What can radar track?

Answer 1

Radar, radio detection and ranging, uses a transmitted electromagnetic signal (usually a pulse) which is sent out to "look at" whatever is in the medium being scanned. And any portion of the returning signal that can be recovered is evaluated to determine what is "out there" in the beam. The long and short of it is that anything that will reflect some of the electromagnetic energy (enough to be detectable and recoverable by a radar receiver) can be seen in some form or another. In air, metal aircraft parts reflect electromagnetic energy, and if they reflect enough of it back to the receiver, they can be detected. We can detect birds, too. Weather phenomenon can also be "seen" in this way. Reflections off water and geographic features will also appear, depending on the way the antenna is pointed and/or the type of radar that is being used. And we've developed different types of radar to better "see" different things we wish to look at. We've even developed ground penetrating radar to look for stuff that's buried. There are some fine points, like the fact that Doppler radar cannot "see" anthing that is stationary relative to the receiver, but there are other questions that speak to this and other particulars about radar. Links are provided to the Wikipedia article on radar and also to related questions.