03.15.14

Understanding “satellite pings”…

Posted in Aeronautical, Inmarsat, Operators, Services at 11:55 am by timfarrar

There’s so much confusion about the satellite communications aspects of the MH370 incident that I thought it would be useful to give a little bit of background and an analogy to aid understanding of what we know and what we don’t. As with all analogies, this is perhaps oversimplified, but may help those without a detailed knowledge of satellite communications. I’m not a satellite designer, so I may also have overlooked some of the intricacies – please feel free to chime in with any corrections or amplifications.

Firstly, it needs to be made clear that the radar transponder “squawks” and the satellite communications “pings” are from completely separate systems (just because its talking about a transponder, that is nothing to do with satellite transponders). The radar transponder sends an amplified signal in response to reception an incoming radar transmission, which has much more power than a simple reflection from the metal skin of the plane, and has additional information about the plane’s ID. If turned off, less sensitive civilian radar will struggle to pick up the plane’s reflection, though military (air defense) radar should still be able to see the plane. But military radar systems are looking for hostile forces and have missed civilian aircraft in the past (e.g. the Mathias Rust incident).

Key point 1: The transponders are nothing to do with the satellite communications system.

So let’s turn to the satellite communications system. There has been talk about ACARS transmissions for monitoring the status of the plane. That is a communications protocol, separate from the underlying satellite (or VHF radio) link. Think of ACARS as like Twitter. I can send a message from my cellphone, which may or may not include my location. When I’m at home, on WiFi, the message goes to Twitter via my home broadband connection. Similarly, when the plane is over land, the ACARS message goes over VHF radio to SITA, who then send it on to the destination (e.g. Rolls Royce if the purpose is engine monitoring, Malaysian Airlines if its an internal airline message, or the Air Traffic Control center if its a navigation related message). [ACARS messages can also be sent over long distances via HF radio, but its not been suggested that was the case on MH370.]

With Twitter, when I leave home, my cellphone connects to the cellular network, and my Twitter messages go over that. But it makes no difference to the message and Twitter doesn’t care. Somewhat similarly, when the plane goes over the ocean, the ACARS system sends its messages over the plane’s satellite connection instead, but it doesn’t affect the content of the message.

Just like I use AT&T for my cellphone service, the plane’s satellite communication system is from Inmarsat, but so long as I have bought the right data service from AT&T, Twitter will work, and so long as I have an Inmarsat data service, ACARS will work fine.

Key point 2: ACARS is an “app” (communications protocol) which can operate over different (satellite and VHF) communications links.

I can sign out of Twitter on my cellphone and then won’t be able to transmit or receive Twitter messages. But that has nothing to do with whether my cellphone is connected to AT&T’s network. Similarly, the pilots can terminate ACARS sessions and stop reporting their position or other data (see for example this document), but that doesn’t affect whether the satellite terminal itself is connected to the Inmarsat network.

Key point 3: ACARS reporting can be disconnected without affecting the underlying satellite communications link.

On my cellphone, even if I’m not sending any data, AT&T needs to know if I’m registered on the network. When I turn on my phone, or move from cell to cell, the network exchanges data with the phone to make sure the network knows which cell the phone is located in. More importantly, even if I stay in one place with the phone in my pocket, the cellphone network checks in occasionally to make sure that the phone is still active (and say the battery hasn’t run out without the phone signing off from the network, or I haven’t gone into an underground car park and the connection has been lost), so that it knows what to do with an incoming call. You don’t normally notice that, because the timescales are pretty long (you don’t usually go into a car park for an hour or two). As another example, if I go to France with my AT&T phone, when I turn the phone on, it is registered in the Visitor Location Register (VLR), but eventually, after I stop using the phone there, my details are purged from the VLR.

Similarly with the Inmarsat connection, the network needs to know if it should continue to assign network resources to a particular terminal in case a communications link needs to be established. Not every aeronautical terminal in the world will be active simultaneously, and indeed there are quite a few that are rarely if ever used, so Inmarsat doesn’t provision resources for all terminals to be used simultaneously. However, once a given terminal are turned on, it needs to be contactable while it is inflight. So the Inmarsat network checks in with the terminal periodically (it appears to be roughly once an hour), to ensure that it should continue to be included in the list of active terminals and gets a message back to confirm that it should remain registered. These are the “satellite pings” that have shown that MH370 was still powered on and active after the ACARS messages and radar transponder were turned off, because the terminal was responding to the requests from the Inmarsat network to confirm it was still connected.

Key point 4: The “satellite pings” are due to the Inmarsat network checking that the terminal on board the aircraft is still connected to the Inmarsat satellite system and the terminal responding in the affirmative.

So now the question is how accurately does the Inmarsat network know where the plane is located? To go back to my cellphone analogy, when the network is checking my phone is still connected, it looks in the last cell it was registered. If I move to a different cell, then my phone should check in with the network to request a new assignment. But AT&T doesn’t need to know my precise position within the cell, it just needs to know where to route an incoming call. Similarly with Inmarsat, there isn’t a need to know exactly where in a cell the plane is located, just that its there and not somewhere (or nowhere) else.

Key point 5: The “satellite pings” indicate the plane is in a cell, but do not intrinsically give specific position information.

How big is a “cell” on the Inmarsat network and why the confusion? First of all, we need to recognize that there are different Inmarsat network architectures for different generations of aeronautical terminals. Think of it like 2G, 3G and 4G phones. If I have a first generation iPhone then I can only use 2G (GSM+EDGE), an iPhone 3G can use 3G, and an iPhone 5 can use LTE. AT&T supports all of these phones, but in slightly different ways. Inmarsat introduced a new SwiftBroadband aeronautical service in 2010, using its latest generation Inmarsat 4 satellites (like AT&T’s LTE network). That has much smaller spot beams (“cells”) than the older Inmarsat 3 satellites. And the Inmarsat 3 satellites (like AT&T’s 3G network) in turn have regional spot beams as well as a “global” beam (covering an entire hemisphere) to support the oldest aeronautical terminals.

As an aside, part of the SwiftBroadband communications protocol (essentially identical to BGAN) conveys (GPS-based) position information to the satellite when establishing a connection, so that the satellite can assign the terminal to the right spot beam. But it isn’t clear that GPS data is required as part of the “pings” which maintain registration on the network. That was one additional source of confusion about whether the specific position was being reported.

In any case, it appears that MH370 had a Swift64 terminal onboard (or possibly an older Aero-H or H+ terminal), not one of the latest SwiftBroadband terminals (that’s hardly surprising since SwiftBroadband is not yet fully approved for aeronautical safety services and is mostly used for passenger connectivity services at the moment, which don’t seem to have been available onboard). This is the equivalent of the iPhone 3G (or the original iPhone), not the newest version.

