you'd probably need active radar if it isn't transmitting anything, unless you had a extremely high gain and directional passive radar system (i do, but i've never tried to track anything small, but i can see commercial jets just fine).
The Hydra (it's changed names so many times) can do passive radar, which you can probably make active with a tx switch and a transmitter. Passive radar works thus: you aim a directional antenna in one direction, toward some transmitting signal (FM radio, television, whatever), and aim your passive detection antenna in the other direction. The signal from behind will hit whatever you're aiming at and possibly reflect some of the signal back to you, and the hydra radio software can detect "echos" of that sort and put them on a chart with relative sizes and speeds and "distance" as well.
