Yeah, Mississauga and Brampton don't let the grass grow under their feet on many different levels... Toronto is the opposite... on many... yeah that's what we get for voting in David Miller. 
Anyway you've got a good chance of beating this at trial. You didn't commit the offence as charged. You've got a good diagram of what you did. Back it up by taking some photographs of the sign that you were ticketed for allegedly disobeying, the area, etc (don't forget the time-date stamp). It just adds a little more evidence. TC already gave you a good foundation to fight it and he knows more than I do, so no point in repeating that. Even better: Go back to Airport Road, park in the parking lot with your camera, and take photographs of people making the same turn.
That way when you're questioning the officer, you can show with undeniable photographic evidence, in addition to the diagram that you already have, that no U-turn was made; therefore, you didn't disobey a sign. (Your Worship, I would like to submit these photographs that I took myself on this date of various drivers performing the same turn that I was ticketed for by Constable So-and-So. As you can see, your Worship, at no point did this manoeuvre involve making a U-turn.) It just augments what TC suggested.
I wouldn't lose any sleep over it. You didn't disobey the sign and you should not have a hard time convincing the JP of that.
By the way has your disclosure package been made available to you yet?