[ProudVet77]

There's a few choices. There is downward looking radar like from an AWACS/JSTAR aircraft from above. You can use a surface search radar (like the Raytheon 1900N) at ground level, that can pick up a plane at 200 feet at 15 miles. And then there are helicopter based radars. They use radars similar to a surface search radar, just are able to get the antenna up to a decent altitude and can move it around for patrols.
Beyond that there are all kinds of whiz bang high tech gadgets (IR for example) which I don't really know much about.

[norraad]

That 40,000 foot fence is a high tech whiz bang gadget I'd like to see!

[BillM]

There are tethered balloons with radar.

[El Gato]

There is downward looking radar like from an AWACS/JSTAR aircraft from above.

JSTARS is for ground targets, although they could probably see helicopters and slow moving aircraft as well. There are also tethered balloon borne radars, similar to ground based air-route surveillance radar, but with longer effective range against low flying targets because of their higher altitude.