19 March 2014
ABC Australia reports – When Malaysia Airlines flight MH370 flew out to sea off Malaysia on a routine flight to Beijing on March 8 it disappeared from all tracking systems.
A total of 239 people were on board and authorities have not been able to pinpoint the exact location of where the flight is or even what direction it was heading.
But with the help of satellites and a number of communication systems on the plane they have been able to refine two major search zones.
The missing Boeing 777-200ER, a model that is about 20 years old, had two main communication systems on board that can be used to help with tracking, the ACARS and a transponder.
The head of aviation at the University of News South Wales, Professor Jason Middleton, helps explain just how these systems work and why there is no exact location for the plane.
ACARS: Satellite-based text messages
The Aircraft Communications Addressing and Reporting System (ACARS) on the plane sends manual or automated text messages from the plane back to base and vice versa.
It is able to send messages using a system of satellites that are run by the British company Inmarsat, which is perfect for a plane at sea as it removes the limitation of needing to be close to ground-based transmitters to send radio messages.
“An ACARS message can be initiated by the pilot who might seek additional weather information or destination information from the company,” Professor Middleton told ABC News Online.
“The company will then respond with a text message which is uploaded and sent.”
Professor Middleton says the ACARS in the missing Malaysia Airlines plane would not have “pinged out” its location in its messages.
However, data from the Inmarsat satellites can help to refine its location. A rough location can be extrapolated by measuring the time it takes for the ping to be picked up by two satellites.
“[Investigators] are actually using not the signal itself, but the time of reception at different receivers,” Professor Middleton said.
“So if you sent out a ping of some sort and it arrives at two receivers and you’re exactly halfway between, you can expect that ping will be received – milliseconds later – at the exact same time at both, because it’s got the same distance to travel.
“On the other hand, if you’re two-thirds closer to one than the other, then that one will receive the signal more quickly than the other.”
Professor Middleton says you can use that satellite data to identify a rough line on the surface from where the signal most likely originated.
But there is no constant line of communication between the ACARS and the satellites which makes it difficult to refine an even tighter location.
“It’s sent in packages so that the line to the Inmarsat isn’t always open,” Professor Middleton said.
“You don’t open a telephone conversation to your friend in London and leave it open hour after hour. You have your message and then you cut it off.”
ACARS messages can be manually initiated by the pilot or ground base, but the system will also send out timed “handshakes” automatically to the satellites, Professor Middleton says.
He says the ACARS sends a handshake to the satellite, which can be every hour, where they ping each other and ask if there is any data to be sent.
This is why investigators know when the last ACARS ping occurred from flight MH370 but not the exact time when it was switched off or disabled afterwards.
Transponder: Radar-based location
The aircraft’s transponder, which alerts air traffic controllers to the plane’s location, is much more limited than the ACARS.
Instead of satellites, it relies on ground-based radar and is useless when outside of signal range – usually about 80-100 kilometres, Professor Middleton says.
Each plane receives a four-digit number – known as a squawk code – which the pilot programs into the transponder at the start of a flight so that air controllers can identify and follow each individual plane.
“A transponder is a simple bit of kit. What it does is when it receives a radar signal of a certain strength is it bleeps back saying: ‘Yes I’ve heard you, and now I’m going to bleep back a response of my own’,” Professor Middleton said.
Out at sea, the transponder on flight MH370 would not have been able to tell authorities where the plane was.
“If the transponder is outside radar range the transponder doesn’t receive anything and if it does receive anything it may not be able to respond with enough signal strength – so the controller can’t see it,” Professor Middleton said.
Black box: Underwater signal
The aircraft’s black box holds important flight data and is built to withstand fires, explosions and underwater pressure.
Its recovery, along with the cockpit voice recorder, would help investigators figure out what went wrong on the plane.
The black box emits an underwater pulse to help authorities locate it, and despite its nickname is actually orange in colour for easy identification.
But even though it sends out a signal, investigators would need to know a rough location on where to search for the wreckage.
“Normally in an accident you know where an airplane is fairly quickly. You run around and find out where [the black box] is and then you pull it out of the water,” Professor Middleton said.