TMF Associates’ Tim Farrar explains issues surrounding the interpretation of satellite ‘ping’ data.
A great deal of useful data has been accumulating in the comments section of my previous blog post on locating satellite pings from MH370 and I’ve greatly enjoyed all the input from many dedicated contributors across various fields of engineering and aviation. If you’re visiting for the first time then you might want to read my original primer on pings first.
In this post I’m going to try to distill this information and explain what we’ve been told [up until March 28], since there is still plenty of confusion out there, and address one thing that we haven’t yet been told, but which should be able to be determined from the analysis that has been conducted. Note that the diagrams shown below aren’t mine – I’ve provided links to original sources in the supporting text.
Almost immediately after the plane disappeared, Inmarsat discovered that the satellite terminal on the plane had continued sending “pings” to the satellite every hour. This was in response to the Inmarsat network checking in with each terminal that it had not seen traffic from, in order to check that it was still connected to the network, just like the cellular network checks every so often that your phone is connected. In technical terms (from the Classic Aero specification), commenter GuardedDon described it well:
The ‘ping’ is a component of the Aero-L [or Aero-H] protocol where the GES [Inmarsat’s Gateway Earth Station] attempts to check the ‘log-on’ state of previously logged on but apparently idle AES [the plane’s Airborne Earth Station]. The GES determines the AES to be idle if a timer ‘tG6′ expires, tG6 is obviously the hourly period.
The GES transmits to the AES over the P channel & receives over the R channel. The initial response burst on the R channel is the timing datum transmitted by the AES ±300 μs of receiving the incoming frame on the P channel. All very deterministic to give us the range to AES from satellite using the Round Trip Timing.
The delay can be measured fairly accurately, since as noted above, the timing is specified to within ±300 μs. This calculation, from PPRUNE [Professional Pilots Rumor Network], shows that the difference in round trip delay between ping arcs 1 degree apart is around 600 μs at the relevant angle for MH370. Thus the location of each arc is known to within 1 or 2 degrees, depending on whether the satellite actually measures the round trip or one way delay to the aircraft.
The arc information was released to the public on March 15 and there was some confusion at that point about why part of the arc close to Malaysia was excluded. Possibilities included:
1) that the area had been checked by radar
2) that the plane’s minimum speed would have meant it could not have been that close to Malaysia
3) that another Inmarsat satellite over the Pacific would have received the signals in this excluded part of the arc.
This issue has still not been clarified, but of these it appears that a combination of the first and second explanations is the most plausible.
Inmarsat measured the arc positions each hour from 2.11am to 8.11am and the possible routes taken by MH370 can be estimated by assuming that the plane was flying at a constant cruise speed, and then noting that the distance between the points at which the plane crossed each successive arc is equal to the distance the plane travelled in one hour. That led to the NTSB’s two potential tracks for the southern route, published by AMSA on March 18, which included two different assumptions for the speed at which the plane was flying.
Several news organisations have published purported ping arcs for the intermediate ping times, including CNN and the Washington Post. However, its important to realize that these arcs are not based on real data, and are purely illustrative, like the chart produced by Scott Henderson.
What was not stated initially by Inmarsat or the investigators was that each of the hourly arcs is further away from the satellite than the previous one. In other words the plane was moving away from the satellite continuously from sometime soon after the 2.11am ping. This statement was made by Inmarsat on Friday (and I have also confirmed it). Once this sequence becomes clear, then it becomes impossible for the plane to have flown out over the Indian Ocean and later have returned to the vicinity of Malaysia. It also has significance for additional reasons that will be discussed below. As Jeff Wise noted, this means that the plane flew only between the green arc (the pink dot where it was at 2.11am) out towards the red arc where the last ping was recorded.
To be more precise, since Inmarsat has indicated that the plane was outside the green arc by 3.11am, the plane did not continue on its northwesterly course for long at all after contact was lost by Malaysian military radar at 2.22am (enabling it to return outside the green arc before the 3.11am ping). That would be consistent with avoiding Malaysian radar, but heading south the plane would have very likely crossed Indonesian radar coverage (something that the Indonesians have denied).
