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How British satellite company Inmarsat tracked down MH370
- zaroth 13y agoengineers were able to narrow the location of the plane down with an initial analysis of the Doppler effect on the signal from the flight’s pings and the plane’s approximate altitude. How and why would they have a record of this to be able to study it?
- Serow225 13y agoI'm curious about that too, it seems very unlikely that the actual digitized RF waveform was available for analysis. My guess, based on working with somewhat similar systems in the past, is that the ping packet content was also tagged by the satellite with the measured carrier frequency (or offset from nominal) and this value had enough precision that it let them calculate the Doppler effect. The reason I would say this is because I've done similar things in the past for telemetry applications, it can be really handy for debugging when things aren't working right.
- sliverstorm 13y agoOr, what if they studied the time dilation between pings? The pings are probably on a precise, known schedule.
- mtkd 13y agoApparently it was hourly and Inmarsat calculated two arcs (north and south) which it could have taken, but then focus seems to have moved to the southern one in last few days. I wonder if they flew test flights along both arcs and compared the data.
- lutusp 13y ago> I wonder if they flew test flights along both arcs and compared the data. No, but they compared the present data with recorded data from other flights along the same routes for comparison.
- waterlesscloud 13y agoWell, there wouldn't be flights along the Southern route, exactly, since it led nowhere. So I'm guessing they extrapolated from routes that were in that general direction, like Sri Lanka - Perth. It's likely there's a lot of assumptions at work in their calculations, but it might be good enough.
- nroets 13y agoSome possibilities http://www.gcmap.com/mapui?P=jnb-per http://www.gcmap.com/mapui?P=jnb-per http://www.gcmap.com/mapui?P=per-rio http://www.gcmap.com/mapui?P=per-rio
- lutusp 13y agoI should have been more clear -- they compared routes and aircraft velocities that could be expected to produce the same radial velocity and Doppler effects, given the satellite's geostationary position.
- cynwoody 13y agoThey didn't need to fly test flights. That would be big $$ and take time. But apparently, their hardware is installed on plenty of airplanes, so they just needed to study normal flights north and south in the region to establish a baseline to which to compare MH370's data. And it was that data that lead them to conclude that the southern route was likely the one MH360 actually took.
- waterlesscloud 13y agoSo the idea is that there's a shift on the intra-ping timescale? So there are numerous packets and each packet would have the frequency of that particular packet attached to it? Obviously I have no clue what the protocol for the ping looks like, just trying to understand how it would work. Also curious how they matched it to planes flying the southern route, since I wouldn't have guessed there were any other planes flying on the southern route (which goes...nowhere).
- DrStalker 13y agoI might be as basic as: Logged ping, base carrier frequency 1375.003762 MHz. Logged ping, base carrier frequency 1375.003761 MHz. Logged ping, base carrier frequency 1375.002961 MHz. Then with some maths and comparing to the known positions of the plane (pre-contact loss) and known positions of the satellite you can work out which way the plane would have had to move to get the observed results once contact was lost. (numbers in example made up, and likely aren't appropriate for satellite communications but you get the idea)
- Serow225 13y agoExactly, although this is all completely conjecture :)
- bradgessler 13y agoConservatively there are tens of thousands of commercial flights per day. They could use each flight as a test case to understand and refine the model of the original analysis and come up with something more accurate.
- waterlesscloud 13y agoWell sure, but I don't know if doppler information between pings buys you much new info. The pings, as I understand it, came an hour apart, and continued for 7-8 hours total. With just the pings themselves they were able to extract the distance from the sat, which is what led to those arcs they published last week. I dunno if doppler shifts from pings an hour apart add much to that.
- rz2k 13y agoWhat is a likely way for that to work with something launched in 1996? If the plane was going 50 MPH away or toward satellite, that's ~7e-8 * c. If I remember, the frequency would be shifted by a factor of the square root of (1 - 7e-8)/(1 + 7e-8), or its inverse. Would the satellite always relaying that level of precision to the ground? If it is, why would they have prioritized that over what would have presumably be a dramatic increase in capability if it were discarded instead? Could they have designed it to store that much precision in rotating logs available on request? And furthermore, how is a relativistic Doppler shift on those magnitudes discernible from the frequency shift naturally occurring from the plane's transmitter? I suppose my point is that while it's easy to understand the idea of a train whistle changing pitch as it passes by, how they would do it in practice sounds like magic, being unfamiliar with the capabilities and the equipment that geostationary satellites generally carry.
- taejo 13y agoAnother poster pointed out that the satellite just reflects the signal back to base stations on earth, which could have been upgraded since 1996.
- chronomex 13y agoInmarsat is a "bent pipe" system, meaning that the satellite just amplifies and sends down to the base station everything that it receives. So the operator's RF fanciness is limited only by the noise factor in the satellite's amplifiers.
- rz2k 13y agoAh, that makes a lot more sense, though nevertheless impressive precision on the part of the base station. However, given the corridors, it still seems like the suggestion is that the Doppler measurements said it was moving rightward or leftward, when it should only say how much the aircraft was getting closer or getting further away. Looking at the map alone, it doesn't seem like that information would help distinguish which corridor it was in.
- Tuna-Fish 13y agoThe satellite was also moving (south or north), and the measured redshift/blueshift could be used to determine if the plane was north or south of the satellite.
- rz2k 13y agoI thought it was geostationary rather than polar orbiting.
- Tuna-Fish 13y agoGeostationary satellites don't actually stay completely stationary for any length of time without active stationkeeping. They very quickly develop a north-south motion which makes them trace out an analemma, a sort of figure eight on the ground.
