Los Angeles airports have a site, http://www.lawa.org . From there you can go to the LAX Airport Monitor at http://www1.passur.com/lax.html . This tracks the positions of aircraft derived from actual radar. As I understand it, it is used in noise abatement so people can identify flights that create too much noise.
You can get 10-minute delayed displays of current data or replay data that is at least 1 hour old.
While looking at this, I noticed that there are oddly-moving aircraft on the display. If you replay data for April 30, 2002 at 22:25, you will notice three aircraft (black icons) on the right side of the screen that wander around very strangely. Clicking on an aircraft icon changes it to red and displays flight information details on the far right. The flights I was looking at have the track IDs 12, 37, and 92. They’re flying pretty low, under 1000 feet, and the aircraft types are all “general aviation”, with no other information.
I didn’t see anything on the lawa site that explained these. From their behavior, I find it hard to believe these are actual aircraft, even helicopters. So what are they? Are they just spurious signals? Or are they really some kind of aircraft?