that are near it.


What are Squawk Codes?

Transponder transmission usually requires a discrete code to identify the aircraft. These codes are assigned by the ATC to each aircraft in their departure clearance.

The squawk codes are 4 digit octal numbers from 0 to 7 and range from 0000 to 7777. Once the pilot receives his squawk code , he has to enter the 4 digit code so that he is visible on the radar .

Let’s take an example, the controller on your departure clearance assigns the pilot to squawk 1234. The pilot can enter 1234 via the numbers shown in the image below and the screen will display the squawk code entered.

Reserved Codes

There are a few transponder codes that have a predetermined meaning and should be used when the aircraft faces that occurrence.

In the above mentioned reserved codes, it is always a good idea to remember the last three codes as they notify the ATC immediately of the problem. As a good rule of thumb, I remember the word ice, that is, interference communication emergency and corresponding to those are the codes, which are, 7500 7600 7700.

Check the image on the right to have a better understanding.

FACT OF THE WEEK: This week we go back to the years where the Wright Brothers were busy making their first powered airplane . A powered airplane would require an engine for the aircraft to take flight. Charles “Charlie” Taylor a mechanic who worked at the Wright Brothers bicycle shop stepped up to help them in their pursuit and became the first man to build an engine that powered an airplane and the first aviation mechanic in history. If it hadn’t been for Charlie the first powered airplane would never have gotten off the ground.

Please click on the link to know more about Charlie Taylor

We are done for this week. I hope you gained some knowledge from this post and if you did please like and share it with your fellow aviators.Please feel free to comment if you have any doubts or suggestions for further posts. Until then stay safe, stay healthy.




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