Tag Archives: Aircraft Communications Addressing and Reporting System

Two years after MH370 there is a way for aviation enthusiasts to obtain location data from aircraft flying in remote areas

Amateur tracking software can monitor the signals sent by the aircraft to the Inmarsat network.

On Mar. 8, 2014, Malaysia Airlines flight MH370, a Boeing B777-200 aircraft (registration 9M-MRO), operating from Kuala Lumpur and Beijing, disappeared from radars about 40 minutes after take off from Kuala Lumpur.

The flight, carrying a total number of 239 passengers and crew members, was regularly transmitting ADS-B data until contact was lost over the Gulf of Thailand, when the wide body was cruising at 35,000 feet at 474 knots in reportedly good weather.

Between 1:19 and 1:20AM local time, the aircraft turned right, changing heading from 25 to 40 degrees.

The transponder stopped transmitting at 1:21AM LT.

According to the Malaysian authorities, there were subsequent primary radar returns to the west of the Malaysian peninsula, over the Strait of Malacca and then north-west. This is assumed to be a real return from MH370 even if based on primary radar echo.

For reasons we still don’t know the aircraft radio systems did not work while the plane flew westwards back towards Malaysia.

Even if information was incoherent and sometimes contradictory, we know for certain that military radars in both Malaysia and Thailand saw the plane.

MH370 route with initial search (credit: Wiki)

MH370 route with initial search (credit: Wiki)

SATCOM (a radio system that uses a constellation of satellites used to transmit voice, data or both)  system pings linked to the INMARSAT network continued for 7+ (last ping at 08:11 local) hrs after LOS (loss of signal).

A Ping is a quite common term for IT Networking. It refers to a utility used to test the reachability of a host on an IP network and measure the round-trip time (RTT) of the packets even if it is more frequently associated to the data messages themselves, or “pings”.

Similarly to what happens on a Local Area Network, satellites send pings (once an hour) to their receiving peers that respond to it thus signaling their network presence. Hence, these pings are no more than simple probes used to check the reachability of SATCOM systems aboard the planes.

Based on the round-trip times of such pings, two arcs made of all the possible positions located at the same distance from the INMARSAT satellite were drawn.

But further analysis on Doppler Effect, as well as correlation among the “signatures” of other B777s, clearly indicated that aircraft had followed a southbound route, towards the South Pole.

Whilst search efforts have not been able to find the wreckage of the Boeing 777, the mysterious disappearance of the MH370 flight highlighted the need for tracking aircraft flying over high seas or remote locations, where radar coverage does not exist (such as southern Indian Ocean where aircraft, ships and submarines from 26 nations, have searched for any debris).

Two years on from the loss of MH370 and six months after Inmarsat completed their initial trials of a free tracking network for commercial traffic across Autralasia and the Pacific region COAA’s team who develop PlanePlotter, an aircraft position plotting program that decodes the digital messages transmitted from aircraft to display their content and plot their position on a radar-like chart, have successfully completed their own trials to see if we can obtain location data from the Inmarsat fleet and then plot it on their own tracking system.

Here’s how they describe their recent developments:

“This had been on our minds for some time, but we didn’t have the decoding knowledge to fully understand the signals from the satellite fleet,” says John, from PlanePlotter support.

“In stepped “Jonti” from New Zealand, who co-incidentally had been decoding the L band signals in the Aero band from Inmarsat. Those signals did not contain location data as the ACARS style messages at L band are from the earth stations, up to the satellite and “down” to the aircraft.

Thinking back to the days when we monitored the Inmarsat analogue phone circuits we realised that to get both sides of the “conversation” we needed to monitor both L band [ 1.5 ghz ] …and C [3.5 ghz] band. With Jonti’s help we set about some trials.

Inmarsat uses 8m dishes to monitor the tracking signals, we only had 1.8m and 2.4m dishes, but with Jonti’s guidance we managed to get good signals with just 1.8m antenna.

