Radar waves travel at the speed of light. Supes may not be able to. We don't know how fast he could go in the L&C universe. Clearly supersonic, but who can say how much faster than sound? As you say, there's a big gap between the speed of light (300,000,000 m/s) and the speed of sound (340 m/s, depending on air pressure and such).
In any case, unless he was flying directly away from the radar tower, the system would be able to see him. He might be somewhat blurry. He might be moving fast enough that his position on the screen would be unreliable (i.e. by the time the radar wave bounces back to the dish and the echo is processed, he'd be somewhere else), but he'd still be seen.
The question is what they'd make of him.
The difference between a large bird and a human being is very small compared to the difference between a human being and even a small airplane. Going supersonic would be a dead giveaway that he's not actually a bird, but it's possible that a human being wouldn't stop to notice that one tiny bird-sized blip was moving faster than it should.
It's also possible that he might be dismissed as several birds. The dish circles around. It spots him. Screen tells the operator that there's a bird-sized blip over there. Supes moves super fast, and is in another location when the dish's next sweep picks him up. Oh, there's another bird. The first one is gone again. Must have dropped below detectable altitude...
Paul
