New array at Leitrim?
The new cleared space overlaps but is not quite concentric with a large circular area that hosted a small direction-finding array in the mid-1960s. That array can be seen in the 1965 air photo of the station viewable on the City of Ottawa's geoOttawa site. The 1965 system looks like the high-band part of a GRD-6 array, but it is not immediately obvious what a U.S. Navy system was doing at a Canadian Army SIGINT site. The Canadian Navy began operating a modified GRD-6 system at its new Bermuda site in 1965, and it is possible that the Leitrim site had something to do with that (although one might have expected the system to be built at HMCS Gloucester if that were the reason).
Whatever the explanation for the earlier array, a more modern Pusher CDAA was built nearby at the station in the early 1970s, and the large graded area then fell into disuse.
Large CDAAs have gone almost completely out of style in the intervening years. Canada operates the only two FRD-10 arrays remaining in the world. The U.S. still operates two even larger CDAAs called FLR-9s, and a third may still be in service with the German BND. And the Russians still operate a half dozen or so large Krug arrays, with Ukraine and perhaps one or two other former Soviet countries possibly also keeping a few in service.
But the days when more than 50 large CDAAs were in operation around the world are long gone.
We can be quite confident, therefore, that whatever system is built in the new circle at Leitrim, it won't be one of the classic "Elephant Cages" of the Cold War.
One possibility is that the new space is intended simply to host the beverage rosette array already at the station. That array was built around 1998 in two distinct sections: a 180-degree fan to the north of the main station buildings and a second 180-degree fan on the south side of Leitrim Road.
The Department of National Defence has been working on a plan to re-route Leitrim Road further south of the main station buildings, and it may be that the location of the southern fan conflicted with that plan, leading to a desire to move the array to a new location.
Presumably time will tell.
Which is good, because DND generally doesn't.
[Update 24 July 2012: It's the beverage array. More details here.]
[Update 8 September 2013: Added information about the earlier array at Leitrim in the mid-1960s.]