Malaysian Airlines MH370 contact lost

[neilki]

ACARS 'Ping"

Not a PING (TCP/ICMP); more a handshake with the ability to include free text. That 'Free Text" -really formatted data, in this case about Donkey Health wasn't transmitted due to the lack of subscription. It's Possible the ACARS handler also included some other info, but more likely not. It's not impossible any other data was transmitted indadvertantly or, (conspiracy theorists, collected by a non aviation actor) it's software; bugs et al happen; this is systems management & monitoring data so not likely subject to the rigors of safety of flight code..


128 16 16 8 8 56 8 16 8 8 32 48 8 128 8
pre-key bit sync char sync SOH mode address ack/nak label block ID STX Seq No. Flight No. Text ETX CRC BCS


pre-key A maximum of 128 bits is transmitted to allow for the transmitter to key. If the transmitter keys up and settles on frequency quickly, it is possible to see most of the 128 bits, if the transmitter is slow then much less than 128 bits is seen. The received signals therefore does not always contain all 128 bits during this pre-key sequence. The sequence is also designed to compensate for variable response times of receiver squelches and AGC settling times. At 2400bps, 128 bits equates to about a 20th of a second (53.333mS to be precise).
bit sync 16 bits of alternate bits to allow the decoder to lock into the bit speed.
char sync 2 bytes with a unique bit pattern allows the decoder to precisely character syncronise to the following 8 bit bytes.
SOH Start of header marker byte.
mode Type of transmission. (AGCS-1, AGCS-D etc).
address Aircraft Registration number. Filled with a special sequence if a squitter burst.
ack/nak Acknowledged/Not acknowledged. Advises other station if a re-send is necessary.
label Label identifies type of message being sent. Usually a letter and a number. See labels table.
block ID Single byte which increments on each successful transmission.
STX If STX present, then text message follows.
Seq. No. Message sequence number. Usually made from the current time (minutes/seconds). Only transmitted in downlink data blocks.
Flight No. Aircraft carriers international flight number. Only transmitted in downlink data blocks.
Text The text message itself. Just as with AX25 packet, the actual length is variable but limited to a maximum of 220 characters.
ETX End of text marker. If ETB is transmitted instead then more text follows in a subsequent data packet.
CRC Checksum.
BCS End of packet block marker byte.

Its much more likely position & speed calculated by doppler/multi station trig than anything else.