Well, being invisible for the eye doesn't mean being invisible for the radar, altough it's an entirely different matter if a rapidly flying Superman could be picked up because of some blip. Imagine the speed of radar waves being about 300000 km/s or ~ 180000 miles/second - there's no way Superman will be able to travel at the speed of light or electromagnetic waves.
In order to avoid being picked up on radar means being able to outfly radar waves which is hardly possible (one of Wendy's stories pictures Superman flying superluminal in outer space).
There do exist different types of radar technologies whereas impulse-radar is the way to go for flight and weather radar. Instead of those alternatives which send a constant stream of radar waves, impulse-radar sends single pulses in a certain cycle which can be used by Superman to slip in undetected.
I did some reading up on radar technology
And I imagine that the most difficult part about avoiding being tracked will be the way to detect radar waves and ranges of several installations. But in the end, a human controlled and observed radar station will not detect a rapidly flying superman simply because of its limited range.