In the Indian Ocean, Inmarsat’s Classic Aero services, which are provided over both Swift64 and Aero-H/H+ terminals, operate on the Inmarsat 3F1 satellite located at 64E (equivalent to AT&T’s 3G network not its latest LTE network), and can use both the regional and global beams, but it appears that Inmarsat’s network only uses the global beam for the “pings” to maintain network registration. Otherwise it would have been possible to rule out a location in the Southern Ocean.

Key point 6: The “satellite pings” were exchanged with the Inmarsat 3F1 satellite at 64E longitude through the global beam.

So how can anyone find the position within this enormous global beam? There are two potential ways to measure the location:
1) Look at the time delay for transmission of the signal to the satellite. This would give you a range from the sub-satellite point if measured accurately enough, which would be a circle on the Earth’s surface.
2) Measure the power level of the signal as received at the satellite. The antennas on the satellite and the plane amplify the signal more at some elevation angles than others. If you know the transmission power accurately enough, and know how much power was received, you can estimate the angle it came from. This again would produce a similar range from the sub-satellite point, expressed as a circle on the Earth’s surface.

[UPDATE: I believe that the first of these approaches is more likely to produce an accurate estimate. See my new blog post for more information on locating satellite pings.] We can see in the chart below (taken from a Reuters Aerospace News photo of the search area posted at the media center) that the search locations are based on exactly these curves at a given distance from the sub-satellite point. However, it is unlikely that the measurements are more accurate than within say 100 miles.

We can also see that the arcs are cut off at each end. The cutoff due east of the sub-satellite point may be due to the fact that the transmissions would also potentially be received by Inmarsat’s Pacific Ocean Region satellite at that point, and if they weren’t, then that region would be ruled out (although others have suggested that military radar plots have already been checked in these regions). Its possible that the boundaries to the north and south have been established similarly by the boundaries of Inmarsat’s Atlantic Ocean Region satellite coverage, but they may instead be based on available fuel (or simply the elapsed time multiplied by the maximum speed of the plane), rather than the satellite measurements per se.

UPDATE (Mar 18): I originally attributed the picture below to a Malaysian government release, based on information from a journalist in Kuala Lumpur. As a commenter below notes, the diagram was put together based on an interpretation of what was stated in a briefing (indicating that the ends of the arcs were determined based on the minimum and maximum speed of the aircraft, rather than being based on the overlap of the Inmarsat satellite coverage areas) and is not an official document. Apologies for any confusion.

Key point 8: The position of the aircraft is being estimated based on the signal timing/power measured at the satellite. Its not based on the data content of any message and is not highly accurate.

ADDITIONAL POINT (Mar 17): Many have asked why it took so long to figure out where these satellite pings were coming from. Taking an extension of the analogy above, assume you have a friend staying in a hotel. The hotel catches fire and burns to the ground and your friend’s regular Twitter updates cease. For the first few days, the fire department is trying to find his body in the hotel. When he can’t be found the police check to see when his iPhone was last turned on. It turns out the phone was still connected to AT&T’s network hours after the fire. So then the police ask AT&T to figure out where the phone was operating by looking at their database of network records.

That’s exactly the sequence of events here. The plane’s ACARS (and radar) communications suddenly ceased and in the first few days, everyone assumed there had been a crash and was looking for the crash site. After no debris was found, investigators started to look at other possibilities. Inmarsat discovered the plane’s terminal was still connected to their network even after the ACARS messages ceased. Then it took a bit more time to calculate the location of the pings from Inmarsat’s network data records.

Finding missing people this way using cellphones is well known, but no-one’s ever had to do it before in the aeronautical satellite world, so its hardly surprising that this would be not be standard practice in an air accident investigation. I’m sure it wasn’t standard practice for cellphone companies in the 1980s either.

UPDATE (Mar20): The WSJ is reporting that Inmarsat had this information very quickly but the Malaysian government delayed making use of ping arc data to revise the search area for several days.

I hope that’s helpful. Let me know of any questions or need for further explanation.

77 Comments »

  1. the keyboard of geoff goodfellow said,

    March 15, 2014 at 4:00 pm

    Key point 9: “Vanished Malaysia Airlines flight leaves relatives with anger and phantom phone calls”

    “… One of the most eerie rumors came after a few relatives said they were able to call the cellphones of their loved ones or find them on a Chinese instant messenger service called QQ that indicated that their phones were still somehow online.

    A migrant worker in the room said that several other workers from his company were on the plane, including his brother-in-law. Among them, the QQ accounts of three still showed that they were online, he said Sunday afternoon.

    Adding to the mystery, other relatives in the room said that when they dialed some passengers’ numbers, they seemed to get ringing tones on the other side even though the calls were not picked up…”

    washingtonpost.com/world/vanished-malaysia-airlines-flight-leaves-relatives-with-anger-and-phantom-phone-calls/2014/03/10/fdb78642-a862-11e3-b61e-8051b8b52d06_story.html

    The home carrier of these unanswered/”calls that were not picked up” would know exactly where these “roaming” cellphones are located, just as the Chinese instant messenger service called QQ (or any other IM service) server logs would know from the IP address these cellphones were using their respective IM services from!

    geoff

  2. timfarrar said,

    March 15, 2014 at 6:42 pm

    Sorry Geoff, but I think this is probably a red herring. Registration maintenance is an intermittent process, made more complicated by international roaming. Switching a phone to airplane mode performs none of the usual shutdown procedures.

    So it would not be in the least bit surprising if the Home Location Register of a cellphone is not updated for many hours after a roaming cellphone is switched to airplane mode, and calls are still forwarded to the visited network and ring out.

    I’d be prepared to bet that if you go abroad, connect your US phone to roam on the foreign network and then tell someone to call you back and switch to airplane mode before they do, they would experience the exact same issue.

  3. MH 370 KUL-PEK Missing: Search operations ongoing [PLEASE SEE WIKI] - Page 302 - FlyerTalk Forums said,

    March 15, 2014 at 6:57 pm

    [...] a definitive explanation on what ACARS is, and how it works with VHF or Satellite systems. http://tmfassociates.com/blog/2014/0…tellite-pings/ Highlight: "Think of ACARS as like Twitter." The analogy actually works. Someone [...]

  4. andyhull said,

    March 16, 2014 at 1:37 am

    A very interesting article, it raises a couple of points in my mind.

    The first is why has the distance of 2200 miles been quoted? The range of the aircraft must have been further than this,

    By my back of the envelope calculation, the planned flight distance was around 2700 miles (2346 nautical miles), the plane therefore would have been carrying fuel for this distance, plus a minimum safe additional fuel load of around 5-10%, so its safe operating range would most likely have been in excess of 3000 miles. The plane potentially could fly further, especially given that it would potentially glide some distance after fuel starvation set in.

    Second question is, if it flew north then it should have been spotted by the ground military of at least four different countries, most of whom would have taken an interest in it, assuming they could not identify it as a commercial flight. None of them seem to have notices anything unusual.. why not? Most likely because the aircraft didn’t travel this way.

