Ralph Jones wrote:

Signal strength is not the problem: a fiberglass ship with a one-foot
corner reflector inside it will look bigger than a metal sailplane.
The bad news: air traffic control radars are "moving target" systems,
which means they filter out returns that don't have any Doppler shift
to indicate a moving object. I don't know what the minimum detectable
speed is, but if you're under it, they just won't see you.

rj

They do not see sailplanes that are thermaling -- tested by Halifax
International Airport controllers