Amateur tracking software can monitor the signals sent by the aircraft to the Inmarsat network.
On Mar. 8, 2014, Malaysia Airlines flight MH370, a Boeing B777-200 aircraft (registration 9M-MRO), operating from Kuala Lumpur and Beijing, disappeared from radars about 40 minutes after take off from Kuala Lumpur.
The flight, carrying a total number of 239 passengers and crew members, was regularly transmitting ADS-B data until contact was lost over the Gulf of Thailand, when the wide body was cruising at 35,000 feet at 474 knots in reportedly good weather.
Between 1:19 and 1:20AM local time, the aircraft turned right, changing heading from 25 to 40 degrees.
The transponder stopped transmitting at 1:21AM LT.
According to the Malaysian authorities, there were subsequent primary radar returns to the west of the Malaysian peninsula, over the Strait of Malacca and then north-west. This is assumed to be a real return from MH370 even if based on primary radar echo.
For reasons we still don’t know the aircraft radio systems did not work while the plane flew westwards back towards Malaysia.
Even if information was incoherent and sometimes contradictory, we know for certain that military radars in both Malaysia and Thailand saw the plane.
MH370 route with initial search (credit: Wiki)
SATCOM (a radio system that uses a constellation of satellites used to transmit voice, data or both) system pings linked to the INMARSAT network continued for 7+ (last ping at 08:11 local) hrs after LOS (loss of signal).
A Ping is a quite common term for IT Networking. It refers to a utility used to test the reachability of a host on an IP network and measure the round-trip time (RTT) of the packets even if it is more frequently associated to the data messages themselves, or “pings”.
Similarly to what happens on a Local Area Network, satellites send pings (once an hour) to their receiving peers that respond to it thus signaling their network presence. Hence, these pings are no more than simple probes used to check the reachability of SATCOM systems aboard the planes.
Based on the round-trip times of such pings, two arcs made of all the possible positions located at the same distance from the INMARSAT satellite were drawn.
But further analysis on Doppler Effect, as well as correlation among the “signatures” of other B777s, clearly indicated that aircraft had followed a southbound route, towards the South Pole.
Whilst search efforts have not been able to find the wreckage of the Boeing 777, the mysterious disappearance of the MH370 flight highlighted the need for tracking aircraft flying over high seas or remote locations, where radar coverage does not exist (such as southern Indian Ocean where aircraft, ships and submarines from 26 nations, have searched for any debris).
Two years on from the loss of MH370 and six months after Inmarsat completed their initial trials of a free tracking network for commercial traffic across Autralasia and the Pacific region COAA’s team who develop PlanePlotter, an aircraft position plotting program that decodes the digital messages transmitted from aircraft to display their content and plot their position on a radar-like chart, have successfully completed their own trials to see if we can obtain location data from the Inmarsat fleet and then plot it on their own tracking system.
Here’s how they describe their recent developments:
“This had been on our minds for some time, but we didn’t have the decoding knowledge to fully understand the signals from the satellite fleet,” says John, from PlanePlotter support.
“In stepped “Jonti” from New Zealand, who co-incidentally had been decoding the L band signals in the Aero band from Inmarsat. Those signals did not contain location data as the ACARS style messages at L band are from the earth stations, up to the satellite and “down” to the aircraft.
Thinking back to the days when we monitored the Inmarsat analogue phone circuits we realised that to get both sides of the “conversation” we needed to monitor both L band [ 1.5 ghz ] …and C [3.5 ghz] band. With Jonti’s help we set about some trials.
Inmarsat uses 8m dishes to monitor the tracking signals, we only had 1.8m and 2.4m dishes, but with Jonti’s guidance we managed to get good signals with just 1.8m antenna.
Jonti identified the short bursts of data which looked like they were from the a/c and after some stirling work he produced a version of his software JAERO, which decoded the bursts. Sure enough, in the data down from the aircraft was information akin to that which is contained in ADS-B signals. Using that and satellite dishes in Europe, America and Australasia we can now locate aircraft from the US
eastern seaboard into Asia and across the Pacific, via five different satellites.”
PlanePlotter are in the middle of further tests and have recently rolled out a new version of their software which, for their satellite ground stations, will provide full information, from location, heading etc. to outside air temperature: in fact, everything you would expect from ADS-B data.
“For our sharers it will show the usual ADS-B style information and plot sat comms equipped Oceanic traffic in real-time. Whilst we are not seeing reports from aircraft on a 15 minute basis yet, presumably as more airlines comply the reports will become more regular,” says John.
“Over the next few weeks more of our satellite ground stations will come online providing real-time coverage for tracking enthusiasts. The Inmarsat data, combined with ACARS, live PiReps and ADS-B will provide seamless global tracking.”