    It therefore seems reasonable to suggest the aircraft flew south, most likely in a failed attempt to return to land, following an incident that knocked out its ground communications, most likely a slow decompression, caused possibly by failure of the roof based satellite antennae pod. This failure may have frozen the electronics in this antennae, or partially destroyed them.

    The outside temperature at 35000ft would knock out all but the most hardened of electronic devices.

    If the cabin temperature also dropped, the comatose passengers and crew would have succumbed to hypothermia within a few minutes.

    This would have left the aircraft flying on autopilot heading south till eventual fuel starvation dropped it in the ocean 2000+ miles from its last contact point (the exact distance south would depend on how far north the aircraft flew before turning back).

    This sadly puts its impact zone in one of the most remote parts of the southern ocean.

    Another thing that suggests this, is the picture of coverage zone of the Inmarsat-3 transponders, these suggest that the flight should have continued to be “pingable” (assuming the aircraft was still airborne) if it flew north, for several more hours, but the southern coverage of Inmarsat-3 looks to run out at about the 3 hour range… the aircraft may have been still airborne, but no longer in satellite range.

  5. andyhull said,

    March 16, 2014 at 1:45 am

    Sorry, missed a link for the transponder info…

    http://www.satbeams.com/footprints?norad=23839

    This suggests Inmarsat-3F1 (64 E) (Inmarsat 3 F1, I3F1, IOR) can “see” much further north, than it can south… for obvious reasons, nobody tends to fly south, because there is nothing there.

  6. peter said,

    March 16, 2014 at 2:38 am

    Very interesting read, thanks! A question I still have : I see Inmarsat uses the L band for communication. What happens when they send a ping request to the plane and the plane replies on it. Can that reply be picked up by satellites of other operators operating on the L band? Are there any others, other then Inmarsat?

  7. ruthpkelly said,

    March 16, 2014 at 6:11 am

    Really helpful, Tim. Good stuff.

  8. timfarrar said,

    March 16, 2014 at 6:52 am

    Andy,

    The picture in your link is for an Intelsat C and Ku-band satellite at the same orbital slot. The coverage of the Inmarsat 3F1 satellite is shown in the diagram in my post above. The slightly darker blue-gray shading is the coverage of the regional beams, while the dark blue circle is the coverage of the global beam. As I noted, the “pings” appear to have been exchanged with the global beam, because otherwise there would only be a short southern track. It also makes sense technically to do the registration maintenance via the global beam rather than pinging multiple regional beams.

    So just to emphasize, the Inmarsat coverage area includes the entire dark blue circle and an extensive region in the southern Indian Ocean.

    On your other points, the 2200 (nautical) mile range originated in this WSJ article when the plane was thought to have flown for 4-5 hours, before the latest statement that the last ping was detected at 8.11am. 2200 miles divided by 480 knots is 4 hours 35 mins.

  9. timfarrar said,

    March 16, 2014 at 7:08 am

    Peter,

    In theory the signals could be picked up by other satellites operating in the same frequencies. There are other operational Inmarsat satellites (e.g. the Inmarsat 4 satellites) in orbit. Thuraya also shares the L-band spectrum band in the Middle East. But that is a question of Inmarsat having certain frequencies allocated to them in some places and Thuraya having different frequencies allocated in those places. Just like AT&T and Verizon have different license allocations in the PCS spectrum band in different parts of the US.

    However, because some L-band services (e.g. satellite phones) operate with non-directional antennas, satellite operators are careful not to re-use those frequencies on different satellites covering the same geographic location in order to avoid a satellite phone interfering with a satellite other than the one its supposed to be communicating with.

    The global beam services use somewhat more directional antennas, but I don’t know if the same frequency is allocated to the global beam registration maintenance function on the Inmarsat 3 satellites adjacent to the 3F1 satellite at 64E. In any case, in the locations of interest at present there is no overlap in the global beam coverage.

  10. christianmlong said,

    March 16, 2014 at 7:33 am

    If there were pings every hour, does that give a set of hourly arcs?

    Put another way, can an arc be calculated for each ping? Or, are the two arcs we’re seeing the result of some kind of aggregate calculation?

    I’m envisioning a series of concentric arcs, one for each ping. Perhaps that could narrow the cone in which the plane traveled.

  11. cnnwatcher said,

    March 16, 2014 at 7:36 am

    “We can also see that the arcs are cut off at each end. The cutoff due east of the sub-satellite point is potentially due to the fact that the transmissions would also potentially be received by Inmarsat’s Pacific Ocean Region satellite at that point, and if they weren’t, then that region would be ruled out. Its possible that the boundaries to the north and south have been established similarly by the boundaries of Inmarsat’s Atlantic Ocean Region satellite coverage, but they may instead be based on available fuel, rather than the satellite measurements per se.”

    FWIW, CNN was claiming that these western limits were due to the the top speed of the aircraft. A 777 which was known to be over the Gulf of Thailand at 1:07 simply could not make it to The Caspian Sea by 8:11, even if they had enough fuel.

  12. polarbreeze said,

    March 16, 2014 at 9:22 am

    Hi Tim,
    Excellent clarification of the satellite ping thing, thank you.
    It seem to me that more information can be gleaned from this.
    The “likely spots” are at the intersection of the red circle (plane’s range) and the green arcs (locus of satellite pings).

    Here’s the thing though: only one instance is shown – the instance at 2200km, which is assumed to be the fuel range of the aircraft. However, there are actually a number of instances of possible intersects – one for each ping. Therefore it should be possible to plot the likely path of the plane for the full 8 hours it was missing, by making an assumption that it was flying at a certain fixed speed.

    If these sets of intersects were to be plotted, it is likely that only one of the two paths (the north path vs the south path) would emerge as the only one that delivers a physically possible track from the available data. This would be achieved by incorporating the additional information from plotting multiple pings and from knowledge of the time of those pings, along with an assumption about the plane’s speed.

    Is anyone in possession of that data to give it a try?

  13. polarbreeze said,

    March 16, 2014 at 9:25 am

    Oh, yes, Christian, I see you had the same idea!

  14. polarbreeze said,

    March 16, 2014 at 9:30 am

    Ah Christian, I see you had the same idea! And also, if the track of the aircraft could be plotted, it could be extrapolated beyond the point where it went out of satellite range, which could narrow down the search some.

  15. timfarrar said,

    March 16, 2014 at 11:03 am

    Polarbreeze,

    The symmetry involved in the measurements (when the plane started very close to the equator) makes it hard to rule out one of the two possible tracks. However, I would agree that there are probably some areas of focus within each track based on the known correlation of aircraft speed and distance between the arc for each point.

    The difficulty is that no-one knows if the plane actually did maintain a constant speed and direction during this period, or if it was taking action to avoid detection, as seems to have been the case when it was flying from the east side to the west side of the Malay Peninsula.

  16. chrishk said,

    March 16, 2014 at 4:45 pm

    Very interesting information.

