Bjorn’s Corner: Transponders, the kingpin of safe air navigation

By Bjorn Fehrm

By Bjorn Fehrm

May 19, 2016, ©. Leeham Co: In my recent Corners, I have been describing how a modern airliner navigates using a Flight Management System, (FMS or Computer, FMC) to navigate along the flight plan and how it finally uses an instrument landing system to safely land the aircraft even in bad weather.

When looking into instrument landing systems, we have described the legacy systems which require large ground installations (such as ILS) and how these can be replaced in the future with smarter concepts using GPS based procedures.

I will now continue on this path and describe some of the additional cornerstone technologies needed to implement a modernized Air Traffic Control (ATC) system, which can replace today’s systems that have their roots in World War 2 (WW2) technology.

We will start today with how aircraft can be seen from the ground or other aircraft without visual sight or Radar contact.

The classical way to detect an aircraft irrespective of visual sight conditions has been to use Radar to spot the aircraft. Radar was developed during WW2 and uses a high frequency radio pulse to bounce off an aircraft and detect its return to the Radar. Only a tiny fraction of the pulse goes back exactly in the direction of the radar so the range of a classical Radar is limited, although the receiver of the Radar is hypersensitive.

By measuring the bearing of the response pulse and the time it took for the round trip, the aircraft’s range and position can be determined. To understand who is behind that tiny response, a coded secondary pulse was added in WW2 and if the aircraft understood the code and responded it should be friendly (Identification Friend or Foe, IFF systems).

The box that understood the coded pulse and responded was called the IFF transceiver. After the war, civil Air Traffic Control (ATC) adopted the Radar to track airliners and other aircraft and pretty soon they added the IFF part as well, now called a Transponder (the whole concept is called a Secondary Radar System but we stick to Transponder systems to simplify things).

Initial Transponders were just doing what the IFF was doing, telling who was there and improving the responses radio quality (even a weak radio emitted pulse from a Transponder is much stronger than the Radar’s own reflected signal, bouncing of the aircraft skin). The frequencies used and codes were the same, so military aircraft could be seen as well with the Transponder interrogation from the Radar station (the military then added its own complicated and secret stuff, but can act as normal Transponder equipped aircraft using basic codes).

Gradually the Transponder was made mandatory for all aircraft, including the simplest general aviation types like gliders. They were also made more capable. An A type Transponder can only say who it is, a C type adds the altitude (read off the altimeter) and an S type adds more information so that it can also serve as a Traffic Collision Avoidance System (TCAS) in the aircraft.

The advantage of Transponders
Transponder returns from aircraft have many advantages compared to Radar:

  • It extends the range of position and identification info. The Transponder return is so much stronger than a Radar return and is independent of rain, snow, etc., which can degrade a Radar’s range a lot.
  • It tells who is behind the position and by using special return codes can signal distress or, e.g., hijacking.
  • The type C Transponders (which today is the minimum level) gives altitude back, something which most civil Radars have difficulty in providing.
  • It can respond with more information than just ID and altitude. When this gets expanded it forms the basis for TCAS systems (collision avoidance) but also the future of smart ATC systems.

In following Corners, we will go into how more capable Transponder technology together with GPS is absolutely essential to give modern air traffic a raft of clever and well needed functions. This is necessary; otherwise our ATCs and aircraft cannot handle the future denser, busier air traffic in a safe way.

10 Comments on “Bjorn’s Corner: Transponders, the kingpin of safe air navigation

  1. Very interesting, as usual. Did the information about the slightly strange descent profile of the Egyptair plane come from a transponder or would primary radar be able to provide it?What are the limits of information that a transponder is able to provide?

    • Hi grubbie,

      it would have come from the transponder, Radar is notoriously bad in resolving altitude. Even if it could (which I doubt) the resolution is coarse, such as within 500m accurate. But in this case it seems it was a S type transponder with enhanced capabilities. We will go through in next week what such a transponder can deliver in terms of info, quite a lot.

      • Looking forward to the next installment.
        “LNC urges caution in drawing conclusions “-can’t say that we weren’t warned.

    • I would guess that it was an ADS-B signal that for sure will be explained later by Bjorn.

  2. Bjorn: At some point could we discuss how US aircraft found their Carriers in WWII?

    I came across a reference to it, electronic of some type, had not a clue they could do that other than they generally managed to do so and with combat ops it seemed strange they could that consistently with single fighter pilots not being able to track their own let alone the carrier shenanigans.

    PS: I liked the Stuff reference. We called it PFM. No polite way to put that in the blog!

    • Hi Transworld,

      I found a really good description of the system. It was a very clever form of an encrypted VOR type of transmitter-receiver system where a directional transmitter on the carrier would send out the bearing sector you were in with morse letters, to be received by your radio receiver. The morse letter for the bearing sector you were in changed several times a day (so that enemy aircraft could not use the transmission to home onto the carrier). You as a carrier pilot carried a page with the time of changes and a key table to show which sector you were in. As you then steered the counter-course you would have increased precision as you got closer to the carrier until you found it.

      Here the link:

      • Thank you, pretty amazing things they did back then with what they had to work with.

  3. Hi Bjorn

    Thank you again for an interesting and easy to read article on the technical aspects of aviation.

    An idea for another Corner series. A few months back you wrote a series on weight, range and payload using the OEM published payload-range charts is the Aircraft characteristics manuals. A few pages deeper into these manuals are charts on take-off runway lengths. What is the possibility of a Corner on these charts and how runway altitude affects maximum take off weight.

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