I think it's to do with ABBIE

http://dlr.thexhunters.com/info/janet_routes_1.jpg

Not sure how - but I think aircraft fly north towards ABBIE then
somehow establish their position from some another source. Is there
another NDB somewhere that could be selected to triangulate? The
Basecamp NDB could be too far away

I guess you use a loop for DFing NDB's?