Handshakes
Satellite Communications: Handshakes and flight MH370
Satellite communications (SATCOM) between Malaysia Airlines flight MH370 and the Inmarsat earth station near Perth, Western Australia, were normal until the aircraft diverted west across peninsula Malaysia.
Position Reports sent by the aircraft via SATCOM were received by Malaysia Airlines up to 0107:29 MYT. The next position report due 30 minutes later at 01:37:29 MYT was never received. It is presumed that the SATCOM link was 'lost' or unavailable. However, at 02:25:27 MYT a Log-On request was sent from the aircraft, ending the 'lost link period'.
The communication from the aircraft Satellite Data Unit (SDU) and the automatic response from the Ground Earth Station was called a 'handshake'.
In seven subsequent events, also termed handshakes, no meaningful data was transmitted from, or acknowledged by, the SDU on MH370.
Metadata from the seven handshakes, the Burst Timing Offset and the Burst Frequency Offset was used by Inmarsat to calculate the location of the aircraft within either of two corridors, a northern corridor and a southern corridor, and eventually focussed in the southern Indian Ocean along a 'seventh arc'.
Satellite Communications Link
The diagram below illustrates the communications path between an aircraft and a Ground Earth Station (GES) via a satellite, referred to as a Satellite Communications link or SATCOM Link.
Transmissions from the Ground Earth Station up to the satellite is an Uplink.
Transmissions from the aircraft via the satellite down to the Ground Earth Station is a Downlink.
Satellite Ground Station Facility near Perth, Western Australia
Inmarsat is one of several service providers using this facility.
Source: Google earth
GLE-IMG-040
Land or Ground Earth Station
The diagram for a typical Satellite Communications Link shows Inmarsat's 'Land Earth Station' in Perth, which has a huge dish antenna firmly anchored to the ground. Inmarsat calls it a Land Earth Station. The Annex 13 Investigation Team calls it a Ground Earth Station or GES. There is no difference. But since 'ground' and 'land' are both on the 'earth' why isn't it just an Earth Station?
A satellite in a geostationary orbit is about 35,786 km above the equator so it can remain above the same spot on the earth below. SATCOM systems on the ground, on ships, and in aircraft are all called Earth Stations, because they are not in space. So on MH370 there was an Aircraft Earth Station. The ships searching for MH370 had SATCOM systems called a Ship Earth Station.
Other terms used are Aeronautical Earth Station, Maritime Earth Station, Mobile Earth Station, and earth stations defined by purpose like Meteorological Earth Station.
Pings and Handshakes
The Satellite Communication Logs have been significant in the story of flight MH370. Without this data the search for MH370 would probably never have been focussed on the southern Indian Ocean. However, that significance also leads to the use of communications terminology which was new to journalists, authors and even pilots who became authors on the subject of MH370.
Did you know that your mobile phone and nearest communications tower periodically send test messages to check that the phone is still on or connected and within range? Satellite communication systems do the same thing. The terms used are a Ping and a Handshake.
The quote below from the Safety Investigation Report illustrates this point:-
If the GES has not heard from an aircraft for an hour after the last communication, it automatically transmits a ‘log on interrogation’ (“ping”) message on the common access frequency using the aircraft’s unique identifier. If the aircraft receives its ‘unique identifier’, it returns a short message that it is still logged onto the network. Both the initial log-on request and the hourly ping have been termed as a ‘handshake'.
Source: 1.9.5 1) Satellite Communications System Description SIR-2018
The Table below lists the Handshakes, and the data is then expanded into the story of the final hours of flight MH370.
When did the Handshakes Occur?
A list of the significant SATCOM transmissions from MH370 was published in the Safety Investigation Report as Table 1.9B. This has been modified slightly, as below, so that the time of each event can be linked directly to a corresponding explanation in the Timeline for Flight MH370.
| SATCOM TRANSMISSIONS | TIME | ||
|---|---|---|---|
| UTC | MYT* | ||
| 1. | Aircraft departed KLIA | 1642:04 | 0042:04 |
| 2. | Last ACARS transmission | 1707:48 | 0107:48 |
| 3. | 1st handshake - log-on initiated by the aircraft | 1825:27 | 0225:27 |
| 4. | Unanswered ground-to-air telephone call | 1839:52 | 0239:52 |
| 5. | 2nd handshake initiated by ground station | 1941:00 | 0341:00 |
| 6. | 3rd handshake initiated by ground station | 2041:02 | 0441:02 |
| 7. | 4th handshake initiated by ground station | 2141:24 | 0541:24 |
| 8. | 5th handshake initiated by ground station | 2241:19 | 0641:19 |
| 9. | Unanswered ground-to-air telephone call | 2313:58 | 0713:58 |
| 10. | 6th handshake initiated by ground station | 0010:58* | 0810:58 |
| 11. | 7th handshake - log-on initiated by the aircraft | 0019:29* | 0819:29 |
| 12. | Aircraft did not respond to ‘handshake’ from Satellite Earth Ground Station | 0115:56* | 0915:56 |
| *08 March 2014 | |||
Adapted from Safety Investigation Report SIR-2018 Table 1.9B - SATCOM ‘Handshakes’
What Does the Data Tell Us?