PlanePlotter global coverage
Top image credit: Laurent Errera (Wiki)
All the articles about MH370 can be read here (scroll down).
On Dec. 28, an Airbus A320-200, registration number PK-AXC, flying as AirAsia Indonesia flight QZ8501 from Juanda International Airport, Surabaya, to Changi Airport, Singapore, lost contact with Air Traffic Control at 06:24 LT over Java Sea.
155 passengers were on board the aircraft: 137 adults, including 2 pilots and 4 cabin crew and 1 engineer, 17 children and 1 infant.
The aircraft was piloted by a captain with an experience of 20,537 flying hours, 6,100 of which were with AirAsia Indonesia on the Airbus A320. The first officer had a total of 2,275 flying hours with AirAsia Indonesia.
According to Indonesia’s Transport Ministry, while flying at FL320 (32,000 feet), QZ8501 requested to deviate and climb to FL380 to avoid very bad weather in the area. Clearance to climb could not immediately be granted because of nearby air traffic.
The Ministry said the aircraft could be tracked by ADS-B until 06:18, when it went missing from radars. Flight crew did not radio any mayday or emergency message.
Noteworthy, a leaked ATC image published by Gerry Soejatman on Twitter shows the AirAsia flight climbing through 36300ft with a Ground Speed of only 353 knots: provided the image is genuine, the radar screenshot would show an airplane much slower than expected at that altitude (a nearby Emirates flight at FL360 – 36,000 feet – was flying at 503 knots).
Although any attempt to explain the reason for the disappearance of the AirAsia flight is pure speculation at this time, we can’t but notice at least one apparent similarity with another famous crash: Air France 447.
AF447 was an Airbus 330 from Rio de Janeiro to Paris that plummeted 38,000 feet in 3 minutes and 30 seconds and crashed into the Atlantic Ocean in 2009. In that case, pilots responded to a stall, induced by inconsistencies between the airspeed measurements likely due to pitot tubes being obstructed by ice, by pulling the nose up instead of pushing it down to attempt a recover.
Even though a low Ground Speed can be caused by strong head winds, the fact that nearby Emirates was cruising at 36,000 feet at a speed of 503 knots, seems to suggest that the missing Airbus 320 was probably too slow and closer to the stall speed than it should have been.
Anyway, although no sign of wreckage, oil, debris were found so far, experts believe there are more chances to locate the aircraft than the Malaysia Airlines MH370 which vanished in March this year and has not be found yet.
Search by air for the missing Malaysia Airlines Boeing 777 has been called off. Most (if not all) the aircraft tupes that undertook visual search from Australia, were grouped for a farewell picture.
52 days into the search, anything would have sunk and for this reason the authorities have decided to focus on underwater search.
But before patrol and support aircraft and crews from Australia, New Zealand, China, South Korea, Japan, Malaysia and U.S. left Perth, RAAF Pearce, and RAAF Learmonth, planes from each participating nation were grouped for one last farewell photo.
A Chinese ship involved in the hunt for the missing Malaysia Airlines MH370 in the Indian Ocean reportedly detected an underwater ping like those emitted by the aircraft black boxes.
On Apr. 5, the sonar detector-equipped Haixun 01 picked up an acoustical signal on 37.5 kHz frequency, the same as emitted by the Underwater Locator Beacon of flight recorders.
According to Xinhua news agency, the “ping” was detected at about 25 degrees south latitude and 101 degrees east longitude, within the search area of 88,000 sq. miles in the Indian Ocean to the west of Australia, where aircraft, ships and submarines from 26 nations, are currently searching for any debris from the missing Malaysia Airlines Boeing 777 mysteriously disappeared since Mar. 8.
On the same day the signal consistent with the aircraft black box was picked up, a Chinese patrol plane (most probably an Il-76 deployed to Perth), spotted some floating debris (see image below).
At the moment, there is no confirmation that the signal and the pieces are related to the missing MH370.
Lessons Learned
Is there a way to prevent a plane from disappearing from the skies as happened to the Malaysian B777?
Several.
Current airplanes make several different kind of services available to passengers: interactive media, movie, games, music, but also Internet and telephone. The latter use satellite channels. This links could be used to stream CVR (Cockpit Voice Recorder) and FDR (Flight Data Recorder) data (or just a subset of flight parameters) or, to reduce transmissions and save much money, simply report the black boxes position (coordinates) to ground stations in real time.
Another option is to make Underwater Locator Beacons more powerful and capable to operate for longer periods (they are currently limited to 30 days).
Then there’s another problem to be addressed: the capability of pilots to switch off all communication and navigation systems to make the plane (almost) invisible to radars. Since we can’t be completely dependent on aircrews to track airplanes wherever they fly, any “new” system should be designed in such a way pilots can’t switch it off.
