How does satellite track the plane?

Each signal sent from the satellite to the plane is reflected by another signal, including the code to identify the aircraft, enabling the authorities to keep track of flight time and landing position.

>>> How is theplane tracked?

According to Malaysian government officials, MH370 continued to fly at least until 8:11 am on March 8, nearly 7 hours after disappearing from the civil radar screen at 1:21. They thought that someone intentionally disabled the communication system to hide the plane's location, among them ACARS, the system uses satellites to transmit information to the ground.

Although it is possible that someone in the cockpit has turned off ACARS, the system's antenna still continues to operate, receive and send an hourly test response signal from a satellite earth station, said Inmarsat. Inmarsat is an English satellite communications company, owning and operating 10 satellites. All satellites are on geostationary orbit, at an altitude of about 35,400km.

Activity of Inmarsat

Inmarsat provides contact satellite service for ACARS, an automated data transmission system on aircraft. ACARS sends digital data links between the aircraft with an airline center, air traffic control station and other related units.

ACARS can be used to send messages and all kinds of data. It could be a text message from the pilot to the dispatcher or reports on the status of the aircraft.

Diagram simulating the process of receiving signals and signaling from air traffic control stations and airplanes, with the participation of satellites.(Photo: BBC)

ACARS messages are usually sent by VHF waves if the plane is on the ground.In the case of aircraft flying into remote areas or waters beyond the VHF range, the ACARS signal will automatically switch to satellite."Pilots don't have to do anything , " said David Coiley, an Inmarsat expert.

According to the basic understanding, satellites as a simple transition point, transmit signals from ground stations to aircraft and vice versa. It is not much different from the mobile wave tower, Coiley adds.

According to an official of Inmarsat, the satellite system is highly reliable. Each signal sent from the satellite to the plane is reflected by another signal, including the code to identify the aircraft. However, the official declined to discuss specifically about MH370.

The change of the identification code or the possibility of mistaking the aircraft for other aircraft is "almost impossible" , the official said. Furthermore, after the satellite signal line is established at the beginning of the flight, it will automatically check periodically until the aircraft lands. This helps investigators to determine flight time if the plane location is unknown.

How does satellite tracking the plane?

At startup, the aircraft automatically sends a signal to the communication network. After that, the ground station sends "control signal" to the plane via satellite. People call it a "handshake" when the plane has a response signal. The information conveyed in the "handshake" is very limited, but it includes a special code to identify the aircraft.

Hourly "handshakes" allow the satellite to calculate the approximate location of the aircraft, serving for subsequent transmissions. To solve this problem, satellites need to know its angle compared to airplanes. Accordingly, the plane just below the satellite is conceived as a 90 degree angle, at the two poles of the Earth is 0 degrees.

Authorities said the last message sent from MH370 was at an angle of 40 degrees. This information can help them determine the possible location of missing aircraft.

A complete "handshake" also shows that the aircraft is operating, because the satellite reply signal needs to have power to send. "The reply as saying" yes, I'm still here "," Coiley said.

The area where the flight MH370 could reach if it flew another 4 hours from the missing position from the Malaysian civil radar screen.(Graphics: Malaysia Insider)

This explains why Malaysian authorities say they "believe" that MH370 continues to fly after disappearing from civil radar screens.

"Inmarsat experts participated in the investigation. We have employees in Kuala Lumpur , " Coiley said. "We are focusing on this issue so that we can fully support the investigation process, because there seems to be no more data sets."