"Anyone know about the technologies that go into detecting low flying airplanes?"
Yes, a little. I had a bro-in-law in AWACS, Gulf War 1.
I haven't heard or read about any planes with look-down capability, patroling (Except right after 911). Just talk about it. Of course that means nothing these days! I mean my not hearing about it.
But they may be patroling the borders and we are not aware of it. But, isn't it more probable that this plane was tracked on FAA radar at normal altitude because it had no flight plan, or was acting in a suspicious manner? Like it suddenly popped up on RADAR after low leval flying.
Many stories here in Florida about tracking the drug smugglers, as you can imagine. I like the infrared videos of planes/boats dumping out the bales.
I'm guessing this plane was treated the same as any unidentified plane would be treated, when Homeland Security is looking for Chinese illegals that may have radiological material in their posession.
I went to a Chinese buffet for dinner tonight. I love Chinese food.
"If we had a 40,000 foot high fence across the border they couldn't have gotten over it."
Altitude, above 29,000 feet.
The E-3 Sentry is a modified Boeing 707/320 commercial airframe with a rotating radar dome. The dome is 30 feet (9.1 meters) in diameter, six feet (1.8 meters) thick, and is held 11 feet (3.33 meters) above the fuselage by two struts. It contains a radar subsystem that permits surveillance from the Earth's surface up into the stratosphere, over land or water. The radar has a range of more than 250 miles (375.5 kilometers) for low-flying targets and farther for aerospace vehicles flying at medium to high altitudes. The radar combined with an identification friend or foe subsystem can look down to detect, identify and track enemy and friendly low-flying aircraft by eliminating ground clutter returns that confuse other radar systems.
U.S.Air Force