    However there are 2 things that you don’t explain :

    1) Look at the time delay for transmission of the signal to the satellite. This would give you a range from the sub-satellite point if measured accurately enough, which would be a circle on the Earth’s surface.

    How is this time delay estimated. That would mean that the “ping” signal is dated (which is possible) and that the 2 clocks (plane and satellite) are PERFECTLY SYNCHRONIZED. I can imagine that the satellite has a perfectly accurate clock (atomic clock?). But I doubt the the plane’s one is too….

    2) Measure the power level of the signal as received at the satellite. The antennas on the satellite and the plane amplify the signal more at some elevation angles than others. If you know the transmission power accurately enough, and know how much power was received, you can estimate the angle it came from.

    How can you accurately know the transmission power level?

    And in both cases, what is the precision of the estimation or calculation. For example a difference of 1/1000 th of second in the time delay estimation translate in a difference of 300 kms on the ground position

  17. polarbreeze said,

    March 16, 2014 at 4:50 pm

    It should be a good starting assumption that the plane was flying at a “normal” cruising speed in a straight line. The person in control would be believing that they had evaded detection (having overlooked the ping thing) and so they would be most likely to head in a straight line towards wherever they wanted to end up – whether that be in the depths of the southern Indian Ocean or on some clandestine airstrip somewhere – which seem to be the two remaining options. Figuring out from the pings a track (pair of tracks) based on an assumption of “normal” cruising speed in a constant straight line would seem to be a good step. The only thing we’re hearing about is attempts to deduce the end point – that’s wasting a lot of valuable information.

  18. elese said,

    March 16, 2014 at 5:38 pm

    Great article and comments. Thank you!

    Is there a reason there are no reported pings from the Pacific Ocean Region Satellite? Or, can you clue me in to any findings from that satellite?

  19. timfarrar said,

    March 16, 2014 at 7:19 pm

    Polarbreeze,

    That may be the correct assumption in the middle of the ocean, but evasive maneuvers appear to have been undertaken to avoid Malaysian air defense radars. On the northerly track there would certainly be similar considerations. In addition, some have suggested that MH370 could have tried to blend in with the radar signature of another commercial airliner, by closely following its track. That would also potentially result in a non-straight line flight.

  20. timfarrar said,

    March 16, 2014 at 7:22 pm

    Elese,

    The aircraft’s satellite terminal was in the footprint of and apparently connected to the Indian Ocean Region satellite. It was not exchanging data with the Pacific Ocean Region satellite. Whether any signals were received at the POR satellite is unknown, and even if they were, if those signals looked like noise (because they weren’t transmissions intended for that satellite), it may not be feasible to identify them.

  21. timfarrar said,

    March 16, 2014 at 7:40 pm

    Chrishk,

    I would expect that the time delay measurement is more likely to produce a useable estimate than the power level, since any change in orientation (banking to change course) would alter the antenna gain significantly.

    With respect to the timing, it is possible to perform geolocation for identification of interference with a high degree of accuracy. But that is usually based on real time continuous measurements and potentially also altering the orientation of the satellite slightly to measure the signal variation. None of that is possible in this case.

    However, the transmissions to and from the terminal are made in timeslots (similar to the timeslots in the GSM cellular protocol), which need to be synchronized (accounting for the lengthy transmission time to geostationary orbit) and established accurately to avoid interference with adjacent timeslots. I would expect that measuring how much “slippage” there is within a timeslot relative to the nominal start of the slot could give reasonably good range information (but I would be surprised if the accuracy was as good as 100 microseconds = 30km).

    Let’s also not forget that the plane could have flown for another 20-30 minutes after the last ping. That alone would be a couple of hundred miles at the nominal cruising speed of 480 knots.

  22. koopai said,

    March 16, 2014 at 10:27 pm

    Tim this is the most informative article on the subject I have seen so far! One thing you didn’t explain though, is the possible reasons why the pings have stopped after 8:11. Would they have stopped if the plane had safely landed somewhere and its engines had been turned off, or do they normally keep being emitted until battery power runs out as is the case on a cell phone?

  23. rah335 said,

    March 17, 2014 at 9:42 am

    Good explanation. Question (which you may have answered but I missed) – when communicating via Inmarsat 3, does it/can it go through both Inmarsat 3-F1 and Inmarsat 3-F3? If so, that either significantly narrows down the arc to two points or if Inmarsat 3-F3 did not receive the ping, then cuts down the possible portions of the arcs.
    Also – any word of the hourly pings between 0215 and 0811? For each one of them there should be a similar diagram which should constrain the final solution, as fuel/range is taken into consideration.
    Lastly – the black cutout in the red arc – is it a valid assumption to rule out a portion of the arc because it is too close to the last radar hit? Who is to say they flew in a straight line for the next 6 hours? Seems like a bad assumption when we know so little.

  24. Flight MH370 — search data in Google Earth | Ogle Earth said,

    March 17, 2014 at 9:53 am

    [...] March 17, 17:45 UTC: As per this blog post on TMF associates, the interruption in the arc near the Gulf of Thailand is due to the Malaysian government’s [...]

  25. rpattay said,

    March 17, 2014 at 9:58 am

    Is the actual received frequency (in Hz) logged/recorded by the receiving sat or ground station? Since the plane is moving and the sat is moving they have a relative speed between them and hence the received ping frequency will be doppler shifted. Since we know the speed/location of the sat at the time of the ping we could theoretically determine possible speeds/locations of the plane at that time. This would be additional information that could be added to the TDOA arcs already found for additional accuracy.

  26. toboev said,

    March 17, 2014 at 10:02 am

    As others have asked, should we not expect a different arc for each ping over the several hours in question? And yet only one arc is public information, relating to the final ping.
    For each previous ping I can only see that the arc generated would be different unless either:
    the aircraft was travelling along the line of the arc itself, so they overlapped,
    or
    the aircraft was stationary, and still alive – i.e. it landed.
    Is that analysis tenable?

  27. Malaysia U-turns on satellite data evidence; unsure when system went off line | Ka communication systems said,

    March 17, 2014 at 10:04 am

    [...] [...]

  28. Laboul said,

    March 17, 2014 at 10:33 am

    Thanks Tim for this interesting analysis.

    All that you say is true, but you all forget one thing : it’s possible to analyze the datas (power level, delay of transmission in this case) only if you are real time watching them or if you save them in specific logs.

    In the case of Inmarsat 3, the LES (Low Earth station for Inm 3) and GES (for I3 et I4) – certainlmy Perth in this case – moreover the satellite don’t save these data but only the registration date and the satellite beam for a long time.

    So I think that nobody was specifically observing the specific datas of the MH370 when they lose it from the civilian radar screens.

    All that we can say is theorically completely right but as many times we must have be prepared before than asking what we could have done after a disaster.