Jonti identified the short bursts of data which looked like they were from the a/c and after some stirling work he produced a version of his software JAERO, which decoded the bursts. Sure enough, in the data down from the aircraft was information akin to that which is contained in ADS-B signals. Using that and satellite dishes in Europe, America and Australasia we can now locate aircraft from the US
eastern seaboard into Asia and across the Pacific, via five different satellites.”

PlanePlotter are in the middle of further tests and have recently rolled out a new version of their software which, for their satellite ground stations, will provide full information, from location, heading etc. to outside air temperature: in fact, everything you would expect from ADS-B data.

“For our sharers it will show the usual ADS-B style information and plot sat comms equipped Oceanic traffic in real-time. Whilst we are not seeing reports from aircraft on a 15 minute basis yet, presumably as more airlines comply the reports will become more regular,” says John.

“Over the next few weeks more of our satellite ground stations will come online providing real-time coverage for tracking enthusiasts. The Inmarsat data, combined with ACARS, live PiReps and ADS-B will provide seamless global tracking.”

PlanePlotter global coverage

PlanePlotter global coverage


Top image credit: Laurent Errera (Wiki)

All the articles about MH370 can be read here (scroll down).

 

Doppler effect analysis on Satellite pings disclosed MH370’s final route

Even if the aircraft’s crash position could not be determined, Doppler effect analysis on SATCOM pings enabled INMARSAT to determine MH370’s final route over South Indian Ocean until a final, “partial ping,” received 8 minutes after the last known one.

Whereas search for debris from the missing Malaysia Airlines MH370’s Boeing 777 continues in a wide area located about 2,300 kilometers to the southwest of Perth, Australia, INMARSAT released new details to explain how the British satellite telecommunications company was able to exclude the so-called “north route” and focus on the southern one, pointing towards the South Pole.

As already explained on a previous post, hourly SATCOM system pings continued for more than 7 hours since the Loss Of Contact with MH370, until 08.11 AM LT.

Based on the round-trip times of such pings, two arcs made of all the possible positions located at the same distance from the INMARSAT satellite were drawn.

But it was further analysis, on Doppler Effect, as well as correlation between the “signature” of other B777s, that clearly indicated the aircraft southbound route.

Doppler Effect

The Doppler effect is something we are familiar without even knowing it. The sound of the ambulance’s siren or the train whistle are among the most common examples of how Doppler Effect works: the high pitch of the siren of an approaching ambulance suddenly drops as the vehicle passes you. Even if the source wavelength and speed do not change, movement of the source alters the wavelength and frequency of the sound.

You can use several online tools to calculate the frequency change induced by motion.

Since satellite pings are carried on a radio wave, the sensed wavelength, frequency increase or decrease depending on the fact the aircraft is moving towards or away from the satellite.

The difference between the expected received frequency and the actual measured one due to Doppler Effect is known as Burst Frequency Offset.

By comparing the Burst Frequency Offset due to Doppler on MH370 against the predicted one based on six B777s flying on the same day, INMARSAT could determine close correlation for the southern route and eliminate the northern one.

BFO MH370

Here’s an excerpt from UK Air Accidents Investigation Branch (AAIB) release that explains how INMARSAT calculated the route.

INFORMATION PROVIDED TO MH370 INVESTIGATION BY UK AIR ACCIDENTS INVESTIGATION BRANCH (AAIB)
25/03/14
[…]
As you have heard, an aircraft is able to communicate with ground stations via satellite.
If the ground station has not heard from an aircraft for an hour it will transmit a ‘log on / log off’ message, sometimes referred to as a ‘ping’, using the aircraft’s unique identifier. If the aircraft receives its unique identifier it returns a short message indicating that it is still logged on. This process has been described as a “handshake” and takes place automatically.
From the ground station log it was established that after ACARS stopped sending messages, 6 complete handshakes took place.
The position of the satellite is known, and the time that it takes the signal to be sent and received, via the satellite, to the ground station can be used to establish the range of the aircraft from the satellite. This information was used to generate arcs of possible positions from which the Northern and Southern corridors were established.

