Aerokid From Belgium, joined Jun 2000, 348 posts, RR: 0
Reply 1, posted (13 years 6 months 3 weeks 2 days 21 hours ago) and read 2156 times:
Appearence on a radar of an object depends mostly on the radar cross section (RCS) of that object and off course also on the sensitivity of the radar. RCS of aircraft can vary a lot: a 747 has a RCS of a few dozen square meters, whilst an F-117 has a RCS smaller than that of a compact disc. For birds: I don't think a single bird will appear on a radar screen but a swarm of birds might. Just like you can't see a single raindrop on the screen whilst clouds are visible.
Actually, you really don't wanna see birds and clouds on a radar screen (at least not an ATC radar, the on-board weather radar is another case off course), so various techniques are used to filter these echo's out. They can monitor the speed of the objects and everything that moves slower than an aircraft would normally do will be taken off the screen. Clouds can be eliminated by using circular polarisation of the radar waves, reflection against rainddrops turns the direction of polarization and these echo's are not detected by the radar antenna.
I've been visiting a radar station a few years ago (it was a high school excursion) and they turned the filters off to show us how sensitive the radar is: you could see power lines on the screen: cables of only a few inches thick.