- rz2k 13y agoI first wrote geosynchronous and approximately geostationary, then decided it was too cumbersome. What sort of magnitude does the analemma have? Also is the effect amplified by it being a "bent pipe"? Even if it isn't intentional, in scenarios such as measuring a Doppler shift I can see how it can be a useful tool. Almost analogous to microsaccades we make with our eyes.
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- wallflower 13y agoA pretty detailed explanation from Reddit user XenonOfArcticus. http://www.reddit.com/r/MH370/comments/218i36/how_the_satellite_company_inmarsat_tracked_down/cgapbao?context=1 http://www.reddit.com/r/MH370/comments/218i36/how_the_satell...
- waterlesscloud 13y agoAh! Now I get it. Thanks for the link. :-)
- ynniv 13y agoA better one: http://www.reddit.com/r/MH370/comments/218i36/how_the_satellite_company_inmarsat_tracked_down/cgb7b6j http://www.reddit.com/r/MH370/comments/218i36/how_the_satell...
- cynwoody 13y agoThe Doppler stuff is new (as of the 24th). What I had understood before was that the airplane carried an Inmarsat satellite transceiver which remained active, even though certain subsystems that use it (notably ACARS) were disabled. While active, the transceiver communicates with the satellite once an hour (the "pings" at 11 minutes past), even if it has no traffic to send. Failing to disable the transceiver in addition to the other systems would have been an oversight on the part of the plane's hijackers, assuming it was hijacked. Apparently, Inmarsat had available accurate ping timing data. Combined with knowledge of turnaround delays, this would enable them to calculate the plane's distance from the satellite during each ping. That defines an imaginary sphere in space, which intersects with the earth's surface to form a circular locus of where the plane could have been at the time. Presumably, there were eight such ping interactions, ending with the one at 0811 Kuala Lumpur time. There was no 0911 ping, meaning the plane had either crashed or landed and powered off before that time. And so we had the northern and southern arcs. These were obtained by cropping the circle based on factors such as how far the plane could have flown in the available time, whether or not military radars, if any, had detected the plane, etc. Now comes the Doppler data. Apparently, Inmarsat also has an accurate read of the plane's transmit frequency. The extent to which this frequency differs from the nominal frequency would indicate the plane's velocity towards or away from the satellite at the time of the ping. Today, Chris McLaughlin revealed that Inmarsat had studied the Doppler patterns of normal flights in the same region and concluded that the Doppler data for MH370 indicated that the southern arc was the right one. Hopefully, Inmarsat has it right, and the black boxes will be found before their beacon batteries run out.
- sixQuarks 13y agoCall me a conspiracy nut, but here's what I believe may have happened: It's almost a certainty that the US government has spy satellites video recording the entire planet at all times, at a level good enough to distinguish individual aircraft. The US spy agencies probably knew where the plane was right away, but they can't just tell everybody or the jig is up. So they may have just contacted this satellite company, told them where the plane is, and came up with a technical explanation of how it was located.
- ewoodrich 13y agoThe US certainly has a bloated defense budget, but I strongly doubt that we have satellites constantly trained on the typically empty oceans that cover 70% of the Earth.
- marshray 13y agoWhile I don't subscribe to GP's view that satellites have bandwidth for "video recording the entire planet at all times", it seems plausible that SIGINT satellites would have a relatively constant bandwidth available for observation. As a (non-geostationary) satellite passes over relatively unpopulated areas (e.g., the southern Indian Ocean), it will be "bored" and thus far more likely to notice a single low-power transmission. On the other hand, if it is storage and downlink bandwidth that is limited, sats would probably be programmed to ignore or discard uninteresting signals. This seems likely to me. So the question may be whether commercial airliner pings from the southern Indian Ocean area were considered 'interesting' at the time and, if not, did anyone realize there was a rogue 777 in time to re-task the satellites? Regardless, I'd bet those signals are considered interesting going forward.
- dingaling 13y agoThe other complication is that Inmarsat up-link is a steered spot-beam ( the antenna on the aircraft is stabilsed and driven, or phased-array in recent implemenations ). So the SIGINT sat would have to be passing through the up-link 'beam' precisely as it was transmitting. Down-links are ( depending on generation and usage ) either from one of an array of narrow-beam antennae on the Inmarsat or from the wide-area general broadast antenna; much easier to intercept, you and I can do it with a sat-TV dish and an RTL dongle.
- topbanana 13y agoAs an interesting aside, Inmarsat are based on the Old Street roundabout in London. This is reputedly the centre of the startup scene.
- awalton 13y ago** is thought to have tracked down. They still haven't actually found the plane, they've just got a very well reasoned assumption of the area the plane should be in.
- sebcat 13y agoWhat people write in the article's comment section is certainly an interesting read. Irrational at best.
- Patient0 13y agoIn the actual video he seems to have misunderstood what the Doppler effect actually is. He gives the example of a train whistle "getting louder as it comes towards you and fainter as it moves away" but he should have said something along the lines of "the same way that a train whistle has a higher pitch as it moves towards you and a lower pitch as it moves away from you". The intensity of the signal is much harder to measure/calibrate than the frequency of the signal.
- Tloewald 13y ago...and by the way, we offer a service that only costs $1/h/plane and we'd like governments to force everyone to pay us. Now, I have no idea whether $1/h/plane is reasonable (is that to cover the satellite comms, or is that on top of them?) but it is kind of stunning that planes aren't tracked automatically and as a matter of course the way, say, most people with cell phones are.