    PS: sorry for the english mistakes, I’m just a little frenchy!! ;o)

  29. TMF Associates MSS blog » Locating “satellite pings”… said,

    March 17, 2014 at 11:30 am

    [...] it would be helpful to give a bit more detail on how the location of a ping can be identified. In my previous post I indicated that you could potentially measure range (based on timing) or angle (based on power). [...]

  30. timfarrar said,

    March 17, 2014 at 11:40 am

    Toboev,

    Yes, there would be different arcs for different pings. There should also be relative speed data for each ping, as I discuss in my new blog post here: http://tmfassociates.com/blog/2014/03/17/locating-satellite-pings/

  31. timfarrar said,

    March 17, 2014 at 11:46 am

    Laboul,

    I agree that the precise data on timing offset might or might not have been saved as part of standard operating procedures. But the fact that precise timing appears to have been used for this calculation suggests that it was and most likely frequency offsets also ought to have been available. If that’s the case then it should also be possible to calculate relative speed.

  32. Learn Everything About MH370 Here - Comprehensive List | Coffeeticks.my said,

    March 17, 2014 at 1:00 pm

    [...] What is Satellite Ping?  [...]

  33. Peetee said,

    March 17, 2014 at 1:03 pm

    @andyhull
    andyhullsaid…
    “The outside temperature at 35000ft would knock out all but the most hardened of electronic devices.”

    If the outside temperature reading of about -50degC was correct from the last time I flew on a plane at about FL350 and I was looking at a screen, this wouldn’t knock out a lot of electronics. Even older run-of-the-mill components had specified minimum operating temperatures of -55degC. Sure, some components start to suffer down at those temperature, electrolytic capacitors start to rise in impedance and fall in capacitance as the dielectric cools and anything programmed can start to fail, but this type of component probably sees much less use in Avionics. Combine better specified components and testing and I think functional at -55degC would be perfectly possible.

  34. LarryC said,

    March 17, 2014 at 1:30 pm

    Thanks for that explanation of the ‘arcs’. It’s so frustrating when the media either a) lacks the curiosity to find out ‘where exactly did those arcs come from?’ or b) assumes the audience has no curiosity.

    I figured at first the arcs were based on signal strength received by a geostationary satellite, but synchronized clock info of satellite receive time v plane transmit time, kind of inverse GPS method, seems more likely.

    I also wondered why the arcs were cut off where they were, but the actual reason there seems oversimplified. It’s certainly possible the plane the plane went in a straight line from the last known radar position to the 8:11AM position, but what if it didn’t? I don’t see how you can eliminate the portion of the circle between the arcs just based on one ping and the assumption of varying speed along a constant course.

    I had assumed the gap between arcs was deductive elimination or probability cut off based on the arcs of earlier pings, which made it unlikely or impossible for the plane to get back to the eastern most point of the theoretical circle. Does any source show implied arcs from earlier pings?

    I think Occam’s Razor, assuming the 8:11 arc info is fairly correct, points to the plane going to a remote area of Indian Ocean and crashing. A narrowly focused terror plot, or just irrational personal act, to fly the plane to destruction where it might never be found (those responsible might not have known the pings would give any hint of position) is no more unlikely than some elaborate plot involving landing somewhere. And the former theory doesn’t require a complicated explanation (terrain masking flight and/or following another airliner at night… how? it’s a 777, not a B-1 with FLIR and radar with a/c detection and terrain following modes) why the plane wasn’t detected after 2 AM: it was outside radar coverage.

  35. removebeforeflight said,

    March 17, 2014 at 1:49 pm

    Hi

    Very interresting article ! I have a blog and I would like to translate your article in french. Do you allow me to do that. Thank you.

  36. timfarrar said,

    March 17, 2014 at 4:22 pm

    LarryC,

    I agree the explanation for cutting off the arcs where they are would only work if the plane flew in a straight line during the ~6 hour period. Irregular movements would mean the minimum distance could be less than shown here.

    As I noted in my other post, combining the relative speed and range data for all of the pings should allow investigators to determine whether or not the plane was moving in a straight line at a constant speed during this period.

  37. Mark Fraser said,

    March 17, 2014 at 8:26 pm

    Well, Hello Geoff! Been almost 20 yrs!
    Back on topic, sort of: I’ve been wondering if the pings could be from a false-flag terminal, programmed with the (like IMEI) same device ID as the one on the 777. Also, the strange absence of any (repoted) mobile phone sign-ins during what must have been transit of at least one terrestrial GSM or other system, suggests either on-board jammers or submersion. (If phones can’t hear cell system beacons, they won’t speak up.)

    Leaving at least two scenarios – craft was alive with active jamming on board, or craft was dead and a spoofed ping was being sent. Or alive with spoofed pings and jammers. Which nation or group have the motivations, the skills and the resources to pull off the latter two? One of the emerging nuclear powers maybe?

  38. Malaysia Airlines Flight 370—Understanding Terminology | Petchmo said,

    March 17, 2014 at 9:00 pm

    [...] satellite communications, provides an excellent and detailed overview of the satellite pings on the TMF Associates blog. MH370′s onboard terminal contacted an Inmarsat-3 satellite roughly every hour allowing [...]

  39. garyf said,

    March 17, 2014 at 9:41 pm

    Excellent article – Thank you. The additional ping data discussed by several above, would ideally help reduce the search area, by validating the straight line assumptions or not. In addition it would allow improved focus for searching radar data and satellite images to validate a route. It is also clear that uncertainty in the actual route flown is much less at the western end of the 08:11am (L) arc’s (must approach a straight line), and is at a max on the end of the arc closest to Malaysia (could include loops & turnarounds). The additional ping data (with their arc’s & min/max speed ranges) is essential to minimize search uncertainty. Finally, if MH370 did loop’s or zig zags somewhere close the origin, then it seems to me the flight could be on the arc between the proposed north and south 8:11am arc’s. Excellent article & excellent feedback!

  40. volcanicash said,

    March 18, 2014 at 1:23 am

    Please note that the diagram illustrating that “…the ends of the arcs were determined based on the minimum and maximum speed of the aircraft..” was absolutely NOT released by the Malaysian government. It is no way an official document of any kind, simply a diagram that I put together to illustrate, for the readers of pprune.org, what had been implied by the authorities at the press briefing on Monday 17th in answer to a question about the arcs.

    Would be grateful if you would either remove the diagram or at least make it very clear that this is just one interpretation of what has been said. Many thanks..

  41. pthephy said,

    March 18, 2014 at 1:52 am

    Excellent analysis timfarrar and good comments.

    I am not a satellite communication geek, but even after chrishk question and timfarrar answer, it still don’t clearly understand the mechanics of how the arc based on the last 8.11 ping was actually calculated.

    I agree that it cannot be based on signal strength – too much influences.

    Now if it is based on time measurement, it seems required that the satellite and the plane use an exact world time clock. I would assume this is definitely the case for the satellite, and if I’m not wrong, any GPS is synchronized with the exact world time to be able to locate its position, so let’s assume the plane ACARS has that possibility too.