Even if the aircraft’s crash position could not be determined, Doppler effect analysis on SATCOM pings enabled INMARSAT to determine MH370’s final route over South Indian Ocean until a final, “partial ping,” received 8 minutes after the last known one.
As already explained on a previous post, hourly SATCOM system pings continued for more than 7 hours since the Loss Of Contact with MH370, until 08.11 AM LT.
Based on the round-trip times of such pings, two arcs made of all the possible positions located at the same distance from the INMARSAT satellite were drawn.
But it was further analysis, on Doppler Effect, as well as correlation between the “signature” of other B777s, that clearly indicated the aircraft southbound route.
Doppler Effect
The Doppler effect is something we are familiar without even knowing it. The sound of the ambulance’s siren or the train whistle are among the most common examples of how Doppler Effect works: the high pitch of the siren of an approaching ambulance suddenly drops as the vehicle passes you. Even if the source wavelength and speed do not change, movement of the source alters the wavelength and frequency of the sound.
You can use several online tools to calculate the frequency change induced by motion.
Since satellite pings are carried on a radio wave, the sensed wavelength, frequency increase or decrease depending on the fact the aircraft is moving towards or away from the satellite.
The difference between the expected received frequency and the actual measured one due to Doppler Effect is known as Burst Frequency Offset.
By comparing the Burst Frequency Offset due to Doppler on MH370 against the predicted one based on six B777s flying on the same day, INMARSAT could determine close correlation for the southern route and eliminate the northern one.
Here’s an excerpt from UK Air Accidents Investigation Branch (AAIB) release that explains how INMARSAT calculated the route.
INFORMATION PROVIDED TO MH370 INVESTIGATION BY UK AIR ACCIDENTS INVESTIGATION BRANCH (AAIB)
25/03/14
[…]
As you have heard, an aircraft is able to communicate with ground stations via satellite.
If the ground station has not heard from an aircraft for an hour it will transmit a ‘log on / log off’ message, sometimes referred to as a ‘ping’, using the aircraft’s unique identifier. If the aircraft receives its unique identifier it returns a short message indicating that it is still logged on. This process has been described as a “handshake” and takes place automatically.
From the ground station log it was established that after ACARS stopped sending messages, 6 complete handshakes took place.
The position of the satellite is known, and the time that it takes the signal to be sent and received, via the satellite, to the ground station can be used to establish the range of the aircraft from the satellite. This information was used to generate arcs of possible positions from which the Northern and Southern corridors were established.
In recent days Inmarsat developed a second innovative technique which considers the velocity of the aircraft relative to the satellite. Depending on this relative movement, the frequency received and transmitted will differ from its normal value, in much the same way that the sound of a passing car changes as it approaches and passes by. This is called the Doppler effect. The Inmarsat technique analyses the difference between the frequency that the ground station expects to receive and that actually measured. This difference is the result of the Doppler effect and is known as the Burst Frequency Offset.
The Burst Frequency Offset changes depending on the location of the aircraft on an arc of possible positions, its direction of travel, and its speed. In order to establish confidence in its theory, Inmarsat checked its predictions using information obtained from six other B777 aircraft flying on the same day in various directions. There was good agreement.
While on the ground at Kuala Lumpur airport, and during the early stage of the flight, MH370 transmitted several messages. At this stage the location of the aircraft and the satellite were known, so it was possible to calculate system characteristics for the aircraft, satellite, and ground station.
During the flight the ground station logged the transmitted and received pulse frequencies at each handshake. Knowing the system characteristics and position of the satellite it was possible, considering aircraft performance, to determine where on each arc the calculated burst frequency offset fit best.
The analysis showed poor correlation with the Northern corridor, but good correlation with the Southern corridor, and depending on the ground speed of the aircraft it was then possible to estimate positions at 0011 UTC, at which the last complete handshake took place. I must emphasise that this is not the final position of the aircraft.
Here below is an INMARSAT image which shows the southern tracks for a ground speed of 400 and 450 knots ground speed.
Last “Partial” Ping
Noteworthy, INMARSAT collected evidence of a partial handshake between the aircraft and ground station at 00:19 UTC, 8 minutes since the last acknowledged response. This partial ping is currently being investigated: there are several different theories, including the one that the final handshake was attempted outside of the hourly window, possibly at fuel starvation because of power fluctuations.
At 0115 UTC, when the ground earth station sent the next log on / log off message, no response was sent by the plane, indicating that the MH370 was no longer logged on to the network (because already crashed).
Lessons Learned
1) Pilots have the power to make aircraft almost invisible to radars. This will have to be addressed in some way, with some system capable to track the plane regardless of the aircrew’s willingness.
2) Black Box data have to be streamed via satellite and stored for the shortest time possible (until the next flight, then automatically erased) somewhere (for instance, in a Cloud Network architecture, to save money and have it immediately available, should the need arise).
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