Almost immediately after the Germanwings Airbus A320 had crashed in the Alps a very detailed plot of its flight path was available worldwide.
The flight of the Airbus was tracked from takeoff to a final position report that ended very near where the airplane hit the mountains. The plot showed altitude, groundspeed, vertical speed and track in detail. It was immediately apparent that the airplane made a fairly rapid but steady descent from cruise altitude to impact with the speed essentially constant.
It’s the first major accident I can think of where such flight path detail was immediately available to everybody and it’s all thanks to ADS-B.
Radar has, of course, tracked a number of doomed flights in the past. But radars are under the control of government agencies and it usually takes weeks, months or even years before radar plots are released. And radar is surprisingly imprecise and coverage gaps are common.
But the details of the A320 flight path were posted on websites around the world and hashed over on TV and internet sites very shortly after the accident. The onboard flight data recorder will have more information such as indicated airspeed, flight control positions, power settings and so on, but the recorder won’t have any better, or even as good, a track of the actual flight than what was available to all instantly.
What is at work here is the automatic operation of ADS-B. That’s what the “A” stands for–automatic. Once ADS-B equipment is installed it goes about its job of automatically sending out the identity, position, altitude and velocity of the airplane, including while the airplane is still on the ground. ADS-B updates that information with a new broadcast at least once per second and the accuracy is within a few meters.
Compare that to how radar and transponders work. A radar requires a directional antenna to determine the azimuth from the radar site to the target. Radar also needs to time the round trip of a transmission and reply to determine distance from the site. That yields a bearing and distance from the radar site to the target.
For its part the transponder in our airplanes does nothing until it is interrogated by a radar or traffic detection system in another airplane. When it is interrogated the transponder can only reply with our identification–the four-digit discrete code and the unique ICAO code if the transponder is Mode S–plus the pressure altitude data of Mode C.
So the radar equipment must plot the bearing and distance information to establish the actual location of the airplane. Then the electronics mix in the Mode C reply to determine altitude. After several radar “hits” an approximate flight path can be determined including ground speed, vertical speed, altitude and track.
But even short range radars get new “hits” only every couple or three seconds, while long range radars may get a new return only every seven or more seconds. That’s a long time when an airplane is moving at more than 400 knots as the Airbus was for the entire descent.
ADS-B is, however, reporting its actual position and velocity. There is no need to plot anything. ADS-B automatically says “I’m here, at this altitude, flying this fast, with this vertical speed, and along this track” all based on the global lat-long grid. All you need is a very simple and inexpensive receiver to know everything about an airplane with ADS-B. Radar needs a vast network of huge hardware and computers that only governments can afford.
The FAA has installed more than 600 ground stations to receive and process ADS-B data. But private networks are installing thousands more receivers around the world and linking them via the internet. Individuals participate by assembling a receiver and hooking it up to their computer. That means soon there will be complete ADS-B reception coverage nearly everywhere except over the open oceans.
The wide open nature of ADS-B data worries many who are concerned about privacy and security. And that’s a valid concern.
But there is an ADS-B benefit I hadn’t considered before this accident. For the first time accident investigators will have precise and detailed information about the flight path of an accident airplane that is nearly as useful as a flight data recorder. In most general aviation accident investigations there is very little to go on. Sometimes there is a radar plot but it’s usually incomplete and certainly not precise. But with ADS-B installed in most airplanes in a few years we can know much more about what happened in an accident, even though, as in the case of the A320, we can’t immediately know why it happened.
Every new technology is designed to solve a problem or create a new product or service, but there are always unexpected issues. It is the same with ADS-B. The system is designed for traffic surveillance and separation, but the immediately available flight path data will also bring unexpected and unintended other results. What will they be? Only the years ahead will tell.