    That leads to two possibilities as far as I see it:

    1. The satellite sends a ping and from the plane’s answer that the satellite registers, it calculates the plane’s position from the time difference between send and receive on the satellite side. This requires that the ACARS ping protocol defines a specific, exact time span between the incoming ping at the plane’s side and the sent ping of the plane (if this wouldn’t be the case, the satellite can’t know the signal runtime between the satellite and the plane and hence not the distance).

    2. The satellite sends a ping and from the plane’s answer that the satellite registers, it calculates the plane’s position from the time difference of the ping signal’s time stamp and the satellite receive time. Clearly this requires that the plane’s ACARS ping protocol defines a timestamp in the signal that represents the plane’s sent time.

    Can anybody shed some light which of the two possibilites are true? Or am I missing something here?

  42. timfarrar said,

    March 18, 2014 at 7:52 am

    Volcanicash,

    Sorry about that. I was sent the diagram by a journalist in KL who told me it was from the Malaysian government. I will correct accordingly.

  43. timfarrar said,

    March 18, 2014 at 8:03 am

    Pthephy,

    There is no need for precision timing of a round trip signal or even for a precise time source to be available on the plane (just like a precise time signal is not needed on your cellphone). Like all TDMA/FDMA air interfaces (e.g. GSM), the Inmarsat service has a broadcast channel which provides the timeslot information to the terminal so it knows when to transmit. What would be measured by the satellite network is the offset of the signal from the center of the designated timeslot, not the elapsed time for the signal to travel from the terminal to the satellite.

    If it is desired to eliminate this offset then the network could tell the terminal how much correction is required as part of a handshaking protocol and/or the terminal could estimate the correction needed (this often happens for Doppler frequency correction). It is unclear if any of that happened in this case (I’m still trying to find more information about the protocol itself).

  44. andyhull said,

    March 18, 2014 at 8:57 am

    @Peetee

    I suspect that the devices in question are designed to operate within the cabin environment, and not at sub zero temperatures.

    My point was that a sudden switch from 20C to -55C or less, coupled with a drop in pressure and a wind speed equal to the speed of the aircraft was likely to cause component failure.

    If you choose to believe it would have no effect, then I suspect you haven’t considered the matter very closely.

    As a man who in the course of his career has emptied a few cans of freezer spray (about -40C) on to electronic components in the course of fault finding, I know they will possibly operate at low temperatures, I also know the extreme weirdness that this can cause.

    Blowing a can of freezer spray on single component however is not quite what we are dealing with when there is a rapid exposure of a critical system to the extremes of altitude relative humidity change and wind.

    Next time you are sitting at FL350, here’s a puzzle for you, what is the temperature of the skin of the aircraft, inside and outside… and the air pressure… and the airspeed… how much static electricity would be generated should that high velocity dry air suddenly come in to contact with your iPhone…

  45. andyhull said,

    March 18, 2014 at 9:01 am

    @Peetee.. You might also like to consider the temperature co-efficient of the oscillator used to drive the transmission circuitry. It might just drift a tad, over the +20C to -55C range.

  46. DosMaster said,

    March 18, 2014 at 12:07 pm

    This must be an airplan in the air, right?
    Just besides a dam.

    http://www.tomnod.com/nod/challenge/malaysiaairsar2014/map/652734

  47. [MH370] Comprendre les « ping » satellite » | La tête dans les nuages ! said,

    March 18, 2014 at 1:28 pm

    [...] m’était faite de ces « pings satellite », et je suis tombé récemment sur un article en anglais qui expliquait très bien ce que sont ces pings, sur le blog de TMFAssocia…. J’ai donc décidé de traduire ici ce [...]

  48. FarmerCharlie said,

    March 18, 2014 at 2:12 pm

    Ever since I first learned of the Inmarsat ping data at 8:11AM, I thought we would soon see six more similar plots for the hours between 2:11 and 8:11, and would be able to use those plots for crude estimates of possible flight paths. Or at least check out some of the speculations like the one proposing that MH370 may have shadowed another flight across India and Pakistan. But there has been almost nothing more for several days, and even the talking heads on the 24/7 news shows have barely mentioned the missing reports. It was comforting to stumble on this site and learn that other folks are interested in this.

    I do have one question, though. I assume the arcs are based on a single measurement of the time delay for one ping at 8:11AM. If so, how much confidence is there in that single measurement? Are there control data to assure that the time delays from electronics within the satellite and the plane are constant and have been accounted for in the calculations?

  49. timfarrar said,

    March 18, 2014 at 2:23 pm

    FarmerCharlie,

    Do check out my other post on locating the satellite pings.
    The time delay measurement is based on a time difference relative to the allocated timeslot. It is not dependent on measuring some sort of round trip delay.

    The electronics in the terminal are carefully calibrated to ensure that the signal is transmitted in the right time slot. Otherwise the system would be subject to significant interference between transmissions in adjacent time slots and would simply not be capable of operating reliably. Remember that this is a terminal designed and certified for aeronautical safety services and therefore must operate correctly at all times.

  50. FarmerCharlie said,

    March 18, 2014 at 3:10 pm

    Thanks for the clarification. Since the system apparently was never designed to be used for this type of distance measurement, I did not realize that it did account for the types of variations I was referring to. Your information has been very useful. I listened to most of the Australian press conference today. They did refer to additional pings, and they implied that those data had been used to come up with their search area. But they did not show data from other pings.

  51. 'Unidentified plane is MH370' | Ka communication systems said,

    March 18, 2014 at 4:31 pm

    [...] [...]

  52. What if Malaysian Airlines Flight 370 is in Malaysia – or its wreckage is? | Gerry Lynch's Thoughts... said,

    March 18, 2014 at 4:49 pm

    [...] That signal arrived at the satellite at an angle of around 40 degrees. The circles show where signals arriving at the satellite from different angles would have originated on the ground. Malaysian authorities claimed possible final positions for MH370. Credit to the TMF Associates blog – see http://tmfassociates.com/blog/2014/03/15/understanding-satellite-pings/ [...]

  53. 7 Facts that may actually make the Maldives Islands the perfect haven for hijackers of Malaysia Airlines Flight 370 - Celebzter.com said,

    March 21, 2014 at 2:26 am

    [...] area for the past week. At a Palo Alto California mobile satellite communications consulting site TMFAssocites.com, many engineers chatted openly about  about the almost unbelivability  that the media has not [...]

  54. Fear of Landing » Considering the Probabilities of the Fate of MH370 said,

    March 21, 2014 at 2:15 pm

    [...] TMF Associates MSS blog » Understanding “satellite pings”… Firstly, it needs to be made clear that the radar transponder “squawks” and the satellite communications “pings” are from completely separate systems (just because its talking about a transponder, that is nothing to do with satellite transponders). The radar transponder sends an amplified signal in response to reception an incoming radar transmission, which has much more power than a simple reflection from the metal skin of the plane, and has additional information about the plane’s ID. If turned off, less sensitive civilian radar will struggle to pick up the plane’s reflection, though military (air defense) radar should still be able to see the plane. But military radar systems are looking for hostile forces and have missed civilian aircraft in the past. [...]

