What causes the random fuzz in the area right around the center of the radar sweep? Is it pollution from the city the radar tower is in, or is it just overly sensitive at short distances and is picking up things like dust and birds?
Yeah, it’s stuff like debris, insects and birds. And here in Central Texas if you look at it around sunset… bats!
That kind of stuff is much more visible when the radar is in clear air mode, which is more sensitive and will generally be used whenever it’s not raining or about to rain.
The reason you only see fuzz close to the tower is because the radar signal travels upward as it moves away from the tower, and the higher you go the less of that stuff there is.
I wouldn’t say it’s pollution from the city because the radar towers aren’t usually located in a city. They’re usually out in the country or at a military base.
-fh
This radar orinthology tutorial is pretty darn cool and informative.
-fh
I imagine quite a bit of it is ground clutter caused by the sidelobes of the antenna. Radar doesn’t emit a true “beam” but instead radiates outward in a pattern that is strongest in front of the antenna, but also has small areas of radiation off to the sides of where the beam is pointing (hence the name sidelobes). The antenna can’t distinguish between an echo from the main beam or a sidelobe, so you end up with a bunch of clutter near the antenna.
Some of it also probably is birds and bats and other flying things, as previously discussed.
Ah, thanks everyone.
From the link that hazel-rah posted, I ended up at http://ww2010.atmos.uiuc.edu/(Gh)/guides/rs/rad/basics/cltr.rxml which describes it quite well.
I think that most weather radar is Doppler Radar so that echoes from stationary objects are filtered out. However if the wind is blowing hard enough, trees and stuff will move around giving a Doppler return and will show up ground clutter. That which is close to the transmitter will give a strong return and will show up close-in to the center of the sweep.
Darn. Stupid URL parser. http://ww2010.atmos.uiuc.edu/(Gh)/guides/rs/rad/basics/cltr.rxml
Next question: if the concept of ground clutter is known to the people who amke doppler radar systems, why not do some simple digital post processing to remove the ground clutter before piping the signal out to the viewing public?
Simple signal processing is done. For example, stationary targets are eliminated by a simple, high pass filter. However, anything that provides a doppler frequency within the pass band of that filter will show up as a target. Separating the moving targets that are weather related from non-weather related targets giving the same doppler frequency is anything but “simple” signal processing.