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

Mar 16 2014 - 120 Comments

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
  • rauschpfeife

    Now what we’re hearing is that Inmarsat has somehow eliminated the northern half of the arc from consideration, based on some additional analysis of the ping data. How, I wonder, could they have done that, if all they have is the RTT of the ping/response?

  • Ong Eu Soon

    Simple question that everyone should ask but seem like nobody knew how to ask. The radars from Phuket International airport, Hat Yai International airport, Penang International airport, Langkawi International airport and Alor Setar air port all failed to detect MH370. How can the Butterworth military radar detected MH370 with it’s primary radar when it has no mean to identify the plane?

    • charlesjannuzi

      Did they really i.d. the plane? They detected A plane, but didn’t really i.d. it. BTW, the Australians / RAAF operate surveillance planes out of Butterworth.

  • Ong Eu Soon

    The other issue that need to ask is how INMARSAT determine which dynamic IP address belong to MH370? How do INMARSAT know which pings belong to MH370?

  • toberd

    I don’t know if anyone is still paying attention to this thread after all the time that has elapsed, but information has recently been put out by news organizations that the satellite people had definitely placed the 777 in the south Indian ocean at it’s last ping. I would really like to know how they think they did that.
    If one knew the exact time between the ping and the reply, then one could draw a circle on a globe representing where the plane could have been at that time. Of course the time measurement (which would include the reaction time of the transponder to the ping) would have to be very accurate, since a one millisecond error would mean a positional error of the circle of about 1000 miles.
    One story mentioned doppler effect, which, if they could have measured it, could have given the speed of the aircraft toward or away from the satellite.
    But I have been unable to imagine any technique that would allow them to place it at a certain position ON the circle. Can anyone enlighten me on that?

    • RocketRob

      If they only had a single ping to work from, then as you point out they would at best only be able to confine the plane’s location to a ring of some thickness centered on the Inmarsat’s sub-satellite point. (A significant fraction of that ring would be eliminated due to the impossibility of the plane reaching it from Kuala Lumpur.) Even with Doppler shift of the ping-back frequency, all that would tell them is whether the range ring was increasing or
      decreasing in radius.

      But of course, they don’t have just the one ping — they have 8 of them. The progression of radii over time combined with Doppler frequency shifts ought to be more than sufficient to differentiate between the northern solution and the southern solution.

  • Blue

    Can a plane still receive and send a ping if under water??? What happens to the ping time ??? What happens to the ping time in a damaged aircraft?? Seems like way too much reliance is being placed on one data point.

    • JohnnyQD

      If you are referring to the ‘hourly ping’ that relies on satellite communications then the answer is no. That particular frequency of RF energy will not penetrate water.

  • Ian Howard

    Does the aircraft have dual ACARS transponders? If, so, can one transponder be transferred to another aircraft? Can pre-recorded information be used as a feed for the second aircraft? Using this as a basis, can one aircraft be purposely used to imitate the aircraft the transponder came from? Is this transfer capable of being done in the field by field maintenance at the normal pre-flight check? Can one transponder be field addressed to imitate another operating system?
    Any takers? Please contact via email. Many thanks.

  • Steve Svensson

    Directly below satellite is 0 deg, and each circle is 10 deg. The 4th circle is 40 deg. Why is the degrees backwards? 40 degrees puts mh370 at Diego Garcia.