This sequence of ping arcs led inexorably to either a northern or a southern track, but there was still some uncertainty about which one was correct. The analysis that Inmarsat undertook took into account that the I3F1 satellite is in a slightly inclined orbit, which moves north and south of the equator each day. In other words it is only station-kept in the east-west direction, not north-south. While this situation is often the case for old FSS satellites, where the fuel is nearly exhausted, even new MSS geostationary satellites do not use strict north-south station-keeping because the beam width of a small L-band antenna is pretty wide and so accurate pointing is not required.
DuncanSteel noted that the satellite was actually north of the equator at the time in question and Inmarsat was able to use the fact that the satellite was moving relative to the aircraft to calculate the resulting Doppler effect that shifted the frequency of the ping as measured at the satellite. If the satellite was moving towards the south, then the frequency of pings from airplanes flying in the southern hemisphere would be shifted up in frequency, while the frequency of pings from airplanes in the northern hemisphere would be shifted slightly down in frequency.
Inmarsat performed an analysis of pings received from other aircraft flying in the Indian Ocean region to confirm that this effect is consistent across all of these planes and therefore concluded that MH370 must have been to the south of the satellite at the time of the last ping, not to its north. This led up to the announcement that the plane must have crashed in the Southern Ocean.
Now for an interesting piece of information that does not appear to have been considered in detail. A pilot on PPRUNE pointed out that there are two different modes of operation of the 777 flight management computer. A programmed route will take a straight line (great circle) route to the next programmed waypoint, but if there is no longer any waypoint in the computer, then the plane will fly on a magnetic bearing. While this is not material around Malaysia, it becomes highly significant in the Southern Ocean.
As a result, a magnetic heading would need to start out going significantly further west (and would also fly much further) to end up at the same point as a great circle route.
It is easy to see that in combination with Jeff Wise’s chart of the ping lines, a magnetic bearing heading is highly unlikely to have resulted in the 3.11am ping arc lying outside the 2.11am ping arc. Once this is realised, the hypothesis that the plane suffered an accident that left it flying on autopilot becomes rather less likely than the plane being deliberately directed towards a part of the southern ocean where presumably whoever was in charge believed the aircraft would never be found.
Indeed the NTSB tracks appear to implicitly assume an absolute not a magnetic heading, so would require the plane to be flying in a pre-programmed direction. Of course we need to see the ping arcs themselves (or at least get absolute confirmation about the trend in the ping arcs) before reaching a definitive conclusion, but this issue appears quite significant for any assessment of what might have happened onboard MH370.
UPDATE (March 25): The Malaysia government has just released this full picture of the potential southern route tracks. The red track appears to be a magnetic bearing heading which would have required a slower speed (400 knots) and would result in a location far to the northeast of previous estimates. The yellow track is apparently the originally assumed programmed heading at cruising speed of 450 knots and is consistent with the current search area. There is clearly an enormous difference in where the plane ended up.
UPDATE (March 25): The Doppler shift data released by the Malaysian Government gives full details of the ping times (note that they are in UTC so add 8 hours for local Malaysian time which is used above). Several pings were received at just before 2.30am, then at 3.40am, 4.40am, 5.40am, 6.40am and 8.11am, not at 2.11am, 3.11am, etc as surmised above.
It seems clear from the Doppler information that the plane made a sharp turn very shortly after it was lost from Malaysian radar coverage at 2.22am. There is also much more time for the plane to move outside the 2.30am arc by 3.40am so this does not impose as much of a constraint on the possible routes of the plane.
The question has been raised about the apparent “partial” ping shortly after the 8.11am ping was recorded. Was that a partial ping because the plane lost power during the course of that handshake? Its hard to tell, but I note that there were several pings quite close together around 2.30am after the “possible turn”. Those appear to have occurred for a different reason than the regular pings (and also from the more frequent earlier handshakes after take off which I assume relate to regular ACARS messages being transferred).
So an understanding of why those occurred is likely to shed some light on why a ping might have been attempted so soon after 8.11am. In particular, could it have been initiated from the plane’s terminal rather than the satellite network? And if so why – for example, could it be due to the plane’s terminal trying to re-establish contact with the satellite after a sharp change in direction?