In recent days Inmarsat developed a second innovative technique which considers the velocity of the aircraft relative to the satellite. Depending on this relative movement, the frequency received and transmitted will differ from its normal value, in much the same way that the sound of a passing car changes as it approaches and passes by. This is called the Doppler effect. The Inmarsat technique analyses the difference between the frequency that the ground station expects to receive and that actually measured. This difference is the result of the Doppler effect and is known as the Burst Frequency Offset.
The Burst Frequency Offset changes depending on the location of the aircraft on an arc of possible positions, its direction of travel, and its speed. In order to establish confidence in its theory, Inmarsat checked its predictions using information obtained from six other B777 aircraft flying on the same day in various directions. There was good agreement.
While on the ground at Kuala Lumpur airport, and during the early stage of the flight, MH370 transmitted several messages. At this stage the location of the aircraft and the satellite were known, so it was possible to calculate system characteristics for the aircraft, satellite, and ground station.
During the flight the ground station logged the transmitted and received pulse frequencies at each handshake. Knowing the system characteristics and position of the satellite it was possible, considering aircraft performance, to determine where on each arc the calculated burst frequency offset fit best.
The analysis showed poor correlation with the Northern corridor, but good correlation with the Southern corridor, and depending on the ground speed of the aircraft it was then possible to estimate positions at 0011 UTC, at which the last complete handshake took place. I must emphasise that this is not the final position of the aircraft.

 

Here below is an INMARSAT image which shows the southern tracks for a ground speed of 400 and 450 knots ground speed.

 

MH370 tracks

 

Last “Partial” Ping

Noteworthy, INMARSAT collected  evidence of a partial handshake between the aircraft and ground station at 00:19 UTC, 8 minutes since the last acknowledged response. This partial ping is currently being investigated: there are several different theories, including the one that the final handshake was attempted outside of the hourly window, possibly at fuel starvation because of power fluctuations.

At 0115 UTC, when the ground earth station sent the next log on / log off message, no response was sent by the plane, indicating that the MH370 was no longer logged on to the network (because already crashed).

Lessons Learned

1) Pilots have the power to make aircraft almost invisible to radars. This will have to be addressed in some way, with some system capable to track the plane regardless of the aircrew’s willingness.

2) Black Box data have to be streamed via satellite and stored for the shortest time possible (until the next flight, then automatically erased) somewhere (for instance, in a Cloud Network architecture, to save money and have it immediately available, should the need arise).

Image credit: INMARSAT via AAIB

Enhanced by Zemanta

All you need to know about MH370: facts, timelines, systems, findings, theories

Let’s try to summarize all what is known to date about the mysterious MH370 flight.

This post contains a complete review of all the Malaysia Airlines 370 story, with new and revised details, update data and findings.

Facts

1) MH370 took off normally and headed on course to Beijing as planned. Just before Loss of Contact (LOS) the aircraft turned right to HDG040°. At the time the transponder stopped transmitting, based on ADS-B data, the aircraft was flying at FL350.

2) MH370 was following the usual route to Beijing.

3) Based on reports, the weather in the area was good.

4) The last ACARS transmission was 01:07AM local.

5) Reports surfaced yesterday that the aircraft either: turned westwards before the last voice transmission – or- entered two new waypoints in the FMS (see ADS-C section).

6) The last radio comms were “All right, good night” transmitted to Malaysia Air Traffic Control at hand-off to Vietnam control. Vietnam was not contacted. It has been reported it was the First Officer’s voice. Although it is not a standard phraseology reply, the “good night”, “ciao”, “au revoir” etc way to greet ATC at hand-off is quite common and, per se, it does not constitute any evidence of something wrong in the cockpit.