MH370 established a connection with Inmarsat's Indian Ocean satellite I-3 in the pre-flight period from 23:54:31 MYT Friday, 7 March 2014. A valid Flight-ID was entered by one of the pilots by 23:55:57 MYT. Before then the Inmarsat system would have recognised a transmission coming from the aircraft 9M-MRO because the aircraft earth station (AES) has it's own unique identifier, just as a mobile phone has an International Mobile Equipment Identity (IMEI) and other unique identifiers. The Flight Number is entered manually because it changes.
So by 23:55:57 MYT while MH370 was still on the ground at Kuala Lumpur, the Aircraft Earth Station had logged-on to the Inmarsat Ground Earth Station via the I-3 satellite. The Ground Earth Station was programmed to contact the logged-on Aircraft Earth Station at hourly intervals to check 'are you still there?' 'do we maintain the connection?'
Every time a message was sent from MH370 via SATCOM, like the take-off report and the position report, the Ground Earth Station 'clock' would reset and wait another hour to 'ping' the Aircraft Earth Station.
The last communication from MH370 was at 0107:48 MYT so the Ground Earth Station should have 'pinged' the aircraft at 0207 MYT. But there would have been no response. The aircraft systems may have been powered off. The connection was 'lost'.
However, at 0225:27 MYT the aircraft SATCOM initiated a Log-On. There was no Flight ID but the Ground Earth Station Responded, completing a 'handshake' and the 'clock' would have reset. The next ping was scheduled to occur an hour later. Except for a telephone call.
At 0239:52 MYT Malaysia Airlines attempted to call the flight deck (pilots) on MH370 using the satellite telephone service. The call was not answered (there was no 'handshake')[1]. However, the attempted phone call caused the Ground Earth Station 'inactivity timer' or 'clock' automatically reset.
The Ground Earth Station pinged the aircraft an hour later at 03:41:00 MYT and MH370's SATCOM responded, counted as the 2nd Handshake. The GES pinged the aircraft three more times, each getting a response, so the ping at 06:41:19 MYT and a response was the 5th Handshake.
MH370 was supposed to have landed at Beijing at 6:30 MYT but had enough fuel to stay in the air for longer. Malaysia Airlines attempted another satellite phone call at 07:13:58 MYT but, like the previous attempt, the call went unanswered. The GES inactivity timer automatically reset.
At 08:10:58 MYT the GES automatically sent a ping to the aircraft and got a response. This counted as the 6th Handshake.
The final surprise from the satellite communication logs was a Log-On initiated from the aircraft at 08:19:29 MYT. The GES responded so this was the 7th Handshake. The GES inactivity timer automatically reset.
The next ping from the Ground Earth Station in Perth was sent to MH370 at 09:15:56 MYT. This time, there was no response. The aircraft would have run out of fuel shortly after the 7th Handshake and, with nowhere to land, would have crashed in the southern Indian Ocean. There were no survivors.
How did Handshakes tell us where MH370 went?
Military radar tracked MH370 as far as the Andaman Sea. None of the satellite transmissions from MH370 after that contained any position information. No GPS data, no altitude, air speed, heading, nothing useful.
Inmarsat engineers and scientists used metadata - data about data - to solve the puzzle of the aircraft's direction and estimated locations at the time of each handshake.
This metadata was recorded as timing differences and frequency offsets for each burst of data in the handshake transmissions.
These are explained in separate articles:-
- Burst Timing Offset (BTO), and
- Burst Frequency Offset (BFO)
The calculations based on this metadata enabled Inmarsat to suggest that at the time of each Handshake, MH370 must have been on an arc at a calculated distance from the satellite; and from the BFO metadata confirmed that the aircraft travelled south towards the southern Indian Ocean.
A significant international search involving a fleet of aircraft, ships and a submarine, however, failed to find any trace of MH370 in the target area. This outcome was depressing for friends and family who needed closure, but also fed into persistent doubts that MH370 was there. Debris found since has, however, confirmed the validity of Inmarsat's initial calculations, which other researchers have attempted to refine.
Notes
- ↑ Early versions of this page indicated that the telephony call counted as a 'handshake'. That was no correct.