  55. solotau said,

    March 21, 2014 at 10:24 pm

    I would like to address the Ping circles. What information can be derived from them? These circles are real data not speculation but known facts.

    How can we use them?

    We have known radar tracks of the planes position. Do these tracks intersect the first ping circle? If so we would have a known position on a Ping circle.

    A reasonable assumption would be that the auto-pilot was flying the plane–Probably from way-point to way-point, so most of the time the plane will be on a direct or straight path.

    What about the distance between circles? If we make an assumption of the planes speed and use the known position and heading on the first circle, we can get three estimates of the position on the next circle. One by extending the known path and two by by drawing a circle intersecting with the next circle from the known position on the first circle, using a distance estimated from the planes estimated speed.

    We can also get some information from the distance between circles as the maximum distance they could be apart is if the plane was flying perpendicular to the circle. Also if the circles were the same size (concentric) it would indicate the plane was not moving.

    With a limited amount of fuel It does not appear likely that the plane would be flying in differing directions but taking the most direct route to its destination.

    Extending this technique would yield an estimated path that is at least base on real data. While an estimate, its probability is still better than guessing and perhaps might coincide with some of the speculation.

    Additional thoughts??

  56. corntrollio said,

    March 22, 2014 at 9:54 am

    “It therefore seems reasonable to suggest the aircraft flew south, most likely in a failed attempt to return to land, following an incident that knocked out its ground communications, most likely a slow decompression, caused possibly by failure of the roof based satellite antennae pod. This failure may have frozen the electronics in this antennae, or partially destroyed them.”

    Whatever happened, it’s definitely not based on “failure of the roof based satellite antennae pod.” According to Boeing, this 777 didn’t have the satellite antenna that is the subject of the AD. Sorry to burst your theory.

  57. FatFriday said,

    March 22, 2014 at 3:06 pm

    Beginner here. I attempted to post the following on reddit but failed. Furthermore the severe lip flapping there is discouraging. The absence of info about the earlier ping-rings has bothered me immensely. However, I concluded the following:
    The search for MH370 is too far south. The last radar contact with MH370 was at 2:15AM at N7.2 E98 and, according to NTSB analysis, at 5:11AM the aircraft was at S20 E90. If I am to believe Google Maps, the shortest distance between these is 2000 miles. To traverse this (unlikely) distance in 2.93 hours would require an average speed of 680 mph, far greater than the official max speed of 590 mph for the 777. Conclusion: MH370 was considerably north of S20 latitude at 5:11AM… or possibly Google Maps is bonkers.

  58. timfarrar said,

    March 22, 2014 at 3:48 pm

    FatFriday,

    Check out the other post where this is being discussed. I concluded that the Washington Post graphic showing the plane at 20S at 5.11am was in error, and appeared to have been based on spacing out the arcs over 7 hours rather than 6.

  59. Articles like this are why people think the cloud is oversold | Gareth Klose said,

    March 23, 2014 at 3:29 pm

    [...] the plane been fitted with a newer Inmarsat system, it would have been connecting to a satellite beams with smaller footprints, which could have [...]

  60. Malaysia Airlines MH370: March 23 as it happened – Telegraph.co.uk | Ka communication systems said,

    March 23, 2014 at 4:06 pm

    [...] [...]

  61. FOFOA said,

    March 23, 2014 at 11:52 pm

    Dear Tim,

    Thank you for this forum. I don’t want to interrupt the current thread, so I will post my question here. Above, you wrote:

    Key point 4: The “satellite pings” are due to the Inmarsat network checking that the terminal on board the aircraft is still connected to the Inmarsat satellite system and the terminal responding in the affirmative.

    So now the question is how accurately does the Inmarsat network know where the plane is located?

    Isn’t the prerequisite question, how accurately does the Inmarsat network know that this particular ‘handshake ping’ came from the B-777 known as 9M-MRO?

    As far as I can tell from following media reports, Inmarsat started looking for pings that fit a certain profile (time and general location) in response to early reports that the Malaysian military radar showed that the plane “may” have made a sharp left turn. That Malaysian report has, to this day, remained as a “may have made” that turn. But if true, that report meant that the plane flew on beyond its last known location over the Gulf of Thailand. So it makes sense to look for anything that fits that profile of time and general location.

    The only media report I’ve seen that stated why Inmarsat thought those pings were from 9M-MRO was one of the first. It said that the pings came from a 777 with RR engines, but that same report also said that such pings would contain location, speed and altitude coordinates as well, which turned out to be false since it turns out that 9M-MRO had an older terminal and was communicating with a single Inmarsat-3 satellite.

    This draws into question the detail in that report that Inmarsat knows these pings came from a 777 with RR engines. Does Inmarsat know for certain that these pings came from a specific aircraft or even a specific type of aircraft? Or do the pings just fit a certain profile which covers a 6 to 7 hour timeframe and expands from eastern Mongolia down to Australia?

    Maybe I missed the report where Inmarsat explained how they know those pings came from a 777, but all I’ve seen indicates that everyone has accepted that as fact because the NTSB seems to be following that assumption. And I haven’t seen anyone state categorically that the plane continued flying (or even functioning) until 8:11AM—they all say they are still open to all possibilities—which seems to support that it is merely an assumption.

    Is it merely an assumption that these pings came from a 777 as far as you know? Or is there something within the pings themselves that would have revealed the type of aircraft sending them?

    Thank you.

    Sincerely,
    FOFOA

  62. Cracking the Code on Malaysia 370: The Ping is the Thing…. | Liberty Pulse said,

    March 24, 2014 at 4:12 am

    [...] ping does not show a precise location, it just interpolates the angle from which the ping hit the satellite. Straight down is 0 degrees, off the poles of the [...]

  63. ldh said,

    March 24, 2014 at 1:36 pm

    Dear Tim,

    I have an idea which I’d like to run by you please. I am wondering whether the plane crashed somewhere close to the last primary radar fix. I’m not an expert in signal propagation but i do know that if the plane was slowly sinking then the signal would be shifted in frequency, delayed slightly and attenuated. What i don’t know is whether this would be sufficient to account for the arc’s generated by whatever algorithm is used to calculate distance. Also, there is no data on the earlier satellite pings only the later ones. This makes me a little suspicious, would the plane need to be circling?

    By the way, I used to work for NATS in the UK analysing radar positional accuracy.