7) The transponder stopped transmitting at 1:21AM LT.

8) There are reports of a climb to 45,000 ft, uneven descent and some changes in altitude. However, this changes are based on primary radar, and altitude data is uncertain at that distance from radar.

9) According to the Malaysian authorities, there were subsequent primary radar returns to the west of the Malaysian peninsula, over the Strait of Malacca and then north west. This is assumed to be a real return from MH370 even if based on primary radar echo.

10) SATCOM (see below for details) satellite system pings continued for 7+ (last ping at 08:11 local) hrs after LOS (loss of signal)

11) SATCOM pings do not locate the aircraft but based on correlation to signal strength latency, satellite height, it is possible to draw arcs (of a circumference centered on the satellite with radius = distance from the satellite) where the last ping may have been located. The arcs identify a series of points at the same distance from the satellite and are located along two directions, the first is north from Andaman Sea to Turkmenistan; whereas the second is south, over the Indian Ocean from southwest Malaysia to southwest of Australia. The last primary radar reply came from a point that is coherent with the northern arc.

12) The last SATCOM ping replied by the aircraft reportedly was at 8:11 am Malaysian time. At that time it would be dark on the north arc and light over the south arc.

13) SATCOM pings are hourly – so the 8:11 ping could be up to 1 hour before the aircraft stopped ‘pinging’: the aircraft onboard systems could reply to ping (for more details read below) even if the aircraft had landed, while it is difficult to believe they could reply after crashing.

14) A pilot of another plane in the air when the aircraft failed to contact Vietnamese control, heard a weird “mumble” or noise seemingly coming from a stuck microphone from MH370 but there is no way to confirm this.

15) According to the Malaysian PM, the way the aircraft has flown since LOS make investigators believe it was a deliberate action.

16) Reports of sightings in Maldives, landing in China etc have been debunked. Thailand has admitted to have possibly tracked the plane on Mar. 7, after LOS,  but only gave this information some 10 days after the disappearance.

17) All the previous alleged sightings of the aircraft debris (Chinese satellite images, Greek ship reports, Tomnod crowdsourcing initiative etc) did not find anything that could be related to the missing plane.

18) Based on satellite imagery collected on Mar. 16, search and rescue aircraft, including a P-8A are investigating possible debris located 2,300 km to the southwest of Perth, Australia.

MH370 update 2

Timeline

1.07 am – Last ACARS transmission.
1.19 am – Last verbal communication “All right, good night” from the plane; believed to be the co-pilot
1.21 am – Transponder stopped transmitting (turned off or failed)
1.30 am – Civilian (primary) radar lost contact
1.37 am – Expected ACARS transmission; not received
2.15 am – Last military primary radar contact
8.11 am – Last (hourly) satellite handshake

ACARS

ACARS is the acronym for Aircraft Communications Addressing and Reporting System. It’s an automated communication system used by commercial planes to transmit and receive messages from ground facilities (airline, maintenance department, aircraft or system manufacturer, etc). Therefore, along with the general information about the flight (callsign, speed, altitude, position, etc), these messages may contain what we can consider systems health checks.

ACARS is a service: airlines have to pay for it. According to the information available to date, it looks like Malaysia Airlines subscribed only to engine health monitoring that enabled MH370 to send data to Rolls Royce.

The ACARS system aboard MH370 last trasmission was at 01:07 LT.

However, it has emerged on Mar. 18, that last trasmission included new way-points that were possibly entered before the loss of contact, using the FMS. This would require ADS-C (see below).

ACARS rely on VHF frequencies (indeed, you can track planes and decode messages with a simple radio receiver tuned on the proper ACARS frequencies and a software running on your computer) or SATCOM (SATellite COMmunication).

Although this is still debated, according to several pilots the ACARS transmissions can be switched off by the pilot from inside the cockpit, by disabling the use of VHF and SATCOM channels. This means that the system is not completely switched off, but it can’t transmit to the receiving stations.

