Yes, but of course there's a generic problem for air defence of primary-only tracks. There are thousands of them, mostly Bloggs in his microlight, gliders etc. You can't track every one, or indeed any of them, on the assumption that they're TNT-packed and heading for Canary Wharf. In the good old days it was simply a question of spotting something coming from the east at 500 knots with no flight plan.
NS