I understand that Gander take data from the adjacent NavCanada domestic (radar) centres to calculate the forward estimate for the Oceanic entry fix. Using this radar-derived information, together with the level/speed held in the FPL, Gander can prepare a provisional clearance ahead of time, without input from the Crew.
The flight data processing system used is the same in both Shanwick and Gander, and the VHF ACARS-based clearance request/delivery kit is likewise similar, so I can only assume the use of radar derived info and the reference to the FPL to produce a form of provisional clearance might account for the difference.
We (Shanwick) base the clearance purely on the info from the Crew in the request, without any radar-derived data (Too many separate ANSPs to the east) or reference to the speed/level in the FPL.
Also, most of our traffic departs within 2 hours flying time of our boundary and is fuel-heavy, so we're not in the same situation as Gander, where the traffic has often been up in the cruise for well in excess of 3 hours before entering Oceanic, so the traffic presents itself differently at each centre and that has created different methods of operation in each - that and the fact that Shanwick doesn't (yet) get the radar-derived estimates received at Gander.
So in short, Gander know you're on your way, approximately when you're going to cross the entry fix and what your speed/level is going to be before you've even made the request. As was hinted at in a previous post, they may keep that provisional clearance on hold to maximise capacity, and to do so they may possibly wait until all the FANS (CPDLC/ADS-C) logging on processes are complete before sending you the clearance.
Shanwick only know you're on your way when we receive the request, and we'll get the clearance up to you ASAP and on a 1st-come-1st-served basis.
Hope that helps a little.
Last edited by rab-k; 8th Jun 2016 at 21:02.