SATCOM

SATCOM is a radio system that uses a constellation of satellites used to trasmit voice, data or both. As said, ACARS can make use of SATCOM to transmit its data to ground stations. Dealing with ACARS, the SATCOM system used by MH370 was linked to the INMARSAT network.

Inmarsat is a British satellite telecommunications company, which offers global, mobile services through a constellation of three geostationary satellites.

The system relies on “pings”.

Ping

A Ping is a quite common term for IT Networking. It refers to a utility used to test the reachability of a host on an IP network and measure the round-trip time (RTT) of the packets even if it is more frequently associated to the data messages themselves, or “pings”.

Similarly to what happens on a Local Area Network, satellites send pings (once a hour) to their receiving peers that respond to it thus signaling their network presence. Hence, these pings are no more than simple probes used to check the reachability of SATCOM systems aboard the planes.

Based on details recently disclosed, the last response to a satellite ping, was sent by the SATCOM aboard MH370 at 08.11AM Malaysia time, some 7 hours past the loss of contact with the Boeing 777.

From the analysis of the time between request and response it is possible to work out the distance of the plane which is a circumference of certain radius from the satellite based on which, two possible arcs containing all the final points were drawn by the investigators.

Search MH370 WaPo

ADS-C

ADS-C stands for Aircraft Dependent Surveillance – Contract. It is a dependent system (depending on the aircraft using it) that sends information based on a “contract” agreed by the controllers and crew. Information can include altitudes, estimates, coordinate for next waypoints and subsequent waypoint.

ADS-C can be programed to report periodically, on demand, on event. It can be initiated by the crew in an emergency.

Based on the revelations that the waypoint past the point of LOS were known to the authorities, it can be assumed that either, ACARS report at 1:07 included the “Predicted Route Group” or that they just meant that the path followed by MH370 was seemingly flown automatically, as if the plane’s primary radar tracks match with waypoints on a published airway.

ADS-C is transmitted via ACARS (which, as said, can use SATCOM, VHF or HF channels).

ELT

ELT – Emergency Locator Transmitter. It is a battery powered system that transmits on the guard frequency (121.5 MHz on VHF and on 406MHz to satellites. If can’t transmit from under water.

Mobile Phones

Although this topic is quite debated, reports say that no cell phone belonging to people on board registered with network towers. This may depend on the altitude the aircraft, the route (above the sea), or the fact that they were either seized to passengers or these were unconscious hence unable to switch them on.

Pressurization / Hypoxia

Cabin pressurization can be regulated by the pilots. This means that cabin could be depressurized to the airplane’s current altitude using manual pressurization.

In the event of the reported climb to 45,000 feet (determined based on primary radar – hence, to be confirmed), masks would deploy providing passengers Oxygen for 12 to 20 minutes (usually, just the time required to descend to below 10,000 feet). After that, passengers would lose consciousness and, at some point, they would die. Pilots O2 lasts more.

The same effect would be achieved at much lower altitudes: the main difference would be that death would arrive earlier at higher altitude (45K above the aircraft max altitude).

Theories

There are so many active theories that is almost impossible to list them all. We’ll try to list the most significant ones. Please consider that a “composite theory” made by hijacking and subsequent failure is possible as well. We will not consider such mixed theories.

Anyway, the main question here is: did the aircraft suffer an in-flight emergency or not? Although this author still believes that the aircraft was diverted from its initial planned route by a pre-planned action, the possibility it experienced a catastrophic emergency can’t be completely ruled out. Still, it seems to me a bit far fetched that the B777 suffered a failure that disabled the transponder, radio, ACARS, etc. but (possibly) left the plane capable to fly (on autopilot?) for 7 hours.

Hijacking

The aircraft is hijacked. Then it crashes after 7 hours for fuel starvation.