    Best regards
    Lee Hazelwood
    Lee.Hazlewood@gmail.com

  64. ldh said,

    March 24, 2014 at 1:39 pm

    Dear Tim, Sorry incorrect email. Please see below.
    Lee Hazelwood
    Lee.Hazelwood@gmail.com

  65. FatFriday said,

    March 24, 2014 at 3:02 pm

    Thanks Tim for pointing out that I fell for an erroneous 5:11AM ping-ring handshake representation. Regarding the latest assertion that the Northern track hypothesis is invalidated because of doppler data being inconsistent with ping-ring progression, I am not a believer. The geometric plane including the Inmarsat satellite, the earth center, and the location of MH370 at 1:20AM really seems all that can be trusted – all is mirror symmetric with respect to this geometric plane….including doppler! (IMHO) Of course if there is one more reliable later datum obtained on either side of this geometric plane then the N/S ambiguity might be resolved. Precisely, which such datum do the Inmarsat/UK folks believe?

  66. TMF Associates MSS blog » Understanding the “satellite ping” conclusion… said,

    March 24, 2014 at 9:19 pm

    [...] engineering and aviation. If you’re visiting for the first time then you might want to read my original primer on pings [...]

  67. FatFriday said,

    March 25, 2014 at 9:07 am

    A few of the proposed trajectories show overflight of Butterworth AFB and a simultaneous change of course to the WNW, seemingly to avoid flying over Sumatra. Is there radar data showing MH370 over Butterworth? Why is not a simple single turn at the moment of crisis (i.e. 1:20AM Malay) sufficient? (Yes, this would mean overflight of Sumatra without detection, something I find easy to believe.)

  68. airlandseaman said,

    March 26, 2014 at 5:02 am

    It occurs to me that the IOR Inmarsat S/C, with a significant inclination, could provide the means to solve for a crude absolute path, not just hemisphere ambiguous LOPs and Doppler. If one used a single “Grand Solution” for all the equations of motion, for all 12 observation sets, in a LSF/Kalman filter type solution, you get math not unlike what is used for older LEO navigation satellites like Transit. IOW…A single moving satellite is essentially like having 12 quasi independent satellites from which we can triangulate. Oversimplified, but it helps to visualize what is going on. Of course, the geometric dilution associated with a geo s/c moving a few degrees is much greater than what you get with a 15 minute Transit pass, but potentially informative. Note that this is apparently the information used by Inmarsat to conclude the aircraft went south, not north. But there may be more that can be squeezed out.

  69. seanhelmi said,

    March 27, 2014 at 12:55 pm

    I still do not understand how the distance can be determined from time period. I do infer from one of timfarrar’s replies above, that scheduled very precise timeslots are involved. So if the plane is scheduled to transmit precisely on the hour, then the time delay between 1am, 2am GMT (precise) and receipt of the signal by the satellite would permit calculation of the distance. But doesn’t this require either a very precise clock onboard the plane or synchronization of the satellite clock with the plane clock, which requires periodic or continuous transmission. I understand GPS works because the satellites all have synchronized atomic clocks on board and the signal from the satellite includes timestamp information. The GPS device (e.g. mobile phone) does not have an accurate clock but can derive the time duration of transmission by comparing timestamp information from multiple satellites, all of which send the transmission at precisely the same time. But here there was only one satellite, and the airplane did not process the data let alone return any content to the satellite.
    I understand the handshake process begins with a transmission from the satellite to the aircraft. Does each satellite transmission include a “timeslot” for the return transmission? For example the satellite transmits a signal to the plane at 1am that states, reply at 1:01am. That would still require synchronized clocks. Or does the satellite transmission state, say, send a return signal exactly 1 minute from receipt of this transmission? That last would only require a timepiece onboard the plane that can accurately time a 1 minute interval. Assuming (not a big assumption here?) that the plane equipment was functioning, the time delay between the scheduled transmission time and the receipt by the satellite can be used to determine distance. Is this correct?
    Please excuse my query. I have no expertise in this area and am trying to find out how this works.

  70. What if they are wrong? Satellite Company Inmarsat reversed position on where MH370 plane flight may be - Celebzter.com said,

    March 28, 2014 at 7:23 am

    [...] stumbled upon a bunch of experts in satellite and mobile consulting  like  Palo Alto USA based TMF Associates and others  on the ‘net. Chatting it up in forums were other engineers, physicists, former [...]

  71. blacksabre said,

    March 30, 2014 at 7:11 am

    Thanks Tim for your explanations. Could you or someone else please explain why Inmarsat are so sure MH370 flew South and not North? I understand the ping in relation to distance with the Doppler effect but I have not been able to find any explanation as to how they determined direction of travel just that they based their opinion on analyzing 6 or so other 777′s in the area at the time but they fail to say what exact information they gained from them. My concern is they have not explained how this was arrived at and whether it is 100% without question correct and not 99% as that is leaving a doubt. I wonder why the Northern path was ruled out so quickly. As I understand they can shadow or fly very low to avoid radar detection. We know the flight was sabotaged but not by whom, a lot of effort to conceal the plane took place as well as changing routes etc It just poses the question if on a presumed suicide mission would he/they not have just flown it straight into the ocean or land straight after take off. I believe there is a very good chance it went North to the middle east area. Valuable plane and perhaps valuable cargo.

  72. blacksabre said,

    March 30, 2014 at 8:59 am

    Whilst on hijack possibilities, what if the plane did go to Mauritius, with the satellite above the equator is it possible or what makes the pings sound or look different travelling East/West as opposed to North/South?

    Is it possible that they could have used a decoy plane or drone to emit a false signal?

  73. Links to Previous Posts on the Inmarsat-3F1/MH370 Ping Analysis Saga | Duncan Steel said,

    April 4, 2014 at 3:17 am

    [...] Understanding “satellite pings” … [...]

  74. SyGunson said,

    May 14, 2014 at 4:26 pm

    Malaysia published a map with a track for MH370 on 1st May which portrays the aircraft turning south off Aceh province, Sumatra at 18:25 UTC then flying towards the satellite to a point west of Sumatra at 19:41 UTC.

    Yet the Burst Offset Frequency chart shows the opposite between 18:25 and 19:41 UTC, that the frequency is reducing rapidly as if MH370 were flying away.

    These two conditions are mutually irreconcilable therefore MH370 could not have flown through the Straits of Malacca as stated.

  75. MH370 Flight reconstruction (Part 7) | The MH370 Skeptic said,

    May 24, 2014 at 7:52 pm

    [...] Understanding “satellite pings”… (–click here for link) [...]

  76. The Locations of Inmarsat-3F1 when Pinging MH370 | Duncan Steel said,

    August 27, 2014 at 11:10 am

    [...] about one millisecond at the most. A blog to which I have been contributing (but see also this related blog) has rendered the information that the precision is ±300 microseconds (0.3 milliseconds). I am [...]

  77. Flight MH370 — search information in Google Earth – Adrod Sanm said,

    October 2, 2016 at 11:13 pm

    [...] March 17, 17:45 UTC: As per this blog post on TMF associates, the interruption within the arc close to the Gulf of Thailand is as a result of Malaysian [...]

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