What Is the Aircraft Transponder System?

One of the most critical elements of aviation safety is communication. Over the years, aerospace engineers have developed multiple sophisticated measures to help aircraft maintain verbal and nonverbal communication with ground authorities and other aircraft. One such piece of technology that has stood the test of time is the transponder, which remains a vital piece of aircraft safety and functionality even 60 years after its invention. In this blog, we will discuss everything you need to know about transponders, including their history, design, and current implementation.

Transponders came about during World War II when allied aviators discovered the need to properly identify friendly and non-friendly aircraft. Early radar systems helped identify the location of targets in general, but if two aircraft began to fly close in proximity, they would appear as a single blur. As a result, an adjunct system was needed to show the location of friendly forces. The solution decided upon was to create a device that could return the signal sent from a radar station with a stronger signal than what would've been usually produced by another vessel.

After the use of transponders gained widespread traction in military aviation, commercial aviation agencies realized that the same technology could be used to manage local air traffic. As a result, the secondary surveillance radar (SSR) was quickly adopted by air traffic control stations worldwide. Unlike older primary radar techniques, which were only compatible with large aircraft composed of metal skin, SSR allowed air traffic controllers to receive accurate real-time location information about departing and arriving flights. To accomplish this, airport surveillance teams installed new radar antennas that could transmit and receive the exact frequencies compatible with standard transponders. The same was also achieved through upgrades to legacy primary radar systems since these beacons could still be used as a backup.

Mode A

When aircraft enter a particular flight information region, the flight crew is provided a unique code that will be used to uniquely identify the plane upon its approach. The four-digit code, which is commonly referred to as the squawk code, only contains numbers from 0 to 7. Thus, using this identification system, there may be 8^4 or 4096 unique codes in use at any time. Universally, there are three special codes that allow the pilots to portray pertinent information to ATC. Squawk code 7500 may be set during a hijacking situation, 7600 is reserved for radio failure, and 7700 is recognized as a general emergency. Once the pilot inputs the assigned squawk code, ATC will have access to real-time location information.

Mode C

In addition to aircraft position, it is also helpful for ATC to receive the plane's real-time altitude in order to adequately separate nearby aircraft in all axes.

Mode S

Mode S is an advanced, frequently updating, selective interrogation system designed to help take the stress off conventional SSR systems, most of which operate at capacity regularly. Unlike modes requiring the pilot to input a squawk code, mode S communication relies upon a unique 24-bit aircraft address that every model holds.


When you are in need of high-reliability aircraft transponder components, look no further than NSN Purchasing. Owned and operated by ASAP Semiconductor, we give customers direct access to an inventory of over 2 billion ready-to-purchase parts for the civil and defense aviation industries. As an AS9120B, ISO 9001:2015, and FAA AC 00-56B accredited enterprise, we regularly subject much of our inventory to rigorous testing to help screen for potential issues before shipping. Additionally, with distribution centers located across the globe, we always provide expedited shipping to domestic and international customers alike. Get started on the purchasing process today with an RFQ to learn why many customers continually choose NSN Purchasing as their strategic sourcing partner.

Recent Twitter Posts

Why Choose Us?

Low Price Warrantied inventory at competitive prices

Find it fast Search the world's largest inventory of electronic components by manufacturer, category or part number

Quality guaranteed We sell only warrantied and traceable parts

Get it fast All inventory ready to ship from our sellers

Semiconductor's Certifications and Memberships
Thank You for Visiting NSN Purchasing!

Before You Go, Please Take a Look at NSN Parts Catalog!

Request for Quote

We use cookies to ensure that we give you the best experience on our website. If you continue to use this site we will assume that you are happy with it.