Communications:SATCOM Logs/Notes

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The following Notes were provided by Inmarsat to assist understanding of the Data Communication Logs.


Update to Signalling Unit Logs December 2014

Requests have been received from external interested parties to release any information from the SU logs (recorded at the ground earth station for the communications with the terminal on 9M-MRO) containing BTO and BFO values prior to those originally published. There are two logs that contain BTO and BFO data prior to 1600 UTC while 9M-MRO was powered-up at the gate in Kuala Lumpur. The two additional log messages have been incorporated into the first two lines of the table below. As the two message are from Channel R600, a 4,600 microseconds calibration needs to be subtracted (19380 – 4600 = 14780) when comparing with subsequent R1200 messages. This is the same as applied for the messages at 18:25:27 and 00:19:29. The remainder of this document is as previously released by Malaysia in May 2014.


Notes for Signalling Unit Log for (9M-MRO) Flight MH370

This set of notes and accompanying tables is intended to provide a readable summary of the data communication logs from the lnmarsat satellite system provided to the authorities in connection with the ongoing investigation and search and rescue activities relating to MH370.

  • All times are in UTC
  • The table below is a readable form of the logfile recorded at the ground earth station for the communications with the terminal on MH370 from 16:00 to the last Handshake Request (Log On Interrogation) attempt at 01:16 UTC
  • Some columns which contain no material information have been removed from the tables below for readability

An example of a complete message for a single log entry:

Field Headings:

Time, AES ID, DP AES Owner, Channel Name, Ocean Region, GES ID (octal), Channel Bearer, Channel Unit ID, Channel Type, Superframe Number, Frame Number , Slot Number, SU Number, SU Type, SU Type Code , SU Contents, Q Number, Reference Number, Ack Control, SDM Figure, CRC Correct, Missed T-Channel Burst, Broadcast, Rx Power (dBm), C/No, Frequency Offset (Hz), Estimated BER, Burst Timing Offset (microseconds)


Example message:

7/03/2014 16:42:04.408, 35200217, SITADP (3047), 10R-R1200-0-36ED, IOR, 305, Rx Primary, 4, R-Channel RX, 12184, 5, 0x62- Acknowledge User Data (R-channel), 62, 1F DO 62 75 00 SF CS 7A 4E 00 00 00 00 00 00 00 00 7F 89 , D, O, E, S31, Yes, No, No, -57.29, 40.09, 146, 0, 14920


Understanding the Burst Timing Offset (BTO) values:
  • The round trip time for a message is a combination of:
    1. Time from the ground station satellite aircraft satellite ground station
    2. Processing time within the ground station, satellite and aircraft terminal, which are constant
  • The BTO is a value (in microseconds) relative to a terminal at a nominal fixed location. Only R-Channel messages are used.
  • The BTO therefore allows the determination of the distance between the satellite and the aircraft. It does not provide the actual aircraft location


Understanding the Burst Frequency Offset (BFO) values:
  • The transmission signal path from the aircraft has two components affected by Doppler shift; between the satellite and aircraft, and between the satellite and ground station
  • The following need to be considered when computing the expected frequency offset:
    • lnmarsat Classic Aero mobile terminals are designed to correct for aircraft Doppler effects on their transmit signals. The terminal type used on MH370 assumes a stationary satellite at a fixed orbital position
    • An individual aircraft terminal will have a fixed frequency bias
    • Satellite, terminal and ground station oscillator stability
    • The correction applied by the Automatic Frequency Control (AFC) system in the ground station partially compensates for the satellite to ground station Doppler