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:
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.