CPDLC Air Initiated Transaction Delay for ATN

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The time provided in the header of the CPDLC LACK message sent from the aircraft is considered as giving a fairly accurate indication of when the associated uplink message has been processed and is available to the pilot. Similarly the timestamp in the header of the CPDLC request from the aircraft can be considered as giving a reasonable indication of when the pilot made the request. The CPDLC Transaction Delay (CTD) is the time taken between the message that initiates a transaction being sent and the corresponding message that closes the transaction being received. Air initiated transactions and ground initiated transactions will be analysed separately since they have different performance requirements (ET is different). Each different type of transaction will have to be considered separately to determine which message initiates the transaction and which closes it. If the initial response is an ERROR message then the transaction should not be included in the statistic (since the transaction will not be closed) but the error should be counted. Also if the initial response is a LACK but an ERROR message is received subsequently for this transaction (because the pilot or the controller did not respond before the timer expired) then the transaction should also not be included in the statistic but again the error should be counted. The ground system would record all ground initiated requests and associated closing responses and all aircraft initiated requests and associated closing responses.