Based on the current information, this is a likely scenario (at least for the first part of the flight – when the transponder was switched off). Hijacking may have been attempted by one of the pilots, then fighting occurred, one or both might be wounded and unable to complete the diversion to another country, the asylum request as happened for Ethiopian flight last month, or his terrorist attack.

It looks like the theory that passengers with stolen passports could be involved in a hijacking attempt was debunked.

Fire

Fire in the cockpit or cabin.

As mentioned before, fire in the cockpit that would selectively disable some aircraft communication systems but didn’t prevent the aircraft from flying is unlikely.

Progressive series of failures

First failure forces pilots to turn then other issues arise until the aircrew and passengers succumb and the aircraft crashes for fuel starvation.

As above, fire or smoke in the cockpit that would either selectively disable some aircraft communication systems or make aircrew unable to react, without affecting the aircraft’s capability to fly (most probably under FMS inputs) is unlikely.

Attempted landing at divert field

System failure or failure aboard. The incapacitated pilot points the aircraft towards the coast to land at Langawi but then fly until fuel starvation and crashes.

Unlikely, for the same reasons mentioned above plus the fact that a pilot trying to land at nearest airport would not switch off transponder.

Aircraft stolen

The aircraft was hijacked and moved in some rogue state to be used for future terrorist attacks.

Aircraft must be hijacked, secretly flown to a secret location escaping radar detection in the shadow of a larger plane, then landed on an unprepared, hidden landing field. Passengers killed or moved elsewhere. Quite unlikely, considered the maneuver to join another aircraft enroute, but not impossible.

Subscenario: passengers kidnapped for ransom: unlikely, because no claim nor ransom request after all these days.

Subscenario 2: passengers kidnapped for their skill and know how.

Subscenario 3: cargo stolen. Cargo manifest doesn’t list anything special and would you organize such a risky operation just to steal the plane’s cargo? Unlikely.

On board systems hacked/aircraft remotely flown

Malware was installed on the aircraft onboard systems. It enabled remote access to hackers that maneuvered the plane.

Although hacking an aircraft system is theoretically possible, it seems that shutting off some of the onboard systems leaving no time for the crew to use any emergency one is a bit too much. Even if the possibility to hack remotely piloted aircraft is actual, it would require a massive operation to use satellites to give inputs to the aircraft once under remote control. Such operation would leave traces (on satellites, for instance).

Aircraft shot down

The aircraft is hijacked, is turning towards a sensitive target. It is shot down.

We’ve extensively discussed it here. Unlikely. Even less likely, considering where search forces are currently focusing.

Suicide

One of the pilots hijacked the plane to commit suicide

It can’t be ruled out but it seems unlikely, considered where the aircraft could have crashed. There are no messages left behind by pilots.

Subscenario: the pilot decided to do something about Anwar Ibrahim, Malaysia’s opposition leader. First he disables the systems, turns the aircraft west out of radar and cell phone range, then, using a satellite phone, contacts PM and demands they reverse the Anwar conviction (or some other demand) or he will crash the plane.

Conclusions

An airworthy aircraft disappered from radars and stopped communicating with Air Traffic Control at the boundary between Malay and Vietnamese ACC areas of responsibility.

For reasons we still don’t know the aircraft radio systems did not work while the plane flew westwards back towards Malaysia.

Even if information was uncoherent and sometimes contradictory, we know for certain that military radars in both Malaysia and Thailand saw the plane.

In spite of all the disabled onboard systems, satellite got a signal of presence of the aircraft for each hour until 8:11AM LT, +7 hours after take off.

For several days, search efforts focused on the wrong area.

The aircraft wreckage was not found but searches in the South Indian Ocean have been intensified. US sources pointed to the Indian Ocean since the beginning. USS Kidd moved there few days after the aircraft disappeared.

This incident could cause some aviation procedures to change to reflect the inability of authorities to react to hijacking attempts conducted by experienced crew members (regardless of whether either of the pilots is found guilty or not).

