The system I saw 30 years ago were more ground based and covered tens to hundreds of KMs.

But even if you need a drone every 5km (you don't). That's all of 400 drones, 800 if you want to have pairs doing charge-work duty. For an 2000km border guarding operation, that's really not much. An order of magnitude or two cheaper than the fence they would guard.

How did that ground based system solve the horizon problem ? Drones don't fly very high, so you'd need one of these systems, say at 50 meters drone height, ~ every 50km.

That's probably worse than those drones watching from above, seeing as you'd need actual infrastructure at every interval. And, that'd be fixed infrastructure, with fixed blind spots.

I don't know exactly how it worked (beyond IIRC that it was a SAR = Synthetic Aperature Radar), but I know for sure that it worked perfectly well.

Ground stations and aerostats.