But it IS relevant when flying a FL. It still happens that flight crews forget to switch to QNE during the climb. Isn't the whole point of the advisory tool to alert the ATCO in these cases?
Sorry - you're right of course. What I should have said is that the altitude data being sent by Mode S isn't affected by whether the BPS is QNE or QNH.
In fact NATS have acknowledge the potential wider use of the BPSAT above the TA:
"The benefit of this tool is currently limited to detecting altimeter setting errors below the transition altitude. The tool has the potential to deliver much greater benefit if Airbus and Fokker updated their implementation of Mode-S BPS such that downlinked data reflected when flight crews are using the standard pressure setting above the transition altitude."