All the articles about MH370 can be read here (scroll down).

H/T to all my visitors and readers for providing tons of information and hints. A big thank you for the help by Airliners.net staff member “rcair1” who filed an almost daily summary with the latest findings on A.net. His Sanity Checks helped shed some light on tecnical and non-technical details.

malaysia-mh-370 debris

Image credit: Reuters/Jason Lee, Washington Post, Australian Governement

 

Enhanced by Zemanta

What SATCOM, ACARS and Pings tell us about the missing Malaysia Airlines MH370

Eight days since the Malaysian Airlines flight MH370 disappeared somewhere over southeast Asia, the last known details about the missing Boeing 777 come from the onboard SATCOM system.

Very few details about the MH370 flight from Kuala Lumpur to Beijing, disappered from the skies on Mar. 7, 2014 have emerged. First of all we don’t know if it crashed or landed somewhere. Second, provided it was really hijacked as Malaysian authorities suggest, we don’t really know whether the pilot or co-pilot played an active role in the operation even if investigators are scrutinizing their profile and personal life.

Based on the information that were released so far, we have been able to draw several different scenarios, each featuring a certain degree of likeliness. The few details that were gathered past the loss of radio contact with the aircraft, come from satellites.

Let’s see why.

ACARS

ACARS is the acronym for Aircraft Communications Addressing and Reporting System. It’s an automated communication system used by commercial planes to transmit and receive messages from ground facilities (airline, maintenance department, aircraft or system manufacturer, etc). Therefore, along with the general information about the flight (callsign, speed, altitude, position, etc), these messages may contain what we can consider systems health checks.

ACARS is a service: airlines have to pay for it. According to the information available to date, it looks like Malaysia Airlines subscribed only to engine health monitoring that enabled MH370 to send data to Rolls Royce.

The ACARS system aboard MH370 was switched off some minutes before the transponder.

ACARS rely on VHF frequencies (indeed, you can track planes and decode messages with a simple radio receiver tuned on the proper ACARS frequencies and a software running on your computer) or SATCOM (SATellite COMmunication).

Although this is still debated, according to several pilots the ACARS transmissions can be switched off by the pilot from inside the cockpit, by disabling the use of VHF and SATCOM channels. This means that the system is not completely switched off, but it can’t transmit to the receiving stations.

SATCOM

SATCOM is a radio system that uses a constellation of satellites used to trasmit voice, data or both. As said, ACARS can make use of SATCOM to transmit its data to ground stations. Dealing with ACARS, the SATCOM system used by MH370 was linked to the INMARSAT network.

Inmarsat is a British satellite telecommunications company, which offers global, mobile services through a constellation of three geostationary satellites.

The system relies on “pings”.

Ping

A Ping is a quite common term for IT Networking. It refers to a utility used to test the reachability of a host on an IP network and measure the round-trip time (RTT) of the packets even if it is more frequently associated to the data messages themselves, or “pings”.

Similarly to what happens on a Local Area Network, satellites send pings (once a hour) to their receiving peers that respond to it thus signaling their network presence. Hence, these pings are no more than simple probes used to check the reachability of SATCOM systems aboard the planes.

Based on details recently disclosed, the last response to a satellite ping, was sent by the SATCOM aboard MH370 at 08.11AM Malaysia time, some 7 hours past the loss of contact with the Boeing 777.

From the analysis of the time between request and responce it is possible to work out the distance of the plane which is a circumference of certain radius from the satellite based on which, two possible routes were drawn by the investigators.

The question is why the hijacker(s) did not prevent the plane from responding to pings: most probably, being a networking detail, not even pilots know that their system/antenna respond “I am here” even if the SATCOM is not being used by any onboard systems (i.e. ACARS).

INMARSAT positions

 

Top image: Boeing; above, Office of the PM of Malaysia

All the articles about MH370 can be read here (scroll down).

 

Enhanced by Zemanta