June 15, 2018, ©. Leeham News: The worldwide aviation is heading for its largest navigation change since VOR and Radar was introduced after the Second World War.
When scheduled airline traffic started in the late 1920s, navigation was with maps and when the weather was bad, through Low-Frequency Radio beacons. Then the VOR and Radar were introduced. Now we will replace these as well.
We have navigated civil air traffic with a combination of VOR, Radar and gradually GPS over the last decades. GPS will be with us going forward, as the way to know where we are and where we shall be going. But it doesn’t keep us from running into one another.
We have been using Radar for this and where it has no coverage we rely on time separation of aircraft flying after one another in corridors (over the Atlantic and Oceans). Lately, collision warning systems have warned for aircraft nearby your own aircraft.
At first, the surveillance with ground-based radars relied on its own signals, needing no cooperation from the aircraft. But a radar return on a display doesn’t say who is up there. For military purposes the radar was complemented with an IFF (Identification Friend or Foe) interrogator, triggering the aircraft to send a response if it was friendly.
Soon the civil radars were equipped with a civil version of the IFF, triggering a response from the aircraft’s civil IFF, the Transponder.
We now had radar traces and a coded signal from the poked Transponder who said whom it was (Mode A Transponder). Soon the Mode C Transponder was added which could return the altitude of the aircraft to the radar operator as well.
Gradually several radars could paint an aircraft at the same time, triggering several Transponder responses, thus saturating the system. The Selective Transponder S was introduced, which only triggered a response to the radar addressing it directly (with a code, getting it a coded response).
Combined with VOR or GPS, who helped with the navigation, this is how we have navigated and informed the Air Traffic Control (ATC) where and who we are.
Today’s system relies on the aircraft being asked to respond with its code and information (altitude for mode C Transponders, Heading, Speed, Vertical speed … for Mode S Transponders).
A new system will be mandatory on the aircraft side from 1 January 2020 in the US and it is gradually introduced for the rest of the world.
The fundamental change for the new system is; it requires no interrogation (poking) of the aircraft. Instead, the aircraft sends a complete information package every second of whom it is, what it’s doing and what its intentions are to anyone who cares to listen.
This means everyone who has a receiver for these signals; other aircraft, ATC, Flightradar 24 enthusiasts,… have full knowledge of:
The function in the aircraft sending these pieces of information is called ADS-B (Automatic Dependent Surveillance-Broadcast).
How the aircraft sends own ADS-B signals and receive others and what all the players do with these signals we will cover in the next Corners.